Photodegradation of acetaminophen in TiO2 suspended solution

https://doi.org/10.1016/j.jhazmat.2007.12.098Get rights and content

Abstract

This study investigated the photocatalytic degradation of acetaminophen (APAP) in TiO2 suspended solution under a 250 W metal halide lamp. The influence of some parameters on the degradation of acetaminophen was studied and described in details, such as initial APAP concentration, initial pH value and TiO2 dosage. After 100 min irradiation, about 95% of APAP is decomposed in the 1.0 g L−1 TiO2 aqueous solution with an initial concentration of 100 μmol L−1. The effect of adsorption at three different pH values has also been analyzed and it has been conducted that pH 3.5, at which APAP was readily adsorbed also degraded at a faster rate. Reaction rate at pH 6.9 and pH 9.5 was 2.84 and 2.96 μM min−1, respectively. Direct hole (h+) oxidation and ipso-substitution was found to be the main initial step for APAP degradation. Main reaction intermediates and products were identified by GC/MS analysis. The mechanism of acetaminophen photocatalytic degradation in TiO2 suspended solution was studied not only experimentally but also theoretically by calculating the frontier electron density of APAP. The results obtained indicated that TiO2 photocatalytic degradation is a highly effective way to remove APAP from wastewater and drinking water without any generation of more toxic products.

Introduction

In recent decades, the presence of Pharmaceuticals and Personal Care Products (PPCPs) in the environment is emerging as a new environmental concern to the scientists as well as the public, which has low concentration in the environment and usually does not show acute toxicity. After digestion and metabolism of these compounds in body, residual parts with their metabolites are excreted in human urine and manure, which are the main inlets of them to enter the environment [1], [2]. There are still some residual parts of them getting into the surface and groundwater during and after the sewage treatment [3], [4], [5].

Acetaminophen (paracetamol, abbreviated as APAP) is one of the top 200 prescriptions in the United States in 2003, which is widely used as pain relief. Muir et al. [6] found 58–68% of acetaminophen was excreted from the body during therapeutic use and the median concentrations of 0.11 μg L−1 was detected in US streams by Kolpin et al. [7]. Most researches on APAP are focused on its mechanism of analgesic action and its toxicity [8], [9], [10], [11], [12]. Only a few researches on its treatment have been published. Lorphensri et al. found acetaminophen exhibited virtually no sorption and no retardation in aquifer sand studies [13]. Based on HPLC, 2D 1H, 13C, 15N NMR and GC/MS analysis, Vogna et al. found the main degradation pathways for acetaminophen were derived from three different hydroxylation steps [14]. Andreozzi et al. found acetaminophen could be mineralized up to 30 and 40% while using ozonation and H2O2 photolysis, respectively [15]. When treated with hypochlorite, acetaminophen has significant transformation efficiency in 1 h. The two quinoidal oxidation products, 1,4-benzoquinone and N-acetyl-p-benzoquinone imine (NAPQI), which are more toxic than acetaminophen, accounted for 25 and 1.5% of the initial acetaminophen concentration, respectively [16].

China is the second largest APAP manufacture country and many kinds of cold medicine used in China contain APAP, which turn APAP into an important environmental research object. In this work, kinetic study of photocatalytic degradation of APAP in TiO2 suspended solution at three different pH values was demonstrated, adsorption experiments were carried out in order to explaining the pH effect on the photodegradation rate. The mechanism of photocatalytic degradation of APAP was put forward on the basis of the nature and content of the final reaction products, which was verified by the quantum calculation using Gaussian 98 soft package [17].

Section snippets

Reagents

Acetaminophen (98%) was purchased from Alfa Aesar (Alfa Aesar, Britain) and was used without further purification. Methanol was spectroscopic grade (TEDIA, USA). The photocatalyst nano-TiO2 (20–40 nm, surface area 120 m2 g−1) was purchased from High Technology Nano Co. Ltd. (Nanjing, China). BSTFA + TMCS (99:1, Supelco, USA) were used as derivatization reagent. HClO4 and NaOH aqueous solutions were used to adjust pH. The reaction solutions were prepared in doubly distilled water.

Photocatalytic degradation experiment

A 250 W metal halide

Control experiment

Aqueous solutions of APAP were irradiated using a 250 W metal halide lamp. The effect of oxygen was carried out as well as TiO2, which was shown in Fig. 2. The concentration of APAP in the absence of TiO2 hardly had any change during the 80 min irradiation. This was because APAP has weak absorption in the range from 350 to 800 nm, which means only few amount of APAP could have direct photolysis when a metal halide lamp was used as the irradiation source. However, oxygen had showed its importance

Conclusion

In summary, we studied the photocatalytic degradation of APAP in TiO2 suspended solution. Under the experimental condition, TiO2 system demonstrated the capability of removing APAP. After 100 min irradiation, about 95% of APAP decomposed in the 1.0 g L−1 TiO2 aqueous solution with an initial concentration of 100 μmol L−1. Initial APAP concentration and TiO2 dosage had effects on degradation efficiency of APAP, which increased with increasing catalyzer concentration, and decreased with increasing

Acknowledgements

The authors thank Miss Y. Li for the help of vacuum freeze drying work and the anonymous reviewers are also gratefully acknowledged. This work was financed by the Natural Science Foundation of PR China (Nos. 40503016 and 20777057) and NSFC-RFBR cooperation project (No. 20611120016 from NSFC and Grant No. 05-03-39007 from RFBR). Thanks also go to the Water Environment Research & Data Sharing Platform in the Middle Reaches of the Yangtse River (Wuhan University) for an initative grant on the work

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