Simultaneous Removal of Anionic and Cationic Species by Ionic Hydrogels via Positively Charged Complex: Adsorption Mechanism of Arsenic(V) and Chromium(III) in Highly Acidic System

Abstract

This study presents a method for efficient simultaneous adsorption by anionic hydrogels via changing the form of cations and anions into the stable positively charged complex rather than chemically modifying the adsorbent materials extensively. The method utilizes ion exchange adsorption mechanism for the simultaneous adsorption in strongly acidic aqueous environment. The effect of mass ratio of the gel in single and binary systems demonstrated the existence of the positively charged complex adsorption in the As and Cr aqueous mixture. The gel exhibits higher adsorption capacities of the binary systems (As > 2.0, Cr > 3.5 mM) than those of the single systems (0, Cr < 2.0 mM). The physicochemical factors, such as pH, initial concentration, time, and coexisting ions, were examined to confirm the key situations affecting adsorption performance. The adsorption capacities of the anionic gel could reach more than 2.0 and 3.5 mmol L −1 for As(V) and Cr(III) in acidic and neutral environments. Gel adsorption behavior, analyzed by fitting data to various adsorption isotherms and adsorption kinetic models, suggests that the adsorption is a homogeneous, chemical adsorption process. Moreover, coexisting metal ions could form positively charged complexes to replace ions on the adsorption sites where the As−Cr adsorbate complex previously existed.