NANOSCALE MAPPING OF ARSENIC BINDING ON GREEN RUST

MATTER AT EXTREMES

20 ESRF

Green rusts (GR) are special iron minerals that form in oxygen-poor environments, where they influence the mobility of contaminants such as arsenic (As). High-resolution microscopic and synchrotron X-ray spectroscopic analysis reveals how the main As-species bind to GR surfaces for removal from the environment.

Green rust (GR) is a redox-sensitive mineral phase consisting of Fe(II)-Fe(III) hydroxide sheets with hydrated interlayer anions (e.g., Cl-, CO32-, SO42-) [1]. GR phases usually form in oxygen-poor and Fe2+-rich environments, where they can heavily influence nutrient availability and contaminant dynamics. Among the most toxic contaminants, arsenic (As) can still be found in high concentrations in drinking and groundwaters, especially in developing countries in south and southeast Asia [2]. It has been shown that synthetic GR sulfate can take up high amounts of As(III) and As(V) from water under anoxic and circum-neutral pH conditions [3,4]. While geochemical controls of As uptake and/or re-release have been identified [5], a clear mechanistic understanding of the As-GR interaction is still lacking. Synchrotron X-ray absorption spectroscopy (XAS) data have helped identify a possible local bonding environment for As sorbed on GR surfaces [6,7], but the lack of independent, cross-confirming analyses to validate these molecular-scale interactions led to contrasting interpretations.

This study examined the interactions between synthetic GR sulfate (GRSO4) and As species [As(III) and As(V)] at anoxic and circum- neutral pH conditions. Scanning transmission electron microscopy (STEM) was coupled with

energy-dispersive X-ray (EDX) spectroscopy to determine the elemental distribution of As and the morphological changes in GR surfaces. High-resolution elemental imaging was complemented with As oxidation state and local bonding environment data from As K-edge XAS collected at beamline BM23, which has the high energy resolution necessary for probing the structure of dilute sorbed species on mineral surfaces.

STEM-EDX maps revealed oxidation state- dependent As distribution and morphological changes in GRSO4 after five days of reaction. The As(III)-reacted GRSO4 (Figure 7a) revealed bright rims (~10 nm wide, indicated by arrows) at the crystal edges, separated by a dark band from the rest of the reacted particle. The EDX map (Figure 7c) revealed an apparent decrease in the Fe and O signal intensities in the dark band, which was best explained by GRSO4 crystal dissolution. Enrichment of the As signal was also observed at the bright rims, indicating that As(III) was preferentially adsorbed at the GRSO4 crystal edges. In contrast, the As(V)-reacted GRSo4 (Figure 7b) did not exhibit any bright rims or dark bands, but was characterised by small dissolution cracks at the particle edges and secondary thread-like structures that were closely associated with the GRSo4 plates. The thread-like structures observed were enriched in As (Figure 7d), suggesting the formation of an As-bearing precipitate. The high-energy X-ray diffraction (XRD) pattern of the As(V)-reacted GRSO4 sample revealed that the As-bearing phase formed was parasymplesite (ferrous arsenate). In addition to the As-bearing precipitates, the GRSo4 crystal edges also exhibited high As signal intensities, indicating that these are the preferred adsorption sites of As(V) on GRSO4.

The direct visualisation of As binding sites was complemented by As oxidation state and local bonding information from As K-edge XAS data collected at BM23. X-ray absorption near-edge structure (XANES) data (Figure 8a) showed that GRSO4 cannot oxidise As(III) or reduce As(V).

Fig. 7: High-angle annular dark-field (HAADF)-STEM images of the (a) As(III)- and (b) As(V)-reacted GRSO4 and their corresponding (c-d) EDX maps showing the elemental distribution of As (red), Fe (blue) and O (green).