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Adsorption of Pb2+ by Alkali-Treated Citrus limetta Peels Umesh Suryavanshi and Sanjeev R. Shukla* Department of Fibres and Textile Processing Technology, Institute of Chemical Technology, (UniVersity Under Section 3 of UGC Act 1956) Nathalal Parekh Road, Matunga, Mumbai-400 019, India
The biomass Citrus limetta fruit peels was subjected to Pb2+ adsorption from aqueous PbNO3 solutions of different concentrations. Pretreatment of the biomass using cold alkali enhanced the adsorption by 87% even after considering the percentage weight loss. The biomass was characterized by scanning electron microscopy (SEM), electrokinetic analysis, and FT-IR spectroscopy. The equilibrium sorption capacity was observed to be 630 mg/g and the adsorption obeyed the Langmuir, Dubinin-Radushkevich (D-R), and ion-exchange models. The optimum pH and biomass concentration for maximum adsorption capacity were observed to be 4.5 and 0.1 g/L, respectively. Almost 80% sorption takes place within the first 15 min. The role of carboxylic and other functional groups was revealed to be ion exchange with a 1:2 stoichiometry between Pb2+ and carboxylic groups. Introduction Heavy-metal pollution has become an important issue in the past few decades, leading to extensive research in the area of its remediation. Metal ions released into the aqueous environment by industrial activities tend to persist indefinitely, circulating and eventually accumulating in the food chain. Although many techniques have been reported for removing metals from solutions, such as chemical precipitation, adsorption, ion exchange, filtration, chemical oxidation or reduction, electrochemical treatment, membrane processes, and evaporation, each of them is associated with one or another drawback, including cost of operation, especially when the initial metal-ion concentrations are as low as 10-100 mg/L.1,2 The biosorption technique has been found to be quite promising, as low-cost nonliving biomaterials have the potential to adsorb the heavymetal ions from wastewater.3 Use of agricultural byproducts such as jute, coir, nut shell, wood bone, peat, rock straw, modified wool, modified cotton, saw dust, waste tea, walnut shells, rice bran and hulls, and sugar cane bagasse is well documented.3–5 Biosorption is known to occur through the interaction of metal ions with the functional groups present in biopolymers. Groups present in the biosorbents such as amide, hydroxyl, carboxylate, sulfonate, phosphate, and amino have been reported to be quantitatively responsible for adsorption.6,7 Carboxyl groups are generally the most abundant functional groups present in biomass, followed by sulfonate and hydroxyl groups. The accessibility of biosorbent as well as chemical/physical modifications performed on it also play important roles in enhancing the metal-ion sorption capacity. A variety of mechanisms such as complexation, coordination, chelation, ion adsorption or exchange, and microprecipitation have been proposed.8–10 Although extensive literature has been published during the past 20 years, very few of the works have reported adsorption capacities greater than 10% of the weight of the biomass.11–16 To become a potential competitor for alternate remediation techniques, biosorbents should have much higher metal sorption capacities. Earlier work using coir, jute, and saw dust in both their original and chemically modified forms have been found to * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +91 22 3361 1111. Fax: +91 22 33611020.
exhibit good sorption capacities for Pb(II), Cu(II), Ni(II), Zn(II), Fe(II),17,18 and Ga(III),19 with modifications such as dye loading, oxidation, and swelling being found to be responsible for enhancing the sorption capacities. The carboxylic group was found to be the major metal-binding site, involving predominantly an ion-exchange mechanism and leading to almost complete recovery of metal ions. A fixed-bed column made up of oxidized coir could be used repetitively for adsorption without any significant loss of sorption capacity.18 In this work, Citrus limetta peels, a waste biomass with abundant availability, was subjected to lead-ion sorption after various pretreatments. The pretreated biomass was characterized to establish the mechanism of metal-ion sorption and its enhancement. Materials and Methods Materials. Lead stock solution of 1000 mg/L concentrartion was prepared from lead nitrate (certified A.C.S., Fisher Scientific, Mumbai, India). pH adjustment of metal-ion solution was achieved by appropriate addition of 0.1 M NaOH or 0.1 M HCl. The peels of Citrus limetta fruit (also commonly known as bitter orange, sweet lemon, or sweet lime) used in this study were procured from a local juice shop. Methods. Preparation of Adsorbent. The fibrous mass attached to the Citrus limetta peels (CLPs) was removed by hand. The peels were first washed with distilled water with agitation for 20 min and then with deionized water at room temperature (30 °C) for 2 h and were subsequently filtered. The residual solids were dried in an oven at 70 °C for 24 h and ground to a fine powder with a size of less than 1 mm. Pretreatments. The CLP particles were agitated slowly with different chemicals as indicated in Table 1. After completion of the pretreatment, the CLP particles were washed with distilled water until neutral pH, dried in an oven at 70 °C for 24 h, and weighed to evaluate the biomass loss due to each pretreatment. Metal Estimation. Pb2+ and Ca2+ concentrations were determined at 217.0 and 422.7 nm, respectively, using a GBC 932 Plus atomic absorption spectrometer with an acetylene flame. Attenuated Total Reflectance Infrared (ATR-IR) Spectroscopy. To identify surface functional groups responsible for binding Pb2+, infrared spectra of pretreated biomass were recorded using a Shimadzu 8400S FT-IR spectrometer having
10.1021/ie101491w 2010 American Chemical Society Published on Web 10/08/2010
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Table 1. Conditions of Various Pretreatments for CLP Biomass pretreatment
enhancement in sorption capacity (%)
solution
temperature (°C)
duration (h)
biomass loss (BL, %)
without considering BL (%)
considering BL (%)
10% isopropanol 1 M citric acid 1 M citric acid 0.1 M NaOH 0.1 M NaOH oxidation with 50% H2O2 (at pH 11.0)3
30 30 80 30 80 80
24 1 1 6 0.5 2
8.50 10.50 38.00 14.50 84.40 54.00
11.00 22.40 34.70 115.00 72.40 75.00
-2.31 10.76 -2.42 87.77 -4.23 13.63
an ATR sampling unit with 50 scans on each sample with resolution of 2 cm-1. Scanning Electron Microscopy (SEM). Scanning electron microscopy images of the CLP particles were recorded on a JEOL JSM 6380 LA spectrometer (JEOL, Tokyo, Japan) to reveal the surface morphology. Electrokinetic Analysis. The zeta potential of cold-alkalitreated (CAT) CLP biomass was measured using Anton Paar Electro Kinetic Analyzer (EKA) (Anton Paar GmbH, Graz, Austria), which is based on the streaming potential method. An electrolyte solution was pumped via an electrolyte circuit through the measuring cell filled with CAT sample. Because of the pressure difference and the relative movement of the charges in the electrical double layer, the streaming potential was detected by Ag/AgCl electrodes placed at both sides of the sample cell. The pH dependence of the zeta potential was determined in the presence of 0.001 M KCl solution in the range of pH 2-7. Speciation of Pb2+ Ions. Speciation studies of Pb2+ solution at different concentrations and pH values and also before and after adsorption were performed using an ion-selective electrode (ISE) (ASTM 9213).20 The results of this analysis were compared with those obtained by atomic absorption spectrometry (AAS). Batch Experiments. Batch adsorption studies were carried out to determine the effects of various parameters on the adsorption efficiencies of variously pretreated biosorbents for Pb2+ ions. Unless otherwise stated, all of these studies were carried out with 100 mg of CAT biomass mixed with 50 mL of metal-ion solution. In dealing with the natural factors affecting metal adsorption by CAT biomass, two approaches were considered: the solidphase approach consisting of various pretreatments of the CLP and the liquid-phase approach investigating the effect of solution pH and metal concentration. (a) Effect of CLP Pretreatment on Metal Removal. To evaluate the effect of pretreatments on the adsorption capacity of the CLP, about 100 mg of a sample was mixed with 50 mL of Pb2+ solution (100 mg/L) in a flask kept at 30 °C and agitated on an orbital shaker at 100 rpm for 6 h. The suspension was then filtered through Whatman filter paper no. 42, and the solution was acidified with 1 M HNO3 and subjected to estimation of the Pb2+concentration. (b) Kinetic Study. A kinetic study was performed by stirring 20 mg of CAT sample in 200 mL of 200 mg/L Pb2+ solution in a beaker. Samples were removed at specified time intervals between 5 and 360 min by a syringe and filtered through Whatman filter paper no. 42 for estimation of the Pb2+ concentration. (c) Isotherm Experiment by the Sequential Addition Method (SAM). To obtain the whole sorption isotherm at a constant equilibrium pH using only one sample, to save time, reagents, and adsorbent, an equilibrium study was performed using the sequential addition method (SAM).21
Thus, 10 mg of the CAT biomass was suspended in 100 mL of deionized water. A small volume of 5 g/L Pb2+ solution was added to the suspension for each additional step and agitated with a magnetic stirrer for 120 min to reach equilibrium. Then, 1 mL of the suspension was removed and filtered using Whatman filter paper no. 42, and the Pb2+ concentration was estimated. (d) Effect of Initial Solution pH. The effect of pH on Pb2+ adsorption by CAT biomass was studied over a pH range of 2.0-6.0 using 100 mg/L Pb2+ solution. (e) Effect of CAT Biomass Concentration on Pb2+ Adsorption. The effect of the metal-to-biosorbent ratio on lead adsorption was studied at room temperature (30 °C) and pH 4.5, keeping the volume (50 mL) and the initial concentration of the metal solution (100 mg/L) constant. (f) Blocking of Functional Groups. To understand the involvement of the carboxyl groups of the CAT biomass in the sorption process, these groups were esterified using methanol as follows: Nine grams of CAT biomass was suspended in 633 mL of 99.9% pure methanol, and 5.4 mL of concentrated hydrochloric acid was added to give a final acidic concentration of 0.1 M HCl. Trimethoxymethane was used to shift the equilibrium toward esterification with methanol. With continuous stirring, the sample was kept at 60 °C for 48 h, filtered, washed twice with distilled water, and dried in an oven at 70 °C for 24 h. It was then used for sorption experiments using 50, 100, and 200 mg/L Pb2+ solutions. Similarly, to check the involvement of amine groups in the sorption process, these groups were methylated using formaldehyde+formic acid treatment as follows: Five grams of CAT biomass was refluxed with 15 cm3 of formaldehyde and 40 cm3 of formic acid at 100 °C for 4 h, after which it was washed several times with distilled water and dried in an oven at 70 °C for 24 h. A sorption experiment was carried out according to the procedure described above to study the effect of methylation. (g) Preparation of Biomass in Various Ionic Forms. To study the effect of the ionic form of the biosorbent on the uptake capacity of Pb2+ and to reveal the ion-exchange mechanism, biosorbent was prepared in H- and Ca- forms. For H-biosorbent, 5 g of CLP biomass was stirred in 250 mL of 0.2 M HNO3 in a beaker for 2 h. It was washed with distilled water until the pH of the rinse water was neutral, filtered, and dried in an oven at 70 °C for 24 h. For Ca-Biosorbent: H-biomass was further added to distilled water in a 500 mL beaker, followed by 0.2 g/L Ca(OH)2 solution until pH 10.0; the mixture was stirred for 3 h and then washed and dried according to the previously described process. Sorption experiments were carried out by contacting 100 mg of either Ca- or H-biomass for 2 h with 50 mL of Pb2+ solution with a concentration of 10, 20, 30, 50, 100, 150, and 200 mg/ L. No pH adjustment was done at any stage. Results and Discussion Effect of Pretreatments on Pb2+ Adsorption. Preliminary experiments showed that the cleaned peel particles of Citrus
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Figure 1. FT-IR spectra of CAT biomass before and after Pb2+ sorption and of esterified biomass.
limetta fruit (CLP) have the capacity to adsorb Pb2+ ions from lead nitrate solution. CLP contains almost 30% (on a dry weight basis) pectins, the complex polysaccharides bearing free carboxyl groups and are known to bind Pb2+ strongly.23 Various pretreatments were applied to CLP with the aim of enhancing the metal adsorption capacity by opening up the physical structure to expose more functional groups and/or by achieving suitable chemical modification of the available functional groups. Isopropanol was thought to remove some soluble components from the biomass without affecting the nature and amount of metal binding sites already present in the biomass. Citric acid might dissolve the polysaccharides present in the cell walls of the biomass, thereby opening up the physical structure and increasing the number of available binding sites such as carboxylic groups. Also, it might result in the binding of some carboxylic groups to free alcoholic groups in pectin. Alkali treatment will have the stronger effect of rupturing the cell walls and exposing more functional groups. Groups such as carboxylic and sulfonic groups will also be converted into their Na salts, which were shown earlier to promote heavy-metal-ion adsorption through preferential ion exchange.17 In both acid and alkali treatments, higher temperature will enhance the results due to the severity of action. Oxidative pretreatment has been shown to enhance the adsorption capacity of cellulosic biomass such as jute and coir.18 All of the pretreatments tested in this work resulted in a loss of biomass to different extents. Even the cleaning of CLP particles with deionized water led to a certain loss of biomass. The maximum loss of 84.4% was registered when the biomass was treated with 0.1 M NaOH at 80 °C for 30 min. The loss in the amount of CLP sample during a pretreatment is likely to lead to some misinterpretation during the quantitative assessment of the biosorption performance. This is because any enhancement in the sorption capacity after a pretreatment would be offset, at least to some extent, by the biomass loss. Batch experiments on the sorption of Pb2+ ions were carried out with untreated as well as variously pretreated CLP biomass. Comparative biosorption efficiency results are reported in Table 1. It can be observed that, although cold alkali pretreatment (CAT) resulted in some biomass loss (14.5%), it significantly
enhanced the sorption capacity for Pb2+. In all other cases, the increase in sorption was heavily offset by the large biomass loss. During batch sorption studies on the pretreated CLP samples, it was observed that the solution became brown in color, except for the CAT sample. This can be attributed to the leaching out of the carotenoids from the biomass (giving a characteristic color). Carotenoids are responsible for binding the metal ion within the solution and forming a complex that is unable to be adsorbed by the biomass. This was also confirmed by using an ion-selective electrode, which indicated that only about 25% of the Pb2+ left in the solution was in the free form. In the case of CAT biomass, however, the free available Pb2+ in solution was more than 90%. Thus, although most of the pretreatments leach out the carotenoids, some more ligands that are capable of binding Pb2+ are also extracted in the solution from the biomass. Only a fraction of Pb2+ in equilibrium with biomass is available for sorption. Similar results were obtained by Maria et al.22 in a study of the biosorption of Cd2+ by dealginated seaweeds. Alkali treatment of CLP at room temperature (CAT biomass) generates ionic sites without significant modification of the cell wall structure and also improves its accessibility. This was clearly observed from the porous structure presented in SEM micrographs (Supporting Information). The limonoidaglycones and limonoidglycosider leach out from the peels.24 Also, the protonation of carboxylate groups of the pectin molecules is prevented, which restricts the formation of hydrogen bonds, a condition that is favorable for hydration. The carboxylic groups on the surface of the cell wall are exposed and the H+ type of functional groups convert into the Na+ type so that Pb2+ ions can bind with them more easily.18 This improves the capacity of the biomass to chelate the metal ions. With hot alkali treatment, however, polysaccharide chains from the cell walls break away, resulting in higher biomass loss. Because these chains also contain metal binding ligands, the amount of surface groups available for metal binding decreases, as reflected by the lower adsorption capacity than for CAT biomass. FT-IR Spectra of CAT Biomass. Figure 1 shows the FTIR spectra of CAT biomass before and after Pb2+ adsorption. Both spectra are characterized by the presence of peaks for -OH
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Figure 2. Change in zeta potential of CLP and CAT biomass with respect to pH: (O) CAT biomass, (b) CLP biomass.
at 3300 cm-1 and -CH at 2900 cm-1. The spectra typically display a peak at 1733 cm-1 corresponding to the stretching band of the free carbonyl double bond from the carboxyl group, which disappears completely after Pb2+ sorption, indicating the role of the -COO- groups present in the pectin molecule in binding the metal ions. Zeta Potential. The solid-liquid interfaces are characterized by chemical and electrochemical potential values different from those of the bulk phase because the molecules at the boundary are subjected to interactive forces from the adjacent phases. The net charge at the interface is generally attributed to the dissociation of functional groups present on the solid surface. The resulting surface charge is balanced by counterions present in the solution. Sorption of heavy-metal ions by a biomass is affected by several factors that include specific surface properties of the biomass and the behavior of a metal ion in an aqueous solution. Electrical properties of the biomass are related to the chemical composition of the cell wall. The charge on its surface originates from the dissociation of acidic groups such as carboxyl, phosphate, amino, hydroxyl, and sulfhydryl, of which carboxylic acid groups are the major contributors. Metal-ion adsorption can be brought about by the ionization of these carboxyl groups, which serve as binding sites. Figure 2 shows the electrokinetic properties of the CLP and CAT biomass as a function of pH. The graph can be seen to record an increase in the negative zeta potential with increasing pH in the acidic region as the degree of dissociation of acidic functional groups enhances. The CAT biomass shows a higher magnitude of change than the CLP biomass. This trend can be explained on the basis of the comparatively higher availability of dissociable functional groups in the CAT biomass. It is interesting to note that, within the entire acidic pH range (2-5) studied, the surface of the biomass remained negatively charged. The presence of some dissociable groups is indicated by a negative charge even at a low pH 2.0. Effect of Initial Solution pH. The solution pH is an extremely important factor affecting the amount of heavy-metal ions adsorbed by the biomass. It not only governs the speciation of the metal ions in aqueous solution, but also determines the status of the groups present in the biomass (i.e., protonated or deprotonated). At higher pH, the metal ions would precipitate and reduce the extent of biosorption, and then it would be difficult to determine the actual sorption performance of the biomass. Also, most of the industrial effluents containing heavy metals are acidic. Thus, the study on the effect of pH was limited to pH values less than 6.0.
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Figure 3. Effect of solution pH on adsorption capacity of Pb2+ by CAT biomass.
Figure 4. Effect of solid-to-liquid ratio on percentage sorption and adsorption capacity. Pb2+ sorption: (2) mg/g, (b) %.
Figure 3 shows that the biosorption of Pb2+ was strongly affected by the solution pH. The Pb2+ sorption capacity of CAT biomass increased from 39 mg/g at pH 2.0 to 285 mg/g at pH 4.5, after which it remained almost constant up to pH 6.0. The mechanism of the pH dependence of Pb2+ biosorption can be explained on the basis of the nature of the metal binding sites on the cell surface and the solution chemistry of metal ions in water. At acidic pH, the surface functional groups of the biomass become protonated and enhance the electrostatic attraction. At low pH, the cell wall ligands are closely associated with H3O+ ions, restricting the approach of metal cations because of the repulsive force. As the pH increases, more types and numbers of groups carrying negative charges would get exposed, with subsequent attraction and sorption of metal ions onto the biomass surface. Another possible explanation is that the solubility of many metal ions in solution decreases with increasing pH and, therefore, the degree of hydration of a metal ion reduces (i.e., less energy is required for removal or reorientation of water molecules associated in hydration with a metal ion). Effect of Biomass Concentration. Biomass concentration determines the extent of metal sorption from solution. Figure 4 indicates a steep rise in the percentage sorption with increasing biomass concentration. This is attributed to the increase in the surface area available for adsorption and hence the active sites. On the other hand, the amount of adsorbed metal ion, per unit weight of biosorbent, decreases with its increasing quantity as a result of complex interactions of several factors such as limited availability of solute, electrostatic interactions, interference between binding sites, and reduced mixing. It has been reported
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Figure 5. Sorption kinetics of Pb+ on CAT biomass: (b) pseudo-secondorder model, (- - -) experimental data.
that higher specific sorption at lower biomass concentration could be due to an increased metal-to-biosorbent ratio.25 At higher concentrations, the biomass particles could get attached to each other, causing agglomeration. The surface of biomass is negatively charged, and the presence of multivalent metal cations could bridge the biomass particles together, resulting in reduction in its surface area and, hence, in the binding sites available to metal ions. This affects the sorption capacity adversely. Kinetic Study. A kinetic study is essential to establish the rates of metal-ion uptake as well as metal-ion release by the adsorbent. The kinetics of sorption of Pb2+ was modeled using a pseudo-second-order rate equation developed by Ho and MacKay.26 Figure 5 shows the sorption kinetics of Pb2+ using CAT biomass at a constant pH of 4.5. Nonlinear regression analysis was used to fit the batch kinetic data to the model. The rate constant and sorption capacity were found to be 78.82 g/(mg min) and 595 mg/g, respectively. The adsorption process following diffusion-controlled dynamics27 with time can be presented as qe ) 2CeS
Dcπ ) k t
0.5
d
where D is the diffusion coefficient, S is the specific surface area of the adsorbent, and kd is the rate parameter. The possibility of fitting the intraparticle diffusion model was tested by plotting metal-ion uptake (qe) results against square root of time (t0.5), wherein the linear portion of the plot is attributed to the intraparticle diffusion. Sorption of Pb2+ on the CAT biomass is time-dependent: most of the sorption (about 80%) observed in the first 15 min. Thereafter, the rate decreases progressively. Equilibrium is reached in approximately 2 h. Sorption in the first rapid adsorption step results from external mass transfer, leading to most of the Pb2+ ion being sorbed on the bare surface of the biomass. The hindered diffusion inside the particle influences the rate of sorption after 15 min, which is slow. Rapid metal uptake allows for a shorter solution-biosorbent contact time and would result in the use of much shallower contact beds of sorbent material during column applications. Modeling Adsorption Isotherm. (a) Langmuir Model. The Langmuir isotherm can be derived from the equilibrium of chemical potentials in two interfacial surfaces or dynamic equilibrium between solute and solvent.28 Hall et al.29 proposed an equilibrium parameter R, also known as the separation factor,
Figure 6. Equilibrium absorption isotherm for the sorption of Pb2+ on CAT biomass: (- - -) Langmuir model, (s) Freundlich model, (() adsorption of Pb2+.
to express the Langmuir constant b. Based on the value of R, important information about the nature of adsorption can be explored. (b) Freundlich Model. For a long time, the Freundlich isotherm was thought to be of little theoretical value. More recently, it has been shown that it can be derived theoretically by considering the heterogeneous nature of adsorption sites.30 The Freundlich isotherm was used to fit the adsorption data. The equilibrium data were simulated using both linearized and nonlinearized isotherms. Figure 6 shows a plot of the biosorption capacity q of the CAT biomass against the residual Pb2+ concentration in the solution, Ce, at equilibrium. The Pb2+ uptake of almost 630 mg/g obtained for CAT biomass is almost 60% of its weight. Various parameters derived from the Langmuir and Freundlich models are presented in Table 2. The model parameters obtained were statistically significant at the 95% confidence level. Adsorption of Pb2+ was favorable, as the value of the separation factor R is between 0 and 1. Dubinin and Radushkevich31 developed an adsorption isotherm (D-R isotherm) that is more general than the Langmuir and Freundlich isotherms, as it does not assume an energetically homogeneous or a constant sorption site potential. A Gaussian distribution of small energetically favorable sites is implied, wherein the ionic species first bind with the energetically most favorable sites and multilayer adsorption takes place over it. The D-R isotherm has the form ln X ) ln Xm - βε2
(1)
where X is the adsorbed concentration of metal at the adsorbent (mg/L), Xm is the the maximum sorption capacity of adsorbent (mg/g), β is a constant related to the sorption energy, and ε is the Polayni potential. ε can be calculated using the equation
(
ε ) RT ln 1 +
1 Ce
)
(2)
where R is the gas constant (8.3144 kJ/mol) and T is the absolute temperature (K). A straight line for a plot of X versus ε2 indicates that the adsorbent has energetically favorable sorption sites on its surface. The sorption capacity (Xm) agrees well with the experimental data, and the maximum value of 614 mg/g was obtained. Also, the energy of sorption (β) was found to be 10.36 kJ/mol, which is in the energy range of an ion-exchange reaction,
Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010 2+
Table 2. Comparison of Linearized and Nonlinearized Models of Pb
Biosorption Isotherm of CAT Biomass
Langmuir model qmax (mg/g)
b (L/mg)
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Freundlich model r2
k
n
r2
0.982 0.986 0.984 0.002
0.643 0.640 0.641 0.0015
2.127 1.987 2.057 0.07
0.949b 0.935b 0.942 0.007
0.589 0.613 0.601 0.012
3.872 4.126 3.995 0.125
0.923a 0.918b 0.9205 0.0025
R Linearized Model
test 1 test 2 mean SD
628 632 630 2.0
0.142 0.138 0.140 0.0028
test 1 test 2 mean SD
648 654 651 3.0
0.102 0.112 0.107 0.005
0.0123a 0.0116a 0.0119 0.00035
Nonlinearized Model 0.927a 0.935a 0.931 0.004
0.0163 0.0148 0.01555 0.00075
a Correlation is statistically significant (using t test at the 95% confidence level). confidence level).
b
Correlation is not statistically significant (using t test at the 95%
also showed marked reductions in the peak height assigned to the carboxylic group at 1750 cm-1, although the peak did not vanish totally (Figure 1), even after 48 h of esterification. However, the reduction in sorption capacity was much less and not in proportion to the amount of COOH groups blocked. This means that the carboxylic groups alone are not responsible for metal binding, and other functional groups might also be involved. This could also imply that a mechanism other than ion exchange with functional groups could be involved in the sorption process.
Figure 7. Ion-exchange isotherm for the sorption of Pb2+onto CAT biomass.
namely, 8-16 kJ/mol. These results support the conclsion that the biosorption of Pb2+ on CAT biomass is an ion-exchange reaction. Many researchers have reported that the principle mechanism of biosorption is ion exchange.32 Biosorbent can be viewed as a natural cation-type ion exchanger containing both strong and weak acidic functional groups. The ion-exchange model used by Kratochvilet al.33 for the sorption of Cu2+ on sargassum biomass was used to model the equilibrium sorption data. The dimensionless metal uptake ym and dimensionless metal concentration xm are obtained by normalizing qm and Cf with the total concentration of binding sites in the biomass Q (mequiv/g) and the normality of solution (mequiv/L), respectively ym )
qm Q
xm )
Cf Co
xm and ym represent equivalent fractions of species m in the liquid and solid, respectively. In the case of ion exchange, where the species present on biomass is displaced by the one in solution, equilibrium adsorption data can be represented by a curve showing ym as a function of xm, where m denotes the sorbing species. This curve is called a dimensionless isotherm, given in Figure 7. The convex shape of the isotherm depicts the situation when ym > xm, that is, when Pb2+ adsorption is favored by the biosorbent. Blocking of Functional Groups. The esterification of carboxyl groups in the CAT biomass reduced Pb2+ binding by 23%, 36%, and 43% for initial concentrations of 50, 100, and 200 mg/L, respectively. The FT-IR spectra of the esterified adsorbent
To further demonstrate that the partial esterification of carboxyl groups decreased the Pb2+ binding and that this was not due to the destruction of the sorbent during esterification, alkaline hydrolysis (shaking 5 g of esterified biomass with 1 M NaOH for 2 h, followed by washing to a neutral pH) of the esterified CAT biomass was carried out, followed by Pb2+ binding. It was observed that, within the experimental error, the total binding capacity for lead was recovered at pH 5.2. Within the limits of experimental error, no reduction in Pb2+ uptake was observed after blocking of the amine groups, which shows that the amine groups do not contribute to the metal sorption process. Evidence of an Ion-Exchange Mechanism. Recent studies on biosorption30 have revealed that biosorbents can be viewed as natural ion-exchange materials that contain strongly acidic, weakly acidic, and weakly basic functional groups. They are very much like synthetic ion-exchange resins and can be prepared in different ionic forms, such as H+, Ca2+, and Na+, by washing the biomass with mineral acids, salts, and/or alkalis. To investigate the role of ion exchange in biosorption by CLP material, sorption experiments were carried out using the H-form and Ca-form of the biomass. Protonation of the biomass with a strong acid such as HNO3 gives a more uniform sorbent by displacing all of the light-metal ions. Figure 8 shows the stoichiometric plot for adsorption of Pb2+ accompanied by desorption of Ca2+ and H+. The adsorption and desorption curves can be observed to form mirror images of each other, suggesting that Pb2+ was exchanged with Ca2+ and H+ and that the exchange was stoichiometric. As can be seen in Figure 8, the adsorption capacity of the H-biomass is almost four times lower than that of the Cabiomass. The disadvantage of using the H-biomass is that the biosorption by protonated biomass would be accompanied by
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Literature Cited
Figure 8. Stoichiometric plot for adsorption of Pb2+ accompanied by desorption of Ca2+ and H+: (2) Pb2+ sorption by Ca-biomass, (4) Ca2+ release from Ca-biomass, (9) Pb2+ sorption by H-biomass, (0) H+ release by H-biomass.
release of protons, resulting in a lowering of the solution pH, which decreases the sorption capacity. Conclusions The biomass derived from CLPs was found to exhibit an excellent sorption capacity for Pb2+ ions. Cold alkali treatment resulted in increased uptake of Pb2+ owing to de-esterification of the galacturonic acid present in the biomass. Carboxylate groups in the biomass played an important role in Pb2+ binding as demonstrated by selective blocking experiments, as well as by FT-IR spectroscopy. A higher uptake of metal ions was found with increasing pH, increasing initial metal-ion concentration, and decreasing biomass concentration. Ho and MacKay’s pseudo-second-order reaction rate model was capable of describing the kinetics of Pb2+ sorption, whereas the initial stage of sorption was well fitted by diffusion kinetics. Biosorption of Pb2+ was better described by the Langmuir, D-R, and ionexchange models than by the Freundlich isotherm model. Ion exchange was identified to be the dominant mechanism. Pb2+ binds with the carboxylic groups of galacturonic acid chain, forming a stable gel. The Langmuir sorption capacity of 630 mg/g is much higher than the capacities of most biosorbents and comparable to those of synthetic ion-exchange resins. Thus, alkali-treated Citrus limetta fruit peel is a promising biosorbent that could find successful commercial application because of its low cost, porous structure, high sorption rate, and relatively high uptake capacity for lead. Acknowledgment Mr. Umesh Suryavanshi acknowledges University Grant Commission (UGC), Government of India, for providing fellowship. Supporting Information Available: SEM image of CAT biomass and “egg-box” model for binding of Pb2+ by galacturonic acid. This material is available free of charge via the Internet at http://pubs.acs.org.
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ReceiVed for reView July 12, 2010 ReVised manuscript receiVed August 24, 2010 Accepted September 11, 2010 IE101491W
