Photocatalytic degradation of humic acid in aqueous solutions

Abstract

Recent interest has been focused on the destructive removal of humic acids innatural waters since these substances are known to be major precursors of carcinogenicdisinfection by-products like trihalomethanes produced during chlorination. Variousmethods have been proposed to control the precursors of trihalomethanes in drinking water.Advanced oxidation processes (AOPs) have been developed in order to achieve completeminerilization of organic compounds. Photocatalytic degradation process usmgsemiconducter oxides such as titanium dioxide has gained attention recently. The mainmechanism of this process is the generation hydroxyl radical which is known as a the mostpowerfull and non-selective oxidizing agent.In this study the effects of pH dependent phosphate species on the photocatalyicdegradation of humic acid (10 mg L-1) in aqueous medium was investigated. Bench scaleexperiments, were carried out using titanium dioxide as the photocatalyst and Black LightFluorescent Lamp as the irradiation source. The degradation was followed in terms ofColor436, Color4oo, UV280, UV254 and TOe. In acidic medium, after Ih irradiation time 95%Color436 and 62% TOC were removed with Ti02 concentration of 0.25 mg mL-1 in theabsence of phosphate ion whereas in the presence of low concentration of phosphate ions« 0.005 M) the corresponding values were found to be 36% and 24%. In spite of the factthat acidic medium was favorable for the removal of humic acids, 49-79% reduction in therates of degradation was observed due to the presence of phosphate ions. Increasingphosphate concentration up to fivefold did not affect the degradation rate of humic acid,whereas degradation rates increased with increasing photocatalyst concentrations in therange of 0.25 -l.0 mg mL-1.The decrease in the photocatalytic degradation rates in terms offirst order reaction kinetics can be explained by an inhibition effect due to the presence ofphosphate species in aqueous medium.