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

The formation of colloid- and nanoparticle-boundAs representsan important geochemical process, which could alter As mobility andbioavailability in paddy environments. The association of arsenic (As) with colloidal particlescouldfacilitate its transport to adjacent water systems or alter its availabilityin soil-rice systems. However, little is known about the sizedistribution and composition of particle-bound As in paddy soils,particularly under changing redox conditions. Here, we incubated fourAs-contaminated paddy soils with distinctive geochemical propertiesto study the mobilization of particle-bound As during soil reductionand subsequent reoxidation. Using transmission electron microscopy-energydispersive spectroscopy and asymmetric flow field-flow fractionation,we identified organic matter (OM)-stabilized colloidal Fe, most likelyin the form of (oxy)hydroxide-clay composite, as the main arseniccarriers. Specifically, colloidal As was mainly associated with twosize fractions of 0.3-40 and >130 kDa. Soil reduction facilitatedthe release of As from both fractions, whereas reoxidation causedtheir rapid sedimentation, coinciding with solution Fe variations.Further quantitative analysis demonstrated that As concentrationspositively correlated with both Fe and OM concentrations at nanometricscales (0.3-40 kDa) in all studied soils during reduction andreoxidation, yet the correlations are pH-dependent. This study providesa quantitative and size-resolved understanding of particle-bound Asin paddy soils, highlighting the importance of nanometric Fe-OM-Asinteractions in paddy As geochemical cycling.