Electrochemical and thermochemical destruction of chlorinated solvents
In this work, an electrochemical method was developed to destroy liquid chlorinated solvents, and a thermochemical technique was invented to reduce gas-phase chlorinated solvents to hydrocarbons. The developed electrochemical method belongs to the most potential technique for wastewater treatment--only electrochemical method is possible to simultaneously destroy all water contaminants; the invented thermochemical method is the fastest way to destroy chlorinated solvents. In the first part of this work, the anodic oxidation of trichloroethylene (TCE) on an Ebonex ceramic electrode was investigated. TCE could be oxidized to CO₂, CO, Cl⁻, and ClO₃⁻. The disappearance of TCE was first-order, independent of pH, and initial TCE concentrations. TCE oxidation occurred only on the anodic surface and was limited by mass transport at Ea > 4.3 V vs SSCE ( i > 5 mA cm⁻²). Hydroxyl radicals generated on anode surface were detected using a spin trap. A kinetic model was successfully correlated with experimental results. In the second part of this work, TCE and CF were rapidly dechlorinated to hydrocarbons on the Ebonex ceramic cathode using Pt or Pd as catalyst. Pt was found to possess great resistance to sulfur and chlorine poisoning. Pd was quickly deactivated in sulfate solution. TCE and CF cathodic transformation were strongly dependent of pH. The reaction rate was limited by mass transport at Ec<-1.6 V (i > 5 mA cm⁻²). The mass transfer to cathode surface was found to be three times faster than to anode surface. The main products of TCE reduction were ethane, ethylene, and chloride, and for CF were methane and chloride. The proposed reaction mechanism and kinetic model were consistent with experimental results. In the third part of this work, a new hydrodechlorination method was invented for gas-phase chlorinated solvent destruction. Gas-phase chlorinated solvents, such as PCE, TCE, 1,1-DCE, VC, and CF were rapidly reduced to ethane, ethylene, and methane in a continuous-flow column reactor at ambient temperature and pressure. This is the fastest way to destroy chlorinated solvents. The catalyst could be easily regenerated and had a long-life time (over one and half year). The reaction mechanism and kinetics were studied. In the forth part of this work, the first three parts of work were combined together to invent a new destruction method for chlorinated solvents in real wastewater. Ebonex ceramic materials served as electrodes and Pt was plated on cathode used as catalyst in the electrolytic cell. The headspace of the electrolytic cell was connected to the Pd/Ni catalyst column. Liquid chlorinated solvents were destroyed on electrode surface, and gas-phase chlorinated solvents were reduced to hydrocarbons in the Pd/Ni catalyst surface in the presence of hydrogen. Water scale or other deposited materials on Pt coated Ebonex cathode could be removed, and the catalyst could be regenerated by reversing electrode polarity.
Engineering, Chemical., Engineering, Sanitary and Municipal., Engineering, Environmental.
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