Coupled effects of iron and sulfate reduction on the mobilization, thionation, and sequestration of diphenylarsinic acid in a paddy soil

Purpose Diphenylarsinic acid (DPAA) tends to mobilize in flooded soil due to iron (Fe) reduction and further thionate under sulfate-reducing conditions. Fe and sulfate reduction are natural processes in paddy soil, but their coupled effects on the behavior of DPAA remain unclear. This study investigated the coupled impacts of Fe and sulfate reduction on the mobilization, thionation, and sequestration of DPAA in a paddy soil. Methods Four treatments were prepared in the soil incubation experiment: (1) treatment S-C- was created without the addition of S or C sources; (2) treatment S+C- was only amended with 426 mu g S g(-1); (3) treatment S+C+ was amended with 426 mu g S g(-1) and 1300 mu g C g(-1); and (4) treatment S+C++ was amended with 426 mu g S g(-1) and 2170 mu g C g(-1). Results DPAA was significantly mobilized in treatments S-C- and S+C- due to Fe reduction. Treatments S-C- and S+C- exhibited a more rapid and final enhanced DPAA transformation than did treatments S+C+ and S+C++, with the final transformation rate reaching > 80% and < 15%, respectively, for the former and the latter two treatments, and the major products of this transformation being diphenylthioarsinic acid (DPTAA) and phenylarsonic acid (PAA). It appears that DPAA thionation was favored when sulfide was supplied at a low concentration but consistently, under conditions where no additional C source was added. However, when both Fe and sulfate reduction occurred prominently and simultaneously, Fe(II) produced consumed a significant amount of sulfide to form FeS, which subsequently reduced DPAA thionation under conditions where an additional C source was added. The produced DPTAA (> 95%) was mainly associated with the soil solid phase, and sulfate reduction appears to promote DPAA sequestration on the one hand by enhancing DPAA thionation and hence its sequestration, and on the other hand by producing FeS that sorbs DPAA. Conclusion Our results demonstrate the coupled effects of Fe and sulfate reduction on the mobilization, thionation, and sequestration of DPAA, and that sulfate addition may be a promising strategy to reduce DPAA contamination in Fe-rich paddy soil.

Automated summary

This study investigated the coupled impacts of Fe and sulfate reduction on the mobilization, thionation, and sequestration of DPAA in a paddy soil. The results showed that both Fe and sulfate reduction significantly enhanced the mobilization of DPAA, but sulfate addition can reduce this contamination.