In situ stabilization of antimony and arsenic in co-contaminated soil using organic matter-Fe/Mn (hydr)oxides colloids and their mineral phase transformation.

Natural organic matter (NOM) containing Fe/Mn (hydr)oxides effectively stabilizes antimony (Sb) and arsenic (As) in soils. However, the specific type of NOM that limits the mobility of Fe/Mn (hydr)oxides and how NOM-Fe/Mn colloidal properties can be modulated for better Sb and As stabilization remains unclear. This study suggests that the degree of stabilization of the colloidal structure formed between NOM and Fe/Mn (hydr)oxides is crucial for Sb and As stabilization. It was found that straw-derived (SD), compared to humic acid (HA) with a high content of carboxyl groups, forms more stable colloidal structures with Fe/Mn (hydr)oxides. HA-Fe/Mn colloids show greater mobility and less deposition than SD-Fe/Mn colloids. In soil remediation simulations, SD-Fe/Mn colloids more effectively stabilized Sb and As. After 35 days, SD-Fe/Mn achieved nearly complete stabilization (100 %) of water-soluble and decarbonate-extracted bioavailable fractions at depths of 1-12 cm, with high rates for other fractions as well. Even at depths of 23-34 cm, SD-Fe/Mn outperformed HA-Fe/Mn, showing higher stabilization rates for Sb and As by 12.6 % and 20.4 %, respectively. Morphological analysis suggests that the stabilization of Sb and As by SD-Fe/Mn primarily involves adsorption onto or incorporation within the Fe/Mn (hydr)oxides. This study offers guidance for optimizing NOM-Fe/Mn for in situ stabilization of Sb and As, enhances the understanding of different types of NOM that affect the behavior of Sb and As soil contamination, and presents new perspectives for developing effective in situ remediation materials.