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Lead (Pb) sorption onto oxide surfaces in soils may strongly influence the risk posed from incidental ingestion of Pb-contaminated soil. Lead was sorbed to model oxide minerals of corundum (alpha-Al(2)O(3)) and ferrihydrite (Fe(5)HO(8).4H(2)O). The Pb-sorbed minerals were placed in a simulated gastrointestinal tract (in vitro) to simulate ingestion...
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... Arsenic is often associated with Fe oxides in soil but can also be adsorbed to CaCO 3 surfaces in calcareous soils. Arsenic sorbed to CaCO 3 is easily dissolved under acidic conditions of the gastrointestinal tract and becomes potentially bioavailable to humans whereas As sorbed to Fe oxide may not readily dissolve in the gastrointestinal track and will potentially be less bioavailable [14]. Soil amendments for As immobilization typically involve the application of Fe or Fe oxide to soil to bind As as either insoluble minerals or strong complexes [15]. ...
... Soil amendments for As immobilization typically involve the application of Fe or Fe oxide to soil to bind As as either insoluble minerals or strong complexes [15]. Once stabilization is achieved via chemical alteration, the next challenge is assessing the resulting changes in human health and environmental risk associated with soil Pb/As [14]. ...
Purpose of Review
The ubiquity of soil contamination by lead (Pb) and arsenic (As) has prompted the development of numerous techniques for its remediation. For human health exposure assessment, oral bioavailability-based methods are the most suitable to assess the efficacy of these treatment strategies, including in vivo relative bioavailability (systemic absorption relative to a toxicity reference) and in vitro bioaccessibility (dissolution in simulated gastrointestinal solutions). This paper provides a critical review of opportunities and challenges associated with the immobilization of Pb and As in contaminated soil.
Recent Findings
This review identified that the major inorganic and organic amendments used to reduce Pb and As exposure include phosphate, industrial by-products, metal oxides, organic matter, biochar, and treatment with iron sulphate to promote the formation of plumbojarosite in soil. In addition to RBA and IVBA assessment, investigating changes in Pb/As speciation in untreated vs treated soil can provide additional confirmation of treatment efficacy. The results of this review showed that immobilization efficacy may vary depending on amendment type, Pb, and As speciation in soil and the approach used for its assessment.
Summary
Reducing childhood exposure to Pb and As is a significant challenge, given the variety of contamination sources and treatment strategies. A lines-of-evidence approach using standardized methodologies is recommended for the assessment of immobilization efficacy to ensure exposure and risk reduction
Graphical Abstract
Bioavailability-based remediation strategies. Popular soil amendments to reduce Pb exposure include phosphate, industrial by-products, metal oxides, organic matter, and biochar; however, these may increase As exposure. The plumbojarosite formation technique has been recently developed to mitigate Pb and As exposure simultaneously. Multiple lines-of-evidence approach is recommended to assess treatment efficacy
... Based on a mouse model, Juhasz et al. (2014) reported that Pb-RBA in a contaminated soil from a shooting range site containing Pb oxide and Pb carbonate was much higher than nonferrous slag contaminated soils enriched with Pb phosphate and hydroxylpyromorphite (77 vs. 10%). Similarly, in vitro Pb bioavailability is generally higher for Pb sorbed onto corundum (>85%) and ferrihydrite (53e88%) than that onto birnessite (<0.01%) (Beak et al., 2006;Beak et al., 2008). However, advanced spectroscopic speciation techniques are not easily accessible to most researchers as they require special training besides their limited availability. ...
... Two reasons may help explain the lower Pb-RBA in soil A. The soil was contaminated by mining activity where insoluble minerals such as Pb phosphate and galena have been found Sun et al., 2018), which was supported by the fact that high P concentration in the residual fraction of soil A (911 mg kg À1 for AF 5 vs. 129e589 for BF5 and CF 5 ). In addition, soil A was more enriched with Al (77.9 g kg À1 ) and Fe (104 g kg À1 ) than soil B and C (17.2e20.2 and 20.7e25.1 g kg À1 ) (Table 1), which may sorb more Pb and lower its bioavailability (Beak et al., 2006;Beak et al., 2008). Li et al. (2017a) also found that Fe concentrations were negatively correlated with Pb-RBA in 12 contaminated-soils based on a mouse kidney model. ...
... It was reported that Pb in the exchangeable and carbonate bound fraction contributed 52% variance in Pb-RBA (Li et al., 2015), therefore removal of these two fractions lowered Pb-RBA in the soils. Studies also reported that 53e85% of Pb absorbed onto ferrihydrite was bioavailable (Beak et al., 2006). Based on X-ray absorption near-edge structure, Smith et al. (2011) showed that Pb associated with Fe minerals and organic matter was solubilized in the gastric phase. ...
Lead bioavailability in contaminated soils varies considerably depending on Pb speciation and sources of contamination. However, little information is available on bioavailability of Pb associated with different fractions. In this study, the Tessier sequential extraction was used to fractionate Pb in 3 contaminated soils to exchangeable (F1), carbonate-bound (F2), Fe/Mn oxides-bound (F3), organic-bound (F4), and residual fractions (F5). In addition, soil residues after F1–F2 extraction (F345), F1–F3 extraction (F45), and F1–F4 extraction (F5) were measured for Pb relative bioavailability (RBA) using a mouse kidney model. Based on the mouse model, Pb-RBA in the soils was 44–93%, which decreased to 43–89%, 28–75%, and 15–68% in the F345, F45, and F5 fractions, respectively. Based on Pb-RBA in the soil residues, Pb-RBA in different fractions was calculated based on a mass balance. The data showed that Pb-RBA was the highest (∼100%) in the exchangeable and carbonate fraction, and the lowest (15–68%) in the residual fraction. In addition, Pb in the first three fractions (F1–F3) contributed most (83–89%) to bioavailable Pb in contaminated soils. Our study shed light on oral bioavailability of Pb in contaminated soils of different fractions based on sequential extraction and provide important information for soil remediation.
... The values for Pb TERs estimations in BO were <1, falling in the low to median range of 0.53 to 0.75 at pH 2.5 and 0.66 to 0.79 at pH 1.5 ( Table 3). The presence of Pb sorbed to ferrihydrite in BO might have contributed significantly to the bioaccessible Pb pool, possibly because of dissolution and re-precipitation during IVBA extraction procedure [51][52][53]. Treatment effect ratio (TER) represents the bioaccessibility in amended soil divided by bioaccessibility in the untreated soil at that particular pH. BO in the label represents Barber orchard soil, "L" stands for low P-amendment, "M" for medium P-amendment, and "H" for high P-amendment. ...
Soluble lead (Pb) can be immobilized in pure systems as pyromorphite through the addition of phosphorus (P) sources; however, uncertainties remain in natural systems. Knowledge of point
zero charge (PZC) is important to predict the ionization of functional groups and their interaction with metal species in solution. This study utilized 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 as well as Pb and arsenic (As) bioaccessibility as impacted by speciation changes. Solution
chemistry analysis along with synchrotron-based Pb- and As-speciation as well as 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 pH < pHPZC (� = 0.05). In contrast, the TERAs was significantly higher at pH > pHPZC compared to the other two treatments for the tested soils. The lack of conversion of Pb to pyromorphite may be attributed to several reasons including the presence of highly stable minerals, such as plumbojarosite, limiting soluble Pb availability to react with phosphates, high Fe and S content in IKS, high organic matter in BO, and high Ca content in NW.
... The Pb-XAS data analysis on BO soil, which is contaminated with Pb-arsenate (PbHAsO 4 ) pesticide, indicated Pb as sorbed phases to Fe (hydr) oxides as the dominant species and Pb complexed with humic acids as the minor component, which is expected in soil with high organic matter and high Fe oxides. Lead sorbed to ferrihydrite contributes significantly to the bioaccessible Pb pool, possibly due to dissolution and reprecipitation during the IVBA extraction procedure (Beak et al., 2006(Beak et al., , 2008Smith et al., 2011). This explains the reason behind detecting higher %IVBA Pb, ranging from 68.12 to 75.31% in BO soils (Supplemental Table S3). ...
... Presence of As(V) sorbed to birnessite is common, as As(V) ions present under oxidized conditions are often sorbed to Fe and Mn hydr(oxides), organic matter, and clays in soils. Presence of higher Mn (1600 mg kg −1 ) in BO soils supports the formation of As(V) sorbed to birnessite, which has bioaccessibility of 16 to 28%, as reported by Beak et al. (2006). High organic carbon (~7.2%) in BO soils may also have resulted in increased arsenic bioaccessibility in BO soils (29-30%). ...
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 mm. This
size fraction was widely used in testing efforts for both in vivo
and in vitro experiments. However, recent studies indicate the
<150-mm 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 mm
moving to the <150-mm fraction. The objectives of this study were
to investigate <250-mm versus <150-mm 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 mm. Sieved versus ground subsamples of <250-mm and
<150-mm bulk soils were also used for IVBA and synchrotronbased
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-mm fraction will not undermine currently validated
IVBA protocols in future bioavailability studies.
... Amorphous Fe in soil C (15.4 g kg −1 ) was much higher than it in soil B (5.10 g kg −1 ; Table 1), possibly contributing its lower Pb-RBA in soil C. This is because, in acidic stomach environment, dissolved Fe may inhibit Pb absorption across the GI barrier via divalent metal transporter [34] whereas in neutral intestinal condition, the dissolved Fe may co-precipitate with Pb, thereby lowing bioavailable Pb [35]. ...
... 0.40-5.75, and 0.24-9.79% for UBM, SBRC, IVG, and PBET, respectively. This was probably due to Pb resorption on soil matrix and/or Pb coprecipitated with Fe oxides [34,42,44]. In addition, P amendment significantly decreased Pb bioaccessibility to 0.03-1.63, ...
Hand-to-mouth activity is an important pathway for children’s exposure to contaminated soils, which is often co-contaminated by Pb and As in mining and smelting sites. To reduce soil Pb risk to humans by oral exposure, phosphate amendments have been used to reduce Pb relative bioavailability (RBA), but its efficiency has not been investigated using validated in vitro assays and its influence on As-RBA has not been studied. Here, 5 contaminated soils were amended with 0.5% phosphoric acid (PA) to study its effect on Pb and As availability using a newly-developed mouse kidney model and 4 in vitro assays including UBM, SBRC, IVG, and PBET. Based on the mouse kidney model, Pb-RBA in PA-amended soils decreased from 14.2–62.5% to 10.1–29.8%. In contrast, As-RBA decreased from 26.5% to 15.9% in soil B but increased from 27.5 to 41.2% in soil D, while changes were insignificant in 3 other soils (35.8–58.8 to 28.1–61.1%). When assessing Pb bioaccessibility in PA-amended soils, comparable results were found using PBET and SBRC. For As, its bioaccessibility increased in PA-amended soils, inconsistent with in vivo data. Our results shed light on the importance of method selection to assess risk in Pb- and As-contaminated soils amended with phosphate.
... The low Pb-RBA values in soils 3, 6, 7, and 8 may be due to their high Fe concentration (Table 1), which was supported by the strong negative correlation (r 2 = 0.55) between Pb-RBA and Fe concentration for the 12 soils (Fig. S2). Previous estimation of soil Pb-RBA using in vitro bioaccessibility assay showed decreased Pb dissolution in the neutral intestinal tract compared to the acidic stomach due to co-precipitation of Pb with Fe [6,14,23]. Higher Fe in soils 3, 6, 7, and 8 might enhance its co-precipitation with Pb in the intestinal tract, thereby leading to lower Pb-RBA. In addition, higher amount of soluble Fe in the stomach may inhibit Pb absorption across the gastrointestinal barrier through divalent metal transporter [24]. ...
Mouse is an acceptable animal model to measure lead (Pb) relative bioavailability (RBA) in contaminated soils; however, there is a lack of comparisons among Pb-RBA measurements based on different endpoints and dosing approaches. In this study, 12 soils (47.8-8123mg Pbkg(-1)) were assessed for Pb-RBA using Pb accumulation in mouse liver, kidneys, and/or femur following a 10-d steady state soil dose via diet, with 6 soils being measured using mouse bioassays with area under the mouse blood Pb concentration time curve (AUC) following a single gavaged dose as the endpoint. Based on individual endpoints of the steady state method, Pb-RBA in soils was 2.1-83.4%, being generally consistent among liver, kidneys, and femur with strong linear correlations between them (r(2)=0.74-0.89). To compensate variation in Pb distribution among different tissues, Pb-RBA was further calculated using a combined endpoint (e.g., sum of Pb accumulation in liver, kidneys, and femur). Compared to Pb-RBA based on individual tissue showing relative standard deviation (RSD) of 11.9-15.8%, Pb-RBA using the combined endpoint showed lower RSD (10.8%), thereby being more robust. For the 6 soils with Pb-RBA based on both mouse single gavaged and steady state dosing approach, no significant difference was observed; however, steady state approach was more repeatable among animals with lower RSD (11.4% vs. 34.5%). To ensure robustness of in vivo data, the steady state dosing approach with Pb accumulation in combined tissues is recommended.
... Adding lignin PyOM with a pH of 10.8 (the highest pH value among the PyOM samples) would increase negative surface charge of most mineral surfaces, which might lower their affinity to electron-rich oxygen in lignin PyOM, leading to lower interaction of OM to minerals such as quartz. On the other hand, corundum, with a high point of zero charge above pH 9 (Beak et al., 2006), would still have positive surface charge and may interact with negatively charged lignin PyOM, thereby leading to lower mineralization extent. Unfortunately, the original or bulk pH does not fully capture which pH and therefore which charge occurs at the interface (Lehmann et al., 2015b), which would be desirable, albeit experimentally challenging, information to obtain. ...
The objectives of the study were to (i) investigate whether short term mineralization varies to a greater extent by charring organic matter (OM) or by exposure to different mineral surfaces,(ii) examine the effect of clay mineralogy on mineralization of either charred or uncharred OM, and (iii) quantify these differences in mineralization in the context of C evolution during charring. Mineralization of different combinations of OM types (corn stover, cellulose, glucose, lignin and arginine) or their corresponding pyrogenic organic matter (PyOM) with minerals (quartz, corundum, goethite, kaolinite, and kaolinite mixed with goethite) were assessed in incubation experiments at 30 °Cfor 196 days. All PyOM materials showed lower mineralization extent (3.8 - 7.5% of initial C) than uncharred OM (7.6-89% of initial C). Charring decreased mineralization to a greater extent (by 4.7-95%) than differences in the minerals (by 3.7 -70%). The relative standard deviation of C mineralization of individual charred OM materials caused by differences in mineralogy was smaller (8.7-16%) than that of uncharred OM (8.7-65%). For the ratio of C remaining after both mineralization and charring to mineralization of uncharred biomass, the relative standard deviation was higher for different OM types (51-84%) than mineral types (1.1-47%), suggesting that mineralogy was less important than OM properties. Therefore, the importance of mineralogy was lower for charred than uncharred OM and the type of PyOM more strongly controlled mineralization than the type of mineral during the initial decomposition process.
... No significant relationship was obtained between Pb bioaccessibility and soil Mn content. A number of studies have demonstrated that Fe/Mn oxides are very efficient sorbents and control solubility of Pb at pH 5.5 (O'Reilly and Hochella, 2003), and Fe(OH) 3 and ferrihydrite (Fe 5 HO 8 Á4H 2 O) were very effective in reducing bioaccessible Pb (Beak et al., 2006). ...
... Fe oxyhydroxides are highly soluble at the gastric pH values used this study, and the (co-) precipitation of Pb and amorphous Fe may have resulted from the oversaturation of hydrolyzed Fe species following the transition from the acidic environment of the stomach to the neutral environment of the intestinal tract. 14,20 Lead RBA in SH15 was significantly lower (10.1 ± 1.2%) than in PP2. Low Pb RBA may be attributed to the low solubility of Pb present in the SH15 15,21 and the elevated phosphate concentrations (as a result of agricultural practices) in this soil (Table 1). ...
In this study, the effect of phosphate treatment on lead relative bioavailability (Pb RBA) was assessed in three distinct Pb-contaminated soils. Phosphoric acid (PA) or rock phosphate (RP) were added to smelter (PP2), non-ferrous slag (SH15) and shooting range (SR01) impacted soils at a P:Pb molar ratio of 5:1. In all phosphate amended soils, Pb RBA decreased compared to untreated soils when assessed using an in vivo mouse model. Treatment effect ratios (i.e. ratio of Pb RBA in treated soil divided by Pb RBA in untreated soil) ranged from 0.39-0.67, 0.48-0.90 and 0.03-0.19 for PP2, SH15 and SR01 respectively. The decrease in Pb RBA following phosphate amendment was attributed to the formation of poorly soluble Pb phosphates (i.e. chloropyromorphite, hydroxypyromorphite and Pb phosphate) identified by X-ray absorption spectroscopy (XAS). However, a similar decrease in Pb RBA was also observed in untreated soils following the sequential gavage of phosphate amendments. This suggests that in vivo processes may also facilitate the formation of poorly soluble Pb phosphates which decreases Pb absorption. Furthermore, XAS analysis of PA treated PP2 indicated further in vivo changes in Pb speciation as it moved through the gastrointestinal tract resulting in the transformation of hydroxypyromorphite to chloropyromorphite.
... 29 Additionally, XAS has been used to examine the effects of digestion on the speciation of Pb in contaminated geomedia with one result being that Pb associated with Fe (oxyhydr)oxides, particularly ferrihydrite, contributes significantly to the bioaccessible Pb pool, possibly due to dissolution and reprecipitation during GI fluid extraction. 23,36,37 The overall goal of the current project was to assess Pb speciation in the near-surface depths of uncapped mine tailings that have undergone weathering in a semi-arid desert environment. The Klondyke State Superfund Site is the subject of intense interest because it comprises mine tailings devoid of vegetation located at the mouth of Aravaipa canyon, an important riparian corridor in southeastern Arizona. ...
... Mine tailings Pb solubilized by H 2 O and AA extractions is also soluble in stomach acid, and Pb associated with ferrihydrite has been shown to be 53-88% solubilized during the gastric portion of in vitro extraction. 3,36 Hence, accumulation of this bioavailable Pb pool at the tailings-atmosphere interface may constitute a health hazard. ...
Mine tailings can host elevated concentrations of toxic metal(loid)s that represent a significant hazard to surrounding communities and ecosystems. Eolian transport, capable of translocating small (micrometer-sized) particles, can be the dominant mechanism of toxic metal dispersion in arid or semiarid landscapes. Human exposure to metals can then occur via direct inhalation or ingestion of particulates. The fact that measured doses of total lead (Pb) in geomedia correlate poorly with blood Pb levels highlights a need to better resolve the precise distribution of molecularly speciated metal-bearing phases in the complex particle mixtures. Species distribution controls bioaccessibility, thereby directly impacting health risk. This study seeks to correlate Pb-containing particle size and mineral composition with lability and bioaccessibility in mine tailings subjected to weathering in a semiarid environment. We employed X-ray absorption spectroscopy (XAS) and X-ray fluorescence (XRF), coupled with sequential chemical extractions, to study Pb speciation in tailings from the semiarid Arizona Klondyke State Superfund Site. Representative samples ranging in pH from 2.6 to 5.4 were selected for in-depth study of Pb solid-phase speciation. The principle lead-bearing phase was plumbojarosite (PbFe(6)(SO(4))(4)(OH)(12)), but anglesite (PbSO(4)) and iron oxide-sorbed Pb were also observed. Anglesite, the most bioavailable mineral species of lead identified in this study, was enriched in surficial tailings samples, where Pb concentrations in the clay size fraction were 2-3 times higher by mass relative to bulk. A mobile and bioaccessible Pb phase accumulates in surficial tailings, with a corresponding increase in risk of human exposure to atmospheric particles.
