Chemical Immobilization Bench-scale Studies On In-situ Remediation Of Multi-heavy Metals Contaminated Soils

Heavy metals contamination in soils is increasingly under environmental concern, as a result of the rapid industrialization and urbanization. Accumulation of heavy metals in soils and subsequently in waters or in food chain is potential threat to human health. Hence, increasing awareness of the hazard makes it necessary to remediate metal contaminated soils. Heavy metal contaminants cannot be destroyed like organic contaminants but only be relocated from one place to another or transferred its environmental existing speciation. Conventional soil remediation technologies based on the excavation, transport, and landfill of metal contaminated soils are highly effective at a low risk, but the cost is high. In situ chemical immobilization technique is of particular interest because it is relatively cost-effective and less disruptive to the environment.The amendments selection and immobile mechanism are the two key aspects which should be taken into account on chemical immobilization study. However, heavy metals speciation distributions in soils between emergency contaminated style and accumulative contaminated style are quite different. Water soluble and exchangeable fractions are the predominant existing species in emergency contaminated soils. In order to hinder the spread of heavy metals in the environment immediately, amendments quick immobile effectiveness should be considered firstly. In accumulative contaminated soils, on the other hand, secondary stable fractions, e.g. carbonate associated fraction, are the predominant existing species, and pose a permanent threat to the environment. Thus, the immobile efficiencies and the long-term stabilities should be taken into account. Unfortunately, discriminatively chemical immobilization research on soils contaminated between these two contamination styles is limited. The purpose of this paper are to select the most effective amendments for emergency multi-heavy metals contaminated soils, to identify immobile efficiencies of different amendments combination for accumulative ones, and to predict metals long-term stability mended by chemical immobile technique.In the present study, four soils with various pH values have been sampled. Experimental soils, mimicking emergency multi-heavy metals contaminated soils, synthesized by adding the corresponding soluble metal salts. The quick immobile effectiveness of CaCO₃, Na₂S₂O₃, Fe (OH) 3, K₃PO₄, K₂HPO₄, Ca (H₂PO₄)2 and phosphate rock for each target metal has been examined by using TCLP (Toxicity Characteristic Leaching Procedure) method. The accumulative multi-heavy metals contaminated soils, sampled from an inoperative smelter site in Shenyang, were immobilized with three chemical immobilization treatments, i.e. T1: H₃PO₄+CaO, T2: Na₂S+CaCO₃ and T3: H₃PO₄+Na₂S+CaCO₃. The effectiveness of every treatment for the tested soils was evaluated by using water extraction, TCLP, and Tessier's sequential extraction method. Metals bioaccessibilities were evaluated by an SBET (simple bioaccessibility extraction test) method mimicking metal uptake in the acidic environment of human stomach. The possible mechanisms for metal immobilization were elucidated using XRD (X-ray diffraction), EPMA (electron probe micro-analyzer) and chemical speciation program Visual MINTEQ. Results listed as follow:In emergency multi-heavy metals contaminated soils:1. CaCO₃ could effectively hinder the spread and translocation of Cu, Zn, Cd, Hg, Ni and Cr in the environment. With the same dosage, the higher clay contents the higher immobile effectiveness.2. Na₂S₂O₃ could effectively hinder the spread and translocation of Cr in the environment. Added 2% (w/w) Na₂S₂O₃ to acidic and alkaline soils, the immobile effectiveness of Cr were above 74% and 98%, respectively.3. Fe (OH)₃ could effectively hinder the spread and translocation of As in the environment. Added 6% (w/w) Fe (OH)₃, the concentrations of As in TCLP leachates decreased below 0.18 mg·L^-1 in all tested soils.4. Phosphorus-containing materials could effectively hinder the spread and translocation of Pb in the environment. The presence of Ca element would be favor to immobilize Cd, Ni, Cu and Zn, added 2% (w/w) Ca (H₂PO₄)₂ to acidic soils, the immobile effectiveness of soil Pb achieved above 90%.5. CaCO₃, Na₂S₂O₃ and phosphorus-containing materials increased the concentrations of As in TCLP leachates.In accumulative multi-heavy metals contaminated soils:a. In T1-treated soils, the immobile effectiveness of Pb, Cd, Cu and Zn evaluated by TCLP are at the range of 80.77-93.16%, 64.64-73.07%, 5.95-62.47% and 9.67-38.17%, respectively.b. In T2-treated soils, the immobile effectiveness of Pb, Cd, Cu and Zn evaluated by TCLP are at the range of 47.33-75.44%, 13.30-59.89%, 54.75-83.08% and 26.08-57.41%, respectively.c. In T3-treated soils, the immobile effectiveness of Pb, Cd, Cu and Zn evaluated by TCLP are at the range of 75.60-97.24%, 74.75-78.77%, 75.99-95.12% and 25.67-42.41%, respectively.d. T1 and T3 treatments significantly increased the availability of soil As.e. Sequential extraction results indicated that the residual fraction conversion ratio of soil Pb and Cd in T1 treated soils at the range of 19.71-44.64% and 5.89-35.32%, respectively, and in T2 treated soils at the range of 23.39-45.03% and 8.13-39.95%, respectively.f. SBET results indicated that T1 and T3 treatments were effective in reducing soil Pb and soil Cd bioaccessibilities while significantly increasing soil As bioaccessibility, T2 treatment was effective in reducing Pb, Cd and As bioaccessibilities in all tested soils.g. MINTEQ model and activity-ratio diagram indicated that PbHPO₄ controlled Pb²⁺ activities in T1 and T3 treated soils. However, PbCO₃ controlled Pb²⁺ activities in T2 treated soils.h. XRD and EPMA elucidated that the mechanism for Pb immobilization were via formation of insoluble chloropyromorphite minerals in T1 and T3 treated soils. The mechanism for Cu, Zn and Cd immobilization were via formation of multi-metal sulfide precipitates (minerals). However, the Cu-sulfide is predominant in multi-metals sulfide precipitates (minerals).In conclusion, T3 treatment proved an effective approach to immobilize Pb, Cd, Cu and Zn for accumulatively multiple contaminated soils. Further research needs to be done on the sequential extraction procedure for Fe-rich industrial contaminated soils.