Removal of pechlorate from dilute aqueous solutions: synthesis, characterization and testing of perchlorate selective and permselective membranes and bimetallic catalysts
Date
2015
Authors
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Publisher
University of Delaware
Abstract
Perchlorate is an emerging inorganic contaminant in the United States. The Environmental Protection Agency (EPA) expects to issue a national primary drinking water regulation (NPDWR) for perchlorate in March 2017. Therefore, methods that are effective in eliminating perchlorate from water are needed. The objective of this study is to synthesize a perchlorate selective and permselective membrane for detection and separation of perchlorate and to integrate electrodialysis and catalytic electrochemical techniques for the simultaneous separation and reduction of perchlorate.
First, a highly sensitive polymeric membrane electrode was synthesized for the detection of perchlorate in water. The membrane electrode exhibited favorable selectivity toward perchlorate over interfering anions such as chloride, nitrate, sulfate, and bicarbonate in water and had a response time of ca. 5 s over the perchlorate concentration (activity) range of 10-6 to 10-1 M and a Nernstian slope of 58.5 ± 0.4 mV at room temperature. The potentiometric response of the electrode was pH independent in the range of 3.0 to 11.0 and had a perchlorate detection limit of 7.0 ×10-7 M (or 70 ppb). The polymeric membrane electrode was able to detect perchlorate ion at the sub-micro-molar level under conditions mimicking those of natural water systems.
Next, perchlorate permselective membranes were synthesized and characterized in this study. A membrane with a thickness of ~300?m was prepared with polyvinyl chloride (PVC) and quaternary ammonium salts in solvent under room temperature. Among 12 different quaternary ammonium salts, methyltributylammonium chloride (MTBA) showed superior perchlorate permselectivity due to in part to the favorable steric effect of the alkyl chain length. In addition, results from contact angle measurements indicated that modification with quaternary ammonium salts rendered the membranes hydrophobic. Results from Fourier transform infrared (FTIR) spectrum analysis showed that the functional groups responsible for ion exchange were incorporated in the membrane matrix successfully. The surface roughness, averaged pore radius, and ion exchange capacity of the MTBA membrane were 3.23±2.58 (nm), 83.6 (Å), and 0.12 (meq/g), respectively. In the presence of an electric field, about 60% of perchlorate ions were separated from the solution while only less than 9% of other anions, specifically, nitrate, sulfate and bicarbonate passed through the membrane under otherwise identical operation conditions simultaneously.
After the membranes were prepared for the separation of perchlorate, catalysts for the reduction of perchlorate were also examined. Monometallic and bimetallic catalysts were prepared for perchlorate reduction in the study. The electrodes were characterized for surface structure and chemical composition using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that with applying the same current and pH in the system, perchlorate reduction efficiency of monometallic-doped electrodes were in the order of Rh>Cu>Ru>Mo>Pd. For bimetallic-doped electrodes, Rh-Cu (75% reduction) was found to be more effective than Rh-Ru (60% reduction). The mass balance of chlorine species (chloride and perchlorate) in the system can be up to 95%. The imbalance part of chlorine species mass would be due to the adsorption of perchlorate by the electrode. The testing results of the catalytic electrochemical technique showed its suitability for reduction of perchlorate. X-ray photoelectron spectroscopy (XPS) analysis of the bimetallic catalyst indicated the presence of metal oxide and elemental metals. X-ray diffraction (XRD) spectra showed the presence of both crystalline and amorphous metallic catalysts.
Finally, an integrated process for the separation and reduction of perchlorate from water was constructed. With the perchlorate permselevtive membrane, perchlorate can be exclusively separated and concentrated from water in presence of other anions. Followed by using perchlorate reduction electrodes, perchlorate can be reduced to chloride. A proposed model for perchlorate reduction was developed and successfully used to calculate the rate constants of anions passing through the membranes. Thus, it has been shown that 80% of perchlorate could be separated by MTBA membrane in electrodialysis process and up to 95% of separated perchlorate can be readily reduced to chloride within 6 hours. No other oxyanions of chlorine, such as ClO3-, ClO2- or ClO- was detected as the intermediates during the reduction process. This integrated ED-CECR process can be applied for the removal of low concentrations of perchlorate from drinking water sources.