Organosilicon-mediated interfacial behavior of cadmium on ultisols clay colloids: Kinetics, thermodynamics, and complexation mechanisms for soil remediation

The growing environmental prevalence of liquid organosilicon compounds necessitates a deeper understanding of their soil chemical behaviors and underlying mechanisms. This study systematically investigated the kinetic and thermodynamic characteristics of cadmium (Cd) adsorption by Ultisols clay colloids modified with three typical liquid organosilicons: dimethyldichlorosilane (DMDCS), dodecamethylcyclohexasiloxane (D6), and 3-(methacryloyloxy)propyltrimethoxysilane (KH-570), with comprehensive evaluation of the impacts of pH, ionic strength, coexisting ions, and humic acid (HA) on the adsorption process. Key findings revealed a maximum Cd adsorption capacity of 4150.81 mg/kg by the unmodified clay colloids, with DMDCS reducing adsorption by 4.18 % while D6 and KH-570 enhanced it by 5.38 % and 3.45 %, respectively, and all three compounds retarding the adsorption kinetics; D6 significantly promoted Cd adsorption at higher pH (≥6.0) whereas DMDCS and KH-570 showed negligible pH-dependent effects; high ionic strength (0.1 mol/L NaCl) strongly inhibited Cd adsorption though HA mitigated DMDCS-induced reduction by forming soluble Cd-HA complexes. Mechanistic insights indicated a two-step adsorption process—initial rapid electrostatic attraction and ion exchange with clay-bound cations (e.g., Al 3+ , Fe 3+ ), followed by slow diffusion of Cd 2+ into interlayers and surface complexation with hydroxyl groups—with organosilicon compounds enhancing adsorption via Si O bond hydroxylation to increase reactive site density and formation of stable surface complexes (e.g., Si-O-Cd 2+ /Si-O-Cd(OH) + ). These results highlight the critical role of organosilicon chemistry in modulating heavy metal retention in acidic soils, offering novel perspectives for designing efficient soil passivators to mitigate Cd contamination in Ultisols and analogous environments.