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.

[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.