Microscopic mechanisms for the effects of ionic strength and DOM on the adsorption and transport of cadmium in red soil

DOM C fraction and KCl influence Cd adsorption by altering electrostatic interaction. • K⁺ competes with Cd²⁺/CdCl + for mineral sites, but not for organic functional group. • HA C has more lignin-like, condensed aromatics but less tannin-like compounds than FA C . • HA C has more electron-rich groups than FA C , boosting Cd adsorption stronger. • DOM C exhibits inadequate performance for Cd immobilization in saline red soils. Dissolved organic matter (DOM) can affect the fate of cadmium (Cd) in soils, yet the mechanisms governing its impact on the adsorption and transport of Cd under varying ionic strength remain unclear. Here, compost-derived DOM (DOM C ) was extracted and fractionated into humic acid (HA C ) and fulvic acid (FA C ) to examine their effects on the mobility of Cd in red soil at varying potassium chloride (KCl) concentrations. DOM C fractions enhanced Cd adsorption, with HA C showing a stronger effect than FA C , due to the higher abundance of lignin-like compounds and condensed aromatics in HA C . The Cd mobility in red soil in the presence of HA C exceeded that of FA C at 1 mmol/L KCl due to the stronger ability of HA C to interact with Cd. DOM C exhibited a weakening inhibitory effect on Cd mobility at elevated KCl concentrations, and the difference in inhibitory effects between HA C and FA C diminished. The effects of increasing KCl concentration on Cd mobility contrast sharply with those of DOM C and underscore the dominance of electrostatic interactions in controlling Cd mobility in the presence of DOM C . Density functional theory calculations suggested that the interacting ability of HA C with Cd was higher than that of FA C , because the number of aromatic rings, carboxyls, and hydroxyls in HA C are higher than those in FA C . K + is more likely to compete with Cd 2+ for adsorption sites on kaolinite and hematite rather than for DOM functional groups. Our findings demonstrate the combined effects of DOM C and KCl on Cd mobility and highlight that DOM C may exhibit limited effectiveness in immobilizing Cd in saline red soils. Download: Download high-res image (373KB) Download: Download full-size image