Effects of ferric nitrate and ferric sulfate on arsenic immobilization in paddy soils: A comparative study

Arsenic (As) behavior in paddy soils is tightly linked to the redox cycling of electron acceptors such as iron (Fe), nitrate (NO₃^-), and sulfate (SO₄^2-), but the role of counterions (NO₃^- vs. SO₄^2-) in ferric salt-mediated As immobilization remains poorly understood. This study compared ferric nitrate (Fe(NO₃)₃) and ferric sulfate (Fe₂(SO₄)₃) through complementary incubation experiments: paddy flooding simulations and controlled redox gradient tests. Supported by sequential extraction, XRD, and XPS analyses, both ferric salts enhanced soil oxidizing conditions and reduced extractable As concentrations. The reductions ranged from 33.6 to 62.1 % for Fe(NO₃)₃ and 2.1 to 90.3 % for Fe₂(SO₄)₃ with As sequestered into less bioavailable, highly crystalline fractions. Critical differences emerged under controlled redox gradient: Fe(NO₃)₃ stably maintained low porewater As across all redox regimes, whereas Fe₂(SO₄)₃ posed an early As release risk, with concentrations exceeding the control treatment. At equal application rates, Fe(NO₃)₃ outperformed Fe₂(SO₄)₃ by lowering soluble As by 81.9-92.5 % and extractable As by 13.2-69.9 %. Novel mechanistic insights reveal that NO₃^- and SO₄^2- differentially regulate redox status, dissolved Fe dynamics, and mineral-As interactions, which are key drivers of As immobilization efficacy. This work clarifies the overlooked counterion effect, providing a scientific basis for selecting optimal iron-based amendments to mitigate As contamination in paddy ecosystems.