Remediation of Cadmium-Contaminated Soil Using Sodium Alginate-Embedded Rectorite and Organoamine-Modified Rectorite Microspheres

Cadmium (Cd) contamination in agricultural soils poses serious risks to ecosystem health and food safety, highlighting the urgent need for efficient and environmentally stable immobilization materials. In this study, sodium alginate-based rectorite microspheres (REC beads) and triethylenetetramine-modified rectorite microspheres (TETA-REC beads) were fabricated and applied for the immobilization of Cd(II) in contaminated soils. Structural characterization confirmed that the ionic cross-linking encapsulation process preserved the layered structure of rectorite, while TETA modification introduced abundant amino functional groups that enhanced the interaction between the material and Cd(II). Immobilization experiments demonstrated that both microspheres exhibited rapid and stable Cd(II) passivation performance, with TETA-REC beads showing significantly higher efficiency. The maximum removal efficiencies of water-soluble and available Cd(II) reached 83.87% and 93.33%, respectively. Sequential extraction analysis revealed that the microspheres effectively transformed labile Cd fractions, including exchangeable and water-soluble forms, into more stable species. Mechanistic investigations indicated that Cd immobilization was governed by a synergistic combination of interlayer ion exchange, physical adsorption within the porous alginate structure, and coordination complexation between Cd(II) and amino groups introduced by TETA. Furthermore, microbial community analysis showed that the remediation process promoted the recovery of soil microbial diversity and metabolic functions. The developed TETA-REC microspheres provide a promising strategy for the efficient immobilization of Cd in contaminated soils and offer potential for the sustainable remediation of heavy metal-polluted farmland.