Removal of humic acid by photocatalytic process: Effect of light intensity

Abstract

Humic acid is commonly found in natural water as it is one of the by-products from decomposition of plants and animal residues. In a conventional water treatment process, which chlorine is common used as a disinfectant, the presence of humic acid could lead to the formation of carcinogenic substances, such as trihalomethanes and haloacetic acids. Thus, removal of humic acid from raw water before disinfection process is necessary. Photocatalytic reaction using Titanium Dioxide (TiO2) as a photocatalyst is one of the most effective techniques for degrading humic acid. The efficiency of this process depends on several factors; and, one of these factors is light intensity. This research investigated the effect of light intensity (35, 225 and 435 μW/cm2) and studied the kinetic of photocatalytic degradation of humic acid, using commercial TiO2 Degussa P25 as a photocatalyst. The concentration of humic acid was monitored by using UV254 absorption technique and the concentration of total organic compound was measured using a Total Organic Carbon Analyzer (TOC) every 30 min. The results showed that the removal efficiency of humic acid increased with increasing light intensity and then becoming asymptotic. At light intensity of 435 μW/cm2 and initial humic acid concentration of 4 mg/L with TiO2 loading of 100 mg/L, the highest humic acid removal efficiency was found at 99%; however, the removal efficiency of the total organic compound was found only 20%-indicating incomplete mineralization to the end product. The kinetic of the humic acid degradation process was further explained using a Langmuir - Hinshelwood (L-H) model. The reaction rate constants (kL-H) at the light intensity of 35, 225 and 435 μW/cm2 were 0.049, 0.152 and 0.178 mg L-1 min-1, respectively; while the adsorption coefficients (Kads) were relatively unchanged with light intensity. These findings imply that light intensity has an effect only on the oxidation process.