Elizabeth A. Dayton

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.

Effect of soil properties on lead bioavailability and toxicity to earthworms.

Soil properties are important factors modifying metal bioavailability to ecological receptors. Twenty-one soils with a wide range of soil properties (USA; http://soils.usda.gov/technical/classification/taxonomy/) were amended with a single concentration of Pb (2,000 mg/kg) to determine the effects of soil properties on Pb bioavailability and toxicity to earthworms. Earthworm mortality ranged from 0 to 100% acute mortality following exposure to the same total concentration of Pb (2,000 mg/kg) in amended field soils. Internal Pb concentrations in earthworms ranged from 28.7 to 782 mg/kg, with a mean of 271 mg/kg. Path analysis was used to partition correlations in an attempt to discern the relative contribution of each soil property. Results of path analysis indicated that pH was the most important soil property affecting earthworm mortality (p < 0.01) and internal Pb (p < 0.05). Soil pH was related inversely to mortality and internal Pb, soil solution Pb, and Pb bioavailability. The most important soil property modifying reproduction was amorphous iron and aluminum oxides (FEAL). Because FEAL is rich in pH-dependent cation-exchange sites, several soil properties, including pH, FEAL, and cation-exchange capacity, have a causal effect on Pb adsorption and soluble Pb. Path analysis is useful for assessing contaminated soils with a wide range of soil properties and can assist in ecological risk assessment and remediation decisions for contaminated sites. Soil properties are important factors modifying metal bioavailability and toxicity and should be considered during the ecological risk assessment of metals in contaminated soils.

The effect of dosing vehicle on arsenic bioaccessibility in smelter-contaminated soils

In vitro gastrointestinal (IVG) methods have been developed to provide an expedient and inexpensive means to estimate bioavailability of arsenic and other contaminants from ingestion of contaminated soil. Both in vivo and in vitro techniques have used a fasting model when determining Pb bioavailability/bioaccessibility as a conservative estimate of risk. Some IVG procedures have incorporated a dosing vehicle (DV) or food (i.e., milk) to simulate in vivo conditions. Potential differences in the bioaccessibility of contaminants between fasting and fed states remain a concern for those interested in adopting in vitro procedures for regulatory purposes. In this study, the effect of eliminating a dough-like DV on As bioaccessibility (BA), and this effect on the relationship between in vitro bioaccessible and in vivo relative bioavailability (RBA) As is determined. Also, the effect of phosphate from the DV on IVG BA is investigated. Two types of smelter-contaminated soils, calcine and iron slag, were used to examine the effect of dosing vehicle (DV) on BA determined by IVG. Dosing vehicle did not affect BA in the gastric extraction (GE) or intestinal extraction (IE) for 3 of the 5 calcinated contaminated soils. Inclusion of DV in the GE slightly increased BA for 2 of the 5 slag-contaminated soils. Increases in BA from DV may be attributed to ligand exchange of arsenate with phosphate. Strong relationships between BA and in vivo RBA As were found with or without DV. Bioaccessible As measured by the GE was strongly correlated with in vivo RBA As (IVG without DV: r = 0.92, P < 0.01; IVG with DV: r = 0.96; P < 0.01). Similarly, BA measured by the IE was strongly correlated with in vivo RBA As (IVG without DV: r = 0.90, P < 0.01; IVG with DV: r = 0.96, P < 0.01). The IVG method, with or without DV, is a reliable method to use as a rapid screening tool to provide an estimate of BA in contaminated soils. Further studies should be conducted to determine the influence of foodstuffs on BA for different types of As contaminated soil (i.e., non-smelter soil).

Evaluating the contribution of soil properties to modifying lead phytoavailability and phytotoxicity.

Soil properties affect Pb bioavailability to human and ecological receptors and should be considered during ecological risk assessment of contaminated soil. We used path analysis (PA) to determine the relative contribution of soil properties (pH, organic C [OC], amorphous Fe and Al oxides [FEAL], and cation-exchange capacity [CEC]) in modifying Pb bioavailability. The response of biological endpoints (bioaccumulation and dry matter growth [DMG]) of lettuce (Lactuca sativa) grown on 21 Pb-spiked (2,000 mg/kg) soils were determined. Lettuce tissue Pb ranged from 3.22 to 233 mg/kg, and relative DMG ranged from 2.5 to 88.5% of their respective controls. Simple correlation showed strong relationships between CEC and OC (p < 0.01) and weaker relationships between pH and FEAL (p < 0.05) and Pb bioaccumulation. Results of PA suggest that soil pH increased the negative surface charge of organic matter and clay, thereby increasing CEC and decreasing Pb bioaccumulation. Also, the direct effect of OC on tissue Pb can be attributed to formation of surface Pb complexes by organic matter functional group ligands. Increased OC and/or CEC reduced Pb solubility and bioavailability in the 21 soils in the present study. The relative importance of soil properties likely will vary between studies employing different soils. Soil properties should be considered during the ecological risk assessment of metal in contaminated soils. Path analysis is useful for ecological studies involving soils with a wide range of physicochemical properties and can assist in site risk assessment of metals and remediation decisions on contaminated sites.

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.

REMEDIATION TECHNIQUES FOR MANURE NUTRIENT LOADED SOILS

Many soils in the United States contain excessive levels of nutrients, especially phosphorus (P), due to repeated heavy applications of animal manure. Also, soils with a history of long-term poultry litter or swine manure applications have elevated levels of copper (Cu), zinc (Zn), selenium (Se), and arsenic (As). Runoff and eroded soils carry dissolved and sediment-associated nutrients to water bodies and degrade their quality. Manure-treated fields can also impair air quality by emitting odorous compounds and dust. Several best management practices (BMPs) have the potential to reduce nutrients in runoff water and loading to surface waters. The BMPs were grouped into two broad categories: (1) technologies to reduce excessive nutrient levels in the soil, and (2) technologies to reduce edge of field discharges of nutrients via runoff or sediment loss from overapplication of manure or other organic biosolids. Potential remedial approaches for nutrient-loaded soils include: • Phytoremediation (P, nitrate, metals) with plant species that preferentially bioaccumulate nutrients or metals and use of deep-rooted crops in novel rotations for subsurface nitrate-N recovery; • Soil and manure amendments with P immobilization chemicals and municipal or industrial byproducts to reduce dissolved reactive P and metal bioavailability (water treatment residuals, aglime, coal combustion by-products); • Addition of soil aggregation promoters, coagulants or flocculants such as polyacrylamide polymers to reduce sediment and particulate nutrient offsite discharges (organic matter, N, P, metals); • Deep mechanical tillage to dilute near-surface zone elevated nutrient concentrations and reduce odor emissions (P, metals, odor, trace greenhouse gases); and • Conservation buffer strips to remove dissolved reactive P from runoff and reduce edge-of-field losses of sediments and particulate nutrients and metals. Growing high biomass-yielding plants can remove large amounts of nutrients and may be a promising remedial strategy to export and reduce excess soil nutrients. Bermudagrass and certain warm-season annual grasses produce large dry matter yields, and thus take up large quantities of applied nutrients. Cool-season grasses and certain legumes have a higher uptake of certain nutrients, such as P, and may remove more specific nutrients than bermudagrass, although their yield potential is not as high. Various plant species, including Brassica, preferentially concentrate Cu, Se, and As from high metal soils. Using forage to extract P and specific metals in problem soils has been an effective approach, but is slow to lower soil levels. Grazed-only systems will not effectively remove nutrients from an over-application site since most of the applied nutrients, especially P and K, are redeposited on the land during grazing. Research using soil amendments has shown that land application of drinking water treatment residuals potentially reduces dissolved P in runoff water by up to 70% from land with excessive levels of soil test phosphorus. Other materials such as fly ash and flue-gas desulfurization products from coal combustion in electric power generation and aglime are readily available and also effec-tively reduce P solubility by up to 98% in manure and manured soils. The reactivity of fly ash components with manure P suggests that co-blending will result in reduced discharges upon land application of treated manure, and amending high P soils with coal combustion by-products can reduce soil P availability and the environmental impact of recycling manure on agricultural lands. Reducing particulate nutrient transport from nutrient-loaded fields depends heavily upon soil erosion control practices. The most widely studied and used methods to control erosion by water and wind involve a variety of conservation tillage and crop residue management methods for the wide range of soils and climatic conditions. When used in combination with metal salts, water treatment polymeric flocculants are a promising component of an effective set of management tools to decrease sediment and sediment-associated nutrient loss. Land management practices such as deep tillage and conservation buffers also provide relief from offsite discharges and reduce the ecological risks of the excessive nutrient levels. Many remedial technologies exist to reduce the environmental impact of agricultural land with excessive nutrient levels following repeated applications of manure or organic by-products. Emerging technologies for nutrient immobilization and alternative nutrient recovery using chemical barriers at the research and exploratory stages are being developed into practical BMPs. To further advance soil remediation research and technology transfer, we feel that areas of critical needs should include urgent efforts to: • Identify and develop efficient nutrient and metal accumulator plants and profitable crop rotations for efficient nutrient and metal removal, • Identify and develop efficient nutrient immobilizing chemicals and municipal or industrial byproducts for manure-derived P and metals, • Identify and develop soil treatment and recovery technologies to produce value-added specialty products, • Develop and apply geo-reference techniques to target remediation at the field and watershed scales, and • Develop and evaluate the effectiveness of specific BMP systems in reducing manure nutrient export to the surrounding environment. Integrated solutions are needed for managing excess manure nutrients in crop and livestock production systems. A combination of load reduction techniques and structural and cultural practices may be required to effectively balance the need to reduce soil nutrient levels and discharges from nutrient-loaded fields with the benefits of sustainable production of food and fiber.

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.