Degradation of Tannic Acid Using TiO 2 Nanotubes as Electrocatalyst

Structured TiO 2 nanotubes were grown on 2mm thick titanium sheet by anodization of titanium in ethylene glycol medium containing 0.025M NaF. The morphology of TiO 2 nanotubes (TNT) was characterized using field emission scanning electron microscope. The potential of TNT as anode and also as photocatalyst for the degradation of tannic acid was studied. The mineralization of tannic acid was measured in terms Total Organic Carbon (TOC). Only 50% of TOC could be removed by exposing the tannic acid solution to UV-radiation (photolysis), whereas it was improved to 70% by electrooxidation (EO) using TNT as anode. Maximum degradation of 83% was achieved when electrooxidation was conducted under the influence of UVradiation (photoelectrocatalytic process (PEC)). Among the electrolytes tried, Na 2 SO 4 was observed to be very effective for the degradation of tannic acid.The kinetics of tannic acid degradation by photoelectrocatalytic process was found to follow zero-order rate expression.


Introduction
Recent researchers have demonstrated the treatment of wastewater containing toxic and refractory organic pollutants by electrochemical and photocatalytic methods.The photocatalytic activity of TiO 2 nanoparticles for the degradation of organics was found to be encouraging [1,2].The TiO 2 nanoparticles were also tried in various applications like hydrogen sensors [3], solar cells [4], and biocompatible materials [5].Because of superior photocatalytic activity, the TiO 2 nanofilms were also attempted for the degradation of dyes, detergents, and organic acids [6].Since the properties of the materials vary with their microstructure, the nanotubes are expected to perform much better than the commercially available TiO 2 powders.Variety of techniques such as hydrothermal, template synthesis, magnetic sputtering, and sol-gel have been tried for the synthesis of nanoscaled TiO 2 wires, dots, particulates, and tubes [7].Preparation of titanium nanotubes (TNT) by chemical precursor route fails to control the thickness and shape of the nanotubes.On the other hand, precise control over the nanotube wall thickness could be achieved by template method.The potentiostatic anodization technique was found to yield highly ordered titanium nanotubes with uniform wall thickness.The shape, wall thickness, and length of the nanotubes are easily controlled by electrolyte and applied voltage.Albu et al. [8] modified the applied voltage to prepare new bamboo type titanium nanotubes and Ji et al. [9] used alternating voltage to produce double walled titanium nanotubes.Very recently, titanium nanotubes having structures like bamboo leaves, squares, triangles, and flowers were achieved by varying the parameters [10,11].The potentiostatic anodization of titanium plate is considered advantageous because TNT could be grown directly on the substrate.The TiO 2 nanoparticles prepared by other methods are just deposited on the substrate unlike titanium nanotubes which are directly bound to Ti substrate.The nanotubular microstructures are perpendicular to the electrically conductive Ti substrate, forming a Schottky-type contact [7] thereby providing two separate channels for efficient electron and positive hole transport from interface to electrode.This is in sharp contrast to electron hopping in TiO 2 nanoparticles, which were widely used as electron accepting species in dye sensitized solar cells [12].Moreover, the ordered tube like architecture can provide unidirectional electric channel for superior charge separation and charge transport.Further, the TiO 2 nanotubes prepared by anodization technique have better anatase to rutile ratio which enhances the electrocatalytic property.The oxidation of acetaldehyde [13], glucose, dopamine, ascorbic acid, and hydrazine [14] was studied using TNT electrode.It was reported that the photocatalytic activity and photodegradation can be enhanced by incorporating minor amount of active carbon in TNT electrodes [15].The degradation of humic acid was observed to be better by photoelectrocatalytic process compared to conventional photocatalytic method.The humic acid was selectively removed before chlorine generation by using TNT photoanode [16].
The photoelectrocatalytic degradation of methyl orange dye [17], Disperse Orange 1, Disperse Red 1, and Disperse Red 13 [18] using TNT electrode, was also investigated and found to be advantageous than simple photocatalytic method.In this paper, the degradation of tannic acid was studied by photolysis, electrooxidation, and photoelectrocatalytic process using TNT electrode.Tannic acid was considered because it is toxic to aquatic organisms such as algae, phytoplankton, fish, and invertebrates [19,20] and highly refractive to conventional degradation.Also tannic acid can form metal complexes which could alter the aquatic ecosystem [21].

Materials.
All the chemicals used in the present study were of analytical grade.Double distilled water free from organic carbon was employed for all the experiments.The pH of the aqueous solutions was adjusted using dilute H 2 SO 4 and NaOH solutions.

Preparation of TNT Electrode.
The TiO 2 -nanotube arrays were grown on Ti sheet by anodization method [22].Titanium sheet (2 mm thick) was cut into 2 × 2 cm pieces and mirror polished using abrasive sheets of different grain size and then ultrasonically degreased using deionized water and finally in 1 : 1 mixture of acetone and ethanol before drying in air stream.These sheets were anodized using 95% ethylene glycol (99% purity) and 0.025 M NaF electrolyte.The electrolysis experiments were conducted using a potentiostat/galvanostat system (Model-KM064, K-Pas Instronics Engineers, India).The electrodes were connected to the respective terminals of the potentiostat and the anodization was continued at a constant current for a period of six hours by keeping the voltage at 20 V.For better templating of nanotubes with Ti-substrate, the titanium sheet was etched in Kroll's reagent (HF: HNO 3 : H 2 O) for 30 seconds just before anodizing.Platinum foil was used as counter electrode in all the experiments.The distance between the two electrodes was maintained at 1.0 cm.The temperature of the electrolytic bath was maintained between 15 and 20 ∘ C and the solution was kept under agitation with the help of magnetic stirrer.After anodization, the samples were washed with water and ultrasonicated in acetone to remove surface debris (precipitate falling out of solution).The anodized amorphous TiO 2 nanotubes were annealed at 480 ∘ C in oxygen atmosphere for six hours with heating and cooling rate of 1 ∘ C min −1 to obtain crystallized samples.The morphology and array of nanotubes were studied using field emission scanning electron microscope.Top view and side view of Ti nanotubes are presented in Figure 1.The SEM images indicate that the average diameter and wall thickness of the nanotubes are around 80 nm and 15 nm, respectively.Similar results have been reported earlier with ethylene glycol as the medium [23].The height/length of each tube is observed to be more than 500 nm.

Experimental Setup.
The high pressure mercury vapor lamp (HML-LP 88) which emits a monochromatic light of 365 nm was used as radiation source.The sample holder made of silica glass of the photoreactor was slightly modified to have an inlet and outlet, so that the experimental solution can be recirculated.The light power inside the reactor was measured using the energy meter (OPHIR, NOVA make) and it was observed to be 130 microwatts.The solution was circulated through the cooling condenser to the reactor by peristaltic pump.Cold water was continuously passed through the inlet of the condenser jacket and let out through the outlet.The temperature of the tannin solution was maintained around 25 ∘ C. The tannin solution was kept in agitation by the bubbles generated by electrolysis and by its recirculation.In addition to this, magnetic stirrer was also used for thorough mixing of the contents.The surface area of TNT anode exposed to electrolysis was estimated to be 4.07 cm 2 .Platinum was used as cathode and the gap between anode and cathode was maintained at 0.5 cm.The schematic diagram of experimental setup is shown in Figure 2. Prior to each experiment, the working electrode was anodically polarized for 60 seconds in 0.01 M H 2 SO 4 electrolyte solution at constant current of 100 mA, while Pt electrode was soaked in concentrated HNO 3 for 10 minutes to remove any impurities deposited on the surface.

Analysis.
During the course of the experiment, samples (2.0 mL) were collected at regular intervals and analyzed by TOC analyzer (Shimadzu VCSN/CPN Model).The homogenized diluted sample was injected into the reaction chamber packed with catalyst.The carbon is oxidized to CO 2 in the reaction chamber and the CO 2 gas generated is quantitatively estimated by nondispersive infrared analyzer.The TOC was deduced from the measurements of total carbon and inorganic carbon.

Results and Discussion
3.1.Tannic Acid Degradation by Various Techniques.Degradation of tannic acid by electrooxidation (EO) using TNT electrode as anode, photolysis (DP), and photoelectrocatalytic process (PEC) that is electrooxidation under UV radiation (with  value of 365 nm) was followed over a period of 6 hours and the results are presented in Figure 3.The cell voltage (open circuit) was measured to be 9.0 V during electrolysis.The potential of the working electrode is 1.31 V versus SCE.From the results, it is apparent that only 50% of TOC could be removed when the tannic acid solution was exposed to UV radiation alone (photolysis).The removal of tannic acid was improved to 70% by electrooxidation (EO) process using TNT as anode.Electrooxidation was carried out in 0.1 M Na 2 SO 4 electrolyte solution at a current density of 36.85 mA/cm 2 .The degradation of tannic acid was improved further (83%) by photoelectrocatalytic process (PEC).It may be attributed to the enhanced generation of photon holes on TiO 2 nanotubes under the combined effect of UV radiation and electrolysis.
Better degradation in the presence of TNT electrode may also be attributed to the higher band gap energy and surface area.Because of high band gap energy, generation of powerful photocarriers is expected when exposed to UV light.Further, the specific surface area of TiO 2 nanotubes is several folds more than the normal electrode and hence adsorbs more organic matter for degradation.Considering the dia and length of each TiO 2 nanotube as 40 × 10 −7 cm and 50 × 10 −7 cm, the actual surface area of the TNT electrode was estimated to be 72.4 cm 2 .This is nearly 20 times more than the surface area calculated by taking in to account the length and breadth of the TNT electrode.The large effective surface area is in close proximity with the electrolyte solution, and thus the photogenerated electrons can transfer more effectively to the counter electrode via the external circuit [12].Thus the enhanced photoelectrochemical activity of the TNT electrode may be attributed due to the effective separation of electronhole pairs on the surface of the electrode.The special structure and light scattering characteristics may also contribute positively to the transportation of photogenerated holes.Also large number of OH groups available on the surface can prevent the agglomeration of nanotubes.Though better degradation of tannin by PEC process is expected, only marginal improvement was observed.It may be attributed to the formation of stable intermediate compounds that resist degradation even at higher concentration of oxidants.It is known that organic compounds having-COOH, -OH (like tannic acid) display high degree of stability because of resonance effect.Thus complete mineralization of compounds like tannic acid appears to be difficult.

Effect of Supporting Electrolyte.
Mineralization of tannic acid in the presence of NaCl, Na 2 SO 4 , and NaNO 3 electrolytes using TNT electrode was studied by photoelectrocatalytic process.The initial tannic acid concentration of 100 mg L −1 and applied current density of 36.86 mA cm −2 were maintained uniformly in all the experiments.From the experimental results presented in Figure 4, it is apparent that the mineralization of tannic acid is effective in the presence of NaCl and Na 2 SO 4 electrolytes.Better degradation in the presence of NaCl and Na 2 SO 4 is explained due to the oxidizing species such as Cl 2 , OCl − , SO 4 −• S 2 O 8 2− , and free radicals produced during electrolysis.It was reported [24,25] that the active chlorine on Ti/TiO 2 under UV irradiation can be generated as per ( 1)-( 5).The OH * radicals are also produced anodically due to the oxidation of adsorbed water on the electrode surface: The reaction products TiO 2 -e VB + and TiO 2 -OH * oxidize chloride ion according to (3) and ( 4).The adsorbed chloride ions on the semiconductor surface acting as anode could be oxidized by holes generated under UV irradiation.As a result, Cl * and HOCl * are formed: In the presence of Na 2 SO 4, oxidizing agents such as OH * , SO 4 − , and S 2 O 8 2− are generated according to the reactions: In addition to the above, oxy chloride species are generated by anodic discharge of chlorine and its reaction with water molecules: HOCl → The rate of TOC removal is better at the initial stages of oxidation that is up to 3 hours of electrolysis time.Beyond this point, the removal was found to be marginal.The slow rate of mineralization at the later stage may be ascribed to mass transfer limitations [26].The results suggest that the in situ generation of hydroxyl (OH * ), sulfate (SO 4 − * ) radicals, and S 2 O 8 2− species plays a vital role in the mineralization of the tannic acid by indirect oxidation.The degradation was observed to be poor in the presence of NaNO 3 because there is no generation of oxidant from the supporting electrolyte.Though the degradation rate of tannic acid in the presence of NaCl was observed to be better, the TiO 2 nanotube arrays were found to be affected (Figure 5) on a long run.Though it was demonstrated that the TiO 2 nanotube arrays possess self-cleaning property under UV light [3], the electrode surface was seen fully covered with debris.In the case of phenol degradation, the polymeric products formed during electrooxidation are found to readsorb on electrode surface thus reducing the efficiency of the electrode.The low efficiency due to blocking of electrode surface by pollutant molecules/reaction products and fouling of electrode material is some of the drawbacks of electrooxidation process.

Effect of Na 2 SO 4
Concentration on the Mineralization of Tannic Acid.The mineralization of tannic acid at different Na 2 SO 4 concentrations of 0.01, 0.05, and 0.1 M was recorded using photoelectrocatalytic process.Current density of 18.43 Ma cm −2 and tannic acid concentration of 100 mg L −1 were maintained uniformly during the experiment.A slight increase of 0.5 V beyond 3 hours of electrolysis may be attributed to the depletion in ionic concentration of SO 4 2− in the bulk of the electrolyte.From the results presented in Figure 6, it is apparent that the TOC removal increases by increasing the concentration of Na 2 SO 4 .Thus the mineralization of tannic acid in the presence of Na 2 SO 4 suggests the role of mediated oxidants in addition to direct electron transfer reaction at the surface of the anode [27].From the results shown in Figure 6(b), it is evident that the mineralization of tannic acid by photoelectrocatalytic process follows the zero-order kinetics during the initial stage.This implies that electrooxidation is current control process [28] where the pollutant molecules arrive at the anode faster than the electrochemical production of oxidant.This is particularly so at low current densities and high concentration of pollutant molecules.

Effect of Current Density on TOC Removal.
The effect of current density on TOC removal was studied at four different applied current densities, namely, 12.28, 24.56, 36.86, and 49.14 mA cm −2 by PEC process and the results obtained are plotted in Figure 7(a).From the results it is apparent that the applied current density has marked influence on TOC removal.The TOC removal was found to increase with current density.However, it should be noted that the TOC removal beyond 36.86mA cm −2 is very marginal.The TOC removal was observed to be better at initial stage of electrolysis at all current densities.The reduction in TOC was observed to be marginal beyond 4 hours of electrooxidation.The goodness of fit of linear regression ( 2 values) suggests that the process of electrooxidation can be described in terms of zero-order reaction and current controlled [29] during the initial stage.

Effect of pH on Mineralization.
Since the initial pH of the electrolyte is an important variable to achieve the total mineralization, the photoelectrocatalysis was carried out in  acidic, basic, and neutral pH.The effect of initial pH on the depletion of TOC by PEC process using TNT anode in the presence of Na 2 SO 4 is shown in Figure 8.The TOC removal was observed to be relatively better at pH 6.0.In general, the pH of the electrolyte mostly affects the indirect mediated oxidation.In alkaline condition, the evolution of oxygen is comparatively high and thus prevents the diffusion of pollutant molecule to the electrode surface [30].Further, inactive hydroperoxide (HO 2 − ) anion which acts as scavenger for OH * is formed in alkaline pH [31]: It was also observed that the change in pH affects the solubility of the oxygen which may adversely affect the generation of free radicals and mediated oxidants in the bulk electrolyte.At an applied current density of 36.86 mA cm −2 , energy required for tannic acid mineralization was calculated to be 2.14 Wh L −1 .

Conclusions
Mineralization of tannic acid by direct photolysis, electrooxidation, and photoelectrocatalytic process was studied using TNT electrode.The degradation of tannic acid is only 50% by direct photolysis process, whereas it was improved to 70% by electrooxidation process using TNT as anode.The mineralization of tannic acid was observed to be maximum (83%) by photoelectrocatalytic process.The nature of the electrolyte was found to influence the anodic oxidation of tannic acid.Among the three different electrolytes used during photoelectrocatalytic process, Na 2 SO 4 and NaCl were found to be effective than NaNO 3 .The electrode surface particularly the array of nanotubes was affected on a long run particularly in the presence of NaCl.An increase in the concentration of electrolyte and applied current density was found to increase the degradation of tannic acid.The mineralization is found to be effective under neutral pH.The photoelectrocatalytic process was found to obey zero-order kinetics at the initial stage.

Figure 1 :
Figure 1: (a) Top view and (b) cross-sectional FESEM images of the TiO 2 nanotube arrays.

Figure 5 :
Figure 5: SEM images of the TNT electrode after photoelectrocatalytic process in 0.1 M NaCl after 30 hours.

Figure 7 :
Figure 7: (a) Effect of current density on TOC removal and (b) kinetics of tannic acid mineralization (initial concentration of tannic acid  0 : 100 mg L −1 ).