Abiotic As(III) oxidation by hydrated Fe(III) oxide (HFO) microparticles in a plug flow columnar configuration

Scientific evidence pertaining to selective sorption of As(V) species or arsenates and As(III) species or arsenites onto Fe(III) oxide microparticles is widely available in the literature. Practical implications of this phenomenon are well recognized for both natural and engineered systems. The previous investigations to this effect have clearly established that such selective sorption processes are accompanied by formation of inner-sphere complexes between the surface functional groups of Fe(III) oxides and As(V) or As(III) species in question. Although thermodynamically favorable, oxidation of As(III) or arsenite by Fe(III) oxides has not been reported to date. Experimental studies for all such investigations were, however, carried out in continuously stirred batch reactors. The results of the present study confirm that, in a plug flow configuration with stationary Fe(III) oxide microparticles in a column, aqueous-phase As(III) undergoes near-complete conversion to As(V) at neutral to slightly alkaline pH. The stationary phase consists of porous polymeric particles within which submicron hydrated Fe(III) oxide particles have been irreversibly dispersed. As the mobile liquid phase slowly percolates through the stationary column, each As(III) solute progressively binds to a multitude of Fe(III) sorption sites favoring As(III) oxidation. The resulting As(V) and Fe(II) are subsequently sorbed onto iron oxide particles. The plug flow configuration of the system allows thousands of contacts with Fe(III) sorption sites for each As(III) molecule, thus enhancing As(III) oxidation in accordance with law of mass action effect. Also, the absence of Fe(II) in the aqueous phase offers highest possible oxidizing environment near the HFO sorption sites. The reactor configuration approximating plug flow is postulated to be the major contributor for As(III) oxidation by Fe(III) oxide microparticles. This observation may have major ramifications for arsenic-containing groundwater percolating through iron-rich soil. At acidic pH, conversion of As(III) to As(V) is, however, much less pronounced due to unfavorable thermodynamics.