Removal of Cr(VI) using polyaniline based Sn(IV), Ce(IV) and Bi(III) iodomolybdate hybrid ion exchangers: Mechanistic and comparative study

Abstract

The adsorption performance of Cr(VI) on newly synthesized hybrid ion exchangers, polyaniline-Sn(IV) iodomolybdate, polyaniline-Ce(IV) iodomolybdate and polyaniline-Bi(III) iodomolybdate has been studied. The physico-chemical characteristics of the synthesized hybrid ion exchangers viz., structure, surface area, surface morphology, composition and thermal stability were studied by FT-IR, UV–vis, XRD, SEM-EDS, TGA, DSC and XPS studies. Percentage removal of Cr(VI) from aqueous solution is found to increase with initial adsorbent dose, contact time, surface area and decrease with Cr(VI) concentration, temperature and pH. The removal of Cr(VI) is highly pH dependent, maximum at pH 1 and negligible at alkaline pH. A suitable mechanism has been proposed for Cr(VI) removal, involving adsorption of Cr(VI) on hybrid exchangers in a rate controlling step followed by reduction of Cr(VI) to Cr(III). Very low % of Cr(VI) desorption shown by spent hybrid materials with NaOH and NH4OH was explained by the reduction of major portion of adsorbed Cr(VI) to Cr(III) species. The observed order of % removal of Cr(VI) among different iodomolybdate hybrid ion exchangers has been explained by their surface area and surface morphology. Validation of pseudo-first-order, pseudo-second-order and Elovich kinetic models demonstrates that adsorption follows chemisorption process. Among Freundlich, Langmuir and Temkin adsorption isotherm models tested, the adsorption data are not fitted with Freundlich isotherm model. The calculated thermodynamic parameters reveal that the nature of Cr(VI) adsorption is spontaneous and exothermic. No appreciable decrease in Cr(VI) removal efficiency is observed in the presence of other ions like Cu2+, Zn2+, NO3− and SO42-, and with regenerated materials using 2 M HCl even after three cycles. Studies with industrial effluents proved that the synthesized materials can be used for industrial wastewater treatment.