Ind. Eng. Chem. Res. 2009, 48, 3589–3594
3589
Biosorption of As(V) onto the Shells of the Crab (Portunus sanguinolentus): Equilibrium and Kinetic Studies K. Vijayaraghavan,† Mahadevan Arun,‡ Umid Man Joshi,† and R. Balasubramanian*,‡ Singapore-Delft Water Alliance and DiVision of EnVironmental Science and Engineering, National UniVersity of Singapore, 1 Engineering DriVe 2, Singapore 117576, Singapore
Worldwide concerns over inorganic arsenic in water bodies have prompted much research and policy development focusing on removal of this chronic human carcinogen. In the present study, the potential use of shell particles of crab (Portunus sanguinolentus) for removal of arsenic(V) from aqueous solution was investigated on the basis of systematic equilibrium and kinetic studies. Crab shells favor the removal of arsenate ion, especially under acidic pH conditions, because of the presence of CaCO3 and chitin in the biosorbent. The scanning electron micrographs together with energy dispersive X-ray analysis (EDX) confirmed the presence of arsenic on the crab shells. A series of isotherm experiments conducted at different pH conditions revealed that pH 3 favored arsenic biosorption. Among the four isotherm models (Langmuir, Freundlich, Redlich-Peterson, and Toth) employed in the study, the Toth model provided a better fit with the experimental data than others as revealed by high correlation coefficients, low % error, and root-mean-square error (rmse) values. The arsenic biosorption kinetics was very fast, and the kinetics data were successfully modeled using nonlinear pseudo-second-order model. As the ionic strength increased, arsenic uptake declined to a great extent. Desorption experiments were conducted to explore the feasibility of regenerating the biosorbent for further use. Results indicated that 0.1 M NaOH was sufficiently strong to remove the biosorbed arsenate ions from the crab shell with elution efficiency of 98.2%. 1. Introduction Arsenic toxicity has become a global concern owing to the ever-increasing contamination of water, soil, and crops in many parts of the world. In nature, arsenic is released to the environment through weathering and volcanism. Arsenic is also released by anthropogenic activities, such as mining, smelting, and agricultural activities.1 The presence of arsenic in the environment, its toxicity, and health hazards are well-known and have been reviewed extensively.2-4 It is highly toxic and known to cause skin, liver, lung, and kidney or bladder cancer. Because of the high toxic effects of arsenic, the World Health Organization (WHO) has revised the guideline for arsenic in drinking water from 50 to 10 µg/L.5 Arsenic forms inorganic and organic complexes in the environment, but the two biologically important species are As(V) and As(III), which are interconvertible depending on the redox status of the environment.6 Under atmospheric or more oxidizing environment, the predominant species is As(V), which, in the pH range of 6-9, exists predominantly as deprotonated oxyanions, namely H2AsO4- or HAsO42-. Under mildly reducing conditions, As(III) is thermodynamically stable and exists predominantly as H3AsO3 at pH levels below 9.0.7 Since arsenic, like heavy metals, cannot be destroyed or degraded, its decontamination can only be envisioned as its sequestration in a nonbioavailable form or its conversion into less toxic forms. Biosorption has emerged as a promising remediation technology, which uses inactive/dead biomaterials to remove pollutants from contaminated environments. Biomaterials come under the class of bacteria, fungi, algae, and other industrial wastes. However, only very few biosorbents have been examined for their potential to remove arsenic from contaminated solutions.8-13 * To whom correspondence should be addressed. E-mail: eserbala@ nus.edu.sg. Tel.: +65-65165135. Fax: +65-67744202. † Singapore-Delft Water Alliance. ‡ Division of Environmental Science and Engineering.
In recent years, few investigators reported that crab shell showed excellent metal binding ability toward lead,14 copper,15 nickel,16 and cadmium.17 The effectiveness of crab shell as a biosorbent is attributed to its rigid structure, excellent mechanical strength, and ability to withstand extreme conditions employed during regeneration processes.15 The additional advantage is that crab shells can be obtained in large quantities at low or no cost from seafood industries. The abundance of crab shells arises from the current situation that seafood wastes disposal routes have become increasingly restricted; for example, landfill is not permitted for raw or untreated seafood waste disposal. Hence, an alternative route such as efficient reutilization of these seafood wastes for alternative processes or environmental applications is highly desired by seafood industries. Thus, the major process cost involved is transportation, which strongly depends on the locations where crab shells are available and the point of use. In view of these attributes, it is of great fundamental interest to study the potential application of the crab shell particles for the removal of toxic elements such as arsenic from aqueous solutions. With this goal in mind, the present research was conducted with crab shell particles to investigate their biosorption capacity for As(V), provide insights into the binding mechanism responsible for removal of As(V) from aqueous solution, and identify the major parameters affecting its biosorption. Results obtained from this study are presented and discussed. 2. Materials and Methods 2.1. Crab Shell and Arsenic Solution. Waste shells of Portunus sanguinolentus were collected from the Marina beach (Chennai, India) and were sun dried and crushed to a particle size in the range of 0.5-1 mm using ball mill. The shell particles were then treated with 0.1 M HCl for 4 h followed by washing several times with deionized (DI) water and then dried in an
10.1021/ie801570v CCC: $40.75 2009 American Chemical Society Published on Web 02/25/2009
3590 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009
oven at 60 °C overnight. This pretreatment process was carried out to ensure the removal of excess calcium carbonate on the shell surface.15 The pretreated shell particles were used in all biosorption experiments. The complete demineralization of crab shell was accomplished by extraction with 1 M HCl at 25 °C for 3 h to dissolve calcium carbonate, followed by washing with deionized water and drying at 25 °C for 24 h. Subsequent to washing with deionized water, the demineralized shell particles were soaked overnight in 1% NaOH at 25 °C to remove the bulk of their protein content. The residue (chitin) was then rinsed with deionized water and dried at 100 °C.18,19 All chemicals used in the experiments were of analytical grade. The As(V) solution was prepared by dissolving Na2HAsO4 in DI water. 2.2. Experimental Procedure. Biosorption experiments were conducted by bringing 0.5 g of crab shell into contact with 100 mL of As(V) solution, at desired pH, in 250 mL Erlenmeyer flasks kept on a rotary shaker at 160 rpm. The pH of the solution was initially adjusted using 0.1 M HCl or NaOH, with the pH of the reaction mixture being controlled in the same manner during experimental runs. After 8 h of contact, the reaction mixture was filtered through a 0.45 µm PTFE membrane filter and analyzed for arsenic concentration using inductively coupled plasma-atomic emission spectrometry (ICP-AES; Perkin-Elmer Optima 3000 DV). Kinetic experiments were conducted using the same method as above, except that the samples were collected at different time intervals to determine the time point at which biosorption equilibrium was attained. The amount of metal sorbed by biosorbent was calculated from the differences between the metal quantity added to the biosorbent and the metal content of the supernatant using the following equation: Q ) V(C0 - Cf)/M
(1)
where Q is the metal uptake (mg/g); C0 and Cf are the initial and equilibrium metal concentrations in the solution (mg/L), respectively; V is the solution volume (L); and M is the mass of biosorbent (g). The metal-loaded crab shell, which was previously exposed to 10 mg/L of As(V) solution at pH 3, was separated from the solution by filtration. The biosorbent was then brought into contact with a known volume of 0.1 M NaOH or 0.1 M HCl for 1 h, on a rotary shaker at 160 rpm. The remaining procedure was the same as that employed in the biosorption equilibrium experiments. 2.3. Mathematical Modeling of Experimental Data. The biosorption isotherms were studied using the following four models, which can be expressed in their nonlinear forms: QmaxbCf 1 + bCf
Langmuir model:20
Q)
Freundlich model:21
Q ) KFCf1/n
Redlich-Peterson model:22
Q)
Toth model:23
Q)
KRPCf 1 + aRPCfβRP QmaxbTCf [1 + (bTCf)1/nT]nT
the Redlich-Peterson isotherm constant (L/g), aRP is the Redlich-Peterson isotherm constant (L/mg)1/βRP, βRP is the Redlich-Peterson model exponent, bT is the Toth model constant (L/mg), and nT is the Toth model exponent. The experimental biosorption kinetic data were modeled using pseudo-first and -second-order kinetics, which can be expressed in their nonlinear forms, as follows: pseudo-first-order model
Qt ) Qe(1 - exp(-k1t)) (6)
pseudo-second-order model
Qt )
Qe2k2t 1 + Qek2t
(7)
where Qe is the amount of metal sorbed at equilibrium (mg/g), Qt is the amount of metal sorbed at time t (mg/g), k1 is the pseudo-first-order rate constant (1/min) and k2 is the pseudosecond-order rate constant (g/mg · min). All the model parameters were evaluated by nonlinear regression using Sigma Plot (version 4.0, SPSS, USA) software. The average percentage error between the experimental and predicted values is calculated using N
∑ (Q
exptl,i
ε(%) )
- Qcalcd,i /Qexptl,i)
i)1
× 100
N
(8)
where Qexptl and Qcalcd represents experimental and calculated metal uptake values, respectively, and N is the number of measurements. The residual root-mean-square error (rmse) was also used to measure the goodness-of-fit; rmse can be defined as rmse )
1 m-2
m
∑ (Q
i
- qi)2
(9)
i)1
where Qi is the observation from the batch experiment, qi is the estimate from the model for corresponding Qi, and m is the number of observations in the experimental isotherm. The smaller the rmse value is the better the curve fitting would be. All experiments were done in triplicates and the data presented are the average values of three replicate experiments. Error bars are indicated wherever necessary. 2.4. SEM Analysis. To determine the major mechanism responsible for As(V) removal, the As(V)-loaded crab shell was dried, coated with a thin layer of platinum, and analyzed by scanning electron microscopy (SEM) equipped with energy dispersive X-ray analysis (JEOL, JSM-5600 LV). 3. Results and Discussion
(2) (3) (4)
(5)
where Qmax is the maximum metal uptake (mg/g), b is the Langmuir equilibrium constant (L/mg), KF is the Freundlich constant (mg/g) (L/mg)1/n, n is the Freundlich constant, KRP is
3.1. Effect of pH. In the first set of experiments, the influence of equilibrium pH on the biosorption of As(V) was examined (Figure 1). The solution equilibrium pH severely affected the As(V) biosorption capacity of crab shell, with strong acidic pH values resulting in maximum uptake. As the pH increased from 3, the As(V) uptake decreased and the crab shell exhibited no biosorption capacity after pH 9. The crab shell comprises mainly calcium carbonate and chitin along with some proteins.15,18 Calcium carbonate in crab shell favors microprecipitation of metal ions as CaCO3 dissociates to Ca2+ and CO32-.14 In the present study, the crab shells were washed extensively with 0.1 M HCl, which practically removed excess calcium carbonate. Repeated exposure to strong acidic conditions may disrupt the structure of crab shell as CaCO3 plays a vital role in the rigidity
Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3591
Figure 1. Effect of pH on arsenate uptake by crab shell (initial As(V) concentration ) 104 mg/L; temperature ) 25 ( 1 °C, agitation speed ) 160 rpm).
of crab shell. Structurally, crab shell mainly consists of chitin, which is a straight-chain polymer composed of β-1,4-Nacetylglucosamine.24 In chitin, hydroxyl (-OH) and acetamido (-NHCOCH3) groups are prevalent; in particular, the acetamido group of chitin acts as a nonspecific chelator and tends to establish weak hydrogen bonds with arsenic in solution. In acidic conditions (pH > 2.3), As(V) exists as H2AsO4-, HAsO42-, and AsO43-.25 Also, under these acidic conditions, dissolution of CaCO3 from the crab shell is strongly favored. The free calcium ions can form pharmacolite (or calcium hydrogen arsenate, CaHAsO4). The calcium arsenate is known to precipitate at low pH (3-6) and seems to be stable in media with high dissolved CO2.26 Microprecipitates formed were then adsorbed to the chitin on the surface of the crab shell particles. Our study showed that at pH 2 low As(V) removal occurred as compared to pH 3. The reason for this behavior can be explained as follows. At strong acidic pH values (pH < 2.3), As exists mainly in the form of H3AsO4,25 and this neutral species is unlikely to react with calcium ions or interact with chitin, which therefore resulted in decreased arsenate biosorption. 3.2. Effect of Calcium Carbonate and Chitin During As(V) Biosorption. To confirm the role of calcium carbonate and chitin on the arsenic(V) removal, the crab shell particles were exposed to various chemical treatments. The results indicated that arsenic biosorption was severely affected when calcium carbonate was completely removed from the crab shell particles. Arsenic uptake capacity of 4.26 ( 0.08 mg/g at pH 3 for an initial As(V) concentration of 104 mg/l by crab shell decreased to 0.36 ( 0.07 mg/g when crab shell was completely demineralized. This observation confirms the important role of CaCO3 in the crab shell during the removal of As(V). The role of chitin in As(V) biosorption was examined by removing CaCO3 and protein from crab shell. The results indicated that chitin was unable to bind significant quantity of negatively charged arsenate ions. The As(V) uptake decreased to 0.32 ( 0.03 mg/g when CaCO3 and proteins were removed from the crab shell. These results indicate that the formation of calcium arsenate is the main mechanism responsible for the removal of arsenate using crab shell. 3.3. SEM Examination. Figure 2 depicts the SEM micrographs of crab shell particles after arsenic biosorption. The surface is not smooth and contains lumps of calcium salts. In EDX analysis, strong Ca peaks were observed, which implies that shell particles contain calcium carbonate. The peaks corresponding to carbon, nitrogen, oxygen, sulfur, and phos-
Figure 2. SEM picture (a) and EDX spectrum (b) of crab shell loaded with arsenate ions.
Figure 3. Isotherms during As(V) biosorption onto crab shell (temperature ) 25 ( 1 °C, agitation speed ) 160 rpm). Curves were predicted by the Toth model.
phorus were recorded in the EDX spectrum. These elements are present in the crab shell as the main constituents of chitin and protein. EDX spectra also show the existence of arsenic and thereby confirm the biosorption of As(V) onto crab shell. The presence of calcium and arsenic through EDX spectra confirms the microprecipitation of calcium arsenate onto the surface of crab shell particles. 3.4. Isotherms and Modeling. Isotherms pertaining to the biosorption of As(V) onto crab shell particles were determined at different pH conditions (Figure 3). A critical analysis of the shape of isotherms revealed that all the isotherms were favorable and can be classified as “L-shaped”.27 This means the ratio between the As(V) concentration in the solution and that sorbed onto the biosorbent decreases with increase in the As(V) concentration, providing a concave curve without a plateau. Under the experimental conditions, pH 3 was found to be the optimum pH at which the highest As(V) uptake took place.
3592 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 Table 1. Biosorption Isotherm Model Parameters at Different pH Conditions pH
r2
ε (%)
rmse
Qmax (mg/g)
b (L/mg)
2.0 3.0 4.0
0.996 0.998 0.997
5.64 4.26 4.21
0.299 0.251 0.234
11.6 12.8 8.83
0.012 0.012 0.012
2.0 3.0 4.0
0.996 0.998 0.997
5.64 4.26 4.21
0.299 0.251 0.234
2.0 3.0 4.0
0.992 0.994 0.992
4.23 3.75 3.69
0.455 0.407 0.359
2.0 3.0 4.0
0.999 0.999 0.999
0.84 0.88 0.29
0.079 0.14 0.084
aRP (L/mg)1/βRP
KRP (L/g)
βRP
KF (L/g)1/n
n
0.336 0.354 0.287
1.55 1.53 1.61
bT
nT
0.016 0.016 0.015
0.206 0.334 0.221
Langmuir Model
Redlich-Peterson Model 0.139 0.150 0.111
0.012 0.012 0.012
0.954 0.989 0.973
Freundlich Model
Toth Model 6.15 7.22 4.92
Table 2. Comparison of Arsenic(V) Uptake Capacities of Various Sorbents sorbent
As(V) uptake (mg/g)
reference
fish scale FeSO4-treated Aspergillus fumigatus natural siderite iron-coated zeolite activated carbon from oat hulls crab shell chitosan
0.027 (pH 4.0) 0.054 0.516 (pH 7.0) 0.680 (pH 4.0) 3.09 (pH 5) 12.8 (pH 3.0) 58.0 (pH 4.0)
32 33 34 35 36 this work 37
Experimental isotherms related to the biosorption of As(V) onto crab shell at different pH conditions were tested using the Langmuir, Freundlich, Redlich-Peterson and Toth models. The isotherm model constants, along with the correlation coefficient (r2), % error, and rmse are presented in Table 1. Initially, the Langmuir model was applied to the present system, with the assumptions that adsorption sites are identical, each site retains one molecule of the given compound, and all sites are energetically and sterically independent of the adsorbed quantity. Even though these assumptions are not valid for the biosorption system, the model was able to describe the isotherm data with high r2, low % error, and rmse values. A maximum As(V) biosorption capacity of 12.8 mg/g was observed at pH 3; whereas increasing or decreasing the pH from 3 resulted in decreased biosorption capacity. The Langmuir affinity constant (b), which characterizes the initial slope of the isotherm, was almost constant at different pH conditions examined. A high “Qmax” and steep initial isotherm slope (i.e., high b) are desirable for a good biosorbent. However, the curves predicted by the Langmuir model significantly deviated from the experimental isotherms at all examined pH conditions. A comparison of the maximum sorption capacity of crab shell for As(V) obtained in the present study with those included in Table 2 indicates that crab shell shows higher sorption capacity relative to many of the sorbents. A similar trend was also observed with the Freundlich and Redlich-Peterson models, which described the isotherm curves with high r2, low rmse, and % error values. However, the curves predicted by these models significantly deviated from the experimental isotherms. The Freundlich isotherm was originally empirical in nature, but was later interpreted as sorption to heterogeneous surfaces or surfaces supporting sites of varied affinities. It is assumed that the stronger binding sites are occupied first and that the binding strength decreases with the increasing degree of site occupation. Both the Freundlich constants (KF and 1/n) were observed to have maximum values
at pH 3. High KF and 1/n values indicate that the binding capacity reached its highest value, and the affinity between the biosorbent and As(V) was also high. The Redlich-Peterson model incorporates the features of both the Langmuir and Freundlich isotherms into a single equation. There are two limiting behaviors: the Langmuir form for βRP ) 1 and the Henry’s law form for βRP ) 0. The isotherm constant (KRP) and model exponent (βRP) were observed to have maximum values at pH 3, whereas the constant aRP remained unaltered at all pH conditions examined (Table 1). Finally, the Toth model was examined for the As(V) biosorption isotherms and this resulted in very good prediction of isotherm curves (Figure 3) along with high r2 and very low rmse and %ε values (Table 1). The Toth isotherm,23 derived from potential theory, has proven useful in describing sorption in heterogeneous systems such as phenolic compounds on carbon. It assumes an asymmetrical quasi-Gaussian energy distribution with a widened left-hand side, that is, most sites have sorption energy less than the mean value. The successful application of the Toth model to the present data supports the fact that the surfaces of the biosorbent are heterogeneous. 3.5. Kinetics and Modeling. The sorption kinetics plays a very important role in the treatment of wastewater, as it provides valuable insights into the reaction pathways and mechanisms of sorption reactions. Since biosorption is a metabolismindependent process, it would be expected to be a very fast reaction. Experimental kinetic data at different initial As(V)
Figure 4. Biosorption kinetics of As(V) uptake onto crab shell (pH ) 3; temperature ) 25 ( 1 °C, agitation speed ) 160 rpm).
Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3593 Table 3. Biosorption Kinetic Model Parameters at Different Initial As(V) Concentration pseudo-first-order
pseudo-second-order
initial concentration (mg/L)
(Qe)exp (mg/g)
K1 (1/min)
Qe (mg/g)
r2
ε (%)
rmse
K2 (g/mg min)
Qe (mg/g)
r2
ε (%)
rmse
14.1 28.2 42.3 56.4
1.11 2.06 3.14 3.62
0.097 0.048 0.060 0.057
1.06 1.97 3.04 3.54
0.998 0.999 0.999 0.999
0.169 0.607 0.229 0.093
0.047 0.063 0.082 0.091
0.161 0.032 0.028 0.022
1.11 2.15 3.27 3.81
0.997 0.999 0.999 0.999
0.017 0.386 0.449 0.681
0.010 0.035 0.069 0.101
concentrations coincided with this expectation, with more than 90% of As(V) ions removed in the first 1.5 h (Figure 4). This initial quick phase was followed by slow attainment of equilibrium as a high amount of calcium ions were initially available for microprecipitation of arsenate, but thereafter, the formation of calcium arsenate would be difficult because of a decrease in calcium release from the crab shell. On changing the initial As(V) concentration from 14.1 to 56.4 mg/L, the uptake increased from 1.1 to 3.6 mg/g, whereas the removal efficiency decreased from 39.2 to 32.1%. This anomaly can be attributed to the fact that the ratio of the initial moles of metal ions to the available surface area was low, and subsequently the fractional sorption became independent of initial concentration of As(V).28 However, at higher concentrations the available surface area becomes less compared to the moles of metal ions present. Hence, the percentage of metal removal is dependent upon the initial metal ion concentration.15 The experimental kinetic data were described using pseudofirst and -second-order models. Application of both models resulted in very high correlation coefficients and low % error and rmse values (Table 3). However, the pseudo-second-order model predicted the equilibrium uptake values and kinetic curves better compared to the pseudo-first-order model. The pseudofirst-order model under-predicted the equilibrium uptake values at all initial concentrations examined. The reason for these differences in the Qe values is that there is a time lag, possibly due to a boundary layer, or external resistance controlling the initiation of the sorption process.29 In most cases in the literature, the pseudo-first-order model does not fit the kinetic data well for the whole range of contact time and generally underestimates the Qe values.30,31 The pseudo-second-order model is based on the sorption capacity on the solid phase. Contrary to other well established models, the latter model predicts the behavior over the whole range of studies, and it is in agreement with a chemisorption mechanism being the rate controlling step.28 However, the equilibrium uptake values were slightly overpredicted by the pseudo-second-order model. The curves predicted by the pseudo-second-order model are shown in Figure 4.
Figure 5. Effect of ionic strength on the uptake of As(V) by crab shell (initial As(V) concentration ) 10.5 mg/L; pH ) 3; temperature ) 25 ( 1 °C, agitation speed ) 160 rpm).
3.6. Ionic Strength. An important experimental parameter to be investigated in biosorption experiments is the ionic strength, which influences the binding of solutes to the biomass surface. The effect of ionic strength on the As(V) biosorption onto crab shell was studied by adding NaCl in the concentration ranges of 0 - 5000 mg/L (Figure 5). As the ionic strength increased, arsenic uptake was severely affected. This decrease may be ascribed to the competition between ions, changes in metal activity, or in the properties of the electrical double layer. The anion Cl- may compete with arsenate anion for the positively charged Ca2+ ions from the crab shell. To confirm this, ion chromatography was used to analyze the Cl- concentration. From Figure 5, it was clear that as the ionic strength increased, Cl- uptake by crab shell also increased. Also, it is worth noting that crab shell also showed potential in binding Na+ ions onto its surface. As a consequence, the Na+ uptake increased with ionic strength. However, compared to the initial concentrations of Na+ and Cl- ions used, the uptake of these ions by crab shell is not significant. Based on these observations, it can be concluded that the As(V) uptake by crab shell was strongly dependent on ionic strength as evident from the competition of Cl- ions during interaction of arsenate with calcium ions. 3.7. Desorption. Desorption is of utmost importance as it is possible to decrease the treatment process cost and also the dependency of the process on a continuous supply of biosorbents. Since maximum As(V) biosorption was observed in the pH ranges of 2-7, elution was attempted using both acidic (0.1 M HCl) and basic (0.1 M NaOH) solutions. Interestingly, both eluents showed superior desorption efficiencies, which were 87.1 and 98.2% by HCl and NaOH, respectively. Also, the weight loss of the biosorbent was insignificant at the end of desorption. 4. Conclusions The current study demonstrated the feasibility of utilizing a seafood industry waste, namely crab shell, for the removal of arsenic(V) from aqueous solutions. The following important findings arise from the study: (a) Crab shell particles, because of the presence of CaCO3 and chitin, performed well in As(V) biosorption. Microprecipitates formed due to calcium ions of crab shell and arsenate ions in the solution were adsorbed to the chitin on the surface of the crab shell particles. (b) Isotherm experiments revealed that crab shell can bind As(V) with as much as 12.8 mg/g biosorption capacity at pH 3, according to the Langmuir model. However, examining the isotherm with different models revealed that the Toth model described the experimental data with very high r2, low % error, and low rmse values. (c) Kinetic experiments indicated that more than 90% of As(V) ions were removed in the first 1.5 h, which implies that the material is well suited for column applications. Application of kinetic data to different kinetic models revealed that the pseudo-second-order model predicted the equilibrium As(V) uptake values satisfactorily with very high correlation coefficients, low % error, and low rmse values.
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(d) Ionic strength strongly affected the As(V) uptake by crab shell owing to Cl- competition during interaction of arsenate with calcium ions. (e) The biosorbed arsenate ions were effectively eluted using 0.1 M HCl and 0.1 M NaOH, with the latter exhibiting 98.2% elution efficiency. (f) Thus, crab shell being low-cost and easily available biomaterial can be considered for decontamination of wastewater containing As(V). Because of its high mechanical strength and ease in desorption, crab shell possesses all prerequisites for column applications involving wastewater treatment. The potential interference from other contaminants in the aqueous medium to the removal of As(V) needs to be rigorously tested. Acknowledgment The authors gratefully acknowledge the support and contributions of this project to the Singapore-Delft Water Alliance (SDWA). The research presented in this work was carried out as part of the research programme (R-264-001-002-272) of the Singapore-Delft Water Alliance (SDWA). Literature Cited (1) Mohan, D.; Pittman, C. U., Jr. Arsenic Removal from Water/ Wastewater Using Adsorbents-a Critical Review. J. Hazard. Mater. 2007, 142, 1. (2) Jain, C. K.; Ali, I. Arsenic: Occurrence, Toxicity and Speciation Techniques. Water Res. 2000, 34, 4304. (3) Matschullat, J. Arsenic in the GeospheresA Review. Sci. Total EnViron. 2000, 249, 4304. (4) Bissen, M.; Frimmel, F. H. ArsenicsA Review. Part I. Occurance, Toxicity, Speciation, Mobility. Acta Hydrochim. Hydrobiol. 2003, 31, 9. (5) WHO, Guidelines for Drinking Water Quality. 1. Recommendations; World Health Orgnaization: Geneva, 1993. (6) Tripathi, R. D.; Srivastava, S. S.; Singh, N.; Tuli, R.; Gupta, D. K.; Maathuis, F. J. M. Arsenic Hazards: Strategies for Tolerance and Remediation by Plants. Trends Biotechnol. 2007, 25, 158. (7) Ferguson, J. F.; Gavis, J. A Review of Arsenic Cycle in Natural Waters. Water Res. 1972, 6, 1259. (8) Sathishkumar, M.; Murugesan, G. S.; Ayyasamy, P. M.; Swaminathan, K.; Lakshmanaperumalsamy, P. Bioremediation of Arsenic Contaminated Groundwater by Modified Mycelial Pellets of Aspergillus fumigatus. Bull. EnViron. Contam. Toxicol. 2004, 72, 617. (9) Hansen, H. K.; Ribeiro, A.; Mateus, E. Biosorption of Arsenic(V) with Lessonia nigrescens. Miner. Eng. 2006, 19, 486. (10) Murugesan, G. S.; Sathishkumar, M.; Swaminathan, K. Arsenic Removal from Groundwater by Pretreated Waste Tea Fungal Biomass. Biores. Technol. 2006, 97, 483. (11) Boddu, V. M.; Abburi, K.; Talbott, J. L.; Smith, E. D.; Haasch, R. Removal of Arsenic (III) and Arsenic (V) from Aqueous Medium Using Chitosan-coated Biosorbent. Water Res. 2008, 42, 633. (12) Sathiskumar, M.; Binupriya, A. R.; Swaminathan, K.; Choi, J.-G.; Yun, S.-E. Arsenite Sorption in Liquid-phase by Aspergillus fumigates: Adsorption Rates and Isotherm Studies. World J. Microbiol. Biotechnol. 2008, 24, 1813. (13) Chen, C.-Y.; Chang, T.-H.; Kuo, J.; Chen, Y.-F.; Chung, Y.-C. Characteristics of Molybdate-impregnated Chitosan Beads (MICB) in Terms of Arsenic Removal from Water and the Application of a MICB-packed Column to Remove Arsenic from Wastewater. Biores. Technol. 2008, 99, 7487. (14) Lee, M. Y.; Park, J. M.; Yang, J. W. Micro Precipitation of Lead on the Surface of Crab Shell Particles. Proc. Biochem. 1997, 22, 671.
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ReceiVed for reView August 4, 2008 ReVised manuscript receiVed December 5, 2008 Accepted December 23, 2008 IE801570V
