Shane Whitacre

Arsenic metabolism by human gut microbiota upon in vitro digestion of contaminated soils.

Background Speciation analysis is essential when evaluating risks from arsenic (As) exposure. In an oral exposure scenario, the importance of presystemic metabolism by gut microorganisms has been evidenced with in vivo animal models and in vitro experiments with animal microbiota. However, it is unclear whether human microbiota display similar As metabolism, especially when present in a contaminated matrix. Objectives We evaluated the metabolic potency of in vitro cultured human colon microbiota toward inorganic As (iAs) and As-contaminated soils. Methods A colon microbial community was cultured in a dynamic model of the human gut. These colon microbiota were incubated with iAs and with As-contaminated urban soils. We determined As speciation analysis using high-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry. Results We found a high degree of methylation for colon digests both of iAs (10 μg methylarsenical/g biomass/hr) and of As-contaminated soils (up to 28 μg/g biomass/hr). Besides the formation of monomethylarsonic acid (MMAV), we detected the highly toxic monomethylarsonous acid (MMAIII). Moreover, this is the first description of microbial thiolation leading to monomethylmonothioarsonic acid (MMMTAV). MMMTAV, the toxicokinetic properties of which are not well known, was in many cases a major metabolite. Conclusions Presystemic As metabolism is a significant process in the human body. Toxicokinetic studies aiming to completely elucidate the As metabolic pathway would therefore benefit from incorporating the metabolic potency of human gut microbiota. This will result in more accurate risk characterization associated with As exposures.

Characterization of physical and chemical properties of spent foundry sands pertinent to beneficial use in manufactured soils

As of 2007, of the 2,000 United States foundries, 93% produce ferrous or aluminum castings, generating 9.4 million tons of non-hazardous spent foundry sand (SFS) annually. Only 28% of the SFS is beneficially used. The U.S. EPA Resource Conservation Challenge identifies SFS as a priority material for beneficial use, with soil blending as a potential reuse option. The objectives of this work were to measure: (1) select chemical and physical properties important to soil quality and function and (2) total and soluble elemental content of 39 SFSs, in order to evaluate SFS suitability as a component in manufactured soils. Total elemental concentration of the SFS was lower than natural background soil levels for most elements analyzed, suggesting limited to no contamination of the virgin sand during metal casting. Pore water elemental concentrations were generally below detection. However, both total and soluble elemental content indicate a potential contribution of plant nutrients. Lettuce (Lactuca sativa) planted in SFS mixtures had a median germination rate of 96.9% relative to the control. Blending SFS at varying ratios with other materials will allow “tailoring” of a manufactured soil’s chemical and physical properties to meet specific growing needs. The SFS organic carbon, clay, and plant nutrient content are benefits of SFS that may make them good candidates as manufactured soil components.

Bioaccessible and non-bioaccessible fractions of soil arsenic

In order for in vitro methods to become widely accepted as tools that accurately assess soil arsenic (As) exposure through the oral ingestion pathway, a better understanding is needed regarding which fractions of soil As are being measured in the in vitro extraction. The objective of the current study is to (1) identify in vitro bioaccessible (IVBA) and non-IVBA fractions of soil As using sequential extraction; and (2) determine the sorptive phases of soil in non-IVBA As soil fractions. Nineteen soils with a range of soil properties were spiked with 250 mg/kg of sodium arsenate and aged. In vitro bioaccessible As (IVBA As) was then determined using The Ohio State University in vitro gastrointestinal method (OSU-IVG), and soil As was fractionated using sequential extraction into: (F1) non-specifically sorbed; (F2) specifically sorbed; (F3) amorphous and poorly crystalline oxides of Fe and Al; (F4) well-crystallized oxides of Fe and Al and residual As phases. The IVBA As across the 19 soil ranged from 0.36 to 2.75 mmol/kg (12 to 86%) with a mean of 1.26 mmol/kg (42%) in the gastric phase and from 0.39 to 2.80 mmol/kg (13 to 87%) in the intestinal phase with a mean of 1.32 mmol/kg (43%). The results of the sequential extraction showed that IVBA As extracted by the OSU-IVG is the As present in the first two fraction (F1 and F2) of the sequential extraction. In the non-IVBA fractions, highly significant relationships (P < 0.01) exist between F3 As and log transformed F3 Fe (r 2 = 0.74), but not F3 Al. In addition, the gastric extraction dissolves a significant fraction of soil Al, but not soil Fe, therefore As sorbed to Al oxides likely contributed to IVBA As and is accounted for in the F2 fraction of the sequential extraction. In vitro methods that demonstrate the ability to extract the similar soil fractions that occur in vivo across a wide range of soil types and As-contaminant sources is an important criteria for in vitro method validation. Further research that includes soils with multiple As-contaminant sources (mining, pesticide, etc.), soil As fractionation, and in vivo bioavailability is needed in order to determine if F1+F2 are the bioavailable As fractions in soils that vary in total As content and sorbed As species.