Synthesis optimization and X-ray absorption spectroscopy investigation of polymeric anion exchanger supported binary Fe/Mn oxides nanoparticles for enhanced As(III) removal

Abstract

A macroporous anion exchange resin containing high density of positively-charged quaternary ammonium functional groups and supporting binary Fe/Mn oxides nanoparticles (HA502P-Fe/Mn) was synthesized for simultaneous As(III)oxidation and As(V) adsorption within a single bead. Various preparation parameters were investigated and optimized to achieve the highest As(III) removal efficiency, including metal oxides type, supporting materials, preparation solvents, and number of loading cycles. The results from five imaging techniques (SEM-EDX, TEM, XRD, XANES, EXAFS) confirmed that amorphous ferric and manganese oxides nanoparticles were successfully dispersed throughout the matrix of the parent macroporous anion exchanger. The changes in oxidation state of the Mn oxides from +4 to +2 and + 2 to +4 were observed by XANES during As(III) removal and regeneration runs, respectively. The EXAFS was used to investigate the neighborhood atoms surrounding arsenic during As(III) removal by HA502P-Fe/Mn. The bond distance between AsFe and AsMn of HA502P-Fe/Mn are 3.35 and 2.94 Å, respectively, suggesting that As(III) was oxidized to As(V) and adsorbed on the surface of Fe(III) and Mn(IV) oxide nanoparticles by formation of inner-sphere complex. As(III) was oxidized by MnO2 nanoparticles under the absence of oxygen environment and only As(V) was observed after adsorption. The equilibrium As(III) sorption isotherms, effect of pH, competing ions, and NOM were also evaluated. Fixed-bed column experiments using NSF challenge water standard 53 (300 μg As(III)/L, pH 6.5) were carried out and found that the material can remove As(III) equivalent to 4300 bed volumes (BVs) compared with 2000 BVs using commercial arsenic selective material (Layne-RT). The exhausted material can be regenerated using 10 BVs of regeneration solution and 98% of adsorbed arsenic can be recovered.