Removing Heavy Metals In Contaminated Soils By Combination Washings And Recovery Technology Research Of Eluent

Laboratorial experiments were conducted on multi-metal contaminated soil samples collected from Chenzhou Hunan to find out an optimal eluant, elution conditions and eluant combination. In order to accumulate techniques and experience of heavy metal contaminated soil recovery within a larger scope, both eluant recovery and germination experiments on washed soil to verify its ecotoxicity were lauched at the same time.The main research results are:（1） From experiments on soil samples, the content of Cu, Zn, Pb, Cd, As are 46 mg·kg^-1, 243 mg·kg^-1,322 mg·kg^-1,1.32 mg·kg^-1,32 mg·kg^-1. Cd pollution is the most serious,4.4 times over national qualified standard and it comes with a certain degree of Zn, Pb and As contamination.（2） From the comparation among washing effects of Na-EDTA,NH4-EDTA and Ca-EDTA, they have certain effects on Cu, Zn, Pb and Cd, but nearly no removal effect on As. Removal rates on different heavy metals are:Cd>Pb>Cu>Zn> As; washing efficiencies on same heavy matal are:NH4-EDTA>Na-EDTA>Ca-EDTA.（3） Determined eluant turbidities of Na-EDTA、NH4-EDTA and Ca-EDTA were 35NTU^ 33NTU、2.5NTU,Ca-EDTA. For Ca-EDTA has a advantage of settleability, Ca-EDTA was selected as the certain eluant for convenience.（4） Optimal conditions of Ca-EDTA are:concentration of 2.5 mmol·L^-1, solid-to-liquid ratio of 1:5, washing time is 2h, washes 3 times. Under these conditions, accumulative 26.30% Cu,12.37% Zn,45.18% Pb and 73.28% Cd are removed in soil sample.（5） Because Ca-EDTA nearly has no removal effect on As, Ca-EDTA oxalate and Ca-EDTA+ K2HPO4 are introduced to washing agents combination experiments. Results show that K2HPO4 has a poor removal effect on As while oxalate has a better removal effect on As and also removes Cu, Zn, Pb and Cd. After 3 times washing of Ca-EDTA and 2 times of oxalate, accumulated 42.49% Cu,21.98% Zn,44.21% Pb,83.46%Cd and 17.74% As can be removed. Every heavy metals index to washed soil is lower than the secondary heavy metal limits of national soil environmental quality standard. For EDTA’s recalcitrance, EDTA is preferred and oxalate comes then.（6） Conducts of Na₂S solution on EDTA eluant show that removal rates of Cu, Zn, Pb and Cd can reach 100% when Na₂S dosage is 3mmol.Recovery EDTA solution still has removal effect on heavy metals in soil after conducts of Na₂S, however, its efficiency falls 30% approximately.（7） Chloro-type anion exchange resin has a preferable absorbing effect on heavy metals in eluant. Removal rates of Cu, Zn, Pb and Cd in eluant are 100%,74%,96% and 95% when its solid-to-liquid ratio is 1:20. Chloro-type anion exchange resin can reborn,it also has a preferable absorbing effect on heavy metals in eluant. Removal rates of Cu, Zn, Pb and Cd in eluant are 95.65%,74.56%,99.69%,100%.（8） Germinate experiments of astragalus sinicus on washed soil show that washed soil will inhibit astragalus sinicus’s germination because nutrient element lost in washing. Soil’s fertility can be regained by organic fertilizer and compound fertilizer to enhance the germination rate of astragalus sinicus.