Ultrasonic-Assisted Soil Washing with Cysteine for Remediation of Heavy Metal-Contaminated Soil: Efficiency, Speciation Transformation and Selective Mechanisms

In order to identify an efficient and environmentally friendly washing agent for heavy metal-contaminated soil, this study selected seven natural amino acids—arginine (Arg), alanine (Ala), glycine (Gly), cysteine (Cys), lysine (Lys), threonine (Thr), and glutamic acid (Glu)—based on their water solubility, effectiveness, and functional group characteristics. According to the removal efficiencies for zinc (Zn), lead (Pb), and cadmium (Cd), Cys, which contains a specific sulfhydryl group (-SH), was chosen as the target leaching agent for the remediation of composite-contaminated soil. The optimal process conditions were determined as follows: 0.02 mol/L of cysteine concentration, liquid-to-soil ratio of 20:1 (mL/g), 10 min of ultrasonic time, and pH = 8.0. The order of removal efficiency was Pb (40.8%) > Zn (21.6%) > Cd (19.9%). The leaching process selective effects on the speciation fractions of Zn, Pb, and Cd in the soil, and these differences can be explained by the hard–soft acid–base theory and the strength of coordination between the metals and cysteine. Mechanism analysis revealed that soil washing essentially achieves selective extraction of the target metal through strong chemical interactions between functional groups of cysteine and active metal sites of secondary minerals in the soil. Cysteine is a green remediation agent with high selectivity and environmental compatibility for contaminated soil. Its application requires precise design and risk assessment based on the chemical properties of the target metals, while ensuring the sustainability of the soil to maintain the ecological functions and long-term health of the remediated soil.