Assessment and optimization of As(V) adsorption on hydrogel composite integrating chitosan-polyvinyl alcohol and FeO nanoparticles and evaluation of their regeneration and reusable capabilities in aqueous media.

A modified chitosan-polyvinyl alcohol (PVA) hydrogel was developed by incorporating FeO nanoparticles. Four chitosan-FeO (ChFe) hydrogel types were developed based on chitosan:FeO ratio as 1:0, 1:1, 1:0.5 and 1:0.25. Batch sorption experiments were conducted with different pH, dosage, kinetics, and isotherms. The exhausted ChFe hydrogels were evaluated for their regeneration and reuse capability with different acids and bases. The best hydrogel for arsenic (V) [As(V)] adsorption was 1:0.5 ratio ChFe hydrogel. The highest As(V) adsorption (89 %) was at pH 4 and the adsorption capacity gradually decreased with increasing solution pH. Within the pH 4-6 range, the hydrogel surface became positively charged due to protonation of NH and OH groups in the polymer chain and the positive surface attracted HAsO and HAsO oxyanions. The experimental kinetic data was well-fitted to the Elovich model (R of 0.99) while the Freundlich isotherm model best described the isotherm data (R of 0.97). The models predicted chemisorption mechanisms on ChFe hydrogel composites. Electrostatic attractions with NH and OH, ligand-exchange inner-sphere complexes formation and bidentate corner-sharing (C) and bidentate edge-sharing (E) trimetric surface complexes formation have been proposed as the adsorption mechanism of As(V) into ChFe hydrogel. 0.1 M CHCOOH showed the best regeneration pattern with 75, 96, 81, 53 and 43 % of 1, 2, 3, 4 and 5 adsorption respectively. Because of this re-usable capability, the As(V) adsorption capacity is not a single value from one adsorption cycle, but a cumulative value of several adsorption cycles and it was 17.39 mg/g for five adsorption cycles. Open for regeneration and reuse, no post-treatment is needed for adsorbent-water separation, allow applications of the ChFe hydrogel composite in a wide range of applications such as water filtration and purification systems. The modification with ChFe further expands the application capacity since the ChFe can remove other contaminants as well.