Design of porous strong base anion exchangers bearing N,N-dialkyl 2-hydroxyethyl ammonium groups with enhanced retention of Cr(VI) ions from aqueous solution

Porous strong base anion exchanger (SBAEx) microspheres bearing either N,N-dimethyl 2-hydroxyethyl ammonium, or N,N-diethyl 2-hydroxyethyl ammonium functional groups attached to styrene-divinylbenzene matrix were comparatively used as sorbents for the removal of Cr(VI) oxyanions from aqueous solutions, in batch mode. The remarkable retention capacity of Cr(VI) ions was slightly influenced by the solution pH, when pH varied in the range 3-6, this being very convenient for the applications of these sorbents. Sorption kinetics results at 25 °C, fitted with pseudo-first order, pseudo-second order (PSO), Elovich, and intra-particle diffusion models evidenced that the sorption of Cr(VI) onto SBAEx obeyed the PSO kinetic model, and this supports chemisorption as the main mechanism of sorption. The experimental data for the Cr(VI) sorption at equilibrium were fitted with five isotherm models, Langmuir, Freundlich, Dubinin-Radushkevich, Temkin, and Sips, and found that the Langmuir, Temkin and Sips isotherms described very well the sorption process. The maximum sorption capacity for Cr(VI) oxyanions, evaluated by the Langmuir isotherm, ranged from 278.2 mg/g to 323.7 mg/g, being higher when the functional groups were N,N-dimethyl 2-hydroxyethyl ammonium chloride than in the case of N,N-diethyl 2-hydroxyethyl ammonium chloride. The negative values of ΔG° showed that sorption process was spontaneous and thermodynamically favorable, while more negative values of ΔG° in the case of the SBAEx having methyl substituents indicated the preference of Cr(VI) oxyanions for this sorbent, at each temperature. A small decrease of the sorption capacity of Cr(VI) after five sorption/desorption cycles was observed, supporting the great potential of these SBAEx in the removal of Cr(VI) oxyanions.