Co-Transport of Cr(VI)/Ni(II) in Solution in Alkaline Soil and their Simultaneous Immobilization

The co-transport of Cr(VI) and Ni(II) in alkaline soil under different soil characteristics and hydrological/hydrochemical variables and immobilization of aqueous and soilborne Cr(VI)/Ni(II) in alkaline soil were investigated. Ni(II) transport was more sensitive to soil particle size and influent pH than Cr(VI). Humic acid (HA) and magnetite (Fe₃O₄) reduced the transport of Ni(II) more obviously, compared with Cr(VI). Reduction and adsorption were responsible for the retention of Cr(VI) in treatments with HA and Fe₃O₄, while complexation and electrostatic attraction dominated in the retention of Ni(II). The increase of IS to 16 mmol/L increased Cr(VI) penetration with 8.8%, and a low pH favored the removal of Cr(VI) but impaired that of Ni(II). Soil barriers spiked with HA + Fe⁰ was efficient in retaining about 93.8% Cr(VI) and 94.3% Ni(II) from influent. The reaction of HA and Fe₃O₄ with Cr(VI) was much slower than Fe⁰ in mixed Cr(VI)/Ni(II) solution. Cr and Ni in co-contaminated soil could be effectively stabilized by 5% Fe⁰ and 5% HA + 5% Fe⁰ groups, converting them from easy-mobile species to more stable species. The leaching experiments of the soil at the 30th day after the remediation showed that there was little amount of Cr, Ni and Fe was washed by deionized water in HA + Fe⁰ treatment. Therefore, soil characteristics and hydrological/hydrochemical variables have significantly different impacts on Cr(VI) and Ni(II) transport in alkaline soil, which should be considered seriously in their risk assessment and the pollution control. Furthermore, HA + Fe⁰ is a promising approach in long-term immobilization of Cr(VI)/Ni(II) in their co-polluted soil.