Electrochemical Reduction of Triclocarban at a Silver Cathode

Triclocarban (TCC) is an environmentally persistent antimicrobial agent commonly used in personal care products such as hand soaps, lotions, and toothpastes.  Although the use of TCC and similar compounds (such as triclosan) is intended to minimize human exposure to harmful bacteria, recent research has indicated that these substances may themselves cause health problems.  TCC, in addition to being a likely endocrine disruptor, also probably contributes to the development of drug-resistant bacteria as it builds up in municipal wastewaters.¹  As of 2013, TCC accounted for more than 50% of pharmaceuticals in municipal sludge, making the possible dangers of its accumulation a primary concern.²  A need for removal and remediation of TCC has inspired research into many methods of degradation, including the use of ozone, photolysis, and electro-Fenton processes.^3–5  Other promising methods, however, have attracted less attention.  For example, earlier literature shows that other organohalogen pollutants can be electrochemically dehalogenated, particularly at silver cathodes, leading to less harmful products.⁶

In this study, we have explored the electrochemical reduction of TCC at a silver cathode as a possible method of remediation.  Cyclic voltammograms for the reduction of TCC show three cathodic peaks at both glassy carbon and silver cathodes in dimethylformamide (DMF) containing 0.050 M tetramethylammonium tetrafluoroborate (TMABF₄).  In comparison with a glassy carbon electrode, each peak is shifted approximately 30 mV toward more positive values at silver, the respective peak potentials being –1.27, –1.37, and –1.80 V vs. a cadmium amalgam reference electrode (with a potential of –0.76 V vs. the saturated calomel electrode).  Whereas the two most positive cathodic peaks for reduction of TCC are nearly merged at a typical scan rate of 100 mV s^–1, lower scan rates can be employed to increase the separation of these peaks.  Three comparable cathodic peaks for TCC are seen for a silver cathode in dimethyl sulfoxide, but only two cathodic peaks are observed in acetonitrile.  In a solvent consisting of 10% water–90% DMF, only one cathodic peak, corresponding to the most positive peak in the other solvents, is seen before the onset of electrolytic breakdown of the solvent–electrolyte.  Diphenylurea, the product anticipated from complete dechlorination of TCC, exhibits a single cathodic peak at –1.80 V in DMF, with a significantly smaller current than the corresponding peak for reduction of TCC.

Preliminary results obtained from controlled–potential (bulk) electrolyses in DMF with a silver gauze cathode indicate that partial dechlorination of TCC does occur over a range of initial concentrations.  Analyses of catholytes by means of HPLC–MS strongly suggest that the major products of electrolyses at a potential corresponding to the second reduction peak are the expected di- and monochlorinated species; under these conditions, no completely dechlorinated product has been identified, and unreduced starting material is present.  In future work, bulk electrolyses of TCC will be conducted at potentials corresponding to the most negative cathodic peak in DMF and DMF–water mixtures, and products will be identified and quantitated with the aid of HPLC–MS.

 

References

1. Halden, R. Environ. Sci. Technol. 2014, 48, 3603–3611.
2. Deo, R.; Halden, R. Water 2013, 5, 1346–1365.
3. Tizaoi, C.; Grima, N.; Hilal, N. Chem. Eng. Process. 2011, 50, 637–643.
4. Ding, S.; Wang, X.; Jiang, W.; Meng, X.; Zhao, R.; Wang, C.; Wang, X. Environ. Sci. Pollut. Res. 2013, 20, 3195–3201.
5. Sirés, I.; Oturan, N.; Oturan, M. A.; Rodríguez, R. M.; Garrido, J.A. Brillas, E. Electrochim. Acta 2007, 52, 5493–5503.
6. Peverly, A.A.; Karty, J.A.; Peters, D.G. J. Electroanal. Chem. 2013, 692, 66–71.