SYNTHESIS OF THIN-FILM ELECTRODES: APPLICATIONS IN WASTEWATER TREATMENT

Abstract

Tanneries generate a large amount of wastewater containing a variety of organic and inorganic compounds along with some heavy metals used during the tanning process. The organized tannery wastewater treatment (TWW) plant usually relies on biological treatment combined with pre/post-chemical treatment steps. Huge land and labor demand, the requirement for additional (tertiary) treatments, and the excessive use of chemical coagulants develop the need for utilizing advanced treatment approaches. Advanced oxidation processes like electrochemical treatment have received huge attention due to the low land and labor requirements, the possibility of treating any persistent pollutants, and compatibility with the existing wastewater treatment processes. The electrochemical treatment can be broadly classified into four categories. First is electrochemical oxidation, which allows the mineralization of pollutants in a broader domain. It can remove organics, ammoniated compounds, sulfides, and mercaptans, among other contaminants. It also helps coagulate the solids and is very efficient in water disinfection. The major obstacle is the utilization of costly electrodes and high operating costs. Electrocoagulation is another type of electrochemical treatment similar to chemical coagulation and works on the principle of coagulating or flocculating the organic contaminants with the help of Fe or Al. The major drawback of this process is that it produces a high amount of toxic solid sludge and is inefficient in water disinfection. The mineralization capability of this process is also poor. The electro-Fenton process works on the principle of in-site H2O2 generation at the cathode to carry out the Fenton reaction in the bulk solution. The Fenton reaction homogenously produces hydroxyl radicals known to have the highest oxidizing potential and are very efficient in mineralizing organic contaminants. The major drawback of this process is the use of costly electrodes and operation in an acidic pH environment. On the other end, the photo electro-Fenton process can provide better cost-effectiveness and activation of oxidants. However, poor penetration of photo-radiation in highly turbid or colored environments limits its use in real wastewater treatment applications. Based on these criteria, the electro-Fenton treatment is taken as a primary base for wastewater treatment. However, the synergy of electro-oxidation and electrocoagulation with the electro-Fenton treatment is also studied. The key challenge in this work is to select a suitable electrode material and synthesize a stable electrode with high electrocatalytic properties to obtain better efficiency. The second challenge is the selection of proper operating conditions. The electro-Fenton treatment is best suited at an acidic pH of around 3.0. Then, the selection of a cathode for sufficient H2O2 production is also necessary to carry out the mineralization. Carbonaceous materials like graphite felt, and gas diffusion electrodes have been found to be suitable for these operations. However, the hydrophilicity of the material needs to be improved to enhance the 2-electron oxygen reduction reaction at the cathode. The electrodes are prepared using various advanced synthesis approaches pointing to higher stability and economy. The effect of using solvothermal, Pechini, and an ionic liquid as precursor solvents and the impact of implementing titanium oxide nanotubes as an intermediate layer and lead oxide as a top layer is also evaluated. Further, the process is optimized in terms of pH, current density, pollutant concentration, and catalyst loading (in combined treatment). For assessing the performance of electro-Fenton treatment, the mineralization of pollutants, stability and repeatability of the process, and effectiveness of cathode material for H2O2 production are analyzed. The efficiency of the electrodes was first measured by using the model pollutants. Chlorophenols are generally detected in TWW; hence, different chlorophenols are selected as a model pollutant. Moreover, to understand the reliability of the proposed process, tetracycline, which is considered a persistent organic pharmaceutical pollutant and detected in the surface water, is also treated. The study compares the effect of using different solvents on the electrochemical properties of the reduced TiO2 nanotubes (TiO2-rNTs) layered Ti/TiO2-rNTs/SnO2-Sb/PbO2 anodes. The electrodes are prepared using three different solvent-based precursors: (i) isopropanol, (ii) ethylene glycol and citric acid (Pechini method), and (iii) 2-hydroxyethylammonium acetate (2HEAA) ionic liquid (IL) via the thermal decomposition route. The decomposition mechanism of precursor solutions was explored using the thermogravimetric (TGA) analysis. Further, the physicochemical properties of the electrodes are examined using Field emission Scanning Electron microscopy (FE-SEM), X-ray diffraction spectroscopy (XRD), and X-ray photoelectron emission spectroscopy (XPS). The results revealed that solvents with higher viscosity and slower decomposition rates support better film uniformity and higher stability of the electrode. The TiO2 -rNTs bottom layer and PbO2 top layer helped obtain higher film stability, increased working potential window (2.2 V vs. SHE) of the electrode, and the repeatability of the results. The performance of different electrodes based on the precursor solution is found as IL>> Pechini > Isopropanol. 4-chlorophenol (4-CP) is used as a model pollutant to test the performance of IL-Ti/TiO2-rNTs/SnO2-Sb/PbO2 anode in an anodic oxidation (AO) coupled electro-Fenton (EF) treatment. Further, the reliability of the electrode is evaluated by mineralizing other persistent organic pollutants (POPs) like tetracyclin, phenol, 2-chlorophenol (2-CP), and 2,4-dichlorophenol (2,4-DCP). Under the optimized conditions, the proposed system was able to mineralize the tetracyclin, phenol, 2-CP, 2,4-DCP, and 4-CP up to 78.91, 82.07, 74.96, 78.78, and 69.3%, respectively. Moreover, the degradation mechanism of chlorophenols is proposed. Further, TWW is selected as a real industrial pollutant to check the efficiency of the process in the real environment. The objective was to investigate the degradation and mineralization of organic pollutants using the electro-Fenton and combined approaches with prime emphasis on the performance of antimony-doped tin oxide and lead oxide electrodes. Further, the effectiveness of graphite felt as an effective cathode for 2-electron oxygen reduction reaction is evaluated. The influence of anode materials on the electrochemical treatment of TWW was evaluated using Pt, Ti/RuO2-IrO2 (DSA), Ti/SnO2-Sb, Ti/PbO2, and Ti/SnO2-Sb/PbO2 electrodes. The comparison of the degradation mechanism of these electrodes in the electro-Fenton (EF) treatment was evaluated. The Ti/SnO2-Sb/PbO2 anode was efficient, with high electrocatalytic activity, stability, and reproducibility of the degradation results. Further, the study was extended to define the ability of sequential EF and electrocoagulation (EC) processes to clean TWW. The EC treatment was conducted using Al electrodes, and the performance of the combined treatment was evaluated by the removal of chemical oxygen demand (COD), turbidity, total suspended solids (TSS), sulfide, and Cr removal. The role of chlorides and sulfate salts during both treatments was evaluated by monitoring the concentration changes of these anions during the whole treatment using ion chromatography (IC). A sequential 1.5h EF and 1h EC treatment were applied to achieve a satisfactory degradation of (81.2 ± 3.9)% COD, >98% Cr, >99% turbidity, TSS, and sulfide removal. Additionally, the combined treatment was found to be more efficient towards the COD removal, achieving about 22.5 % higher COD removal consuming almost the same amount of electrical energy.