Enhancing the metal ion binding characteristics and reversal of selectivity of crosslinked chitosan sorbents through functionalisation for targeted applications

In this study, we report optimised synthesis of N-carboxymethylated chitosan (CM-Cts) and its crosslinking to obtain, for the first time, glutaraldehyde crosslinked N-carboxymethylated chitosan (CM-Cts-Glu) as a metal ion sorbent. CM-Cts and CM-Cts-Glu were characterised using FTIR and solid state ¹³C NMR techniques. As compared to epichlorohydrin, glutaraldehyde was found to be better suited for efficient synthesis of the crosslinked functionalised sorbent. CM-Cts-Glu showed better metal ion uptake properties compared to the crosslinked chitosan (Cts-Glu). Metal ion removal by CM-Cts-Glu was studied in detail under different conditions such as different initial solution concentrations, pH, presence of complexants and competing ions. Further, sorption-desorption kinetics was studied and it was shown that complete desorption and multiple cycles of reuse without any loss in capacity was feasible. The maximum Co(II) uptake obtained for CM-Cts-Glu was found to be 265 μmol/g, while for Cts-Glu it was 10 μmol/g. Metal ion sorption by CM-Cts-Glu was found to be through chelation by the carboxylic acid functional groups present over the chitosan backbone. Utility of the CM-Cts-Glu under complexing decontamination formulations used in nuclear industry was ascertained. While Cts-Glu generally preferred iron over cobalt under complexing conditions, it was shown that the selectivity was reversed in favour of Co(II) in the functionalised sorbent, CM-Cts-Glu. N-carboxylation followed by crosslinking with glutaraldehyde was found to be a feasible approach for the generation of superior chitosan-based sorbents.