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.

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.

[Oxidation of Humic Acid Complexing As(â ¢) by As(â ¢)-Oxidizing Bacteria].

The interaction between soil arsenic and soil microorganisms has been identified as one of the important parts of the morphological transformation of soil arsenic. In order to investigate the interaction between Humic Acid complexing As(Ⅲ)[HA-As(Ⅲ)] and As(Ⅲ)-oxidizing bacteria (HN-2), the changes in arsenic speciation in the liquid phase and the solid phase, with different pH, were studied. The results indicated there was better As(Ⅲ) oxidation efficiency in the pH 7 reaction system. A part of As(Ⅲ) would be discharged from the HA-As(Ⅲ) solid phase during hours 0-10 in the reaction system, with or without HN-2, and meanwhile it was found that HN-2 oxidized As(Ⅲ) to As(Ⅴ) rapidly, while As(Ⅲ) was oxidized into As(Ⅴ) by HA gradually. As(Ⅲ) complexing HA can be transformed into free-As(Ⅲ), and then oxidized into free-As(Ⅴ) by HN-2 over hours 10-24 of the reaction. The system achieved the equilibrium state after 48 h. The results of the X-ray absorption near edge structure (XANES) further confirmed the conclusions above.

[Effects of Human Gut Microbiota on Bioaccessibility of Soil Cd, Cr and Ni Using SHIME Model].

The influence of human gut micobiota on bioaccessibilities of soil Cd, Cr, and Ni were investigated in this study. Five soil samples were collected from some sites of China, and the bioaccessibilities of soil Cd, Cr, and Ni in the gastric, small intestinal, and colon phases were determined using the PBET method (physiologically based extraction test) combined with SHIME model (simulator of human intestinal microbial ecosystem). The results showed that the bioaccessibilities of Cd, Cr, and Ni in the gastric phase were 4.3%-94.0%, 6.4%-21.6%, and 11.3%-47.3%, respectively. In the small intestinal phase, the bioaccessibilities of Cr and Ni were either congruent or slightly increased, while for Cd, the values were reduced by 1.4-1.6 folds except for soil 2. In the gastric and small intestinal phases, the mean bioaccessibility of Cd was higher but that of Cr was lower. In the colon phase, the bioaccessibilities of Cr and Ni were 1.3-2.4 and 1.0-2.1 times higher than those in the small intestinal phase. Furthermore, the bioaccessibility of Cd also increased except for soil 3 and 4. Human gut micobiota could induce Cd, Cr, and Ni release from soils and increase their bioaccessibilities, which may result in high risk to human health.