Soil flushing coupled with aminated-nanocellulose/MOF hydrogel nanocomposites adsorbents: A novel sustainable remediation strategy for Cr(VI)-contaminated agricultural soils

Soil flushing and adsorption-based soil remediation are promising options for removing heavy metals in agricultural soil. However, they encounter limitations in low removal efficiency, secondary pollution potential, or the need for a purification/recovery process. Optimizing flushing solutions or the functional properties of adsorbents could fundamentally eliminate the drawbacks of these methods. Yet, their practical application remains a formidable challenge. Herein, a new combined remediation strategy of multiple-pulse water soil flushing (MF) with novel aminated cellulose nanofibers (A-NFCs)/AEM/AM@MIL-100(Fe) nanocomposite hydrogel adsorbent (ANCMH) was adopted to remove Cr(VI) from agricultural soil. The characteristic analyses approved Cr(VI) adsorption-coupled reduction/swelling-based mechanisms on ANCMH with the maximum theoretical adsorption capacity of 338.24 mg g⁻¹ at pH 6.8 governed by pseudo-second-order and Freundlich models. Instead of conventional MF application using environmentally problematic extractant fluids, a 5-day soil column simulation trial of MF coupled with ANCMH adsorbent layers (MF@ANCMH) was applied in medium-to-highly Cr(VI)-contaminated soil followed by easy ANCMHs separation, preventing secondary pollution. Innovatively, the water flushing-stop flux events favorably decreased the pH of neutral soil by 0.83 folds and allowed Cr(VI) release from the soil matrix to be adsorbed by ANCMHs, achieving a high Cr(VI) removal efficiency of 98.9 %. Compared to the soil treated with ANCMH individually and soil flushed with water or Na₂EDTA, the MF@ANCMH-treated soil reached the corresponding regulation of permissible levels for Cr(VI) residential scenarios in agricultural soil of 5.6 ppm with 3-cycles of ANCMH reuse. The residual Cr in MF@ANCMH-remediated soil displayed minimal bioavailability at root-zoon soil of ≤4.2 ppm and a 97.66 % reduction in its exchangeable-carbonate fraction compared to untreated soil, corresponding to a risk assessment code score below 1 %. Also, the application of MF@ANCMH resulted in Cr bioaccumulation below 0.1 ppm in cultivated wheat plants, prevented the physiological phytotoxicity symptoms, and elevated their antioxidant enzyme activity, photosynthesis, growth, and yielding indicators compared to that grown in untreated soil. Furthermore, it improved soil organic matter, microbial community, diversity, and microecological functions. This study provides a promising alternative remediation strategy for neutral soil that can maintain soil/crop quality and soil health with high chemical/environmental stability of pollutants after remediation, demonstrating its sustainability and feasibility in agricultural soil.