Immobilization effects and remediation mechanisms of montmorillonite-based amendments for Hg(II) contamination in soil-pakchoi (Brassica chinensis L.) system

Mercury (Hg) contamination in vegetables has emerged as a critical challenge globally, due to its neurotoxicity and irreversible health impacts. To effectively reduce Hg(II) exposure risk associated with vegetables intake, remediation measures should be taken urgently, especially for the severely polluted scenarios. In this study, the remediation performance and mechanism of montmorillonite (Mt) and Mt-based amendments (ISH-Mt, GSH-Mt and BSH-Mt) for pakchoi planted on heavily Hg(II)-contaminated soil (5.60 mg kg⁻¹, threefold of screen value of GB 15618-2018) were investigated. Compared to CK, soil application of Mt-based amendments significantly decreased Hg(II) concentration in pakchoi shoot and root by 68.00 %-74.23 % and 57.3 %-62.8 %, respectively, of which Hg(II) concentration in edible shoot was below the Chinese standard of fresh vegetables (10 μg kg⁻¹, GB 2762-2022). Soil available Hg(II) extracted by HCl and Na₂S₂O₃ under Mt-based amendments treatments were significantly decreased by 58.40 %-66.34 % and 80.31 %-98.66 %, compared with CK (control group), respectively, which is responsible for the inhibition of Hg(II) translocating from soil to pakchoi plant as the conversion of soil liable Hg(II) species into stable and unavailable Hg(II) species. Meanwhile, the alleviation of Hg(II) toxicity on soil-pakchoi system under Mt-based amendments remediation improved promoted plant growth and soil quality, characterized by enhancement in enzymes activities (47.61 %-62.61 % of urease, 26.69 %-44.02 % of sucrase, 15.45 %-31.20 % of catalase, and 17.67 %-28.07 % of arylsulfatase) and shoot biomass (37.27 %-63.68 %). This study provides an effective and sustainable alternative solution for the remediation of heavily Hg(II) contamination in soil-vegetables system, with simultaneously improvement on plant growth and soil quality.