Mobilization of Colloid- and Nanoparticle-Bound Arsenic in Contaminated Paddy Soils during Reduction and Reoxidation

The association of arsenic (As) with colloidal particles could facilitate its transport to adjacent water systems or alter its availability in soil–rice systems. However, little is known about the size distribution and composition of particle-bound As in paddy soils, particularly under changing redox conditions. Here, we incubated four As-contaminated paddy soils with distinctive geochemical properties to study the mobilization of particle-bound As during soil reduction and subsequent reoxidation. Using transmission electron microscopy–energy dispersive spectroscopy and asymmetric flow field-flow fractionation, we identified organic matter (OM)-stabilized colloidal Fe, most likely in the form of (oxy)hydroxide–clay composite, as the main arsenic carriers. Specifically, colloidal As was mainly associated with two size fractions of 0.3–40 and >130 kDa. Soil reduction facilitated the release of As from both fractions, whereas reoxidation caused their rapid sedimentation, coinciding with solution Fe variations. Further quantitative analysis demonstrated that As concentrations positively correlated with both Fe and OM concentrations at nanometric scales (0.3–40 kDa) in all studied soils during reduction and reoxidation, yet the correlations are pH-dependent. This study provides a quantitative and size-resolved understanding of particle-bound As in paddy soils, highlighting the importance of nanometric Fe–OM–As interactions in paddy As geochemical cycling.