Photochemical reduction of chromium(VI) by phenol and its halogen derivatives

Photoreduction of chromium(VI) has been investigated in solutions containing phenol or monohalogenophenol (4-bromophenol, 4-chlorophenol and 3-chlorophenol) under conditions mimicking the environmental ones. The effects of radiation wavelength, pH and molecular oxygen are analysed. The results show that chromate(VI) in excited state is reduced by phenol in its ground state via photoinduced electron transfer (PET). Moreover, solvated electrons generated by the phenol photoreaction reduce the $CrO_{4}^{2-}$ anions in their ground state. Phenol oxidation in both processes is assumed to proceed via generation of the $C_{6}H_{5}O\cdot$ radical dot radicals, which participate in further redox processes. Acidic and anoxic conditions favour the Cr(VI) photoreduction, whereas alkaline oxygenated solutions are preferable for phenol photooxidation followed by Cr(VI) thermal reduction by means of $e_{solv}^{-}$. Bromophenol appears more readily photodegradated, whereas chlorophenols are more resistant to it than unsubstituted phenol. The system consisting of chromate(VI) and phenol demonstrates the synergetic photodegradation as the phenolic substance enables photoreduction of environmental Cr(VI) and vice versa the Cr(VI) pollutant enhances photooxidation of phenol and its derivatives.