Study of benzylparaben photocatalytic degradation by TiO₂

Abstract

The photocatalytic degradation of benzylparaben in TiO₂ suspension under UV irradiation was investigated and different experimental parameters were optimized. The degradation apparently followed a Langmuir–Hinshelwood kinetic relationship and the apparent first order rate constant was found equal to k_app = 0.017 min⁻¹. It was found that alkaline pH was beneficial for benzylparaben photocatalysis, and the highest degradation efficiency was obtained at pH 9. A TiO₂ loading of 2.5 g L⁻¹ was enough to reach an optimal conversion rate while a higher loading (3.0 g L⁻¹) had obvious inhibition phenomenon. The pseudo first order kinetic constant increased from 0.0043 to 0.025 min⁻¹ while oxygen concentration increased from 0 to 36 mg L⁻¹. In addition, the removal efficiency decreased while initial benzylparaben concentrations increased from 5 to 25 mg L⁻¹. Light intensity is a quite important parameter influencing benzylparaben photocatalysis since for photonic flux ranging from 1.2 to 5.0 × 10¹⁵ photons s⁻¹ cm⁻², the rate constant was directly proportional to the photonic flux, while for higher intensity (from 5.0 to 5.8 × 10¹⁵ photons s⁻¹ cm⁻²) the rate of degradation varied as a square root of the photonic flux. Different intermediates of benzylparaben photocatalysis were identified or characterized by GC–MS and HPLC and total organic carbon (TOC) analyses showed an almost total mineralization. These observations suggest that photocatalysis may be envisaged as an efficient method for treatment of diluted waste waters containing emerging paraben pollutants.

Introduction

Pharmaceutical and personal care products have recently been detected in sewage effluents [1], [2], [3], [4], surface and ground water [3], [4], [5], [6], and even drinking water [3], [7]. Frequent occurrence of pharmaceuticals in aquatic environments and drinking water has raised a concern about their potential effects on environment and human health. Some adverse health effects of pharmaceutical pollutants include aquatic toxicity, resistance development in pathogenic bacteria, genotoxicity, and endocrine disruption [8], [9]. The presence of trace pharmaceutical and personal care products in drinking water is also of public concern since little is known about potential chronic health effects associated with long-term ingestion of these compounds through drinking water [7]. These compounds make their way to water bodies through various sources, such as direct disposal of surplus drugs in households, excretion by humans and animals, and inadequate treatment of manufacturing effluents [4], [10]. To avoid undesired accumulation of drugs in aquatic environments, development of powerful oxidation techniques is underway to remove these compounds from water [10], [11], [12].

Heterogeneous photocatalysis is an advanced oxidation process (AOP) where semiconductors acting as photocatalysts, are exposed to UV irradiation. During irradiation, a multi-step process involving the formation of reactive species such as hydroxyl radicals (HO), holes (h⁺) able to oxidize and mineralize almost all kind of organic compounds occurs. The most extensively used photocatalyst is titanium dioxide (anatase) due to its optical and electronic properties, low cost, abundance, chemical stability and non-toxic nature. The TiO₂-mediated photocatalysis has been successfully employed to oxidize many organic pollutants present in aqueous systems that cannot be treated by conventional techniques due to their chemical stability and low biodegradability [13], [14].

Benzylparaben (BNP, Fig. 1) is one of a homologous series of parabens including methyl-, ethyl-, propyl-, butyl- and benzyl-parabens. These molecules have an antimicrobial activity which increases as the chain length of ester group increases, while their solubility decreases with increasing chain length. Parabens are extensively used as cosmetic preservatives present in a large variety of products including face, body and hand creams, lotions and moisturizers, eye makeup products; night creams, lotions, cleansing products hair conditioners, bubble baths, shampoos, mud packs, underarm deodorants, skin lighteners, sachets, etc. [15]. Rastogi et al. [16] analyzed 215 cosmetic products for the methyl-, ethyl-, propyl-, butyl- and benzyl paraben esters and found that nearly all (99%) of leave-on products and 77% of rinse-off products contained parabens. However, parabens have been suggested as agents in body care formulation potentially involved in breast cancer [17], [18] because of their absorption through skin [19], [20] as intact esters [21], their hormonal activity and their reproductive toxicity. Moreover, because of wide usage, parabens continuously released into aquatic media through domestic wastewater and have been detected at ng L⁻¹ level in river water samples [22]. Therefore, parabens have been classified as emerging environmental pollutants by the U.S. Environmental Protection Agency (USEPA).

Recently the photocatalytic degradation of methylparaben by TiO₂ using a chemometric approach based on a surface methodology to evaluate possible interactions between four different experimental parameters [23] was investigated. It was found that the two most important parameters were light flux and pH values and strong interactions could take place between two or three parameters (dissolved oxygen concentration and light flux or TiO₂ loading and light flux, etc.).

The present work reports on the investigation of the photocatalytic degradation of benzylparaben, a molecule slightly different from methylparaben due to the presence of benzene ring instead of methyl. Benzene ring can induce different physicochemical effects with respect to methylparaben, including lower water solubility, a higher light sensitivity due to the second benzene ring and a new target for hydroxyl radical attack. In this study several parameters were investigated individually, such as initial pH, TiO₂ loading, dissolved oxygen concentration, light intensity, and initial benzylparaben concentration. In addition, the identification of the reaction intermediates as well as carboxylic acids and total organic carbon generated during the degradation were carried out.