The influence mechanism of fulvic acid on Fe(II)-catalyzed transformation of Cr(III)-bearing schwertmannite: pH effect and nanoscale redistribution of Cr and C

[Display omitted] •Fe(II) does not affect the complexation order of schwertmannite and fulvic acid.•Goethite was formed in Fe(II)-low fulvic acid treatments.•Fulvic acid could inhibit transformation of schwertmannite to produce lepidocrocite.•Part of the sequestered Cr is found with the bulk of goethite.•FA entered the porous and defective structures of lepidocrocite. The interactions between schwertmannite (Sch), chromium (Cr), and dissolved organic matter play a significant role in the cycling of Cr and carbon (C) in acid mine drainage (AMD) contaminated soils. However, mechanism insights into the immobilization of Cr and C during the Fe(II)-catalyzed, fulvic acid (FA)-mediated reductive transformation of Cr(III)-bearing Sch (Cr-Sch) remain unclear. In this study, we investigated the mechanisms underlying sequestration of Cr and C during the Cr-Sch transformation under various FA concentrations and pH conditions. Solid-phase transformation analysis revealed that the pH increasing from 3.5 to 7.0 favored the transformation of Cr-Sch to lepidocrocite (Lep) and/or goethite (Goe), while the transformation rate and degree decreased with increasing FA concentration from 10 to 100 mg/L. By means of the distribution of elements observed in spherical aberration corrected scanning transmission electron microscopy, Cr was predominantly adsorbed on the Lep and Goe surfaces, and Cr may be partially sequestered by Goe. Meanwhile, FA was adsorbed onto the surfaces of Goe and Lep, and the incomplete defect structure of Lep provided space for C immobilization. This study provides a basis for an in-depth understanding of the nanoscale mechanisms of Cr and FA redistribution during Cr-Sch transformation and can be used for developing strategies for the immobilization of heavy metals and C in AMD-polluted environments.