Preparation Of Polymer Coated Nanoiron Particles For Remediation Of Contaminated Groundwater

In recent years, halogenated hydrocarbons, including chlorinated hydrocarbons such as trichloroethylene(TCE), have been accumulating in the environment, particularly in groundwaters. As a result, pollution of water by halogenated hydrocarbons has become an important environmental problem. Contaminated groundwaters represent a large portion of environmental remedial action plans throughout the world.Due to their small particle size and high reactivity, the nanoscale zero valent iron (nZVI) has increasingly been utilized in the remediation of soils and groundwater.nZVI has been proved to be one of the latest innovative technologies for environmental remediation. Laboratory studies have demonstrated that nZVI can effectively transform a wide array of environmental contaminants including chlorinated organic compounds, toxic metals, and nitrate. The use of nZVI in the reaction affords little environmental threat. Thus, it is desirable to be able to efficiently dechlorinate groundwater contaminates using nZVI.However, Fe nanoparticles were easily oxidized, e.g. by air, and thereby significantly lost their main advantage of a very high surface reactivity. Freshly synthesized iron nanoparticles often ignited spontaneously upon exposure to air. When slowly exposed to air, a coating of iron oxide was formed on the surface of particles. Furthermore, it was hard for nanoiron to touch TCE in solution. The characters of this material have been serious drawbacks in practical application.To overcome these drawbacks, the study aims to: (1) synthesize polymer coated nanoiron particles and assess their air stability; (2) research the removal efficiency of TCE by polymer coated nanoiron particles; and (3) study the kinetic and mechanisms of the TCE removal by the synthesized polymer coated nanoiron particles. The research work is a part of the project named"The In-situ Remediation of Chlorinated Hydrocarbons in Groundwater with Supported Nanoscale Iron Emulsion", which is funded by the National Natural Science Foundation of China (NNSFC).There were three main parts in our study:1. nanoscale iron particles were entrapped successfully in poly(methyl methacrylate) (PMMA) by emulsion polymerization. It was found that the freshly synthesized polymer coated nanoiron particles did not ignite spontaneously when they were rapidly exposed to air. However, upon subsequent exposure in air for one week, a coating of iron oxide was formed on the surface of particles. And then, we synthesize PMMA coated nanoiron particles via a modified in-situ inverse microemulsion polymerization. The remarkable characteristic of nanoparticles in-situ synthesizing is which can synchronously bring to success in a single system for the growth of particles, granularity control, agglomeration control, high degree dispersion and long lasting stability. In this study, the polymer coated nanoiron particles produced by microemulsion polymerization have been shown to be more stable in air.2. The polymer coated nanoiron particles were used for TCE removal from aqueous solution. The effects of some factors were discussed, such as initial pH, dispersion intensity, nanoiron particles content, initial concentration of TCE, dissolved oxygen and so on.The batch studies showed that the reaction between the polymer coated nanoiron particles and TCE was composed of two processes: (a) TCE molecules was adsorped on the surface of particles; (b) Adsorped TCE molecules moved through the polymer membranes and participate in the redox reaction. The reaction of TCE removal by nanoiron particles was fater than that by polymer coated nanoiron particles.The TCE removal rate was strongly dependent on intensity of dispersion. Great intensity would accelerate the reaction when a rotary shaker was used. The greater the polymer coated nanoiron particles content was, the greater the reaction rate would be. Under the same content of polymer coated nanoiron particles, reaction rate would decrease with the increase of the pH and the initial nitrate concentration. Dissolved oxygen would retard the reduction of TCE by consuming electrons released from iron. The co-exist ion could weaken the dechlorination efficiency of polymer coated nanoiron particles. The generation of OH- in the reaction would cause the precipitation of Fe2+ during the experiment.The column studies indicated that the greater the dechlorination efficiency was, the longer the contact time in column was. 3. The kinetics and mechanisms of TCE removal by the synthesized polymer coated nanoiron particles investigated.In this study, the experimental data could be fit to pseudo first-order kinetic model. In the solid-liquid interface reaction between polymer coated nanoiron particles and TCE, the main factors that control the reaction rate was the chemical reduction. The nanoiron is the main reducing agent in the reaction. The possible reaction pathways for TCE reduction by nanoiron were hydrogenolys and reductiveβelimination, but the hydrogenolysis was the main reaction pathway for TCE. The polymer membranes appeared to have no obvious impact on the final efficiency and products of the denitrification. In anaerobic system, Fe3O4 was the final iron corrosion product.