Pharmaceuticals and personal care products (PPCPs) have been regarded as one of the most serious environmental problems in recent years, due to their potential negative impact on aquatic ecosystems and human health [1
]. PPCPs include a series of organic compounds, such as non-steroidal anti-inflammatory drugs, antibiotics, hormones, fragrances, and cosmetics. Diclofenac (DCF) is a typical PPCP with anti-inflammatory, analgesic, and antipyretic effects. DCF has been widely detected in the effluent of sewage treatment plants, rivers, lakes, and other environmental water bodies and has attracted attention around the world [3
]. Currently, DCF is widely used in the pharmaceutical industry, which consumes approximately 940 tons globally each year. DCF is difficult to degrade completely through conventional water treatment processes, such as the activated sludge or anaerobic fermentation process [5
]. Most of the removal rates of DCF in wastewater from sewage treatment plants were below 40% [7
], which led to the cumulative concentration of DCF in the effluent and receiving waters of some sewage plants reaching g/L. DCF could have a negative effect on the growth of terrestrial animals, and aquatic organisms, and bring potential harm to human health. Reportedly, an increase in the amount of DCF will change the activity of biological mitochondria and the DNA, especially for aquatic organisms [8
]. For example, 1 μg/L of DCF can cause the kidney failure of Indian vultures or change the gills of rainbow trout [9
], affect the early embryonic development of marine bivalve mussels [10
], and cause tissue damage in fish.
In the past few years, advanced oxidation processes (AOPs) have attracted broad attention because of their high efficiency in degrading various refractory organics. Ozonation, ultraviolet oxidation, and photocatalysis are the most researched AOPs [11
]. Among them, photocatalytic technology has a great potential in treating organic monomers due to its indicators, and environmental protection and sustainability advantages [15
]. Photocatalytic technology destroys organic pollutants in drinking water or sewage by oxidizing free radicals and reducing hydration electrons (eaq−
]. Titanium dioxide (TiO2
) is considered an excellent photocatalytic material because of its biological properties, chemical inertness, high efficiency, low cost, and non-toxicity [19
can promote the photocatalytic degradation of various organic micro-pollutants in the aqueous solution through oxidation processes (for example, the formation of hydroxyl radicals [OH] and superoxide radicals [O2−
]) at the appropriate wavelength (λ < 380 nm) [20
]. In addition, the two-phase TiO2
composed of anatase and rutile, which exhibits a higher photocatalytic activity than the single-phase TiO2
anatase. The enhanced photocatalytic activity of this two-phase TiO2
was ascribed to the interfacial charge transfer from the anatase conduction band to the rutile, which promotes the photo-induced charge separation [24
]. P25 is a highly dispersed gas phase nanometer TiO2
composed of anatase and rutile, which has been widely used in industry to degrade DCF. Achilleo et al. [26
] studied the factors affecting the decomposition of DCF in water by UV-A/TiO2
photocatalysis, using six different commercially available TiO2
samples, and found that the Degussa P25 is the most effective for degrading DCF.
Inorganic ions and dissolved organic matter (DOM) are significant factors affecting the photocatalytic degradation ability for various PPCPs [27
] depending on their properties, and concentration and the acidity of the solution. DOMs could scavenge OH to inhibit degradation, compete for adsorption on the surface of the catalyst [28
] and quench AOPs [29
]. However, few studies have systematically discussed the impact of actual sewage on the photocatalytic degradation of DCF. Many inorganic ions and DOM are present in actual sewage, affecting the photocatalytic degradation of organic pollutants. Thus, the effect of DOM on the degradation of DCF in actual sewage must be investigated.
In this study, the photocatalytic degradation of DCF by adding TiO2 in the secondary effluent from two sewage treatment plants was studied. The effects of the actual effluent sewage on the photocatalytic degradation of DCF were investigated through the analysis of the degradation efficiency of DCF, the influence of dissolved substance, and the change in biological toxicity. Furthermore, various indicators, including chemical oxygen demand (COD), total organic carbon (TOC), three-dimensional fluorescence, ultraviolet-visible light spectrum analysis, and molecular weight, were used to analyze the organic matter in the sewage during the photocatalysis process.
2. Material and Methods
2.1. Material and Reagents
TiO2 (Aeroxide® P25, Degussa-Evonik, Germany) was used as the photocatalyst without any pretreatment. DCF (C14H11Cl2NO2, CAS: 15307-86-5, analytical grade, purity ≥ 99.0%) was purchased from American Sigma-Aldrich Company. HPLC-grade water was obtained from Watsons Water. In the HPLC analyses, formic acid (HCOOH, p.a.) and acetonitrile (CH−3CN, HPLC-grade), which obtained from Sinopharm Chemical Reagent Co., Ltd. Shanghai, China, were used as components of the mobile phases. Sodium-type 732 cation exchange resin and chlorinated 717 anion exchange resin (Soleibao Technology Co., Ltd., Beijing, China, >95%) were used to exchange and remove cations and anions in actual sewage, respectively.
In this study, two different secondary effluents from two sewage treatment plants were selected to explore the changes in organic matters and their impact on the degradation of DCF in the photocatalytic process. The secondary effluents from sewage treatment plants A and B are referred to as SE-A and SE-B, respectively. The water quality indicators of SE-A and SE-B are respectively shown in Table 1
and Table 2
, and meet the level A standard of discharge standard for pollutants form municipal sewage treatment plants in China (GB 18918-2002). The comparison of various water quality indicators, shows that the pH, COD, total nitrogen (TN), total phosphorus (TP), and ammonia nitrogen (NH3−
N) values of SE-A are higher than those of SE-B. This may because the sewage quality of plant A is more complicated than that of plant B, since plant A contains a certain proportion of industrial water. The main anion ions in both effluents are Cl-
, and SO42-
, and the chromatography contents of these three anion ions are roughly similar.
2.2. Photocatalytic Degradation Experiment
In the photodegradation experiment, the initial concentration of DCF was set to 10 mg/L, and a BL-GHX-V multifunctional photochemical reactor (a xenon lamp with power of 1000 W), which has a similar solar radiation spectrum energy distribution, was used as the light source (Figure 1
). An air-cooling system and a circulating condensate water device were used to control the temperature of the reaction environment during the experiment (25 ± 0.5 °C). A dark reaction was carried out first for a period until the adsorption equilibrium was established to eliminate the influence of TiO2
adsorption on the degradation of DCF. During the degradation process, 1000-μL samples were taken at a fixed time.
During the entire experimental procedure, samples were taken at fixed times, filtered through a 0.22-μm filter (Durapore-PVDF-Millipore), and stored at room temperature. Subsequently, the suspensions were analyzed consecutively by HPLC to measure the DCF concentration. In addition, the suspensions were used to analyze other indicators, including the DOM, the biological toxicity, the absorbance of UV254 and UV365, the three-dimensional fluorescence, and the apparent molecular weight (AMW).
After each experiment, TiO2 was recovered from the solution and washed with ethanol and deionized water to remove impurities on the surface. The TiO2 was separated by a centrifuge and dried in an oven at 50 °C. The experiment was repeated five times to ensure the repeatability of TiO2.
Analysis was performed with three parallel samples throughout the procedure. The data are expressed as the average value.
2.3. Analytical Methods
The concentration of DCF was determined by HPLC (Shimadzu, Kyoto City, Japan) using a C18 RP trace Extrasil OD52-5 Micromet 25 × 0.46 Teknockloma column, and a Waters 996 photodiode array detector, and the Empower Pro software 2002 (Water Co). The DCF samples (20 μL) were injected into the C18 column. The determination conditions of HPLC were adopted at an absorbance of 276 nm, and the retention time for the DCF samples was 7.8 min. Furthermore, acetonitrile and 0.1% glacial acetic acid (75:25 v/v) were used as the constituents of the mobile phases at 0.8 mL/min. The DCF concentration was quantified by the external standard peak area method.
The conventional indicators in sewage, including COD, TN, TP, and NH3-N were measured by the dichromate, alkaline potassium persulfate digestion UV spectrophotometer, ammonium molybdate spectrophotometer, and Nessler’s reagent colorimeter methods, respectively.
Three-dimensional fluorescence was used to determine the changes in the organic matters of the secondary effluent during the photocatalytic process. The reaction was measured at a 1 cm four-way quartz fluorescence cuvette scanning speed of 12,000 nm/min, a date interval of 2.0 nm, an excitation light bandwidth of 3.0 nm and emission light bandwidth of 3.0 nm. The excitation wavelength (Ex) was 200–500 nm, and the emission wavelength (Em) was 300–600 nm.
The AMW of DOM in the sewage was measured at 260 nm using a Shimadzu high performance size exclu-sion chromatography (HPSEC) UV detector. The 100-μL solution was added into a Shodex kw 802.5 size exclusion chromatography column (effective separation range of 50–50,000 Da) and measured using a 0.1 mol/L phosphate solution as the mobile phase with a flow rate of 1 mL/min.
Microtox biological toxicity test technology (Modern Water M500, Modern Wate, UK) may quickly test the toxicity of the sample solution through Vibrio fischeri freeze, which is a kind of luminous bacteria. EC50 is the concentration of a half effective inhibitory, indicating the concentration of the sample solution when the light intensity of Vibrio fischeri is reduced by half. The smaller the EC50 is, the more toxic the solution is.
In this study, a photocatalysis experiment was conducted using a laboratory-scale simulated solar experimental device to investigate the degradation of DCF. This study focused on exploring the effects of actual secondary effluent on the photocatalytic degradation of DCF and the changes in DOM during the photocatalytic degradation process.
When SE-A and SE-B were used as the background water of the DCF solution, they had a significant inhibitory effect on the degradation of DCF, and the inhibitory effect of SE-A was stronger than that of SE-B due to its higher COD, TN, and TP contents. Among them, DOMs exerted the main inhibitory effect on the photocatalytic degradation of DCF in sewage, and the anions and cations had relatively small inhibitory effects on degradation. In addition, sewage as the DCF background water contributed a certain biological toxicity to the solution, but the complex inorganic and organic substances in the sewage had fewer toxic effects on the DCF mixed solution as the degradation reaction proceeded.
UV254 can be used as an indicator to predict the removal effect of COD for SE-B. The molecular weights of the DOMs in the two kinds of sewage were mainly small molecules. After the photocatalytic reaction, the organic matters in the sewage were greatly degraded. Among them, TiO2 first catalyzed the degradation of macromolecular organic matters and then degraded the small molecular organic matters.