Studies On In-situ Remediation Of Multi-heavy Metals Contaminated Soils

Remediation of Multi-heavy metals contaminated soils is increasingly becoming a hot topics in environmental sciences. Although single-heavy metal pollution happens, the majority of heavy metal contaminations are caused by multi-heavy metals. It is more complicated to remediate multi-heavy metal contaminated soils than to remediate single-heavy metal contaminated soils because of the differences in nature of various heavy metals. Stabilization is the most widely used methods in in-situ heavy metal remediation. However, there are still some problems about immobilization mechanism and evaluation of the effectiveness. In this paper, four soil samples with different pH values were collected and the synthetically heavy metals contaminated soils were obtained by adding the corresponding metal salts. The applicability of TCLP, Tessier and BCR extraction procedures in the evaluation of stabilizing effection were evaluaetd. And a new sequence extraction procedure which is called MM methods and suitable for soils with different pH values was created. H3PO4, Na2S, CaCO3 and FeSO4 were used to stabilizing the heavy metals in soils which were collected from Shenyang, a smelter plant. Through the experiment we found that: 1) H3PO4 was recognized as the most suitable reagent for stabilizing lead in the polluted soils, Added H3PO4 two times as the molar concentration of non-residual Pb into contaminated soils. The TCLP leaching concentration rapidly reduced after added H3PO4. 20 weeks later, more than 81% of Pb was stabilized. The effects of H3PO4 on chemical fractions of lead in soils using Tessier sequence extraction procedure were evaluated. In the three soils simples the non-residual fractions Pb turned into residual fraction. It may formed a new mineral which is called pyromorphite [Pb5(PO4)3Cl] in tested soils. The conversion rate of residual was more than 32%. But the effectiveness of H3PO4 on Cu, Zn, Cd were insignificant. To make matters worse, it increased the bioavailability of some As. 2). Added Na2S, of which the molar concentration is the same as the TCLP leaching molar concentration of all heavy metals, into tested soils. Na2S can stabilizing Cd、Cu、Zn、Pb in multi-heavy metals contaminated soils. The effect on Cu was best, of which more than 83% was stabilized. Pb, Zn followed, and the effect on Zn was insignificant. The active fractions of four heavy metals turned into stability fractions after added Na2S. As Na2S increased the soil’s pH value, a part of arsenic in soils was activated which increased in environment risk. 3) Complex reagent, added as the followed orders: H3PO4, Na2S, FeSO4, CaCO3, can effectively stabilizing heavy metals in tested soils. Not only H3PO4 and Na2S make the corresponding heavy metal more stabile, the arsenic activated by H3PO4 and Na2S was also stabilized by FeSO4. Reagent complex is better than single reagent in remediating multi-heavy metals contaminated soils.