A comparison of novel ozone-based systems and photocatalysis for the removal of water pollutants

Three ozone-based systems and a photocatalytic system have been compared for the removal of a reactive dyestuff, orange RO16, and 2-chlorophenol in relation to degradation performances and ozone and energy consumptions. These systems were (1) liquid/gas–ozone (LGO): ozone was applied as it is produced in the gas phase; (2) liquid/solid–ozone (LSO): ozone was adsorbed on particulate silica-based material and then applied to water; (3) liquid/liquid–ozone (LLO): ozone was dissolved in a water-immiscible solvent and then applied to water; and (4) photocatalytic system using titanium dioxide catalyst (PHC). All four systems were capable of degrading the pollutants but presented different characteristics. The LSO system offered the possibility of using long contact times for slow ozone reactions and the LLO system is most suitable for fast ozone reactions. Both systems offer the prospect of more efficient use of ozone by extracting specific pollutants away from the water phase to the solid or the solvent phases. The PHC system presented the lowest rates and the highest energy consumptions by a factor of up to 400 times as compared to the ozone-based systems. The four systems were classified on the basis of their energy consumption as follows: for the degradation of RO16 (LLO<LSO<LGOPHC) and for the degradation of 2-CP (LLO.LGO<LSOnPHC).

All four systems studied in this work were capable of degrading RO16 and 2-CP though they presented different characteristics. Substantial variations in ozone consumption between the three ozone-based systems were found. These variations may due to changes in reaction mechanisms as well as mass transfer limitations. In addition, ozone decay reactions due to the presence of the third medium may also have contributed to variations in ozone consumption. The LSO system offers the possibility of using long contact times to match slow reactions but currently suffers the problem of long and potentially expensive drying times of the adsorbent between water contacts. This may be tolerated if the silica adsorbent is very effective for the pollutant in question. The LLO system is most suitable for fast reactions and offers the prospect (as also does the LSO system) of more efficient use of ozone by extracting specific pollutants away from bulk components. The power consumption varied largely between the four systems and it was found that PHC system consumed the highest energy. It was estimated that only 1% of the supplied energy is actually used to activate the catalyst and the remaining energy is dissipated as heat. The PHC system used 400 times more energy to decolourise RO16 and 55 times to degrade 2-CP as compared to the LLO system. The PHC system used 10 times more energy to decolourise RO16 as compared to 2-CP due to light absorption capabilities of the dye, which reduced the light intensity that would reach the catalyst. Higher optimum catalyst concentration was also obtained for RO16 decolourisation (5 g/L) as compared to 2-CP degradation (0.25 g/L). A plausible explanation for this was linked to the differences in sizes of the two molecules and their absorption capabilities of light. All systems studied in this work appear to have merits as alternative oxidation processes. LSO and LLO systems are expected to offer better performances in terms of removal rates and ozone utilisation.