Abstracts Category : Other

The impact of natural organic matter (NOM) on photocatalytic process for the degradation of micropollutants

by Reza Rezaei

Photocatalytic oxidation process has been demonstrated as an effective technology for the removal of micropollutants in water. This process, however, is greatly affected by the presence of natural organic matter (NOM) in natural water, which interferes with treatment process by absorbing UV radiation and scavenging oxidant species. This research focused on investigating the effect of NOM on the photocatalytic oxidation of 2,4-dichlorophenoxy acetic acid (2,4-D), as target contaminant at different pH. Experiments were performed in fluidized photocatalytic reactor using template free photocatalyst spheres.Changes in solution pH were used to decouple the effects of NOM on adsorption and major oxidative mechanisms, e.g., reactions on the surface of the photocatalytic spheres via positive hole mediation and in the solution via hydroxyl radicals (OH) reaction. At pH 3, due to electrostatic attraction between solutes (2,4-D and NOM) and photocatalyst surface, photocatalytic oxidation mostly occurred via charge transfer on the surface of the photocatalyst. At pH 7, on the other hand, electrostatic repulsion between solutes and photocatalyst surface reduced adsorption and the process was primarily driven by hydroxyl radical reactions. The removal of 2,4-D reduced from 49% in the absence of NOM to 7% in the presence of 5 mgL-1 TOC NOM in neutral pH. At pH 3, this reduction was from 88% to 58%. It was observed that at neutral pH, due to higher aromatic moieties concentration and lower NOM adsorption, the effect of NOM on scavenging OH was considerable. This effect substantially decreased at low pH due to high adsorption of NOM.Higher 2,4-D removal at low pH was also due to the effect of pH on the kinetic of photocatalytic oxidation. Photocatalytic oxidation at pH 7 followed first order kinetic model. At pH 3, on the other hand, the rate of oxidation was a combination of first order and L-H models. Furthermore, the dependence of rate constants on UV intensity changed with pH; the rate constant was directly proportional to UV intensity at pH 3; whereas it is proportional to the square root of intensity at pH 7.