Electrochemical Photocatalysis on Nanostructured TiO2 Meshes : Degradation of Emerging Organic Pollutants in Wastewater Effluents

An immobilized photoactive TiO2 coating grown directly on titanium meshes was successfully exploited for the electrochemical photocatalytic degradation of carbamazepine in a real secondary wastewater effluent. The catalyst was prepared by Plasma Electrolytic Oxidation (PEO) and during the photocatalytic water treatment an electrical polarization (bias) was applied to the catalyst. The new process was compared with a conventional one employing suspended TiO2 powder (Degussa P25). Results showed that carbamazepine degradation rate follows the order UV/supported TiO2+bias ≈ UV/TiO2 Degussa P25 > UV/supported TiO2 > UV. The investigation also included the identification of other micropollutants and the degradation products. This allowed the detection of 201 compounds present in the secondary wastewater effluent employed for the photocatalysis tests, 51 of them also successfully associated to compounds of emerging concern (CECs), and 194 transformation products. The degradation of detected compounds followed firstorder kinetics and the mean kinetic constant values of the 51 CECs resulted to be 0.048, 0.035 and 0.043 min-1 for the TiO2+Bias+UV, TiO2+UV and UV, respectively. As for TPs, results showed that the TiO2+Bias+UV treatment is much more efficient than both TiO2+UV and UV in minimizing the intensity of the organics in the real wastewater. Such a better performance was more pronounced at higher reaction time reaching 60 % reduction of mean peak area of TPs at 90 min of reaction. Among the detected TPs also compounds belonging to known carbamazepine TPs were found. This allowed to propose a degradation pathway of carbamazepine. Experimental set-up Titanium dioxide coatings were obtained by PEO of grade I titanium meshes (328 cm2) in 1.5 M H2SO4, at constant potential of 150 V, for 5 mins. Morphology, crystallography and photocarrunet Experimental set-up characterization SEM surface image showed that oxide layer is porous with an interconnected sponge like morphology. The cross-section SEM micrograph (inset) revealed that the coatings are increasingly compact in depth. According to the XRD patterns, the asprepared TiO2 films are crystalline in structure. The mass fraction of the anatase and rutile phases calculated following Spurr are 58 % and 42 %, respectively. Linear sweep voltammetry at dark and under irradiation (UVC-254nm) showed that as-grown TiO2 films was highly photoactive without any post-treatment. The photocurrent reached to a plateau at 4 V cell potential having a value of 480mA. Degredation of Carbamezapine/Actinometry test 1- Carbamezapine was completely removed within 45 min by both suspended Degussa P25 and electrochemical TiO2 photocatalysis. 2- The beneficial effect of applying a polarization potential (bias) to the supported TiO2 catalyst is clearly evident. 3- Degradation rate of UV/supported TiO2+bias ≈ UV/TiO2 Degussa P25. According to actinometric measurements, the mesh reduced by 50% the UV dosage on the waters. Proposed degredation pathway, detection of compounds, kinetic constants comparison and monitoring/controlling the process Proposed degradation pathway of carbamazepine (the above image) based on transformation reactions detected by linkage analysis. Based on positive match of both the isotopic cluster of molecular ion and the product ion spectrum with that of the available MS library, 201 compounds were identified among which 51 suspect CECs were found (below). Box-whisker plots of the peak area for the 194 TPs (a) and for all 395 compounds (b) including 194 TPs + 201 compounds) , and kinetic constants for the 51 suspect CECs identified by analytical screening procedure (c), present in real secondary effluent wastewater. In addition to avoiding to remove the suspended catalyst at the end of the process, another beneficial advantage of using TiO2 mesh is capability of monitoring the efficiency of the process by measurement of photocurrent. In spite of high durability, it is possible to re-anodize the TiO2 mesh to re-adjust the efficiency. Conclusion Electrochemical photocatalytic degradation of carbamazepine in a real secondary wastewater effluent was successfully exploited and the result was compared with conventional suspended TiO2 powders (Degussa P25). The catalyst was grown directly on titanium mesh using Plasma Electrolytic Oxidation (PEO) technique. SEM images and XRD patterns showed that the surface of the catalyst has a porous structure with sponge-like morphology, and it consisted in a mixture of the two allotropes anatase (58%) and rutile (42%). Results showed that carbamazepine degradation rate follows the order UV/supported TiO2+bias ≈ UV/TiO2 Degussa P25 > UV/supported TiO2 > UV. Actinometric measurements showed a reduction of 50% of the UV dosage when a supported TiO2 mesh was used, meaning that a higher efficiency could be arguably obtained by reducing this shielding effect. In addition to carbamazepine, the investigation also included the identification of other micropollutants and of the degradation products. This allowed the detection of 201 compounds present in the secondary wastewater effluent, 51 of them also successfully associated to compounds of emerging concern (CECs), and 194 transformation products.