Surface-loaded magnesium and phosphorus-modified lignite adsorbents: Efficient adsorption and immobilization for remediation of Cd-contaminated water and soil

Lignite is a resource-rich material with favorable adsorption properties and can be used as an environmentally friendly material for removing heavy metals. In this study, we loaded magnesium and phosphate ions onto the surface of lignite (LM) by using chemical modification to generate modified lignite materials, Mg-LM and P-LM, which were used for the adsorption and immobilization of Cd (II) in polluted water and soil. Characterization analysis showed the introduction of exchangeable Mg ions and enhancement of the pore structure in Mg-LM; the increase in the number of oxygenated functional groups and, exchangeable calcium ions and P content of P-LM may favor the adsorption and immobilization of Cd (II). Cd (II) adsorption on Mg-LM and P-LM was consistent with the pseudo-secondary kinetics and Langmuir isotherm models. The maximum adsorption capacity of Cd (II) on Mg-LM and P-LM was 1033 mg/g and 55 mg/g, respectively. The reduction in the soil DTPA-Cd content (32.9%) was greater under P-LM treatment than under Mg-LM (20.2 %) and LM (11.1 %) treatments. In addition, successive BCR extractions confirmed that Mg-LM and P-LM promoted the transformation of unstable Cd fractions to stable Cd fractions in the soil. The XRD, FTIR, and XPS results indicated that electrostatic interactions, ion exchange, surface complexation and precipitation might be the main mechanisms involved in the adsorption and immobilization of Cd (II) by Mg-LM and P-LM. Our results suggest that Mg-LM is more suitable than P-LM for the remediation of Cd-contaminated water, and less applicable than in Cd-contaminated soil.