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.

Introduction

Iron (Fe) oxides such as ferrihydrite, lepidocrocite, and goethite are common components in soil (Larsen and Postma, 2001, Cornell and Schwertmann, 2003). Among the Fe oxides, poorly crystalline ferrihydrite, a thermodynamically unstable Fe oxide, can transform into a more crystalline Fe oxides of lepidocrocite, goethite, or hematite in soil environments. Fe oxides can interact with organic matter (OM) (Tipping, 1981) and heavy metals/metalloids such as arsenic (As) (Jain et al., 1999, Appelo et al., 2002, Dixit and Hering, 2003, Roberts et al., 2004), affecting their fate in the environment. The adsorption and/or coprecipitation of OM with Fe oxides changed the reactivity and surface charge characteristics of Fe oxides (Tipping, 1981, Franchi and O'Melia, 2003, Vindedahl et al., 2016) and promoted the sequestration of C in the environment (Lalonde et al., 2012). As(V) can form various complexes with the surface binding sites of Fe oxides (Jain et al., 1999, Appelo et al., 2002, Dixit and Hering, 2003, Roberts et al., 2004), which is a significant natural attenuation process for As contamination in soil and groundwater. In addition to the surface desorption process, As(V) release in the environment was observed to be closely associated with the dissolution and transformation of Fe oxides (Rawson et al., 2016, Sun et al., 2018). Therefore, understanding the dynamic interactions among Fe oxides, As, and OM is crucial for predicting the geochemical cycling of Fe, As and C in the environment.

Previous studies have found that, induced by Fe(II) under anoxic conditions, the amorphous ferrihydrite can convert to the more crystalline Fe oxides (Zachara et al., 2002, Jang et al., 2003, Hansel et al., 2005, Pedersen et al., 2005, Boland et al., 2014), which was affected by Fe(II) concentrations (Hansel et al., 2005) and pH (Boland et al., 2014). The presence of As (Ford, 2002, Wang et al., 2015) and/or OM (ThomasArrigo et al., 2018) as well as the variations of the As/Fe or C/Fe molar ratios can significantly affect the abiotic ferrihydrite transformation pathways and rates (Ford, 2002, Gomez et al., 2013, Chen et al., 2015, Chen and Sparks, 2018, ThomasArrigo et al., 2018, Aeppli et al., 2019). In turn, the transformation process of Fe oxides will ultimately affect the biogeochemical cycling of As and the stability of OM associated with Fe oxides. The temporal changes of As speciation and distributions on Fe oxides varied during the ferrihydrite transformation process, such as the incorporation of As(V) into the structure of the newly formed Fe oxides (Boland et al., 2013), the occlusion in the defect sites (Ford, 2002), or the formation of ferric arsenate precipitates (Paktunc et al., 2008). Previous work has demonstrated that, during the transformation from ferrihydrite to hematite, As(V) can be sequestrated by hematite, leading to the significant immobilization of As(V) (Bolanz et al., 2013). During Fe(II) induced lepidocrocite transformation to magnetite, As(V) could be incorporated into magnetite structures while As(III) only formed surface complexes on magnetite (Wang et al., 2014).

Iron oxides transformation process also affected the sequestration of C associated with the coprecipitates of OM and Fe oxides (Chen et al., 2015, Chen and Sparks, 2018). Coprecipitation of Fe oxide and OM has been suggested to promote the long-term stabilization of OM (Lalonde et al., 2012). However, less is clear about the dynamic changes of OM characteristics and stability during the ferrihydrite transformation to more crystalline Fe oxides. The dynamic distributions of OM within Fe oxides will also affect the mobility, solubility, and bioavailability of metals/metalloids, nutrients, and organic pollutants in the environment over time (Jambor and Dutrizac, 1998, McDowell, 2003, Schmidt et al., 2011). In the As contaminated, OM-rich subsurface environments, both As and OM may be adsorbed by ferrihydrite to form ferrihydrite-As-OM complexes (ThomasArrigo et al., 2014, ThomasArrigo et al., 2016), which may affect the speciation of As(V) in the environment. Previous studies reported that the coexisted As(V) or As(III) and OM strongly impeded the ferrihydrite transformation rates and changed the transformation products compared with the transformation process with ferrihydrite only (Chen and Sparks, 2018, Hu et al., 2018). During the transformation process, OM competed with As(V) for the surface sorption sites of ferrihydrite, and potentially increased As(V) mobility (Sharma et al., 2010). Overall, to our best knowledge, little is known about the dynamic distributions and speciation of both As and C on Fe oxides during the transformation process of ferrihydrite at anoxic conditions with the presence of Fe(II). Understanding the dynamic transformation of ferrihydrite-As-OM coprecipitates induced by Fe(II) is critical for evaluating the sequestration of both As and C in anoxic soils and sedimentary environments.

The objectives of this study are: (1) to elucidate the effect of the co-existence of As(V) and fulvic acids (FA), a typical dissolved OM in soil solution, on ferrihydrite transformation induced by Fe(II); (2) to image the temporal distributions of As(V) and determine its speciation in Fe oxides, As(V) and FA ternary system; and (3) to provide a direct visualization of the temporal C distributions on the Fe oxides at nano and sub-nano scales. The results obtained in this study can provide mechanistic and quantitative understanding of the sequestration of As(V) and C during the ferrihydrite transformation process in natural anoxic environments.