Sodium arsenate heptahydrate

The National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 2710a soil is certified for highly elevated As levels (1540 mg/kg) and was purchased from NIST (NIST, Gaithersburg, MD). US Geological Survey (USGS) soil was collected by USGS near an abandoned lead mine in Flat Creek, Montana (USGS, Denver, CO). Sodium arsenate heptahydrate served as a water-soluble positive control (Sigma-Aldrich). Remaining As-contaminated soils were collected from mining and pesticide-impacted sites. Soils were dried at <40 °C and sieved to either <250 μm or <74 μm. Particles <250 μm are most likely to adhere to hands which poses a concern for young children who often engage in hand-to-mouth activity (Xue et al., 2007 (link)). Soils were homogenized and riffled, and physicochemical properties were recorded (Table 1). Arsenic soil concentrations were determined using Instrumental Neutron Activation Analysis (INAA) in the Department of Nuclear Engineering, North Carolina State University (NCSU, Raleigh, NC; mean As mass detection limit, 0.035 μg).

Arsenopyrite (FeAsS, As oxidation state As⁰), realgar (AsS), orpiment (As₂S₃, As oxidation state As³⁺), sodium arsenite (NaAsO₂, Sigma-Aldrich, >99%), sodium arsenate heptahydrate (Na₂HAsO₄·6H₂O, Sigma-Aldrich, 99.995%) and scorodite (FeAsO₄·2H₂O) were obtained from the Certified Reference Material Center, China, with their identities confirmed by XRD analysis.