Treatment of Chromium, Mercury, Lead, Uranium, and Arsenic in Water by Heterogeneous Photocatalysis

Treatment of chromium, mercury, lead, uranium, and arsenic in water by heterogeneous photocatalysis with TiO2 or other semiconductors is reviewed in the present contribution. This chapter focuses on the mechanisms taking place in the processes, together with experimental physicochemical evidences that support the involved pathways. Most metals react through successive one-electron pathways producing unstable intermediates until the most stable species are formed. On this basis, three types of mechanisms have been considered: (a) direct reduction by photogenerated conduction band electrons; (b) indirect reduction by intermediates generated from electron donors present in the system; (c) oxidative removal by holes or hydroxyl radicals. Chromium and mercury react generally by direct reduction (pathway (a)), their removal being enhanced by the presence of electron donors. Lead and arsenic react mostly by the oxidative pathway (c), but indirect pathway (b) can be driven by the addition of suitable electron donors. The uranium system has been less studied and its reductive removal from water in anaerobic conditions in the presence of electron donors has been described. Practical applications for the removal of the four elements related to the protection of the environment are included.