Catalytic ozonation for the removal of anthropogenic organic contaminants in water

The ZSM-5 zeolites with varying silica to alumina ratios and with both hydrogen and sodium counter ions (Z1000H:SiO2/Al2O3 = 1000, Z900Na:SiO2/Al2O3 = 900, Z25H:SiO2/Al2O3 = 25 and Z25Na:SiO2/Al2O3 = 25) and γ-alumina have been selected as catalysts. The first investigation was initiated to study the mechanisms of catalytic ozonation on zeolites and alumina. The formation of reactive oxygen species (ROS) such as hydroxyl radicals (oOH), hydrogen peroxide (H2O2) and superoxide ion radical (oO2-) have been investigated by using coumarin (COU), amplex red and 4-chloro-7-nitrobenzo-2-oxa-1,3-dizole (NBD-Cl) as probes respectively. The effects of hydroxyl radical scavenger and phosphates have also been studied to investigate the mechanism. The results show that alumina catalyses radical pathways involving ROS, showing its highest activity at pH close to the point of zero charge. However, zeolites do not promote the formation of ROS. The presence of phosphates and t-butanol (TBA) significantly reduces the formation of ROS in the case of alumina. However, in the case of zeolites TBA and phosphates do not have a significant effect on ROS formation.

The second investigation involved the study of the efficiency of catalysts to remove organic contaminants. The ibuprofen and volatile organic chemicals (VOCs) such as cumene, 1,2-dichlorobenzene and 1,2,4-trichlorobenzene have been selected as target pollutants. The results show that within the family of zeolites, silica to alumina ratio is important for the adsorption of pollutants and for catalytic efficiency of zeolites. Therefore, Z1000H and Z900Na have been found to effectively catalyse the removal of VOCS and Z25H and Z25Na were the better catalysts for the removal of ibuprofen in its ionized form. The alumina was found to be ineffective for the removal of VOCs. However, alumina effectively removes ibuprofen. This is because of high adsorption of ibuprofen on alumina. Therefore, it is hypothesized that zeolites operate through a simple mechanism involving the direct reaction of adsorbed species on their surfaces; their activity depends upon their silica to alumina ratios and is insensitive to the nature of counter ions. The alumina operates through a radical mechanism involving the formation of ROS. Furthermore, the adsorption of pollutants plays an important role in the catalytic ozonation process.