The regulation of Cd and As behavior by the iron oxide-organic matter interaction: The simultaneous immobilization mechanism of coal gasification slag

Cadmium (Cd) and arsenic (As) co-contamination in agricultural soils is challenging to remediate due to their opposing geochemical behaviors. Although iron-modified gasification slag (IGS) can simultaneously adsorb Cd and As, its mechanisms in inhibiting the migration of Cd and As within the soil-plant system, as well as its interactions with dissolved organic matter (DOM), remain unclear. Through a pot experiment, it was demonstrated that the application of IGS significantly reduces the bioavailability of Cd from 0.40 mg/kg to 0.18 mg/kg and As from 12.66 mg/kg to 6.49 mg/kg, while promoting their transformation into more stable soil fractions. Consequently, IGS effectively blocked Cd and As transformation, reducing Cd and As concentrations in plant leaves from 0.96 mg/kg and 0.37 mg/kg to 0.12 mg/kg and 0.14 mg/kg, respectively. Mechanistic analysis revealed that the oxidation and dissolution of zero-valent iron in IGS drives the formation of active iron oxide phases. These new oxides serve as primary sinks, immobilizing Cd and As through adsorption and incorporation. Furthermore, IGS altered soil DOM, reducing its total content and specific fulvic/humic components, as characterized by excitation-emission matrix fluorescence spectroscopy. This reduction is attributed to the formation of iron oxide-DOM complexes, which diminishes the pool of ligands available for metal mobilization, thereby enhancing the net sequestration of Cd and As. Our findings elucidate the coupled Fe-DOM processes underpinning IGS efficacy, advancing its potential application for the simultaneous remediation of Cd and As. • Proposed a novel strategy of utilizing iron-modified industrial waste to co-immobilize Cd and As in soil. • Unravel the synergistic mechanism of suppressing Cd/As bioavailability via driven iron cycling and DOM transformation. • Demonstrated effective reduction of Cd/As bioavailability in soil and their plant accumulation.