•In vitro digestion/Caco-2 cell was used to estimate Cd and Pb bioavailability.•Cadmium and Pb bioavailability were higher in the cooked vegetables.•Bioaccessibility values may overestimate health ...risks of Cd and Pb in vegetables.
The estimation of heavy metal bioaccessibility and bioavailability in vegetables is helpful for human health risk assessment. Using an in vitro digestion/Caco-2 cell model, the bioaccessibility and bioavailability of cadmium (Cd) and lead (Pb) in raw/cooked pakchoi (Brassica rapa L., Chinensis Group) and Malabar spinach (Basella rubra L.) were studied. The effect of the addition of iron, calcium and acetic acid to the samples was also determined. The results indicated that Cd bioaccessibility was higher in the gastric phase and Pb bioaccessibility was higher in the small intestinal phase. Cadmium and Pb bioavailability were 11.2% and 9.4% in the raw vegetables, respectively, and found to be higher significantly than the cooked vegetables with 6.1% for Cd and 3.2% for Pb. The results showed that it will be overestimating the risk of Pb and Cd based on the data of raw vegetables ingestion. Using bioavailability values, average Cd and Pb daily intake by adult were 23% and 28% respectively, of the base bioaccessibility values. Our study will be better understanding the possible health risks of some vegetables base on the bioaccessibility or bioavailability.
The pH dependent solid-solution distribution of arsenate and phosphate in five Dutch agricultural soil samples was measured in the pH range 4–8, and the results were interpreted using the LCD (ligand ...and charge distribution) adsorption modeling. The pH dependency is similar for both oxyanions, with a minimum soluble concentration observed around pH 6–8. This pH dependency can be successfully described with the LCD model and it is attributed mainly to the synergistic effects from Ca adsorption. The solubility of phosphate is much lower than that of arsenate. This big difference cannot be sufficiently explained by the reduction of small amount of As(V) into As(III), neither by slow desorption/adsorption. The difference between phosphate and arsenate in their solid-solution distribution becomes larger with the increase of aluminum (hydr)oxides (Al-oxides) contribution to the total amount of metal (Al and Fe) (hydr)oxides. The influence of Al-oxides is much larger than its relative amount extracted from the soils. When Al-oxides account for >40% of the soil oxides, the whole adsorbents behave apparently similarly to that of pure Al-oxides. These results indicated that surface coating and substitution may have modified significantly oxyanion adsorption to Fe-oxides in soils, and how to account for this complexity is a challenge for geochemical modeling.
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•As(V)-reducing bacteria may reduce and release arsenic bound to all soil fractions.•The redistribution pathway of released As is dominated by soil Fe and S contents.•In soil with low ...Fe, As sequestration depends largely on the formation of As-sulfide.•In Fe-rich soil, released As preferentially adsorb on Fe-oxides, despite S content.•Microbial reduction of solid-phase As(V) was enhanced in organic-rich soils.
Microbially-mediated mobilization of soil arsenic (As) is greatly influenced by the soil properties. However, in soils with contrasting iron (Fe), sulfur (S), and organic matter (OM) contents, the biogeochemical pathways controlling As transformation and distribution remain unclear. Using sequential soil As extraction and X-ray absorption spectroscopy (XAS), we investigated the causal mechanisms of As reduction and redistribution in five soils during microbial incubation. Incubation of arsenate (As(V))-reducing bacteria resulted in a significant arsenite (As(III)) release (21.6–61.9% of total soil As (Astotal)). Thereafter, the re-immobilization of released As(III) was controlled by contrasting biogeochemical pathways, which were mainly dominated by soil Fe and S. For soil with high Fe content (191.1 g/kg), As immobilization is attributed to As(III)-readsorption by (neoformed) Fe-(oxyhydr)oxides, despite the presence of abundant S (10.3 g/kg); while in soils with relatively low Fe content (25.9–35.6 g/kg) and high S content (1.4–1.7 g/kg), As-sequestration depends largely on As-sulfide formation (5–47% of solid-phase As), including realgar and orpiment-like phases. In contrast, released As remains in solution in soils with relatively low Fe (27.5–52.4 g/kg) and S contents (0.6–1.0 g/kg). Arsenic-XAS results show that all soil As fractions, including residual As(V), can potentially be reduced (34–92% of Astotal), and solid-phase As(V) reduction was enhanced at higher OM content. Collectively, these results elucidate the dominant biogeochemical pathways controlling As fate in soils with different Fe, S, and OM contents.
The oxidation of aqueous arsenite (As(III)) by As(III)-oxidizing bacteria is known to attenuate the mobilization and toxicity of arsenic, and is regarded as potential method for As(III)-pollution ...remediation. However, during the interactions between As(III)-oxidizing bacteria and different As(III)-adsorbed soil Fe-minerals, the oxidation and partitioning of solid-phase As(III), as well as the controlling mechanisms, remain unclear. In this study, we therefore incubated three As(III)-adsorbed Fe-minerals with a typical As(III)-oxidizing bacteria (Pseudomonas sp. HN-1) at different pH conditions. After microbial oxidation, the percentage of arsenate (As(V)) was significantly higher at pH 7 (15–94%) and 9 (12–89%) than at pH 4 (6–50%) in all Fe-minerals. Incubation of As(III)-oxidizing bacteria promoted As-immobilization under acidic-conditions but As-mobilization under alkaline-conditions. Arsenic-X-ray adsorption spectroscopy results showed that solid-phase As(V) fraction in goethite, hematite and magnetite was 27–64%, 5–12% and 50–91%, respectively. Compared with the corner-sharing As(III)-adsorption complexes formed on magnetite, the edge-sharing complexes on hematite were significantly more stable towards microbial-oxidation. Additionally, the strong adhesion between strain HN-1 and hematite probably limit bacterial-activity and mobility, thereby inhibiting microbial As(III)-oxidation. Our findings elucidate the controlling mechanisms of microbial As(III)-oxidation in different As(III)-adsorbed Fe-minerals and demonstrate strain HN-1 is an excellent candidate for As(III)-remediation in soils containing goethite and magnetite.
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•Microbial oxidation of mineral-adsorbed As(III) is magnetite > goethite > hematite.•Edge-sharing (2E) As(III) complexes are more stable towards microbial oxidation.•Strong bacterial adhesion to Fe-minerals may inhibit solid-phase As(III)-oxidation.•Strain HN-1 is an excellent candidate for soil As(III)-remediation.•pH is a critical factor controlling the bioremediation of soil As(III) pollution.
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•The As biosorption by bacteria was due to hydroxyl, amino, and carboxyl groups.•Escherichia coli had the strongest tolerance to As and the highest reducing ability.•Seven strains had ...As(V)-reducing ability, which was mainly regulated by arsC gene.
There is growing evidence that human gut microbiota can metabolize arsenic (As); however, which bacteria play roles in this metabolism is unclear. In this study, we measured the abilities of 21 human gut bacteria strains from diverse clades to adsorb and transform As using in vitro method with the aim of determining which bacteria play a role in As metabolism. Seven strains showed high biosorption of As, ranging from 20.1 to 29.8%, which was attributed to functional groups on the bacterial surfaces, such as hydroxyl, amino, and carboxyl groups. Moreover, six of these seven strains were versatile, as they also had roles in reducing As(V) to As(III), which is mainly regulated by the arsC gene. Escherichia coli had the strongest tolerance to As and the highest reducing ability, with a value of 71.04–73.13 µM As/h. This study reveals that gut bacteria play essential roles in As biosorption and biotransformation, and provides a better understanding of which strains are involved, which has implications for the regulation of As toxicity-based gut bacteria and provides basic data for regulating arsenic to human health.
Despite rice consumption, rice bran as a byproduct of rice milling contains higher arsenic (As). The present study evaluated the metabolic potency of in vitro cultured human colon microbiota toward ...As from five rice bran products with 0.471–1.491 mg of As/kg. Arsenic bioaccessibility ranged from 52.8 to 78.8% in the gastric phase, and a 1.2-fold increase (66.0–95.8%) was observed upon the small intestinal phase. Subsequently, a significant decline of As bioaccessibility (11.3–63.6%) and a high methylation percentage of 18.5–79.8% were found in the colon phase. The predominant As species in the solid phase was always As(V) (49.6–63.4%), and As–thiolate complexes increased by 10% at the end of colon incubation. Human gut microbiota could induce As bioaccessibility lowering and As transformation in rice bran, which illustrated the importance of food-bound As metabolism in the human body. This will result in a better understanding of health implications associated with As exposures.
Propose
In the soil environment, the existence of Cr(VI)-reducing microorganisms might affect the adsorption, desorption and reduction of Cr(VI) adsorbed on soil minerals (such as Fe oxides). The ...behaviour of Cr(VI)-reducing microorganisms might affect the adsorption and reduction of Cr(VI) by soil minerals.
Materials and methods
Goethite and haematite with saturated adsorbed Cr(VI) were incubated with
Microbacterium
sp. QH-2, a Cr(VI)-reducing bacteria. Scanning electronic microscopy (SEM) and X-ray diffraction (XRD) were used to detect the changes in goethite and haematite. X-ray absorption near-edge structure (XANES) was performed to detect the changes in Cr(III) and Cr(VI) on goethite and haematite.
Results and discussion
Strain QH-2 adhered to the surfaces of goethite and haematite. No morphological changes in goethite and haematite were detected after incubation. No secondary Fe minerals formed. Cr(III) was the dominant species of Cr on goethite and haematite (78.5% for goethite and 96.7% for haematite) after incubation. Furthermore, the reduction rate of Cr(VI) by strain QH-2 in the liquid phase was faster than that of Cr(VI) adsorbed on goethite and haematite.
Conclusions
The existence of strain QH-2 could promote the adsorption of Cr by goethite and haematite. Strain QH-2 could affect the morphological distribution and transformation of Cr in Fe oxides such as goethite and haematite.
Effects of vitamin C supplementation on the oral bioaccessibility of lead (Pb) present in contaminated soils were examined using a number of in vitro assays (PBET, SBRC, UBM and IVG). In the presence ...of vitamin C, an increase in Pb bioaccessibility was observed in the gastric phase by 1.3-fold (30.5%−85.5%) and in the intestinal phase by 3.1-fold (0.9%−58.9%). Lead mobilization was regulated by reductive dissolution of Fe(III) and sequestration of Pb on secondary Fe minerals. Sequential extraction by the Bureau Community of Reference (BCR) provided more evidence that reducible fraction and residual fraction were major contributor of gastric Pb bioaccessibility, as well as reduced fractions in intestinal Pb bioaccessibility. In addition, higher non-carcinogenic risks may occur based on target hazard quotient (THQ ≥ 1). For people exposed to Pb present in soil, the management of vitamin C supplements is of serious concern.
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•Effects of vitamin C on in vitro bioaccessibility of Pb in soil were examined.•Soil Pb bioaccessibility increased by 1.3-fold (gastric) and 3.1-fold (intestinal).•Fe reduction was observed significantly in the presence of vitamin C.•Higher non-carcinogenic risk for children, especially with vitamin C supplement.•Nutritional management of vitamin C is of serious concern for people exposed to Pb.
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•Fe reduction coupled with soil particle size control As metabolism by gut microbiota.•A high degree of As reduction and methylation up to 53.4 and 0.074 μg/(log CFU/mL)/hr.•Increased ...As methylation with increasing soil organic matter and decreasing pore size.•Higher As bioaccessibility of colon phase from Fe(III) oxide reductive dissolution.•Arsenic reduction mediated by gut microbiota carrying arrA and arsC genes.
Gut microbiota provides protection against arsenic (As) induced toxicity, and As metabolism is considered an important part of risk assessment associated with soil As exposures. However, little is known about microbial iron(III) reduction and its role in metabolism of soil-bound As in the human gut. Here, we determined the dissolution and transformation of As and Fe from incidental ingestion of contaminated soils as a function of particle size (<250 μm, 100–250 μm, 50–100 μm and < 50 μm). Colon incubation with human gut microbiota yielded a high degree of As reduction and methylation of up to 53.4 and 0.074 μg/(log CFU/mL)/hr, respectively; methylation percentage increased with increasing soil organic matter and decreasing soil pore size. We also found significant microbial Fe(III) reduction and high levels of Fe(II) (48 %−100 % of total soluble Fe) may promote the capacity of As methylation. Although no statistical change in Fe phases was observed with low Fe dissolution and high molar Fe/As ratios, higher As bioaccessibility of colon phase (avg. 29.4 %) was mainly contributed from reductive dissolution of As(V)-bearing Fe(III) (oxy)hydroxides. Our results suggest that As mobility and biotransformation by human gut microbiota (carrying arrA and arsC genes) are strongly controlled by microbial Fe(III) reduction coupled with soil particle size. This will expand our knowledge on oral bioavailability of soil As and health risks from exposure to contaminated soils.
Sixteen soil samples were collected from the vicinity of an abandoned lead–zinc mine in Shangyu City, eastern China, and the heavy-metal speciation and wheat phytotoxicity in the soils were studied. ...The results showed that the concentrations of free Cu
2+
, Zn
2+
, Cd
2+
and Pb
2+
were highly variable and ranged from <0.01 to 0.32, 0.06 to 10.62, <0.01 to 1.40 and 0.02 to 37.10 μmol l
−1
, respectively. The concentrations of soluble Cu, Zn, Cd and Pb ranged from 0.38 to 3.24, 0.72 to 78.74, <0.01 to 1.95 and 0.15 to 639.34 μmol l
−1
, respectively. The general trend of mean solid/liquid partition coefficient and percentage of free metal ion to total soluble metal concentration were Cu > Pb > Zn > Cd and Cd > Zn > Cu > Pb, respectively. Stepwise multiple linear regression with pH, log(total metal) and log(organic matter) showed that log(total metal) was an important factor that controlled log(free metal ion) and log(soluble metal). Of the variability in log(free Cu
2+
), log(free Cd
2+
) and log(free Pb
2+
), 55.2, 58.6 and 64.3% could be explained by log(total Cu), log(total Cd) and log(total Pb) alone, respectively. Of the variability in log(soluble Cu) and log(soluble Cd), 77.1 and 72.5% could be explained by log(total Cu) and log(total Cd) alone, respectively. Wheat root length was controlled by the various metals with different free and soluble concentrations, and 99.2% of the variability in root length could be explained by concentrations of free and soluble Pb, soluble Cu and total Zn in the soils.
