Investigating changes in arsenic speciation through the combination of in vitro simulated gastric fluid extractions and EXAFS spectroscopy

Alphabetical by first author Investigating changes in arsenic speciation through the combination of in vitro simulated gastric fluid extractions and EXAFS spectroscopy Mining conducted throughout the state of California has left an environmental legacy of exposed mine wastes containing elevated levels of toxic metal(loid)s including arsenic (As), a known carcinogen. The relationships between speciation, bioaccessibility, location, and particle size of As are critical to calculating and predicting the As exposure risks associated with these wastes. Bulk mine tailings, waste rock, and surface sediment samples were collected from a number of gold mine sites across California and separated through dry sieving to generate eleven distinct size fractions which were then analyzed to determine initial As concentration. Bulk EXAFS spectroscopic analysis using principal component analysis, target transform and linear combination fitting of size-separated samples from various mine sites in California were then used to quantify the speciation and distribution of arsenic (As) both before and after exposure to a simulated gastric fluid extraction. Across a majority of samples analyzed, results show little change in speciation between preand post-extraction, particularly in samples with low bioaccessibility. However, certain samples show quantitative changes in speciation after extraction. In Tropico mine tailings samples, amorphous ferric arsenate is preferentially removed, correlating closely with bioaccessibility values. Expressing As speciation changes in terms of the mass of As lost shows distinct differences between expressing it simply as percent change in As phase. Analyzing only percent change in the surface sediment samples, scorodite is the predominant species removed post-extraction, while some released As appears to reprecipitate as sodium arsenate. Expressing this solely as percentage change in speciation shows a slight loss in percentage of scorodite, but expressing it as mass lost shows over 70% of As released as scorodite. Other trends observed include that As bioaccessibility generally increases as particle size decreases, tailings samples generally have higher As bioaccessibility than waste rock samples, and As bioaccessibility decreases as time after a rain event increases. These calculations and trends observed help better understand the mechanisms of As bioaccessibility than simply calculating total As released. Using this information can help better understand which variables best predict As toxicity in mine waste samples, leading to more accurate and expedited calculations in predicting As exposure. Anthony Tyler, Chapman University