Plumbojarosite Remediation of Soil Affects Lead Speciation and Elemental Interactions in Soil and in Mice Tissues.

Lead (Pb) contamination of soils is of global concern due to the devastating impacts of Pb exposure in children. Because early-life exposure to Pb has long-lasting health effects, reducing exposure in children is a critical public health goal that has intensified research on the conversion of soil Pb to low bioavailability phases. Recently, plumbojarosite (PLJ) conversion of highly available soil Pb was found to decrease Pb relative bioavailability (RBA <10%). However, there is sparse information concerning interactions between Pb and other elements when contaminated soil, pre- and post-remediation, is ingested and moves through the gastrointestinal tract (GIT). Addressing this may inform drivers of effective chemical remediation strategies. Here, we utilize bulk and micro-focused Pb X-ray absorption spectroscopy to probe elemental interactions and Pb speciation in mouse diet, cecum, and feces samples following ingestion of contaminated soils pre- and post-PLJ treatment. RBA of treated soils was less than 1% with PLJ phases transiting the GIT with little absorption. In contrast, Pb associated with organics was predominantly found in the cecum. These results are consistent with transit of insoluble PLJ to feces following ingestion. The expanded understanding of Pb interactions during GIT transit complements our knowledge of elemental interactions with Pb that occur at higher levels of biological organization.

Bioavailable soil Pb minimized by in situ transformation to plumbojarosite.

Exposure to lead (Pb) during early life has persistent adverse health effects. During childhood, ingestion of bioavailable Pb in contaminated soils can be a major route of Pb absorption. Remediation to alter physiochemical properties of soil-borne Pb can reduce Pb bioavailability. Our laboratory-based approach for soil Pb remediation uses addition of iron (Fe) sulfate and application of heat to promote formation of plumbojarosite (PLJ), a sparingly soluble Pb-Fe hydroxysulfate mineral. We treated two soils with anthropogenic Pb contamination and samples of clean topsoil spiked with various Pb compounds (i.e., carbonate, chloride, phosphate [P], or sulfate) to convert native Pb species to PLJ and used a mouse assay to assess relative bioavailability (RBA) of Pb in untreated (U) and remediated soils. Bone and blood Pb levels were significantly lower (P < 0.001, Student's t test) in mice that consumed diets amended with remediated soils than with U soils. Estimated RBA for Pb in both remediated natural soils and Pb-mineral spiked soils were reduced by >90% relative to Pb RBA for U soils, which is substantially more effective than other soil amendments, including P. X-ray absorption spectroscopy showed that >90% of all Pb species in remediated soils were converted to PLJ, and ingested PLJ was not chemically transformed during gastrointestinal tract transit. Post treatment neutralization of soil pH did not affect PLJ stability, indicating the feasibility in field conditions. These results suggest that formation of PLJ in contaminated soils can reduce the RBA of Pb and minimize this medium's role as a source of Pb exposure for young children.

Correlation between lead speciation and inhalation bioaccessibility using two different simulated lung fluids.

This study investigated the relationship between lead (Pb) speciation determined using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy in <10 µm particulate matter (PM10) from mining/smelting impacted Australian soils (PP, BHK5, BHK6, BHK10 and BHK11) and inhalation exposure using two simulated lung fluids [Hatch's solution, pH 7.4 and artificial lysosomal fluid (ALF), pH 4.5]. Additionally, elemental composition of Pb rich regions in PP PM10 and the post-bioaccessibility assay residuals were assessed using a combination of Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) to provide insights into how extraction using simulated lung fluids may influence Pb speciation in vitro. Correlation between Pb speciation (weighted %) and bioaccessibility (%) was assessed using Pearson r (α = 0.1 and 0.05). Lead concentration in PM10 samples ranged from 782 mg/kg (BHK6) to 7796 mg/kg (PP). Results of EXAFS analysis revealed that PP PM10 was dominated by Pb adsorbed onto clay/oxide, while the four BHK PM10 samples showed variability in the weighted % of Pb adsorbed onto clay/oxide and organic matter bound Pb, Pb phosphate, anglesite and galena. When bioaccessibility was assessed using different in vitro inhalation assays, results varied between samples and between assays, Pb bioaccessibility in Hatch's solution ranged from 24.4 to 48.4%, while in ALF, values were significantly higher (72.9-96.3%; p < 0.05). When using Hatch's solution, bioaccessibility outcomes positively correlated to anglesite (r:0.6246, p:0.0361) and negatively correlated to Pb phosphate (r: -0.9610, p:0.0041), organic bound Pb (r: -0.7079, p: 0.0578), Pb phosphate + galena + plumbojarosite (r: -0.9350, p: 0.0099). No correlation was observed between Pb bioaccessibility (%) using Hatch's solution and weighted % of Pb adsorbed onto clay/oxide and between bioaccessibility (%) using ALF and any Pb species. SEM and EDX analysis revealed that a layer of O-Pb-Ca-P-Si-Al-Fe formed during the in vitro extraction using Hatch's solution.

Relationship between Pb relative bioavailability and bioaccessibility in phosphate amended soil: Uncertainty associated with predicting Pb immobilization efficacy using in vitro assays.

In this study, an in vitro in vivo correlation (IVIVC) between Pb in vitro bioaccessibility (IVBA) and relative bioavailability (RBA) was explored to determine whether the efficacy of Pb immobilization in phosphate amended soils could be predicted using an in vitro approach. Mining/smelting impacted soil from Broken Hill, Australia (582-3536 mg/kg of Pb in the <250 µm soil particle fraction) was amended with Phosphoric Acid (PA), Mono Ammonium Phosphate (MAP) or Triple Super Phosphate (TSP) at Pb:P molar ratios of 1:1-1:5. Pb speciation in pre- and post-treated soil was assessed using X-ray Absorption Spectroscopy (XAS), Pb IVBA was measured using the Solubility Bioaccessibility Research Consortium (SBRC) assay (gastric and intestinal phases), and Pb RBA was determined in mice using blood Pb concentration as the bioavailability endpoint. XAS analysis revealed a 3.75-6.00 fold increase in the weighted % of Pb phosphates in soil containing >1000 mg/kg Pb while treatment effect ratios of 0.89-0.99 (SBRC-G), 0.09-0.71 (SBRC-I) and 0.27-0.80 (RBA) were observed in PA amended soil (Pb:P = 1:5). Although significant (p < 0.05) correlation were obtained between Pb RBA and IVBA (%) determined using SBRC-G (r = 0.64) and SBRC-I (r = 0.67), the strengths of the relationships were weak (r2 = 0.41-0.45). This research highlights the complexities associated with the prediction of Pb RBA in phosphate amended soil.

Methodological factors influencing inhalation bioaccessibility of metal(loid)s in PM2.5 using simulated lung fluid.

In this study, methodological factors influencing the dissolution of metal(loid)s in simulated lung fluid (SLF) was assessed in order to develop a standardised method for the assessment of inhalation bioaccessibility in PM2.5. To achieve this aim, the effects of solid to liquid (S/L) ratio (1:100 to 1:5000), agitation (magnetic agitation, occasional shaking, orbital and end-over-end rotation), composition of SLF (artificial lysosomal fluid: ALF; phagolysosomal simulant fluid: PSF) and extraction time (1-120 h) on metal(loid) bioaccessibility were investigated using PM2.5 from three Australian mining/smelting impacted soils and a certified reference material. The results highlighted that SLF composition significantly (p < 0.001) influenced metal(loid) bioaccessibility and that when a S/L ratio of 1:5000 and end-over-end rotation was used, metal(loid) solubility plateaued after approximately 24 h. Additionally, in order to assess the exposure of metal(loid)s via incidental ingestion of surface dust, PM2.5 was subjected to simulated gastro-intestinal tract (GIT) solutions and the results were compared to extraction using SLF. Although As bioaccessibility in SLF (24 h) was significantly lower than in simulated GIT solutions (p < 0.05), Pb bioaccessibility was equal to or significantly higher than that extracted using simulated GIT solutions (p < 0.05).

An inhalation-ingestion bioaccessibility assay (IIBA) for the assessment of exposure to metal(loid)s in PM10.

Although metal(loid) bioaccessibility of ambient particulate matter, with an aerodynamic diameter of <10µm (PM10), has recently received increasing attention, limited research exists into standardising in-vitro methodologies using simulated lung fluid (SLF). Contradictions exist regarding which assay parameters should be adopted. Additionally, potential continuation of metal(loid) dissolution once PM10 is cleared from the lungs and passed through the gastro-intestinal tract (GIT) has rarely been addressed. The objective of this study was to assess parameters that influence inhalation bioaccessibility in order to develop a conservative assay that is relevant to a human inhalation scenario. To achieve this aim, the effect of solid to liquid (S/L) ratio, extraction time, agitation and five major SLF compositions on the bioaccessibilities of arsenic (As) and lead (Pb) was investigated using PM10 from three Australian mining/smelting impacted regions. Using the biologically relevant parameters that resulted in the most conservative outcomes, bioaccessibility of metal(loid)s in PM10 was assessed in SLF, followed by simulated GIT solutions. Results from this study revealed that fluid composition and S/L ratio significantly affected metal(loid) dissolution (p<0.05). The highest Pb bioaccessibility resulted using simulated lung-gastric solution, while that of As resulted using simulated lung-gastric-small intestinal tract solutions. Compared to SLF alone, metal(loid) dissolution using the inhalation-ingestion bioaccessibility assay (IIBA) was significantly higher (p<0.05) for all PM10 samples.

Point of zero charge: Role in pyromorphite formation and bioaccessibility of lead and arsenic in phosphate amended soils.

Soluble lead (Pb) can be immobilized in pure systems as pyromorphite by adding sources of phosphorus (P), but uncertainties still remain in natural systems. Knowledge of PZC is important to predict the ionization of functional groups and their interaction with metal species in solution. This study utilized the Pb- and As-contaminated soils to determine the combined effect of pH with respect to PZC and different rates of P-application on pyromorphite formation, and Pb and arsenic (As) bioaccessibility as impacted by speciation changes. Solution chemistry analysis along with synchrotron-based Pb- and As-speciation, and bioaccessibility treatment effect ratios (TERs) were conducted. Results indicated no significant effect of PZC on pyromorphite formation in P-amended soils; however, the TERPb appeared significantly lower at pH>pHPZC and higher at pHpHPZC, compared to the other two treatments, for the tested soils. The lack of conversion of soil Pb to pyromorphite may be attributed to presence of stable minerals limiting soluble-Pb availability and high organic matter content of the tested soils.

State of the science review: Potential for beneficial use of waste by-products for in situ remediation of metal-contaminated soil and sediment.

Metal and metalloid contamination of soil and sediment is a widespread problem both in urban and rural areas throughout the United States (U.S. EPA, 2014). Beneficial use of waste by-products as amendments to remediate metal-contaminated soils and sediments can provide major economic and environmental advantages on both a site-specific and national scale. These waste by-products can also reduce our need to mine virgin materials or produce synthetic materials for amendments. Waste by-products must not be hazardous or pose unacceptable risk to human health and the environment, and should be a suitable replacement for virgin and synthetic materials. This review serves to present the state of science on in situ remediation of metal-contaminated soil and sediment and the potential for beneficial usage of waste by-product materials. Not all unintended consequences can be fully understood or predicted prior to implementing a treatment option, however some realized, and potentially unrealized, benefits and unintended consequences are explored.

Lead and Arsenic Bioaccessibility and Speciation as a Function of Soil Particle Size.

Bioavailability research of soil metals has advanced considerably from default values to validated in vitro bioaccessibility (IVBA) assays for site-specific risk assessment. Previously, USEPA determined that the soil-size fraction representative of dermal adherence and consequent soil ingestion was <250 µm. This size fraction was widely used in testing efforts for both in vivo and in vitro experiments. However, recent studies indicate the <150-µm size fraction better represents the particle size that adheres to skin for potential ingestion. At issue is the relevance of validated in vivo and in vitro methods developed with <250 µm moving to the <150-µm fraction. The objectives of this study were to investigate <250-µm versus <150-µm particle size and particle size groups for evaluating lead (Pb) and arsenic (As) IVBA and speciation. Soils with different properties were homogenized, oven dried, and sieved: <250 to > 150, <150 to >75, <75 to >38, and <38 µm. Sieved versus ground subsamples of <250-µm and <150-µm bulk soils were also used for IVBA and synchrotron-based Pb and As speciation. Although we observed an increase in total and IVBA-extractable Pb and As with decreased soil particle size, changes in %IVBA of Pb and As (dependent on the ratio extractable:total) remained consistent in all of the tested soils. No significant changes in Pb and As speciation were observed across the soil fractions. The results suggest that using the more relevant <150-µm fraction will not undermine currently validated IVBA protocols in future bioavailability studies.

Alterations of lead speciation by sulfate from addition of flue gas desulfurization gypsum (FGDG) in two contaminated soils.

This is the first study to evaluate the potential application of FGDG as an in situ Pb stabilizer in contaminated soils with two different compositions and to explain the underlying mechanisms. A smelter Pb contaminated soil (SM-soil), rich in ferrihydrite bound Pb (FH-Pb), cerussite and litharge with a total Pb content of 65,123mg/kg and an organic matter rich orchard soil (BO-soil), rich in FH-Pb and humic acid bound Pb with a total Pb content of 1532mg/kg were amended with 5% FGDG (w/w). We subjected the two soils to three leaching tests; toxicity characteristic leaching protocol (TCLP), synthetic precipitation leaching protocol (SPLP), kinetic batch leaching test (KBLT) and in-vitro bioaccessibility assay (IVBA) in order to evaluate the FGDG amendment on Pb stabilization. Solid residues of original and FGDG amended soil were analyzed using X-ray absorption spectroscopy (XAS) to identify changes in Pb speciation after each leaching test. The leachate Pb concentrations of FGDG amended soil were lower compared to those of in non-amended soil. The linear combination fitting analysis of XAS confirmed the formation of anglesite and leadhillite in FGDG amended soil. FGDG reduced the Pb desorption from ferrihydrite (FH), by forming FH-Pb-SO4 ternary complexes. FGDG decreased the Pb adsorption onto humic acid (HA) possibly due to the release of divalent cations such as Ca and Mg, which can compete with Pb to get adsorbed onto HA. The FGDG can successfully be used to remediate Pb contaminated soil. The efficiency of the treatment highly depends on the soil composition.

Synchrotron-based X-Ray Spectroscopy Studies for Redox-based Remediation of Lead, Zinc, and Cadmium in Mine Waste Materials.

Several studies have examined the effect of submergence on the mobility of metals present in mine waste materials. This study examines the effect of organic carbon (OC) and sulfur (S) additions and submergence time on redox-induced biogeochemical transformations of lead (Pb), zinc (Zn), and cadmium (Cd) present in mine waste materials collected from the Tri-State mining district located in southeastern Kansas, southwestern Missouri, and northeastern Oklahoma. A completely randomized design, with a two-way treatment structure, was used for conducting a series of column experiments. Two replicates were used for each treatment combination. Effluent samples were collected at several time points, and soil samples were collected at the end of each column experiment. Because these samples are highly heterogeneous, we used a variety of synchrotron-based techniques to identify Pb, Zn, and Cd speciation at both micro- and bulk-scale. Spectroscopic analysis results from the study revealed that the addition of OC, with and without S, promoted metal-sulfide formation, whereas metal carbonates dominated in the nonamended flooded materials and in mine waste materials only amended with S. Therefore, the synergistic effect of OC and S may be more promising for managing mine waste materials disposed of in flooded subsidence mine pits instead of individual S or OC treatments. The mechanistic understanding gained in this study is also relevant for remediation of waste materials using natural or constructed wetland systems.