Effects Of Low Molecular Weight Organic Acids On Immobilization Of Soil Heavy Metal By Biochar

Biochar has been widely used in soil heavy metal pollution remediation due to its rich carbon content,developed pore structure and rich surface functional groups.However,the influence of low molecular weight organic acids,as important root exudates,on the stability and effect of biochar passivation has not been clearly yet.In this experiment,the effects of low molecular weight organic acids on biochar adsorption of metals and passivated polluted soil were studied by batch adsorption and laboratory incubation in a copper mining area of Daye city,Hubei province,China.The findings are given below:（1）An immersion experiment was conducted with three kinds of organic acid,viz,citric acid,tartaric acid,malic acid.The organic acid solutions at three concentrations（5,10 and 20 mmol/L）were soak biochar for 7-days.The results showed that organic acid can improve the C,H elements of biochar and cation to release a quantity,the higher organic acid concentration increased,as compared to CK,The citric acid under the biochar C element is increased by 7.85%-9.51%,Ca and Mg ion release quantity increased 66.5 times and 9.6 times respectively.All the three organic acids at 5mmol/L could promote the adsorption of Cd²⁺by biochar,but all the treatments had an inhibitory effect on the adsorption of Cu²⁺by biochar,and the inhibitory effect was enhanced with the increase of organic acid concentration.（2）In second experiment,1% rice straw biochar and 20 mmol/L of three organic acids were added into the soil for 80 days.Results revealed that soil pH in the presence of organic acids decreased in the early stage and was higher than that without organic acids in the late stage,and increased by 0.6 unit compared with BC on average.By the complexing of heavy metals,organic acids affect the adsorption and passivation effect of biochar on heavy metals,and promote the transformation of soil heavy metals from residual fractions to acid-soluble and reducible fractions.Under citric acid treatment,residual state contents of Cd,Cu and Zn decreased by 15%,7%and 4%,respectively.Except for Zn,the toxicity leaching and ecological risk index of Cd and Cu were higher relative to BC,these findings indicate that the presence of 20 mmol/L organic acids had a negative effect on Cd and Cu in biochar passivated soil.The inhibitory effect was citric acid>tartaric acid>malic acid.（3）In the third experiment,different addition sequences of rice straw biochar and citric acid in heavy metal compound contaminated soil was set to simulate the effect of biochar before and after crop planting on soil Cd,Cu and Zn speciation and potential ecological risks in the actual passivated process.The results showed that addition of biochar could significantly increase the soil pH（0.3-0.6 unit）and the residual content（3%-5%）of the three heavy metals in the contaminated soil,and reduce the content of the weak acid extraction fractions and the toxic leaching concentration of the three heavy metals.Under the different order of addition,first plus biochar on three kinds of heavy metals toxicity leaching concentration of heavy metals and the lowest impact,three kinds of heavy metal toxicity leaching quantity are lower than the original soil pollution（2%-6%）,through the ecological risk assessment found that add this way did not change the original pollution soil ecological risk rating,on soil heavy metal still has certain passivation effect.After being added organic acid,due to its activation,follow-up joining biochar leaching toxicity of three kinds of heavy metals in soil volume remained at a high level,compared with CK,TCLP extractable Cd and Cu concentration on increased by21.6%and 41.5%respectively.Similarly,the three kinds of heavy metal in the index and potential ecological risk level significantly improved,which make the cadmium in metro index（medium）from 5.18 to 7.1（serious）.By electron microscope observation,it was found that biochar could adsorb soil particles into the internal pores and reduce the dissolution and activation of minerals by organic acids.In conclusion,medium and high concentration of organic acids can inhibit biochar’s adsorption of soil and aqueous solution metals in water and soil heavy metals.The stability of soil heavy metals was improved by adding biochar and the effect of organic acids on soil heavy metals was decreased.After being added organic acids,the activity and ecological risk of heavy metals were maintained at a high level,but the effect of biochar was poor.In this paper,the effects of different types and concentrations of organic acids on the passivation of heavy metals in soil by biochar and its mechanism were investigated by isothermal adsorption and soil incubation experiments.This study has a certain theoretical value for improving the efficiency of biochar in the application of passivation remediation of heavy metal pollution in farmland soil.