Nicholas T. Basta

Evaluation of chemical immobilization treatments for reducing heavy metal transport in a smelter-contaminated soil

Three chemical immobilization materials, agricultural limestone (AL), mineral rock phosphate (RP), and diammonium phosphate (DAP), were evaluated using solute transport experiments to determine their ability to reduce subsurface heavy metal transport in a smelter contaminated soil. Percent reductions in metals transported were based on comparison with cumulative totals of metal species eluted through 60 pore volumes from an untreated soil. Reductions of metal eluted from the AL treatment were 55% for Cd, 45.2% for Pb, and 21.9% for Zn. Rock phosphate mixed with soil at 60 and 180 g kg(-1) was generally ineffective for reducing Cd, Pb, and Zn elution with <27% reduction for Cd, Pb, and Zn. Rock phosphate placed under contaminated soil as a reactive barrier (i.e. layered RP) at 180 g kg(-1) reduced Cd 53% and Zn 24%, and was the most efficient treatment for reducing Pb (99.9%) transport. DAP treatments were superior to all other materials for reducing Cd and Zn elution with reduction >77% for Zn and >91% for Cd from the 90 g DAP kg(-1) treatment. Increasing DAP from 10 to 90 g kg(-1) increased total arsenic released from 0.13 to 29.5 mg kg(-1) and total P eluted from 2.31 to 335 mg. DAP at 10 g kg(-1) was the most effective treatment for immobilizing the combination of Cd, Pb, and Zn, with reductions of 94.6, 98.9, and 95.8%, respectively.

An inter-laboratory trial of the unified BARGE bioaccessibility method for arsenic, cadmium and lead in soil

The Bioaccessibility Research Group of Europe (BARGE) has carried out an inter-laboratory trial of a proposed harmonised in vitro physiologically based ingestion bioaccessibility procedure for soils, called the Unified BARGE Method (UBM). The UBM includes an initial saliva phase and simulated stomach and intestine compartments. The trial involved the participation of seven laboratories (five European and two North American) providing bioaccessibility data for As (11 samples), Cd (9 samples) and Pb (13 samples) using soils with in vivo relative bioavailability data measured using a swine model. The results of the study were compared with benchmark criteria for assessing the suitability of the UBM to provide data for human health risk assessments. Mine waste and slag soils containing high concentrations of As caused problems of poor repeatability and reproducibility which were alleviated when the samples were run at lower soil to solution ratios. The study showed that the UBM met the benchmark criteria for both the stomach and stomach & intestine phase for As. For Cd, three out of four criteria were met for the stomach phase but only one for the stomach & intestine phase. For Pb two, out of four criteria were met for the stomach phase and none for the stomach & intestine phase. However, the study recommends tighter control of pH in the stomach phase extraction to improve between-laboratory variability, more reproducible in vivo validation data and that a follow up inter-laboratory trial should be carried out.

Estimation of Cd, Pb, and Zn bioavailability in smelter-contaminated soils by a sequential extraction procedure.

Chemical fractionation methods may be capable of providing an inexpensive estimate of contaminant bioavailability and risk in smelter-contaminated soil. In this study, the relationship between metal fractionation and methods used to estimate bioavailability of these metal contaminants in soil was evaluated. The Potentially BioAvailable Sequential Extraction (PBASE) was used for Cd, Pb, and Zn fractionation in 12 soils contaminated from Pb and Zn mining and smelting activities. The PBASE procedure is a four-step sequential extraction: extraction 1 (E1) is 0.5 M Ca(NO3)2, E2 is 1.0 M NaOAc, E3 is 0.1 M Na2EDTA, and E4 is 4 M HNO3. Metal bioavailability for two human exposure pathways, plant uptake (phytoavailability) and incidental ingestion (gastrointestinal, Gl, availability), was estimated using a lettuce (Lactuca sativa L.) bioassay and the in vitro-Gl Physiologically Based Extraction Test(PBET). Metal in the PBASE E1 fraction was correlated with lettuce Cd (P < 0.001) and Zn (P < 0.05) and was the best predictor of Cd and Zn phytoavailability. Only total metal content or the sum of all PBASE fractions, ΣE1–4, were correlated (P < 0.001) with PBET gastric phase for Pb. The sum of the first two PBASE fractions, ΣE1–2, was strongly correlated (P < 0.001) with Pb extracted by the PBET intestinal phase. The PBASE extraction method can provide information on Cd and Zn phytoavailability and Gl availability of Pb in smelter-contaminated soils.

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.

Chapter 1 Advances in Assessing Bioavailability of Metal(Loid)s in Contaminated Soils

The term bioavailability has many different meanings across various disciplines of toxicology and pharmacology. Often bioavailability is concerned with human health aspects such in the case of lead (Pb) ingestion by children. However, some of the most contaminated sites are found in nonpublic access facilities (Department of Defense or Energy) or in remote regions as a result of mining or industrial practices in which ecoreceptors such as plants, animals, and soil organisms are the primary concerns as well as the potential for food-chain transfer. In all cases, the endpoint requires movement of the element across a biological barrier. The still utilized approach to base risk assessment on total metal content in soils is an outdated endeavor and has never been proved to be scientifically sound. Yet to reverse this trend, much work is required to establish baseline bioavailability measurements and to develop complementary methods that are capable of predicting bioavailability across a whole range of impacted media in a cost-efficient manner. Thus, regulators have recognized site-specific human health risk assessments play a key role in decision-making processes at contaminated sites. Bioavailability issues surrounding metal-contaminated soils and media have been an area of intense research. For obvious ethical reasons, we cannot solicit humans, in particular the sensitive population of children, from the general population for experimental purposes to examine the long-term harmful effect of metals in soils. However, some adult human feeding studies have been accomplished under tight medical supervision and with very small doses. One option to understand and relate bioavailability in humans is to employ animal surrogates; however, the physiology of most animals is different than that of humans but good correlations have been achieved despite the dose–response paradigm not being identical. The biggest drawback of in vivo studies to examine metal bioavailability to an appropriate ecoreceptor, be it human, plant, or soil organism, is the tremendous cost and time involved relative to chemical and physical surrogates. Chemical surrogate methods generally only require knowledge of the total metal content so that a percent bioaccessible number can be generated from in vitro extractions that simulate digestive systems or mimic responses to sensitive ecoreceptors. However, there is not a consensus as to which of the many in vitro methods is the best analogy to an ecoreceptor uptake and the same can be said for in vivo animal models to mimic human response as well. Further, there is yet to be a single in vitro method that can account for more than a few elements for a specific exposure pathway (e.g., Pb and/or arsenic (As) for human health). These in vitro tests require honest and accurate validation against in vivo bioavailability measurements, but most of all would benefit from metal speciation methods to identify the forms of metals allowing their release. Adaptation of spectroscopic speciation techniques to identify metal(loid) phases is extremely beneficial in bioavailability research to understand the variability of biologically available metal uptake, to manipulate the ecosystem to reduce bioavailability via in situ amendments, to monitor the long-term stability of elements to ensure bioavailability indicators do not change over time, and to develop comprehensive predictive models based on speciation.

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).

Variability of bioaccessibility results using seventeen different methods on a standard reference material, NIST 2710

Bioaccessibility is a measurement of a substances solubility in the human gastro-intestinal system, and is often used in the risk assessment of soils. The present study was designed to determine the variability among laboratories using different methods to measure the bioaccessibility of 24 inorganic contaminants in one standardized soil sample, the standard reference material NIST 2710. Fourteen laboratories used a total of 17 bioaccessibility extraction methods. The variability between methods was assessed by calculating the reproducibility relative standard deviations (RSDs), where reproducibility is the sum of within-laboratory and between-laboratory variability. Whereas within-laboratory repeatability was usually better than (<) 15% for most elements, reproducibility RSDs were much higher, indicating more variability, although for many elements they were comparable to typical uncertainties (e.g., 30% in commercial laboratories). For five trace elements of interest, reproducibility RSDs were: arsenic (As), 22–44%; cadmium (Cd), 11–41%; Cu, 15–30%; lead (Pb), 45–83%; and Zn, 18–56%. Only one method variable, pH, was found to correlate significantly with bioaccessibility for aluminum (Al), Cd, copper (Cu), manganese (Mn), Pb and zinc (Zn) but other method variables could not be examined systematically because of the study design. When bioaccessibility results were directly compared with bioavailability results for As (swine and mouse) and Pb (swine), four methods returned results within uncertainty ranges for both elements: two that were defined as simpler (gastric phase only, limited chemicals) and two were more complex (gastric + intestinal phases, with a mixture of chemicals).

Bioavailability-Based In Situ Remediation To Meet Future Lead (Pb) Standards in Urban Soils and Gardens

Recently the Centers for Disease Control and Prevention lowered the blood Pb reference value to 5 μg/dL. The lower reference value combined with increased repurposing of postindustrial lands are heightening concerns and driving interest in reducing soil Pb exposures. As a result, regulatory decision makers may lower residential soil screening levels (SSLs), used in setting Pb cleanup levels, to levels that may be difficult to achieve, especially in urban areas. This paper discusses challenges in remediation and bioavailability assessments of Pb in urban soils in the context of lower SSLs and identifies research needs to better address those challenges. Although in situ remediation with phosphate amendments is a viable option, the scope of the problem and conditions in urban settings may necessitate that SSLs be based on bioavailable rather than total Pb concentrations. However, variability in soil composition can influence bioavailability testing and soil amendment effectiveness. More data are urgently needed to better understand this variability and increase confidence in using these approaches in risk-based decision making, particularly in urban areas.

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.