Field assessment of carboxymethyl cellulose bridged chlorapatite microparticles for immobilization of lead in soil: Effectiveness, long-term stability, and mechanism

Abstract

Lead (Pb) contamination in soil has been a major concern. Yet, cost-effective remediation technologies have been lacking. This study pilot-tested a new type of carboxymethyl cellulose (CMC) bridged chlorapatite microparticles (CMC-CAP) for immobilization of Pb at an abandoned chemical plant site, and monitored long-term (1 year) stability of Pb under the field conditions. Concentrated CMC-CAP (18 g L−1) was prepared at the pilot scale and then used for amending the soil on the site. CMC-CAP appeared as interwoven flocks of CAP microparticles with a hydrodynamic diameter of 2442 nm, which is 2.2 times smaller than that for bare CAP. CMC-CAP at a dosage of 0.5 wt% resulted in the highest immobilization of Pb, and decreased the acid-leachable Pb in the soil from 1.46 mg L−1 to 0.008 mg L−1 after 1 day of reaction, and further to 0.005–0.006 mg L−1 when aged for 180 and 365 days under the field conditions. The long-term aging consolidated the Pb immobilization, securing the compliance with the Chinese regulatory limit (0.010 mg L−1 for Pb). CMC-CAP served as a long-term sink for leachable Pb in soil and a pH buffer to maintain an alkaline soil pH. Sequential extraction data confirmed that the CMC-CAP amendment converted the more leachable Pb fractions into the less available Pb species. The soil amendment reduced the risk assessment code from “High risk” for the untreated soil to “Low risk” after 120 days of aging. XRD and FTIR results revealed the immobilization of Pb by CMC-CAP was achieved through the formation of the highly stable pyromorphite, surface complexation, and/or ion exchange reactions. Based on the field data, the remediation cost of the CMC-CAP based technology was estimated to be $36.2/m3. This study provides important field data on the effectiveness of CMC-CAP for cost-effective immobilization of Pb in soil.