Development, validation and deployment of a novel chemo-trapping method to assess arsenic biovolatilisation

• Main Author: Mestrot, Adrien
• Published: University of Aberdeen 2011
• Subjects: 549.25
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.554243

Arsenic (As) biovolatilisation occurs in a variety of environs, including soils and volcanic hot springs. Nonetheless, it is a scarcely studied phenomenon due to inherent sampling issues associated with a lack of validated method for field measurements. This situation led us to validate a low-level, field deployable, chemo-trapping technique based on silver nitrate impregnated silica gel traps. This new method allows for total measurements (above 80% recovery) and speciation analysis for the four most important species involved in the As biovolatilisation pathway: arsine (AsH3), mono-, di- and trimethylarsine (AsH2(CH3), AsH(CH3)2, As(CH3)3). The traps were then used on microcosms and it was found that biovolatilisation only occurred when the soil was flooded (reduced conditions) and amended with organic matter. The method was then deployed to assess As levels emanating from four field sites: a mangrove in Bangladesh, a mine tailing in Spain and a rice paddy in both countries. Significant volatilisation was measured only in the rice paddies, with As(CH3)3 being the main species. Further experiment were designed in order to test an array of soils of various properties under different organic matter amendments, in presence of different forms of As. The results confirmed that As(CH3)3 seems to be the end product of As biovolatilisation but the very toxic AsH3 was also measured. Moreover, the quantity of As trapped was strongly correlated with the total porewater As concentration and the dissolved organic carbon (DOC) concentration (p < 0.0005). The data from both field and laboratory studies was then used to estimate the first global fluxes of As based on actual field measurements. Calculations show that As volatilisation represents 2 to 6% of natural As emissions to the atmosphere. Finally, in a bid to gauge the importance of As biovolatilisation with regards to the As biogeochemical cycle, the stability in the dark and under UV light of arsine and methylarsines was investigated. Their stability was higher than previously reported with half-lives of 8 hours for methylated arsines under oxidising UV light while all species are stable in the dark over a 7 day period. AsH3 was stable under both conditions.