Egypt. J. Chem. Vol. 61, No.5 pp. 799 - 812 (2018)
Adsorption of Chromium(Vi) from Aqueous Solution by Glycine Modified Cross-linked Chitosan Resin Asmaa S. Hamouda1, Sayed A. Ahmed1, Nahla M. Mohamed1, Mostafa M.H. Khalil2* 1 Faculty of Postgraduate Studies of Advanced Sciences (PSAS), Beni Suef University, Beni Suef, Egypt. 2 Chemistry Department, Faculty of Science, Ain Shams University, Abbassia, 11566, Cairo, Egypt.
HE ADSORPTION of Cr(VI) onto glycine-modified crosslinked chitosan (GMCCR) resin has been investigated. Batch experiments were performed to examine kinetics, adsorption isotherm, pH effect, and thermodynamic parameters. The effect of pH for the adsorption of Cr(VI) was studied at range from 2 to 6 and the equilibrium was accomplished within 150 minutes and maximum removal was achieved under the optimum conditions at pH 3 . The result obtained from equilibrium adsorption studies are fitted Langmuir and Freundlich adsorption models and the data was found that the equilibrium data agreed very well with the Langmuir model . The maximum uptake was found to be 1.5 mmol/g (calc 1.75 mmol/g) at 250C. Thermodynamic parameters for the adsorption system were determined at 298 K, 308 K and 318 K (ΔH° =22.85 kJ•mol−1; ΔG° = −33.17 to −36.93 kJ•mol−1 and ΔS° = 188 J•K−1•mol−1). The positive values of ΔH° and ΔS° suggest an endothermic reaction and increase in randomness at the solid-liquid interface during the adsorption. The negative values of ΔG° indicating a spontaneous adsorption process. The kinetic process was described very well by a pseudo-second-order rate equation. Keywords: Modified chitosan, Adsorption, Kinetics, Thermodynamics, Cr(VI).
Introduction Chromium occurs mainly in the oxidation states trivalent Cr(III) and hexavalent in the environment Cr(VI) state. Whereas Cr(III) is essential inhuman nutrition (specially in glucose metabolism) as well as for plants and animals at trace concentrations, the hexavalent Cr(VI) has been considered more hazardous to public health due to its mutagenic and carcinogenic properties . It is also move readily through soils and aquatic environments and is a strong oxidizing agent capable of being absorbed through the skin . A wide range of physical and chemical processes is available for the removal of Cr(VI) from drinking water, such as electrochemical precipitation, ultraﬁltration, reverse osmosis and ion exchange [3-5]. Most of these methods suffer from high operational costs. Therefore, it is necessary to develop new treatment processes that are not only effective, but also feasible in terms of cost [6–8]. Adsorption
is one of the most economically favorable and a technically easy method . Chitosan has been reported to have high potential for adsorption of chromium(VI) [10,11]. It is an amino-polysaccharide constituted of both acetylglucosamine and glycosamine moieties. Chitosan has been widely applied to the fields of pharmacy processing biotechnology, food and analytical chemistry. Amino group and hydroxyl group in chitosan exhibit good ability to chelate metal ions. On the other side, chitosan represent suitable materials for binding of metal oxo-anion species because of numerous functional groups (e.g., -OH and -NH2) with their suitable H-bond acceptor and donor sites. Adsorption capacity of chitosan can be improved by chemical means such as addition of functional groups, crosslinking and by physical conditioning of the biopolymer as gel beads or fibers [12-14]. Several chemical changes have been applied to chitosan in order to enhance
*Corresponding author e-mail: [email protected]
©2017 National Information and Documentation Center (NIDOC)
ASMAA S. HAMOUDA et al.
its uptake of Cr (VI) from the solution [15-19]. Novel chitosan resins possessing chelating moieties have been developed by using acrosslinked chitosan resin as a base material. The development of chelating resin is important from the view point of the collection and separation of metal ions .The cross-linked chitosan are found to be very stable and maintain their strength even in acidic and basic solutions. These characteristics are very important for an adsorbent so that it can be used in a lower pH environment .
All other chemicals were Prolabo products and were used as received. Preparation of glycine modified chitosan resin The glutaraldehyde-crosslinked chitosan glycine type was prepared as in literature . Three grams of chitosan was dissolved in 20% aqueous solution of acetic acid and stirred until the solution became homogenous. Then 1 mL of glutaraldehyde solution (50%) was added and the solution was stirred with heating for two hours. The pH of the solution was raised to 6 and the obtained gel was washed with distilled water several times and kept to dry. The obtained crosslinked chitosan gel from the previous step was suspended in 60 mL isopropyl alcohol. Then 7 mL epichlorohydrine (62.5 mmol) dissolved in 100 mL acetone/water mixture (1:1 v/v) was added. The above mixture was stirred for 24 h at 60 oC. The obtained solid product was filtered off and washed several times with water followed by ethanol. The product and glycine (10 g) were suspended in dioxane (100 ml), then 40ml NaOH (1M) was added and the mixture was refluxed for 3 h. the final product was filtered and washed 3 times with ethanol and with deionized water. The synthesis steps are shown in Scheme 1.
In the present work, we prepared modified glutaraldehyde-crosslinked chitosan glycinetype and used it to adsorb Cr (VI) ions in a batch system. The effects of the process parameters such as pH, temperature on the removal were investigated. In order to have better understanding of the adsorption process, some isotherm, kinetic and thermodynamic models were employed. Experimental Chemicals Chitosan (from crab shell), glycine, glutaraldehyde, isopropyl alcohol, epichlorohydren, K2Cr2O7were Aldrich products.
Scheme 1 Egypt. J. Chem. 61, No.5 (2018)
ADSORPTION OF CHROMIUM(VI) FROM AQUEOUS SOLUTION...
Characterization of the resin Infrared spectra were performed using Nicolet 6700 FT-IR Spectrometer. The surface morphology of the absorbent was visualized with a scanning electron microscope (JEOL-1200, Japan). The scanning electron microscopy (SEM) enabled us to directly observation of the changes in the surface microstructures of the absorbent. X-ray diffraction (XRD, JCPDS No. 03-0921) was used to characterize the crystal structures of chitosan and GMCCR. Water Regain For water regain determination, resin sample was centrifuged for 30min at 1000rpm to remove excess water and then weighed. The sample was then dried at 50–60 0C until complete dryness then weighed again. To calculate this factor, the following equation was applied: Where Ww and Wd are weights (g) of the wet and dried resin, respectively. Water regain values are (37±3%). This value reflects the hydrophilic character of the resin type. Uptake experiments using batch method Preparation of solutions Stock solution (1×10-2 M) of chromium (VI) was prepared by dissolving 1.47g K2Cr2O7 in 1L bi-distilled water. All batch experiments were carried out with adsorbent samples in a 250 mL conical ﬂasks with 100 mL Cr (VI) aqueous solutions on a rotary shaker at 200 rpm. The concentration of Cr(VI) ions was determined spectrophotometrically at 540 nm using diphenylcarbazide as the complexing agent. Effect of pH The uptake of Cr (VI) by the investigated resins was studied at different pH values from 2 to 6. The pH was adjusted using HCl or NaOH. 0.1 g of investigated resin was placed in a series of flasks. To each flask 100 mL of Cr (VI) solution (5×10-3 M) was added. The contents of each flask were shaken for 150 min on a shaker at 200 rpm and at temperature 20 ± 1 oC at desired pH. The resin was separated from the solution by filtration. Then the residual concentration of Cr (VI )was determined. Adsorption isotherms Complete adsorption isotherms were carried out by placing 0.1 g portions of dried resin in a series of flasks containing 100 mL of Cr (VI) ions at pH 3. The temperature was thermostatically kept
at 25±1, 35±1 or 45±1°C and equilibrium time 150 min for studied resin. The residual concentration of Cr (VI) was determined. The adsorption data were treated according to Langmuir equation . qe =
Q max K L C e 1 + K L Ce
Where qe the adsorbed value of Cr (VI) ions at equilibrium concentration (mmol/g), Qmax is the maximum adsorption capacity (mmol/g) and KL is the Langmuir binding constant which is related to the energy of adsorption (L/mmol), Ce is the equilibrium concentration of Cr (VI) in solution (mmol/L). Its linearized equation is shown as below Ce qe
Ce 1 + Q max K L Q max
Plotting Ce/qe against Ce gives a straight line with slope and intercept equal to 1/Qmax and 1/KLQmax, respectively. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor RL that is given by 1 R
Where Ci (mmol/L) is the highest initial concentration of adsorbate and RLvalues 0