| Transportation and transformation of heavy metals may cause the pollution of soil and groundwater simultaneously. The remediation of contaminated soil or groundwater is often implemented separately for a specific site. The main objective of this study was to demonstrate a simultaneous remediation technology for heavy metal contaminated soil and groundwater by integrating the chemical immobilization and pump-and-treat methods. The study focused on the technical feasibility of this simultaneous remediation technology, the remediation efficiency and the immobilization mechanism of heavy metals in contaminated soil and groundwater, as well as the influence of plant growth to the simultaneous remediation system. The bichar- and phosphate- induced immobilization of heavy metals in the soil-water interface as well as the transportation and transformation of heavy metals in the new remediation system were studied by both laboratory experiment and computer numerical simulation. The potential heavy metal immobilization mechanisms in the simultaneous remediation system will be elucidated by experiment and spectroscopic analysis technique(XRD, SEM). The main contents are as follows:(1) An incubation experiment was first conducted to determine how phosphate materials(triple superphosphate, TSP; phosphate rock tailing, PR and their mixture, PT) and biochar(dairy-manure-derived biochar, DM; rice- straw-derived biochar, RS) induced Pb, Cu, Zn, and Cd immobilization in contaminated soils. The results showed that phosphate amendments reduced soil Pb, Cu and Zn leachability in TCLP extraction up by 89.2%, 24.4% and 34.3%, respectively. The PT, DM and RS amendment reduced Pb, Zn, and Cd in TCLP extraction by 35.8-77.6%, 25.0-31.5% and 4.4-27.9%, respectively. The heavy metal remediation effect followed the decreasing order of phosphate materials>dairy-manure-derived biochar>rice-straw-derived biochar. The Pb/Cd-P precipitation was the main mechanism for Pb and Cd immobilization by phosphate amendment. The precipitation, adsorption and ion exchange would be responsible for Pb, Cd immobilization by biochar. PT and DM could be effective soil amendments for heavy metals immobilization(2) The study on the stability of initially immobilized metals in contaminated soils was conducted by a column leaching experiment. After 10 Pore volumes(PV) simulated acid rain leaching, the Pb, Cu and Zn in contaminated soil showed the risk of down migration to deeper soil. Compared to the control,phosphorous amendments could significantly decrease the risk of Pb, Cu and Zn down migration by 45.2-99.5%, 49.2-99.0% and 17.8-84.9%, respectively. At the end of leaching, 92.5-99.9% of Pb, 88.9-99.7% of Cu, 73.5-99.8% of Zn and 78.3-97.7% of P were locked in contaminated soil, which further demonstrated the feasibility of simultaneous remediation of contaminated soil and groundwater combining chemical immobilization and pump and treat technology.(3) The new simultaneous remediation technology of both heavy metal contaminated soil and groundwater was developed by integrating the chemical immobilization and pump-and-treat methods. In the phosphate-induced simultaneous remediation system, the disposal effect of Pb, Zn and Cd from simulated contaminated groundwater was 96.4%, 44.6% and 49.2%, respectively. And also the Ca Cl2 extracted Pb, Zn and Cd concentration in soils was decreased by 60.5%, 9.3% and 12.3%, respectively, compared with CK. In the DM biochar-induced simultaneous remediation system, the disposal effect of Pb, Zn and Cd from simulated contaminated groundwater was 97.5%, 53.4% and 54.5%, and their Ca Cl2 extracted concentration was decreased by 98.2%,72.6% and 67.8%, respectively, compared with CK. Consequently, the research provided a theoretical basis and technical support for the simultaneous heavy metals remediation technology of contaminated soil and groundwater. The amendment material, leaching speed, agrotype and heavy metal had influence on remediation efficiency. X-ray diffraction(XRD) and scanning electron microscopy(SEM-EDS) analysis demonstrated that the phosphate- and DM amenments showed different reaction processes and mechanisms for heavy metals immobilization in the simultaneous remediation system. The phosphate-induced Pb immobilization in this system was via the Pb-P precipitation, while biochar was via both sorption and precipitation mechanism; The phosphate-induced Cd immobilization in this system was through co-precipitation of Pb, P and Cd, while biochar was only through adsorption.(4) The transportation of Cd and Zn in simultaneous remediation system was simulated by both experiment and Hydrus modeling. The Two-site non-equilibrium adsorption model was better to simulate the transportation of Cd and Zn in simultaneous remediation system. The simultaneous remediation system incorporated with phosphorus or biochar could increase disposal efficiency and adsorption capacity of Cd and Zn. Two reasons contributed to the phenomenon: 1) incorporated phosphorus or biochar could decrease saturated conductivity, increase Hydrodynamic dispersion degree and soil saturation moisture content, which could increase the contact time of water and soil, and then increase the soil adsorption capacity; 2) incorporated phosphorus or biochar could increase the adsorption capacity and adsorption speed.(5) In ecological simultaneous remediation based on chemical immobilization and phytoextration, the disposal efficiency of simulated contaminated groundwater was 97.5-99.7% for Pb, and 86.9-97.7% for Cd. In the plant-soil-water system, most of heavy metals from simulated contaminated groundwater were retained in the top layer of contaminated soil, only 0.03% of Pb and 0.01% of Cd were extracted to the plant shoot. Compared to CK, the incorporation of phosphate or DM biochar in simultaneous remediation system could benefit the species change of Pb from the labile fraction to the more stable fraction, increase biomass of reeds, and decrease the Pb, Cd concentration in shoot. The increased soil porosity by plant root could enhance the soil seepage velocity,and then decrease the contact time between soil and contaminated water, leading to the decrease of soil adsorption ability for heavy metal from aqueous solution. The exudates from rhizosphere and microorganisms(bacteria and mycorrhiza) living in the rhizosphere could increase DOC and decrease soil p H, which were responsible for decreasing heavy metal adsorption and re-mobilizing them to more liable fraction. However, the incorporation of phosphate or biochar could reduce heavy metal re-mobility, compared with CK. |
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