Research On Washing Of Soil Contaminated By Vanadium And Associated Metals Via Low Molecular Weight Organic Acids

China is one of the most important participator in the industry of vanadium over the world,along with which comes severe environmental pollution owing to the extensive mining of vanadium ore.This research conducted a thorough field investigation on a contaminated site related to vanadium ore slag,and examined the usage of low molecular weight organic acids for the washing remediation of vanadium contaminated soil under the method of ex situ vibration washing and simulated in situ column leaching.The main research results are as follows.The average concentration of V,Cr,Cu,Mn,Zn and Pb in the contaminated soil related to vanadium ore slag were 931?721?279?223?163 and 11 mg/kg,respectively.Based on single and Nemerow pollution index,86.8% of soil on the contaminated site was heavily polluted,owing to the high content of V,Cr,Cu and Zn.These 4 heavy metals were found significantly positively correlated.Under the method of ex situ vibration washing,among the 9 types of low molecular weight organic acids studied,oxalic acid,malonic acid,tartaric acid,malic acid and citric acid were found capable of eluting almost all the extractable state of V,Cr,Cu and Zn,which were 63.5%,7.0%,36.3% and 20.2% respectively based on soil metal speciation analysis.Acidity was found paramount for the removal of metals rather than chelating ability.At pH 1,all extractable proportion of metals could be washed out by all 9 acids.At pH 6,metals were hardly removed.At pH 3,binary acids and tribasic acid with greater chelating ability removed higher amount of metals than monoacids.The optimal concentration of acids was above 0.1 mol/L.Fixing the initial pH at 3,the ultimate removal efficiency of metals was found merely decreased by <5% compared to the initial pH uncontrolled scenario,while the pH of leachate was increased to 2.9~3.4 from 0.7~3.3.Under the method of simulated in situ column leaching,removal of extractable metals was achieved after 5 volumes of washing.When adjusting the initial pH of washing agents to 3,or replaacing acids with water after certain amount of acid washing,the decrease of metal cumulative removal efficiency was negligible,and the pH of leachate and soil were increased to 3.2~4.3 and 4.9~6.1 from 2.1~3.1 and 2.7~3.9,respectively.These results demonstrated that pH control or assistant water washing after acid washing can effectively alleviate the acidification of leachate and soil during acid washing with negligible loss of metal removal efficiency.After washed by low molecular weight organic acids,the content of hydrolysable nitrogen,rapidly available potassium and cation exchange capacity in soil were decreased by 51.8%~86.4%,73.1%~91.0% and 10.3%~65.3%,respectively.The content of available phosphorus,however,increased by 0.5-fold ~12.0-fold,possibly owing to the excitation of non-available phosphorus by acids.The residual acids and their salts in soil also induced the increase of organic content by 0.9-fold ~15.3-fold.