Lead Immobilization in Soil and Uptake Reduction in Brassica chinensis Using Sepiolite-Supported Manganese Ferrite

Lead (Pb) in soil poses serious environmental and health risks, and its removal requires complex and costly treatment methods to meet strict regulatory standards. To effectively address this challenge, innovative and efficient techniques are essential. Sepiolite-supported MnFe2O4 (MnFe2O4/SEP) composites were synthesized via a chemical co-precipitation method. The effects of MnFe2O4/SEP on soil pH, cation exchange capacity (CEC), available Pb content, Pb2+ uptake, and the activities of antioxidant enzymes in Brassica chinensis (Pak Choi) were examined. MnFe2O4/SEP showed superior Pb2+ adsorption compared to SEP alone, fitting Langmuir models, Dubinin-Radushkevich (D-R) models, Temkin models and pseudo-second-order kinetics. The maximum adsorption capacities at 298, 308, and 318 K were 459, 500 and 549 mg·g−1, respectively. XPS analysis indicated that chemisorption achieved through ion exchange between Pb2+ and H+ was the main mechanism. MnFe2O4/SEP increased the soil pH by 0.2–1.5 units and CEC by 18–47%, while reducing available Pb by 12–83%. After treatment with MnFe2O4/SEP, acid-extractable and reducible Pb in the soil decreased by 14% and 39%, while oxidizable and residual Pb increased by 26% and 21%, respectively. In Brassica chinensis, MnFe2O4/SEP reduced Pb2+ uptake by 76%, increased chlorophyll content by 36%, and decreased malondialdehyde (MDA) levels by 36%. The activities of antioxidant enzymes—superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)—were decreased by 29%, 38% and 17%, respectively. These findings demonstrate that MnFe2O4/SEP is an efficient Pb2+ adsorbent that immobilizes Pb in soil mainly through ion exchange, thereby providing a highly effective strategy for remediating Pb-contaminated soils and improving plant health.