Effect of Iron on the Release of Arsenic in Flooded Paddy Soils

Abstract

Amorphous iron oxides in paddy soil are critical adsorbents of arsenic. The flooding period during rice cultivation contributes to the reductive dissolution of these amorphous iron oxides, which releases sorbed arsenic into the paddy soil solution. However, more detailed work should be conducted to evaluate quantitatively arsenic immobilization, release, and transformation regulated by metastable amorphous iron oxides. In previous studies, arsenic in the soil solution phase and solid phase were classified into F1 (exchangeable arsenic), F2 (specifically sorbed arsenic), F3 (amorphous iron oxide bound arsenic), and F4 (crystalline iron oxide bound arsenic), according to a sequential extraction procedure using reagents of increasing dissolution strength. In this study, soil samples were collected from the vicinity of a silver smelting plant in Chenzhou, Hunan Province, and the contribution of different arsenic speciation (F1, F2, F3, and F4) to arsenic release during anaerobic enrichment incubation of paddy soil was investigated. Sample analysis was conducted at the end of the first phase (day 15) and the second phase (day 30). The effects of amorphous iron oxides in paddy soil on migration and transformation of arsenic were discussed. Results showed significant elevation of dissolved Fe(Ⅱ) and arsenic concentration (P<0.05) in enrichment solutions in the second phase compared with that in the first phase. Arsenic released in the soil solution in both phases originated from exchangeable arsenic and specifically sorbed arsenic, as indicated by its significantly positive correlation with F1 and F2 (r=0.73, P<0.05; r=0.657, P<0.05). However, an insignificant positive correlation was found between the arsenic released and F3. Moreover, HCl-extractable Fe(Ⅱ) was significantly and positively correlated with arsenic (r=0.577, P<0.05; r=0.613, P<0.05), while amorphous iron oxides were significantly and negatively correlated with arsenic (r=-0.428, P=0.126; r=-0.564, P<0.05). In conclusion, arsenic in the F1 and F2 fractions acted as the major source of released arsenic. Despite elevated levels of HCl-extractable Fe(Ⅱ) that might result from the slight reductive dissolution of amorphous iron oxide, the significant negative correlation between dissolved arsenic and amorphous iron oxides indicated that metastable amorphous iron oxides in anaerobic paddy soil can generally sorb dissolved arsenic effectively, resulting in lower mobility of arsenic. Increasing the level of amorphous iron oxides in paddy soil is conducive to inactivation of arsenic.