Research Of Thioglycolic Acid On Elution Of Pb From Mining Contaminated Soil

In this paper, thioglycolic acid was used as the new soil eluent,and the eluting effect of lead in mining contaminated soil of Chenzhou as well as influencing factors were studied.Preparing a water-solubility lead complex under the alkalescent environment by using lead nitrate solution and thioglycolic acid as the raw materials,and its related chemical properties were studied by solubility experiment,elemental analyzer,infrared spectroscopy,UV-Vis spectrometer,differential thermal analysis and electrochemical testor,meanwhile,the stability of lead complex solution was tested and the optimal conditions of preparation were determined,offerring theoretical basis for eluting remediation of lead in the soil by thioglycolic acid.The basic properties of contaminated soil was analyzed,and the chemical speciation distribution of lead was studied by BCR.This paper also systematically researched the eluting effect of thioglycolic acid to lead in soil with different conditions of pH values,eluting time,concentrations of eluant,liquid-solid ratio,eluting temporature and eluting times and so on.The optimal conditions for eluting Pb from the soil were determined by orthogonal experiment,and then analyzed morphological changes of Pb before and after eluting the contaminated soil under optimum conditions.The main results of this study were given as follows:(1)The product’s molecular formula was Pb(SCH2COOK)2, difficult to dissolve in organic solvent and acids, soluble in water.A strong coordinating bond formed between Pb and the mercapta group of the thioglycollic acid and its characteristic adsorption wave length was 276nm.The product was thermally stable until 170 ℃ and its redox stability was good under the condition of low concentration and normal temperature.The stability of lead complex solution was not very good,influenced by air,storing period and temperature.The solution should be preserved airtight and dark,storing period not too long,temperture not exceeding 55℃.The optimal conditions of preparation were as follows:reaction ration was 3:1,reaction temporature was 30℃reaction time was 120min.(2)Pb content in upland soil near Shizhuyuan Mining of Chenzhou is 3497.3mg/kg, far beyond the third standard limits (Pb<500) in the "national soil environmental quality standards" (GB156182-1995).Soil is neutral to slightly acidic with medium fertility, belonging to sandy silt loam. Fe-Mn oxides and residue are the main existence form of Pb in the soil, the organic content also high, the carbonate content relatively small.(3)The results of eluting experments showed that the removal rate of Pb increased first and then tended to remain stable with the increase of pH value; With the increase of the oscillation time, the removal rate of Pb was generally holding steady after the first rapid increase trend, when the oscillation time was 4h, the removal rate reached 74.53%, then continued to extend the time,the removal rate changed slightly;The removal rate of Pb increased significantly with the increasing concentration of thioglycolic acid,the highest removal rate reached 78.82%;Improving eluting liquid-solid ratio from 5 to 10,the removal rate of Pb increased from 24.06% to 72.61% rapidly,with liquid-solid ratio in the range of 10～25,the removal rate increased a little extent,when exceeding 25,the removal rate decreased a narrow range; The removal of Pb showed a slow downward trend after the first increase with the increasing eluting temperature;The increasing eluting frequency did not much affect on the removal rate of Pb.(4)The optimal conditions for eluting Pb from the soil were as follows: concentration of thioglycolic acid was 0.50mol/L;liquid-solid ratio was 12;elution temporature was 30℃;elution frequency was 1; pH value of thioglycolic acid and elution time were 10 and 4h,respectively.Under the optimal eluting conditions, carbonate, Fe-Mn oxides fraction, organic fraction and residual Pb in the contaminated soil decreased by 98.97%,99.08%,98.90% and 23.42%,respectively.A lead complex was prepared with stable nature,which confirms the feasibility of thioglycolic acid as the soil eluent initially and riches the kinds of water-solubility lead compound and the system of eluting remediation,offerring powerful theoretical basis for remediation of lead in contaminated soils with thioglycolic acid. Lead pollution of soil was serious in the studying area.The soil had good permeability, suit to take eluting remediation technology,and Pb was in conjunction with clay minerals in the soil or forms a stable lattice structure with soil.Thioglycolic acid could significantly increase the mobility of Pb in the soil, promoting Pb to desorb from the soil, and the eluting effect to Pb was influenced by pH value, liquid-solid ratio,concentration, eluting temperature,eluting frequency and eluting times.The optimal conditions for eluting was determined,and all species of Pbin the soil had different degrees of reduction after eluting.