This article was downloaded by: [Iowa State University] On: 28 September 2013, At: 13:04 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Energy Sources, Part A: Recovery, Utilization, and Environmental Effects Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ueso20 Chromium(VI) Adsorption by Brown Coals Fethiye Gode a & Erol Pehlivan b a Department of Chemistry, Suleyman Demirel University, Isparta, Turkey b Department of Chemical Engineering, Selcuk University, Konya, Turkey Published online: 23 Aug 2006. To cite this article: Fethiye Gode & Erol Pehlivan (2006) Chromium(VI) Adsorption by Brown Coals, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 28:5, 447-457, DOI: 10.1080/009083190927156 To link to this article: http://dx.doi.org/10.1080/009083190927156 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
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This article was downloaded by: [Iowa State University]On: 28 September 2013, At: 13:04Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
Energy Sources, Part A: Recovery, Utilization, andEnvironmental EffectsPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/ueso20
Chromium(VI) Adsorption by Brown CoalsFethiye Gode a & Erol Pehlivan ba Department of Chemistry, Suleyman Demirel University, Isparta, Turkeyb Department of Chemical Engineering, Selcuk University, Konya, TurkeyPublished online: 23 Aug 2006.
To cite this article: Fethiye Gode & Erol Pehlivan (2006) Chromium(VI) Adsorption by Brown Coals, Energy Sources, Part A:Recovery, Utilization, and Environmental Effects, 28:5, 447-457, DOI: 10.1080/009083190927156
To link to this article: http://dx.doi.org/10.1080/009083190927156
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
Department of ChemistrySuleyman Demirel UniversityIsparta, Turkey
EROL PEHLIVAN
Department of Chemical EngineeringSelcuk UniversityKonya, Turkey
The ability of brown coals (leonardites) to remove chromium(VI) from aqueous so-lutions was studied as a function of pH, contact time, adsorbent nature, and con-centration of metal solutions. The adsorption of Cr(VI) was higher between pH 2.0and 3.0 for all brown coals and maximum sorption was observed at pH 3.0. TheLangmuir adsorption isotherm was used to describe observed sorption phenomena,and the constants were evaluated. The maximum adsorption capacity of 0.92 mmolof Cr(VI)/g for YK, 0.98 mmol of Cr(VI)/g for KK was found from experimental data.KK was the best among the selected adsorbents for the sorption of Cr(VI) at pH 3 andthe sorption was 81% out of 100 ppm Cr(VI) after 120 min of stirring.
Keywords brown coal, adsorption, chromium, Langmuir’s
Chromium in the wastewaters from plating and metal finishing, tanning, and photographicindustries causes environmental pollutions. The chromium release into the environment isregulated by the Environmental Protection Agency (EPA). Commercially available browncoals were used for the removal of hexavalent chromium (Cr(VI)) from aqueous solu-tion. Chromium occurs most frequently as Cr(VI) or Cr(III) in aqueous solutions. Thetwo oxidation states have different chemical, biological, and environmental properties.Chromium has widespread industrial applications; hence, large quantities of chromiumare discharged into the environment. Chromium compounds are essential to many indus-tries. The major industries that contribute to water pollution by chromium are mining,electroplating, aluminum conversion coating operations, leather tanning, textile dyeing,and plants producing industrial inorganic chemicals and pigments, and wood preserva-tives (Dakiky et al., 2002; Lalvani et al., 2000). For all of these industries, chromium intheir wastewater is a major environmental problem. For example, hexavalent chromiumcompounds are known to be very toxic although trivalent ones are somewhat less toxic.
Several methods are utilized to remove chromium from industrial wastewater. Theseinclude: reduction followed by chemical precipitation, ion exchange, membrane separa-
Address correspondence to Fethiye Gode, Department of Chemistry, Suleyman Demirel Uni-versity, 32260, Isparta, Turkey. E-mail: [email protected]
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tion, reduction, adsorption, electrochemical precipitation, solvent extraction (Pagilla andCanter, 1999; Chakravarti et al., 1995; Cimino et al., 2000; Lalvani et al., 2000). Adsorp-tion is an effective and efficient method for removing chromium from aqueous solution,particularly when combined with appropriate regeneration steps. This solves the prob-lems of sludge disposal and renders the system more economically viable, especially iflow-cost adsorbents are used (Bailey et al., 1999). However, the literature is still insuf-ficient to cover this problem, and more work and investigations are needed to deal withother locally available and cheap adsorbents to eliminate Cr(VI) from industrial waste-water samples with different compositions and characteristics. It is known that Cr(VI) ispresent as an anionic species in the solution.
Although brown coals are naturally occurring polymers, they have a significant se-lectivity for toxic metals in aqueous solution. In this study, commercially available youngbrown coals from the southwest region of Anatolia (Yarıkkaya-YK; Kasıkara-KK) wereused for sorption of Cr(VI) from aqueous solution. Brown coals differ considerably inmode of occurrence and in their physical and chemical properties, and it is generallyaccepted that microorganisms have played a prominent part in the process of coal for-mation. Phenolic structures may have occurred from lignin in the lignocellosic materials(Demirbas and Demircioglu, 2000; Demirbas, 2002; Wieber et al., 1988).
Brown coals are amorphous, aromatic biopolymers which offer a valuable resourcethat is inexpensive, renewable, and available in abundant quantities. Brown coals arecharacterized by their low calorific value, as well as high oxygen and moisture contents.The high oxygen content of low-rank coals allows, however, the unique capability forbrown coal to remove metals from solution via ion exchange with carboxylic acid andphenolic hydroxyl functional groups on the surface of coals. Recent studies have shownthat brown coals and other naturally occurring biomass can serve as effective adsorbentsfor metal removal from wastewater (Lalvani et al., 2000; Karabulut et al., 2000).
The adsorption selectivity of brown coals for first-order transition metal groups fol-lows that of an Irving-Williams order, indicating that the group involved in the metaladsorption contains either a nitrogen or oxygen atom, and is most likely a carboxylic site(Karabulut et al., 2000; Yavuz and Küçükbayrak, 2001). Brown coals are considered asnatural polyelectrolyte organic compounds of complex structure involving a proportionof more or less condensed aromatic rings with a large number of attached −OH and−COOH groups. Carboxyl or hydroxyl groups are able to take part in the ion exchangeaccording to Eqs. (1) and (2).
R-COOH + Me+ ↔ R-COOMe + H+, and (1)
R-OH + Me+ ↔ R-OMe + H+. (2)
Brown coal contains several functional groups such as methoxyl, hydroxyl groups,aliphatic and phenolic, and carbonyl groups. It is hypothesized that these functionalgroups serve as sites of adsorption of metal ions (Rengaraj et al., 2001; Brown et al.,2000; Raji and Anirudhan, 1998; Lalvani et al., 1997). These groups are active centersof brown coal. Because of the polar character of brown coal, the specific adsorptionpotential for dissolved metals is quite high.
Experimental
All solutions were prepared from analytical grade chemicals and Mili-Q filtered deion-ized water. Cr(VI) stock solution (1.10−3 mol/l) was prepared by dissolving K2Cr2O7
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Chromium(VI) Adsorption by Brown Coals 449
Table 1Elemental analysis of YK and KK brown coals
salt (from Merck) in deionized distilled water. For sorption experiments, concentrationsranging from 1.10−4–1.10−8 mol/l were prepared and standard acid and base solutions,1 M HCl and 1 M NaOH, were used for the pH adjustments. The electrolyte used tomodify the ionic strength in the adsorption experiments was 0.1 N NaNO3.
Isparta-Yalvaç-Yarikkaya(YK) and Kasıkara(KK) brown coal samples were takenfrom the coal field of the mines (southwest region of Anatolia). The coal samples wereground to −65-mesh size using a ceramic mill and stored under a nitrogen atmosphere.Both samples were dried in a vacuum oven at 60◦C for one h before use. The character-istics of the brown coals are summarized in Tables 1 and 2 and Figures 1 and 2.
Batch sorption experiments were conducted using 100 mL Erlenmeyer flasks keptat a constant temperature (30 ± 1◦C) for a period of 60 min. A Gallenkamp OrbitalIncubator thermostated shaker was used to mix the solution. The reaction mixture al-ways consisted of a total volume of 30 ml, and when necessary, the initial pH valuewas adjusted before starting the experiments. Then, the desired amount of adsorbentwas weighed and added to the solution. Each flask was removed after the required re-action time, and the solution mixture was filtered through Whatman filter paper. Theinitial and final metal concentrations were measured in triplicate using a Perkin ElmerAA800 Model AAS. The concentration of chromium metal ions was calculated fromthe change in metal concentration in the aqueous solution before and after equilibriumsorption. In all cases, mass balance was confirmed. The metal removal efficiency is de-fined as the percent change in the concentration of the solution upon treatment withadsorbent.
Contact time adsorption experiments were conducted at 30 ± 1◦C in a well-mixedPyrex glass vessel with the incubator (thermo-stated shaker). A solution of 30 ml, 1.10−3
mol/l of the Cr(VI) was used and the initial pHs of the samples were adjusted in therange of 2–6 by using diluted 0.1 N HCl or 0.1 N NaOH solution. All pH measurementswere performed with an Orion 900S2 model digital pH meter. After equilibrium wasreached, the pH of the solution was measured and recorded. Sorption isotherms werecarried out with different initial metal concentrations varying from 1.10−4–1.10−8 mol/l
Table 2Chemical properties of YK and KK brown coals
Calorific values Total Total Volatile FixedBrown of dry coal wet Ash sulphur matter carboncoals (cal/g) (%) (%) (%) (%) (%)
Figure 1. (a) Scanning electron microscopy of YK lignite coal. (b) Scanning electron microscopyof KK lignite coal.
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Chromium(VI) Adsorption by Brown Coals 451
Figure 2. FTIR spectrum of a) YK and b) KK brown coals.
while holding the sorbent amount at constant value at room temperature (30 ± 1◦C). Thesorbent amount in the batch vessel was varied from 0.01 to 0.1 g for adsorption studies.
SEM(JEOL 5600-LV Model instrument) photographs of coals are given in Figure 1.The composition of (YK) and (KK) coals was determined from the elemental-analysisdevice (Leco, CHNS-932) and their classification and ultimate analyses are given inTable 2. FTIR spectrum of each brown coal was acquired using a Perkin Elmer BXModel FTIR Spectrometer (Figure 2).
Results
Hexavalent Chromium Adsorption
Kinetic Studies. The kinetics of Cr(VI) sorption were studied by varying the contact timefrom 1 to 1440 min using about 0.05 g of brown coal and an initial metal concentrationof 1.10−3 mol/l (Figures 3a and 3b). The metal sorption involved rapid metal uptakewithin the first 3 min of contact. This study also illustrates that equilibrium for the metaluptake process is attained in about 20 min.
Experiments were also directed at an attempt to understand the kinetics of chromiumremoval by YK and KK lignite coals. It is a well-established fact that the adsorption ofions in an aqueous system follows reversible first-order kinetics, when a single species isconsidered on a heterogeneous surface. Details of the kinetics of chromium removal aredescribed in an earlier study (Gode and Pehlivan, 2003). Using the kinetic equations, theoverall rate constant, the forward and backward rate constants were calculated accordingto the equation given in the earlier work. The equilibrium constant Kc, forward andbackward rate constants k1 and k2 were calculated, as shown in Table 3. From Table 3,it can be seen that the forward rate constants for the removal of chromium were muchhigher than the backward rate constants, namely the desorption process (Rengaraj et al.,2003).
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(a)
(b)
Figure 3. (a) Adsorption rate of Cr(VI) ions by YK from aqueous solutions. Adsorption conditions:initial concentration of Cr(VI), 1.10−3 mol/l; amount of coal, 0.05 g; volume of adsorption medium,30 ml; temperature, 30◦C; stirring time, 150 rpm; pH, 3.0. (b) Adsorption rate of Cr(VI) ions byKK from aqueous solutions. Adsorption conditions: initial concentration of Cr(VI), 1.10−3 mol/l;amount of coal, 0.05 g; volume of adsorption medium, 30 ml; temperature, 30◦C; stirring time,150 rpm; pH, 3.0.
Table 3Rate constants for the removal of chromium with YK and KK coals
Sorption Equilibrium. The influences of initial chromium concentration on the efficiencyof the removal of Cr(VI) at an initial solution pH of 3.0 using brown coals are shown inFigure 4a and 4b. The amount of absorbent was kept at 0.05 g, and the contact time of60 min was employed in this study. The sorption was found to increase with the initialmetal concentration, then it reached a maximum very quickly, and thereafter it reachedequilibrium. A maximum removal of about 85% hexavalent chromium was observed atthe initial metal concentration of 1.10−3 mol/l. The sorption equilibrium is established
(a)
(b)
Figure 4. (a) Adsorption capacity of YK coal for Cr(VI) ion from aqueous solutions. Adsorptionconditions: initial concentration of Cr(VI), 1.10−3–8.10−4 mol/l; amount of coal, 0.05 g; volumeof adsorption medium, 30 ml; temperature, 30◦C; stirring time, 150 rpm; adsorption time, 60 min;pH, 3.0. (b) Adsorption capacity of KK coal for Cr(VI) ion from aqueous solutions. Adsorptionconditions: initial concentration of Cr(VI), 1.10−3–8.10−4 mol/l; amount of coal, 0.05 g; volumeof adsorption medium, 30 ml; temperature, 30◦C; stirring time, 150 rpm; adsorption time, 60 min;pH, 3.0.
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Table 4Freundlich and Langmuir adsorption
isotherm parameters of Yarıkkaya andKasıkara lignite coals for Cr(VI)
Langmuir isothermBrowncoals Q0 b R2 RL
YK 0.92 15.58 0.90 0.07KK 0.98 15.01 0.98 0.07
Q0 = mmol/g coal; R2 = correlation coefficient.
when the concentration of adsorbate in the bulk solution is in dynamic balance with thatof the interface. Table 4 contains the experimental data that relate the adsorption capacityof the coal used to the equilibrium concentration.
The experimental results obtained for the adsorption isotherms were found to obey theLangmuir adsorption isotherms. Langmuir isotherm is used for a monolayer adsorptionsurface containing a finite number of identical sites. The maximum adsorption capacityfrom the Langmuir isotherm equation was found to be 0.98 mmol/g coal for KK and0.92 mmol/g coal for YK coals.
The Langmuir isotherm is represented by the following equation.
Langmuir Isotherm:
Ce
qe
= Ce
Q0+ 1
Q0b, (3)
where Ce (mg/l) is the equilibrium concentration, qe (mg/l) is the amount of Cr(VI) sorbedat equilibrium, and Q0, and b are Langmuir constants related to adsorption capacity andenergy of adsorption, respectively.
A plot of Ce/qe vs Ce is linear and shows that the sorption follows the Langmuirisotherm model. Q0 and b were determine from the slope and intercept of the plots(Bayat, 2002; Dakiky et al., 2002; Lakatos et al., 2001). The adsorption intensities, b forCr(VI), calculated from the Langmuir adsorption isotherm were found to be 15.58 and15.01 for the brown coals, YK and KK, respectively. It was found that more than 85%removal was achieved using these coals.
Influence of pH, Cr(VI) Concentration. The relation between the initial pH of the solutionand Cr(VI) sorption was researched. It was found that when pH is low, the sorption ofCr(VI) increases, and it was maximum at pH 3 for both brown coals. In addition, it wasfound that a relatively significant metal sorption by the coals may also be possible athigh pH values between 4.0 and 5.0. Increased pH resulted in a slower rate of Cr(VI)reduction, indicating that an abundance of protons was required for the rapid removal ofCr(VI). The reduction rate was increased as the Cr(VI) concentration increased.
Effect of Amount of Adsorbent. The influence of adsorbent concentration on the removalefficiency of hexavalent chromium was studied at an initial pH of 3.0, 30 ± 1◦C andinitial metal concentration of 1.10−3 mol/l. Figures 5a and 5b plot metal sorption (%)
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Chromium(VI) Adsorption by Brown Coals 455
(a)
(b)
Figure 5. (a) The effects of the amount of YK coal for Cr(VI) ion from aqueous solutions. Ad-sorption conditions: initial concentration of Cr(VI), 1.10−3 mol/l; amount of coal, 0.02–0.1 g;volume of adsorption medium, 30 ml; temperature, 30◦C; stirring time, 150 rpm; adsorption time,60 min; pH, 3.0. (b) The effects of the amount of KK coal for Cr(VI) ion from aqueous solutions.Adsorption conditions: initial concentration of Cr(VI), 1.10−3 mol/l; amount of coal, 0.02–0.1 g;volume of adsorption medium, 30 ml; temperature, 30◦C; stirring time, 150 rpm; adsorption time,60 min; pH, 3.0.
versus adsorbent dosage. When the brown coal amount increases in bulk solution, thereduction rate correspondingly increased. As a result, protons and electrons from browncoals were consumed in the reduction of Cr(VI).
Discussion
Factors influencing the adsorption rate are mainly, among others, initial concentration,pH, contact time, and amount of coal. The adsorption of Cr(VI) by coals is very high, inthe range of 80–85% at low initial concentrations, and reaches equilibrium very quickly.This indicates the possibility of the formation of monolayer coverage of the metal ions
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at the outer interface of coals and suggests that these YK and KK coals can removemost of the chromium ion from aqueous solution if their concentrations are low, be-low 1.10−3 mol/l. At a fixed coal quantity, the amount sorbed increased with increasingconcentration of solution, but the percentage of sorption decreased. In other words, theresidual concentration of Cr(VI) ions will be higher at a higher initial concentration ofthe Cr(VI) under study. In the case of lower concentrations, the ratio of initial numberof Cr(VI) moles to the available sorption sites is low, and subsequently, the fractionaladsorption becomes independent of initial concentration. At higher concentrations, how-ever, the available sites of sorption become fewer and subsequently the removal of metalsdepends on the initial concentrations of Cr(VI). At higher initial concentration, it is notlikely that metal ions are only adsorbed in a monolayer at the outer interface of coals.As a matter of fact, the diffusion of exchanging ions within coal particles may governthe sorption rate at higher initial concentrations.
The brown coals are not suitable for the anions since the carboxyl functionalitiesuse the ion exchange and chelation mechanism, able to bound only cations. A positivelycharged surface can form at very low pH, which is a possible way for Cr(VI) anionsorption. Considering the structure of brown coal, during the interaction of Cr(VI) withthe coal, Cr(VI) is supposed to be reduced to Cr(III), and this metal ion will be boundto the skeleton. Still, the nature of metal binding onto brown coal has received extensiveinvestigation, but a common conclusion on the exact mechanism has not yet to be reached(Lakatos et al., 2002; Adria-Cerezo et al., 2000).
Hexavalent chromium (Cr(VI)) may exist in the aqueous phase in different anionicforms, such as chromate (CrO2−
4 ), dichromate (Cr2O−7 ), or hydrogen chromate (HCrO−
4 ),with total chromate concentrations and pH dictating which particular chromate speciesis predominant (Lalvani et al., 2000). Brown coals, on other hand, are able to adsorbCr(VI), presumably due to the presence of a multitude of anionic sites. Cr(VI) sorptioncould be attributed either to the physical adsorption mechanism or to the presence ofpositive charges, especially at low pH. The data show that the removal efficiency ofCr(VI) increased with lowering pH (at pH values < 3). This observation is consistentwith the hypothesis that the low pH renders the coal surface with relatively more positiveand less negative charges, thus resulting in greater adsorption of the anions of chromium.Increasing the pH resulted in a decrease in the removal efficiency of chromium because,at these pH values, the high concentrations of OH− ions compete more effectively withthe chromate anions for the positive surface charges present in the coal surface.
Analysis of adsorption results obtained at various concentration showed that theadsorption pattern on the coals followed Langmuir isotherm as seen Table 2. The max-imum Cr(VI) adsorption at an optimum pH of 3.0 is 0.92 mmol/g of weight for YK,0.98 mmol/g of weight for KK brown coals, respectively (Table 2). The Langmuir con-stant b can serve as an indicator of isotherm rise in the region of lower residual metalconcentrations, which reflects the strength of the coal for the solute. The adsorption ofCr(VI) by coals is very high, in the range of 80–85% at low initial concentrations, andreaches equilibrium very quickly. We can conclude that the KK and YK brown coalsmay be an efficient material for purification in wastewater containing chromium metal.
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