Effecet Of Iron Oxides Reduction On The Arsenic Species In Paddy Soils Under Anaeronic Condition

The process of iron reduction by the microorganism has an important significance in the soil environmental chemistry of paddy slurry. As a sort of electron acceptor in the reduction process, iron oxides have not only an important effect on the species of the volatile metals in the paddy slurry, but also affect its availability. The proportion of electron acceptors and donors will change by adding volatile metals to the soil, which can affect the potential of oxidation and reduction process markedly at the same time, as well as the electrons transfer. And also, iron oxides with high surface activity, may promote the adsorption and precipitation of heavy metals in the paddy slurry, which determine the availability and mobility of the heavy metals effectively. Making use of the characteristic of iron oxides may remove or reduce soil contamination as an efficient means. In this experiment, some representative paddy slurries from four provinces were used. The soluble arsenic extraction and determination method were studied,and through the experiment of constant temperature incubation under airtight studied the effect of anaerobic incubation on arsenic species by adding As(III) and As(V) to the paddy slurry. The electron acceptors of SO42-,NO3-and Fe(OH) 3 on iron reduction and As(III) in the paddy slurries also were compared in this study. The primary results were as follows: 1. Concentration of the soluble arsenic in the four paddy soils was very poor, and the available arsenic was determined by the characteristic of paddy soils. Using 1.0 mol. L-1 HCl extract the soluble arsenic can show the availability of arsenic effectively in 60 minutes extraction. 2. In the determination method, the samples were treated by adding sodium borohydride (KBH4) as the reduction by peristaltic pump injection, As (III) and As (V) can be detected by adjusted pH of the samples solution, and the produced arsine (AsH3) absorbed by the solution of AgNO3-PVA-C2H5OH (1:1:2) was determined directly by colorimetric analysis in the wavelength of 400nm. The results showed that the detection range was 0~0.5mg/L, relative error of the standard solution determination were among -3.60%~2.20%, and the detect limit was 0.6μg .The recovery of simulate the environmental polluted water samples was ranged in 97.14%~103.4%. The method was found no significant difference compared with Ag-DDC standard method. The recoveries of total Arsenic and As (III) in the paddy soil samples were from 98.36% to 102.8% and from 56.02% to 65.34% in simulate the environmental polluted water samples respectively. 3. When added As (III) to the soil in the experiment, the iron reduction was stimulated by As(III)in the former 10 days incubation and stopped till the iron reduction finished. For the depleting of the oxidants in the 10 days incubation, the As (III) reached the lowest concentration and the oxidation process was terminated. In the following incubation the As (III) concentration increased as the results of As(V) reduction. 4. When added As (V) in the experiment, the Fe (II) increasing was procrastinated, the iron oxides reduction was not restrained by the As (V), but for the reoxidizing of Fe (II) by As (V), and this oxidation –reduction reaction given the impetus to the As (V) reduction. And the results suggested that the balance of iron reduction was consistent with the balance of As (V) reduction in the term of incubation time. 5. The total arsenic concentration in paddy soil would decrease to 87% and then keep in balance regardless the addition of As(V).Adding 100mg/kg As (V) to the paddy soil, the As (III) can be detected and the concentration was increased to 53 mg/kg and kept in balance during the following incubation, while the As (V) detected was 33mg/kg in the soil. There was a dynamic balance between the As(III) and As(V)in the paddy soil. This suggested that the As (V) cannot be reduced to As (III) completely in the anaerobic incubation and the As (III) produced was finite. The iron reduction process controlled this phenomenon during the oxidation-reduction potential changing process in the soil. 6. After adding SO42-or NO3-to the soil, NO3-can obviously restrain iron reduction, while SO42-hasn't an obvious effect on iron reduction. When added NO3-or SO42-to the soil, the concentration of As(III) would decreased, and the concentration of As(III) would decreased more remarkable when added iron oxides at same time. When added NO3-to the soil, the As(III) decreased in the former 20 days and reached the lowest ,and then the As(III) would increase. But the concentration of As(III) would decreased and could not be detected after 20days when added SO42-to the soil, and formed kind of FeAsS precipitation in the soil reported in some researches. The result suggested that using the evolution of sulfide and other reduced species such as iron oxides to stabilize bound arsenic is feasible, by adding iron oxides and SO42-can effectively decrease the arsenic pollution in the soil.