Contrasting effects of iron reduction on thionation of diphenylarsinic acid in a biostimulated Acrisol

Diphenylarsinic acid (DPAA) is an emerging phenylarsenic compound derived from chemical warfare agents. It has been suggested that biostimulation of sulfate reduction decreases the concentrations of DPAA in soils. However, biostimulation often induces Fe(III) reduction which may affect the mobility and thereby the transformation of DPAA. Here, a soil incubation experiment was carried out to elucidate the impact of Fe(III) reduction on the mobilization and transformation of DPAA in a biostimulated Acrisol with the addition of sulfate and lactate. DPAA was significantly mobilized and then thionated in the sulfide soil (amended with sulfate and sodium lactate) compared with the anoxic soil (without addition of sulfate or sodium lactate). At the start of the incubation period, 41.8% of the total DPAA in sulfide soil was mobilized, likely by the addition of sodium lactate, and DPAA was then almost completely released into the solution after 2 weeks of incubation, likely due to Fe(III) reduction. The relatively low fraction of oxalate-extractable Fe in Acrisol, which contributes significantly to DPAA sorption and is more active and reduction-susceptible, may explain the observation that only < 40% of the Fe(III) (hydr)oxides were reduced when DPAA was completely released into the solution. A more rapid and final enhanced elimination of DPAA was observed in sulfide soil and the fraction of total DPAA decreased to 60.1 and 91.0%, respectively, at the end of the incubation in sulfide soil and anoxic soil. The difference appears to result from increased DPAA mobilization and sulfate reduction in sulfide soil. On the other hand, the formation of FeS precipitate, a product of Fe and sulfate reduction, may reduce the efficiency of DPAA thionation. Accordingly, the potentially contrasting effects of Fe(III) reduction on DPAA thionation need be considered when planning biostimulated sulfate reduction strategies for DPAA-contaminated soils.