Aerobic and anaerobic TiO2-photocatalysed purifications of waters containing organic pollutants

Developments reported by European research groups in removal of dilute organic pollutants from water under the combined action of TiO2 particles and solar-illumination are briefly outlined. Instances are given of poor correlation between trends actually observed for selected TiO2*-photocatalysed conversion in aerobic conditions and trends expected within the context of two-parameter Langmuir-Hinshelwood-type rate expressions. The necessity of recognising important kinetic roles of several parameters is emphasised not only for aerobic, but also for anaerobic conditions, e.g. TiO2*-photocatalysed conversions of chloromethane pollutants in deoxygenated aqueous solutions. Comparisons between initial TiO2-sensitized photocatalytic degradation (TiO2*-PCD) rates versus dark-equilibrated [C](eq) profiles for one strongly-absorbed and one weakly-adsorbed pollutant in aerated TiO2 suspensions at concentrations <100 ppm imply that, under high photon flux, the former can be more strongly inhibited in consequence of: (i) chemisorption-induced depletions of surface OH- groups; (li) adsorbate-enhanced hole-electron recombinations on the TiO2* surfaces (iii) mass transport limitations within the TiO2 particle aggregates. Insights into the nature of strongly chemisorbing pollutants (e.g. hydroxybenzoic acids or catechol) onto TiO2(P25) were obtained by DRIFTS measurements upon freeze-dried TiO2 powder samples after their dark-equilibration with 3-100 ppm solutions. (C) 1999 Elsevier Science B.V. All rights reserved.