INTERRELATIONSHIPS BETWEEN POLLUTANT CONCENTRATION, EXTENT OF ADSORPTION, TIO2-SENSITIZED REMOVAL, PHOTON FLUX AND LEVELS OF ELECTRON OR HOLE TRAPPING ADDITIVES .1. AQUEOUS MONOCHLOROPHENOL-TIO2(P25) SUSPENSIONS

Results are presented for the equilibrium extent of dark adsorption and the initial rates of TiO2(P25)-sensitized photocatalytic decomposition (PCD) for monochlorophenols (Cp) in aqueous suspensions at various initial concentrations. When a high flux (2 x 10(18) min-1 per 30 ml of suspension) of photons at 365 +/- 10 nm was used, a zero-order dependence of PCD on eq[4-Cp]aq was observed, allied to a low apparent efficiency. Apparent efficiencies were much higher under a much lower UV flux (1.7 x 10(16) min-1) and the dependence of the initial rate on q[4-Cp], then resembled that frequently associated with a Langmuir-Hinshetwood-type saturation. However, the equilibrium extent of dark adsorption of the monochlorophenols onto TiO2 determined experimentally was very small at eq[Cp]aq less-than-or-equal-to 100 ppm, above which it increased slowly in a manner consistent with an adsorption isotherm of the multilayer type. Additions of Ag+ to the suspension greatly increased the initial PCD rates and the apparent efficiency attainable with a high photon flux, such increases being consistent with decreased hole-electron recombination due to electron localization on the adsorbed Ag+. Results are also reported from the competition-type kinetic experiments at moderate intensities in which 3-chloro-4-hydroxybenzoic acid (CHBzA), isopropanol or furfuryl alcohol were added to the suspensions to compete against 4-Cp for holes and/or OH. radicals photogenerated at the TiO2-aqueous solution interface. These results cannot be rationalized solely in terms of hole scavenging by well-adsorbed CHBzA or of homogeneous scavenging of OH. radicals by alcohols in the bulk aqueous phase.