The Study Of As Speciation Transformation By Chlorella Vulgaris In Paddy Soil

In recent years,with the rapid progress of industrialization and urbanization in China,industrial waste gas,waste water and waste slag discharge,mineral smelting,sewage irrigation and irrational application of pesticides and fertilizers have caused the soil to face different levels of heavy metal pollution,of which arsenic has it has become the primary class of metal pollutants in agricultural soil in many places in China.With the advancement of science and technology,the composition and characteristics of algae have been well known to humans.Among them,Chlorella vulgaris is a common green unicellular algae,which is easy to obtain,small in size,fast in reproduction,etc.The material is sensitive and has achieved good results in environmental arsenic pollution control.In this paper,we will first study the transformation of Chlorella vulgaris on the chemical forms of arsenic in solution system,and further explore whether Chlorella vulgaris cells have accumulation of arsenic and redox conversion,and confirm whether the oxidation of arsenite in aqueous solution comes from algae.In order to clarify whether the oxidation or reduction of arsenic by Chlorella vulgaris cells occurs mainly in cells or outside the cells,the arsenic morphology and total amount of Chlorella vulgaris cells were tested.On this basis,we explored the transformation of Chlorella vulgaris on the chemical form of soil As in the flooded soil system and the flooded soil-rice system.The main research contents and results are as follows:(1)In the arsenic solution system,the photosynthesis of Chlorella vulgaris in the shallow water system under light conditions increased the oxygen content in the water,Eh increased,Chlorella vulgaris promotedthe conversion of As(?)to As(?),and the oxidation rate of As(?)it increases with the increase of Chlorella vulgaris concentration,which reduces the toxicity of arsenic in water and facilitates the adsorption and fixation of arsenic.It can be seen that Chlorella vulgaris can promote the oxidative conversion of arsenic in the upper layer of natural water,making As(?)dominant,thereby reducing arsenic toxicity.(2)Chlorella vulgaris can oxidize As(?).In the case of arsenic concentrations below 200 ?g/L,chemical oxidation dominates;in As(?)solution,when As(?)concentration is 200 ?g/L the reduction efficiency of Chlorella vulgaris to As(?)is as high as 97%,and the reductivity of algae is stronger than that of oxidization at higher concentrations.(3)The arsenic experiment inside the Chlorella vulgaris cells showed that the Chlorella vulgaris cells not only accumulate arsenic,but also redox transformation.When the concentration of arsenic in aqueous solution is high,the oxidation of As(?)in algae cells is dominant,the maximum amount of accumulated arsenic in algae cells is about 138 ?g/g,and the accumulation of arsenic in the cells of Chlorella vulgaris varies with As(?)concentration.The increase is reduced.(4)Accumulation of arsenic in Chlorella vulgaris also excretes arsenic.The efflux causes only organic arsenic to remain inside the Chlorella vulgaris cells,and the total amount of arsenic in the cells is still decreasing,while the total amount outside the cells is increasing.At the end of the experiment,the total amount of arsenic outside the cells of Chlorella vulgaris was twice as high as that of the cells.It can be observed that the rate of arsenic excretion of Chlorella vulgaris cells is greater than the rate of accumulation of arsenic.(5)Through the comparison experiment in As(?)solution,the concentration of As(?)in the filtrate after Chlorella vulgaris cultivation was calculated to be 68.37 ?g/L and 69.67 ?g/L,respectively.It is knownthat the reduction of arsenic by Chlorella vulgaris cells is mainly occurs inside the cell.(6)In flooded soil systems,lower concentrations of Chlorella vulgaris(1%)are beneficial to promote arsenic methylation in soil pore water,while high concentrations of Chlorella vulgaris(5-10%)accelerate the demethylation process.Inorganic As(?)is the most important form of arsenic in the soil solution;alkaline characteristics of each concentration of Chlorella vulgaris are beneficial to promote the desorption and dissolution of As(?)in soil.(7)In the submerged soil-rice system,Chlorella vulgaris increased the content of soluble organic carbon in the soil,accelerated the formation of As(?)and DMA,and transformed the solid phase arsenic in rhizosphere soil from stable passivation to non-steady state.This leads to an increase in arsenic content in rice seedlings.