Crosslinked poly(ionic liquid)s as selective receptors for Cr(VI) – Counter anion effect and application in treating drinking water and tannery effluents

Abstract

A series of crosslinked poly(ionic liquid)s (C-PILs), with five different counter anions, were synthesised and investigated for their Cr(VI) binding properties. C-PIL with chloride as the counter anion (C-PIL-Cl) showed the highest sorption capacity (2.96 ± 0.03 mmol/g), while bis(trifluoromethanesulfonyl)imide led to minimum uptake. The chromium binding properties of C-PIL-Cl were investigated in detail, which revealed remarkable Cr(VI) selectivity against other anions. It could reduce the Cr(VI) concentration from 980 ppb to 28 ppb in a typical drinking water sample, which is below the WHO prescribed acceptable limit. The polymer could also remove about 90% of Cr(VI) from synthetic tannery effluent. SEM-EDX studies clearly revealed the chromium uptake and regeneration of the chromium sorbed polymer to be through anion exchange at the binding sites. Quantitative elution of the bound chromium from C-PIL-Cl was feasible with 1 M NaOH and 1 M NaCl solution. While the polymer regenerated with NaCl showed sorption properties similar to the virgin polymer, regeneration with NaOH led to reduced capacity. Microscopic investigations showed no effect on polymer morphology during desorption. The sorption kinetics followed pseudo second order and showed poor fit with pseudo first order model.