Purpose The mobility of arsenic (As) in soils is fundamentally affected by the clay mineral fraction and its composition. Diphenylarsinic acid (DPAA) is an organoarsenic contaminant derived from chemical warfare agents. Understanding how DPAA interacts with soil clay mineral fractions will enhance understanding of the mobility and transformation of DPAA in the soil-water environment. The objective of this study was to investigate the speciation and sorption structure of DPAA in the clay mineral fractions. Materials and methods Twelve soils were collected from nine Chinese cities which known as chemical weapons burial sites and artificially contaminated with DPAA. A sequential extraction procedure (SEP) was employed to elucidate the speciation of DPAA in the clay mineral fractions of soils. Pearson's correlation analysis was used to derive the relationship between DPAA sorption and the selected physicochemical properties of the clay mineral fractions. Extended X-ray absorption fine structure (EXAFS) L III -edge As was measured using the beamline BL14W1 at Shanghai Synchrotron Radiation Facility (SSRF) to identify the coordination environment of DPAA in clay mineral fractions. Results and discussionThe SEP results showed that DPAA predominantly existed as specifically fraction (18.3-52.8%). A considerable amount of DPAA was also released from non-specifically fraction (8.2-46.7%) and the dissolution of amorphous, poorly crystalline, and well-crystallized Fe/Al (hydr)oxides (20.1-46.2%). A combination of Pearson's correlation analysis and SEP study demonstrated that amorphous and poorly crystalline Fe (hydr)oxides contributed most to DPAA sorption in the clay mineral fractions of soils. The EXAFS results further demonstrated that DPAA formed inner-sphere complexes on Fe (hydr)oxides, with As-Fe distances of 3.18-3.25 Å. It is likely that the steric hindrance caused by phenyl substitution and hence the instability of DPAA/Fe complexes explain why a substantial amount of DPAA presented as weakly bound forms. Conclusions DPAA in clay mineral fractions predominantly existed as specifically, amorphous, poorly crystalline, and crystallized Fe/Al (hydr)oxides associated fractions. Amorphous/poorly crystalline Fe rather than total Fe contributed more to DPAA sorption and DPAA formed inner-sphere complexes on Fe (hydr)oxides.

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.