Research Journal of Chemistry and Environment______________________________________Vol.15 (2) June (2011) Res.J.Chem.Environ
Biosorption of Chromium from Aqueous Waste Water using Chitosan and Desorption of Chromium from Biosorbent for Effective Reuse Bhuvaneshwari S.,1* Sivasubramanian V. 1 and Senthilrani S. 2 1. National Institute of Technology Calicut, Kerala 673 601, INDIA 2. Seethalakshmi Ramaswamy College, Trichy, Tamil nadu 620008, INDIA *
[email protected]
of the column packed with chitosan biomass for Cr (VI) detoxification was demonstrated.
Abstract Wastewater reclamation has often been overlooked in the study of water resources. There is pressure by the public and media with regards to environmental discharges of toxic effluents, Toxic metal pollution can be a much more serious and insidious problem, as these are intrinsic components of the environment. At high concentrations, all the metals are harmful to human life. Therefore there is a requirement for newer and effective methods which are also cost-effective. Biosorption is a feasible option because it is both efficient and cheap. Compared with conventional methods for removing toxic metals from effluents, the biosorption process has the advantages of low operating cost, minimization of volume of chemicals and biological sludge to be disposed off and high efficiency in detoxifying very dilute effluents.
Keywords: Chitosan, chromium, adsorption, desorption and metal recovery.
Introduction The term “Heavy metal” is often used to cover a diverse range of elements which constitute an important class of pollutants. Such pollutants have received the attention of researchers all over the world, mainly due to their harmful effects on living beings. Human biology is full of instances where heavy metal toxicity has led to mass deaths. Heavy metals enter into the environment by three routes: (i) deposition of atmospheric particulates, (ii) disposal of metal enriched sewage sludge and sewage effluents and (iii) by-products from metal mining process.8 Chromium is one of the heavy metals where its compounds are considered highly toxic to humans. Chromium (III) occurs naturally in the environment. Chromium (VI) is generally produced by industrial processes and used in such industries as pigment manufacturing, leather tanning, wood treatment and chrome plating. The primary use for chromium compounds containing chromium (VI) is in the metal finishing industry for both decorative and functional purposes. The dumping of industrial waste materials significantly increases chromium concentration in soil and is usually accompanied by groundwater contamination. Hexavalent chromium is known as the most mobile chromium form in soil and water systems, whereas Cr(III) is generally not transported over great distances because of its low solubility and tendency to be absorbed by solid particles in the appropriate pH range.20 Redox conversion of Cr(III) to Cr(VI) can increase the chromium dislocation from the soil into the water system. Chromium is often chosen as a surface finish because it possesses a low coefficient of friction, high hardness, good corrosion resistance, high heat resistance and anti-galling properties. Due to its toxicity and suspected carcinogenicity, however, chromium is heavily regulated for the protection of human health and of the environment.12 Hence chromium is taken for this study.
The present study was conducted with the major objective of using chitosan for the removal of metal(s) from synthetic wastewaters. Experiments were conducted in a batch adsorption system for the removal of chromium ions from aqueous solution by chitosan. The influence of different experimental parameters; metal concentration, adsorbent concentration, agitation time, agitation speed, temperature and pH was evaluated. A removal of 99.257% was achieved under optimized conditions. The mechanism of metal adsorption by chitosan gave good fit for Langmuir and Freundlich isotherms models. Desorption of metals from chitosan were conducted in batch system using eluants like nitric acid hydrochloric acid , EDTA & sodium thio sulphate to regenerate the chitosan for effective reuse. The best desorption results were obtained with 0.1 M EDTA and the adsorbent was restored to the original state without any physical damage to the adsorbent. The adsorption studies repeated with regenerated chitosan and the sorption efficiency was 99 %, this study confirms the reusable potential of chitosan. The kinetic parameters were determined for chromium adsorption, based on the correlation coefficients the adsorption of chromium is best described by the pseudo second order equation. Finally, the potential
Biosorption (Adsorption by biomass) can be defined as a non-directed physicochemical interaction that may occur between metal species and microbial cells. It is a biological method of environmental control and can be an (185)
Research Journal of Chemistry and Environment______________________________________Vol.15 (2) June (2011) Res.J.Chem.Environ experiments were carried out using dead form (pelletized) of chitosan.
alternative to conventional contaminated water treatment facilities. It also offers several advantages over conventional treatment methods including cost effectiveness, efficiency, minimization of chemical/ biological sludge and regeneration of biosorbent with possibility of metal recovery.22 The biosorption process involves a solid phase (sorbent or biosorbent; usually a biological material) and a liquid phase (solvent, normally water) containing a dissolved species to be sorbed (sorbate, a metal ion). Due to higher affinity of the sorbent for the sorbate species the latter is attracted and bound with different mechanisms. The process continues till equilibrium is established between the amount of solidbound sorbate species and its portion remaining in the solution 27. While there is a preponderance of solute (sorbate) molecules in the solution, there are none in the sorbent particle to start with. This imbalance between the two environments creates a driving force for the solute species. The heavy metals adsorb on the surface of biomass thus, the biosorbent becomes enriched with metal ions in the sorbate.
Batch adsorption studies were carried out using adsorbent (chitosan) in 50 ml of synthetic chromium solution in a conical flask with constant shaking using an incubator shaker operated at different speeds. The following operating conditions such as adsorbent dosage, metal concentration, agitation speed, contact time, pH and temperature were investigated. For each of the investigation, the mixture was shaken in an incubator shaker followed by filtration using Whatmann filter paper. The filtrate containing the residual concentration of chromium was determined atomic absorbance spectrophotometer. Batch adsorption studies: Chitosan was added at different concentrations 10, 20, 30, 40, 50, 60 mg in 50 ml of known chromium solution in Erlenmeyer flasks at pH 4.0 using dilute HNO3 and were agitated at 150 rpm at different times (20, 40, 60, 80, 100, 120, 180, 24 min) kept at room temperature, filtered using Whatmann filter paper no.1 and the filtrate containing the residual concentration of chromium was determined at 540 nm using atomic absorbance spectrophotometer. Similarly, time course biosorption experiments were carried out by varying pH (2, 3, 4, 5, 6 and 7) and metal ion concentration (2, 4, 6, 8 and 10 ppm ). Chitosan of 40 mg (optimized value) is added to 50 ml of 6 ppm chromium solution (optimized value) in three different flasks. Various temperatures (30, 35, 40 0C) were maintained for each flask in an incubator shaker. The solutions were maintained at 4 pH and shaker speed at 150 rpm for 90 minutes to study the effect of temperatures on biosorption of chromium using chitosan.
Worldwide, the solid waste from processing of shellfish, crabs, shrimps and krill constitutes large amount of chitinaceous waste. Chitosan (2-acetamido-2-deoxy-βD-glucose-(Nacetylglucosamine) is a partially deacetylated polymer of chitin (2-acetamido-2-deoxy-β-Dglucose-( Nacetylglucan)) and is usually prepared from chitin by deacetylation with a strong alkaline solution21. The cationic property of chitosan offers an opportunity to take advantage of its electrostatic interaction properties. Chitosan is chosen as the biosorbent because it is a cheaply available cationic polymer, 28 a characteristic ideal for the binding of metal anions such as chromate.13
Batch desorption studies: 250 ml of 6 ppm chromium solution inoculated with optimum amount of adsorbent at optimum conditions. After 90 min exposure, a part of this solution was filtered and the concentration of metal ion in the filtrate was determined using atomic absorbance spectrophotometer. Chromium loaded chitosan were collected, gently washed with distilled water to remove any unadsorbed chromium, Then the metal loaded chitosan was dried and treated with various eluants like EDTA, Sodium thio sulphate, HNO3, H2SO4 and HCl 15 in 2:1 volume proportion by varying their concentration (0.05, 0.1, 1 M ), contact time ( 20, 30, 40 mins), temperature (30,35,40 0C) and agitation speed ( 150 and 200 rpm) to recover the chromium ions from it. The solution was then filtered and the concentration of metal ion in filtrate was determined to know the desorption performances of different eluants. And adsorbent regenerated by washing successively with deionised water and 0.1M NaOH until a neutral pH is obtained 14, 25. Regenerated adsorbent was dried and used to initiate another cycle.
Material and Methods Experiment: All the chemicals used are of analytical grade and samples were prepared using deionized water. Chitosan powder was donated by Cochin Institute of Fisheries Technologies, Cochin. Preparation of chromium solution Synthetic wastewater containing chromium is prepared by adding 2.8287 g of potassium dichromate (K2Cr2O7) in 1000 ml of deionized water to prepare 1000 ppm chromium solution. Working standards ranging from 1 ppm to 100 ppm were then prepared by appropriately diluting the stock solution. Batch Studies: Metal uptake by the adsorbent depends on many factors such as metal concentration, adsorbent concentration, time, temperature and pH.15 In order to study the effect of each factor, batch studies were done at different metal concentration, adsorbent concentration, time, pH and temperature to find the optimum condition for removal of metals from wastewater. The adsorbent used for these experiments is chitosan, a copolymer of glucosamine and N-acetyl glucosamine and it has an amine functional group which is strongly reactive with metal ions. Present
Sorption isotherm and kinetics: Isotherm studies were conducted with a constant chitosan (40 mg) and varying (186)
Research Journal of Chemistry and Environment______________________________________Vol.15 (2) June (2011) Res.J.Chem.Environ initial concentration of Cr in the range of 2 to 10 ppm, at an optimum condition of pH 4, agitation speed 150 rpm and contact time 90 minutes 15. The amount of adsorption was then calculated based on the difference of Cr concentration in aqueous solutions before and after adsorption, the volume of aqueous Cr solution (50 ml) and the weight of the chitosan powder (40 mg). The adsorption capacity was calculated from adsorption experiments as qe =( C0Ce)V/W, Where C0 is the initial Cr concentration (ppm), Ce is the final or equilibrium Cr concentration (ppm), V is the volume of the Cr solution (ml) and W is the weight of the chitosan (g). For batch kinetic studies, 40 mg of chitosan with the desired concentration of chromium equilibrated at optimum condition were placed in eight flasks maintained at different time slots (20, 40, 60, 80, 100, 120, 180, 24 min) stirred by an incubator shaker. For every time, after filtration the concentration of Cr in filtrate was determined and the results were analyzed for fitness with first order, pseudo first and second order kinetic models to determine the order and rate constant of Cr biosorption onto chitosan15.
Analytical methods For separation of chromium from aqueous solution, samples were agitated using incubator shaker (Model), The chromium concentration was measured using atomic absorbance spectrophotometer at 540 nm. The metal uptake was calculated from adsorption experiments as: % metal recovery= (Co- Ci / Co)x 100 where Co is Cr concentration in aqueous solutions before adsorption Ci is Cr concentration in aqueous solutions after adsorption The metal uptake was calculated from desorption experiments as: q =V. Cf /M where Cf is the final eluated metal concentration in solution (mg/l);V is solution volume ( L);M is initial mass of used adsorbent (g). The metal uptake capacity (amount of removal of chromium ion) and the adsorption capacity (Percentage of chromium ion removal) for column studies were calculated using the following equations: Metal Uptake Capacity = Initial Chromium ion conc. – Final Chromium ion conc. Adsorption Capacity (%) = (Metal Uptake Capacity X 100) / Initial Chromium ion conc.
Continuous Column Studies Single Stage Column: Continuous column used in this study consists of a 12 cm long and 1 cm internal diameter acrylic column. The column was densely packed with a known amount of dried chitosan upto the specified height of the column and different height of the adsorbent packing (3 cm, 6 cm and 9 cm) was used as specified for each experiment. The process was operated in an up-flow mode and set at constant temperature with the aid of a water bath. The flow rate 90 ml/min was regulated with a variable speed pump, after each column operation, packed biomass was vacated from the column and fresh dried biomass was packed and used for another operation.24 The adsorption capacity was 33 % for a bed height of 3 cm and contact time of 8 hours whereas as the bed height increased to 6cm the adsorption capacity increased to 42 % as shown in figure 1 but further increase in bed height (9cm) decreased the adsorption capacity to 27 %.
Result and Discussion Effect of chitosan concentration on metal binding: Figure 2 shows the effect of concentration of adsorbent on metal binding, it can be seen that increase in concentration of adsorbent lead to a better uptake. However beyond a concentration of 0.8 mg/ml, there is a decrease in chromium removal, Even though the binding capacity of adsorbent is high, this might occur because probably not all binding sites on the polymer are accessible to the metal ions.
Single Stage Column with recycle: In every experiment the metal solution of a known concentration was pumped at a fixed flow rate of 90 ml/min to the column filled with known bed height of adsorbent. The above fixed flow rate into the fixed column was maintained with the help of monitoring a bypass stream. The samples solution after passing through the adsorbent in the fixed bed column was again recycled back into the sample reservoir tank so that there is a continuous adsorption of chromium ion. Samples for analysis of chromium ion concentration were collected at a regular interval of 5 mins from the bottom of the sample reservoir tank. The chitosan used inside the fixed bed column was replaced by a fresh chitosan batch of at the beginning of each of experiment. The adsorption capacity was 63 % for a bed height of 6 cm with the contact time of 8 hours.
Effect of contact time on biosorption: The mechanism of solute transfer to the solid includes diffusion through the fluid film around the adsorbent particle and diffusion through the pores to the internal adsorption sites. In the initial stage of adsorption of chromium, the concentration gradient between the film and the available pore sites is large and hence the rate of adsorption is faster. The rate of adsorption decreases in later stage probably due to the slow pore diffusion of the solute ion into the bulk of the adsorbent as shown in figure 3. Effect of temperature on biosorption: The experimental results obtained from a series of contact time studies for metal ions adsorption with an optimum metal ion concentration of 6 ppm at solution in which temperature was varied from 30 to 40 ◦C. The adsorption of metal ions (187)
Research Journal of Chemistry and Environment______________________________________Vol.15 (2) June (2011) Res.J.Chem.Environ has been found to decrease with an increase in temperature from 30 to 40 ◦C as shown in figure 4. This phenomenon might be explained if adsorbent particles assembled at high temperatures
it affects the solubility of the metal ions, concentration of the counter ions on the functional groups of the adsorbent and the degree of ionization of the adsorbate during reaction. To examine the effect of pH on the chromium removal efficiency, the pH was varied from 2.0 to 7.0. Figure 5 shows that the uptake of free ionic chromium was maximum at pH 4 and then declining at higher pH. At lower pH (4 pH) the adsorbent is positively charged while the chromium anions in solutions are negatively charged. This leads to an electrostatic attraction between the two so better uptakes 11 .At higher pH (5-7 pH) the adsorbent will be less protonated or neutral and hence chromium removal is reduced.
Figure 1 : Adsorption capacity of Continouous column packed with chitosan
Figure 4 : Effect of chromium uptake at different temperature (Adsorbent concentration = 0.8 mg/ml, C0=6ppm and pH=4 Effect of chromium concentration on biosorption: Chromium adsorption is significantly influenced by the metal concentration in aqueous solutions. Figure 6 shows that with increase in chromium concentration the percentage removal decreases, this can be explained by the fact that the adsorbent had a limited number of active sites, which would have become saturated above a certain concentration.
Figure 2: Chromium removal at different adsorbent concentration
Adsorption dynamics and equilibrium studies of chromium onto chitosan: The quantity of adsorbate that can be taken up by adsorbent is a function of both the characteristics and concentration of adsorbate and the temperature. The amount of material adsorbed is determined as a function of concentration at a constant temperature, the resulting function is called an adsorption isotherm. The analysis of the isotherm data is of particular importance to develop an equation which accurately represents the results and could be used for design purpose. In this study, the experimental data of chromium ions onto chitosan were well fitted to the Langmuir and Freundlich, compared to Sips and Redlich-Peterson isotherms as illustrated in table 1.
Figure 3 : Effect of Chromium uptake at different contact time (Adsorbent concentration = 0.8 mg/ml, Co=6ppm and pH=4)
Sorption Kinetics: To find the potential rate controlling steps involved in the process of biosorption of chromium onto chitosan first order, pseudo first and second order
Effect of pH on biosorption: pH is an important parameter for adsorption of metal ions from aqueous solution because (188)
Research Journal of Chemistry and Environment______________________________________Vol.15 (2) June (2011) Res.J.Chem.Environ kinetic models were tested at various chitosan concentrations and at different contact times. Adsorption kinetics of chromium is reported in table 2.The isotherm and kinetic models parameters were determined using the trial and error non linear method.
Acknowledgement The authors are most grateful to Department of Science and Technology, New Delhi for financial support. Project No: SR/FTP/CS_68/2007.
References
Desorption: The chromium bound on the adsorbent was desorbed at different experimental conditions and the results are represented in table 3, the maximum desorption was obtained while using 0.1 M EDTA at 400C with the contact time of 40 minutes. The desorption experiments of Cr ions was repeated two times at optimized conditions. The adsorption capacities of the biosorbent did not change during the reuse, represented in table 4. Hence this study confirms the reusable potential of chitosan.
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Conclusion
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Table 1 Comparison of isotherms based on regression coefficient Isotherm
Equation
Linear form
Regression coefficient (R2)
Langmuir
0.9636
Freundich
0.9652
Redlich-Peterson
0.9371
Sips
0.8689
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Research Journal of Chemistry and Environment______________________________________Vol.15 (2) June (2011) Res.J.Chem.Environ Table 2 Comparison of first order, pseudo first order and pseudo second order rate constants at optimized experimental conditions Pseudo first order Pseudo second order First order -1 2 -1 2 k1 (min ) R k2(min ) R k(min -1) R2 0.0145 0.962 0.01337 0.999 0.003 0.979 Table 3 % Desorption of chromium from chitosan using different eluants at various concentrations by changing contact time and temperature with an agitation speed of 150 rpm 300C
Temperature Time (min)
20
30
350C 40
20
30
400C 40
20
30
40
Eluants
Conc
HCL
0.01M
34
39
38
36
40
44
37
41
47
HNO3
0.05M 0.1 M 0.01M
54 77 4
61 82 5
62 80 7
59 78 9
65 82 12
67 83 16
60 83 10
66 85 13
69 87 19
EDTA
0.05M 0.1 M 0.01M
24 43 34
27 52 41
29 52 45
29 46 38
31 56 42
35 58 43
32 55 41
34 59 45
37 64 47
0.05M 0.1 M 0.01M
61 86 32
75 94 38
77 96 39
64 88 34
79 96 40
79 98 44
76 94 37
82 98 42
82 99 47
0.05M 0.1 M
60 75
65 84
67 84
63 80
66 87
68 88
66 85
67 90
69 93
Sodium thio sulphate
Table 4 Comparison of metal recovery using fresh and regenerated chitosan at optimized condition Run
% metal recovery using fresh chitosan
1 2
99.257 --
% metal recovery using regenerated chtosan (using EDTA) 99.20 99.00
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% metal recovery using regenerated chtosan (using HCL ) 98.67 98.50
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