Preparation And Modification Of Supported Nano Fe/Cu Bimetal And Fundamental Research Of Remediation On The Trichloroethylene Reduction

Groundwater as the important water source of human is now encountered enormous contamination. In our country, the groundwater contamination scale, degree and the variety of pollutants are becoming larger. Halogenated hydrocarbons such as trichloroethylene (TCE), which are strongly toxic and carcinogenic are found the main contaminants in shallow groundwater. Halogenated hydrocarbons are difficult to biodegrade and the conventional treatments technologies for it required long-term operational periods, incurring high operation costs. It is also found that the halogenated hydrocarbons could not degraded completely and produce the more toxic interim products under some remediation technologies. Recent studies show that nano zero-valent iron (ZVI) is the new and highly effective reductant on the transforming halogenated organic contamination to hydrocarbons. The nano scale ZVI particles have the small sizes, the large surface areas and high surface reactivity. The nano ZVI slurry can be directly injected under pressure to contaminated groundwater for remediation. This treatment method is a new in situ remediation technology which can be rapidly removed the pollutants in short term.There are also some drawbacks on the in situ remediation of groundwater with nano ZVI. For example, nano ZVI particles have the potential of congregation in the solution due to their high surface area and high surface energy. Nano ZVI has the lower efficiency of the dechlorination and the poor stabilization in the air. It is not easy to contact and react between the dense nonaqueous phase liquids (DNAPL) such as TCE and the hydrophilic nano ZVI. The pathways and mechanisms of dechlorination by nano ZVI also require the more studies. Accordingly many researchers work on the preparation and modification of nano ZVI. The efficiency and mechanism of nano ZVI on the destroying halogenated hydrocarbons are also the interesting focus.In our research,which is a part of the project funded by National Science Committee named"The in-situ remediation of chlorinated hydrocarbons in groundwater with supported nano scale iron emulsion"the preparation of supported nano ZVI and nano Fe/Cu bimetal are presented. This study is aimed to: research the dechlorination efficiency of supported nano materials with different shapes and composition; research the effect of the surface modified supported nano metal on the removal of TCE and assess their stability; characterize and discuss the dechlorination kinetic and mechanism by supported nano metals.1. Synthesis of supported nano ZVI was achieved by liquid phase reduction and deposition while the graphite was the carrier of iron particles in ethanol/water system. The supported nano Fe/Cu bimetal was prepared by two methods of one-step and two-step.The nano ZVI particles with the size of 50~100nm were well dispersed on the surface of graphite. The individual iron nanocluster was composed by many small particles and is theα-Fe0.The surface area of supported nano iron with Fe loading of 15wt% was 23.8 m2/g and higher than other materials. The supported nano bimetal have the similar shape, size and surface area with the supported nano iron.2. The supported nano iron and nano bimetal were utilized in the dechlorination of TCE.The supported nano bimetal prepared by two-step was the most powerful reductant as compared to the supported nano iron and non-supported nano bimetal on the dechlorination of TCE. The 10mg/L TCE was completely removed in 5 hours by the supported nano bimetal. The non-supported nano bimetal was apt to aggregate in the aqueous phase so that its reactivity decreased in the reduction. The supported nano iron has the lower efficiency on the TCE reduction despite with the high surface area.The supported nano bimetal synthesized while the Fe2+and Cu2+ were reducing at the same time had the poor dechlorination efficiency. The nano bimetal particles were not easy to congregate due to the carrying by support, so the dry and wet treatment methods on nano metal were applicable at the end of synthesizing.The supported nano bimetal with 4wt% Cu on Fe0 had the fastest dechlorination rate. The material with higher or lower Cu mass neither has the better reducing effect on TCE. The nano iron had the whisker shape while the Fe2+ concentration was 0.02mol/L during the synthesizing. The size of nano iron was non-uniformly when the Fe2+ concentration was 0.2mol/L and the nano iron has the uniformly sphere shape when the Fe2+ concentration was 0.05mol/L. The nano bimetal with this shape had the fastest dechlorination rate on TCE.3. The cationic surfactant Cetyltrimethyl Ammonium Bromide (CTAB) that has the better effect of biodegradation and is nontoxic to the environment was used to modify the surface of the supported nano bimetal.The hydrophilic surface of supported nano bimetal was transformed to the hydrophobic after modification. The hydrophobic bimetal was advantageous to contact with the nonaqueous TCE molecule and had the effect of dechlorination and sorption. When the Fe0 was 5g/L, the 10mg/L TCE could be completely removed in 4 hours. When the dosage of Fe0 was larger, the active reaction site of Fe0 was occupied by the excess H2 and the pH was increasing so that dechlorination was be prohibited. The ion of NO3-,SO42- could not weaken the dechlorination efficiency of modified supported nano bimetal because the enhancing ionic intensity facilitated the TCE reduction. During the 36 days of continuous dechlorination, the modified supported nano bimetal has the highly stable property and was not easy to be oxidized.4. The kinetic of TCE dechlorination was studied according the pseudo-first order model. The mechanism of dechlorination was also discussed in the paper.The dechlorination reaction by nano bimetal was better followed the pseudo-first order equation. When the Cu was 4wt% percent of Fe0, the apparent rate constants Kobs of dechlorination was the largest; the apparent rate constants Kobs of dechlorination at the high initial TCE concentration was smaller than the constants at the low initial TCE concentration; the surface-normalized rate constant Ksa had the maximum value when the Fe0 dosage is 5g/L.Theπelectron of TCE could be bonded with empty electron orbit of Cu to become the Cu…Cl…R transition substance and the high reactivity H2* was also produced on the surface of Fe0. The H2* attacked the Cu…Cl…R on the surface of bimetal so the Cl- was broken away. The graphite and the nano iron composed large numbers of galvanic cell. The H2O obtained the electrons to produce H2 in the galvanic reaction and the pH value of solution was increasing.The three kinds of dichloromethane of interim dechlorination products were not detected during the reaction. Because the Cl elimination of TCE was took place on the surface of nano bimetal with catalytic reactivity, the intermediate products were adsorbed on the metal and could not transplant to the liquid phase.It was found that Fe3O4 was the corrosion product of Fe oxidation after dechlorination. The attenuation of reduction activity of nano metal could be attributed to excess oxide precipitates on the surface. Fe3O4 entirely covered on nano metal surface eventually lead the losing of reduction reactivity.