Influence of phosphate and natural organic matter on arsenic solubility in paddy soils: Insights from the NOM-CD model

The contamination of paddy soils with arsenic (As) poses a major threat to ecological systems and food safety due to its toxicity and redox-sensitive nature. In this study, effects of phosphate (PO₄³⁻), humic/fulvic acids (HA/FA), and calcium (Ca²⁺) in regulating As solubility in six paddy soils were investigated by integrating batch adsorption experiments with the Natural Organic Matter-Charge Distribution (NOM-CD) modeling. Results indicated that elevated Ca²⁺ concentrations enhanced both As(V) and PO₄³⁻ adsorption by increasing positive surface charge density on Fe/Al hydroxides. With addition of 20 mmol kg⁻¹ PO₄³⁻, the concentration of As(V) in solution was 2.56 to 14.40 times higher than that without added PO₄³⁻ across the six soils. The presence of 3 g kg⁻¹ HA/FA increased soluble As(V) by a factor of only 1 to 1.79. This difference was mainly attributed to PO₄³⁻ preferential occupation of high-affinity binding sites, facilitation of monodentate complex of adsorbed phosphate, and modulation of surface charge density on metal (hydr)oxide surfaces. The presence of HA and FA increased As(III) mobility by ∼7.08-fold, primarily by stimulating the (bio)reduction of both iron (hydr)oxides and As(V) in the soil. Addition of PO₄³⁻, however, demonstrated dual regulation: improving As(III) mobilization under reducing conditions, while inhibiting soluble As(III) concentrations in aerobic soils. The NOM-CD model indicated that the mobility of As was governed by a hierarchical mechanism involving competitive adsorption, microbial-redox feedback, and ligand loading thresholds. These findings advance predictive understanding of As behavior in rice agroecosystems, offering a quantitative framework to optimize phosphorus fertilization and organic amendments while mitigating As bioavailability.