Improving the predictive value of bioaccessibility assays and their use to provide mechanistic insights into bioavailability for toxic metals/metalloids - A research prospectus

Interactions between environmental pollutants and gut microbiota: A review connecting the conventional heavy metals and the emerging microplastics

Growing epidemiological evidence suggests that the diverse and functional gut microbiota plays a vital role in regulating the health and disease of organisms including human. However, organisms are inevitably exposed to widespread environmental pollutants, and the interactions between their gut microbiota and pollutants are relatively underreported. The present paper considers heavy metals (HMs) and microplastics (MPs) as representatives of traditional and emerging pollutants and systematically summarizes their effects on gut microbiota and the effects of gut microbiota on pollutants. The former refers to the alterations in the gut microbiota's abundance, diversity and composition caused by pollutants, whereas the latter focuses on the changes in the metabolism of pollutants by adjusting the dominant bacteria, specific enzymes, and key genes. In particular, some fields were found to be poorly studied, including extension of research to humans, mechanistic exploration of gut microbiota's changes, and the metabolism of pollutants by gut microbiota. Accordingly, we draw attention to the development and application of in vitro test models to more accurately explore the interactions between pollutants and gut microbiota when assessing human health risks. In addition, by combining state-of-the-art biological techniques with culturomics, more gut microbiota can be identified, isolated, and cultured, which helps to confirm the relationship between pollutants and gut microbiota and the potential function of gut microbiota in pollutant metabolism. Furthermore, the phenomenon of coexposure to HMs and MPs is becoming more frequent, and their interactions with gut microbiota and the influence on human health is expected to be one of the frontier research fields in the future. The key information presented in this review can stimulate further development of techniques and methodologies for filling the knowledge gaps in the relationships between combined pollutants (HMs and MPs), gut microbiota, and human health.

Analysis of changes and influencing factors of stablization treatment effects and bioavailability after freeze-thaw: a case study of Pb-contaminated soil in a non-ferrous metal factory in Northeast China

Introduction

Solidification/Stabilization techniques are commonly used for the containment and isolation of Pb-contaminated soil, but they cannot reduce the amount of contaminants. Freeze - thaw after stabilization may affect Pb's environmental behavior and increase the uncertainty of environmental risk.

Methods

In vitro experiments can simulate the bioavailability of heavy metals to the human body, accurately assessing their environmental health risks. In this study, soil samples from Pbcontaminated site are collected from a non-ferrous metal plant in Northeastern China. Through the results of stabilization and freeze-thaw after stabilization experiments, analyzing the changes of physicochemical property, Pb treatment effects (total concentration, leaching concentration, and occurrence forms) and microbial communities, and studying the influencing factors of Pb's bioavailability.

Result and discussion

The results show that stabilization and freeze - thaw after stabilization directly alter soil physicochemical property, thereby affecting the leaching and occurrence form of Pb and microbial communities, and closely related to changes in bioavailability of Pb. Both stabilization and freeze-thaw treatment reduced the leaching concentration of Pb, decreased the proportion of available Pb (acid-soluble state, oxidation state and reduction state), increased the bioavailability of Pb in the gastric phase, but decreased in the intestinal phase; And the dominant bacterial phylum in the soil changed to Firmicutes, the dominant bacterial genus changed to Bacillus; The analysis of the results shows that the bioavailability of Pb is related to soil pH, cation exchange capacity (CEC), soil organic matter (SOM), soil moisture content (SMC), Pb (leaching, acid soluble state, oxidation state, residual state), types of microorganisms in soil.

Predicting the Bioaccessibility of Soil Cd, Pb, and As with Advanced Machine Learning for Continental-Scale Soil Environmental Criteria Determination in China

Investigating the bioaccessibility of harmful inorganic elements in soil is crucial for understanding their behavior in the environment and accurately assessing the environmental risks associated with soil. Traditional batch experimental methods and linear models, however, are time-consuming and often fall short in precisely quantifying bioaccessibility. In this study, using 937 data points gathered from 56 journal articles, we developed machine learning models for three harmful inorganic elements, namely, Cd, Pb, and As. After thorough analysis, the model optimized through a boosting ensemble strategy demonstrated the best performance, with an average R 2 of 0.95 and an RMSE of 0.25. We further employed SHAP values in conjunction with quantitative analysis to identify the key features that influence bioaccessibility. By utilizing the developed integrated models, we carried out predictions for 3002 data points across China, clarifying the bioaccessibility of cadmium (Cd), lead (Pb), and arsenic (As) in the soils of various sites and constructed a comprehensive spatial distribution map of China using the inverse distance weighting (IDW) interpolation method. Based on these findings, we further derived the soil environmental standards for metallurgical sites in China. Our observations from the collected data indicate a reduction in the number of sites exceeding the standard levels for Cd, Pb, and As in mining/smelting sites from 5, 58, and 14 to 1, 24, and 7, respectively. This research offers a precise and scientific approach for cross-regional risk assessment at the continental scale and lays a solid foundation for soil environmental management.

Different effects of vitamin supplementation on arsenic bioaccessibility in contaminated soils using multiple in vitro methods and their relevant mechanisms

Exposure to arsenic (As) induces adverse effects on human health. Vitamins B1, B6, and C, as indispensable micronutrients for humans, have been proven to influence the metabolism and toxicity of ingested As. To determine the effect of vitamins on health risks associated with soil exposure, As bioaccessibility in 14 soil samples using four in vitro methods of IVG, PBET, SBRC, and UBM was measured with the addition of vitamins B1, B6, and C. With vitamins B1 and B6 addition, the gastric As bioaccessibility in 14 soil samples was reduced by 1.14-3.52 and 1.14-5.02 fold, respectively, and instead an increase in the intestinal bioaccessibility was presented in some cases. Vitamin C supplementation yielded higher As bioaccessibility in the gastric (1.13-13.02 fold) and small intestinal (1.21-33.35 fold) phases, respectively. As evidenced by the X-ray absorption near-edge spectroscopy (XANES) and Fourier transform infrared spectroscopy (FTIR) analysis, arsenic dissolution was promoted by Fe-As and hindered by the formation of Al-As fractions. Soil As dissolution in the simulated gastrointestinal tract was strongly influenced by soil minerals and ingested vitamins, due to the chelation of arsenic with vitamins and soil minerals such as Fe (hydr)oxides, and Fe(III) reductive dissolution to enhance As release by vitamin C as an iron reducer. These findings will expand the knowledge of health risks of exposure to As-contaminated soils and nutritional interventions aiming at the mitigation of As toxicity.

Lead Speciation, Bioaccessibility, and Sources for a Contaminated Subset of House Dust and Soils Collected from Similar United States Residences

Residential lead (Pb) exposure is of critical concern to families globally as Pb promotes severe neurological effects in children, especially those less than 5 years old, and no blood lead level is deemed safe by the US Center for Disease Control. House dust and soils are commonly thought to be important sources of Pb exposure. Probing the relationship between house dust and soil Pb is critical to understanding residential exposure, as Pb bioavailability is highly influenced by Pb sources and/or species. We investigated paired house dust and soil collected from homes built before 1978 to determine Pb speciation, source, and bioaccessibility with the primary goal of assessing chemical factors driving Pb exposure in residential media. House dust was predominately found to contain (hydro)cerussite (i.e., Pb (hydroxy)carbonate) phases commonly used in Pb-based paint that, in-turn, promoted elevated bioaccessibility (>60%). Pb X-ray absorption spectroscopy, μ-XRF mapping, and Pb isotope ratio analysis for house dust and soils support house dust Pb as chemically unique compared to exterior soils, although paint Pb is expected to be a major source for both. Soil pedogenesis and increased protection from environmental conditions (e.g., weathering) in households is expected to greatly impact Pb phase differences between house dust and soils, subsequently dictating differences in Pb exposure.

Digestive fluid components affect speciation and bioaccessibility and the subsequent exposure risk of soil chromium from stomach to intestinal phase in in-vitro gastrointestinal digestion

The simulated in-vitro gastrointestinal method provides a simple way to evaluate the health risk of human body exposed to soil contaminants. Several in-vitro methods have been successfully established for soil As, Pb, and Cd. However, the method development for soil Cr failed up to now, which could be resulted from alteration in the species of Cr (e.g., Cr(VI)/Cr(III)) caused by the gastrointestinal digestion components, ultimately affecting the accessibility of Cr. This study explored the transformation and bioaccessibility of Cr in two Cr-contaminated soils during the physiologically based extraction test. The water-soluble and exchangeable Cr in soil was dissolved in gastrointestinal tract, accompanied with reduction of Cr(VI) into Cr(III), and the reduction occurred after the chemical extraction in two soils rather than during the extraction. Pepsin and organic acids in gastric phase could reduce Cr(VI) into Cr(III) and reduction efficiency were 20.4%- 53.0%, while in intestinal phase, pancreatin and bile salt had little effect on the Cr(VI) reduction, instead, more Cr(VI) was released from soil. In the gastric solution, Cr(VI) was mainly present as HCrO4- and Cr(III) as free Cr3+ ion. In the intestinal phase, Cr(VI) mainly occurred as CrO42- and Cr(III) as Cr(OH)3 (aq). Cr in the soil solid phase was dominated as the precipitates of Cr-Fe oxide, which was hardly extracted. Bioaccessibility of Cr in gastric phase increased as extraction duration increased and decreased in the intestinal phase, the contrary trend was observed for the hazard quotient of Cr in two phases due to Cr(VI)/Cr(III) transformation. This study indicates that the gastrointestinal components could influence the Cr transformation and subsequently affect the Cr bioaccessibility, which would help for a successful establishment of in vitro determination method for soil Cr bioaccessibility.

Potassium jarosite seeding of soils decreases lead and arsenic bioaccessibility: A path toward concomitant remediation

Soils are common sources of metal(loid) contaminant exposure globally. Lead (Pb) and arsenic (As) are of paramount concern due to detrimental neurological and carcinogenic health effects, respectively. Pb and/or As contaminated soils require remediation, typically leading to excavation, a costly and environmentally damaging practice of removing soil to a central location (e.g., hazardous landfill) that may not be a viable option in low-income countries. Chemical remediation techniques may allow for in situ conversion of soil contaminants to phases that are not easily mobilized upon ingestion; however, effective chemical remediation options are limited. Here, we have successfully tested a soil remediation technology using potted soils that relies on converting soil Pb and As into jarosite-group minerals, such as plumbojarosite (PLJ) and beudantite, possessing exceptionally low bioaccessibility [i.e., solubility at gastric pH conditions (pH 1.5 to 3)]. Across all experiments conducted, all new treatment methods successfully promoted PLJ and/or beudantite conversion, resulting in a proportional decrease in Pb and As bioaccessibility. Increasing temperature resulted in increased conversion to jarosite-group minerals, but addition of potassium (K) jarosite was most critical to Pb and As bioaccessibility decreases. Our methods of K-jarosite treatment yielded <10% Pb and As bioaccessibility compared to unamended soil values of approximately 70% and 60%, respectively. The proposed treatment is a rare dual remediation option that effectively treats soil Pb and As such that potential exposure is considerably reduced. Research presented here lays the foundation for ongoing field application.

Bioaccessibility and bioactive potential of different phytochemical classes from nutraceuticals and functional foods

Nutraceuticals and functional foods are composed of especially complex matrices, with polyphenols, carotenoids, minerals, and vitamins, among others, being the main classes of phytochemicals involved in their bioactivities. Despite their wide use, further investigations are needed to certify the proper release of these phytochemicals into the gastrointestinal medium, where the bioaccessibility assay is one of the most frequently used method. The aim of this review was to gather and describe different methods that can be used to assess the bioaccessibility of nutraceuticals and functional foods, along with the most important factors that can impact this process. The link between simulated digestion testing of phytochemicals and their in vitro bioactivity is also discussed, with a special focus on the potential of developing nutraceuticals and functional foods from simple plant materials. The bioactive potential of certain classes of phytochemicals from nutraceuticals and functional foods is susceptible to different variations during the bioaccessibility assessment, with different factors contributing to this variability, namely the chemical composition and the nature of the matrix. Regardless of the high number of studies, the current methodology fails to assume correlations between bioaccessibility and bioactivity, and the findings of this review indicate a necessity for updated and standardized protocols.

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.

Bioaccessibility of arsenic from contaminated soils and alteration of the gut microbiome in an in vitro gastrointestinal model

Arsenic exposure has been reported to alter the gut microbiome in mice. Activity of the gut microbiome derived from fecal microbiota has been found to affect arsenic bioaccessibility in an in vitro gastrointestinal (GI) model. Only a few studies have explored the relation between arsenic exposure and changes in the composition of the gut microbiome and in arsenic bioaccessibility. Here, we used simulated GI model system (GIMS) containing a stomach, small intestine, colon phases and microorganisms obtained from mouse feces (GIMS-F) and cecal contents (GIMS-C) to assess whether exposure to arsenic-contaminated soils affect the gut microbiome and whether composition of the gut microbiome affects arsenic bioaccessibility. Soils contaminated with arsenic did not alter gut microbiome composition in GIMS-F colon phase. In contrast, arsenic exposure resulted in the decline of bacteria in GIMS-C, including members of Clostridiaceae, Rikenellaceae, and Parabacteroides due to greater diversity and variability in microbial sensitivity to arsenic exposure. Arsenic bioaccessibility was greatest in the acidic stomach phase of GIMS (pH 1.5-1.7); except for GIMS-C colon phase exposed to mining-impacted soil in which greater levels of arsenic solubilized likely due to microbiome effects. Physicochemical properties of different test soils likely influenced variability in arsenic bioaccessibility (GIMS-F bioaccessibility range: 8-37%, GIMS-C bioaccessibility range: 2-18%) observed in this study.

Effect of soil particle size and extraction method on the oral bioaccessibility of arsenic

Recent findings indicate that incidental ingestion of soil by humans primarily involves soil particles <150 µm, rather than <250 µm-sized fraction previously used for most oral bioaccessibility and bioavailability studies. It was postulated that a greater soil surface area in the finer fraction (<150 versus <250 µm) might increase oral bioaccessibility of arsenic (As) in soil. Bioaccessibility and concentrations of As were compared in <150 and <250 µm fractions of 18 soil samples from a variety of arsenic-contaminated sites. The two methods used to measure bioaccessibility were compared - EPA Method 1340 and the California Arsenic Bioaccessibility (CAB) method. Arsenic concentrations were nearly the same or higher in the <150 fraction compared with <250 µm. EPA Method 1340 and the CAB method presented significantly different bioaccessibility results, as well as estimated relative oral bioavailability (RBA) based upon algorithms specific to the methods, but there was no marked difference for <150 and <250 µm soil fractions within either method. When compared with RBA determined previously for these soil samples in vivo in non-human primates, EPA Method 1340 was generally more predictive than the CAB method. Data suggest that soil- or site-specific factors control bioaccessibility under either method and that the test method selected is more important than the particle size fraction (<150 or <250) in using these in vitro methods to predict As RBA for use in risk assessment.

Interconnected soil iron and arsenic speciation effects on arsenic bioaccessibility and bioavailability: a scoping review

Extensive research has examined arsenic (As) bioavailability in contaminated soils and is routinely assessed using in vitro bioaccessibility (IVBA) assays. Analysis of differences in bioaccessibility measurements across IVBA assays and phases is expected to provide valuable insights into geochemical mechanisms controlling soil As bioaccessibility and bioavailability. Soil iron (Fe) content and As speciation are expected to significantly influence IVBA gastric and intestinal phases due to fluctuations in precipitation-dissolution chemistry and sorption reactivity as pH and assay chemical complexity changes. The aim of this review was to examine these relationships by 1) conducting a meta-analysis (n = 47 soils) determining the influence of total Fe on As bioaccessibility measurements and 5 IVBA assays and 2) investigating the effect of As speciation on gastric/intestinal phase IVBA and in vitro-in vivo correlations. Our findings indicate that soil Fe content and As speciation heterogeneity are important in elucidating variability of bioaccessibility measurements across IVBA assays and gastrointestinal phases. Greater focus on coupled As speciation and Fe precipitation chemistry may (1) improve our understanding of soil geochemical factors and assay constituents that influence As in vitro-in vivo correlations and (2) resolve variability in the precision of oral relative bioavailability (RBA) estimated using IVBA assays for soils possessing heterogenous As speciation and Fe composition.