Among the available remediation techniques for heavy metal contaminated soils, chemical immobilization and phytoremediation are promising due to the cost-effectiveness and high public acceptance of these techniques; chemical immobilization and phytoremediation can be used for the in situ remediation of large areas of contaminated soil but currently remain in the development stage. Chemical immobilization via amendment is recognized as a viable option for vast industrial areas and the surrounding land, although the heavy metals remain in the soils. Phytoremediation is a cost-effective, environmentally friendly, publicly recognized approach. However, phytoremediation is time consuming and relatively in efficient. Combined remediation techniques may overcome the limitations of a single remediation technique and improve the remediation efficiency of multi-metal-contaminated soil. Chemically assisted phytoremediation is quickly gaining attention as a way to accelerate the remediation efficiency of multi metal contaminated soils soils. Industrial by products with high silicon and iron contents have shown potential in reducing heavy metal a vailability and remediating contaminated soil due to the strong adsorption and chemical precipitation capacit ies of these elements . The recycling of industrial by products c ould not only solve the problem of waste disposal but would also provide a financia lly lucrative market for industrial by products as useful products . In the present Ph.D. project, a nonhazardous and reduced process i s proposed to dispose of copper (Cu) tailings; a silicon iron amendment was prepared from residue originating from acid extracted Cu tailings based on thermal activation; and the influence of the silicon iron amendment on the remediation of multi metal contaminated soils was evaluated through soil incubation and greenhouse vetiver pot experiments. The primary results are summarized as follows: 1. This study proposed a nonhazardous and reduced process to dispose of Cu tailings using zero valent iron (Fe 0 ) to replace Cu 2+2+; the process occurs in an H 2 SO 4 extracting solution of Cu tailings and include s ferrate(VI) precipitation of heavy metals in wastewater. The proposed process generate s f o ur products: nonhazardous extracted residue, valuable Cu concentrate, nonhazardous effluent, and hazardous sludge. A total of 90% of the Cu tailings were transformed into nonhazardous extracted residue, as the extraction efficiencies of Cu, Zn, Ni, Pb and Cd from the Cu tailings reached 92.8%, 95.1%, 48.1%, 99.3%, and 12.6%, respectively, under the optimal conditions of 5% H conditions of 5% H22SOSO44, 0.05 mL/g H, 0.05 mL/g H22OO22 and a liquidand a liquid--solid ratio of 3 mL/g undersolid ratio of 3 mL/g under 900 rpm900 rpm stirring stirring at 85℃ for 1 h. Approximately 93% of at 85℃ for 1 h. Approximately 93% of the the Cu in the Cu tailings was recovered with high purity Cu in the Cu tailings was recovered with high purity ((82.06% 82.06% Cu) when using the FeCu) when using the Fe00 replacement of Cureplacement of Cu2+ 2+ in an Hin an H22SOSO4 4 extractiextractionon solution at 20solution at 20°C°C withwith stirring at 400 rpm,stirring at 400 rpm, a pH of 1.5, a pH of 1.5, an an agitation time of 60 min and agitation time of 60 min and an an Fe/Cu molar ratio of 1.4. The Fe/Cu molar ratio of 1.4. The hazardous sludge collected from wastewater treatment accounted for 0.3% of the sludge collected from wastewater treatment accounted for 0.3% of the weight of theweight of the Cu Cu tailings, tailings, exceeding the hazardous waste landfill pollution control standards of China exceeding the hazardous waste landfill pollution control standards of China (GB18598(GB18598--2001). Because 2001). Because more than 99% of more than 99% of the the CdCd2+2+, Cu, Cu2+2+, Zn, Zn2+2+, and Ni, and Ni2+2+ in the wastewater were in the wastewater were removed by removed by ferrate(VI), the heavy metal content in the effluent was below the industr, the heavy metal content in the effluent was below the industrial pollutant ial pollutant emission standards of China emission standards of China (GB8978(GB8978--1996)1996)..2. A silicon 2. A silicon--iron amendment was prepared from iron amendment was prepared from residue residue originatoriginatinging from acid extracted Cu tailings from acid extracted Cu tailings based on thermal based on thermal chemicalchemical activation.activation. The rThe results showed that the highest soluble silicon (6.11% on esults showed that the highest soluble silicon (6.11% on basis of total Si) and iron (2.3% on basis of total Fe) in basis of total Si) and iron (2.3% on basis of total Fe) in the the prepared amendment was determined prepared amendment was determined based on based on thermal activation at 1150°C for 30 minat 1150°C for 30 min under the optimal additive ratio the optimal additive ratio ofof residue: residue: CaO: CaO: NaNa22COCO33: NaOH = 1: 0.4: 0.4: 0.2: NaOH = 1: 0.4: 0.4: 0.2.. The The amendmentamendment contained abundant Si, Fe and Cacontained abundant Si, Fe and Ca with high pHwith high pH.. In addition,In addition, the the water and soil suspension incubation trialswater and soil suspension incubation trials indicated that the amendment cindicated that the amendment couldould be be safely used in soil remediation.safely used in soil remediation. 3.3. The remediation performance oThe remediation performance of the siliconf the silicon--iron amendment was evaluated through soil iron amendment was evaluated through soil incubation and greenhouse vetiver pot experiments. incubation and greenhouse vetiver pot experiments. The results showed that the contents of The results showed that the contents of available Cd, Cr and Pb in available Cd, Cr and Pb in the the contaminated soils were significantly reduced by contaminated soils were significantly reduced by the the siliconsilicon--iron iron amendment anamendment and decreased with increasing d decreased with increasing amendment amendment raterate. . The amendment The amendment could contribute tocould contribute to pHpH--changechange--inducinduceded immobilizationimmobilization, Fe, Fe--induced chemisorption, Siinduced chemisorption, Si--induced coprecipitation, and induced coprecipitation, and CaCa--induced ion exchange. Therefore, induced ion exchange. Therefore, the siliconsilicon--ironiron amendmentamendment could be a candidate for the remediation of Cd, Cr, and Pb contaminated soil. 4.4. Vetiver grassVetiver grass (Vetiveria zizanioides) could be a candidate for phytostabilization of As, Cr, Cd, Pb could be a candidate for phytostabilization of As, Cr, Cd, Pb in the tested multiin the tested multi--metalmetal--contaminated soil. The mechanism underlying metal tolerance in vetiver contaminated soil. The mechanism underlying metal tolerance in vetiver was that the As, Cd, Cr, Pb accumulated in the vetiver grass were largely retained in the roots.was that the As, Cd, Cr, Pb accumulated in the vetiver grass were largely retained in the roots. The The effecteffectss of the siliconof the silicon--iron amendment on the growth and heavy metal translocation and iron amendment on the growth and heavy metal translocation and accumulation of vetiver grown in contaminated soils waccumulation of vetiver grown in contaminated soils wereere studied. The results showedstudied. The results showed thatthat the the siliconsilicon--iron amendmentiron amendment significantly improved the growth of significantly improved the growth of the the vetivervetiver, which, which induced an increaseVin the absorption and accumulation of Cd, Cr and PbCd, Cr and Pb in the roots and precipitation on the root surface, thereby inhibiting transportation of the metals from the roots to the shoots. The highest The highest biomass of vetiver was observed biomass of vetiver was observed when the amendment was applied at a rateamendment was applied at a rate of 1.0% in the contaminated soils; the concentrations of the metals in the shoots was reduced by 24.2% for Cd, 73.0% for Pb, 58.4% for Cu and 41.1% for Zn. TThe amendment induced accumulation of Cd and Cr in the roots and then enhanced the phytostabilization of Cd and Cr.phytostabilization of Cd and Cr.55. The effects of . The effects of the the siliconsilicon--iron amendment and biochar on the growth and heavy metal iron amendment and biochar on the growth and heavy metal translocation and accumulation of vetiver grown in cotranslocation and accumulation of vetiver grown in contaminated soils were studied through pot ntaminated soils were studied through pot experimentexperimentss. The results showed that. The results showed that the disadvantages of the disadvantages of the the siliconsilicon--iron amendmentiron amendment could be could be offset by combining offset by combining the amendmentthe amendment with biochar.with biochar. Combined application of Combined application of ironiron--siliconsilicon amendment amendment and biocharand biochar markedly improved markedly improved the the vetiver growth, vetiver growth, which was more effective for seeding growth. which was more effective for seeding growth. Combined application of amendment and biochar could furtherCombined application of amendment and biochar could further enhance the phytostabilization of Cr, enhance the phytostabilization of Cr, Cd, Cd, and and Pb.Pb.The results of this study could provide technical support for theThe results of this study could provide technical support for the useuse of tailingsof tailings and a practical basis and a pra