Diclofenac (DCF) is one of the most frequently detected pharmaceuticals in various water samples. This paper studied the effects of aquatic environmental factors (pH, temperature and dissolved organic matter) on photodegradation of DCF under simulated sunlight. The results demonstrate that degradation pathways proceed via pseudo first-order kinetics in all cases and the photodegradation of DCF by simulated sunlight. Thermodynamic study indicated that the photodegradation course is spontaneous, exothermic and irreversible. The rate constant gradually increased when the pH increased from 3 to 5, then decreased when the pH increased from 5 to 8, and finally increased when the pH further increased from 8 to 12. Humic acid inhibited the photodegradation of DCF. Three kinds of main degradation products were observed by high performance liquid chromatography/mass spectrometry and the degradation pathways were suggested. A toxicity test using Photobacterium phosphoreum T3 Sp indicated the generation of some more toxic products than DCF.
Pharmaceutical and personal care products (PPCPs) have been found polluting a wide range of aquatic environments including groundwater, surface water and drinking water (Hofmann et al. 2007). Diclofenac (DCF) is a synthetic non-steroidal drug widely prescribed as an anti-inflammatory, mostly used as its sodium salt in medical care as an antiarthritic and analgesic, and found polluting a wide range of aquatic environments, including groundwater, surface water and drinking water (Lin et al. 2016). Accordingly, this emerging trend of environmental pollutants and their metabolites have the potential to have an adverse impact on aquatic environments (Gao et al. 2016). The behavior and fate of pharmaceuticals in aquatic environments remain poorly understood, and therefore studies of these phenomena would be valuable (Avetta et al. 2016; Jewell et al. 2016; Poirier-Larabie et al. 2016). However, several studies have demonstrated that the rate of degradation of DCF in communal sewage treatment plants is low. In the wastewater treatment plant, the level of DCF removal efficiency is still very uncertain, ranging from 21% to 40% (Zhang et al. 2008). DCF has been detected in maximum concentrations of 28.4 μg/L in surface water. It is also detected in groundwater in concentrations up to 0.59 μg/L. Nowadays, the harmful effects of DCF on different organisms in aquatic environments have been demonstrated (Czech & Oleszczuk 2016; De Oliveira et al. 2016; Lonappan et al. 2016). For example, DCF can cause renal failure in the Indian gyps vulture and alterations of the gills of rainbow trout, with effects observed with concentrations as low as 1 μg/L (Taggart et al. 2007; Wang et al. 2015a, 2015b) and it also can influence the biochemical functions of fish and lead to tissue damage (Mehinto et al. 2010). Recent studies have focused on examining the efficiency of various treatment processes on DCF removal. For example, Wang et al. (2015a) investigated DCF removal via potassium ferrate, Vogna et al. (2004) investigated DCF removal via UV-light irradiation in the presence of H2O2 and Wang et al. (2014) investigated oxidation of DCF in aqueous solution via aqueous chlorine dioxide. Hui et al. (Yu et al. 2013) investigated degradation of DCF by advanced oxidation and reduction processes. Ernest et al. (Marco-Urrea et al. 2010) investigated degradation of DCF by Trametes versicolor pellets. However, there have been few studies of the environmental behavior of DCF in natural water. For instance, Radke et al. (2010) analyzed the short-term dynamics of selected pharmaceuticals (bezafibrate, clofibric acid, DCF, naproxen) in the river downstream from a wastewater treatment plant. The factors affecting the environmental behavior of DCF and its photodegradation in the aquatic environment are clearly of interest, and the study of DCF degradation under simulated sunlight conditions would be of value.
Photodegradation is one of the principal abiotic degradation pathways of DCF in the aqueous environment. It occurs mainly at the water surface, and is affected by various environmental conditions. Therefore, aquatic environmental factors should be included when modeling the photodegradation of DCF under simulated sunlight irradiation. In natural waters, the main variables in the aquatic environment include pH, temperature and dissolved organic matter (DOM). DOM can influence photodegradation by acting as photosensitizers and/or HO· sinks (Koumaki et al. 2015; Poirier-Larabie et al. 2016). It has been proposed that humic acid (HA) in its photo-induced transient excited state (triplet state, 3HA*) reacts with pharmaceutical compounds by energy and/or electron transfer, and/or by hydrogen abstraction (Rigobello et al. 2013; Sadmani et al. 2014; Hu et al. 2016). However, in some cases, HA absorption spectra overlap with the absorption spectra of pharmaceuticals. Moreover, HA can scavenge reactive oxygen species (ROS), which can interfere with the direct photolysis of pharmaceuticals (Guerard et al. 2009).
Various aquatic environmental factors may affect the environmental fate and ecological risk of DCF. Therefore, the objectives of this study were to investigate both the reaction kinetics and the influences of temperature, pH and HA on DCF degradation under simulated sunlight irradiation. A further step was to identify the major transformation product, and the possible degradation pathway was proposed. To evaluate the phototoxicity risks, a toxicity assay by Photobacterium phosphoreum T3 Sp was conducted to monitor the toxicity evolvement of reaction solutions.
MATERIALS AND METHODS
DCF, 2-[(2,6-dichlorophenyl) amino] benzeneacetic acid, sodium salt (98% purity), was purchased from J&K Chemical Co. Ltd (Beijing, China). HPLC-grade methanol was obtained from Suqian Guoda Chemical Reagent Co. Ltd (Jiangsu, China). All of the chemicals used were of analytical grade without further purification. Ultra-pure water from a Milli-Q water process (Millipore, USA) was used for preparing all aqueous solutions.
Fluka HA was purchased from Saint-Quentin Fallavier Co. Ltd (France). The HA was used without any further purification. Stock solution of HA was prepared by weighing a given amount, dissolving it in 0.1 mL 0.1 mol/L NaOH, and diluting to a fixed volume using Milli-Q water; the concentration of HA stock solution was 2.5 g/L. The pH value of HA stock solution was approximately 7.6. This solution was stored at 4 °C in the dark.
A detailed description of the photodegradation processes has been reported elsewhere (Zhang et al. 2011), although the authors provided only a simple description of the experimental process.
The concentrations of DCF solutions were determined via reversed-phase high-performance liquid chromatography (HPLC), which consisted of a Waters 1525 Binary HPLC pump and Waters 2998 Photodiode Array detector (Waters, Massachusetts, USA). Analytical column temperatures were controlled with a Model 1500 Column Heater (Waters, and Product of Singapore). The analytical column was a 150 mm × 4.6 mm Waters C18 column (particle size 5 μm). A Waters guard column (C18, 4.6 × 20 mm, particle size 5 μm) was used to protect the analytical column. The injection volume was 20 μL. The mobile phase was a mixture of 75% HPLC-grade methanol and 25% Milli-Q water (containing 1% acetic acid) at a constant ﬂow rate of 1.0 mL/min, and the detection wavelength was set at 276 nm. The possible degradation products of DCF were analyzed by an HPLC-mass spectrometry (MS) system (Waters Corporation) equipped with a C18 column (100 mm × 2.1 mm, 5 μm) and triple quadrupole detector. The mobile phase was a mixture of 65% HPLC-grade acetonitrile and 35% Milli-Q water (containing 1% acetic acid) at a constant ﬂow rate of 0.3 mL/min. The injection volume was 3 μL. Single MS analysis was performed using an ion trap mass spectrometer equipped with an atmospheric pressure ionization interface and an electrospray ionization ion source. The flow rate of the high purity nitrogen (heater temperature, 350 °C) was maintained at 650 L·h−1.
RESULTS AND DISCUSSION
Effect of temperature on the photodegradation of DCF
Effect of initial DCF concentration on the photodegradation of DCF
Effect of pH value on the photodegradation of DCF
Effect of HA on the photodegradation of DCF
The overall effect of HA on the photodegradation of DCF depends on the balance between these mechanisms, and in this study HA was found to inhibit DCF degradation.
Photodegradation mechanisms and intermediates/products identification
Toxicity of diclofenac to Photobacterium phosphoreum T3 Sp
This paper studied in detail the effects of temperature, pH and HA on the photodegradation of DCF under simulated sunlight conditions. The following conclusions can be drawn:
(1) Degradation pathways proceeded via pseudo first-order kinetics in all cases.
(2) The photodegradation course is spontaneous, exothermic and irreversible. The rate constant gradually increased when the pH increased from 3 to 5 and decreased as the pH increased from 5 to 8, finally increasing when the pH further increased from 8 to 12.
(3) HA exerts inhibiting effects on the photodegradation of DCF.
(4) The transformation products of DCF were identified by HPLC/MS and the possible photoreaction pathways were proposed.
(5) A toxicity test using Photobacterium phosphoreum T3 Sp indicated the generation of some more toxic products than DCF.
This work was supported by the National Natural Science Foundation of China (No. 20677012) and the Scientific Research Key Project of Henan Provincial Education Department (No. 14A610014).