Stabilization of cadmium in polluted soil using palygorskite-coated nanoscale zero-valent iron

Abstract

The palygorskite-coated nanoscale zero-valent iron (PAL-nZVI) was synthesized and used in the remediation of Cd-contaminated soils. The function of PAL-nZVI on fraction of Cd in soils, the growth, Cd translocation, and accumulation in corn (Zea mays L.) were investigated using a pot experiment. Cd-contaminated soils were amended with palygorskite, nanoscale zero-valent iron (nZVI), and PAL-nZVI (with varying mass ratios between PAL and nZVI) and with no amendment as control. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the changes in both the surface and the structure of stabilizers. CaCl2-extractable and toxicity characteristic leaching procedure (TCLP)-extractable Cd were measured in order to estimate the bioavailability and the stabilization efficiency. The Community Bureau of Reference (BCR) sequential extraction method was applied to evaluate the speciation change of Cd in soils stabilized by nZVI, PAL, and PAL-nZVI with different coating mass ratios of PAL to nZVI. The Cd accumulation in root and shoot fresh weight was measured to assess the stabilization efficiency. SEM and XRD indicated that nZVI particles were distributed uniformly and well fixed on the surface of palygorskite. After adding PAL-nZVI, the pH value of soil increased by 1 pH unit; CaCl2-extractable Cd decreased from 2.24 to 0.95 mg/kg; the residual Cd increased by 23.06% after 30-day incubation. The fresh weight of corn reached 43.02 g/plant, and the Cd contents in roots and shoots achieved minimum values of 1.50 and 0.73 mg/kg, respectively, when applying PAL-nZVI composite with a coating mass ratio of 2:1. The application of PAL-nZVI decreased the bioavailability and mobility of Cd in soils effectively. The stabilization showed a better effect when applying PAL-nZVI in Cd-contaminated soils than applying nZVI alone. Hence, it was a feasible and potential material that could be applied to amend the Cd-contaminated soil effectively, which would have a high and valuable utilization in field applications.