Study On Control Of Soil Arsenic Uptake By Rice Using Iron-Containing Materials

Iron-containing material, an important water treatment agent in treating arsenic (As)-containing wastewater, is also used to remediate As-contaminated upland soils and has good effects. But there is less study on remediating As-contaminated paddy soils using iron-containing material. In this research, a chemical stabilization experiment was conducted to compare the stabilization effects of four iron-containing materials (FeCl3, FeCl2, Fe0and Fe2O3) on arsenic in paddy soil, and then the Fe0, which had good stabilization effect on soil As and had minor effect on soil pH value, was selected to carry out a rice pot experiment and verify the control effects of Fe0on soil As uptake by rice. Meanwhile, the effects of Fe0on iron and As removing, transforming, form change and valence change were studied to explain the control mechanisms of Fe0on As uptake by rice from the aspect of form and valence changes. The results were as follow:(1) All of the four iron-containing materials had significant stabilization effects on As in paddy soil, and the capability is followed as the order FeCl3> FeCl2, Fe0> Fe2O3. The Fe0, which had good effects on stabilizing soil As and had minor effects on soil pH value, was the most suitable iron-containing material to remediate lightly or moderately arsenic-contaminated paddy soil. From the analysis of form effects, the mechanism of As stabilization by Fe0may be similar to that of Fe2O3, but it may has certain difference with the FeCl3and significant difference from the FeCl2.(2) Fe0increased the rice yield in general, decreased the total As and inorganic As content in root, husk and brown rice, but had no obvious effects on total and inorganic As content in straw. When the addition amount of Fe0reached the maximum dosage (8.00g/kg), the total As content in root, husk and rice were reduced by54.85%,39.86%,30.07%, and the inorganic As content in root, husk and rice were reduced by59.23%,30.82%,30.26%respectively, compared with control. Fe0also decreased the contents of root trivalent arsenic (As(Ⅲ)), root pentavalent arsenic (As(Ⅴ)), straw As(Ⅲ), husk As(Ⅲ), rice As(Ⅴ), rice (DMA) in varying degrees, but slightly increased the rice As(Ⅲ) and husk As(V) contents.(3) On the whole, Fe0made the As contents decreased or had no obvious changes, made the iron contents in each parts of the soil-rice system increased or had no obvious changes. High addition amount of Fe0(≥2.00g/kg) shortened continuous iron growth in soil solution along with the growth of rice. These functions had positive effects on decreasing the toxic action of As and minimizing As uptake by rice. And Low addition amount of Fe0(≤0.50g/kg) extended the continuous iron growth in soil solution along with the growth of rice.(4) High addition amount of Fe0(≥4.00g/kg) significantly inhibited the As gathering in the root, but stimulated the As transfer from root to straw and had no obvious effects on continuous transfer. Meanwhile, Fe0inhibited iron transfer from soil to root and from straw to husk, and stimulated iron transfer from root to straw.(5) Fe0decreased the soil WE-As, Al-As, Fe-As content and increased the soil RS-As content during the whole growing time of rice. Before the rice planting and after rice harvesting, Fe0had the same effects on decreasing WE-As content and increasing RS-As content in paddy soil, so zero-valent iron could immobilize paddy soil As persistently in a rice growth period.(6) Explaining the immobilization mechanism from fraction aspect, the zero-valent iron was first transformed into these iron fractions which can adsorb or/and coprecipitate with soil As, such as amorphous iron (AO-Fe), carbonate iron (C-Fe) and manganese oxide iron (Mn-Fe). Then, these irons reacted with WE-As, Al-bound As (Al-As), Fe-bound As (Fe-As), and turned them into residual As (RS-As) with better stability. Moreover, C-Fe and AO-Fe was enriched in the Fe plaque of rice root and increased the amount of Fe plaque, then enhanced the enrichment of soil WE-As、Al-As、Fe-As by Fe plaque and decreased the toxicity of soil WE-As、Al-As、Fe-As on environment.(7) Fe0made the Fe2+, Fe3+, As3+, As5+contents in soil-rice system increased or had no obviov changes. There was a dynamic balance of oxidation-reduction reaction as Fe2++As5+(?)Fe3++As3+, but the effects of iron on the oxidation-reduction of As just had taken a back seat in the remediation of arsenic-contaminated paddy soil.