Arsenic speciation, oxidation and immobilization in an unsaturated soil in the presence of green synthesized iron oxide nanoparticles and humic acid

While the availability of arsenic (As) in soil is well known to be highly correlated with the presence of iron (Fe) oxides and humic acid (HA) in the soil, the relationship between Fe oxides and HA and As species in the soil is less well understood. In this study, As speciation in an unsaturated soil in the presence of external HA and green synthesized Fe oxide nanoparticles (FeNPs) showed that As(V) was mainly distributed to the specifically-bound (F2), amorphous and poorly-crystalline hydrous oxides of Fe, Al (F3) and the well-crystallized hydrous oxides of Fe and Al (F4). While As(III). This was the major component in unsaturated soil, and was mainly distributed to F4 and the residual fraction (F5). As bound to F3 and F5 was most sensitive to the addition of HA and FeNPs, while HA/FeNPs treatment increased the F3-bound As(V); however, it decreased the F5-bound As(III). Nonetheless the effect of HA on As is completely different to the HA/FeNPs treatment. The increase of As(V) in F3 resulted from F5-bound As(III) oxidation when treated by HA/FeNPs. Cyclic voltammetry confirmed that HA and Fe3+/Fe2+ redox enhanced As(III) oxidation, while FTIR revealed that HA-bound As(III) was the least available fraction in the soil. Finally, a mechanism involving a combination of HA and FeNPs was proposed for explaining the redistribution of As species in the soil.

Automated summary

This study found that the availability of arsenic (As) in soil is closely related to the presence of iron (Fe) oxides and humic acid (HA). However, the relationship between Fe oxides and HA and As species is not well understood. The experiment showed that As(V) is mainly distributed to certain bound forms of Fe and Al oxides, while As(III) is mainly found in the residual fraction. The addition of HA and green synthesized Fe oxide nanoparticles (FeNPs) affected the distribution of As species in the soil, with HA/FeNPs treatment increasing As(V) in certain bound forms and decreasing As(III) in the residual fraction. The study proposed a mechanism involving the combination of HA and FeNPs to explain the redistribution of As species in the soil.