KINETICS AND THERMODYNAMICS OF CHROMIUM (III) REMOVAL FROM AQUEOUS SOLUTIONS BY THE ORGANIC ION EXCHANGERS

Abstract

Chromium is a toxic element and exists in two stable oxidation states, Cr(III) and Cr(VI) where the later is very toxic to human beings. The presence of strong oxidants in soil and water can change Cr(III) into harmful Cr(VI). Therefore, it is necessary to remove both the chromium species from aqueous solutions. Thus, the present study pertains to the use of commercially available different organic ion exchangers for the removal of Cr(III) ions from aqueous solutions. The exchangers used are macroporous weak acid exchanger Amberlite.IRC-50 and strong acid exchangers, microporous Amberlite.IR-120 and macroporous Amberlyst-15. The sorption studies are conducted employing the concentration in the range of 0.962-19.231 mmol/L at different temperatures of 293, 313, 323 and 333K. It is observed that Cr(III) sorption increases with increasing concentration, time and temperature of the solution. The selectivity of exchangers towards Cr(III) ions is found to follow the order Amberlyst-15 > Amberlite.IR-120 > Amberlite.IRC-50 which is controlled by the surface morphology, functionality and porosity of the resin matrix and mobility of the exchanging ions. The maximum exchange capacity observed for macroporous Amberlyst-15(H+) is 1.20 mmol/g which increases to 1.31 mmol/g at 333K. All the Na+ forms of the exchangers particularly the weak acid exchanger Amberlite.IRC-50 are found to co-sorb H+ along with Cr(OH)2+ ions. This H+ co-sorption is observed to increase with the increase in temperature and is thus endothermic in nature. The equilibrium data is subjected to the Langmuir equation to determine the maximum exchange capacities (Xm) and binding energy constants (Kb). The Amberlyst- 15 has greatest exchange capacity among the all exchangers due to its porous structure and largest contact area, while the weak acid exchanger Amberlite.IRC-50 has the greatest binding energy constants due to stronger interaction of Cr(III) with the carboxylic groups as compared to sulphonic groups in strong acid exchangers. The thermodynamic parameters (ΔH, ΔS and ΔG) for Cr(III) sorption are also evaluated. The values of both ΔH and ΔS are positive showing that process is endothermic and is accompanied by the dehydration of Cr(III) ions. Further, these values are found to be lower for macroporous Amberlyst-15(Na+) due to the presence of abundant water molecules in the resin matrix. The ΔH and ΔS are linearly related showing the process to be entropy driven ion exchange. The kinetics data and the interruption tests suggest the pre-dominance of particle diffusion mechanism. The macropore diffusion rates are higher than micropore diffusion rates in Amberlyst-15. The activation parameters are calculated by Arrhenius and Eyring equations. The lower activation energy of weak acid exchanger is due to the increased co-sorption of H+ ions at higher temperature which facilitates the dissociation of carboxylic group for Cr(III) binding. The IR and XPS studies confirmed the electrostatic interaction is the mechanism of chromium binding with the ionogenic sites of the exchangers. Both the co-ions and counter-ions are observed to have a profound effect on the removal of Cr(III) ions by the Amberlyst-15(H+). To find out the co-ions effect, Cr(III) sorption is undertaken as a function of time and temperature using CrCl3.6H2O and [Cr4(SO4)5(OH)2] solutions, while for counter ions effects, the sorption on H+, Li+, Na+, Ca2+ and Al3+ forms is investigated. The rate is found to be governed by the particle diffusion for both the co-ions chloride and sulphate and is faster for Cl- solution than SO42-. The exchange capacities are, however, found to be higher for SO42- system than Cl- . It is suggested that in case of Cl- solutions, the metal is exchanged as Cr3+, while in case of SO42- solutions, the metal exchanging specie is CrSO4+. The selectivity of Amberlyst- 15 is observed to follow the order univalent > divalent > trivalent forms which is associated to the electrostatic interaction of ions with the fixed group of the exchanger. The thermodynamic and activation parameters reveal that the mechanism of Cr(III) sorption for all the counter ions is the entropy driven ion exchange. The rate of sorption of three metal ions Cr(III), Ca(II) and Al(III) on Amberlyst- 15(H+) at different temperatures (293, 313 and 333K) is also studied from equimolar mixed system. The selectivity of metal ions is observed to be in the order: Ca(II) > Cr(III) > Al(III). The hydration energy changes of metal ions are playing the dominant role in determining the selectivity of the resin. The kinetic and thermodynamic parameters like activation energy, enthalpy and entropy of activation have also been evaluated and their significance is discussed.