Remediation Of Cadmium And Lead Contaminated Farmland Soil By Wasing With Organic Acid And Chlorides

With the rapid growth in population,the development of industrial production and the rapid urbanization,human have long-term and over developments on the natural resources,it caused many serious environmental problems.In China,the problems of heavy metals contaminated soils become increasingly serious,soil remediation and management become essential.Chemical washing technique has been one of the hot in soil remediation because of its low cost and high efficiency.This technique has been applied to site remediation in many other countries and achieved good results.Heavy metals contaminated soil was taken from Hunan Province and composite washing with organic acids and FeCl3 was investigated.The main results are as follows:1.Experiments for selection of suitable chemicals were performed.We compared the washing efficiencies of different organic acids(citric acid,tartaric acid,malic acid,oxalic acid,acetic acid)and chlorides(FeCl.,CaCl2,MgCl2,NaCl,KCl)at the concentrations of 100 mmol·L-1,citric acid,tartaric acid and FeCl2 were selected as the washing agents for our composite washing experiments.2.Considering the cost and damage from high concentrations of compounds to soil properties,20 mmol·L-1 FeCl3 was mixed with 100 mmol·L-1 organic acids was the optimum concentration.For soil of Laowangzhai,citric acid-FeCl3 extracted 43.7%of Cd and 22.9%of Pb,compared with FeCl3 washing,the Cd and Pb extractions were increased by 76 percents and three times,respectively;compared with citric acid washing,the Cd and Pb extractions were raised by 23 and 97 percents,respectively.For soil of Datianwan,citric-FeCl3 extracted 47.3%of Cd and 11.0%of Pb,compared with FeCl3 washing,the Cd and Pb extractions were increased by 16 and 36 times,respectively;compared with citric acid washing,the Cd and Pb extractions were raised by 28 percents and six times,respectively.For soil of Laowangzhai,tartaric acid-FeCl3 extracted 43.2%of Cd and 16.5%of Pb,compared with FeCl3 washing,the Cd and Pb extractions were increased by 71 percents and four times,respectively;compared with citric acid washing,the Cd and Pb extractions were raised by 30 and 45 percents,respectively.For soil of Datianwan,tartaric acid-FeCl3 extracted 30.7%of Cd and 0.8%of Pb,compared with FeCl3 washing,the Cd and Pb extractions were increased by ten times and 80 percents,respectively;compared with citric acid washing,the Cd and Pb extractions were raised by 2 times and 33 percents,respectively.3.The effects of liquid to solid ratio,pH of washing agents,washing time and washing times on heavy metal removal were studied by washing experiments.The Cd and Pb removal increased and levelled off with the liquid to solid ratio increased from 2:1 to 20:1;the Cd and Pb removal decreased with the pH of composite washing agents increased from 2 to 5;the Cd and Pb removal increased and levelled off with the washing time increased from 0.5 to 36 h;the Cd and Pb removal decreased with washing times changed froml to 3 times,but the Pb removal decreased slowly and the cumulative removal of Pb was ideal.According to the results of composite washing experiments,the optimum washing condition was the liquid to solid ration of 5:1,washing time being 24 h and washing times being 3.4.The heavy metal fractionations were studied by Tessier method,the experiment results shows that the heavy metal fractionations were changed after first composite washing,heavy metals bound to iron and manganese oxides,organic matters significantly decreased,carbonate-bound metals were almost zero,heavy metals in residual form almost constant;the heavy metal fractionations were changed after first composite washing,the exchangeable and carbonates decreased with the increasing washing times,heavy metals bound to iron and manganese oxides almost unchanged after FeCl3 washing repeatedly and reduced after citric acid,citric acid-FeCl3 repeatedly washing,the organic matters and residuals almost unchanged;the concentrations of constant elements in eluents were determined,the results of this and sequential extraction shows that composite washing agents can damage the carbonates,Fe-Mn oxides in the soil and form complexation with heavy metals,this process made a lot of Al,Ca,Fe,Mg and Mn ion released;analyze the composite washing mechanism by experiments and the chemical species of Cd and Pb in the extracts were calculated using Visual MINTEQ software,all the data and experiment results illustrates that composite washing mechanisms:formation of stable and soluble metal complexes with Cl-and citrate or tartrate anion,ion exchange with H+ released by citric acid and hydrolysis of FeCl3,the pH reduction by CA was remarkably beneficial to the formation of metal-Cl complexes.5.The physical and chemical properties of two soil samples before and after washing were determined.Soil properties changed after composite washing.When the washing times increased,the soil pH,available phosphorus significantly reduced,the exchangeable calcium,magnesium,CEC,organic matters,available potassium and nitrogen in soil samples slowly reduced,the free oxides(Fe,Al)in soil samples increased slightly and their change was negatively correlated with changes in CEC,the electrical conductivity(EC)increased after first washing and then decreased.