[Influencing Mechanism of Eh, pH and Iron on the Release of Arsenic in Paddy Soil].

The massive release of soil arsenic and its enrichment in rice are significantly associated with the flooded and anaerobic management in paddy soil. Soil redox potential (Eh), pH and iron oxides exert remarkable impacts on arsenic release, which remain to be explored. In this study, long-term aerobic and anaerobic as well as intermittent aerobic incubation treatments were applied to investigate the influences of Eh, pH and iron content on arsenic release. It was found that anaerobic and flooded treatment contributed to the highest arsenic release. With decreasing Eh, significant enhancement in As(Ⅲ) and As(Ⅴ) contents in soil solution was observed. Particularly, As(Ⅲ) and As(Ⅴ) contents during the second phase increased by 1.37 and 0.99 µg·L-1compared with those in the first phase. Conversely, significant reduction in soil arsenic release (P<0.05) occurred when intermittent aerobic treatment was adopted, and the lowest level of arsenic release was observed along with the longest treatment time (6 d). The exponent relationships between arsenic and soil Eh, pH and Fe2+ content were also established, which indicated that arsenic release could be accelerated by lower pH and elevated Eh. In addition, a significant positive correlation was also found between iron(Ⅱ) content and arsenic content in soil solution. Since low Eh and elevated pH served as critical factors driving arsenic release, intermittent and aerobic water management was proved to be an effective method for the inhibition of arsenic release and uptake and accumulation of arsenic by rice.

[Effect of Different Iron Minerals on Bioaccessibility of Soil Arsenic Using in vitro Methods].

To explore the effects of different iron minerals on soil arsenic bioaccessibility, ferrihydrite, goethite and hematite were used in PBET, SBRC and IVG in-vitro experiments in this study. The relationship between arsenic bioavailability in gastric, small intestinal phases and arsenic speciation was also studied. The results showed that when 1% ferrihydrite was added, arsenic bioavailability in gastric phase was 2.22%, 5.11% and 7.43% by PBET, SBRC and IVG methods, respectively, while in the small intestinal phase it was 3.39%, 2.33% and 6.18%. At an elevated ferrihydrite dosage of 2%, significant difference in arsenic bioavailability was observed in both phases (P<0.05). According to in vitro experiments, the addition of the same amount of different iron minerals had contributed to the decrease in arsenic bioavailability to varying extents in contrast with the blank group, in the descending order of ferrihydrite(F1) > goethite(G1) > hematite(H1) (F2 > G2 > H2). Total arsenic in exchangeable (F1) and specifically sorbed (F2) state was found positively correlated with arsenic bioavailability in gastric phase by PBET, SBRC and IVG methods, the correlation coefficient of which being r=0.93, P=0.002, r=0.90, P=0.004 and r=0.89,P=0.006, respectively. It was also found that arsenic bioavailability in gastric phase was positively correlated with total arsenic in F1 and F2 states by PBET(r=0.94,P=0.001) and IVG (r=0.87,P=0.009) methods, but no significant correlation was observed by SBRC method. Additionally, three in vitro experiments showed that amorphous iron bound arsenic had significant negative correlation with arsenic bioavailability in gastric phase and small intestinal phase, except that no correlation was found in small intestinal phase by SBRC method.