Kinetics of As(V) and carbon sequestration during Fe(II)-induced transformation of ferrihydrite-As(V)-fulvic acid coprecipitates

Abstract

The dynamic interactions among iron (Fe) oxides, organic matter (OM) and heavy metals/metalloids play crucial roles in controlling the geochemical cycling of carbon (C) and heavy metals/metalloids in natural environments. Although the inhibitory effects of arsenate (As(V)) or OM on the ferrihydrite transformation process have been studied previously, there is still a lack of mechanistic and quantitative understanding on the kinetics of As(V) and C sequestration during the Fe(II)-induced ferrihydrite transformation. In this study, we employed a suite of techniques to elucidate the underlying mechanisms accounting for the temporal changes of As(V) and C distributions and speciation on Fe oxides during the Fe(II)-induced ferrihydrite transformation process. Characterizations with X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and the spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) at different times indicated that the presence of As and/or FA resulted in the formation of more lepidocrocite than goethite. Besides surface adsorption, a portion of As(V) may be incorporated into the lattice structures of newly formed crystalline Fe oxides, and the amount of As(V) sequestration within Fe oxides were correlated with the formation of crystalline Fe oxides. In comparison, FA molecules were either adsorbed on the surfaces of goethite or diffused into the defects or nano pore spaces of lepidocrocite. Our results shed the light on different nanoscale mechanisms accounting for the sequestration of C and As(V) on Fe oxides. The defects or nano pore spaces formed in the structures of lepidocrocite may provide an effective way to sequester C through physical isolation, while the crystal structures of Fe oxides may sequester As(V) through isomorphic substitution. The knowledge on the dynamic coupling between Fe oxide transformation and C/As(V) sequestration provided the basis for accurately predicting the geochemical cycling of trace elements and OM.