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Comparison between two sequential extraction procedures to assess Arsenic mobility Renato W. Veloso1*, Susan Glasauer2, Jaime W. V.de Mello1, Luísa P. Cardoso1 Department of Soils, Federal University of Viçosa, Viçosa, MG, Brazil School of Environmental Sciences, University of Guelph, Guelph, ON, Canada *Corresponding author: rwveloso@gmail.com - University of Viçosa, Av. P.H. Rolfs, CEP 36570-000, Viçosa, MG, Brazil 1 2
Key-words: Acid Rock Drainage, Arsenic, extraction methods
Arsenic is a common contaminant in mining environments impacted by Acid Rock Drainage (ARD). The process is initiated by the oxidation of sulphide-bearing materials exposed to surface conditions and induces the acidification of drainage waters that may increase metal and metalloid mobility, including arsenic. The As content of drainage waters will be affected by several factors, including the As chemical species, Fe content and pH. Risk is, however, commonly assessed by determining total As content in the associated solid substrate. This approach recognizes that the solid phase forms the largest reservoir of As in typical natural systems. Understanding the chemical form of As in the solid phase is vital to evaluate how reactive As may be remobilized from soils and sediments. Sequential extraction procedures are developed in order to obtain a more precise understanding of As mobility than single extraction. The aim of our research was to test two different sequential extraction for As. Samples of sediments were collected within a gold mining area impacted by AMD, located in Minas Gerais State, Brazil. Four samples were collected from two creeks. One sample was from near the source spring and the other from downstream for each creek. Two sequential extraction methods were performed. The first sequential method, SE.1, described by Keon et al. (2001) was slightly altered. The extraction of As-linked to sulfides was not performed. The extraction of As associated to well-crystalline Fe and Al (hydr)oxides was replaced by the method developed by Wenzel et al. (2001). The
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extraction of As linked to organic matter was added, according to the method described by Tessier et al. (1979). Finally, the final extraction step (silicates) was substituted by acid digestion of the remaining solids after freeze drying. The second sequential method, SE.2 was described by Huang and Kretzschmar (2010). The only alteration was to replace the extraction step of As bound to organic matter by the extraction developed by Tessier et al. (1979). The total As content of the samples was determined by digestion with inorganic acids (HF, HNO3 and HF). The external standard (SRM 2710a) was treated by the same procedures. The remaining As after shaking the samples in a 60 mg L -1 As solution for 1 h was analyzed to assess the As adsorption capacity of these sediments. The As concentrations were measured using graphite furnace atomic absorption spectroscopy (GFAAS, Perkin-Elmer 4100ZL). Results for the external standard yielded recovery rates between 90 and 107 %. The source water springs for both creeks (DC.1 and RC.1) had the highest As contents, 4792 and 3851 mg.kg-1, respectively. The As contents for the downstream samples (DC.2 and RC.2) were lower, at 1153 and 1092 mg.kg-1, respectively. The As content was related to the distance from the As source, since the springs are closer to sites of active mining. The remaining As values were lower for the water springs, having presented values lower than the detection limit, and slightly higher for the downstream samples, 14.3 and 10.8 mg.L-1, for DC.2 and RC.2, respectively. The values indicate a high As adsorption capacity,