A dual-function approach for remediating cadmium-contaminated soil: Prepared gel bead immobilizing Cd and controlling release of nitrogen

Abstract

Soil cadmium (Cd) contamination in farmland is a critical issue related to soil health and food security. Besides, long-term excessive urea fertilization may enhance the mobility and bioavailability of cadmium. In this study, tourmaline (TM) was modified by nanosilica to obtain silica@TM particles, which were subsequently embedded in polyvinyl alcohol (PVA)/sodium alginate (SA) gel containing urea. Millimeter-sized PVA/SA/N-silica@TM gel beads (PNST) were innovatively synthesized and utilized as soil amendment and controlled-release urea carrier for Cd-contaminated soil, subsequently comparing the bioavailability of Cd across composite and individual material treatments, as well as the urea release rate between the composite and unencapsulated urea. The results showed that the 5 % PNST addition increased soil pH by 1.23 units and reduced CaCl₂-Cd and TCLP-extractable Cd by 95.89 % and 35.95 %, respectively, following a 70-day incubation period. Also, the residual Cd proportions were 21.82 % and 25.49 % under tourmaline and PNST treatments, respectively. It indicated that PNST effectively facilitates the immobilization of soil Cd, with the reduced partition index (I_R) of 0.37. XRD, FTIR and SEM-EDS analyses revealed the immobilization mechanisms of Cd on PNST, encompassing ion exchange (e.g., Ca^2 + and Mg²⁺), surface absorption, and complexation processes. Additionally, slow-release of urea was also achieved owing to the highly porous structure of gel beads and the cross-linked network formed by hydrogen bonding among SA, PVA and urea, resulting in only 51 % nitrogen release after 28 days. Therefore, a novel gel particle PVA/SA/N-silica@TM may be gifted for “remediation coupled with agro-production” in slightly Cd-contaminated agriculture land.