The Inhibition Effect of Tert-Butyl Alcohol on the TiO2 Nano Assays Photoelectrocatalytic Degradation of Different Organics and Its Mechanism
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The inhibition effect of tert-butyl alcohol (TBA), identified as the •OH radical inhibitor, on the TiO2 nano assays (TNA) photoelectrocatalytic oxidation of different organics such as glucose and phthalate was reported. The adsorption performance of these organics on the TNA photoelectrode was investigated by using the instantaneous photocurrent value, and the degradation property was examined by using the exhausted reaction. The results showed that glucose exhibited the poor adsorption and easy degradation performance, phthalate showed the strong adsorption and hard-degradation, but TBA showed the weak adsorption and was the most difficult to be degraded. The degradation of both glucose and phthalate could be inhibited evidently by TBA. But the effect on glucose was more obvious. The different inhibition effects of TBA on different organics could be attributed to the differences in the adsorption and the degradation property. For instance, phthalate of the strong adsorption property could avoid from the capture of •OH radicals by TBA in TNA photoelectrocatalytic process.
KeywordsTert-butyl alcohol Photoelectrocatalysis TiO2 nano assays Hydroxyl radical inhibitor Inhibition effect
Titanium dioxide (TiO2) has been demonstrated to be a promising and cost-effective alternative material in the photoelectrocatalytic (PEC) treatment of wastewater that containing refractory pollutants since it was observed [1, 2, 3, 4, 5, 6, 7, 8]. As a typical photoanode material, the TiO2 nano assays (TNA) process the advantages such as the uniform distribution, neat arrangement, large specific surface area, and strong adsorption ability. Therefore, the TNA electrode shows relatively more excellent PEC performance and conversion efficiency comparing with other nano-TiO2 film materials. For this reason, the TNA preparation and its application to pollutants degradation have drawn lots of concerns [9, 10, 11, 12, 13].
The PEC performance could be affected by kinds of factors. As known, the type of photocatalyst is a crucial factor in PEC degradation of organics, and lots of efforts have been made to improve the photocatalyst including the structure, the modification on the surface, and so on . Furthermore, the configuration of reactor is an important factor in the PEC performance, and many researches have been carried on to improve the efficiency of the reactor . In addition, the reactants composition of the reaction system could also affect the PEC performance. It has been found that some small molecule organics could increase the degradation rate of other organics. For example, methanoic acid could enhance the photocurrent values and the reaction activity in the PEC process . However, other chemical substances such as tert-butyl alcohol (TBA), phosphate, and carbonate could inhibit the hydroxyl radical activity of ozone oxidation process .
The effects of TBA on the ozone and Fenton oxidation processes for organics degradation have been studied in detail relatively [17, 18, 19]. Staehelln and Holgne found that TBA served as the •OH radical inhibitor and inhibited the transition path from O2 to peroxy radical in the O3 decompose process . It has been reported that TBA separated the direct molecular ozone reaction pathway in humic acid oxidation by O3 . Dao and Laat found that TBA seriously inhibited the reaction of the hydroxy radicals in the degradation processes of atrazine, fenuron, and parachlorobenzoic acid by FeIINTA/O2, FeIINTA/H2O2, and FeIIINTA/H2O2 Fenton reaction . However, the inhibition effect and mechanism of TBA in the PEC degradation process, especially on the surface of TNA, which is an important advanced oxidation method, has little or no description.
In the present work, the inhibition effects of TBA on different organics, including the weak adsorption of glucose and strong adsorption of phthalate, were studied, and the mechanism on the surface of the TNA in the PEC oxidation was reported. The adsorption and degradation performance of different organics on the surface of TNA were also investigated.
2.1 Material and Sample Preparation
Unless otherwise indicated, all the reagents were analytical reagent grade and purchased from Sinopharm Chemical Reagent Company (Shanghai, China). Potassium hydrogen phthalate was used as the representative of phthalate. All solutions were made up with high-purity deionized water (18 MΩ) purified from a Milli-Q purification system (Millipore Corporation, Billerica, MA), and a NaNO3 solution served as the supporting electrolyte in samples.
2.2 Preparation of the TNA Electrode
2.3 Reactor Used in the Experiment
2.4 Concentration Unit and Degradation Efficiency
The concentration unit of organics in oxygen equivalent (mg L−1) was used, and the computation method of the degradation efficiency was established based on the principle of PEC oxidation of organics in the thin-layer reactor.
This equation suggests the mineralization of 1 mol of glucose generates 24 mol electrons. Thus, the Faraday’s law can be written as Q th = 24FVC. Accordingly, the amount of glucose can be represented by the amount of the net charges. The conversion relationship 4e− + 4H+ + O2 → 2H2O indicates that 4 mol of electrons is equivalent to 1 mol of O2. Therefore, the quantity of net charges could be represented by the oxygen equivalent (mg L−1) which has been used as the measurement unit of the organics concentration for the purpose of comparison in this work.
Based on the measurement of the total number of photoelectrons generated from the photocatalytic oxidation of organics, the photoelectrochemical method can easily quantify the degree of the oxidation according to Faraday’s law, assuming that we can ignore the complicated interim reactions in the traditional photoelectrochemical oxidation method. The rate of electron capturing (i.e., the value of the photocurrent) can directly describe the photocatalytic degradation efficiency.
It has been reported that complete PEC degradation of the glucose in the thin-layer reactor could be achieved . Therefore, the quantity of transferred charges (Q net-glucose) in the glucose oxidation process could be recognized as Q th to measure the oxidation extent of organics in this work. Figure 3 shows the schematic diagram of photocurrent signals of the glucose oxidation. As can be seen, the degradation situation is reflected by the I–t curve which is smooth and not fluctuating.
3 Results and Discussion
The PEC degradation characteristics of organics on the surface of catalyst could be revealed easily by using a thin-layer reactor since both the mass transfer distance and the time it takes to travel from the solution to the surface of the electrode should be shorten. Thus, the oxidation of organic can be carried out quickly in the thin-layer reactor, which is conducive to examine the adsorption and degradation processes of organics on the surface of the catalyst . On the contrary, in a bulky reactor, the large reaction volume and the long reaction–diffusion pathway will greatly prolong the reaction time because the long diffusion distance from the solution to the surface of electrode resulting the long distance and traveling time of the organic molecules. Thus, a PEC thin-layer reactor has been chosen in this work to study the adsorption and degradation performance of organics.
3.1 The Adsorption and Degradation Characteristics of Different Organics on TNA
3.2 The Adsorption and Degradation Characteristics of TBA on TNA
TBA is a kind of tertiary alcohol, its hydrogen atom and oxygen atom in –OH are firmly bonded due to the high electron cloud density. Moreover, there is no hydrogen atom on the carbon atom which attached to the –OH, resulting the stable property of TBA that hardly to be oxidized or dehydrogenated. It has been reported that the reaction rate constant between TBA and •OH is 5 × 108 L (mol s)−1 and could generate the inert intermediate [27, 28].
As mentioned above, TBA is adsorbed onto the surface of the electrode before the PEC oxidation starts, and therefore, the instantaneous photocurrent values could reflect the adsorption and degradation behavior on the surface of the electrode.
The coefficients of different organics in pseudo Langmuir equation
8.344 × 10−6
2.028 × 10−4
3.541 × 10−5
2.147 × 10−4
6.102 × 10−5
1.507 × 10−4
However, the PEC degradation of TBA could not achieve the exhausted mineralization. For illustration, the PEC degradation I–t curves of TBA and glucose (which could be completely oxidized as mentioned above) at the same concentration of 100 mg L−1 are shown in Fig. 5c. It can be seen that the degradations of TBA and glucose take the similar time to achieve the stable state, but both the photocurrent values and the peak area of the I–t curve obtained from TBA are significantly smaller than that of glucose. It could be inferred that Q net of TBA in PEC degradation is much smaller than the corresponding theoretical value. According to Eq. (5), the degradation rate of TBA could be calculated as 28.45%, which suggesting the partly mineralization of TBA. The possible reason is that TBA scavenges the •OH radicals, and the formed inert compound ends the further oxidation.
The Q net obtained from the PEC degradation of glucose and TBA at a series concentrations are shown in Fig. 5d. It can be seen that all of the Q net obtained from the TBA degradation are much smaller than that of glucose, indicating the partly degradation of TBA at different concentrations in the range of 25–200 mg L−1.
3.3 The Inhibition Effect of TBA on the PEC Degradation of Different Organics
3.4 The Inhibition Mechanism of TBA on the Surface of TNA in the PEC Degradation
The adsorbed organics on the surface of the TNA electrode are oxidized by the photogenerated holes in advance as soon as the PEC degradation starts. Considering the adsorption ability varies with the species of organics, there must be competition between two different kinds of organics existing in the solution, resulting inhomogeneous distributions of adsorption on the surface of TNA electrode.
In comparison, the adsorption coefficient of phthalate is significantly higher than that of TBA. Thus, large amount of phthalate, which has the absolute advantage in the distribution, adsorbs on the surface of the electrode within the equilibration time. Therefore, the major oxidation object of photogenerated holes and •OH radicals is phthalate when the PEC degradation begins. As the PEC degradation proceeds, phthalate and TBA in the solution body transfer to the surface of the electrode along with the degradation of the adsorbed organics. In the transition process, the speed of phthalate is faster than TBA because of its strong adsorption property. So the transition and the degradation of TBA are gradually becoming the major reaction as the phthalate concentration decreasing. In other words, the degradation of TBA in the mixture of phthalate-TBA is continue to occur and gradually enhanced. According to the above mentioned, the inhibition effect of TBA is greater on glucose than phthalate which could be inferred from the degradation rate of the mixtures.
More TBA will distribute on the surface of the electrode with its increasing of concentrations, resulting the more notable inhibition effect on the PEC degradation of organics. This phenomenon could be certified by the degradation rates of both the glucose-TBA and phthalate-TBA mixtures shown in Fig. 8.
The inhibition effect of TBA on the PEC degradation of different organics on the surface of TNA and its mechanism were studied by using a thin-layer reactor. Glucose and phthalate were chosen as the object organic matters. The results showed that both glucose and phthalate, with concentrations ranging from 0 to 200 mg L−1, could be exhaustedly mineralized, but a shorter degradation time was taken by glucose at the same concentration. TBA, however, could hardly be completely degraded under the same condition. The adsorption properties of different organics were also studied by the instantaneous photocurrent values, and the adsorption coefficients of TBA, glucose, and phthalate were 3.730, 1.991, and 382.7, respectively. The degradation of both glucose and phthalate could be inhibited evidently by TBA, which was identified as the •OH radical inhibitor. The different inhibition effects of TBA on glucose and phthalate could be attributed to the differences in the adsorption property and the degradation mechanism on the TNA photoanode.
The authors would like to acknowledge the National High Technology Research and Development Program of China (Grant No. 2009AA063003), and the National Nature Science Foundation of China (No. 20677039) for financial support.
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