Fire Promotes Arsenic Mobilization and Rapid Arsenic(III) Formation in Soil via Thermal Alteration of Arsenic-Bearing Iron Oxides

Abstract

Arsenic in oxic surface soils readily associates with Fe(III) oxide minerals such as ferrihydrite and goethite, predominantly as As(V). Fires are a common feature in many landscapes, creating high-temperature soil conditions which drive thermal transformation of these As(V)-bearing minerals. However, it is unknown whether fire-induced transformation of ferrihydrite and goethite can alter the mobility of As, or alter As(V) speciation (e.g., via pyrolysis induced electron-transfer generating the more mobile and toxic inorganic As(III) species). Here, we subject an organic-rich soil (∼15% organic C) mixed (4:1) with As(V)-bearing ferrihydrite and goethite (total As of 2.8-3.8 µmol g −1), to various temperatures (200-800 • C) and heating durations (5-120 min) and examine the consequences for As and Fe via X-ray absorption spectroscopy, X-ray diffraction, 57 Fe Mössbauer spectroscopy and selective extracts. We show that heating transformed both ferrihydrite and goethite to mainly maghemite at temperatures >400 • C and tended to increase exchangeable surface-complexed As (As Ex) in ferrihydrite yet decrease As Ex in goethite. We demonstrate for the first time that ferrihydrite and goethite-bound As(V) can be rapidly reduced to As(III) during heating of organic-rich soil. Electrons were readily transferred to both Fe(III) and As(V), with reduction of As(V) to As(III) peaking at intermediate temperatures and time periods (maxima of ∼88% for ferrihydrite; ∼80% for goethite). Although As(III) formation was fast (within 5-10 min at temperatures >400 • C), it was followed by partial re-oxidation to As(V) at higher temperatures and longer time intervals. Additionally, combusted As-bearing ferrihydrite and goethite soil-mixtures display greatly enhanced (2-3 orders of magnitude) mobilization of inorganic As(III) aq species upon re-wetting with water. Mobilization of As(III) aq was positively correlated with solid-phase As(III) formation and was greater for goethite than ferrihydrite. These findings challenge the current prevailing view that As(V) reduction to As(III) in soil is mainly limited to waterlogged conditions and suggest that moderate-temperature fires of short duration in oxic soils, may generate substantial labile As(III) species and lead to a pulse of As(III) aq mobilization upon initial rainfall and re-wetting. Further investigation is recommended to explore the consequences for arsenic cycling in fire-prone natural landscapes and agricultural systems which involve controlled-burn practices.

Firing Increases Arsenic Leaching from Ceramic Water Filters via Arsenic and Iron Phase Transformations

“…Decreased oxalate- and HCl-extractable Fe after thermal treatment (Figure ) are also consistent with Fe extractions from schwertmannite before and after thermal treatment . Although schwertmannite was likely not present in clay samples used for CWF production, similar changes in As and Fe solubility between schwertmannite, ferrihydrite, goethite, , and samples in this study after treatment at ≥800 °C suggest similar mechanisms that control As and Fe solubility occur during thermal alteration of these minerals. To further characterize the chemical changes that result from firing, we analyzed the clay and CWF samples collected from Cambodia in 2010 using Fe and As X-ray absorption spectroscopy.…”

“…This result is consistent with (1) previous results that showed phosphate did not increase As desorption from CWFs in Cambodia relative to desorption in water and (2) higher water-extractable As from CWF compared to clay (Figure ). A similar 20- to 50-fold increase in water-soluble As after heating to 800 °C was observed in ferrihydrite and goethite amended soils, respectively . Decreased oxalate- and HCl-extractable Fe after thermal treatment (Figure ) are also consistent with Fe extractions from schwertmannite before and after thermal treatment .…”

“…Reduction of As­(V) to As­(III) has been shown to result in the net desorption of As from Fe oxides, , but here we observed that As oxidation coincided with an increase in As solubility (Figures , ), indicating that adsorbed arsenate may not dominate As speciation. A previous study of burned soil (200–800 °C for 5–120 min) that contained ∼15% organic C resulted in the reduction of As­(V) to As­(III), but partial reoxidation to As­(V) occurred with increasing temperature and heating time . During CWF production, clay is mixed with 15–30% by mass burnout material (rice husk and sawdust), and pyrolysis products of burnout material could also reduce As­(V) to As­(III), but As XANES indicates that firing at 866 °C resulted in net As oxidation, consistent with higher temperature and heating duration as favorable for (re)­oxidation of As­(III) to As­(V) .…”

“…A previous study of burned soil (200–800 °C for 5–120 min) that contained ∼15% organic C resulted in the reduction of As­(V) to As­(III), but partial reoxidation to As­(V) occurred with increasing temperature and heating time . During CWF production, clay is mixed with 15–30% by mass burnout material (rice husk and sawdust), and pyrolysis products of burnout material could also reduce As­(V) to As­(III), but As XANES indicates that firing at 866 °C resulted in net As oxidation, consistent with higher temperature and heating duration as favorable for (re)­oxidation of As­(III) to As­(V) . Although As can volatilize at temperatures below 866 °C, a previous study of incinerated soil showed that As primarily partitioned to ash rather than flue gas, and that the leaching characteristics of As from post-incineration ash increased by a factor of 2–12 compared to pre-incineration soil …”

Heavy Metals in Soils and the Remediation Potential of Bacteria Associated With the Plant Microbiome

“…In welldrained (oxic) surface soils, As occurs predominantly as oxyanions of As 5+ (arsenate), whereas As 3+ (arsenite) species are more abundant in reducing environments, such as waterlogged soils (Roberts et al, 2010). The relative abundance of As 5+ and As 3+ critically influences As mobility, toxicity and environmental behavior, with inorganic As 3+ species generally considered to be more mobile and toxic than inorganic As 5+ species (Shumlas et al, 2016;Johnston et al, 2019). As 5+ is less mobile than As 3+ because it forms complexes on mineral surfaces, especially on hydroxides and oxides of iron (III) as well as on calcite (Bowell, 1994;Guo et al, 2009).…”