Photocatalysis Of Parabens By Titanium Dioxide Under UV Irradiation In Aqueous Solution

The environmental impact of pharmaceuticals and personal care products (PPCPs), as emerging environmental pollutants, has been the focus of great concern in recent years.Parabens are esters of 4-hydroxybenzoic acid, only differing in the ester group, which may be a methyl-, ethyl-, propyl-, butyl- or benzyl- group. These ester groups result in the formation of methyl paraben, ethyl paraben, propyl paraben, butyl paraben or benzyl paraben. Parabens are a group of preservatives widely used in cosmetic, food and pharmaceutical products. These compounds have a broad spectrum of antimicrobial activity. However, many manufacturers are reducing the use of parabens because of a growing body of evidence that parabens are estrogenic. The topical formulation industry became even more averse to using parabens after researchers reported that parabens were present in breast cancer tissue and carcinogenic function for skin. In recent years, several studies have reported the presence of the parabens in the environment, and have been detected in various aquatic ecosystems worldwide at the level of ng L-1. However, little has been done to identify the degradation pathways in water.A number of systemic experiments were carried out to investigate the photocatalytic degradation of methyl-, ethyl-, propyl-, butyl-, and benzyl paraben:(a) to assess the extent of hydrolysis, photolysis, adsorption and photocatalysis; (b) to determine photocatalytic degradation; (c) to elucidate the dynamics of the degradative and; (d) to investigate the effects of pH values, TiO2 loading, initial paraben concentration, oxygen concentration, and light intensity; e) apply a multivariable center composite design based on response surface methodology for estimating the individual and interaction factors including these four more important parameters, such as pH, TiO2 loading, oxygen concentration and light flux, and abtain optimal experimental conditions; f) to identify the intermediates and products using HPLC-DAD and GC-MS; and (g) to evaluate the extent of mineralization processes.The photocatalysis of these parabens is nearly pure condition including negligible hydrolysis and photolysis and small amout of adsorption. The heterogeneous photocatalytic degradation of these paraben follows a Langmuir-Hinshelwood kinetic model expression.The effects of several important parameters on the degradation of these paraben were investigated. pH value has an important influence on the parabens photocatalysis, and degradation rate increase with increase in pH value up to 9.0, slight decrease at higher pH value (pH 11.0). The effect of TiO2 loading on the degradation is obvious, and exist an optimal concentration (2.5 g L-1). With the increase of initial parabens concentration, the degradation rate decrease (1/ro-1/Ceq has a satisfactory linear cooperation). Oxygen is an essential parameter to photocatalytic degradation of parabens, which accelerate the degradation rate markedly. However, higher oxygen concentration hinders the further degradation rate increase. Light intensity, as a very important parameter, is necessary to investigate the influence on the model compound remove. The light intensity at 5.0×1015 photons s-1 cm-2 is a turn point. Less than the light flux is a linear relationship range, but more than that is a square-root relationship range.Moreover, a multivariable center composite design based on response surface methodology was applied to estimate the individual and interaction factors including pH, TiO2 loading, oxygen concentration and light flux. An acceptable semi-empirical expression was obtained via data analysis to predict the response, and optimal experimental conditions were also achieved thanks to the experimental design (pH 9, TiO2 loading 2.5 g L-1, dissolved oxygen concentration 18 mg L-1 and light flux 5.8×1015 photon s-1 cm-2). For investigating the behavior of these paraben in photocatalyzed degradation, intermediates and photoproducts were identified by HPLC-DAD and GC-MS qualltatively. Several carboxylic acids, such as oxalic, formic, acetic, malonic, tartaric, and succinic acid were identified by HPLC-DAD, and several aromatic products were detected by GC-MS application. Among these acids, acetic and tartaric acid were also identified by GC-MS. To assess the extent of mineralization during the photocatalytic degradation of these parabens, TOC decrease was monitored during photocatalytic degradation of them at optimal experimental conditions.