Assessing the selectivity of extractant solutions for recovering labile arsenic associated with iron (hydr)oxides and sulfides in sediments

Sequential extractions can provide analytical constraints on the identification of mineral phases that control arsenic speciation in sediments. Model solids were used in this study to evaluate different solutions designed to extract arsenic from relatively labile solid phases. Sequential and single-step extractions were conducted to evaluate the selectivity of these extraction solutions for differentiating arsenic associated with iron (hydr)oxides and iron sulfides. Extraction phases were categorized as 1) MgCl2-extractable, 2) phosphate-extractable, 3) ascorbate-extractable, and 4) HCl-extractable when the procedure is applied in sequence. Arsenic, iron, and sulfur concentrations were compared between the single-step and sequential extractions. Additionally, phosphate extractions were conducted at three pH values for comparison purposes. Application of the extraction protocol to orpiment of varying crystallinity and crystalline arsenopyrite indicated that these phases have limited solubility in the extractant solutions. Extraction with ascorbate at circumneutral pH appears to target dissolution of poorly crystalline iron (hydr)oxides and is largely selective for extracting arsenic associated with this phase. Tests of the ascorbate extraction solution on sediment samples previously characterized for mineralogy and solid phase arsenic speciation by X-ray absorption spectroscopy reveal the utility of ascorbate for differentiating arsenic associated with poorly crystalline iron (hydr)oxides and iron sulfides. Results from tests in which samples were dried by different approaches or were subjected to air exposure prior to or during extraction demonstrate the importance of sample handling on the outcome of extraction protocols.