Control Of As Soil-to-rice Transfer With Nano-manganese Dioxide

Hunan province, southern China is famous for "hometown of nonferrous metals", is also "hometown of fish and rice" in the world. However, paddy soils around mining and smelting areas have been suffered from heavy metal pollution as a result of mining and smelting activities. The high concentration of in paddy soil As caused the decrease of the yield of rice grain and rice quality, and posed potential risk to animals and humans through food chain due to high levels of As accumulated in rice plant. Recently, more and more people paied attention on pollution in soil and rice.Therefore, effective measures should be taken to resistant the availability of arsenic and reduce the arsenic content in rice. In this study, in order to investigate the effects of nano-manganese dioxide on As transfer from soil to rice,taking a typical arsenic-contaminated paddy soils in Hunan Province as the case, preparation and characterization of nano-manganese dioxide, and three methods were adopted,which are the adsorption experiments, potted plants and field experiments. Meanwhile,to understand the mechanism of the effects of nano-manganese dioxide on As transfer from soil to rice,the soil properties,the soil solution,the arsenic speciation and the arsenic concentration in rice were analyzed. The main resultst are as follows:?1?The crystallinity of the synthesized samples is good with a uniform dispersion and agglomeration of nano-particles. The size of cluster microspheres is 1000^-1500 nm, a single nanoneedle is approximately 50 nm in diameter, length of about 500 nm. Its many gaps between the particles with specific surface area.And it is with excellent thermal stability and outstanding oxidation resistance.?2? With the increase of time, the removal rate of arsenic by nano-manganese dioxide increasing at first and then remain stable after reaching the peak value.The effect of dosage is as the same as the effect of time. However, with the increase of the initial pH, the removal rate of arenic decreasing, it is indicated that the acid condition is more conductive to the adsorption of arsenic, while, excessive acidic conditions may cause dissolution of nano-manganese dioxide;The adsorption isotherm was agreed well to the Langmuir adsorption model with maximum adsorption capacities of 27.16 mg·g^-1.?3? The main fractionations of As was the residual form, which accounts for 50%, before and after adding the nano-manganese dioxides, and followed Fe-As and Ca-As.The nano-manganese dioxide could decrease the contents of Ca-As in soil, and when the 2.0% of the nano-manganese dioxide was added to soil, the percentages of Ca-bound was the lowest with 12.25%. While, there were non-significant effects on the loosely bounded As with nano-manganese dioxide. Moreover, the percentage of residual As in the soil increased with nano-manganese dioxide increasing.?4? Under greenhouse condition,the effect of different contents of nano-manganese dioxide on soil solution pH were not significant.The concentrations of As and Mn in soil solution decreased during the rice growth period. The order of distributions of As and Mn in different parts of rice plant were root> stem> leaf> husk> brown rice.There were significantly negative correlation ?p< 0.05? between Mn and As concentrations in rice plant. The transfers of As from soil to rice were effectively controlled by different contents of nano-manganese dioxide. The total As concentration in brown rice was reduced by 65.4% with addition of 1.0% nano-manganese dioxide, and the corresponding inorganic As content was 0.19 mg·kg^-1 lower than the limited content of inorganic As in food.?5? In field experiment, the concentrations of As in root were 2-7 times than the background value of the soil, and significantly higher than those in the stems and leaves, which demonstrated that the content of As in rice followed root> stem> leaf> husk> brown rice. There was a significant positive correlation between amount of iron plaque and As concentration in root at mature stage.The iron plaque decreased with the nano-manganese dioxide treatment due to increasing Mn concentrations.Meanwhile, there was a significant correlation between Translocation factors of As from iron plaque, root,stem and leaf to seed in mature stage indicated that there was significant differences between same part of rice with different nano-manganese dioxide treatments ?p< 0.05?. The effect of different contents of nano-manganese dioxide on soil solution pH were not significant. The concentrations of Mn in the soil solution are the first to rise, there is no obvious regularity arsenic concentration in the treated group; nano-MnO₂ can effectively reduce the arsenic content of rice, and the dosage of 0.2%,0.5%,2.0% arsenic content of rice were below limit of arsenic in the food limited content, indicated that nano-MnO₂ can be used on As transfer from soil to rice.