Synchronous sequestration of cadmium and fulvic acid by secondary minerals from Fe(II)-catalyzed ferrihydrite transformation

As consequence of the dual demands for pollution control and carbon (C) fixation in soils, Fe(II)-catalyzed mineral trans-formation may be a promising method to simultaneously immobilize heavy metals or organic matter (OM), but the underlying mechanisms remain unclear. Here, the synchronous sequestration mechanism of cadmium (Cd) and fulvic acid (FA) during Fe(II)-catalyzed the transformation ferrihydrite with C/Fe molar ratio of 0.21 were examined. Mineral phase analysis revealed that increasing the Fe(II) concentration (1-5 mM) favored the transformation of lepidocrocite and goethite to magnetite, and ferrihydrite transformation rate increased with increasing Fe(II) concentration. Color overlays and line profiles of elements depicted that Cd was dominantly adsorbed on the lepidocrocite and goethite surfaces. A positive correlation between the quantity of nonextractable Cd and magnetite further indicated that Cd may be sequestered by magnetite. Meanwhile, FA molecules were adsorbed on goethite surfaces and magnetite aggregates, and incomplete structure of lepidocrocite provide spaces for immobilizing C. Newly formed iron (Fe) (oxyhydr)oxides may immobilize Cd through surface binding, structural substitution, and physical encapsulation. The OM bound to the newly formed Fe (oxyhydr)oxides was rich in aromatic and carboxyl functional groups, which was beneficial for binding Cd, whereas the presence of Cd promoted the generation of nano pore spaces or defects and consequently enhanced FA sequestration. Therefore, Cd immobilization and FA sequestration can be synchronously achieved during the phase transformation. The findings provide a profound insight into various nanoscale mechanisms accounting for the fate of Cd and FA coupled with mineral transformation. The findings also are very helpful for developing strategies for simultaneously immobilizing heavy metals and C in soils. (c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the synchronous sequestration mechanism of cadmium (Cd) and fulvic acid (FA) during Fe(II)-catalyzed the transformation of ferrihydrite. The study reveals that increasing Fe(II) concentration promotes the transformation of lepidocrocite and goethite to magnetite, and Cd can be sequestered by magnetite. Meanwhile, FA molecules are adsorbed on goethite and magnetite, and the incomplete structure of lepidocrocite provides spaces for immobilizing carbon.