Comparison of U(VI) adsorption onto nanoscale zero-valent iron and red soil in the presence of U(VI)–CO₃/Ca–U(VI)–CO₃ complexes

Highlights: • NZVI can be used for adsorbing U(VI)–CO₃ complexes. • Use of NZVI is feasible for remediation of uranium-contaminated soils. • The mechanism of U(VI)–CO₃ complexes adsorbing onto NZVI has been explained. - Abstract: The influence of U(VI)–CO₃ and Ca–U(VI)–CO₃ complexes on U(VI) adsorption onto red soil and nanoscale zero-valent iron (NZVI) was investigated using batch adsorption and fixed-bed column experiments to simulate the feasibility of NZVI as the reactive medium in permeable- reactive barriers (PRB) for in situ remediation of uranium-contaminated red soils. The adsorption capacity (q_e) and distribution constant (K_d) of NZVI and red soil decreased with increasing pH, dissolved carbonate and calcium concentrations, but the q_e and K_d values of NZVI were 5–10 times higher than those of red soil. The breakthrough pore volume (PV) values increased with the decrease of pH, dissolved carbonate and calcium concentration; however, the breakthrough PV values of the PRB column filled with 5% NZVI were 2.0–3.5 times higher than the 100% red soil column. The U(VI)–CO₃ complexes adsorbed onto the surface of red soil/NZVI (≡SOH) to form SO–UO₂CO₃⁻ or SO–UO₂ (CO₃)₂³⁻. XPS and XRD analysis further confirmed the reduction of U(VI) to U(IV) and the formation of FeOOH on NZVI surfaces. The findings of this study are significant to the remediation of uranium-contaminated red soils and the consideration of practical U(VI) species in the natural environment.