Study On The As Forms Transformation In Soils And Biological Effect

More and more attention is paid to arsenic toxicity with the arsenicals applied extensively and the problems on arsenic contamination increasing around the world. Combined with the process of arsenic-containing water irrigation, application of arsenic-containing pesticides and fertilizer, and atmospheric deposition, arsenic enters farmlands and will threaten people's health through the food chain. In ordinary conditions, the predominant asenicals is As-S compounds in nature environment with less mobility and low bioavailability. But the As(Ⅴ) is the predominant form in soils with more availability. There is a great difference of toxicity and bioavailability among the different As forms. Generally, the greatest toxicity is the tri-organic derivative, secondly is As(Ⅲ), As(Ⅴ), DMA, MMA. The AsB and AsC almost have no toxicity, which mainly exists in marine animal and plant. Under the actions of soil animals and microorganisms, the forms and valence of arsenicals can be changed through physical, physico-chemical, biochemical processes, and the toxicity and plant-availbality also vary. Because of the toxicity varying with the valence chaning, study on the transformation of arsenic forms,plant-avalibality, and the existing forms in plants will make great sense for prevention of the As toxicity, improvement of agriculture product quality and betterly establishing the standards of soil and the quality of agricultural products.In this paper, study on the method for extracting different form of arsenic in soils, transformation of inorganic and organic arsenic in soils, impact of different soil moisture on the asenic forms, influence of organic and inorganic arsenic compounds on the plant toxicity, availability and arsenic forms in plants were conducted through pot experiments and soil incubation experiments.The main results are as following:1. The optimum method for extracting different forms of arsenic in soils is that using ultrasonic extraction. The recovery percentage of different forms of arsenic, excepting As(III), are all above 84%, and the corresponding extracting amount is significantly higher than that using oscillatory and centrifugal extraction method.2. Soil incubation experiment shows: demethylation is the mainly process for all the arsenicals containing As(Ⅴ),DMA,MMA when they entering into soils, while methylation and reduction process was not observed. A series of process of arsenic occurred, like adsorption-desorption process for As(Ⅴ), demethylation from DMA, MMA to As(Ⅴ) combined with adsorption -desorption effect; Different soil moisture content can influence the conversion process directly and no obvious effect of water content on the product of various arsenicals was observed.; Under 35%, 70%, >100% of the maximum field capacity, the completely transformation times of DMA were 120d, 60d, 30d respectively, and 150d, 60d, 60d for MMA transformation;. Under the same condition, the demethylation speed of DMA is apparently faster than MMA; The optimal soil moisture for the transformation of DMA and MMA are >100%, 70% of the maxmun field capacity.3. The arsenic added into soil can partly volatilize into the air with more volatilized amount for DMA and MMA and the inorganic As(Ⅴ) hardly volatiled. The volatilizing percentage of DMA and MMA acctounted for the total arsenic respectively are 18.1% and 12.0%.4. Through the study on arsenic forms in plant, As(Ⅲ)and As(Ⅴ) are the predominant form in small Rape plant in the second season pot experiment, and there are no significantly difference for concentration and arsenic forms in small Rape between the treatments of DMA and As(Ⅴ) with the same concentration, which may be connected with the fact that DMA almost transformed into As(Ⅴ) after the first season pot experiment.5. With the outer source arsenic added into soils, more effects on the biomass of small rape can be observed with the concentration of arsenic increased especially for DMA and MMA treatments. It could cause the death of the small Rape the concentration of DMA reached 60mg/kg. The toxicity of different arsenic decreased in this order: DMA>MMA> As(Ⅴ); The mobility decreased in the order of DMA≥MMA> As(Ⅴ). The total arsenic of the small Rape above ground part for DMA and MMA treatments is significantly higher than that for As(Ⅴ) treatment.6. The As(Ⅴ) and As(Ⅲ) are the predominant form in small Rape for As(Ⅴ) treatment. DMA is the predominant form with less amount of As(Ⅴ) and As(Ⅲ) for DMA treatment. MMA is the predominant form for MMA treatment although less amount of As(Ⅴ), As(Ⅲ) and DMA was also detected. The above results showed that plant mainly absorbed As which is available with highest amount in soils, but the forms of arsenic in plant could vary with plant absorbing processes. Under the same concentration, the dimethylarsinic acid is more toxic than disodium methanearsonate, and the concentration causing plant death were 15(or even lower) and 60 mg/kg,respetively.Through this study, the effect of soil water content on the transformation of arsenic form was proclaimed to some extent, and the dynamic transformation process of DMA, MMA, As(Ⅴ) in soils and the plant-availability of different forms of arsenic, and the arsenic forms in the plant were also explored in some degree;All these obtained results can provide important reference for further research on remediation and transformation of arsenic in soils, and the mechanism for reducing the account by plant uptake and establishment of standard for soil and agricultrural product quality of As concentration and forms in our country or around the world.