Remediation of Heavy Metal-Contaminated Soils in Alpine and Arid Mining Regions: Synthesis and Performance of a Multifunctional Amendment

Abstract

Heavy metal (HM) contamination, freeze–thaw (F-T) cycling, and dry, nutrient-poor conditions jointly degrade the soils of lead–zinc mining areas on the Qinghai–Tibet Plateau (QTP) and severely inhibit plant growth. To address these problems, a multifunctional inorganic–organic composite amendment was developed from locally sourced solid wastes. The composite amendment prepared at the optimal mass ratio of diatomite, biochar, gypsum, magnesium hydroxide, and soil (5:6:1:1.45:103, designated as EK) significantly improved soil physicochemical properties: water saturation increased by 14.71%, whereas daily evaporation decreased by 23.33%. In addition, the cation exchange capacity (CEC), phosphorus retention, and potassium retention increased by 44.88%, 20.34%, and 100.70%, respectively. Under F-T conditions, the modified soil showed a two- to three-fold improvement in both adsorption capacity (Qe) and removal efficiency (R) for different HMs in single- and multi-metal systems when compared to the control (CK). The removal efficiency of Pb2+ was especially high, at over 80%. Isotherm analysis showed that the experimental adsorption data for Pb2+ and Zn2+ were well described by the Freundlich isotherm model, suggesting that the adsorption mainly occurred on energetically heterogeneous surfaces. The kinetic studies showed that the adsorption of Pb2+ followed the pseudo-second-order model, while that of ions with smaller ionic radii (Zn2+, Cu2+, Cd2+) was better fitted by the pseudo-first-order model. The effect of EK on plant growth was further investigated in a pot experiment with rye. EK improved rye growth even under the combined stresses of F–T cycling and HM contamination, demonstrating its application potential for soil remediation in lead–zinc mining areas of the QTP.