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Journal of Hazardous Materials 338 (2017) 102–123

Contents lists available at ScienceDirect

Journal of Hazardous Materials

jo ur nal ho me p ag e: www.elsev ier .com/ locate / jhazmat

eview

dsorption of VOCs onto engineered carbon materials: A review

ueyang Zhanga,b,c, Bin Gaoc,∗, Anne Elise Creamerc, Chengcheng Caoa, Yuncong Lid

School of Environmental Engineering, Xuzhou University of Technology, Xuzhou, 221000, PR ChinaShanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Shanghai, 200433, PR ChinaDepartment of Agricultural and Biological Engineering, University of Florida, Gainesville, FL, 32611, USATropical Research and Education Center, University of Florida, Homestead, FL, 33031, USA

r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 3 April 2017eceived in revised form 7 May 2017ccepted 9 May 2017vailable online 12 May 2017

eywords:OC emissionsrganic vaporsarbon-based adsorbents

a b s t r a c t

Volatile organic compounds (VOCs) severely threaten human health and the ecological environmentbecause most of them are toxic, mutagenic, and carcinogenic. The persistent increase of VOCs togetherwith the stringent regulations make the reduction of VOC emissions more imperative. Up to now,numerous VOC treatment technologies have emerged, such as incineration, condensation, biologicaldegradation, absorption, adsorption, and catalysis oxidation et al. Among them, the adsorption tech-nology has been recognized as an efficient and economical control strategy because it has the potentialto recover and reuse both adsorbent and adsorbate. Due to their large specific surface area, rich porousstructure, and high adsorption capacity, carbonaceous adsorbents are widely used in gas purification,especially with respect to VOC treatment and recovery. This review discusses recent research devel-

ir qualityiltration

opments of VOC adsorption onto a variety of engineered carbonaceous adsorbents, including activatedcarbon, biochar, activated carbon fiber, carbon nanotube, graphene and its derivatives, carbon-silica com-posites, ordered mesoporous carbon, etc. The key factors influence the VOC adsorption are analyzed withfocuses on the physiochemical characters of adsorbents, properties of adsorbates as well as the adsorptionconditions. In addition, the sou

∗ Corresponding author.E-mail address: [email protected] (B. Gao).

ttp://dx.doi.org/10.1016/j.jhazmat.2017.05.013304-3894/© 2017 Elsevier B.V. All rights reserved.

rces, health effect, and abatement methods of VOCs are also described.
© 2017 Elsevier B.V. All rights reserved.
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X. Zhang et al. / Journal of Hazardous Materials 338 (2017) 102–123 103

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1032. Overview of VOCs and the abatement methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

2.1. Sources of VOCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1032.2. Health and environmental effects of VOCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1042.3. VOC abatement methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

3. Engineered carbon materials for VOC adsorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1053.1. Activated carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1053.2. Biochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1053.3. Activated carbon fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1083.4. CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1093.5. Graphene and its derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1093.6. Carbon-silica composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1113.7. Ordered mesoporous carbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1113.8. Other carbon materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

4. Key factors controlling VOC adsorption onto engineered carbon materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1124.1. Characteristics of adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

4.1.1. Specific surface area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1124.1.2. Pore size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1124.1.3. Surface chemical functional groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1134.1.4. Other adsorbent properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

4.2. Characteristics of adsorbates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1134.2.1. Molecular structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1134.2.2. Molecular polarity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1144.2.3. Boiling point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1154.2.4. Other adsorbate properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

4.3. Adsorption conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1174.3.1. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1174.3.2. Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1174.3.3. Other adsorption conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

5. Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

. Introduction

Volatile organic compounds (VOCs) are defined as “any com-ound of carbon, excluding carbon monoxide, carbon dioxide,arbonic acid, metallic carbides or carbonates, and ammoniumarbonate, which participates in atmospheric photochemical reac-ions” by U.S. Environmental Protection Agency. Environment anduman health are seriously damaged by VOCs because most ofhem are toxic and can cause serious environmental problems suchs greenhouse effect, photochemical smog, stratospheric ozoneepletion etc. In recent years, great efforts have been made toevelop efficient VOC abatement techniques, such as catalysisxidation, condensation, biological degradation, absorption, anddsorption, among which adsorption using carbon materials as aOC adsorbent is recognized as one of the most economic andromising control strategy. A great deal of research has been con-ucted to investigate the adsorption of VOCs onto a variety ofarbon materials including novel engineered carbonaceous adsor-ents. The overarching goal of this work thus is to provide a criticaleview of the recent research developments of VOC adsorption ontoovel engineered carbon materials as well as the key factors con-rolling the adsorption processes.

. Overview of VOCs and the abatement methods

.1. Sources of VOCs

VOCs are a big family of carbon-based chemicals with commonembers of more than 300 types. The definition of VOCs varies

pogenic VOC emissions have been increasing dramatically due tothe development of industry. For example, in China, the amountof VOCs emitted by industry increased at an average annual rateof 8.5% since 1980, and further increased from 1.15 Tg to 13.35 Tgtill 2010 [1]. The portion of anthropogenic sources is getting larger,and their influence is getting worse because almost every humandaily activity such as cooking, painting, smoking, driving, buildingwould result in the emissions of VOCs [2]. Table 1 lists the sourcesand health effects of some typical VOCs.

The majority of anthropogenic VOC emissions are from theexploitation, storage, refining, transport and usage of fossil fuels[3]. The major VOC emission sources of liquid fossil fuels areasphalt blowing, coking, and catalytic cracking processes, whichcontribute to 27 kg VOCs/m3 asphalt, 0.4 kg VOCs/m3 feed, and0.25–0.63 kg VOCs/m3 feed, respectively [4]. The total emissionamount of non-point source VOCs, which are mainly caused byleaks and evaporation during production, storage, and transporta-tion, is huge and even more difficult to control. It is reported theevaporative VOC emissions of fuel total approximately 2.9 kg t−1 atservice stations [3]. In addition to the liquid fossil fuel, the produc-tion of coal and gas would also result in vast VOC emissions.

Besides the anthropogenic ones, VOCs of natural sources pro-duced from biogenic emissions of terrestrial and ocean occupy amuch higher proportion of total VOC emissions. The natural VOCemissions can reach as high as 1150 Tg C/year, while the VOC emis-sion of human activities is only about 142 Tg C/year [5]. Typicalbiogenic VOCs are isoprene and monoterpenes and the total emis-sion amount of terrestrial source isoprene is about 500 Tg C each

rom country and organization, but all have the general charactersf low boiling point, high vapor pressure, and strong reactivity,specially with respect to photochemical reactions. VOCs comerom both anthropogenic emissions and natural emissions. Anthro-

year [6]. Besides the huge terrestrial source VOC emission, it is
reported the ocean is another important sources of VOCs. Singhet al. [7] found that seawater microlayers are highly supersaturatedin acetone and acetaldehyde. Subsequently, a group of marine bac-

104 X. Zhang et al. / Journal of Hazardous Materials 338 (2017) 102–123

Table 1Sources and health effects of major VOCs [30,183].

Classification Representatives IDLH* Sources Health effects

Alcohols MethanolEthyl alcoholIsopropyl alcohol

6000 ppm3300 ppm2000 ppm

AntisepticsPreservativeCosmetics and personal careproducts

Throat irritation and shortnessof breathEye irritationCentral nervous systemdepression

Aldehydes FormaldehydeAcetaldehyde

20 ppm2000 ppm

Decorative and constructionmaterialsCosmetics and plasticadhesives Fabrics andbio-waste decompositionBiomass burningDegradation of VOCs inmultiple steps oxidations

Irritation of the throat, eyesand skinNasal tumorsPredecessor of ozone

Alkenes PropyleneEthylene

– Petrochemical synthesesProduction of varnishesSynthetic resins, adhesives,printing inkOrganic intermediates ofpharmaceutical and perfumes

Photochemical ozone creativitypotentialPotentially carcinogenic andadversely affects the odor andtaste of drinking water

Aromatic compounds BenzeneTolueneEthylbenzene

500 ppm500 ppm800 ppm

Petroleum productsIncomplete combustion ofliquid fuelsAdhesivesLacquers

CarcinogenDamage the ozone layerProduce photochemical smog,and pose mutagenic hazards

Halogenated VOCs Carbon tetrachlorideChlorobenzene1,1,2-Trichloroethane1,1,2,2-TetrachloroethaneTrichloroethyleneTetrachloroethyleneDichloromethane

200 ppm1000 ppm100 ppm100 ppm1000 ppm150 ppm

Chemical extractantPaintsAdhesivesPolymer synthesesWater purification systems

Strong bioaccumulationpotentialAcute toxicityDestruction of the ozoneCause greenhouse gas effects

Ketones AcetoneEthyl butyl ketone

2500 ppm1000 ppm

Varnishes, window cleaners,paint thinners, adhesives

Irritation of eyes, nose, andthroatCentral nervous systemdepressionHeadache and nausea

Polycyclic aromatichydrocarbons

PhenanthrenePyrene

– Release from creosote andincomplete combustion oforganic matter, coal, oil, and

Carcinogen

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eria with the ability to produce acetone have been discovered [8,9].hese studies have adequately demonstrated that ocean biologys involved in the synthesis of VOCs. Many other VOCs have beenound in the oceans. For example, dimethyl sulphide and methylodide are two VOCs that are predominately produced from marineources [10]. Some alkanes and alkenes are also emitted from theceans at a rate of 5 Tg C/year [11]. The emission of isoprene fromhe ocean is about 0.1 Tg C/year [12]. Further, some researchersound that the ocean is not only the source but also the sink ofOCs [7].

.2. Health and environmental effects of VOCs

VOCs are harmful for both the human health and ecological envi-onment. As for the human health, most VOCs such as aldehydes,romatic compounds, polycyclic aromatic hydrocarbons, alcoholsnd ketones etc. are highly toxic and carcinogenic (Table 1). Alde-ydes are one of the most commonly encountered VOCs, especially

ormaldehyde and acetaldehyde emitted from building materi-ls, are the major indoor pollutants [13,14]. Low level exposuref aldehyde would cause respiratory issues, such as throat irrita-ion, shortness of breath, eye irritation, and chest tightness [15].igh concentration or long term exposure would increase the risk
f acute poisoning or chronic toxicity as well as nasal tumors16,17]. Similarly, formaldehyde can cause serious diseases suchs nasopharyngeal cancer, pulmonary damage, leukemia, and sickuilding syndrome (SBS) [18–22].
biofuels

afety and Health.

Aromatic compounds belong to another group of VOCs, theymainly include benzene, toluene, and ethylbenzene, which are toxicand carcinogenic [23]. Low concentration exposure would induceconfusion, tiredness, nausea as well as loss of appetite, memory,and sight. High concentration inhalation would result in uncon-sciousness, dizziness, and even death [24,25]. Benzene, the majorcause of leukemia and lymphomas can damage human beingsboth specifically and systematically [22,26]. Only 2% benzene inair would be fatal during 5–10 min exposure, and its maximumpermitted exposure concentration is 16.25 �g m−3 [27]. Polycyclicaromatic hydrocarbons (PAHs) are a group of VOCs containing sev-eral benzene rings. It mainly includes naphthalene, phenanthrene,and pyrene, which have been identified as carcinogenic VOCs [28].

Alcohols, primarily ethanol, isopropanol, and n-butanol, cancause serious nervous system depression. Furthermore, alcoholscontribute to the formation of aldehydes, which is more hazardousto human and the environment [29]. Ketones would induce irri-tation of the eyes, nose, and throat. High concentration inhalationwould result in central nervous system depression, headache, andnausea [16,30].

Besides the damages to human, VOCs are major contributors tostratospheric ozone depletion. Many VOCs can reach the strato-sphere after surviving the tropospheric removal processes, which
mainly include photolysis, physical removal, reaction with atmo-spheric hydroxyl free and ozone [31]. Halogenated VOCs wouldparticipate in the stratospheric photolysis and then release activeozone-destroying chain carriers, resulting in both stratospheric

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zone layer depletion and Antarctic ozone hole formation [32].olychloromethanes (PCMs) are typical halogenated VOCs, whichre hazardous to the ecological environment, due to their strongioaccumulation potential, acute toxicity, and resistance to degra-ation. Most PCMs have significant impact on both the ozoneepletion and global warming because they are a source of new rad-

cals. Their 100-year global warming potential (GWP) is 10–1800imes of CO2 [33]. Further, aldehyde VOCs such as formaldehydend acetaldehyde and aromatic VOCs such as benzene and toluenere also the sources of new radicals that can further influencehe formation of ozone [23,34]. The threshold concentration ofromatic compounds is 200 ppm in air [16]. Propylene is also con-idered to be a serious pollutant due to its large photochemicalzone creativity potential [35]. In addition, Methyl tertbutyl etherMTBE), a semi-volatile VOC, is potentially carcinogenic and wouldffect the odor and taste of drinking water severely [36]. The thresh-ld concentration of MTBE in drinking water is 20–30 ppb [37].

.3. VOC abatement methods

With the persistent increase of VOCs and their harmful impactn human health and ecological environment, stringent emissionegulation of Goteborg protocol has been proposed, which stipu-ates that the reduction of VOC emission by 2020 should be halff the sum amount released in 2000 [38,39]. Therefore, develop-ng effective VOC elimination techniques are of great significancend urgent. For this reason, lots of VOC control techniques havemerged, which can be generally divided into recovery meth-ds and destruction methods based on whether the VOCs can beecovered. The recovery methods include adsorption, condensa-ion, absorption, and membrane separation, while the destructionechniques include incineration, photocatalytic oxidation, ozoneatalytic oxidation, plasma catalysis, and biological degradation,tc. Compared with the destruction methods, which mainly convertOCs into CO2 and H2O, the recovery methods are more economicecause they can realize the recovery of VOCs. Furthermore, incin-ration and most of other destruction methods would expend vastmounts of energy to produce high temperature for the reaction,s well as they will inevitably produce some toxic byproducts, suchs NOx, O3, OH• radicals, secondary organic aerosols, etc. The char-cteristics of VOC removal techniques are summarized in Table 2.

Among the recovery methods, adsorption technology has beenonsidered one of the most favorable methods to treat VOCs, mainlyecause it is low cost and high efficiency [40–42]. Finding the opti-al porous solid adsorbent is crucial for the commercial application

f the adsorption technique. Carbon materials, in spite of somenherent disadvantages such as hygroscopicity and pore blockingresent, are recognized to have the most potential as a low-cost,igh-efficiency, and good-stability adsorbent for VOC abatement43].

. Engineered carbon materials for VOC adsorption

.1. Activated carbon

Activated carbon (AC) is produced from carbon-rich materialsuch as coal, peat, lignite, petroleum pitch, wood, nutshells, etc.y the processes of carbonization and activation. It is one of theost popular adsorbents due to its cost efficiency, excellent adsorp-

ion ability, and acid/base- and thermo-stability. The production,odification, application, adsorption, and generation, along with

ther aspects of gas sorption by AC have been well studied andeviewed in the literature. AC can be generated by conventional andicrowave heating methods [44], bioregeneration [45], and surfaceodification [46,47], using precursors such as lignin [48], agricul-

Materials 338 (2017) 102–123 105

tural residues [49,50], rice husks [51], and waste materials [52].Environmental applications of AC include the adsorption of pheno-lic compounds [53], heavy metals [54,55], dyes [56,57] from wastewater, removal of endocrine disrupting compounds, pharmaceuti-cally activated compounds and cyanobacterial toxins in drinkingwater [58,59], landfill leachate treatment [60], CO2 capture [46],VOC adsorption [61] and support for catalytic removal pollutants[62]. The adsorption mechanisms of pollutants onto AC have alsobeen reviewed in previous studies [63].

AC has been widely used in adsorption to recover most types ofVOCs including alkane, alcohols, ethers, aldehydes, ketones, esters,aromatics, etc. The applications of AC on typical VOC vapor adsorp-tion are summarized in Table 3. Based on those applications, it iseasy to draw the conclusion that the VOC adsorption capacity of ACranges from a dozen to several hundreds of milligrams per gram,depending on AC’s physicochemical properties such as surface area,pore size, pore volume, chemical functional groups, etc., the prop-erties of VOCs such as molecular size and the polarity, as well as theadsorption conditions such as temperature, moisture, etc. Becauseof the overwhelming physical adsorption mechanisms, AC withlarge surface area and rich pore structure shows high adsorptioncapacity to VOCs. While the presence of chemical functional groupson AC surfaces makes it a good adsorbent for polar VOCs throughthe chemical adsorption mechanisms.

There are drawbacks of AC, affecting its adsorption abilitytoward VOCs. AC is a natively nonpolar adsorbent that wouldinevitably limit the adsorption toward hydrophilic VOCs. In addi-tion, the porous structure of AC is primarily within the microporerange (pores size less than 2 nm), which would hinder the trans-port of VOC molecules, especially those with larger molecular sizes,into the pores. Furthermore, the disordered pores of AC would pro-long the adsorption equilibrium because of the increase of diffusionresistance in the irregular pore structures.

3.2. Biochar

Biochar is a group of carbon materials produced from biomassby slow pyrolysis under inert atmosphere (Fig. 1). Although thesource materials and production methods of biochar are similar asactivated carbon, the distinctions between them are obvious. Forexample, the production temperature of biochar is usually less than700 ◦C, which is lower than that of AC. Besides, during the biocharproduction, the process of activation is unnecessary while it is cru-cial for AC production. In addition, the break-even price of biocharis about US $246 t−1, which is only 1/6 of that of commerce AC[64,65]. Biochar has been widely used in various fields such as soilfertility improvement [66–68], CO2 sequestration [69,70], catalyticconversion syngas into biodiesel [71], and pollution remediation[65]. Table 4 summarizes the applications of biochar on environ-mental remediation. It indicates that almost every aspect of thepollution abatement is involved in biochar applications, such astreatment of liquid waste, removal of toxic gas, as well as treatmentof solid waste and soil remediation, however, limited research hasbeen done on VOC vapor adsorption onto biochar (Table 4).

The feedstock type of biochar determines its physiochemicalproperties that would further influence its adsorption performance.As shown in Table 4, under the same pyrolysis temperature of600 ◦C, the surface areas of woody plant biochar derived from pinewood, and hickory are 312 and 256 m2 g−1, respectively, which arelarger than that of herbaceous plant biochars. The surface areas ofherbaceous plant biochars derived from alfalfa, cotton, switchgrass,corn straw, rice husk, and wheat straw at 600 ◦C are 0.2, 2.2, 15,
61, 168, and 182 m2 g−1, respectively. According to the literatures[72,73], the molar ratios of H/C and O/C can be used to indicate thequantization aromaticity and polarity of biochar. The H/C whichreflect the aromaticity of biochar ranged from 0.002 for corn straw


Table 2Technological characteristics of VOC removal methods.

Methods Efficiency Market sales Reuse Waste generation Energyconsumption

VOCsconcentration

Ref.

Incineration >99%(40 min) High No CO, NOx Moderate 20%–25% [224,225]Condensation Moderate High Yes – High >5000 ppm [225]Biological degradation 100% (∼7 months) Nca No Acetaldehyde, Propanal,

AcetoneLow <5000 ppm [226]

Absorption – Low Yes Spent absorbent Moderate –Adsorption >90% High Yes Spent adsorbent Moderate 700–10000 ppm [227]Plasma catalysis 74%–81% Nca No Formic acid, Carboxylic

acids, NOx, O3

High – [228]

Photocatalytic oxidation 100%(5 min) Low-moderate No Strong oxidant OH· radicals Moderate – [229,230]Ozone-catalytic oxidation 100% (2 h) High No Secondary organic aerosols High – [231]Membrane separation – Nca Yes Clogged membranes High <25% [224,225]

a Not widely commercialized.

Table 3Summary of the applications of AC on VOC adsorption.

Adsorbate Formula Formula weight g mol−1 Adsorption capacity mg g−1 Condition Ref.

Acetone C3H6O 58.08 483.09 25 ◦C, 800 L h−1 [232]Acetone C3H6O 58.08 343.89 20 ◦C, 5 L h−1, 50 g m−3 [221]Benzene C6H6 78.11 27.50 25 ◦C, 3.6 L h−1, 6000 ppm [208]Benzene C6H6 78.11 161.42 0.680 P0 [233]Butanol C4H9OH 74.12 262.38 20 ◦C, 625 Pa, [234]Butanone C4H8O 72.11 24.30 25 ◦C, 60 mL min−1, 6000 ppm [208]Butanone C4H8O 72.11 364.88 20 ◦C, 5 L h−1, 50 g m−3 [221]Cyclohexane C6H12 84.16 327.18 25 ◦C, <0.2 P0 [235]Dichloromethane CH2Cl2 84.93 360.95 30 ◦C, 742 Pa, 5 mL min−1 [236]Ethanol C2H6O 46.07 15.90 25 ◦C, 60 mL min−1, 6000 ppm [208]Ethanol C2H6O 46.07 389.84 20 ◦C, 5 L h−1, 50 g m−3 [221]Ethyl acetate C4H8O2 88.11 420.92 25 ◦C, 0.740 P0, 6000 ppmv [237]Ethyl acetate C4H8O2 88.11 450.24 20 ◦C, 9686 Pa [234]Ethyl acetate C4H8O2 88.11 388.65 20 ◦C, 5 L h−1, 50 g m−3 [221]isopropyl acetate C5H10O2 102.13 147.45 25 ◦C, 0.822 P0 [237]isobutyl acetate C6H12O2 116.16 151.71 25 ◦C, 0.7222 P0 [237]Methanol CH3OH 32.04 10.60 25 ◦C, 60 mL min−1, 6000 ppmv [208]methyl acetate C3H6O2 74.08 165.54 25 ◦C, 0.497 P0 [237]m-xylene C8H10 106.16 31.09 0.606 P0 [233]m-xylene C8H10 106.16 292.40 25 ◦C, <0.2 P0 [235]n-Hexane C6H14 86.18 379.90 25 ◦C, <0.2 P0 [235]n-Propanol C3H8O 60.10 30.30 25 ◦C, 60 mL min−1, 6000 ppm [208]n-propyl acetate C5H10O2 102.13 199.64 25 ◦C, 0.5053 P0 [237]n-butyl acetate C6H12O2 116.16 260.80 25 ◦C, 0.6154 P0 [237]o-xylene C8H10 106.16 305.70 22–27 ◦C, 45 mL min−1, 2176–2239 mg m−3 [79]o-xylene C8H10 106.16 90.40 25 ◦C, 60 mL min−1, 6000 ppmv [208]o-xylene C8H10 106.16 111.47 0.741 P0 [233]p-xylene C8H10 106.16 29.46 0.556 P0 [233]Toluene C7H8 92.14 364.96 25 ◦C, 0.8 m3 h−1 [232]Toluene C7H8 92.14 59.20 25 ◦C, 60 mL min−1, 6000 ppmv [208]Toluene C7H8 92.14 109.45 0.818 P0 [233]Toluene C7H8 92.14 366.72 20 ◦C, 2910 Pa [234]Toluene C7H8 92.14 424.40 20 ◦C, 5 L h−1, 50 g m−3 [221]1,1,1-trichloroethane C2HO2Cl3 163.40 765.60 25 ◦C, <0.2 P0 [235]

377.2 ◦ 3 −1

526.6415.6

bttoi

pVssOtiw

1,2-dichloroethane C2H4Cl2 98.96

1,2-dichloroethane C2H4Cl2 98.96

2-ethyl-4-methyl-1,3-dioxolane C6H12O2 116.16

iochar to 0.04 for the rice husk biochar, while the O/C which reflecthe polarity of biochar ranged from 0.03 for hickory wood biocharo 0.37 for switchgrass biochar. More than ten times difference isbserved on both aromaticity and polarity, which would inevitablympact the performance of biochar on VOC adsorption.

Besides the feedstock type, pyrolysis conditions, especially theyrolysis temperature, also affect the performance of biochar onOC adsorption via changing both the morphology structure andurface chemical functional groups. As shown in Fig. 2, the specificurface area of biochar increases with the pyrolysis temperature.n the contrary, the pore size would decrease with the pyrolysis

emperature [74–77]. In addition, the pyrolysis temperature alsonfluences the chemical functional groups of biochar. It has been

idely reported that high pyrolysis temperature facilitates the

2 25 C, 0.8 m h [232]7 20 ◦C, 5 L h−1, 50 g m−3 [221]1 20 ◦C, 5 L h−1, 50 g m−3 [221]

removal of oxygen containing groups and increase the aromaticityof biochar, which would subsequently promote the adsorption ofhydrophobic VOCs [72,73,78–80]. Ahmad et al. [81] compared thetrichloroethylene adsorption onto biochar, and found the biocharproduced at 700 ◦C exhibited higher adsorption efficacy than thatat 300 ◦C. Similar finding has been reported on the adsorption ofnaphthalene, nitrobenzene, and m-dinitrobenzene onto biocharproduced at 100–700 ◦C [78].

In general, the adsorption of organic compounds onto car-bonaceous adsorbents are mainly controlled by five potentialinteractions, i.e., hydrophobic effect, �-� bonds, hydrogen bonds,
Van der Waals interactions, and covalent and electrostatic interac-tions [82,83]. The governing mechanisms of VOC adsorption ontobiochar are summarized in Fig. 2, which mainly include electro-

X.

Zhang et

al. /

Journal of

Hazardous

Materials

338 (2017)

102–123

107

Table 4Summary of physiochemical properties and environmental applications of biochar.

Raw materials Pyrolysistemperature ◦C

Specific surfacearea m2 g−1

Total pore volumecm3 g−1

C%

H%

O%

N%

Adsorbate Applicationphase

Ref.

Alfalfa 600 0.2 0.006 73.25 1.91 19.43 1.91 As, Pb Liquid [238]Bagasse 600 388.3 76.4 2.9 18.3 0.8 Methylene blue Liquid [239]Bamboo 600 375.5 80.89 2.43 14.86 0.15 Methylene blue Liquid [239]Bamboo 500 56.9 39.3 H2S Gas [240]Camphor tree 500 22.6 38.8 H2S Gas [240]Coconut shell** Purchased 1421 1.148 CH2Cl2, CHCl3,

CCl4, CH3IGas [183]

Citrus 600 182 0.013 78.28 2.08 14.90 1.28 As, Pb Liquid [238]Corn stalk* 700 1284 0.597 [BMIM][Cl] Liquid [241]Corn straw 600 61.0 0.036 85.3 1.7 5.2 0.8 Cu2+ Liquid [242]Cottonwoodtree

600 99 0.01 CO2 Gas [243]

Cottonwoodtree*

600 749 0.33 CO2 Gas [243]

Cotton wood 600 2.2 0.01 72.38 1.85 25.31 0.46 Ni Liquid [244]Cotton stalk 600 224.0 0.07 CO2 Gas [245]Hickory 350 0.5 71.6 3.88 23.2 0.25 Pb,

Methylene blueLiquid [74]

Hickory 450 1.6 77.6 3.52 17.3 0.27 Pb,Methylene blue

Liquid [74]

Hickory 600 256 84.7 1.83 11.3 0.30 Pb, Methyleneblue

Liquid [74]

Hickory* 600 873 821 2.25 13.2 0.25 Pb, Cu, Cd, Zn,Ni

Liquid [246]

Hickory wood 600 101 81.81 0.73 2.17 Pb, Cu, Cd Liquid [247]Monterey Pine 300–500 62.2–82.6 0.04–0.22 Ammonia Soil [248]Peanut shell* 700 1347 0.484 [BMIM][Cl] Liquid [241]Pine cone 500 6.6 0.016 67.9 3.9 22.1 0.5 As3+ Liquid [249]Pine cone* 500 11.5 0.028 71.2 3.0 20.4 0.5 As3+ Liquid [249]Pine wood 600 209.6 51.7 1.4 43.1 0.2 As Liquid [250]Pine wood 600 312 85.6 2.84 9.86 0.09 Levofloxaci Liquid [251]Pine needle 500 13.06 0.015 90.10 2.06 3.74 4.10 Trichloroethylene Liquid [81]Pine needle 700 390.52 0.12 93.67 0.62 2.07 3.64 Trichloroethylene Liquid [81]Pinus taeda 600 209 0.003 85.68 2.13 11.40 0.33 As, Pb Liquid [238]Rice husk 600 168 51.8 2.05 10.4 0.56 Levofloxaci Liquid [251]Rice straw Uncontrolled

burn72.1 0.133 18.49 0.71 – 0.69 Pentachlorophenol

(PCP)Soil [252]

Rice hull 500 115 43.3 H2S Gas [240]Switchgrass 600 15 0.024 68.15 2.21 24.99 1.90 As, Pb Liquid [238]Saw dust 500 2.6 0.05 68.7 3.8 – 0.3 Cu2+ Liquid [253]Saw dust* 500 2.5 0.05 62.1 4.2 – 4.6 Cu2+ Liquid [253]Sicyosangulatus L

300 0.9 0.004 66.0 5.6 23.1 5.1 Sulfamethazine Liquid [254]

Tea waste 300 2.3 0.006 71.5 4.8 18.2 5.5 Sulfamethazine Liquid [255]Tea waste 700 342.2 0.022 85.1 2.0 8.9 3.9 Sulfamethazine Liquid [255]Wheat straw 450 9.5 0.012 47.2 2.4 18.4 1.1 Nitrate,

PhosphateLiquid [256]

Wheat straw* 450 50.0 0.038 25.9 1.7 21.6 0.6 NO3•, P Liquid [256]Wheat straw* 700 1283 0.611 [BMIM][Cl] Liquid [241]Wheat straw 600 182 NO3• Liquid [257]Agricultural/forestrywastes

100–500 H2S Gas [240]

* Modified biochar; **VOC adsorption.


lysis te

ssadbicoprmsiotp

3

mopsusieigip

Fig. 1. Production of biochar and the influence of pyro

tatic attraction, interaction between polar VOCs and hydrophilicites, interaction between nonpolar VOCs and hydrophobic sites,nd partition in non-carbonized portion. The mechanisms differepending on the pyrolysis temperature of biochar. Partition haseen suggested to be the primary mechanism for organic contam-

nant abatement by biochar produced at 100–300 ◦C because thehar contains many non-carbonized portion. While the adsorptionnto the surface of biochar is the major mechanism for the biocharroduced at 400–700 ◦C [65,78]. The production of biochar mightesult in the release of VOCs, such as methanol, acetic acid, acetone,ethyl acetone, and acetaldehyde [84–86]. Nevertheless, biochar

till has the great potential to be used in VOC adsorption because ofts excellent adsorption efficiency and low cost. Due to complexityf VOC adsorption onto biochar, further research is in crucial needo better understand the adsorption processes and mechanisms toromote the applications of biochar in VOC abatement.

.3. Activated carbon fiber

Activated carbon fibers (ACFs) are novel fibrous carbonaceousaterials, which are prepared by carbonization and activation of

rganic fibers (such as polyacrylonitrile fibers, cellulose fibers,henolic resin fibers, pitch fibers, etc.) at 700–1000 ◦C in the atmo-phere of steam or carbon dioxide [87]. As shown in Fig. 3, ACFs havenique characteristics compared with activated carbon. Thin-fiberhape with short and straight micropore make ACFs have fasterntraparticle adsorption kinetics than AC. Furthermore, ACFs areasier to be handled into desired form such as felt or fabric, which
s convenient for engineering applications. Therefore, ACFs are aood candidate for gas adsorption for their large adsorption capac-ty and high mass transfer rates during adsorption or desorptionrocess [88–92].
mperature (data are from Table 4 and reference [65]).

For most of the porous solid adsorbents, the morphologycharacteristics decide their adsorption ability, ACFs are no excep-tion. Baur et al. [93] compared the effect of surface morphologyon ACFs adsorbing toluene based on the calculation of adsorp-tion enthalpy. They found the adsorption strength of ACFs withultra-microporous (dpore < 1 nm) was higher than that with supermicroporous (1 nm < dpore < 2 nm). Similarly, Ródenas et al. [94]suggested the narrow microspores (dpore < 0.7 nm) other than totalmicropore (dpore < 2 nm) is the key factor for benzene adsorptiononto ACF. Generally, ACFs with narrow pore exhibit superior VOCadsorption performance.

ACFs are naturally hydrophobic, as reported the content of sur-face oxygen groups is only 735–865 �mol g−1, which is less thanthat of most AC (ranging from 1570 to 4289 �mol g−1) [94,95].The adsorption capacity of VOCs onto ACFs greatly depends on thepolarity of VOC molecule. Nonpolar and weak polar molecules canbe easily adsorbed onto ACFs, while the polar ones are reverse.The adsorption of acetaldehyde, benzene, and toluene onto ACFshas been compared in previous studies [95,96]. The authors foundthe adsorption capacity of polar molecular acetaldehyde is 3.2 wt.%,less than that of nonpolar benzene (31 wt.%) and toluene (53 wt.%).What’s more, the adsorption capacity of nonpolar VOCs onto ACFsis even greater than that on AC with higher specific surface area[97].

As mentioned, the adsorption performance of ACFs toward polarVOCs is terrible. Thus, to increase the polar VOC adsorption, modi-fication by acid is usually carried out onto ACFs. After modificationby HNO3, the acetaldehyde adsorption capacity on ACFs increasedfrom 3.2 wt.% to 9.9 wt.% [96]. Similar improvement of methanol
and ethanol on modified ACFs were also reported [98]. Besides themodification on ACFs, wet impregnation of metal oxides nanopar-ticles onto ACFs can efficiently promote the VOC adsorption either.


F ants. Cb har ann

Bdt

3

aucgnNci0ttfaN[ha[oa

isBhrCti

ig. 2. Postulated mechanisms of the interactions of biochar with organic contaminetween biochar and organic contaminant, II – electrostatic attraction between biocon-polar organic contaminant [65].

aur et al. [96] compared the acetaldehyde adsorption onto ACFsoped with La2O3, CaO, MgO, ZnO, and Al2O3, and found the adsorp-ion amount up to 10 times compared to the original ACFs.

.4. CNTs

Carbon nanotubes (CNTs) are engineered carbon nanomateri-ls in form of rolling up graphene sheets into cylinder. CNTs aresually produced by arc discharge [99], laser ablation [100], andhemical vapor deposition [101], etc. According to the number ofraphene sheets, CNTs can be divided into single-walled carbonanotubes (SWCNTs) and multi-walled carbon nanotubes (MWC-Ts). The SWCNTs are rolled by a single graphene sheet into aylinder while the MWCNTs are rolled by at least 2 graphene sheetsnto a stacking of concentric layer cylinders with an interspacing of.34 nm [102]. Since the discovery of CNTs by Iijima [99] in 1991,hey are widely used in many fields. Using as adsorbent is one ofhe most promising application for CNTs possess large specific sur-ace area, controlled cylindrical hollow structure, hydrophobic wallnd easily modified surfaces [103]. The adsorption of CO2 [104],Ox [105], H2 [106], NH3 [107], methane, and VOC gas molecules

108,109] on CNTs has been widely explored. In addition, CNTsave strong adsorption affiliations to organic compounds such as,nthracene and its derivatives [48], n-nonane and CCl4 [110], dioxin52], thiophene [111], and PAHs [47]. The adsorption capacities ofrganic compounds onto CNTs are usually higher than that onto ACnd other carbon adsorbents.

The mechanisms of VOC adsorption onto CNTs are primary phys-cal adsorption together with slight chemical reaction betweenurface functional groups of CNTs and the VOC molecule [112–114].ecause CNTs are composed of graphene sheets, they are highlyydrophobic, which facilitate their strong adsorption of aromatic
ing VOCs [115]. Generally, the adsorption of nonpolar VOCs ontoNTs is primary controlled by physical processes, while the adsorp-ion of polar VOCs onto CNTs has the participation of chemicalnteractions, which have been proven by the enthalpy changes and
ircles on biochar particle show partition or adsorption. I – electrostatic interactiond polar organic contaminant, and III – electrostatic attraction between biochar and

desorption activation energy. For example, the enthalpy changes ofnonpolar VOCs trichloroethylene, benzene, and n-hexane adsorbedonto CNTs are relatively low indicate they are physical exothermicadsorption processes. On the contrary, acetone has higher enthalpychange and need higher desorption activation energy, indicatingthe polar VOC acetone is adsorbed onto CNTs mainly through chem-ical interactions [113]. As for the adsorption sites, although threelocations, i.e., inside of nanotubes, external surface of CNTs, andinterstitial space between graphite sheet layers of MWCNTs, arepotentially adsorption sites for VOC adsorption, the layer to layerinterspace is generally too small for VOC adsorption. Most VOCsthus are adsorbed on the external surface or internal of the CNTs.which has been confirmed in the literatures [110,116].

Both the defects and amorphous carbon on the surface of CNTsgreatly affect the VOC adsorption processes. Shih and Li [113] foundthat acetone prefer to be adsorbed onto MWCNTs with topologicaldefects, which is in accordance with the observation reported inother literatures [45,57,117]. On the other hand, the amorphouscarbon on the surface of CNTs is a strong adsorbent for hydrocar-bons, which has been widely reported [112,113].

Although CNTs are promising VOC adsorbents, aggregation isone of the most crucial drawbacks that limits their commercialapplications. To overcome the CNT aggregation, surface oxidationand coating with surfactants are effective solutions for dispersionCNTs in liquid phase [118–120]. As for CNT adsorption in gas phase,supporting method is more effective than the oxidation and coat-ing methods, which was demonstrated by Tulaphol et al. [121],who embedded the MWCNTs into SiO2 particles and found the sup-ported adsorbent has excellent adsorption performance to gaseouschlorinated phenolics.

3.5. Graphene and its derivatives

Graphene discovered by Novoselov et al. [122] is the first two-dimensional atomic crystal with the single layer of sp2-hybridizedcarbon atoms arranged in a hexagonal manner. Graphene can


OC ad

ba[o

Fig. 3. ACF characteristics and V

e produced by methods of exfoliation [123], hydrothermal self-ssembly [124], chemical vapor deposition [125], nanotube slicing126], etc. Graphene oxide (GO) is one of the typical derivativesf graphene. GO is functionalized with several oxygen-containing

sorption mechanisms [268,269].

groups such as carboxylic, hydroxyl, and epoxide groups. BothGraphene and its derivatives are widely used in the fields ofelectronics, sensors, photonics, energy storage, and environmentmanagement because of their excellent physical-chemical charac-

rdous

tsRdoi[nr

roaTtMsivbGtdM

ttsocarwtatta

3

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eristics such as huge theoretical surface area, great mechanicaltrength, conductivity, and chemical functionalization [127,128].egarding to their environmental applications, graphene and itserivatives demonstrate excellent performances on the treatmentsf waste water containing heavy metals, dyes, and organic ornorganic pollutants and toxic gases such as NH3, H2S, and VOCs129,130]. Graphene and its derivatives seem to be attractive alter-atives for toxic gas adsorption; however, the severe aggregationestricts their commercial applications as gas adsorbents.

Graphene and its derivatives are a novel adsorbent for VOCsemoval. As reported, Metal-organic frameworks (MOF)/graphenexide composites can successfully adsorb acetone (20.1 mmol g−1

t 288 K) [131] and n-hexane (1042.1 mg g−1 at 298 K) [131,132].heir acetone adsorption capacity is 44.4% larger than MOF’s andheir n-hexane adsorption amount is nearly 2 times greater than

OF’s and 2–11 times greater than that of AC and zeolites [132]. Theuperior adsorption performance of MOF/graphene oxide compos-tes is ascribed to their large surface area (up to 3502.2 m2 g−1), poreolume (1.75 cm3 g−1), and the strong dispersive forces producedy the introduction of graphene oxide with dense arrays of atoms.raphene is an excellent catalyst support for VOC catalytic oxida-

ion reaction. For example, MnO2/graphene can effectively catalyticecomposition toluene for the synergetic effect of graphene andnO2 [133].The water-reresistant performance is one of the most crucial fac-

ors affecting the commercial applications of gas adsorbents. Dueo plenty of oxygen groups present on the surface of GO, it exhibitstrong hydrophilicity. Thus, the performance of VOC adsorptionnto GO would be impact by water vapor. Removal of the oxygen-ontaining groups would facilitate VOC adsorption onto GO in thetmosphere of water vapor. For example, Bai et al. [91] preparededuced graphene oxide (RGO)/carbon composite ultrafine fiber,ith the GO being reduced to RGO by H2 during the carboniza-

ion. Because of the removal of oxygen groups, more sp2 carbontoms emerged on the adsorbent that would weaken the reac-ion between water vapor and the adsorbent [134]. Consequently,he RGO/carbon composite ultrafine fiber shows higher adsorptionbility toward VOCs than water molecular.

.6. Carbon-silica composites

The carbon-silica composites (CSCs) are usually prepared by fill-ng the carbon containing precursor chemicals into the mesoporeilica followed by carbonization (Fig. 4). The obtained CSCs are goodandidates for VOC adsorption because of their large surface area,ontrollable pore size distribution, as well as high ignition tempera-ure. The embeddedness of carbon into mesopore silica can reducehe large pores into micropore and shorten the diffusional path,oth of which are contributed to VOC adsorption. Moreover, theore size of CSCs is determined by the carbon loadings, as reportedy Clippel et al. [135], when the carbon content below 5 wt.% orurpass 22 wt.%, the predominant pore in CSCs is mesoporous oricroporous, respectively, while bi-porous CSCs was synthesized

etween 5 wt.% and 22 wt.%. Clippel et al. [135] suggest the bi-orous CSCs have high VOC adsorption capacity, while others foundhe highest toluene adsorption capacity of 27.6 wt.% was observedn microporous primary CSCs [136]. The difference may be associ-ted with the characteristics of VOC molecules.

Because of their unique pore size, the VOC adsorption abilityf CSCs is enhanced compared with their parent materials. CSCsith 0.5 nm nano-porosity show faster ethane adsorption equilib-

ium than AC and MCM-41 [137]. The methyl ketone adsorption
apacity is 25.7 wt.% on CSCs, which is higher than that of their par-nt material MCM-41 [138]. Benzene adsorption capacity of CSCss 5.06 mmol g−1, which is higher than that of AC (4.37 mmol g−1)139]. Both the benzene and ethyl benzene show higher adsorp-
Materials 338 (2017) 102–123 111

tion onto CSCs than that onto AC [140]. The improvement of CSCs’adsorption performances is attributed to their shorter diffusionalpath, highly dispersion of carbon, strong affinity, and less masstransfer resistance. Besides the excellent adsorption performance,CSCs have higher spontaneous ignition temperature (SIT) than AC,making them more suitable for commercial applications [136].

3.7. Ordered mesoporous carbons

Ordered mesoporous carbons (OMCs) are synthesized for thefirst time by Ryoo et al. [141], who impregnated the carbon pre-cursor into ordered mesoporous silica followed by carbonizationand template removal. Some severe disadvantages of this method,such as need hard template, incompatibility between the templateremoval method and objective materials, and low synthesis effi-ciency, provoke the development of more effective self-assemblymethods, which use low-molecular-weight resol as carbon source,silica as triblock copolymer, and surfactant as structure-directingagent [142,143]. Since the discovery of OMCs, they have beenwidely used in catalysis and energy storage because of theirunique properties, such as tunable pore size, large specific surfacearea, huge pore volume, good chemical inertness and mechani-cal stability, as well as electronic conductivity [143,144]. OMCsalso show great adsorption ability in pollutant treatment, suchas CO2 separation and capture [145,146], disposal of wastewa-ter containing bulky dye [147], 2,4-dichlorophenoxyacetic acid[148], ciprofloxacin [149], Cd(II) [150], dibenzothiophene [151], etc.Although numerous studies have certified OMCs as an adequateadsorbent in environmental remediation, their applications in VOCadsorption are not well studied previously.

In the past, the mesoporous channel is usually regard as only thepassageway for adsorbate molecules. Nowadays, more and moreresearchers have found the mesoporous channel can provide effec-tive adsorption sites for adsorbate molecules [152,153]. Becauseof their larger pore size and less adsorption resistance, OMCs canexhibit higher VOC adsorption than many conventional adsorbents.Wang et al. [153] investigated the adsorption of benzene, cyclohex-ane, and hexane onto OMCs, and found the diffusion rate constantsof VOCs onto OMCs are larger than that onto carbon molecular sieve,and are almost 2 times of that onto AC.

The most attractive VOC adsorbents should possess not onlyeffective adsorption performance but also quick desorption ability.Although microporous adsorbents exhibit superior VOC adsorp-tion ability for their pore sizes are close to the VOCs moleculesizes, the strong affinity would inevitably cost the difficulty of VOCdesorption. OMCs with relatively large pore size successfully over-come this drawback, and the adsorbed VOCs can be easily desorbed[146,147].

3.8. Other carbon materials

Carbon cryogels microspheres (CCMs) are synthesized throughthe sol-gel polycondensation followed by freeze-drying and pyroly-sis in inert atmosphere [154]. The obtained CCMs have high specificsurface areas and rich mesopore, and moreover, their mesoporositycan be tailored by changing the type, proportion, and concentrationof reactant, as well as the pyrolysis temperature. CCMs are suit-able for many applications, such as methane [155] and hydrogen[156] storage, phenol and reactive dyes adsorption from aque-ous solution [157], column packing materials for HPLC [158], andlithium ion batteries [159]. CCMs are also an excellent adsorbentfor VOC adsorption. as reported by Yamamoto et al. [160], both the
amorphous and crystal graphitic structures of CCMs facilitate VOCadsorption, additionally, the disordered graphitic structure locatedinner part of CCMs while the graphitic structure is outside. TheVOC adsorption capacity on CCMs depends on their pore size and


n–silic

mo

twdpMraoo

baiFo

4c

4

iaV

4

sssbfStVs

isaascab[smt

Fig. 4. Schematic of carbo

icropore volume. The influence of moisture on VOC adsorptionnto CCMs is less than that onto AC.

Mesoporous graphite carbon (MGC) is usually synthesized viaemplate-based method. MGC possesses mesoporous structureith excellent hydrophobicity, thermal stability, adsorption andesorption ability. Therefore, MGC is considered as one of the mostromising adsorbents for VOCs [161]. Zhu et al. [162] preparedGC and investigated its adsorption performance on benzene. They

eported that benzene adsorption capacity of MGC was as highs 19.615 mmol g−1. They also found 100% benzene can be des-rbed and no obvious degradation on adsorption performance wasbserved after three times cycle.

Many other carbonaceous materials such as coke [163], carbonlacks [164,165], fullerenes [166], natural graphite [167], etc. arelso widely used as adsorbents. However, most of them are usedn aqueous phase with the adsorbates mainly as inorganic ions.urther investigations thus are needed to explore the adsorptionf VOCs onto those carbonaceous adsorbents.

. Key factors controlling VOC adsorption onto engineeredarbon materials

.1. Characteristics of adsorbents

Specific surface area, pore structure together with surface chem-cal functional groups are three crucial factors of engineered carbondsorbents, which would directly determine their performance onOC adsorption [168,169].

.1.1. Specific surface areaFor any adsorbents, large specific surface area usually means

uperior adsorption performance because surface area provides theites for adsorption processes. Das et al. [170] compared the effect ofpecific surface area on toluene adsorption and found that toluenereakthrough time in ACF of larger surface area (1700 m2 g−1) wasour times of that of smaller surface area (1000 m2 g−1). Similarly,hih et al. [113] compared the adsorption of acetone, benzene,richloroethylene, and n-hexane on two kinds CNTs, and higherOC adsorption capacities were observed on the CNTs with largerurface area.

In order to increase surface area of carbon adsorbents, open-ng of inaccessible pores or creation of new pores are two commontrategies. Modifications of the carbonaceous adsorbents by heat,cid, base, microwave, ozone, plasma, impregnation, etc. thusre often used to modify carbonaceous adsorbents [171,172]. Ashown in Section 2.2 (Fig. 1), the specific surface area of pyrogenicarbon materials increases with elevating the pyrolysis temper-ture because increase the pyrolysis temperature can open thelocked micropores and create more defects on existing pores
76,78,173–175]. Proper acid treatment can effectively enlarge theurface area of many carbon materials; however, overdose aciday decrease the surface area due to the destruction or collapse of
he pores [79,176–178]. Similarly, alkali treatment can also effec-

a composites production.

tively enlarge the surface area of some carbonaceous adsorbents toimprove their VOC adsorption capacity [79].

AC with the highest surface area, however, does not alwaysmean the best adsorption ability for organic compounds. A casein point is batch experiments on toluene adsorption onto AC [179],the tested AC with surface area of 798 m2 g−1 can adsorb 656 mg g−1

toluene, much higher than that (346 mg g−1) of another AC withhigher specific surface area (2719 m2 g−1). Similar phenomenonhas been observed by Bansode et al. [180], indicating adsorption is acomplicated processes controlled by many other factors in additionto surface area.

4.1.2. Pore sizeCarbonaceous materials are porous adsorbents, thus the mor-

phology structure especially the distribution of pore size isresponsible for their VOC adsorption ability [79,181,182]. Theirpores can be dived into micropore (pore diameter <2 nm), meso-pore (2 nm < pore diameter <50 nm), and macropore (pore diameter>50 nm). In general, micropores provide principal adsorption sites,while the mesopore enhance the intra-particle diffusion andshorten the adsorption time. VOC adsorption onto engineered car-bon materials thus is impacted by pore size in various ways.

It has been pointed out that micropore particularly the narrowmicropore dominates VOC adsorption onto carbon materials. Qianet al. [183] compared the adsorption of chloromethanes and CH3Ion AC, and found the adsorption capacities (calculated as mL g−1)of all the adsorbates are close to the micropore volume of theAC. Similar observations have been reported on toluene and ace-tone adsorption by AC [184] and biochar [185]. To figure out theeffect of pore size distribution on VOC capture, it is necessary toeliminate the influence of chemical functional groups and surfacearea. It has been reported that, after removing oxygen functionalgroups by thermal treatment, narrower micropore (size <0.7 nm)volume of AC is a better indicator on benzene adsorption capac-ity than the total micropore volume [94]. Crespo and Yang [111]proposed the conception of normalized adsorption using a per-area basis (�mol m−2) to eliminate the influence of surface areaon adsorption. Although the micropore volume is beneficial for theVOC adsorption onto carbon, the diffusion resistance may increasein the narrow pores, leading to low adsorption rates [186,187].

The adsorption rates of VOCs in mesopore are faster thanthat in micropore due to the higher intra-particle diffusion rate.Tsai et al. [187] compared the adsorption kinetic curves of sev-eral VOCs onto ACF (pore diameter 1.7 nm), AC (pore diameter2.1 nm) and biochar (pore diameter 2.7 nm). They found the dif-fusion coefficients of VOCs onto mesopore AC and biochar rangedfrom 10−5 to 10−4 cm2 s−1, higher than that onto micropore ACF(10−8–10−7 cm2 s−1). Similarly, Wang et al. [153] found the diffu-sion rate constants of VOCs in ordered mesopore carbon are almosttwice of that in micropore AC. Higher diffusion rate of VOCs in
mesopore than that in micorpore are widely reported in the lit-eratures [160].
Optimal adsorption occurs where the pore size fits the adsorbatesize; thus, large pores such as mesopores would be more suitable for

rdous

VwmtTtfietk5matVlwt

4

cpm[baaohectatfinnifi

wsemgo[cf

tct[caoebsts


OCs with large molecules [188,189]. Kim et al. [178] reported that,ith the reduction of pore size, the adsorption capacity of smallolecule VOCs (e.g., MeOH, EtOH and i-Propanol) increase, while

hat of large molecule VOCs (e.g., m-xylene and MEK) decrease.his is in good agreement with the fact that micropore facilitateshe adsorption of small size VOCs while large molecule VOCs pre-er mesopore. It should be noted that, even when VOC molecules smaller than pore size, it not always guaranteed VOCs cannter the pores. Kim and Ahn [186] compared the VOC adsorp-ion onto mordenite and faujasite, and found the VOCs with smallinetic diameters of 3.8–5.3 Å can be easily adsorbed, while that of.8–6.8 Å are difficult to be adsorbed [190,191]. Although the size oficropore is larger than the molecular diameters of VOCs, the VOC

dsorption capacity may be correlated to mesopore volume otherhan micropore or total pore volume. Moreover, even though littleOCs would be adsorbed into the macropore or mesopore, those

arger pores would benefit for the VOC adsorption because theyould provide the necessary transport channel for VOCs especially

he ones with larger molecular sizes [179].

.1.3. Surface chemical functional groupsThe adsorption of VOCs onto carbonaceous adsorbents may be

ontrolled by physical and chemical processes. Besides the mor-hology structure, surface chemical functional groups of carbonaterials may also be responsible for their adsorption of VOCs

79,181,182]. The surface functional groups of carbonaceous adsor-ent are associated with both the nature of raw material andctivation or modification methods such as heating, chemical,nd electrochemical treatments [175,192,193]. The heteroatomsf surface functional groups govern their surface chemistry. Theeteroatoms mainly include oxygen, nitrogen, halogen, hydrogen,tc., among which the oxygen and nitrogen groups on the porousarbon are recognized as the most important species for the adsorp-ion [5,169]. The oxygen groups present three different types ofcidic, basic, or neutral, which are associated with the oxida-ion phase. Generally, oxidation in liquid phase conduces to theormation of carboxylic acids, while the gas phase oxidation facil-tate the generation of hydroxyl and carbonyl groups [168]. Theitrogen containing groups are usually introduced by ammonium,itric acid, and N-containing compounds treatment, which primar-

ly present basic property [73,168,169]. Common surface chemicalunctional groups on carbonaceous materials and their character-stics are summarized in Table 5.

Most of the oxygen groups are the source of surface acidity,hich contributes to the adhesive of hydrophilic VOCs onto carbon

urface [79]. Oxidations by acids and ozone are among the mostfficient methods to introduce surface oxygen groups to carbonaterials. For instance, after modification by H3PO4, more oxygen

roups present on the surface of AC, and the adsorption capacitiesf methanol, ethanol and i-propanol on AC are greatly improved178]. Treated with ozone can increase the acidity of AC signifi-antly, with the pHpzc (pH of the point of zero charge) reducingrom 8.8–9.8 to 3.6–4.3 [194].

The present of oxygen groups may inhibit the specific interac-ions between hydrophobic VOCs and �-electron rich regions onarbonaceous adsorbents [94]. Therefore, hydrophobic VOCs prefero be adsorbed onto the AC without surface oxygen groups. Li et al.79] found all of the acid modified ACs have less o-xylene adsorptionapacity compared with unmodified ACs. Similarly, lower benzenend carbon tetrachloride adsorption capacities have been observedn acid-activated biochar compared with pristine biochar [180]. Tonhance hydrophobic VOC adsorption onto carbonaceous adsor-
ents, suitable modification should be adopted to eliminate theurface oxygen groups. Thermally treatment and alkali modifica-ion are two commonly used methods. It has been reported thaturface functional groups would decompose at high temperature,
Materials 338 (2017) 102–123 113

such as carboxylic groups decomposition at 150–300 ◦C, acid anhy-dride and lactone decomposition at 300–500 ◦C, ethers, phenolichydroxyl, and carbonyl groups decompose at 500–800 ◦C [74,194].Thermal treatment of over 500 ◦C makes the surface of biochar beless polar and more hydrophilic due to the decomposition of Oand H containing functional groups, which directly affect the VOCadsorption [65]. The removal of oxygen groups from carbon sur-faces at 900 ◦C under helium [94], 500–900 ◦C under CO2 [195,196]have also been reported. Beside the thermal treatment, alkali mod-ification is another efficient method to remove the oxygen groupsfrom the surface of carbon adsorbents. For instance, after modifiedby NH3H2O, the adsorption capacity of AC to o-xylene has beenimproved by 26.5% [79].

Some studies, however, suggested that the chemical functionalgroups on the surface of adsorbent are not an important factorin VOC adsorption. Tsai et al. compared the CHCl3 (weak polar-ity) adsorption onto hydrophobic and hydrophilic AC. Based onthe affinity between adsorbate and surface functional groups,hydrophobic AC should have higher adsorption capacity. How-ever, 146 mg g−1 CHCl3 was adsorbed on hydrophilic AC whilethat was only 74 mg g−1 on hydrophobic AC [187]. Similar resultswere observed by Díaz et al. [197], which may be explained bythe hypothesis that the physical characteristics of the adsorbentare more important than the surface oxygen functional groupsfor the nonpolar VOC adsorption [187]. Gil et al. [179] reportedthe adsorption capacities of hydrophobic VOCs such as benzene,toluene, and p-xylene on acid modified AC are even larger thanthat of on the pristine AC, which is attributed to the fact that acidmodification would change the pore size and enlarge the surfacearea to affect the adsorption. Sometimes, correlation analyses failto demonstrate the direct relationship between specific surfacearea, pore size, functional groups of carbon adsorbents and theirVOC adsorption capacities. That is because the above-mentionedcharacteristics never play the role singly [179]. Therefore, the bal-ance between all the morphology and chemical properties shouldbe taken into account, and then the VOC adsorption ability of car-bonaceous adsorbents would be explained reasonably.

4.1.4. Other adsorbent propertiesBulk density is an important characteristic of the carbonaceous

adsorbents, which can also be a key parameter for VOC adsorption,especially in the design of adsorption columns. A high density car-bon may capture more VOCs per unit volume thus it should not beregenerated frequently [180].

4.2. Characteristics of adsorbates

Properties of adsorbate molecule, such as the molecular struc-ture, polarity, and boiling point, also play important roles in VOCadsorption onto carbon materials.

4.2.1. Molecular structureThe structure of VOCs molecule determines their adsorp-

tion capacity on carbonaceous adsorbent, because VOCs of smallmolecule sizes are easy to access the adsorption sites, while thelarger VOCs cannot enter into the smaller pore. Both Qian et al.[183] and Huang et al. [198] found the adsorption capacities ofVOCs onto AC and metal-organic frameworks (MOFs) are negativecorrelated to their molecular size (Fig. 5). Herein, the molecularsize refers to molecule cross-sectional area other than minimumdimension such as molecule width, thickness or length. Besides,
the large size VOCs have lower adsorption capacity are consistentwith the observation reported by Lashaki et al. [199], who foundthe naphthalene molecules (kinetic diameter 0.62 nm) can blockthe entrance of narrow micropores in AC.


Table 5Characteristics of common acidic and basic functional groups on carbonaceous adsorbents [46,47,53,94,169,201,258].

Groups Schematic Property Possible FT-IR peakassignments

Decompositionproduction

Decompositiontemperature

Carboxyl Acidic C O: 1710, 1712, 1720,1720–1750, 1600–1800O H: 3530, 3500

CO2 523 K373 K, 673 K473 K, 523 K

Hydroxyl Acidic O H: 3530, 3500, 2500–3620,3605, 3393C OH: 1000–1220,1100–1400, 1200–1300

CO 873 K, 973 K

Carbonyl Acidic C O: 1560, 1570, 1700, 1705,1707

CO 973 K, 1253 K1073 K, 1173 K

Anhydride Acidic C O: 1740–1880C O:980–1300

COCO2

873 K900 K623 K, 673 K

Lactone Acidic C O: 1710, 1720, 1750, 1760 CO2 523 K473 K, 523 K463 K, 923 K

Quinones Basic C H:1635, 1645, 1650,1550–1680, 1580–1620

CO 1073 K, 1173 K973 K, 1253 K

Pyrone Basic C O: 1650, 1688, 1706, 1740C H: 3025

C O C: 1279

– –

Chromene Basic C H: 2930, 2850, 2924,2600–3000

CH2 O: 2835−28,124980C O C: 1279,1249CH2 : 1470, 720,

1350−1180

– –

Pyrrole Basic N H: 1480, 1560C N: 1190, 1250, 1250

– –

Pyridine Basic C N: 1480–1610, 1512–1570,1566

– –

Pyridinium Basic N H: 1480, 1560 – –

Pyridone Basic C N: 1190, 1250, 1250C N: 1570, 1600, 1600O

– –

N

ca0cTtsa

Pyridine-N-oxide Basic

Adsorbate molecule shape also influences VOC adsorption ontoarbon materials. As shown in Fig. 5, although o-xylene, m-xylene,nd p-xylene possess almost equal cross-sectional area of 0.375,.379, and 0.380 nm2, respectively, the adsorption of p-xylene onarbon materials is significantly higher than that of the others.
he difference can be ascribed to the different molecule shape ofhe adsorbates, which is derived from different position of methylide chains in xylene molecule. The impact of molecule shape ofdsorbates on the adsorption capacities of carbon is summarized
H (stretching): 3530, 3500

O : 1000–1300 – –

in Fig. 6. According to the observation reported by Yang et al.[200], methyl groups other than the benzene ring prefer to inter-act with active sides of carbon adsorbents. Thus, both o-xylene andm-xylene cannot enter into the pore because the distance betweentwo methyl groups is larger than the pores diameters. While the
p-xylene molecule with two methyl groups linking benzene ringwith an angle of 180◦ can easily enter into the pore.


0.15 0. 18 0. 21 0.2 4 0. 27 0.30 0.33 0.36 0.390.0

0.3

0.6

0.9

1.2

1.5

1.8

Acetone Benzene Toluene Ethylbenzene m-Xylene o-Xylene p-Xylene Dichloromethane Iodomethane Tetrachloromethane Trichloromethane

Ads

orpt

ion

capa

city

(ml/g

)

Cross-se cti ona l are a (n m2)

activated carbon metal-organic frameworks

y=-0.669 x+0 .592 8R2=0.62 10

y=-5.84538x+3.2471R2=0.6784

Fig. 5. Relationship between VOC molecular cross-sectional area and their adsorption capacity onto carbon adsorbents [183,198].

ylene

4

ttlpaamatooAmatnwpr

n

Fig. 6. Scheme of ethylbenzene(a), p-xylene(b), o-x

.2.2. Molecular polarityThe molecular polarity of VOCs directly impacts their adsorp-

ion onto carbonaceous adsorbents. Generally, polar VOCs prefero be adsorbed onto adsorbents with polar groups, while nonpo-ar VOCs would like to be adsorbed onto the adsorbents withoutolar groups. For example, the surface of AC is primarily nonpolar,ttached with slightly polar because of the present of oxygen groupsnd some inorganic impurities. The nonpolar dominant featureakes AC not only be qualified to deal with humid gas mixtures but

lso can preferentially adsorb nonpolar or weak polar VOCs in quan-ity [201]. Bansode et al. [180] compared various VOC adsorptionnto carbonaceous adsorbents, the result show that nonpolar VOCsf C6H6 and CCl4 exhibit higher adsorption capacity on biochar andC than other polar VOCs, which is attribute to their zero dipoleoment is well matched with the carbonaceous adsorbents. In

ddition, Tsai et al. [187] compared the weak polar CHCl3 adsorp-ion onto the AC, and found the adsorption capacity of AC containingonpolar C C groups was 146 mg g−1 which was higher than thatith polar lactone groups (74 mg g−1). Similar results that VOCs
refer to be adsorbed onto the same polarity adsorbents have beeneported in the literatures [162,197,202].
Some carbonaceous materials can present strong polarity eitheratively or after modification such as oxidization by nitric acid or

(c), and m-xylene(d) entering MIL-101 pores [200].

ozone. Due to the dipole-dipole interactions between the adsor-bents and polar VOCs, the intermolecular potential energy maydecrease to facilitate the adsorption. Qian et al. [183] comparedthe adsorption capacity of CH2Cl2, CH3I, CHCl3, and CCl4 (withdipole moment of 1.8, 1.59, 1.1, and 0 Debyes, respectively) on acti-vated carbon microspheres (ACMs). They found the ACM adsorptioncapacities of polar VOCs (CH2Cl2 and CH3I) were higher than thatof the weak- and non-polar VOCs (CHCl3 and CCl4). Similar obser-vations have been reported on beaded activated carbon modifiedby nitric acid [203].

4.2.3. Boiling pointThe physical adsorption process of adsorbate on porous adsor-

bent is similar to vapor-liquid phase transitions, where theadsorbates with higher boiling points would be preferentiallyadsorbed than those with lower boiling point because of thestronger intermolecular forces [204]. Additionally, liquid-like con-densation plays an important role in VOC adsorption onto AC, thusthe boiling point of VOCs is a crucial factor to influence the adsorp-
tion process [205]. Giraudet et al. [206] compared the adsorption ofdichloromethane, ethanethiol, and siloxane D4 (with boiling pointsof 313.2, 308.2 and 448.9 ◦C, respectively) on ACFs. They foundthe adsorption capacity of low boiling point dichloromethane and


Table 6Desorption of VOCs from various engineered carbon materials.

Adsorbents Adsorbates Desorptionstrategies

Desorptionconditions

Desorptionefficiency

Cycles Ref.

Activated carbon AcetoneMethyl ethyl ketone

Thermalregeneration

Temperature(80–120 ◦C)

95% at 80 ◦C90% at 120 ◦C

8 [259]

Activated carbon TolueneMethyl ethyl ketone

Thermalregeneration

Temperature(140–180 ◦C)

98.1% at 180 ◦C99.1% at 140 ◦C

5 [260]

Activated carbonfiber cloth

Mixture of styrene, butylacetate, m-xylene,p-xylene, o-xylene,toluene, and ethylbenzene

Electrothermalheating

Gas set-pointconcentrations(40–900 ppm),superficialgas velocity(6.3–9.9 m/s)

Desorption factor(desorptionaverage/inletaverage):0.59–10.3

– [261]


Toluene Electrothermaldesorption

1500 W kg−1 75% 5 [206]


Methylene chloride Electrothermalregeneration

Temperature 60 ◦C,electric power200 W,flow rate 20 m/h

99% six-day period [262]


Methyl ethyl ketone Electrothermaldesorption

10 A for 300 s 70% – [263]


Methyl ethyl ketone Electrothermalregeneration

Voltage wascontrolled between12 and 20 V

92% 40 [264]

Beaded activatedcarbons (modified)

Mixture of n-decane,1,2,4-trimethyl benzene,2,2-dimethylpropylbenzene, n-butyl acetate,2-butoxyethanol,1-butanol, 2-heptanone,naphthalene, anddiethanolamine

Electrothermaldesorption

288 ◦C 93.4–92.1% 5 [203]

Beaded ActivatedCarbon

Mixture of n-decane,1,2,4-trimethyl benzene,2,2-dimethylpropylbenzene, 1-butanol,2-heptanone, n-butylacetate, 2-butoxyethanol,naphthalene, anddiethanolamine

Electrothermaldesorption

400 ◦C VOCs in narrowmicropores werenot fully desorbed

– [199]

Electrospunnanofibers

Acetonen-hexanen-butanolToluene

Thermaldesorption

80 ◦C 96.03%84.41%91.31%86.01%

6 [265]

Microporousactivated carbons

Mixture of toluene, butylacetate, and butanol

Temperatureprogrammeddesorption

150 ◦C Adsorptioncapacity unaltered

7 [266]

MIL-101(Cr)/graphiteoxide composites

n-hexane Vacuum desorption 25 ◦C, 0.07 mbar 96.78% 5 [132]

MOF/grapheneoxide composite

Acetone Vacuum desorption 25 ◦C, 0.04 mbar 91.3% 6 [131]

Monolithic carbonaerogels

Toluene Microwave heating 400 ◦C Completelyrecovered

3 [267]

Silica aerogelactivated carbon

Benzene Solventextraction–thermal

In ethanol for 1 h,dry at 80 ◦C for 1 h

Adsorptioncapacity unaltered

15 [140]

elht[aiaip

aVi

composite treatment method

thanethiol were 0.21 and 0.18 mol kg−1, respectively, which wereess than that of siloxane D4 (1.23 mol kg−1). Similar observations ofigher boiling point VOCs tend to obtain higher saturation adsorp-ion capacity has been reported on AC [207]. Moreover, Oh et al.208] investigated the adsorption of seven VOCs onto granular AC,nd established an empirical formulas between adsorption capac-ty and boiling point of VOCs, with the R2 being as high as 0.988. Inddition to carbonaceous materials, the similar influence of boil-ng point on VOC adsorption has also been observed on the otherorous adsorbents [209].

Because of the strong affinity between high boiling point VOCs
nd adsorbents, they will easily replace the lower boiling pointOCs during the competitive adsorption process. Wang et al. [210]
nvestigated the adsorption of eight VOCs onto AC, and found

only low boiling point components of n-butyl alcohol and n-butylacetate were detected just after breakthrough. This was ascribed tothe displacement of low boiling point VOCs by those with high boil-ing point. Similar observation has been reported by Giraudet et al.[206], who carried out dynamic multicomponent adsorption exper-iment on ACF. The interesting phenomenon was observed near thesaturation that the outlet concentrations of lower boiling pointVOCs such as isopropanol, dichloromethane, and ethanethiol sur-passed their inlet contents, which was attributed to the adsorbedlow boiling points VOCs were replaced by high boiling point silox-ane D4.

The adsorption performance of high boiling point VOCs ontoadsorbent is superior than those of low boiling point, whereas,the desorption of high boiling point VOCs from adsorbents is

rdous

meadhLn

4

owabwo

4

4

baadbfAtataTftoa

lrm[ofttaVoCttCACetocim

4

ot


ore difficult for their strong affinity with adsorbent. Giraudett al. [206] compared the VOCs desorption at 420 K, and foundll of the adsorbed low boiling point VOCs, such as toluene,ichloromethane, and isopropanol were entirely desorbed, whileigh boiling point siloxane D4 was partially desorbed. Similarly,ashaki et al. [199] found higher desorption temperature waseeded for the removal of the high boiling point VOCs from AC.

.2.4. Other adsorbate propertiesIn addition to the molecular structure, polarity, and boiling point

f VOCs, other molecular characteristic such as molecular weightould also impact the adsorption. Tsai et al. [187] compared the

dsorption of CH3CN, C3H6O and CHCl3 on carbonaceous adsor-ents, and found the diffusion coefficients of CH3CN and C3H6Oith higher molecular weight were significantly higher than that

f CHCl3.

.3. Adsorption conditions

.3.1. TemperatureFor most poriferous adsorbents, the adsorption of VOCs can

e mainly attributed to physical exothermic interaction, in whichdsorption temperature plays an important role [187,211]. Gener-lly, the adsorption of VOCs onto carbonaceous adsorbent wouldecreased with elevating the adsorption temperature. As reportedy Qian et al. [183], when the adsorption temperature increasedrom 20 to 60 ◦C, the adsorption capacities of CH2Cl2 and CH3I ontoC decreased by 46.2% and 47.4%, respectively. Similar observa-

ions have been reported on the adsorption of toluene, acetone,lkane, aromatic, and ketone vapors onto AC [184,212]. In addi-ion to AC, other type carbonaceous adsorbents such as biocharnd ACF present similar variation tendency with the temperature.sai et al. [187] compared the adsorption of chloroform onto dif-erent carbonaceous adsorbents, and found when the adsorptionemperature increased from 30 ◦C to 80 ◦C, the adsorption capacityf chloroform onto AC, biochar, and ACF reduced by 7–67%, 17–38%,nd 5–74%, respectively.

Elevating the adsorption temperature can promote the molecu-ar diffusion to enhance the adsorption rate. The adsorption kineticate constants of benzene, cyclohexane, and hexane onto orderedesoporous carbons at 318 K are larger than that at 308 or 298 K

153]. Similar observations have been reported on VOC adsorptionnto AC and carbon molecular sieve [213,214]. High temperatureacilitates the molecular diffusion, facilitating VOCs to enter intohe pores of adsorbents. In addition, in high temperature ranges,he chemical adsorption caused by the interaction between VOCsnd the carbonaceous adsorbents may take place. Therefore, theOC adsorption capacities do not always decrease with the increasef temperature. Chiang et al. [205] compared the adsorption ofCl4, CHCl3, CH2Cl2, and C6H6 onto AC during 0–80 ◦C. They foundhat, for most of the VOCs, the adsorption capacity decreased withhe elevating of temperature. However, an exception occurred on6H6 during 35–45 ◦C, where the adsorption capacity of C6H6 onC increased with the temperature. The cross-sectional area of6H6 is 0.43 nm2, too large to enter into the micropore easily. How-ver, elevating the adsorption temperature can conduce to the C6H6hrough the narrow entrance of micropore. In summary, the effectf temperature on VOC adsorption onto carbon materials is compli-ated. Improving the adsorption temperature, on the one hand maynhibit the physical adsorption, on the other hand may enhance the

olecular diffusion and chemical adsorption.

.3.2. HumidityThe presence of water molecules can influence VOC adsorption

nto carbon materials because the water molecules may occupyhe sites in the pores competitively [90,215,216]. To mitigate

Materials 338 (2017) 102–123 117

the impact of water vapor on VOC adsorption, their adsorptionmechanisms onto carbon materials should be understood. TheDubinin-Serpinsky theory is one of the most popular interpreta-tion on water molecule adsorption by AC, which suggests the watermolecule is first adsorbed on the surface oxygen functional groupsof AC, and then more water molecule is adsorbed via hydrogenbonding. Subsequently, with the increase of water vapor pressure,the water cluster is formed which is followed by capillary conden-sation in pores of AC [217,218]. The water adsorption isotherm onAC generally presents an S-shaped curve, indicating that only littlewater is adsorbed at low vapor pressures [201]. Moreover, the littleadsorbed water molecules are bonded to certain oxygen complexesby hydrogen bonding and electrostatic forces [219]. Therefore,removal the oxygen complexes from the surface of carbon materialwould inhibit the water adsorption effectively [56,220].

The S- shaped curve of water adsorption isotherm also indi-cates abundant water can be adsorbed onto carbon materials athigh vapor pressures, which belongs to the stage of capillary con-densation according to Dubinin-Serpinsky theory [201]. Moreover,the capillary condensation is related with pore structure of carbonmaterials, thus the resistance of moisture differs from carbona-ceous adsorbents. Yamamoto et al. [160] compared the moistureadsorption onto larger pore carbon cryogel microspheres (CCMs)and smaller pore AC. They found the pore filling fraction of water inCCMs was 0.20, which is smaller than that in AC (0.65). The authorssuggest the difference adsorption capacity of water is due to thecapillary condensation prefers occurring in smaller pore. Conse-quently, the influence of water vapor on larger pore carbonaceousadsorbents is less than those with smaller pore.

4.3.3. Other adsorption conditionsBesides experimental temperature and water vapor content,

other adsorption conditions such as VOC concentration, gas veloc-ity, and co-existing adsorbates can also influence VOC adsorptiononto carbon materials. Since the total adsorption amount of VOCsonto carbonaceous adsorbents is constant, higher inlet concentra-tion would result in the elevating of VOCs entering into the pores,thus the breakthrough time decreases with the increase of VOC con-centration [170,183]. Gas velocity influences the VOC adsorptiontoo. Increase the gas velocity can reduce the adsorption capacity,because higher gas velocity will result in the escape of adsorbedVOCs under strong feed flow [221]. The influence of gas veloc-ity on VOC adsorption differs from the pore structure of carbonadsorbents. Adsorbents with few tortuous pore would shorten thediffusion pathway of VOCs to the adsorption sites, thus little effectof gas velocity on them [183]. Besides, the breakthrough time willbe prolonged when the gas velocity is reduced. As reported, whenthe gas velocity reduced by half, the breakthrough times increasedby 109% on the benzene adsorption [222]. The slight change ofthe liner relationship between gas velocity and breakthrough timeis attributed to the more efficient mass transfer under lower gasvelocity and longer contact time. Similar observations have beenreported in previous studies [170,223].

5. Conclusions and perspectives

VOCs are one of the most troublesome air pollutants because oftheir toxicity and precursor role in photochemical smog, as well asextensive sources. Although a number of VOC removal technologieshave been proposed in recent years, many of them are unsuitablefor commercial applications due to the drawbacks of low efficiency,
high energy consumption, or serious toxic byproducts. Up to now,adsorption is one of the most popular VOC treatment methods inpractical applications. The selection of adsorbent is the most cru-cial aspect of VOC adsorption technology. Among numerous novel

1 rdous

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OC adsorbents, carbonaceous materials show excellent potentialecause of their large surface area, plentiful pore structure, hightability, as well as relatively low cost. Furthermore, most of theOC adsorbed onto the carbon materials can be recycled throughesorption processes under various conditions (Table 6).

In this work, the VOC adsorption performance of several novelngineered carbonaceous materials such as AC, biochar, ACFs, CNTs,raphene and its derivatives, CSCs, OMC, as well as carbon cryo-el microspheres and mesoporous graphite carbon are reviewed.urthermore, the key factors governing VOC adsorption onto car-onaceous adsorbents, such as specific surface area, pore size,hemical functional groups, molecule size, polarity, boiling point,dsorption temperature and humidity are discussed.

From what has been discussed, it can be concluded that: 1)lmost all the engineered carbon materials show excellent VOCdsorption ability at suitable condition or after appropriate mod-fication. 2) Both the morphology and chemical functional groupsf adsorbents impact the VOC adsorption. Generally, large specificurface area and small pore size facilitate the adsorption, whilehe influence of functional groups is associated with the polarityf VOCs. Acidic groups such as carboxyl, lactone and anhydriderefer adsorbing polar VOCs, whereas the basic groups such asyrone, chromene, and quinones prefer to adsorb nonpolar VOCs.) The characteristics of VOCs influence their adsorption onto car-onaceous materials to a great extent. Large VOCs molecule size

nhibit their diffusion into the small pore of adsorbents, thus VOCdsorption capacity on adsorbents is negative correlative to theirolecular cross-sectional area. The impact of molecular polarity

epends on the surface functional groups of carbonaceous mate-ials. Polar VOCs prefer to be adsorbed on acidic group covereddsorbents, while nonpolar VOCs like to be adsorbed on the basicroup containing adsorbents. High boiling point VOCs are prefer-ntially adsorbed onto adsorbents than the low boiling point ones.owever, the former is more difficult to be desorbed than the later.) The adsorption conditions such as temperature and water vaporontent influence the VOC adsorption severely. Low temperatures conductive to the VOC adsorption because the process is over-

helming physical exothermic interaction. The present of waterapor will decrease the VOC adsorption capacity because its com-etitive adsorption with VOCs molecules.

Although great progress has been done on adsorption of VOCsn carbon materials, there are still knowledge gaps that need to belled. Additional studies are still need to: 1) further improve VOCdsorption capacity of carbon materials, 2) decrease carbon adsor-ent production cost, 3) increase the low boiling VOC adsorptionfficiency, 4) solve the difficulty of high boiling VOC desorption, 5)ncrease selectivity of carbon adsorbents for the reuse of VOCs, and) improve VOC adsorption onto carbon adsorbents under moistureondition.

cknowledgements

This work was partially supported by the Natural Science Foun-ation of the Jiangsu Higher Education Institutions of China (Granto. 14KJB610010), the Opening Project of Shanghai Key Labora-

ory of Atmospheric Particle Pollution and Prevention (LAP3) (Granto. FDLAP15005), the Key Laboratory Open Fund of Persistent Pol-

utant Control and Resource Recycling of Jiangxi Province (Granto. ST201422010), Science and Technology Plan Projects of Xuzhou
ity (Grant No. KC15SH017), Key Plan Projects of Xuzhou Universityf Technology (Grant No. XKY2015105), and the National Buildingaterials Industry Science and Technology Innovation Plan (Granto. 2014-M4-1).
Materials 338 (2017) 102–123

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