A novel iron-based composite modified by refinery sludge for fixing Pb, Zn, Cu, Cd, and As in heavy metal polluted soil: Preparation, remediation process and feasibility analysis

In this study, we prepared a novel iron-based composite (NIC) via a one-step reduction method utilizing refinery sludge (RS) as the reducing agent and iron ore (IO) as the iron source, allowing for the concurrent stabilization of multiple heavy metals (HMs) in soil. The loose, porous structure of NIC provided ample active sites, including Fe0, CaAl2Si2O8, fixed carbon, and a small amount of active amorphous substances. After 60 days of soil remediation using 5% NIC, the bioavailable content of HMs Cu, Zn, Pb, Cd, and As was reduced by 71.55%, 49.62%, 91.35%, 61.37%, and 41.45%, respectively. Upon magnetic recovery and analysis of NIC in soil, HMs were found to have been enriched in the material, forming new phases like FeAsO4, ZnFe2O4, among others. Thus, the immobilization mechanism of HMs included adsorption, co-precipitation and complexation etc. Furthermore, NIC enhanced soil pH and cation exchange capacity, elevating the soil's ability to retain heavy metal cations. Lastly, cost-benefit analysis revealed that RS as a reducing agent could lower the production cost of environmental remediation materials. Furthermore, HM leaching toxicity analysis verified the safety of its usage. Overall, our findings offer a promising procedure for both the resource utilization of refinery sludge and multi-metal co-contaminated soil remediation.