Pharmaceuticals and personal care products (PPCPs) have been considered as emerging pollutants over the last few decades, which have a low concentration in the environment and typically do not exhibit acute toxicity; however, they may impart potential damage to human and ecological health due to long-term exposure [6, 7]. Acetaminophen, also referred to as paracetamol, is one of the most frequently used over-the- counter analgesic pain relievers and antipyretic fever reducers worldwide [8]. It is generally safe for use at recommended doses, but may be toxic when overdosed [9, 10]. In recently years, acetaminophen has garnered much attention due to its propensity for inducing liver and kidney damage [14]. There have been many reports of acetaminophen being detected in rivers, ambient waterways, the influents of wastewater treatment plants, and the effluents of sewage treatment plants [55]. Valacyclovir is an oral antiviral drug, which is active against the herpes viruses. It is used to treat infections of shingles (herpes zoster), genital herpes (herpes simplex genitalis), and cold sores (herpes labialis) [31, 47]. Because acetaminophen and valacyclovir are of growing concern, a viable approach is urgently required for their effective removal from wastewater.
A number of biological, physical, and chemical methods have been introduced to eliminate organic pollutants from wastewater [56]. Among them, advanced oxidation processes (AOPs), particularly photochemically, electrochemically and photoelectrochemically related, are very promising [1]. A wide range of electrocatalysts and photocatalysts (e.g.,Pt,IrO2,SnO2,TiO2,WO3,C,PbO2, and ZnO) have been explored for the efficient removal of organic toxic compounds [2]. Undoubtedly,TiO2 is one of the most intensively researched semiconductors due to its unique combination of eminent characteristics, such as high efficiency, non-toxicity, biologi cal and chemical inertness, high stability, environmentally compatibility, and comparative cost effectiveness [3]. However,TiO2 does not have the capacity to utilize solar energy effectively, as it is limited by its large band gap (~3.0 eV for rutile and ~3.2 eV for anatase) [4]. In addition, due to the large band gap, unmodified TiO2 has low conductivity and is considered as an inefficient electrocatalyst. Therefore, it is desirable to reduce the band gap in TiO2 to expand its scope of applications in photochemistry and photoelectrochemistry. Doping with metal and non-metal elements, such as Au,Ag,Pt,Zn,Fe,Ni,Cu,Co,C,N,F, and S, has shown to enhance visible light absorption in TiO2 [5]. However, complex synthesis processes may limit its practical application and suitability.
Reduced TiO2 has gained increasing interest recently, and various strategies have been implemented for the reduction of TiO2 [6]. In this study, we have successfully synthesized and electrochemically treated nanoporous TiO2 electrodes for the effective photoelectrochemical degradation of acetaminophen and valacyclovir. Both the kinetics and effect of temperature on the photoelectrochemical oxidation of acetaminophen and valacyclovir have been systemically investigated.
Acetaminophen (99%) and HPLC-grade valacyclovir hydrochloride (≥ 98%) were purchased from Sigma-Aldrich and Sigma, respectively. Generic tablets of acetaminophen (500 mg) and valacyclovir (500 mg) were obtained from the Thunder Bay Regional Health Sciences Center Pharmacy. Titanium plates were purchased from Alfa Aesar. All other reagents were of analytical grade and were used without further purification. All solutions were prepared with pure water (18.2 MΩ cm), which was generated by a Nanopure Diamond water system. All acetaminophen and valacyclovir solutions were freshly prepared and used within 24 h.
Highly ordered nanoporous TiO2 was grown directly onto titanium plates utilizing anodic oxidation. Prior to modification, the titanium plates (1.25 cm × 0.8 cm × 0.5 mm) were sonicated in acetone for 15 min. After being rinsed with pure water, the Ti plates were etched in 18% HCl at 85 ℃ for 15 min; and subsequently immersed in a solution containing ethylene glycol + 0.3 wt% NH4F + 2 wt% H2O in two-electrode cell, to serve as the anode, with a Pt coil as the cathode. The Ti plate was initially anodized at 50 V for 5 h; the rough as-grown nanoporous TiO2 layer was thereafter peeled off through the application of masking tape. For the second-step, anodization was applied at 50 V for 2 h, whereafter the newly formed nanoporous layer on the Ti plate was again removed using the masking tape technique. A third-step included anodization at 50 V for 15 min, whereupon highly organized nanoporous TiO2 was eventually generated on the Ti plate. In order to obtain an anatase crystal structure, the Ti plate was annealed at 450 ℃for 3 h. An electrochemical reduction treatment was then conducted on the obtained nanoporous TiO2 in 0.1 mol/L H2SO4 at a cathodic current of -5 mA/cm2 for 10 min using a three-electrode system, where a Pt coil with a 10-cm2 surface area was utilized as the auxiliary electro de, an Ag/AgCl electrode was employed as the reference electrode, and the prepared nanoporous TiO2 served as the working electrode. The current density was calculated based on the geometric surface area.
The fabricated nanoporous TiO2 was characterized via field-emission scanning electron microscopy (FE-SEM,Hitachi SU 70). The electrochemical measurements were carried out using a Voltalab 40 Potentiostat (PGZ301). The experimental temperature was maintained using a water bath (20, 40, and 60 ℃) or a water/ice mixture (0 ℃). Unless specifically mentioned, all experiments were conducted at 20 ℃. Cyclic voltammograms (CVs), and Mott-Schottky plots were employed to characterize the electrochemical activity of nanoporous TiO2. CVs were recorded in a 0.1 mol/L Na2SO4 solution at a sweep rate of 20 mV/s; Mott-Schottky plots were measured at a fixed frequency of 500 Hz in a Na2SO4 solution (0.1 mol/L).
All experiments were carried out in a three-electrode cell system. The acetaminophen, valacyclovir and mixed acetaminophen and valacyclovir solutions were deaerated with ultrapure argon gas for 10 min prior to and during the experiments. The UV-visible light was supplied using an ADAC SystemsTM Cure SpotTM 50 UV spot lamp with three peaks of emission, at ca. 365, 405 and 435 nm, and two small peaks of emission, at ca. 315 and 330 nm. Light with an intensity of ca. 130 mW/cm2 was introduced into the cell through a fiber optic cable, which was affixed 2 cm above the electrode. For the photoelectrochemical degradation of acetaminophen and valacyclovir, the applied electrode potential was 1.0 V vs Ag/AgCl. Samples were extracted at regular intervals, and variations in the concentrations of acetaminophen or valacyclovir, and the TOC of the solution were analyzed. The UV-vis spectra of acetaminophen and valacyclovir were recorded in the range from 200 to 500 nm during the degradation process using a Cary 50 UV-vis spectrophotometer. The spectra of acetaminophen and valacyclovir displayed strong peaks at ca. 243 and 250 nm, respectively, which were employed for the determination of their concentrations during the degradation process. TOC was determined using a Shimadzu TOC-L combustion analyzer.
To survey the structure and morphology of the fabricated nanoporous TiO2 electrode,FE-SEM was employed, with the results shown in Fig. 1(a) and (b). Highly ordered nanoporous TiO2 was uniformly grown on the Ti substrate. The typical nanopore diameter was ~200 nm, as shown in the high resolution SEM image (Fig. 1(b)). There were instances where several nanopores joined together to form larger nanopores. Strong oxygen and titanium peaks with a 2:1 ratio observed in the EDX spectra of the nanopores (Fig. 1(c)) confirmed the composition of the formed nanoporous arrays.
To assess the electrochemical behavior of the electrodes, the second cycles of the CVs of the nanoporous TiO2 prior to and following electrochemical treatment are presented in Fig. 2(a), which were recorded in the potential range between 0.0 and 1.5 V in a Na2SO4 solution (0.1 mol/L) at 20 mV/s in the absence of UV-visible light irradiation. Two discriminative regions appeared in the CV curve of the untreated nanoporous TiO2: the initial hydrogen adsorption-desorption region between 0.0 and 0.5 V, and the double-layer charging region (0.5 to 1.5 V). As expected, the electrocatalytic properties of the nanoporous TiO2 were low, and the double-layer charging current was very small prior to electrochemical treatment. A significant increase in the current of the nanoporous TiO2 following the electrochemical treatment was observed in Fig. 2(a), particularly in the second region, which indicated that the double-layer capacitance of the nanoporous TiO2 was significantly augmented.
In order to assess the effects of the electrochemical treatment on the photoelectrochemical catalytic activity of the nanoporous TiO2 electrode, the CVs were recorded under UV-visible light irradiation prior to and following the electrochemical treatment and compared in Fig. 2(b). The photocurrent of the untreated nanoporous TiO2 gradually ascended when scanning the electrode potential from 0.0 to 0.6 V, which then attained a platform at ca. 5.3 mA/cm2. This was much higher than the photocurrent generated by TiO2 nanotubes synthesized using one-step anodization [7], which usually suffer from disparity in tube lengths and considerable tube array surface roughness, resulting in the low photoelectrochemical activity. In contrast, as seen in the SEM images (Fig. 1), the formed nanopores were uniformly interconnected, leading to more effective electron transfer. In addition, the photocurrent of the treated nanoporous TiO2 increased continuously across the entire electrode potential range tested, and arrived at 15.2 mA/cm2 at 1.5 V, which was ca. three times higher than that of the untreated nanoporous TiO2. These results indicated that a significant enhancement of the photoelectrochemical activity of nanoporous TiO2 could be achieved through the facile electrochemical treatment.
Mott-Schottky measurements conducted in 0.1 mol/L Na2SO4 were employed to further elucidate changes in the electronic properties of the nanoporous TiO2, prior to and following the electrochemical treatment. The Mott-Schottky plot can be used for estimating the donor density (ND):
where C is the capacitance of the space layer (F/m2),ε is the average value of the semiconductor dielectric constant,ε0 is the permittivity of the free space charge (8.854 × 10-12 F/m), and e is the absolute value of the electron charge (1.602 × 10-19 C). Fig. 2(c) displays the Mott-Schottky plots of the nanoporous TiO2 prior to and following the electrochemical treatment. A sigmoidal plot may be seen from Fig. 2(c) in the investigated electrode potential range, which is a typical characteristic of n-type semiconductors. A good linear relationship was observed between C-2 and the electrode potential in the range from -0.2 to -0.1 V vs Ag/AgCl for the nanoporous TiO2, prior to and following electrochemical treatment, when the curve was enlarged. The donor density of the electrochemically treated nanoporous TiO2 electrode was calculated to be 1.99 × 1020 cm-3 from the Mott-Schottky plot, which is 277 times larger than that of the untreated nanoporous TiO2 (7.18 × 1017 cm-3), showing that the electrochemical treatment significantly increased the conductivity of the nanoporous TiO2 electrode.
In order to evaluate the enhanced activity of electrochemically treated nanoporous TiO2 electrode, the degradation of acetaminophen and valacyclovir was tested under different applied conditions. Fig. 3(a) and (b) presents the UV-vis absorbance spectra of acetaminophen during the photoelectrochemical degradation of acetaminophen when the electrochemically treated or the untreated nanoporous TiO2 electrode served as the photoelectrocatalyst under an applied potential of 1.0 V, respectively. It may be seen that the absorbance of the strong peak, centered at 243 nm, rapidly decreased during the 3-h photoelectrochemical degradation process. Although a similar trend was observed with the untreated nanoporous TiO2 electrode (Fig. 3(b)), the decrease of the peak intensity was much slower. Fig. 3(c) presents kinetic plots of the photochemical, electrochemical, and photoelectrochemical degradation of acetaminophen at the nanoporous TiO2 prior to and following the electrochemical treatment. Almost no change in the concentration of acetaminophen was observed during the photochemical tests (under UV-visible light irradiation, but without an applied electrode potential), or during the electrochemical experiments (at an applied electrode potential of 1.0 V vs Ag/AgCl, but in the absence of UV-visible light irradiation), whether or not the treated or untreated nanoporous TiO2 was used. Interestingly, a linear relationship of the ln(C/C0) vs time with the R2 value close to 1.0 was observed under the UV-visible light irradiation at an applied electrode potential of 1.0 V as summarized in Table 1, showing that the photoelectrochemical degradation of acetaminophen at both the treated and untreated nanoporous TiO2 followed first-order kinetics. The rate constant was determined to be 0.0046 and 0.0086 min-1for the untreated and treated nanoporous TiO2, respectively, showing that the electrochemical treatment enhanced the photoelectrocatalytic activity of the nanoporous TiO2 by 86.96%.
Fig. 4(a) and (b) depicts the UV-vis absorbance spectra during the photoelectrochemical degradation of valacyclovir under the UV-visible light irradiation and an applied electrode potential of 1.0 V, at the electrochemically treated and untreated nanoporous TiO2 electrode, respectively. Fig. 4(c) presents the kinetic plots of the photochemical, electrochemical, and photoelectrochemical degradation of valacyclovir at the nanoporous TiO2 prior to and following the electrochemical treatment. Similar trends to the degradation of acetaminophen were observed. No obvious changes were observed when only the UV-vis light irradiation or the electrode potential of 1.0 V was applied. In contrast, the first-order kinetics for the photoelectrochemical degradation of valacyclovir at both the untreated and treated nanoporous TiO2 was seen. As listed in Table 1, the associated rate constants were calculated to be 0.0064 and 0.0096 min-1, respectively, revealing that a 53.12% enhancement was achieved by the electrochemical treatment of the nanoporous TiO2 for the photoelectrochemical degradation of valacyclovir. This significant promotion might be attributed to the partial reduction of Ti4+ to Ti3+, an increase in oxygen vacancies, as well as disorder on the TiO2 surface structure caused by the electrochemical treatment [8]. The energy level of these reduced states lies in between the valence and conduction band, which reduces the band gap of the TiO2 and increases the number of electron and hole pairs, thus enhancing the photoelectrochemical oxidation of acetaminophen and valacyclovir.
The TOC removal during the photoelectrochemical degradation of acetaminophen and valacyclovir using the electrochemically treated nanoporous TiO2 was further investigated. Fig. 5(a) and (b) displays the photoelectrochemical degradation and corresponding TOC removal of acetaminophen and valacyclovir, respectively. With increases in the removal of acetaminophen and valacyclovir, the related TOC removal was continuously increased, indicating that the photoelectrochemical degradation at the treated nanoporous TiO2 could effectively oxidize acetaminophen and valacyclovir to CO2. As can be seen from Fig. 5(a), subsequent to photoelectrochemical degradation for 180 min, 79.72% of the acetaminophen was removed, and 59.30% of the TOC was eliminated, revealing that 74.38% of the removed acetaminophen was completely oxidized to CO2, which is very promising compared with the results reported in the literature. For instance,Chang et al. [9] investigated the degradation of acetaminophen using a ZSM-5 supported TiO2 as the photocatalyst, and a 14 W UV lamp as the light source, showing that after 180 min of strong UV irradiation, only 58% of TOC was eliminated, although nearly all of the acetaminophen was removed. As seen in Fig. 5(b), 81.47% of the valacyclovir was removed, and 55.47% of the TOC was eliminated, showing that 68.09% of the removed valacyclovir was fully oxidized to CO2.
Fig. 6(a) and (b) exhibits the kinetic curves for the photoelectrochemical degradation of acetaminophen and valacyclovir, respectively, conducted under four different temperatures. These plots fitted well using the first-order kinetic equation. The rate constants of the photoelectrochemical degradation of acetaminophen and valacyclovir, as well as the fitting R2 values are listed in Table 1. The R2 value for all of the kinetic curves was close to 1, further demonstrating that the photoelectrochemical degradation of acetaminophen and valacyclovir at the electrochemically treated nanoporous TiO2 followed the first-order kinetics. With the increase of the experimental temperature, the rate constants of the photoelectrochemical degradation of acetaminophen and valacyclovir were increased, indicating that the applied temperature could significantly affect the removal of acetaminophen and valacyclovir. For acetaminophen, the rate constant was augmented 2.1 times (from 0.0065 to 0.0137 min-1), when the temperature was increased from 0 to 60 ℃; as for valacyclovir, the rate constant was increased 4.2 times, showing that the applied temperature had a stronger impact on the photoelectrochemical degradation of valacyclovir than that for acetaminophen.
To calculate the activation energy of the photoelectrochemical degradation of acetaminophen and valacyclovir,Fig. 7 depicts the logarithm of rate constant (lnk) as a function of inverse of the absolute experimental temperature (1/T). The good linear relationship confirmed that the photoelectrochemical degradation of acetaminophen and valacyclovir at the electrochemically treated nanoporous TiO2 followed Arrhenius' law. Therefore, the activation energies may be determined from the slopes of the plots. The activation energies of the photoelectrochemical degradation of acetaminophen and valacyclovir were calculated to be 9.62 and 18.42 kJ/mol, respectively, which further confirmed that temperature had a greater influence on the photoelectrochemical degradation of valacyclovir than that of acetaminophen.
We have developed an advanced photoelectrochemical approach for the effective degradation of acetaminophen and valacyclovir based on nanoporous TiO2, which was directly grown on titanium substrates using a three-step anodization method. Being electrochemically reduced, the efficiency of the photoelectrochemical degradation of acetaminophen and valacyclovir was significantly enhanced by 86.96% and 53.12%, respectively, when compared with the untreated nanoporous TiO2 electrode. TOC analysis has shown that the photoelectrochemical degradation at the treated nanoporous TiO2 could effectively oxidize acetaminophen and valacyclovir to CO2. The photoelectrochemical degradation of acetaminophen and valacyclovir followed the first-order kinetics and the rate constants were increased with the increase of the applied temperature. The activation energies for the photoelectrochemical degradation of acetaminophen and valacyclovir were further determined to be 9.62 and 18.42 kJ/mol, respectively.
2016,
Vol. 37 Issue (7): 1062-1069 
PDF (586 KB)
2016,
Vol. 37 Issue (7): 1062-1069 PDF (586 KB)