This article elaborates on an earlier communication from this laboratory that described a new approach to the electrochemical remediation of Cr(VI) based on the use of a conducting polymer, namely, polypyrrole. The polymer film is shown to act as a catalyst for Cr(VI) reduction by shuttling reversibly between its two redox states. The thermodynamic and kinetic aspects of the process are described. The influence of solution pH, substrate [Cr(VI)] initial concentration, and the number of pyrrole units in the polymer film, on the extent of reduction of Cr(VT) is presented. Cyclic voltammetry experiments designed to probe the catalytic nature of the reduction process are described as are stability tests for the polypyrrole film electroactivity on extended contact with Cr(VI) containing solutions. InfroductionChromium occurs in two common oxidation states in nature, Cr(III) and Cr(VI). Because it is only weakly sorbed onto inorganic surfaces,1'2 Cr(VI) is notoriously mobile in nature. On the other hand, Cr(III) is readily precipitated or sorbed on a variety of inorganic and organic surfaces at near neutral pH."2 The properties of Cr(III) and Cr(VI) have been reviewed with respect to acute and chronic toxicity, dermal toxicity, systemic toxicity, toxicokinetics, cytotoxicity, genotoxicity, and carcinogencity.4 The hexavalent chromium compounds appear to be 10 to 100 times more toxic than their Cr(III) counterparts when both are administered by the oral route. The toxicology of chromi-
Formaldehyde (HCHO) was decomposed in UV‐irradiated aqueous suspensions of titanium dioxide. Initial HCHO levels in the range 60–1000 ppm, a nominal TiO2 dose of 1 g/liter, and a medium‐pressure Hg lamp were employed in these experiments. Chemical oxygen demand measurements of the solution before and after photocatalytic treatment revealed the mineralization of HCHO to be complete only for the lower end (<100 ppm) of the concentration range. Direct ultraviolet photolysis of HCHO also occurred, but this pathway had an initial lag unlike the photocatalytic route. In the presence of TiO2 and ultraviolet irradiation, the direct photolysis route was bypassed and the reaction proceeded dominantly by the photocatalytic route. The kinetics of the photocatalytic oxidation of HCHO were analyzed using several models. The effect of H2O2 addition to the UV/TiO2 system was also probed as well as the homogeneous ultraviolet H2O2 approach for the treatment of HCHO. The latter exhibited the fastest kinetics for the destruction of HCHO. Finally, the long‐term stability and photocatalytic activity of TiO2 were monitored with two substrates, namely, HCHO and trichloroethylene in two different media: viz. pristine water and a “real‐life” water sample with bicarbonate alkalinity. No degradation in the photocatalyst performance was noted over ten repeat use cycles in either case.
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