The dual role of oxygen in redox-mediated removal of aqueous arsenic(III/V) by Fe-modified biochar.

The Fe-modified biochar (FeBC) was used to remove aqueous As(III/V), and the role of oxygen (O2) in As removal was investigated by integrating aqueous and solid analyses. The removal efficiencies for As(III) and As(V) increased from 86.4% and 99.2% under anoxic conditions, respectively, to >99.9% when O2 was available. FeBC removed As(III) from As(III)-spiked systems by surface-oxidation following adsorption, where oxidation of As(III) was promoted by O2. As(V) was first reduced, re-oxidized in solutions, and then adsorbed to FeBC in As(V)-spiked systems, where reduction of As(V) was inhibited at the presence of O2. Both As(III) and As(V) were bidentate corner-sharing complexed to Fe oxides/hydroxides on FeBC, with As coordinated to Fe at ~3.4 Å according to As extended X-ray absorption fine structure (EXAFS) modeling. These findings identified the effect of ambient O2 in As(III/V) redox transformations and removal, guiding the further application of FeBC in environmental treatment.