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Chemical Engineering Journal 181–182 (2012) 467–478

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j ournal homepage : www.elsevier .com/ locate /ce j

Comparison of activated carbon and iron impregnated activated carbon derivedfrom Gölbası lignite to remove cyanide from water

Tolga Depci ∗

Yuzuncu YilUniversity,Faculty of Engineering – Architecture, Department ofMiningEngineering, 65080 Van, Turkey

a r t i c l e i n f o

Article history:

Received 3 October 2011

Received in revised form 1 December2011Accepted 1 December 2011

Keywords:

Cyanide

Magnetic activated carbon

Non-linear regression

Turkish lignite

a b s t r a c t

The ability of lignite-activated carbon (LAC) and iron-impregnated activated carbon (FeAC) obtained from

Gölbas ılignite to remove cyanide ions from aqueous solution byadsorptionwas researched and comparedwith each other. The same process was applied also with commercial activated carbons which are in bothgranular (CAC-1) and powder forms (CAC-2). The morphologies, structures and properties of the activatedcarbons were determined by BET, XRD, XRF, SEM, zeta meter and magnetometer, respectively. The effects

of various experimentalparameters, such as initial cyanide concentration, pH, adsorbent type and particlesize were researched in a batch adsorption technique at a temperature of 25 ◦C. BET surface area of LAC

is determined as 921 m2/g. The obtained magnetic activated carbon has high surface area of 667 m2/gwith 19 wt.% Fe3O4 coated and perfect magnetic separation performance. Langmuir model was found

to be the best representative for cyanide-adsorption. The maximum monolayer adsorption capacitiesof LAC and FeAC are 60.18 mg/g and 67.82 mg/g at pH values of 7–7.5 and 64.10 mg/g and 68.02 mg/g

at pH values of 10–10.5, respectively. Kinetic evaluation indicated that the cyanide adsorption onto theobtained activated carbons followed the pseudo-second order rate reaction. The diffusion-controlled

kinetic models on the cyanide-adsorbent system showed that the removal rate was controlled not onlyby intraparticle diffusion but also by film diffusion. All experimental results point out that the LAC andFeAC are viable candidates for the removal of cyanide from water and wastewater.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In gold and silver leaching operation, cyanide is mostly used forextracting gold and silver from finely disseminated ores owing to

its potential applicability and low cost [1]. Turkey has epithermaltype gold deposits andit is noteconomicallyfeasibleto operate thistype of ores without using cyanide in today’s conditions. Therefore,gold has been extracted by cyanide (cyanidation) [2,3]. However,

over the past decades, with the rising awareness of environmen-tal consciousness and growing fear because of its extremely toxiccharacter, cyanide has become a matter of considerable debates inTurkey. Therefore, there has been a great pressure on producers of

gold and silver for the disposal of cyanide wastes from gold andsilver plants. Moreover, cyanide is not only employed for gold andsilver discharge but also for the discharge of plating and surfacefinishing [4,5].

Removal of cyanide from aqueous solution is required whenhuman health and protection of ecosystems are considered. Espe-cially, at last decade, various researches have been carried out forthe removal of cyanide using activated carbon [5–9]. Activated

∗ Tel.: +90 432 225 1024; fax: +90 432 225 1732.

E-mail address: [email protected]

carbon (AC), one of the most commonly used adsorbent all overthe globe due to the great specific surface area and pore structure,has a potential for adsorption of molecules from both the liquidand gas phase. Recently, granular activated carbon was impreg-

nated with metallic compounds such as nickel [7], copper [8–10]and silver [7,8,11] to increase the removal efficiency of activatedcarbon. In the literature, however, there is no scientific investiga-tion about removal of cyanide using iron impregnated activated

carbon. Productions of iron impregnated activated carbons forthe removal of inorganic compounds from aqueous solution haveattracted great attention since separation from the medium isexecuted by a simple magnetic process [12–15]. Especially, in

recent years, studies regarding synthesis and characterization of coal based magnetic activated carbon show rapidly increasingtrend [16–18].

The aim of this research is to focus on the removal of cyanide

from aqueous solution at different pH range using Gölbas ı lignitebased activated carbon and iron-impregnated coal based acti-vated carbon as adsorbent materials and is to compare the resultswith commercial activated carbon. As carbonaceous raw material,

Gölbası lignite, was selected to prepare coal based activated car-bon and magnetic activated carbon due to its readily availabilityand low cost. Proven reserve of Gölbas ı lignite (Harmanlı field) is51,325,000ton, and it has high ash and high moisture content with

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468 T. Depci / Chemical Engineering Journal 181–182 (2012) 467–478

Table 1

Proximate and ultimate analysis of the air-dried lignite sample [20].

Proximate analysis Ultimate analysis (d.a.f)

Moisture (%) 20.50 C 55.10

Volatile matter (%) 18.10 H 5.49

Fixed carbon (%) 31.69 N 2.06

Ash (%) 29.71 S 3.02Low calorific value (kcal/kg) 3063 O (diff.) 34.33

High calorific value (kcal/kg) 3248 H/C 1.20

low calorific value [19]. Moreover, the adsorption isotherm datawere fitted to well known two-parameter models Langmuir andFreundlich isotherms as well as three parameter isotherm model

(Redlich–Peterson). Theresults were also considered on thebasisof kinetic models, the pseudo first-order, second-order and diffusion-controlled kinetic models. To find out the best fitting kinetic andisotherm models, error analysis was also conducted. Average rel-

ative error (ARE) and nonlinear Chi-square test ( X 2) for kineticmodels were applied in addition to correlation coefficient (R2).

2. Materials and methods

2.1. Preparation of activated carbons

2.1.1. Preparation of activated carbon (LAC)

Activated carbon was prepared using Gölbas ı – Adıyaman(Turkey) lignite which was chosen for the precursor due to itsavailability and low cost. Proximate and ultimate analyses of theair-dried lignite sample are given in Table 1 [20].

The lignite sample (−60 + 4 0 mesh) was mixed with ZnCl2

(lignite/ZnCl2 weight ratio of 1:1) and the required amount of dis-tilledwaterwas added to this mixture. Then, this mixture wasdriedat 105 ◦C in furnace to obtain an impregnated sample. The impreg-

nated sample was heated to the activation temperature of 500 ◦Cfor 1h under N2 flow (100ml/min) at the rate of 10◦Cmin−1. After

the activation process, the obtain product was cooled down underN2 flow and then 0.5N HCl was added on it. This mixture was fil-

tered and washed with distilled water at several times to removeresidual chemicals and chlorine until filtrated solution did not giveany reaction with AgNO3. Finally, the samples were dried at 105◦Cfor 24h and ground and sieved under 200 mesh sizes.

2.1.2. Preparation of iron-impregnated activated carbon (FeAC)

The experimental procedure is based on the study conductedby Yang et al. [21]. Activated carbon derived from Gölbas ı lignite

(LAC) was modified with nitric acid for 3h at 80 ◦C. Then, beingprepared it was modified with 20ml aqueous solution containing4gFe(NO3)3·9H2Oand0.5goftheLACwasputintosolutionandthemixture wasplaced intoultrasonic bath fordispersion.The mixture

wasfiltered andthe solid dried.Finally,the solid was heated in tubefurnace at 750 ◦C for 3 h in the presence of nitrogen. The obtainedmaterial was denoted as FeAC.

2.1.3. Commercial activated carbon (CAC)

In order to evaluate andgive a better decision about the applica-bilityof LACand FeAC forremovalof cyanide from aqueous solution,commercial activated carbon was used in the same experimental

condition and the obtained results were compared to the results of LAC and FeAC. The commercial activated carbon, which is derivedfrom coconut shell by physical activation, was supplied by Cal-gon Corporation (Calgon GRC 22). It was used in granular form

(−7+12mesh)andpowderform(−200 mesh) andthey were calledas CAC-1 and CAC-2, respectively.

2.2. Characterizationmethods

Surface area, pore size distribution and pore volume of the acti-vated carbons were measured from nitrogen adsorption isothermsat 77 K in the range of 10−6 to 1 relative pressures by a Tri Star

3000(Micromeritics, USA) surface analyzer.BET equation wasusedto calculate total surface area and average pore-diameter. Poresize distribution and the features of pore structure were deter-mined using BJH methods and t -plot, respectively. Prior to the

measurement, the sample was degassed at 400 ◦C for 2 h . X-raypowder diffraction (XRD) was used to characterize and identify thephase compositions and crystallinity of the samples. The XRD pat-terns were recorded by using Rigaku Miniflex Diffractometer with

Cu Ka (30kV, 10mA, k=1.54050 A). An iron content of FeAC wasdetermined using X-ray fluorescence (XRF Spectro IQ). In order toinvestigate the morphology of the samples, LeO EVO 40 scanningelectron microscope was used. The zeta potential of the activated

carbon was measured by a Zeta Meter 3.0 (Malvern Inc.) equippedwitha microprocessorunit.The pHof thetest solution wasadjustedto thedesired value by drop-wise addition of dilute NaOH (0.5%) orHCl (0.1 N). Magnetic measurements of the LAC andFeAC were per-

formed with a vibrating sample magnetometer (VSM; Lake Shore7407).

2.3. Adsorption experiments

Adsorption studies were conducted in routine manner by batchtechnique. First, the effect of pH on the cyanide adsorption on theactivated carbon was investigated. Experiment was carried out at500ml of 100mg/L initial cyanide concentration with 1.5 g adsor-

bent dosage at25 ◦C for 72h at a constant stirring speed of100 rpm.The pH values were adjusted in the range of 5–11. This process isvery dangerousdue to the HCN. Therefore, especially at lowpH val-ues, the reactors were sealed immediately and all the experiments

were done under fume hood.In respect to the result of pH, allexperiments were conducted at

two different pH ranges, 7–7.5 and 10–10.5, respectively. Adsorp-tion of cyanide onto activated carbons with respect to initial

cyanide concentration was determined by equilibrating 1.5 g of activated carbon samples with 500ml of cyanide solution at theselected pH ranges in a 1.2 L plastic reactor provided with tem-perature control and shaker. The adsorption experiments were

conducted at room temperature and samples were put in shakerat 100 rpm for 72h. The experimental procedure was based on thestudy published by Behnamfard and Salarirad [22]. Nine solutionswith different cyanide concentrations were prepared (i.e., 50, 100,

150, 200, 250, 300, 350, 400 and 450 mg/L) by using the cyanidestock solution (2000mg/l). Kinetic experiments were also carriedout under similar conditions (four selected solutions with differentcyanide concentrations; 100, 200, 300 and 400mg/L) and equilib-

riumconcentration was foundat different time intervals. Eachtime,the samples of5 ml were taken by micropipette and stored in vials.Cyanide equilibrium concentration was determined by titratingagainst standardized silver nitrate solution (0.001M) in the pres-

ence of p-dimethylaminobenzylrhodanine (0.02% w/w in acetone)as indicator [8,23].

The amount of cyanide q (mg/g) adsorbed on activated carbonswas calculated by the mass balance Eq. (1).

q = (C 0 − C )×V

W (1)

where C 0 (mg/l) is the initial cyanide concentration andC (mg/l) isunadsorbed cyanideconcentration in solution at time t ,V (L) andW

(g)is the volume of thesolution and theweight of the dry activated

carbons used respectively.



T. Depci / Chemical Engineering Journal 181–182 (2012) 467–478 469

0

50

100

150

200

250

300

350

400

10.80.60.40.20

P/Po

V o

l . A d s o r b e d ( c m / g ) 3

LAC

FeAC

CAC

Fig. 1. Adsorption isotherm of nitrogen on theactivated carbons at 77K.

In the present study, adsorption isotherms and kinetics studieswere explained using linear least squares methods and non-linear

method which were determined using the solver add-in withMicrosoft’s spreadsheet, Microsoft Excel.

2.4. Validity of adsorption isotherm and kinetic model

Average relative error (ARE) and nonlinear Chi-square test ( X 2)

to control consistency of adsorption isotherms and kinetic mod-els were applied in addition to determination coefficient (R2). Theexpressions of the error functions are given below:

ARE, 100

N

N

i=1

qeexp

− qecal

qeexp

i

(2)

X 2,

N

i=1

(qecali − qe

expi

)2

qeexpi

(3)

where qeexpi

and qecali

represent the amount of cyanide (mg/g) on

the activated carbon which are experimentally found and calcu-lated results, respectively. N is the number of observations in the

regression model.

3. Results and discussion

3.1. Characterization of the activated carbons

The N2 adsorption isotherms of the activated carbons are givenin Fig. 1. They can be classifiedas Type 1 – characteristic of microp-ore solids [24]. The isotherms plots exhibit a round knee, indicatinga high adsorption capacity and wider micropores in microporous

solids [25,26]. Besides, Fig. 1 shows that thevolume of nitrogen gasadsorption of FeAC is lower than that of others.

Surface area andporosity valuesof theactivated carbons arepre-sented in Table 2. It was found out that the activated carbons had

a remarkable BET surface area, which was primarily contributedby micropores. The average pore diameters were between 2.08nmand2.16 nm, indicative of itsmicropores character.It appeared that

55453525155

2 (Degree)

I n t e n s i t y ( a . u . )

( 3 1 1 )

( 4 0 0 )

( 5 1 1 )

( 2 2 0 )

FeAC

AC

Fig. 2. Powder X-ray diffraction patterns of activated and magnetic activated car-

bon.

activated carbons were dominantly micropores. The comparison of pore structure of CAC-1 and CAC-2 indicated that particle size of activated carbon had only a very small effect on the specific surface

area because of highly developed internal pore structure. Besides,

the comparison of surface area and porosity values of LAC and FeACshowed that the surface area of FeAC reduced from 921 m2/g to667m2/g. Especially, the decrease in micropore volume and micro-

pore area were 28.45% and 46.13%, respectively, due to the ironimpregnation process. It may be attributed to theformation of ironoxide inside the pores [17,21,27,28].

Fig. 2 shows the XRD pattern of the activated carbon and FeAC.An analysis ofthe powderXRD data provedthat themain structuresof activated carbons are amorphous and the broader reflections inthe range of 10–30◦. The reflections on FeAC could be assigned to

the iron oxide. Yang et al. [21] claimed that the peaks that wereindexed in Fig. 2 show the face-centered cubic (fcc) structure of Fe3O4 which is magnetic nanoparticles. By comparison with thepatterns and d values of the FeAC and those of standard magnetite

(JCPDS Card No: 19-629), it can be found that all peaks correspondto magnetite (Fe3O4 – partially oxidized at the surface of acti-vated carbon), and no characteristic peaks belonging on other ironoxides or iron forms are detected in the XRD pattern. The average

crystallite size of Fe3O4 particles are calculated by Debye–Scherrerequation (D = 0.9/ˇ cos ) and found as 17nm.

Fig.3 showstheSEMimagesofLACandFeAC.Asobservedwithinfigure, LAC has a highly porous structure with a uniform distribu-

tion of thepores. On theother hand,SEM image of FeACreflects themorphological changes. It can be seen that the iron oxide particles(brighter and smallaggregates form)are insidethe porous structureand cover the surface of activated carbon due to the impregnation

process. The coated iron oxide content was determined as 19 wt.%.The hysteresis loops of LAC and FeAC are given in Fig. 4.

The results indicated that the activated carbon gained magneticproperty as a result of the impregnation process. The satura-

tion magnetization of LAC (Ms = 0.16emu/g) showed nearly 656%increase after the impregnation process. The saturation magneti-zation of FeAC was obtained as 1.05emu/g. According to hysteresis

Table 2

Surface area andporosity values of theactivated carbons.

Code S BET (m2/g) S mic (m2/g) S mezo (m2/g) V t (cm3/g) V mic (cm3/g) V meso (cm3/g) Dp (nm)

CAC-1 906 794 112 0.49 0.42 0.07 2.16CAC-2 928 807 121 0.504 0.44 0.064 2.17

LAC 921 812 109 0.476 0.427 0.049 2.11

FeAC 667 581 86 0.27 0.23 0.05 2.08

Dp denotes theaverage pore diameter (4V/A by BET);V t denotes thetotal pore volume.




Fig. 3. SEM imagesof LACand FeAC.

Fig. 4. Thehysteresis loops of LACand FeAC andpicture of FeAC with magnet.

loop, FeAC is a magnetic material and it can be removed from solu-tion by magnetic separator. Besides, remanent magnetization of FeAC was found to be 0.069 emu/g. This value is very close to zero

indicating that FeAC shows super-paramagnetic behaviors at roomtemperature [29]. It means that FeAC can be easily separated fromsolution by a magnet which is seen in Fig. 4. The obtained resultsare compatible with the literature data [16,17,21]. In order to test

the magnetic separability of the FeAC from solution, a magnet isalso put near the glass bottle. As seen in Fig. 4, FeAC stick on thesurface of glass bottlenear themagnetdemonstrating themagneticsensitivity of FeAC.

3.2. Effect of pH on removal of cyanide and adsorption

mechanism

The effect of pH of solution on cyanide adsorption is shown

in Figure 5. The lower limit of pH value was selected as 5 tokeep the magnetic properties of iron impregnated activated car-bon stable [13]. Fig. 5 indicates that the shape of the removal

curve of FeAC is different from the others. It has a strict pH-dependent character. The percentage removal of cyanide increasedwith increasing the pH and maximum retention was obtained atpH values between 6 and 8 and then decreased drastically until pH

11. This observation is very compatible with the study publishedby Noroozifar et al. [30] and Cornell and Schwertmann [31] whoused zeolite–iron oxyhydroxide to remove cyanide from wastew-ater. They mentioned that Fe oxides enter the zeolite channels

or covers the surface of zeolite and they cause additional active

sites ( Fe OH). In addition, Noroozifar et al. [30] claimed that

maximum retention which was obtained at a pH of 7.5 and adsorp-tion mechanism of cyanide could be explained using electrostaticattraction/repulsion “Fes–OH+

2 + CN−↔ Fes–OH+

2 · · ·CN−” or direct

exchange mechanism “Fes–OH+

2 + CN−↔ Fes − CN +H2O”. Fur-

thermore, Adams [6] declared that maximum retention of cyanide(HCN form) is obtained at pH value of 7. At high pH values (higherthan 9), literature survey indicates that when activated carbon is

impregnated with transitional metal such as copper and silver, theadsorption properties of the obtained adsorbant increase due tochemisorption and the catalytic oxidation of cyanide [7,8,32]. At

35

45

55

65

75

1210864

pH

C N R e m o v a l ( % )

CAC-1

CAC-2

LAC

FeAC

Fig. 5. Effect of solution pH on adsorption of CN on CAC-1, CAC-2, LAC and FEAC

(cyanide concentration of 100mg/L).




-60

-40

-20

0

20

121086420

Equilibrium pH

Z e t a P

o t e n t i a l ( m V )

LAC

FeAC

CAC

Fig. 6. Zeta potentials of the activated carbons.

pH values of 10–10.5, cyanide could be removed by forming ironcyanide complexes in the form Fe(CN)−3

6 and Fe(CN)−46 .

In order to check the suggestion of Noroozifar et al.[30] and Cor-nell and Schwertmann [31] about the effect of electrostatic forceson adsorption mechanism of cyanide, zeta potential is taken intoaccount. Thezeta () potentialvariation of the activated carbonwas

estimated considering the pH of solution and this variation is givenin Fig.6. Thezeta potentialof FeAC increasein thenegativedirectionwith increasing pH and isoelectrical points (IEP) is approximately7 for FeAC. FeAC has positive charge until the pH of 7. HCN and

CN− present in the aqueous solution at the pH values in the rangeof 6–8. It can be said that electrostatic attraction may take placebetween cyanide species and FeAC due to different charge. As thepH of the solution increases, the surface of the activated carbon has

negative charge due to the successive deprotonation of positivecharged groups on the surface of activated carbon, and negativecharge density on the surface increases. Therefore, electrostatic

repulsionbetween thenegatively charged sites of FeAC andcyanide

occurred. Depending on the incompatible surface interactions, theadsorption of the cyanide on FeAC decreased with increasing thepH of the solution.

On the other hand, the adsorption of cyanide on CAC-1, CAC-2

and LAC showed the same trend and rose by increasing the pH of thesolution. The pK a ofHCNis 9.0 and thisshowsthat HCN iscom-pletely dissociated to CN− at a solution pH of 10 [33]. Fig. 6 showsthat the CAC and LAC particle surfaces are negatively charged for a

pH over 2.8 and 4 (pHzpc), respectively. Despite the same chargeof the CAC and LAC particles and the dominant specify of cyanide(CN−) in the solution, maximum cyanide retention was obtainedat pH value over 10. It proves that adsorption mechanism not

only depends on the surface charge of adsorbate and adsorbent. Asknown that physical, chemical and exchange adsorptions are threegeneral types of adsorption [34] and the adsorbents surface chem-istry andaqueous phase chemistryaffect an adsorption mechanism

[35]. In the literature, adsorption of cyanide ion at pH values overthan 10 is explained with the properties of chemical ion exchangemechanism and physical adsorption. Dash et al. [36] declared thatCN− ionis a nucleophilicion andit could be bind with thenegatively

charged adsorbent surface due to the anionic functional groupspresent on the adsorbent surface which occur due to the depro-

tonation on the activated carbon surface at pH values over than10 [37]. The prevailing mechanism for the adsorption of cyanide

ions onto CAC and LAC may be explained by chemical ion exchangeand this mechanism improves the cyanide adsorption. In addition,Garg et al. [38] mentioned that maximum cyanide adsorption at pH11 related with complexation of CN− with functional groups, and

physical adsorption due to the precipitations. The results obtainedin the present study are also compatible with recent publicationswhich indicates that maximum cyanide adsorption onto differentadsorbents occurs in the pH range of 9–11 [7,8,22,30,39,40].

With respect to the above-mentioned information and also dueto the pH values of 10–11 of gold cyanide disposals, in the presentstudy, the adsorption isotherm parameters and kinetic data weredetermined at two different pH ranges (7–7.5 and 10–10.5).

CAC-1

0

20

40

60

806040200

Contact Time (h)

C N R e m o v a l ( % )

100 ppm

200 ppm

300 ppm

400 ppm

CAC-2

0

20

40

60

806040200

Contact Time (h)

C N R e m o v a l ( % )

100 ppm

200 ppm

300 ppm

400 ppm

LAC

0

20

40

60

806040200

Contact Time (h)

C N R e m o v a l ( % )

100 ppm

200 ppm

300 ppm

400 ppm

FeAC

0

20

40

60

80

806040200

Contact Time (h)

C N R e m o v a l ( % )

100 ppm

200 ppm

300 ppm

400 ppm

Fig. 7. Effect of cyanide concentration on theadsorption of theactivated carbons at pH 7–7.5.




CAC-1

0

20

40

60

806040200

Contact Time (h)

C N R e m o v a l ( % )

100 ppm

200 ppm

300 ppm

400 ppm

CAC-2

0

20

40

60

806040200

Contact Time (h)

C N R e m o v a l ( % )

100 ppm

200 ppm

300 ppm

400 ppm

LAC

0

20

40

60

806040200

Contact Time (h)

C N R e m o v a l ( % )

100 ppm

200 ppm

300 ppm

400 ppm

FeAC

0

20

40

60

806040200

Contact Time (h)

C N R e m o v a l ( % )

100 ppm

200 ppm

300 ppm

400 ppm

Fig. 8. Effect of cyanide concentration on theadsorption of theactivated carbons at pH 10–10.5.

3.3. Effect of initial cyanide concentration

The effect of cyanide concentration onto adsorbents (CAC-1,CAC-2, LAC and FeAC) was investigated at two different pH rangesof 7–7.5 and 10–10.5 and their plots are illustrated in Figs. 7 and8,respectively.

Figs. 7 and 8 indicate that the percentage of cyanide removal

at the adsorption equilibrium shows a concentration-dependentcharacter. The adsorption capacity of the activated carbonsdecreases with increasing the cyanide concentration for all cases.

This may indicate that the adsorption is limited by number of theavailable active sites which are not sufficient for the high initialconcentration of the cyanide. The obtained results are consistentwith the available scientific data [8,22,36]. Besides, it can be seen

that the shape of the curves of all adsorbents keep almost constant,showing an activated carbon type-independent character.

Literature survey shows that the cyanide adsorption capaci-ties of activated carbons increases with the impregnation of them

with silver, copper, and nickel at high pH level [6–8]. On the con-trary of the literature, it was found that iron impregnation did notenhance the percentage of the removal of cyanide from aqueous

solution at pH values of 10–10.5 (in Fig. 8). On the other hand, bycomparing the percentage of cyanide removal using FeAC depend-ing on the initial pH of solution (Figs. 7 and 8), it was definedthat the removal of cyanide increased with decreasing the pH val-ues from 10 to 7 for FeAC (Fig. 8). The results indicate that iron

impregnation enhanced the removal of cyanide depending on thepH of the solution. This result also supports the previous con-clusion which was obtained the effect of pH section. However,according to Fig. 8b at normal pH conditions (7–7.5) the adsorp-

tion of cyanide onto FeAC is only 30–50% higher than on LAC. Asmentioned before, the obtained magnetic activated carbon (FeAC)shows super-paramagnetic behaviors at room temperature, so itcan be removed easily by a magnetic separator, as opposed to

the traditional screening technology. Although iron impregnation

does notsignificantlyaffect the removal of cyanide, considering the

magnetic property of FeAC, it canbe said that it is a viablecandidatefor the removal of cyanide from water and wastewater.

3.4. Adsorption isotherms

The adsorption isotherm data were fitted with well-known

two-parameter models, Langmuir [41] and Freundlich [42] as well

as three-parameter isotherm model, Redlich–Peterson [43]. Lit-erature data show that these isotherm models have been foundapplicable to cyanide sorption processes [6,8,30], so they were

chosen in the present study. To find out the best fitting kineticand isotherm models, average relative error (ARE) and nonlinearChi-square test ( X 2) for kinetic models were applied in additionto correlation coefficient (R2). The isotherm expressions are given

below:

Langmuir, qe =Q 0bC e

1 + bC e(4)

Freundlich, qe = k f (C e)1/n (5)

Redlich–Peterson, qe =

ARPC e

1 + BRPC g e (6)

where C e is the equilibrium cyanide concentration in the liquidphase (mg/L), qe is the adsorption capacity (mg/g), b (L/mg) and

Q 0 (mg/g) are Langmuir isotherm constants, where Q 0 signifies the

theoretical monolayer capacity, k f (L/g) is the Freundlich constantand 1/n (dimensionless) is the heterogeneity factor, ARP (L/g), BRP

(L/mg) g and g are the Redlich–Peterson isotherm constants.The linearized isotherm parameters were calculated by apply-

ing the linear regression procedure and also non-linear isothermparameters were determined using the solver add-in withMicrosoft’s spreadsheet, Microsoft Excel. The calculated parame-ters for two different pH range (7–7.5 and 10–10.5), the correlation

coefficient and error functions were summarized in Tables3 and 4,

respectively.




Table 3

Isothermsparametersfor thesorption of cyanide on theactivated carbons (cyanide concentration of 100mg/L, pH values in therangeof 7–7.5).

Linearized form Non-linearized form

CAC-1 CAC-2 LAC FeAC CAC-1 CAC-2 LAC FeAC

Langmuir isotherm

Q 0 (mg/g) 61.23 60.6 59.17 66.24 62.12 61.35 60.18 67.82

b×10−3 (L/mg) 8.32 7.92 8.32 5.49 8.11 7.24 7.93 5.33

R2 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99

ARE 2.16 2.33 2.40 2.05 2.37 1.92 2.32 1.38

X 2 0.24 0.34 0.28 0.27 0.21 0.12 0.13 0.17

Freundlich isotherm

k f (L/g) 1.77 1.68 1.82 5.75 2.43 2.04 2.13 6.12

n 1.73 1.72 1.76 2.41 1.92 1.85 1.92 2.73

R2 0.96 0.96 0.95 0.92 0.95 0.97 0.97 0.95

ARE 5.45 4.23 5.73 3.23 6.72 4.72 5.12 4.12

X 2 0.91 0.78 0.8 0.68 0.94 0.81 0.7 0.39

Redlich–Peterson isotherm

ARP (L/g) 0.62 0.52 0.52 2.34 0.57 0.48 0.46 1.71

BRP (L/mg) g 0.013 0.002 0.002 0.923 0.007 0.006 0.006 0.032

g 0.91 1.21 1.32 0.52 1 1 1 0.99

R2 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99

ARE 2.94 2.92 3.19 2.12 3.12 1.85 2.92 1.41

X 2 0.5 0.42 0.52 0.42 0.21 0.12 0.11 0.07

From Tables 3 and 4, it can be inferred that, two parameterlinear isotherm and non-linear isotherm data are very close witheach other and have the minimal deviation, but not in the case

of Freundlich isotherm, the deviation very large for both cases.Adsorption of cyanide on all the activated carbons at two dif-ferent pH ranges is quite well consistent with the Langmuir andRedlich–Peterson models. Especially, Langmuir model has lowest

level of thevalues of error function. It means that cyanide ions coverthe surface of all activated carbons as monolayer and each cyanideion has equal sorption activation energy at both pH values. There-fore, only the plots of linear and non-linear Langmuir isotherms for

both pH values are depicted in Fig. 9.Since Langmuir model is one of the best fitting isotherm mod-

els which is also applied in this study, ‘RL ’ named as dimensionless

separation factor was calculated to determine whether the adsorp-tion system is favorable or not [44]. It is defined by the followingequation:

RL =1

1 + bC o(7)

where RL is a dimensionless separation factor, C o is the initial dyeconcentration and b is Langmuir constant. The feasibility of thereactions are explained using the value of RL (RL > 1 – unfavorable,

RL = 1 – linear, 0 <RL <1 – favorable, RL = 0 – irreversible). The val-ues of RL were found in the range of 0.07–0.8, indicating that theadsorption is favorable.

The results of Redlich–Peterson isotherm also support the

suitability of Langmuir model. This model is related with threeparameters empirical equation and is a hybrid isotherm featuringboth Langmuir and Freundlich isotherms [45]. It can be used tounderstand adsorption mechanism of homogeneous or heteroge-

neous systems due to its versatility [46]. In this model, “ g ” is theexponent varying between 0 and 1. Tables 3 and 4 show that “ g ”values are close to one, and especially they calculated as one using

non-linear form. This indicatesthat Langmuir conditionis ideal onefor the adsorption mechanism [47].

Based on the adsorption capacities (Langmuir (Q 0) and Fre-undlich (k f ) constants) of the activated carbons at both pH range(in Tables 3 and 4), the adsorption capacities of CAC-1, CAC-2 and

Table 4

Isothermsparametersfor thesorption of cyanide on theactivated carbons (cyanide concentration of 100mg/L, pH values in therangeof 10–10.5).



Langmuir isotherm

Q 0 (mg/g) 64.93 63.29 64.10 68.02 64.34 63.22 64.20 68.35

b×10−3 (L/mg) 8.13 9.4 9.37 8.16 8.23 9.3 9.2 8.21

R2 0.99 0.99 0.99 0.98 0.99 0.99 0.98 0.99

ARE 2.45 2.68 2.44 3.32 2.23 2.41 2.34 3.28

X 2 0.29 0.31 0.38 0.8 0.26 0.07 0.32 0.71

Freundlich isotherm

kf (L/g) 1.81 2.12 2.01 1.82 2.49 2.95 2.53 2.96

n 1.74 1.78 1.74 1.67 1.95 2.12 1.95 1.86

R2 0.97 0.96 0.95 0.96 0.96 0.96 0.95 0.96

ARE 7.25 6.42 8.63 6.88 7.13 5.51 6.42 6.13

X 2 0.88 0.97 0.98 0.82 0.81 0.75 0.81 0.7

Redlich–Peterson isotherm

ARP (L/g) 2.17 1.98 1.95 2.18 0.51 1.48 0.51 0.52

BRP (L/mg) g 0.79 0.47 0.81 0.57 0.006 0.14 0.008 0.009

g 0.45 0.52 0.51 0.49 1 1 1 1

R2 0.95 0.97 0.96 0.97 0.99 0.99 0.99 0.99

ARE 5.45 4.18 5.67 4.86 3.25 3.93 3.45 3.16

X 2 0.67 0.52 0.74 0.72 0.25 0.41 0.27 0.21




Linear isotherm plots (pH 7 - 7.5)

0

2

4

6

8

4003002001000

Ce (mg/L)

C e / q e ( m g / g )

CAC-1

CAC-2

LAC

FeAC

Non-linear isotherm plots (pH 7 - 7.5)

0

15

30

45

60

4003002001000

Ce (mg/L)

q e ( m g / g )

CAC-1

CAC-2

LAC

FeAC

Linear isotherm plots (pH 10 - 10.5)

1

3

5

7

4003002001000

Ce (mg/L)

C e / q e ( m g / g )

CAC-1

CAC-2

LAC

FeAC

Non-linear isotherm plots (pH 10 - 10.5)

0

15

30

45

4003002001000

Ce (mg/L)

q e ( m g / g )

CAC-1CAC-2LACFeACLangmuir non-linear

Fig. 9. The plots of linearand non-linear isotherm Langmuir model forthe removal of cyanide at two differentpH values (cyanide concentration of 100mg/L).

LAC are very close with each other at the same pH range and theircapacities increases by increasing the pH of the solution. Equi-librium adsorption between the activated carbons and cyanideindicated that the extent of adsorption was not dependent on the

pore structure of the activated carbons. On the other hand, theadsorption capacity of FeAC shows different trend. FeAC has thehighest adsorption capacity at pH 7–7.5 and its capacity decreaseswith increasing the pH of the solution. It indicates that the adsorp-

tion capacities of the activated carbons forcyanide,especially FeAC,have a strict pH-dependent character. The result confirms the pre-vious results which were obtained in this study. Moreover, the

ultimate carbon-adsorption capacities of CAC-1, CAC-2 and LAC atboth pH ranges show that the adsorption of cyanide onto themis virtually independent of particle size and the activated carbontypes.

It was found that LAC and FeAC exhibited nearly same cyanideremoval efficiency with the commercial activated carbons. In orderto compare the adsorption capacities of LAC and FeAC, the adsorp-tion capacities of some adsorbentsfor cyanide adsorption aregiven

in Table 5. It seems that LAC and FeAC have also greater removalefficiency than lots of the adsorbents which are given in Table 5.Therefore, it seems that LAC and FeAC are attractive candidates for

removal of cyanide from aqueous solution.

Table 5

Comparison of the adsorption capacities of cyanide onto some adsorbents.

Adsorbents Adsorptioncapacity (mg/g)

pH References

Ag-imregnated AC 45.7 11 [7]

Ni-impregnated AC 15.4 >11 [7]

Cu-impregnated AC 19.7 10.5–11 [8]

Ativated carbon 47.62 10–11 [22]

Modified zeolite with iron 6.21 7–7.5 [30]

Pistachio hull wastes 156.2 10 [39]

Pyrophylite 72.4 7 [40]

3.5. Adsorption kinetics

The pseudo-first order kinetic model [48] and pseudo-secondorder kinetic model [49] were used to predict the mechanism

involved in the adsorption process of cyanide at two differentpH ranges (7–7.5 and 10–10.5). The accuracy and applicability of the kinetics model were checked using ARE and X 2 error func-tion model. In addition, intraparticle diffusion model [50] was also

applied for all conditions. The kinetic equations are given below:

Pseudo-first order, qt = qe[1 − exp(−k1t )] (8)

Pseudo-second order, qt =k2qe

2t

1 + k2qet (9)

Intraparticle diffusion, qt = kintt 1/2

+ C (10)

whereqe andqt arethe amounts(mg/g) ofsoluteboundat theinter-face at the equilibrium and after time t (min), respectively, k1 is therate constant of the pseudo-first-orderadsorption(min−1), k2 is therate constant of the pseudo-second-orderadsorption (g/mg min),C is the intercept and kint is the intraparticle diffusion rate constant(mg/g min1/2).

The kinetic linearized and non-linearized model studies werecarried out at initial cyanide concentration of 100 mg/l and the cal-

culated kinetic parameters of the fitted kinetic models for two pHranges, 7–7.5 and 10–10.5,are given in Tables6 and 7, respectively.It can be concluded that the adsorption of cyanide onto the acti-

vated carbons did not follow the pseudo first order model. Thepseudo second order kinetic model with high correlation coeffi-cient, low ARE and X 2 describes the system much better than thepseudo first order kinetic models. Similar results were found by

Deveci et al. [8] and Behnamfard and Salarirad [22]. Therefore onlythe plots validating the pseudo-second order kinetic model for lin-ear and non-linearized kinetic models (between t /qt vs t and qt vs t ) at different pH range of the cyanide of 100 m g/L, at room

temperature (25 ◦C)are given in Fig. 10. From Fig. 10, it can be con-

cluded that pseudo-second order kineticmodelwith linearand non




Table 6

Kinetic parametersfor theeffect of theactivated carbontypeson theadsorption of cyanide (pHvalues:7–7.5; cyanide concentration: 100mg/L).

Pseudo-first-order



qe (cal) (mg/g) 13.08 20.32 12.16 20.37 16.45 17.32 14.44 24.70

k1 (min−1) 0.072 0.14 0.10 0.13 0.08 0.11 0.14 0.15

R2 0.97 0.94 0.98 0.95 0.995 0.99 0.99 0.99

ARE 26.67 29.95 24.84 23.22 4.48 5.85 4.49 6.29

X 2 13.16 11.03 12.55 19.56 0.28 0.58 0.34 2.32

Pseudo-second-order



qe (cal) (mg/g) 19.80 19.19 16.10 26.53 19.93 20.10 16.32 27.71

k2 (g/mg min) 0.00 0.01 0.01 0.01 0.004 0.007 0.011 0.007

h (mg/g min) 1.74 2.86 3.03 6.56 1.75 2.63 3.06 5.66

R2 0.99 0.99 1.00 1.00 0.99 0.98 1.00 0.99

ARE 5.04 7.94 6.57 8.66 3.2 4.82 4.34 5.23

X 2 0.46 1.23 0.47 2.10 0.25 1.03 0.25 1.89

Table 7

Kinetic parametersfor theeffect of theactivated carbontypeson theadsorption of cyanide (pHvalues:10–10.5; cyanide concentration: 100mg/L).

Pseudo-first-order



qe (cal) (mg/g) 14.87 19.31 19.08 18.36 16.86 18.20 18.46 16.94

k1 (min−1) 0.07 0.14 0.14 0.10 0.09 0.12 0.12 0.11

R2 0.98 0.97 0.97 0.98 0.99 0.99 0.99 0.99ARE 22.10 12.39 7.18 7.63 4.78 6.87 6.39 4.72

X 2 8.52 2.61 1.38 1.02 0.42 1.26 0.83 0.48

Pseudo-second-order



qe (cal) (mg/g) 19.72 19.96 18.20 18.90 20.06 20.85 21.05 19.68

k2 (g/mg min) 0.01 0.01 0.01 0.01 0.005 0.007 0.007 0.007

h (mg/g min) 2.09 3.51 3.63 2.72 2.05 3.09 3.32 2.59

R2 1.00 0.99 1.00 0.99 1.00 0.99 1.00 0.99ARE 4.93 7.58 9.48 9.02 4.96 7.00 6.02 8.39

X 2 0.39 1.11 2.87 1.37 0.36 0.97 0.94 1.19

linear forms showing good correlation between experimental andpredicted values.

In the present study, to understand the adsorption mechanismof cyanide on the activated carbons, intraparticle diffusion modelwas also applied for all conditions. Parameters which are obtainedfrom the general equation of intraparticle diffusion model as a

function of different initial cyanide concentrations at two differ-ent pH values (7–7.5 and 10–10.5) are given in Tables 8 and 9,

respectively. Their plots areshownin Figs.11and12 which indicatethat the adsorption mechanism of cyanide onto the activated car-

bons for all conditions follows two steps. The first one covering thetime interval between 1 and 21h (more than 21h for high cyanideconcentration),whichhas a very sharp,is attributed to thediffusionof cyanide through the solution film to the external surface of the

activated carbons (mass transfer). The second range (after 21h) isdescribed as the gradual adsorption limitedby diffusionof solutein

Table 8

Intraparticlediffusion parameters for the effect of the activated carbon types and cyanide concentration on adsorption (pH values: 7–7.5).

Intra particle diffusion

CAC-1 CAC-2

Initial concentration (ppm) 100 200 300 400 100 200 300 400

kint (mg/gmin1/2) 2.05 3.72 5.1 5.7 1.92 3.66 5.2 5.17

C 1.99 2.79 2.8 3.9 4.06 4.54 4.36 7.52

R2 0.88 0.85 0.86 0.88 0.77 0.85 0.86 0.85


LAC FeAC


kint (mg/gmin1/2) 1.48 3.69 5.19 5.19 2.35 3.86 4.43 5.42

C 4.71 2.65 2.46 5.61 9.17 15.42 21.19 14.13

R2 0.76 0.84 0.86 0.86 0.70 0.82 0.79 0.89




Linear forms (pH 7 - 7.5)

0

1.6

3.2

4.8

6.4

806040200

t (hours)

t / q t

CAC-1

CAC-2

LAC

FeAC

Non-linear forms (pH 7 - 7.5)

0

10

20

30

806040200

t (hours)

q t ( m g / g )

CAC-1

CAC-2

LAC

FeAC

pseudo second order non-linear

Linear forms (pH 10 - 10.5)

0

1.6

3.2

4.8

806040200

t (hours)

t / q t

CAC-1

CAC-2

LAC

FeAC

Non-linear form (ph 10 - 10.5)

0

10

20

806040200

t (hours)

q t ( m g / g )

CAC-1

CAC-2

LAC

FeAC

pseudo second order non-linear

Fig. 10. Thepseudo-second order adsorption kinetics forcyanide adsorption onto theactivated carbons at two pH values at cyanide concentration of 100mg/L.

the liquid contained in pores of adsorbent particles and along thepores walls (intraparticle diffusion) [51]. Besides, the adsorption

kinetics reaches the equilibrium at a different time depending onthe initial cyanide concentration. Fast equilibrium obtained at lowinitial cyanide concentration. This observation is supported withthedata publishedby Behnamfard andSalarirad [22] andDashetal.

[36] who explained that cyanide ions at low cyanide concentration

could be adsorbed by more readily available sorption sites. At highcyanide concentration, diffusion of cyanide through the solution

filmto theexternalsurfaceof theactivated carbons areveryfastandthendecreasesdepending on the equilibrium time. This shows that

available sorption sites of activated carbon are saturated depend-ing on the mass transfer and then high level of cyanide ions in thesystem try to diffuse in the pores. It means that the pores showresistance against the diffusionof cyanide ioninto inner adsorption

sites. Brieflyit canbe concluded that the intraparticle diffusionstep

controls the cyanide adsorption on CAC-1, CAC-2, LAC and FeAC forall conditions. Moreover, Moussavi and Khosravi [39] and Hameed

CAC-1

0

15

30

45

60

1086420

t

1/2

(h

1/2

)

q t ( m g / g )

100 ppm

200 ppm

300 ppm

400 ppm

CAC-2

0

15

30

45

60

1086420

t

1/2

(h

1/2

)

q t ( m g / g )

100 ppm

200 ppm

300 ppm

400 ppm

LAC

0

15

30

45

60

1086420

t1/2

(h1/2 )

q t ( m g / g )

100 ppm

200 ppm

300 ppm

400 ppm

FeAC

0

15

30

45

60

1086420

t1/2

(h1/2 )

q t ( m g / g )

100 ppm

200 ppm

300 ppm

400 ppm

Fig. 11. Plots of intraparticle diffusion for adsorption of cyanide onto theactivated carbons at various initial concentration of cyanide (pH values: 7–7.5).




Table 9

Intraparticlediffusion parameters for the effect of the activated carbon types and cyanide concentration on adsorption (pH values: 10–10.5).


CAC-1 CAC-2


kint (mg/gmin1/2) 2.02 3.73 5.17 5.48 1.93 3.68 5.19 5.19

C 2.74 4.13 4.47 5.57 5.04 5.54 5.37 8.52

R2 0.88 0.85 0.87 0.87 0.78 0.82 0.86 0.86


LAC FeAC


kint (mg/gmin1/2) 1.94 3.69 5.19 5.19 1.90 3.64 5.09 5.19

C 5.32 5.99 5.80 8.94 3.93 4.56 5.33 8.23

R2 0.76 0.84 0.86 0.86 0.77 0.84 0.85 0.86

CAC-1

0

15

30

45

60

1086420

t1/2

(h1/2

)

q t ( m g / g )

100 ppm

200 ppm

300 ppm

400 ppm

CAC-2

0

15

30

45

60

1086420

t1/2

(h1/2 )

q t ( m g / g )

100 ppm

200 ppm

300 ppm

400 ppm

LAC

0

15

30

45

60

1086420

t1/2

(h1/2

)

q t ( m g / g )

100 ppm

200 ppm

300 ppm

400 ppm

FeAC

0

15

30

45

60

1086420

t1/2

(h1/2

)

q t ( m g / g )

100 ppm

200 ppm

300 ppm

400 ppm

Fig. 12. Plots of intraparticle diffusion for adsorption of cyanide onto the activated carbons at various initial concentration of cyanide (pH values: 10–10.5).

[52] suggested that the boundary layer thicknesses, “C ” should beconsidered to give a better decision. Tables 8 and 9 show that

C values are positive indicating that cyanide adsorption controlswith not only the intraparticle diffusion but also the film diffusion.BesidesC valuesincrease with increasing cyanide concentration forall activated carbons. It means that the film diffusion is involved in

controlling cyanide adsorption onto the activated carbons at higher

cyanide concentrations [52].

4. Conclusion

In this study, a plain activated carbon and iron impregnatedactivated carbon were prepared from Gölbas ı lignite. BET surfacearea of LAC is determined as 921 m2/g. FeAC has also high surfacearea of 667 m2/g with 19wt.% Fe3O4 coated and perfect magnetic

separation performance. Adsorption of cyanide on the activatedcarbons is quite well consistent with the Langmuir models. Theultimate carbon–cyanide adsorption capacities of the activatedcarbons are virtually independent of particle size, pore structure

and the activated carbon types. The maximum monolayer adsorp-

tion capacities of LAC and FeAC are 60.18mg/g and 67.82mg/g

at pH values of 7–7.5 and 64.10mg/g and 68.02mg/g at pH val-ues of 10–10.5, respectively. They are much higher than some

of the maximum values reported in the literature. The percent-age of cyanide removal at the adsorption equilibrium shows aconcentration-dependent character. The adsorption capacity of theactivated carbons decreases with increasing the cyanide concen-

tration due to the limited available active site of the surface of

activatedcarbons.SolutionpH playsa major role in cyanide adsorp-tion onto FeAC and maximum retention was obtained at pH 7–7.5due to the electrostatic attraction and exchange mechanism. The

experimental results best fitted to the second-order kinetic equa-tion. Intraparticle diffusion model shows that the removal rate iscontrolled by notonly intraparticle diffusionbut also the filmdiffu-sion. Finally, it is possible to affirm that LACand FeAC arepromising

adsorbents, which were obtained from natural resource, for theremoval of cyanide from aqueous solution.

Acknowledgements

Thanks are due to Askale Van Cement Factory for their con-

tributions in performing experiments. Also special thanks to

comparison of activated carbon and iron impregnated from lignite to remove...

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