Immobilisation remediation of arsenic-contaminated soils with promising CaAl-layered double hydroxide and bioavailability, bioaccessibility, and speciation-based health risk assessment.

Arsenic (As)-contaminated soil poses great health risk to human mostly through inadvertent oral exposure. We investigated CaAl-layered double hydroxide (CaAl-LDH), a promising immobilising agent, for the remediation of As-contaminated Chinese soils. The effects on specific soil properties and As fractionation were analyzed, and changes in the health risk of soil As were accurately assessed by means of advanced in vivo mice model and in vitro PBET-SHIME model. Results showed that the application of CaAl-LDH significantly increased soil pH and concentration of Fe and Al oxides, and effectively converted active As fractions into the most stable residual fraction, guaranteeing long-term remediation stability. Based on in vivo test, As relative bioavailability was significantly reduced by 37.75%. Based on in vitro test, As bioaccessibility in small intestinal and colon phases was significantly reduced by 25.65% and 28.57%, respectively. Furthermore, As metabolism (reduction and methylation) by the gut microbiota inhabiting colon was clearly observed. After immobilisation with CaAl-LDH, the concentration of bioaccessible As(Ⅴ) in the colon fluid was significantly reduced by 61.91%, and organic As (least toxic MMA(V) and DMA(V)) became the main species, which further reduced the health risk of soil As. In summary, CaAl-LDH proved to be a feasible option for immobilisation remediation of As-contaminated soils, and considerable progress was made in relevant health risk assessment.

Assessment of the stabilization effect of ferrous sulfate for arsenic-contaminated soils based on chemical extraction methods and in vitro methods: Methodological differences and linkages.

Stabilization of arsenic-contaminated soils with ferrous sulfate has been reported in many studies, but there are few stabilization effects assessments simultaneously combined chemical extraction methods and in vitro methods, and further explored the corresponding alternative relationships. In this study, ferrous sulfate was added at FeAs molar ratio of 0, 5, 10 and 20 to stabilize As in 10 As spiked soils. Stabilization effects were assessed by 6 chemical extraction methods (toxicity characteristic leaching procedures (TCLP), HCl, diethylenetriamine pentaacetic acid (DTPA), CaCl2, CH3COONH4, (NH4)2SO4), and 4 in vitro methods (physiologically based extraction test (PBET), in vitro gastrointestinal method (IVG), Solubility Bioaccessibility Research Consortium (SBRC) method, and the Unified Bioaccessibility Research Group of Europe method (UBM)). The results showed that the HCl method provides the most conservative assessment results in non-calcareous soils, and in alkaline calcareous soils, (NH4)2SO4 method provides a more conservative assessment. In vitro methods provided significantly higher As concentrations than chemical extraction methods. The components of the simulated digestion solution as well as the parameters may have contributed to this result. The small intestinal phase of PBET and SBRC method produced the highest and lowest ranges of As concentrations, and in the range of 127-462 mg/kg and 68-222 mg/kg when the FeAs molar ratio was 5. So the small intestinal phase of PBET method may provide the most conservative assessment results, while the same phase of SBRC may underestimate the human health risks of As in stabilized soil by 51 %(at a FeAs molar ratio of 5). Spearman correlation analysis indicated that the small intestinal phase of PBET method correlated best with HCl method (correlation coefficient: 0.71). This study provides ideas for the assessment of stabilization efforts to ensure that stabilization meets ecological needs while also being less harmful to humans.

Bioaccessibility assessment of arsenic and cadmium in polished and unpolished rice: Comparison of three in vitro methods.

INFOGEST is a standardized in vitro digestion method suitable for foods, but rarely used to study the bioaccessibility of heavy metals in food. This study aimed to explore the differences between INFOGEST and the extensively used Physiologically Based Extraction Test (PBET) and Unified Bioaccessibility Research Group of Europe Method (UBM) methods for determining the bioaccessibility of As and Cd in rice. Intestinal As (79.3 ± 8.5 %, 75.8 ± 12.7 %, and 72.3 ± 12.2 % for INFOGEST, PBET, and UBM, respectively) and Cd (47.0 ± 6.4 %, 40.7 ± 13.8 %, and 38.1 ± 15.7 % for INFOGEST, PBET, and UBM, respectively) bioaccessibilities in the rice samples determined by the three methods were generally similar (p > 0.1, except for As bioaccessibility between INFOGEST and UBM). Furthermore, PBET was significantly correlated with INFOGEST for As bioaccessibility (R2 = 0.416) and with UBM for Cd bioaccessibility (R2 = 0.879). Additionally, PBET indicated that the bioaccessibilities of As and Cd in the polished rice were 17.0 % and 19.8 % higher, respectively, than that in the unpolished rice. This study highlights the influence of in vitro methods and rice matrices on heavy metal bioaccessibility values, necessitating a more accurate assessment of health risks associated with rice consumption.

Effects of foods and food components on the in vitro bioaccessibility of total arsenic and arsenic species from Hizikia fusiforme seaweed.

Seaweed is an important food source, especially in many Asian countries, because of its high nutritional value; however, increasing arsenic (As) accumulation may pose serious hazards to human health. The influence of food components on As bioaccessibility and transformation in the high As-containing seaweed Hizikia fusiforme was determined using an in vitro gastrointestinal digestion method. The results showed that co-digestion with several daily foods (such as celery, broccoli, onion, green chili, tomato) produced a higher As bioaccessibility (approximately 6-11 % increase) compared with that of seaweed alone. Vegetables such as fennel (Foeniculum valgare Mill.), celery (Apium grareolens L.), blanched garlic leaves (Allium sativum L.), scallions (Allium fistulosum L.), ginger (Zingiber officinale Rosc.), and green pepper (Capsicum frutescens L. vat. grussum Bailey) decreased bioaccessible inorganic As (18-35 %) in both the gastric and small intestinal phases. Meanwhile, the process of reducing As(V) to As(III) also occurred during co-digestion with some food matrices. Egg white and other animal proteins were the most effective reducing agents, transforming >70 % As(V) into As(III) in the solution system. These results may have important implications for health risk assessment via co-consumption. The present study provides the first evidence showing that the co-consumption of some vegetables and proteins leads to a higher toxicity of inorganic arsenic-containing food. In addition, the positive and negative effects of co-digestion on the bioaccessibility of essential metals (iron, manganese) compared to single digestion were evaluated in this study.

Effects of calcium supplements on oral bioavailability of fluoride in soil based on In Vivo and In Vitro methods.

Dietary calcium (Ca) intake can alleviate fluoride (F) induced fluorosis to maintain bone health. However, it is unclear whether calcium supplements can reduce the oral bioavailability of F present in contaminated soils. Here we evaluated the effects of Ca supplements on F bioavailability in three soils using an in vitro method (Physiologically Based Extraction Test) and an in vivo mouse model. Seven Ca salts, commonly used in calcium supplements, significantly reduced the F bioaccessibility in the gastric and small intestinal phases. Particularly for Ca phosphate at 150 mg Ca supplementation, F bioaccessibility in the small intestinal phase was reduced from 35.1-38.8% to 0.7-1.9% where soluble F concentrations were less than 1 mg/L. Overall, the eight Ca tablets tested in this study showed greater efficiency at decreasing F solubility. The in vitro bioaccessibility after Ca supplementation was consistent with the relative bioavailability of F. As supported by X-ray photoelectron spectroscopy, a possible mechanism is that freed F can be bound by Ca to form insoluble CaF2 and exchanged with OH groups from Al/Fe hydroxide to strongly adsorb F. These findings provide evidence of Ca supplementation in reducing health risks associated soil F exposure.

Role of microbial iron reduction in arsenic metabolism from soil particle size fractions in simulated human gastrointestinal tract.

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.

Effect of manganese oxides on arsenic speciation and mobilization in different arsenic-adsorbed iron-minerals under microbially-reducing conditions.

The oxidation and immobilization of arsenic (As) by manganese oxides have been shown to reduce As toxicity and bioavailability under abiotic conditions. In this study, we investigate the impact of manganese oxide (δ-MnO2) on the fate of different Fe-minerals-adsorbed As in the presence of As(V)-reducing bacteria Bacillus sp. JQ. Results showed that in the absence of δ-MnO2, As release in goethite was much higher than in ferrihydrite and hematite during microbial reduction. Adding 3.1 mM Mn reduced As release by 0.3%, 46.3%, and 6.7% in the ferrihydrite, goethite, and hematite groups, respectively. However, aqueous As was dominated by As(III) in the end, because the oxidation effect of δ-MnO2 was limited and short-lived. Additionally, the fraction of solid-phase As(V) increased by 9.8% in ferrihydrite, 39.4% in goethite, and 7.4% in hematite in the high-Mn treatments, indicating that δ-MnO2 had the most significant oxidation and immobilization effect on goethite-adsorbed As. This was achieved because goethite particles were evenly distributed on δ-MnO2 surface, which supported As(III) oxidation by δ-MnO2; while ferrihydrite strongly aggregated, which hindered the oxidation of As(III). Our study shows that As-oxidation and immobilization by manganese oxides cannot easily be assessed without considering the mineral composition and microbial conditions of soils.

Arsenic and iron bioavailability in Caco-2 cells: The influence of their co-existence and concentration.

Arsenic (As) exposure in humans is primarily caused through food and drinking water. Iron (Fe) is one of the most common element of the human and can influence the toxicity and bioavailability of As. However, information on the interaction between As and Fe when present together is limited. In this study, the interaction effects of Fe(III) (0, 3, and 10 mg/L) and As (As(III) at 0, 0.05, 0.1 mg/L, and As(V) at 0, 0.1, and 2 mg/L, respectively) on their absorption and bioavailability in Caco-2 cells were analyzed. As(III) absorption significantly decreased with the addition of Fe, while Fe absorption significantly increased. Compared with 0.1 mg/L As(III) addition alone, 3 and 10 mg/L Fe(III) addition significantly reduced the As(III) absorption by 8.6 and 11 µg/L, respectively. The absorption of As and Fe(III) and the bioavailability of Fe(III) significantly increased with the addition of As(III/V). Compared with 10 mg/L Fe(III) alone, the absorption of As(III) was significantly increased by 1 and 1.3 mg/L with 0.05 and 0.1 mg/L As(III) addition, respectively. Furthermore, the absorption and bioavailability of Fe(III) were significantly increased by 1.2 mg/L and 8% and 1.2 mg/L and 8.2%, respectively, after adding 0.1 and 2 mg/L As(V).

Ca Minerals and Oral Bioavailability of Pb, Cd, and As from Indoor Dust in Mice: Mechanisms and Health Implications.

BACKGROUND: Elevating dietary calcium (Ca) intake can reduce metal(loid)oral bioavailability. However, the ability of a range of Ca minerals to reduce oral bioavailability of lead (Pb), cadmium (Cd), and arsenic (As) from indoor dust remains unclear. OBJECTIVES: This study evaluated the ability of Ca minerals to reduce Pb, Cd, and As oral bioavailability from indoor dust and associated mechanisms. METHODS: A mouse bioassay was conducted to assess Pb, Cd, and As relative bioavailability (RBA) in three indoor dust samples, which were amended into mouse chow without and with addition of CaHPO4, CaCO3, Ca gluconate, Ca lactate, Ca aspartate, and Ca citrate at 200-5,000µg/g Ca. The mRNA expression of Ca and phosphate (P) transporters involved in transcellular Pb, Cd and As transport in the duodenum of mice was quantified using real-time polymerase chain reaction. Serum 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3], parathyroid hormone (PTH), and renal CYP27B1 activity controlling 1,25(OH)2D3 synthesis were measured using ELISA kits. Metal(loid) speciation in the feces of mice was characterized using X-ray absorption near-edge structure (XANES) spectroscopy. RESULTS: In general, mice exposed to each of the Ca minerals exhibited lower Pb-, Cd-, and As-RBA for three dusts. However, RBAs with the different Ca minerals varied. Among minerals, mice fed dietary CaHPO4 did not exhibit lower duodenal mRNA expression of Ca transporters but did have the lowest Pb and Cd oral bioavailability at the highest Ca concentration (5,000µg/g Ca; 51%-95% and 52%-74% lower in comparison with the control). Lead phosphate precipitates (e.g., chloropyromorphite) were observed in feces of mice fed dietary CaHPO4. In comparison, mice fed organic Ca minerals (Ca gluconate, Ca lactate, Ca aspartate, and Ca citrate) had lower duodenal mRNA expression of Ca transporters, but Pb and Cd oral bioavailability was higher than in mice fed CaHPO4. In terms of As, mice fed Ca aspartate exhibited the lowest As oral bioavailability at the highest Ca concentration (5,000µg/g Ca; 41%-72% lower) and the lowest duodenal expression of P transporter (88% lower). The presence of aspartate was not associated with higher As solubility in the intestine. DISCUSSION: Our study used a mouse model of exposure to household dust with various concentrations and species of Ca to determine whether different Ca minerals can reduce bioavailability of Pb, Cd, and As in mice and elucidate the mechanism(s) involved. This study can contribute to the practical application of optimal Ca minerals to protect humans from Pb, Cd, and As coexposure in the environment. https://doi.org/10.1289/EHP11730.

Effect of vitamin C supplement on lead bioaccessibility in contaminated soils using multiple in vitro gastrointestinal assays: Mechanisms and health risks.

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.

Role of human gut bacteria in arsenic biosorption and biotransformation.

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.

Controlling microbial arsenite oxidation and mobilization in arsenite-adsorbed iron minerals: The Influence of pH conditions and mineralogical composition.

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.

Stabilization of fluorine-contaminated soil in aluminum smelting site with biochar loaded iron-lanthanide and aluminum-lanthanide bimetallic materials.

Trivalent metals-modified-biochar (BC) has been widely used for the removal of fluorine (F) in water, but little is known about its effects on the stability and mobility of F-contaminated soil. Two types of modified-BC materials (BC-loaded iron-lanthanide (BC/Fe-La) and BC-loaded aluminum-lanthanide (BC/Al-La)) were synthesized and used for the remediation of F-contaminated soil. The forms of BC/LaxFe3x(OH)y in BC/Fe-La and BC/LaxAl3x(OH)y in BC/Al-La were identified by spectroscopy, X-ray dispersion, thermogravimetric, and pore diameter/volume analyses. Following application (4-12%, w/w) to F-contaminated soil for 30 d, water soluble fluoride (WSF) decreased significantly. The modified-BC with a 1:1:1 molar ratio (BC: Al3+ or Fe3+: La3+) were more effective than those at 1:0.5:0.5. The BC/Al-La were the most effective to stabilize F. In particular, the highest decrease in WSF (by 91.75%) was obtained with the application of 12% BC/Al-La-2, while 8% BC/Al-La-2% and 12% BC/Al-La-1 reduced the WSF by 87.58% and 90.17%, respectively; all values obtained were lower than the national standard of China (< 1.5 mg/L). In addition, the sequential extraction results showed that modified-BC promoted the transformation of the other chemical speciation to the Fe/Mn-F.

Predictive capabilities of in vitro colon bioaccessibility for estimating in vivo relative bioavailability of arsenic from contaminated soils: Arsenic speciation and gut microbiota considerations.

Arsenic (As) transformation by human gut microbiota has been evidenced to impact As toxicity and human health. However, little is known about the influence of gut microbiota on As bioavailability from incidental ingestion of soil. In this study, we assessed As relative bioavailability (RBA) using an in vivo mouse model and As bioaccessibility in the colon phase of in vitro assays. Strong in vivo-in vitro correlations (R2 = 0.70-0.92, P < 0.05) were observed between soil As RBA (10.2%-57.7%) and colon bioaccessibility (4.8%-49.0%) in 13 As-contaminated soils. Upon in vitro incubation of human colon microbiota, we found a high degree of As transformation and 65.9% of generated As(III) was observed in soil residues. For in vivo mouse assay, DMA(V) accounted for 79.0% of cumulative urinary As excretion. Except for As(V), dominant As species including As(III), DMA(V) and As sulfides were also detected in mouse feces. Gut bacteria (families Rikenellaceae and Marinifilaceae) could be significantly correlated with As intake and excretion in mice (P < 0.05). Our findings provide evidence that gut microbiota can affect transformation, bioavailability, and fate of the orally ingested soil As in human gastrointestinal tract.

Human health risk assessment in aluminium smelting site: Soil fluoride bioaccessibility and relevant mechanism in simulated gastrointestinal tract.

Incidental oral ingestion is considered to be an important exposure route for humans to soil contaminants, such as fluoride (F). For 25 soil samples containing 4000 mg F/kg from aluminium smelting site in southwestern China, this study investigated F bioaccessibility in the human gastrointestinal tract in vitro. Fluoride bioaccessibility (2.4-48.8%) in the gastric phase was primarily caused by the dissolution of F-Ca and F-Al compounds (assigned to residual phase), identified by X-ray photoelectron spectroscopy and sequential extraction. Following modification to the small intestinal phase, the variation in F bioaccessibility (2.5-38.8%) should be the result of concurrent processes, including the formation of F complexes and competitive adsorption, and inversely the precipitation of fluorite and surface adsorption of formed F-Al complexes. The colon incubation with human gut microbiota yielded a 1.3-fold increase in F bioaccessibility (3.9-45.7%), probably due to the dissolution of F bound to Fe (hydr)oxides. Bioaccessibility adjustment can reduce hazard quotient of fluoride, and non-carcinogenic risk for children should be noted that soil F intake contributed 21.7% on average, up to 76.6% of oral reference dose. This will result in better understanding of human health risk assessment associated with F exposures.

Effect of gut microbiota on in vitro bioaccessibility of heavy metals and human health risk assessment from ingestion of contaminated soils.

To identify the role of gut microbiota in human health risk assessment, the bioaccessibility of heavy metals in 14 soil samples were determined in simulated gastrointestinal fluids. Compared to the small intestinal phase, the bioaccessibility values of the colon phase varied, either increased by 3.5-fold for As, by 2.2-fold for Cr, and by 1.6-fold for Ni, or reduced by 4.4-fold for Cu, respectively. The colon incubation with adult gut microbiota yielded higher bioaccessibility value of As (1.3 times) and Fe (3.4 times) than that of the child in most soil samples. Colon bioaccessibility was about 60% greater of Cd for the adult and 30% higher of Cr for the child. Congruent data on the bioaccessibility of Cu and Ni was observed. In addition, correlation analysis indicated that in vitro bioaccessibility was primarily related to total concentrations of heavy metals in soils, followed by soil pH and active Fe/Mn oxide. Significantly, risk assessment calculated based on colon bioaccessibility indicated that the target hazard quotient (THQ > 1) of As was presented in 3 soil samples for the adult (1.05-3.35) and in 9 soil samples for the child (1.06-26.93). The hazard index (HI) of the child was 4.00 on average, greater than that of the adult (0.62), primarily due to the contribution of As and Cd. It suggested non-carcinogenic risks are likely to occur in children through typical hand-to-mouth behavior. The adjustment of colon bioaccessibility will result in more accurate risk assessment of human exposure to heavy metals from oral ingestion of contaminated soils.

Arsenic speciation and bioaccessibility in raw and cooked seafood: Influence of seafood species and gut microbiota.

Seafood is an important source of arsenic (As) exposure for humans. In this study, 34 seafood samples (fishes, shellfishes, and seaweeds) collected from different markets in China were analysed for total and speciated As before and after boiling. Furthermore, the As bioaccessibility was also assessed using a physiologically based extraction test combined with the Simulator of Human Intestinal Microbial Ecosystems. The results showed that the total As (tAs) contents of seaweeds (raw: 44.12; boiled: 31.13, µg·g-1 dw) were higher than those of shellfishes (raw: 8.34; boiled: 5.14, µg·g-1 dw) and fishes (raw: 6.01; boiled: 3.25, µg·g-1 dw). Boiling significantly decreased the As content by 22.24% for seaweeds, 32.27% for shellfishes, and 41.42% in fishes, respectively (p < 0.05). During in vitro digestion, the bioaccessibility of tAs and arsenobetaine (AsB) significantly varied between the investigated species of seafood samples in gastric (G) and small intestinal phases (I) (p < 0.05). Higher tAs bioaccessibility (G: 68.6%, I: 81.9%) were obtained in fishes than shellfishes (G: 40.9%, I: 52.5%) and seaweeds (G: 31%, I: 53.6%). However, there was no significant differences in colonic phase (C) (p > 0.05). With the effect of gut microbiota, arsenate (AsⅤ) was transformed into monomethylarsonic acid (MMA) and arsenite (AsⅢ) in C. Moreover, as for seaweeds, an unknown As compound was produced.

Assessment of arsenic distribution, bioaccessibility and speciation in rice utilizing continuous extraction and in vitro digestion.

Rice, a staple food for half the world's population, easily accumulates arsenic (As). Research on As distribution in rice protein and starch and its relationship with rice As bioaccessibility remains limited. This study investigated As distribution, chemical composition, As bioaccessibility and speciation in rice by continuous extraction and in vitro digestion. Of the total As, 87.5-94.5% was in rice protein and 5.0-9.8% in rice starch. The As amount in different protein fractions decreased as follows: glutelin > globulin > albumin > prolamin. As(V), As(III) and DMA in rice were more bioaccessible in the small intestinal phase than the gastric phase, and almost all As(V) dissolved in the small intestinal phase. Bioaccessible As in gastrointestinal digestive solution and As mass in protein fractions (albumin, globulin, and glutelin) were significantly positively correlated (p < 0.05). These results illuminate the bioaccessibility of As to humans consuming As-contaminated rice and avoid overassessment.

The role of soil arsenic fractionation in the bioaccessibility, transformation, and fate of arsenic in the presence of human gut microbiota.

Soil arsenic (As) fractionation and its bioaccessibility are two important factors in human health risk assessment. However, data related to the impact of As minerals on the bioaccessibility with human gut microbiota involvement are scarce. In this study, speciation analysis was determined using HPLC-ICP-MS and XANES after incubation with colon microbiota from human origin, in combination with sequential extraction. Significant increase of colon As bioaccessibility was contributed primarily from As associated with amorphous and crystalline Fe/Al (hydr)oxides. We found a high degree of transformation at higher bioaccessibility (ave. 40 % of total As), which was predominantly present as liquid-phase As. In contrast, As transformation occurred mainly in the solid phase at lower bioaccessibility (< 5%), especially for soils containing As-S species. XANES spectroscopy revealed that As(III) increased by about 20 % in soil residues. Finally, the excreted As may be predominantly in association with (alumino)silicate minerals by SEM-EDX. It inferred that the priority sequence in As transformation by human gut microbiota was dissolved As(V), As(V) sorbed to mineral surfaces, crystalline As(V)-bearing minerals and As sulfides. This study will shed new light on the role of As-bearing minerals in evaluating health risks from soil As exposure.

Stabilization of Soil Arsenic with Iron and Nano-Iron Materials: A Review.

Soil arsenic (As) contamination is an important environmental problem, and chemical stabilization is one of the major techniques used to remediate soil As contamination. Iron and iron nanoparticle materials are widely used for soil As stabilization because they have one or more of the following advantages: high adsorption capacity, high reduction capacity, cost effectiveness and environmental friendliness. Therefore, this review introduces the stabilization of soil As with iron and iron nanoparticles, including zero-valent iron, iron oxides/hydroxides, some iron salts and Fe-based binary oxides and the nanoparticles of these iron materials. The mechanism of chemical soil As stabilization, which involves adsorption and the coprecipitation process, is discussed. The factors affecting the chemical stabilization process are presented, and challenges to overcome in the future are also discussed in this review.