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.

[Bioaccessibility of soil cadmium and its health risk assessment].

Sixteen soil samples were collected from different sites of China to study the bioaccessibility of soil cadmium. The relationship between the soil properties and the bioaccessibility as well as the health risk assessment of the oral ingestion soil was also studied. The results showed that comparing with Chinese environmental quality standard for soils, the concentrations of cadmium in 11 soil samples were higher than the standard. The high variability of dissolved and bioaccessible cadmium of soils were observed. Concentrations of bioaccessible Cd ranged from 0.05-20.71 mg x kg(-1) and 0.03-11.99 mg x kg(-1) with a mean of 1.81 mg x kg(-1) and 1.06 mg x kg(-1) in gastric and small intestinal phase respectively. Bioaccessible Cd ranged from 6.37%-69.43% and 3.19%-36.91% with a mean of 25.34% and 14.84% in gastric and small intestinal phase respectively. A significant correlation between dissolved cadmium in gastric stage with the soil pH was also observed. In gastric stage, for children, the highest contribution of the oral ingestion soil cadmium to the provisional tolerable weekly intake (PTWI) that recommended by WHO was 26.90% in the soil sample that was collected from Nanning Guangxi and the contribution rate in 11 soil samples is lower than 1.00%. In small intestinal stage, for children, the contribution of the oral ingestion soil cadmium to PTWI was also variable. The highest contribution rate was 15.57%, the four samples were higher than 1.00% and others were below 1.00%. Health risk from the oral ingestion of soil cadmium was low in most of soils and the high health risk only occurred in the soil sample with high total cadmium concentration and high bioaccessibility.

[Bioaccessibility of lead in urban topsoil and its health risk assessment: a case study of a small area near Shougang Group].

Bioaccessibility and health risk of lead in urban topsoil after unconscious oral ingestion were studied. 65 topsoil samples were collected from a small area near Shougang Group, and then bioaccessibility of soil lead was measured by means of in vitro digestion test. Results showed that the bioaccessibility of soil lead covered a wide range (28.83%-62.50% in gastric phase, and 6.86%-45.71% in intestinal phase), and had no relation to its total concentration. It was also found that the high-value areas of the gastrointestinal dissolved concentration of soil lead distributed in nearly the same way as those of its total concentration. Besides, high gastrointestinal dissolved concentration and low bioaccessibility of soil lead were observed in the sample sites close to traffic and vehicles. Due to low contribution rate of the bioaccessible amount of soil lead to the provisional tolerable weekly intake (PTWI), the health risk of soil lead in the studied area was generally low. Only when health risk assessment was based on the gastric data of children (the worst case), the sample with highest contribution rate, 25.37%, was found in the northwestern part of the studied area, where the other 5 samples with contribution rate over 10% were also located. It was obvious that the health risk of soil lead to children was higher than that to adults. Due attention should be paid to the soil with both high total concentration and high bioaccessibility of lead, as it might pose high risk to the health of general public.

Pilot study of temporal variations in lead bioaccessibility and chemical fractionation in some Chinese soils.

The effect of ageing, following the addition of approximately 400mgkg(-1) lead (Pb) as Pb(NO(3))(2), on Pb bioaccessibility was examined in five typical Chinese soils using a physiologically based extraction test. Sequential extraction was employed to identify the source fraction(s) of bioaccessible Pb in the soils. Pb bioaccessibility decreased exponentially to nearly steady levels in mildly acidic or alkali (pH 6.09-7.43) soils, for both gastric (69.91-71.75%) and small intestinal (7.53-9.63%) phases within the first 2-4 weeks and 1-2 months of incubation, respectively; however, it took only 1-2 weeks for strongly acidic ( approximately pH 4.5) soils to reach nearly steady levels of Pb bioaccessibility (73.01-74.46% and 10.30-10.98% in the gastric and small intestinal phases, respectively). In addition to the water-soluble and exchangeable fractions, the carbonate fraction of mildly acidic or alkali soils appeared to be a third main source of bioaccessible Pb in the small intestinal phase; however, bioaccessible Pb was likely to derive principally from Pb in the water-soluble and exchangeable fractions of strongly acidic soils. Bioaccessible Pb in the gastric phase appeared to derive from all the fractions in all five studied soils, even the residual fraction.

The effect of ageing on the bioaccessibility and fractionation of cadmium in some typical soils of China.

Ingestion of contaminated soil has been recognized as an important exposure pathway of cadmium (Cd) for humans, especially for children through outdoor hand-to-mouth activities. The effect of ageing process following the input of Cd into soil on the bioaccessibility of Cd in five typical soils of China was investigated using physiologically based in vitro test in this study. A sequential extraction procedure was employed with attempt to identify the bioaccessible fraction(s) of Cd in soils. The bioaccessibility of Cd in strongly acidic (approximately pH 4.5) soils reached nearly steady levels (76.5-76.9% and 52.0-52.6% in the gastric and small intestinal phases, respectively) after a sharp decline in the first week of ageing. In contrast, the bioaccessibility of Cd in higher pH (>6.0) soils was found to be much lower (53.3-72.7% and 29.9-43.4% in gastric and small intestinal phases, respectively) and took 2 weeks of ageing to reach steady levels. The freshly spiked Cd was more labile than native Cd. The main proportion of spiked Cd was found in exchangeable Cd which was higher in strongly acidic soils (68.6-71.8%) than in higher pH soils (53.4-61.4%) at day 120 after a sharp decline to the nearly steady state in the first 1 and 2 weeks, respectively. Significant correlations between Cd bioaccessibility and either water soluble and exchangeable Cd individually, or the sum of water soluble and exchangeable Cd throughout the incubation period for all soils, indicate that these forms of Cd are likely to constitute the main proportion of bioaccessible Cd in soils.

Soil contamination and plant uptake of heavy metals at polluted sites in China.

We investigated heavy metal contamination in soils and plants at polluted sites in China including some with heavy industries, metal mining, smelting and untreated wastewater irrigation areas. We report our main findings in this paper. The concentrations of heavy metals, including Cd and Zn, in the soils at the investigated sites were above the background levels, and generally exceeded the Government guidelines for metals in soil. The concentrations of metals in plants served to indicate the metal contamination status of the site, and also revealed the abilities of various plant species to take up and accumulate the metals from the soil. Substantial differences in the accumulation of heavy metals were observed among the plant species investigated. Polygonum hydropiper growing on contaminated soils in a sewage pond had accumulated 1061 mg kg(-1) of Zn in its shoots. Rumex acetosa L. growing near a smelter had accumulated more than 900 mg kg(-1) of Zn both in its shoots and roots. Therefore these species have potential for phytoremediation of metal-contaminated sites. Our results indicate the need to elucidate the dynamics of soil metal contamination of plants and the onward movement of metal contaminants into the food chain. Also our results indicate that the consumption of rice grown in paddy soils contaminated with Cd, Cr or Zn may pose a serious risk to human health, because from 24 to 22% of the total metal content in the rice biomass was concentrated in the rice grain. Platanus acerifolia growing on heavily contaminated soil accumulated only very low levels of heavy metals, and this mechanism for excluding metal uptake may have value in crop improvement. Sources of metal entering the environmental matrices studied included untreated wastewater, tailings or slurries and dust depositions from metal ore mining, and sewage sludge. Pb, Zn or Cd concentrations declined with the distance from metal smelter in accordance with a good exponential correlation (R2>0.9), and this shows that metal dust deposition is an important contributor to metal contamination of soils.

Responses of legume and non-legume crop species to heavy metals in soils with multiple metal contamination.

Field and glasshouse investigations were conducted on the responses of two legumes (field pea and fodder vetch) and three non-leguminous crops (maize, wheat and rapeseed) to the heavy metals Cd, Cr, Zn, Pb, Cu and Mn in soil with multiple metal contamination. In general, the results indicate that the two legumes and wheat were more susceptible to soil metals than were rapeseed and maize. The dry matter yields of field pea, wheat, fodder vetch, rapeseed and maize decreased by up to 169, 123, 113, 93 and 68%, respectively, in metal-contaminated soil. Among the crops, maize had the highest concentrations of Mn, Zn and Cd, rapeseed had the highest concentrations of Cr, the concentration of Cu was highest in fodder vetch, and wheat was the highest accumulator of Pb. The bioconcentration factors (BCF) of the metals decreased as the soil metal loading rates increased except for Cr in fodder vetch and Cd in wheat, whose BCF increased as the metal loading rate increased. Significant linear correlations were found between plant and soil metal concentrations. Patterns of metal distribution in plant parts varied with different crops and metals, with more Cd and Cu accumulating in the grain of wheat than of maize, suggesting that growing wheat would represent a higher risk of food contamination than growing maize in Cd- or Cu-contaminated soil. The results suggest that on sites with multiple metal contamination, growing maize and rapeseed would be safer than growing wheat or legumes. However, maize could perhaps be used for phytoremediation of lightly contaminated soils, providing that the crop residues were safely disposed of.