Iron and phosphate species regulates arsenic speciation and potential mobility in contaminated soils

Arsenic in soils poses a high environmental risk. The understanding of arsenic geochemical speciation, mobility, and other potential factors in contaminated soils is crucial for appropriate remediation strategy development and environmental assessment. The objective of this study was to investigate the arsenic oxidation state and its form in each step of sequential extraction applied to different types of contaminated soils, and to analyze the impact of sequential extraction forms of soil Fe and phosphate. Soil samples were collected from three agricultural regions: acid mine drainage (AMD)-impacted red soils (n = 5, 61.1–248.6 mg As/kg) and As-contaminated groundwater-impacted soil including yellow soils (n = 6, 23.2–32.1 mg As/kg) and chestnut soils (n = 5, 9.0–13.3 mg As/kg). The results of sequential extraction revealed that As was primarily associated with Fe(III) oxyhydroxides. The highest proportion of amorphous Fe(III) oxyhydroxide-bound As was observed in the southern red soils, which was attributable to the coprecipitation/immobilization of high Fe and As concentration levels in AMD during irrigation. The amount of adsorbed As (mass fraction) increased linearly with increasing amounts of As and is related to the presence of both amorphous and crystalline Fe phases in the soils. This demonstrates the immobilization role of reactive Fe phases in controlling the potential mobility of As in contaminated soils impacted by As-contaminated groundwater and AMD. Soil phosphate, with mass concentrations 2–4 orders of magnitude higher than those of As, occupied most of the Fe(III) oxyhydroxide reactive sites. Phosphate-extractable As was 4.3–80.7 mg/kg, accounting for 18.3–76.0 % (median of 33.5 %) of total As, indicating the competitive effect of phosphate on the desorptive release of As. The AMD-impacted paddy soil exhibited much higher proportions of phosphate-extractable As and a predominance of As(III) in the water-soluble extract, revealing the high potential mobility and toxicity of As in flooded soil. The dominant occupation of Fe adsorption sites by soil phosphate likely contributes to low efficiency of soil Fe in immobilizing As. To reduce As mobility, it is imperative to develop future strategies for phosphates used as sustainable fertilizer to support crop culture.