Successful Conversion of Pb-Contaminated Soils to Low-Bioaccessibility Plumbojarosite Using Potassium-Jarosite at Ambient Temperature.

Methods promoting lead (Pb) phase transformation in soils are essential for decreasing Pb bioaccessibility/bioavailability and may offer an in situ, cost-efficient process for mitigating contaminant exposure. Recent plumbojarosite (PLJ) conversion methods have shown the greatest potential to reduce soil Pb bioaccessibility, an in vitro bioaccessibility assay measurement of the proportion of Pb solubilized under gastric chemical conditions. Soils tested utilizing the recent PLJ method were found to have a Pb bioaccessibility of <1%, compared to original soils possessing bioaccessibility of >70%. However, this technique requires heat (95-100 °C) to promote mineral transformation. Jarosite-group minerals may incorporate multiple interlayer cations; therefore, we probed the potential for jarosite to remediate Pb via intercalation by reacting presynthesized potassium (K)-jarosite with aqueous Pb and/or Pb-contaminated soil at room temperature. Both K-jarosite and heated PLJ-treated samples were investigated by pairing bioaccessibility analyses with advanced bulk and spatially resolved X-ray absorption spectroscopy analyses. Samples treated with K-jarosite promoted Pb transformation to low-bioaccessibility (<10%) PLJ, with soil being converted to 100% PLJ using both heated and nonheated techniques. µ-X-ray fluorescence (µ-XRF) and µ-X-ray absorption near-edge structure (µ-XANES) showcase significant differences between elemental interactions for heated and nonheated PLJ-treated samples with anglesite impurities being found on the microscale. Although further development is necessary to accommodate for suitable field conditions, results indicate, for the first time, that K-jarosite may successfully convert soil Pb to PLJ without high-temperature conditions. The newfound utility of K-jarosite is expected to be key to future jarosite-based soil Pb remediation method development.

Interactive effects of biochar amendment and lead toxicity on soil microbial community.

This study determined the interactive effects of biochar and lead toxicity on the soil microbial community in a phytoextraction experiment. Arranged with a completely randomized design in a greenhouse, banana liners were planted singly in a sandy soil spiked with Pb(NO3)2 at 0, 400 and 1200 mg kg-1 and amended with bamboo biochar (pyrolyzing at 600 °C) at 0, 1, 3%. Soil samples were taken from triplicated pots five months after planting and measured for (i) content of lead and organic carbon; (ii) lead speciation; and (iii) microbial community composition through 16S rRNA gene sequencing. DNA sequencing results showed that lead and biochar treatments had significant individual and interactive effects on soil microbial dissimilarities from taxonomic levels of phyla to genera. While some specific taxa were lead resistant, biochar addition apparently alleviated lead toxicity and increased their richness (e.g., Alkanibacter, Muciaginibacter, Burkholderiaceae, and Beggiatoaceae). Soil analysis data indicated that biochar not only helped retain more lead in the soil matrix but created a soil environment inducive for transformation of lead into highly insoluble pyromorphite. This study highlights the effectiveness of biochar for lead remediation and the sensitivity of soil microorganisms in sensing changes in soil environment and lead bioavailability.

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.

Bioaccessibility of potentially toxic elements in mine residue particles.

Mining companies used to abandon tailing heaps in countryside regions of Mexico and other countries. Mine residues (MRs) contain a high concentration of potentially toxic elements (PTE). The wind can disperse dust particles (<100 µm) and once suspended in the atmosphere, can be ingested or inhaled; this is a common situation in arid climates. Nowadays, there is little information on the risk of exposure to PTEs from particulate matter dispersed by wind. The pseudo-total PTE in bulk and fractionated MR after aqua regia digestion, the inhalable bioaccessibility with Gamble solution (pH = 7.4), and the gastric bioaccessibility with 0.4 M glycine solution at pH 1.5 were determined. As and Pb chemical species were identified by X-ray absorption near-edge structure (XANES) spectroscopy. The highest rate of dispersion was observed with 74-100 µm particles (104 mg m-2 s-1); in contrast, particles <44 µm had the lowest rate (26 mg m-2 s-1). The highest pseudo-total As (35 961 mg kg-1), Pb (3326 mg kg-1), Cd (44 mg kg-1) and Zn (up to 4678 mg kg-1) concentration was in the <20 µm particles and As in the 50-74 µm (40 236 mg kg-1) particles. The highest concentration of inhaled bioaccessible As (343 mg kg-1) was observed in the <20 µm fraction and the gastric bioaccessible As was 744 mg kg-1, Pb was 1396 mg kg-1, Cd was 19.2 mg kg-1, and Zn was 2048 mg kg-1. The predominant chemical As species was arsenopyrite (92%), while 54% of Pb was in the adsorbed form. Erodible particle matter is a potential risk for humans in case of inhalation or ingestion.

High Lead Bioavailability of Indoor Dust Contaminated with Paint Lead Species.

House dust and soils can be major sources of lead (Pb) exposure for children. The American Healthy Homes Survey (AHHS) was developed to estimate Pb exposure from house dust and soil, in addition to other potential household contaminants and allergens. We have combined X-ray absorption spectroscopic (XAS) fingerprinting and in vivo mouse relative bioavailability (RBA) measurements for a subset of house dust and residential soils collected in the AHHS, with the primary objective of gaining a better understanding of determinants of house dust Pb bioavailability. Lead speciation was well related to variations in RBA results and revealed that highly bioavailable Pb (hydroxy)carbonate (indicative of Pb-based paint) was the major Pb species present in house dusts. Measured Pb RBA was up to 100% and is likely driven by paint Pb. To our knowledge, this is the first report of in vivo Pb RBA for U.S. house dust contaminated in situ with paint Pb and corroborates results from a previous study that demonstrated high RBA of paint Pb added to soil. We also report a relatively low RBA (23%) in a residential soil where the major Pb species was found to be plumbojarosite, consistent with a previous report that plumbojarosite lowers Pb RBA in soils.

Lead speciation, bioaccessibility and source attribution in Missouri's Big River watershed.

The Southeast Missouri Lead District is among the most productive lead deposits exploited in modern times. Intensive mining conducted prior to regulations resulted in a legacy of lead contaminated soil, large piles of mine tailings and elevated childhood blood lead levels. This study seeks to identify the source of the lead contamination in the Big River and inform risk to the public. Isotopic analysis indicated the mine tailing piles at the head of the Big River are the primary source of the lead contamination. The isotopic signature of the lead in these mine tailings matched the lead over 100 km downstream. All of the other potential lead sources investigated had different isotopic signatures. Lead concentrations in soils and sediments decrease with distance downstream of the mine tailings piles. Additionally, the speciation of the lead changes from predominantly mineralized forms, such as galena, to adsorbed lead. This is reflected in the in-vitro bioaccessibility assay (IVBA) analysis which shows higher bioaccessibility further downstream, demonstrating the importance of speciation in risk evaluation.

Comparison of Zn accumulation and speciation in kernels of sweetcorn and maize differing in maturity.

BACKGROUND AND AIMS: Understanding the speciation of Zn in edible portions of crops helps identify the most effective biofortification strategies to increase the supply of nutrients for improving the health and nutrition of consumers. METHODS: Kernels of 12 sweetcorn and three maize (Zea mays) varieties were analysed for Zn concentration and content. The speciation of the Zn in the embryos, endosperms and whole kernels at 21, 28 and 56 days after pollination (DAP) was then examined for one maize and one sweetcorn variety using synchrotron-based X-ray absorption spectroscopy (XAS). KEY RESULTS: Averaged across all sweetcorn and maize varieties at 21 DAP, the embryo contributed 27-29% of the whole kernel Zn whilst the endosperm contributed 71-73 %. While sweetcorn embryos contributed a lower proportion to the total kernel Zn than those of maize, the proportion of total Zn in the embryo increased as kernels aged for both varieties, reaching 33 % for sweetcorn and 49% for maize at 28 DAP. Using XAS, it was predicted that an average of 90 % of the Zn in the embryos was present as Zn-phytate, while in the endosperm the Zn was primarily complexed with an N-containing ligand such as histidine and to a lesser extent with phytate. However, in maize endosperms, it was also observed that the proportion of Zn present as Zn-phytate increased as the kernel matured, thereby also probably decreasing its bioavailability in these mature maize kernels. CONCLUSIONS: The apparent low bioavailability of Zn supplied in maize at its consumption stage (i.e. mature kernels) probably undermines the effectiveness of biofortification of this crop. Conversely, successful biofortification of Zn in sweetcorn and green maize consumed as immature kernels could potentially provide a good source of bioavailable Zn in human diets.

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.

X-ray Analyses of Lead Adsorption on the (001), (110), and (012) Hematite Surfaces.

Predicting the environmental fate of lead relies on a detailed understanding of its coordination to mineral surfaces, which in turn reflects the innate reactivity of the mineral surface. In this research, we investigated fundamental dependencies in lead adsorption to hematite by coupling extended X-ray absorption fine structure (EXAFS) spectroscopy on hematite particles (10 and 50 nm) with resonant anomalous X-ray reflectivity (RAXR) to single crystals expressing the (001), (012), or (110) crystallographic face. The EXAFS showed that lead adsorbed in a bidentate inner-sphere manner in both edge and corner sharing arrangements on the FeO6 octahedra for both particle sizes. The RAXR measurements confirmed these inner-sphere adsorption modes for all three hematite surfaces and additionally revealed outer-sphere adsorption modes not seen in the EXAFS. Lead uptake was larger and pH dependence was greater for the (012) and (110) surfaces, than the (001) surface, due to their expressing singly- and triply coordinated oxygen atoms the (001) surface lacks. In coupling these two techniques we provide a more detailed and nuanced picture of the coordination of lead to hematite while also providing fundamental insight into the reactivity of hematite.

The impact of particle size on the adsorption of citrate to hematite.

We investigated the adsorption of citric acid on the surface of two different sized hematite nanoparticles using batch adsorption experiments, Fourier-transform infrared spectroscopy, surface complexation modeling and computational molecular modeling. Citrate adsorption reached a maximum between pH approximately 2.5 and 5.5 and declined as the pH was increased or decreased from that range. At high surface loading conditions, the dominant adsorbed citrate structure was outer-sphere in nature with a protonation state that varied with pH. At low pH, there was also evidence of an inner-sphere complex consistent with a binuclear, bidentate structure where the hydroxyl group was deprotonated and played an active role in the adsorption. An inner-sphere complex was also detected at low citrate surface loading conditions. Surface-area normalized surface coverages were similar for both sizes of hematite, however, the inner sphere complex appeared to be slightly more prevalent on the smaller hematite. Based on these structures, a triple layer surface complexation model comprised of two outer-sphere complexes and one inner-sphere complex was used to describe the adsorption data for both hematite sizes across a range of solution conditions with a single set of surface area dependent equilibrium constants.