Optimization of water desalination using carbon-basedadsorbents

Ana Mendeza, Gabriel Gascob*aDepartamento de Ciencia y Tecnologıa del Medio Ambiente, Universidad Catolica de Avila C/Canteros

s/n. 05005, Avila, SpainbDepartamento de Edafologıa, Escuela Tecnica Superior de Agronomos, Universidad Politecnica de Madrid,

Av. Complutense s/n. 28043, Madrid, SpainTel. þ34 91 3365691; Fax: þ34 91 3365680; email: gabriel.gasco@upm.es

Received 15 February 2005; accepted 10 March 2005

Abstract

The objective of this work is to study the removal of Ca, Mg, Na and K from water using carbon-based

materials obtained from sewage sludges pyrolysis. For this reason, carbon-based materials were prepared at two

different pyrolysis temperatures: 450 (SL-450) and 650�C (SL-650). Also, a third material was prepared washing

SL-650 with HNO3:H20 1:1 (vol). These carbon-based material were mixed with solutions with different

concentrations of NaCl, KCl, CaCl2 and MgCl2. Results showed that percentages of metal removal followed

next sequence Mg2þ > Ca2þ > Naþ�Kþ and that the percentage of removal increased with the pH of the initial

solution. Similar percentages of ions removal were obtained by SL-450 and SL-650 increasing the percentage of

metal removal when the SL-650 washed with acid was used. Metals lixiviation from carbon-based materials

decreases with the increment of the pyrolysis temperature. Nevertheless, the properties of raw material and

consequently, of carbon-based materials can have a great influence in the adsorption properties of the final

carbonaceous material, and so, in the removal of metals from water.

Keywords: Pyrolysis; Sewage sludge; Metals

1. Introduction

Over the last few decades desalination tech-nologies have been used increasingly through-out the world to produce drinking water from

brackish groundwater and seawater, to improvethe quality of existing supplies of fresh water fordrinking and industrial purposes, and to treatindustrial and municipal wastewater prior todischarge or reuse [1]. Indeed, the installedwater desalting capacity increased especially

Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005.

European Desalination Society.

0011-9164/05/$– See front matter � 2005 Elsevier B.V. All rights reserved

*Corresponding author.

Desalination 183 (2005) 249–255

doi:10.1016/j.desal.2005.03.038

from 1980, when the improvements in themembrane technologies extended the reverseosmosis (RO) desalting procedure. For exam-ple in Spain, the installed water desaltingcapacity was more than 0.21 km3 y�1, ofwhich 70% was by reverse osmosis (RO) pro-cedure, 23% by a multi-stage flash (MSF)and multi-effect distillation (EMD), 5% byvapour compression (VC), and 2% by elec-tro-dialysis (ED) [2].

The filtration of saline water before the ROprocedure can be used to reduce desalinationcost. Commercial activated carbons are effec-tive adsorbents materials extensively used forwater purification due to their surface area.Commercial activated carbons can be used tothe removal of organic matter [3,4] or metalsions [5] fromwater but their price can limit theiruses. Nevertheless, different waste materials,after an adequate treatment, can be trans-formed and used like carbon-based materials.For e.g., nutshell carbon has been used for theremoval of Cu2þ, Pb2þ and Zn2þ of drinkingwater [6], hazelnut shell activated carbon for theremoval of Cu6þ [7], or agricultural by-pro-ducts for the removal of Pb2þ [8]. Also, car-bon-based materials have the advantage thatcarbon is not sensitive to organic impurities,which in the case of commercial ion-exchangerscause their irreversible deterioration [9].

The increment of the wastewater treatmentplants due to environmental regulation hasincreased sewage sludges production andtheir valorization should be an environmentalobjective. Pyrolysis of sewage sludges permitsto obtain carbon-based adsorbents with sur-face area of 100–400m2 g�1 that can be usedto the removal of organic compounds [10].

Previous works carried out by our researchgroup have showed that carbon-based materi-als obtained from sewage sludge pyrolysiscould be used in the removal of Ca2þ, Mg2þ,Naþ and Kþ from water [11]. The objective ofthis work is to improve the removal of these

ions in brackish water using carbon-basedmaterials from pyrolysis of sewage sludge. Inorder to achieve that, three different materialswere prepared from the same sewage sludgeand different experimental conditions wereused in the metal removal.

2. Materials and methods

2.1. Sewage sludge characterisation

One anaerobic sewage sludge from thewastewater treatment plant of Avila region(Spain) was used for the production of car-bon-based materials. First, sewage sludge wasair-dried, crushed and sieved through 2 mmmesh. Then, sludge was characterized accord-ing to their total humified substances(HA þ FA), humic acids (HA), fulvic acids(FA) and pH. A subsample of the sewagesludge for determination of the total carboncontent (TC), the total content of Ca, Mg,Na, K, Fe, Ni and Cu was crushed and sievedthrough 0.074 mm mesh [12].

Total carbon (TC) of sewage sludges wasdetermined by burning it at 540�C [13].Total humic substances (THS) wereextracted with a mixture of 1 M Na4P2O7

and 0.1 M NaOH, centrifuged at 3000 rpmand filtered using Whatman No. 42 filters[14]. An aliquot of this extract was acidifiedwith concentrated H2SO4 to pH 1, centri-fuged to separate coagulated humic acids(HA) and then, the HA were re-dissolvedwith 0.1 M NaOH [15]. The non-coagul-tated fraction with H2SO4 is referred to asfulvic acids (FA) and it was calculated asthe difference between THS and HA. The Ccontents of the THS and HA were deter-mined by the Walkley-Black method [16]after being dried in a thermostatic bath at60�C. The pH was measured after stirring amixture of sewage sludge and distilled water(4g L�1) for 2 h.

250 A. Mendez, G. Gasco / Desalination 183 (2005) 249–255

Total sewage sludges content in Ca, Mg,Na, K, Fe, Ni, Cu was extracted withHCl and HNO3 following 3051a USEPAmethod [17].

The quantitative estimation of carboxylic-containing surface groups was conductedusing the neutralization method described byTessmer [18]. Solutions of NaHCO3 0.05 Nand standard H2SO4 were prepared usingdistilled water. 70 mL of base solution wasadded to 2 g of sewage sludge and allowedto equilibrate for 3 days in a sealed 100 mLbottle while being mixed with a rotary shaker.At the end of the equilibration period, thesample was filtered and the filtrate titratedusing standardized H2SO4 solution. Theamount of NaHCO3 consumed by the car-boxylic groups was calculated as the differ-ence between the acid required to titrate theblank to pH 4.5 and the acid required totitrate the filtrate to the same end point.

Carbon, hydrogen, sulphur and nitrogencontent of sewage sludges was determinedwith a LECO-CHNS-932 microanalyzer.The oxygen content was obtained using aLECO-VTF-900 furnace coupled to themicroanalyzer.

2.2. Preparation of carbon-based materials

Two different materials were prepared byheating the sludge in an inert atmosphere atdifferent temperatures (450 and 650�C).

20 g of sewage sludge (SL) were placedin a covered ceramic cup placed in a nickelrecipient. The cavity between the two reci-pients was filled up with fuel coke particles(<1 mm). These samples were pyrolysed inan electrical furnace by increasing the tem-perature to 450 (SL-450) or 650�C (SL-650)at a rate of 3�C min�1. The final tempera-ture was maintained for 2 h. As the tem-perature increases, O2 is consumed by fuelcoke particles and the sewage sludge is

pyrolysed in the inert atmosphere generated.External chemical activation was not per-formed to avoid effects on sewage sludgespyrolysis.

2.3. Characterization of carbon-based materials

Carbon-based materials were characterizedaccording to the following parameters: iodinenumber (IN), cation exchange capacity(CEC), carboxylic-containing surface groups,ash content and pH.

The iodine number (mgI2 g�1) defined asthe quantity of iodine adsorbed per gram ofactivated carbon at an equilibrium concentra-tion of 0.02 N was calculated according toD-4607 standard test method [19]. The iodinenumber is considered a simple method toevaluate the surface area of activated carbonsassociated with pores with d >1 nm.

CEC, pH and carboxylic-containing sur-face groups were determined by methodsused for raw material (section 2.1). Ash con-tent of carbon-based adsorbents was calcu-lated after oxidation of sample in anelectrical furnace, at 850�C during 5 h.

To evaluate metal lixiviation from eachcarbon-based material, 0.25 mg of each sam-ple was mixed with 100 mL of distilled waterand agitated during 3 h. After this time, solu-tions were centrifuged at 3000 rpm during10 min to remove suspension particles andthe water content in Na, K, Ca, Mg, Fe, Cuand Ni was evaluated by atomic absorptionusing a Perkin Elmer 2280 atomic absorptionspectrophotometer.

2.4. Metals removal

The carbon-based materials were used forthe removal of Ca2þ, Mg2þ, Naþ and Kþ.Standard solutions of 200 and 100 mg L�1

of NaCl, KCl, CaCl2 and MgCl2 were pre-pared and used in the experiment. Moreover,solutions more concentrated (500 and

A. Mendez, G. Gasco / Desalination 183 (2005) 249–255 251

100 mg L�1) were prepared in the case ofNaþ and Kþ.

To evaluate metal removal, 100 mL ofeach solution was mixed with 0.25 mg ofeach carbon-based materials and agitated at300 rpm during 3 h. After this time, solutionswere centrifuged at 3000 rpm during 10 minto remove suspension particles and the metalfinal concentrations were evaluated by atomicadsorption using a Perkin Elmer 2280 atomicabsorption spectrophotometer. Different pHsolutions (6.5 and 9.0) were used to study thepH effect.

3. Results and discussion

3.1. Properties of raw material

Sludge composition is shown in Tables 1and 2. Total carbon (TC), HA and FA

contents and HA/FA values are typical forthis kind of wastes [20,21]. HA/FA is a mea-sure of the organic matter polymerisation. Pre-vious research carried out by our group haveshowed that sewage sludge with low HA/FA(0.8) ratios produces adsorbents with high sur-face areas [22]. For this reason, this materialwas used in this work. The carboxylic groupscontent of sewage sludge is according withother authors [23]. Metals content is normalfor this type of residues, except the Na contentthat is over the average Na value [20].

3.2. Carbon-materials characterization

Tables 3 and 4 show the main properties ofthe carbon-based materials prepared from sew-age sludge. The pH increased with temperature(from 7.30 at 450�C to 8.20 at 650�C) and thisfact is related to the significant decrease in

Table 1

Properties of sewage sludge

TC(wt%)

THS(wt%)

HA(wt%)

FA(wt%)

HA/FA pH meqCOOH/g

64.90 2.92 1.35 1.57 0.86 7.76 44.31

Na K Ca Mg Fe Cu Ni(g kg�1) (g kg�1) (g kg�1) (g kg�1) (g kg�1) (mg kg�1) (mg kg�1)5.25 3.62 3.71 3.9 2.72 21 49

Table 2

Elemental analysis of sewage sludges

C (wt%) H (wt%) N (wt%) S (wt%) O (wt%) H/Ca O/Ca N/Ca

26.89 5.57 3.85 0.84 32.54 2.48 0.91 0.12

amolar ratio.

Table 3

Properties of carbon materials prepared from sewage sludges

Carbon adsorbent pH meqCOOH g�1 Ash content(wt%)

CEC(cmol(þ) kg

�1)Iodine number(mg I2 g

�1)Pyrolysis yield(wt%)

SL-450 7.30 25.0 65.7 72.31 334 50.73SL-650 8.20 2.5 71.9 47.31 554 46.85

252 A. Mendez, G. Gasco / Desalination 183 (2005) 249–255

carboxylic groups (SL-450: 25.0 meqCOOH g�1

and SL-650: 2.5 meqCOOH g�1).The highest pyrolysis temperature, the

highest is iodine number. These values aresimilar to the iodine numbers obtained byother authors [23].

Table 4 shows the lixiviated metals fromeach carbon-based materials. These resultsshow how metals lixiviation decreases whenpyrolysis temperature increases. Therefore,pyrolysis offers the advantage of concentrat-ing the heavy metals in the final solid residue[24]. Indeed, the risk of metals lixiviation inpyrolysis ashes is lower than in the incinera-tion ashes [24].

3.3. Metals removal

Similar Ca removal (Table 5) were obtainedfor both material (SL-450, SL-650). The high-est is pH, highest is the Ca removal. For theseCa concentrations, the Ca(OH)2 precipitation

takes place at pH > 10, and so, pH effect wasnot related with the metal precipitation. Thesematerials can act as cation exchangers due totheir content in oxygenated-functional groups(carboxyl and hydroxyl) (Table 3) that areactive centers of ion exchange. Indeed, thehighest is pH, highest is the CEC.

Behaviour of Mg2þ was similar to that ofCa2þ. The highest is pH, highest is Mgremoval. Comparison of Tables 5 and 6shows that carbon-based materials preparedin this work were more effective for the Mgremoval with values close to 70% at pH = 9.0.These results show that carbon-based materi-als could be used to reduce the hardness ofwater avoiding problems of salt precipitation,deposition of suspended matter or corrosionof metal surfaces. Nevertheless, the use ofCa2þ and Mg2þ solutions produced the lixivia-tion of dissolved organic matter of the SL-450while this fact was not produced in the SL-

Table 4

Soluble metals content in water (mg L�1)

Carbonadsorbent

Na K Ca Mg Fe Cu Ni

SL-450 15.5 4.2 4.0 1.6 0.3 nda ndSL-650 6.6 1.6 3.5 0.2 nd nd ndGuideline

valuesb200 – – – 0.2 2 0.02

aConcentration below detection level.bGuidelines values of World Health Organisation [26].

Table 5

Percentage of Ca2þ removal

Percentage of Ca2þ removal (%)

pH = 6.5 pH = 9.0

100a 200 100 200

SL-450 18.6 18.8 38.0 31.0SL-650 20.0 25.0 35.0 29.5

aColumn shows the Ca2þ removal (%) from an initialsolution of 100, 200 mg L�1.

Table 6

Percentage of Mg2þ removal

Percentage of Mg2þ removal (%)

pH = 6.5 pH = 9.0

100a 200 100 200

SL-450 56.9 58.0 65.0 65.0SL-650 58.0 60.0 75.0 65.0

aColumn shows the Mg2þ removal (%) from aninitial solution of 100, 200 mg L�1.

Table 7

Percentage of Naþ and Kþ removal by SL-650 at

pH ¼ 9.0

Percentage of removal (%)100a 200 500 1000

Na –b – 19.0 21.0K – 11.0 17.8 18.0

aColumn shows the Naþ and Kþ removal (%) froman initial solution of 100, 200, 500, 1000 mg L�1.bInvaluable.

A. Mendez, G. Gasco / Desalination 183 (2005) 249–255 253

650. For this reason, SL-450 material was notuse to the removal of Naþ and Kþ.

A different behaviour was observed in thecase of Naþ and Kþ solutions (Table 7).Metal removal was not produced with lowconcentration (100 mg L�1 for Kþ and 100and 200 mg L�1 for Naþ). This differencewith Ca2þ and Mg2þ behaviour could berelated to ion exchange generally have higherselectivities for ions with increasing valenceor charge [27]. These affinities relationshipsare reversed in concentrated solutions. Thisfact makes possible regeneration of exhaustedcation resin used for softening of water, thatis predominately in the calcium and magne-sium form, both divalent ions. The resin isrestored to its regenerated condition, thesodium form, by the introduction of elevatedsodium chloride concentrated solutions(100,000 mg Naþ L�1) to reverse selectivity[28]. This fact is according to experimentalresults shown in Table 7. The highest is bothNaþ and Kþ concentration, highest is Naþ andKþ removal. Previous works have showed per-centage of Naþ removal over 70% for concen-trated Naþ solutions (10,500 mg L�1) [11], andso, carbon-based material characteristics couldaffect to the difference in percentage of Naþ

and Kþ removal.In order to improve the use of these mate-

rials, SL-650 material was washed withHNO3:H20 1:1 (vol) at room temperature to

remove the soluble cations (Table 8). Withthis treatment, Hþ replaces to all cationsfrom the exchange positions. The percentageof metal removal by this new material washigher than the percentage obtained usingSL-650 and SL-450. The highest is pH, high-est is the metal removal. In case of Naþ andKþ removal was not produced at low pH.

Finally, it should be pointed out that theuse of carbon-based materials lead to theacidification of the final solutions, speciallyin the solutions agitated with the carbon-based material treated with acid, due to Hþ

exchange by Ca2þ, Mg2þ, Naþ and Kþ.

4. Conclusions

Major findings from this study are sum-marized as follows:� Similar percentages of ions removal were

obtained by the carbon-based materialsprepared from pyrolysis of sewage sludgeat 450 and 650�C. The percentage of ionsadsorption followed next sequence Mg2þ

> Ca2þ > Naþ�Kþ.� The higher was pyrolysis temperature, lower

was the metal and organic matter lixiviationfrom carbon-basedmaterial. Indeed, carbon-based material prepared at 650�C presentedless metal lixiviation than carbon-basedmaterial obtained at 450�C and release oforganic matter was not produced.

Table 8

Percentage of metal removal by SL-650 washed by HNO3:H20 1:1

Percentage of metal removal (%)

Ca2þ Mg2þ Naþ Kþ

100a 200 100 200 100 200 500 1000 100 200 500 1000

pH = 4 27.7 19.0 10.0 71.0 –b – – – – – – –pH = 9 55.0 39.5 55.0 78.0 – 13.6 26.1 28.0 14.1 14.8 18.3 19.0

aColumn shows the metal removal (%) from an initial solution of 100, 200, 500, 1000 mg L�1.bInvaluable.

254 A. Mendez, G. Gasco / Desalination 183 (2005) 249–255

� The percentage of metal removal could beincreased using carbon-based materialwashed by acid.

� In general, percentage of metal removalincreased with the increment of pH.

� A wide study will be necessary to optimizethe preparation of adsorbents and ionsadsorption due to the properties of rawmaterial can have a great influence in theadsorption properties of the final carbo-naceous material.

Acknowledgements

The authors of this work want to acknowl-edge the Diputacion Provincial de Avila-Institucion Gran Duque de Alba-CSIC forthe economical support.

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