The Development and Application of a Method to Measure Volatile Antimony Emissions from Soils and Antimony Release from Contaminated Soils

Caplette, Jaime N. (2022). The Development and Application of a Method to Measure Volatile Antimony Emissions from Soils and Antimony Release from Contaminated Soils (Unpublished). (Dissertation, Geography Institute, Faculty of Sciences)

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In the environment, antimony (Sb) is a ubiquitous, but toxic, element. Due to the increased use of Sb in flame retardants, ammunition, as a PET catalyst, and its potential for applications in green energy, the exploitation and associated contamination of Sb has increased and likely will continue to increase. Due to its assumed geochemical similarities to the toxic element arsenic (As), the environmental fate of Sb has become an area of public concern. In general, the biogeochemical cycle of Sb remains to be elucidated with many unknowns. The behavior of inorganic Sb in the aerobic environment is relatively well understood but, Sb behavior in anaerobic environments has been poorly investigated. Even less studied are the microbial-mediated transformations of Sb in the environment such as its methylation and volatilization, although these transformations may have an impact on its mobility and toxicity. Environments susceptible to anoxic conditions (e.g., wetlands) constitute a major part of Earth’s surface and all soils are prone to periodic flooding therefore, the fate of Sb, behavior, and its transformations require further investigation. This dissertation aimed to investigate Sb transformations, such as its volatilization, and release from different soils under laboratory and field conditions from soils susceptible to waterlogging.
First, to investigate Sb volatilization in laboratory and field conditions a method had to be developed that had the following criteria 1) easy to deploy in the field, 2) cost-effective, and 3) user-friendly. Commercially available activated charcoal sorbent tubes (Sb-traps) were tested for their ability to capture stibines (trimethylstibine ((CH3)3Sb) and stibine (SbH3)) produced by hydride generation. The traps were extracted using a closed-vessel aqua regia extraction. The extraction recoveries, as total Sb, were 93.2 ± 2.3% for (CH3)3Sb and 89.6 ± 3.8% for SbH3 with a limit of detection < 2 ng. This method was then applied to microcosm experiments on Swiss shooting range soils to test its ability to capture naturally produced stibines. The 42-day microcosm experiment showed that volatile Sb was only detected in waterlogged conditions, highlighting the importance of waterlogging on volatilization. By amending the waterlogged soils with 5% (w/w) manure a five-fold increase in volatile Sb was produced from the soils. This experiment indicated that common agricultural practices on contaminated soils such as waterlogging, potentially anoxic conditions, and organic-matter amendments (e.g., manuring) strongly influence the formation of stibines from soils by likely stimulating microorganisms in the soil. As the first experiment showed that volatile Sb was only produced under waterlogged conditions, there was a focus in the next experiments on soils commonly impacted by waterlogging.
Rice-paddy soils are agricultural soils impacted by varying hydrologic conditions and rice is an important staple food for many nations. In the second experiment, Sb release from three rice paddies and Sb volatilization from two rice paddies (low and high Sb contaminated sites) was investigated in an Sb-mining region in the field. In the field campaign, porewater Sb was associated with Fe at two sites and with Eh, SO42-, Fe, and Mn at the third site. Volatile Sb was detected (18.1 ± 5.2 to 217.9 ± 160.7 mg ha−1 y−1) in both investigated rice paddies but the high Sb-contaminated site had higher volatile Sb emissions. The same soils from the field campaign were complemented with a 56-day microcosm experiment in the laboratory using two treatments, waterlogging-only and waterlogging with 0.8% (w/w) manure, to better understand the potential drivers of Sb volatilization and release. The lowest-producing soil was the low Sb site from the field campaign which produced relatively negligible volatile Sb. The two high-Sb contaminated soils produced more volatile Sb when manured and agree with the first experiment where agricultural practices may influence Sb inputs to the atmosphere. Volatile Sb in the microcosm was correlated with the surface water Sb concentration indicating that the surface water Sb maybe the source of volatile Sb. The release of Sb in the porewaters was not influenced by manuring, and was always high with the beginning of waterlogging, indicating that with the start of the rice growing season relevant amounts of Sb may be released into waters and pose a risk to the environment. A rapid decrease of porewater Sb was observed in all experiments correlated with a decrease in porewater SO42-, indicating potential sulfide precipitation. In some microcosms, a temporary release of Sb was observed, likely associated with the reductive dissolution of Fe-(oxyhydr)oxides.
The third experiment was a 27-day microcosm experiment investigating Sb release, methylation, and volatilization from two different wetland soils with a high soil Sb (organic-rich wetland) and a low soil Sb (Fe-rich floodplain soil). The Sb release patterns between the two soil types varied. In the organic-rich wetland soil, Sb release was a complex interplay with DOC, SO42-, and Mn while the Fe-rich floodplain soil showed a strong coupling of Sb to the Fe-cycle and potential immobilization by sulfide precipitation. Both soils released environmentally relevant concentrations of Sb to the waters indicating that waterlogging of these soils may pose risk to the surrounding environment. In the porewaters of the organic-rich wetland soil, after 6 days of waterlogging an unknown species was detected. Spiked porewaters with TMSb did not aid in species identification. Additionally, poor column recoveries for the waters indicate the potential formation of retained Sb species. In soil TMSb extractions before and after incubation and the surface waters from the organic-rich wetland soils, a species with a similar retention time as the unknown species in the porewaters was detected. Spiked extracts and surface waters with TMSb overlapped with the unknown species but the identification remained unclear due to potential matrix effects causing a TMSb retention time shift in the samples. These results highlight the importance of future investigations into Sb speciation outside of typical SbIII, SbV, and TMSb analysis. Both soils produced volatile Sb more (organic-rich wetland soil) or comparable (Fe-rich floodplain soil) amounts as the previous experiments even though these soils were not nutrient amended, indicating that naturally high organic carbon may promote volatilization. In this experiment, relationships between volatile Sb production and physicochemical properties of the soils and waters were not detected indicating a more complex, likely microbial, mechanism.
In conclusion, the release of Sb from waterlogged soils is controlled by the Fe and S cycles in rice-paddy and floodplain soils. In organic-rich wetland soils, Sb solubility is a more complex mechanism. By manuring rice-paddy soils there was no impact on the release of Sb, an interesting finding as manuring soils typically enhances the mobility of the geochemically similar element, As. In all soils, except for the Fe-rich floodplain soil, Sb is quickly immobilized in the porewaters likely due to sulfide precipitation. In the floodplain soils, Sb is released with Fe and subsequently immobilized after a longer periodic of waterlogging. All soils released environmentally relevant concentrations of Sb into the waters. Additionally, speciation analysis of the porewaters indicated the presence of an unknown Sb species in the organic-rich wetland soils and highlights the importance of future investigations. Volatile Sb was detected for the first time in the rice-paddies and rice paddy, organic-rich, and floodplain soils microcosms and is influenced by agricultural practices (e.g., waterlogging and manuring) and may represent an important, but overlooked, source of Sb to the atmosphere.

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