Different contributions of crystalline and non-crystalline iron (hydr)oxides on the mobilization and thionation of diphenylarsinic acid in a flooded paddy soil

Iron reduction impacts the mobilization and thionation of diphenylarsinic acid (DPAA) in soil, but the contribution of crystalline and non-crystalline iron remains unknown. A paddy soil deficient in non-crystalline iron (P-Fe o ), crystalline and non-crystalline iron (P-Fe d ) were incubated with sulfate-plus-lactate, and the results were compared with paddy soil (P) in our previous study. For treatments without ferrous sulfide (FeS) precipitation, the solution-to-solid ratio ( R L/S ) of DPAA increased slightly and dramatically with iron reduction, respectively, for P-Fe o and P, suggesting that the reduction of non-crystalline iron contributes more to DPAA mobilization than crystalline iron. The R L/S was > 0.667 and ≤ 0.667 for treatments without and with FeS, respectively, in P and P-Fe o but not P-Fe d , indicating that crystalline, rather than non-crystalline iron contributes more to DPAA sequestration through reduction and the subsequent FeS precipitation. The degree to which iron reduction facilitates DPAA mobilization or sequestration is likely determined by the composition and amount of iron in paddy soil, respectively. For treatments with FeS, the DPAA transformation rate was < 44 % and 74–86 %, respectively, in P-Fe o and P-Fe d after 90 days, with the maximum value comparable to those without FeS in P after 30 days. This suggests that the crystalline iron in P-Fe o and the residual, small amount of non-crystalline iron in P-Fe d could reduce DPAA thionation by forming FeS, and such effect seems to depend on both the composition and the amount of iron in paddy soil. The results would deepen our understanding of the “As-Fe-S” interactions in soil.