N-Rich Organic Matter–Iron(II) Complexation Drives Efficient Roxarsone Stabilization in New Paddy Soils

Roxarsone (ROX), a widely used organic arsenic feed additive, enters paddy soils through animal manure fertilizer, posing serious risks to environmental and human health. However, the underlying mechanisms of ROX stabilization, particularly the role of soil cultivation duration, remain poorly understood. Here, we compared ROX stabilization in new paddy soil (NPS, 10-year cultivation) and old paddy soil (OPS, 100-year cultivation). NPS exhibited higher ROX stabilization ability than OPS, as evidenced by a 1.5-fold higher adsorption rate constant, a 28-fold higher organic carbon-normalized partition coefficient (K oc ) and retention of 13-fold more arsenic in the residual fraction (non-extractable arsenic). The improved adsorption and stability in NPS were mainly due to complex formation between nitrogen-rich organic matter and Fe(II). In-situ spectroscopic and electrochemical analyses revealed that the complex promote ROX fixation by forming a Fe(II)-ROX-N ternary configuration. This structure was derived from the bidentate binuclear binding of As(V) with iron sites and deprotonated N-containing functional groups. These findings provided mechanistic insights into organic arsenic adsorption in soils and offered theoretical guidance for optimizing soil composition management strategies.