Molybdenum and Antimony immobilization by iron oxalate salt in contaminated soils

This study introduces iron oxalate dihydrate (FeOx) as an innovative and highly effective treatment for the chemical stabilization of trace levels of Mo and Sb in soils. The effectiveness of the FeOx treatment was evaluated for a Mo-contaminated limestone tunnel sludge (LTS) and Mo/Sb-contaminated industrial wasteland soil (IWS) with organic matter. FeOx reactivity and oxyanion retention were investigated using TGA, XRD, optical microscopy and standardized US/EU leaching tests. Mineralogical changes and the pH-dependence on Mo sorption were further analyzed by geochemical modeling. Under near-neutral pH and oxidizing conditions, FeOx dissociates, releasing Fe 2+ and C 2 O 4 2- ions. The Fe 2+ is subsequently oxidized to Fe 3+ , forming poorly crystallized hydrous ferric oxides (HFOs) that are uniformly distributed. This process induces acidification, partially dissolving calcite and releasing Ca 2+ , which reacts with C 2 O 4 2- to form calcium oxalate monohydrate (whewellite). Mo and Sb immobilization occurs through sorption onto HFOs, achieving effective retention at ≥1 wt.% FeOx for LTS and ≥5 wt.% for IWS, meeting European inert waste standards. The fast transformation of FeOx into HFOs and whewellite enables Mo and Sb retention within 6 hours. Overall, leaching decreased by 67% for Mo in LTS with 2 wt.% FeOx, and by 88% for Sb and 90% for Mo in IWS with 6 wt.% FeOx. Retention stability was observed at pH ≤8, with no significant competition with sulfate or acetate ions. These results highlight FeOx as a promising solution for industrial applications requiring rapid chemical stabilization of contaminants. However, further research is needed to assess long-term stability under variable redox conditions. • Innovative Sb and Mo oxyanion soil treatment using iron oxalate salt • Uniformed dispersion and precipitation of hydrous ferric oxides (HFOs) in the soil • Molybdate and antimonate immobilization by sorption on HFOs • Precipitation of calcium oxalate prevents detrimental effect of counter-ions • Fast and high Sb and Mo retention at acidic-neutral pH suitable for industrial use