Studies on the treatment of biorecalcitrants present in pulp & paper mill effluents using photoelectrocatalysis

Abstract

Huge consumption of fresh water and generation of huge quantity of effluent having high toxic potential towards environment is one of the most important environmental concerns associated with pulp and paper (P & P) industry. Advanced oxidation processes (AOPs) and electrochemical advanced oxidation processes (EAOPs) such as photocatalysis (PC) and electrochemical oxidation (EC) has proven their potential against the treatment of highly toxic and recalcitrant pollutants present in wastewater. However, these techniques possess few drawbacks which results in the restriction of their application. Photoelectrocatalysis (PEC), a combination of photocatalysis and electrochemical oxidation has shown its viability by overcoming the shortcomings observed in these two treatment techniques. In the present study, efforts have been made to study the potential viability of PEC treatment process against the biorecalcitrant pollutants commonly found in P & P industrial effluent. For PEC treatment method, novel electrodes have also been synthesized at laboratory scale to overcome the issues associated with present electrodes. The synthesized electrodes have been evaluated for their possible application in the PEC treatment of highly stable contaminants. PEC treatment of pentachlorophenol (PCP) and 4-chloroguaiacol (4-CG) and P & P mill simulated effluent have been studied using prepared electrodes. TiO2 and GO/TiO2 nanotube array electrodes have been synthesized on a titania plate by a simple in-situ anodization method. The physicochemical characteristics of synthesized TiO2 and GO/TiO2 electrodes has been determined by Field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Raman Spectroscopy, Ultraviolet-vis diffuse reflectance spectroscopy (UV-vis DRS), Fourier Transform Infrared spectra (FTIR), Photoluminescence (PL) spectroscopy and X-ray photoelectron spectroscopy (XPS). Synthesized GO/TiO2 nanotube electrode has been adjudged for its efficacy in the PEC degradation of PCP aqueous solution. Box-Behnken design (BBD) has been used to optimize the effect of graphene oxide (GO) loading (0.005-0.25 g/L), pH (3-8), applied current (20-60 mA) and degradation time (10-120 min) on the decomposition of PCP and energy consumption. At optimum conditions, 91% degradation and 85% mineralization of PCP has been achieved after 90 min under UV-A illumination with 0.015 g/L of GO concentration, 20.68 mA applied current and at pH 3.14 by consuming 0.00068 kWh energy. Effect of reactive species scavengers on the degradation of PCP has also been studied and presence of hydroxyl radical has been determined. Based on the LC-MS analysis results possible intermediates has been identified and corresponding decomposition pathway of PCP has been proposed. The other model compound (4-CG) has been then targeted using novel Cesium (Cs) doped TiO2 nanotubes photoelectrode (Cs/TiO2NTs). The electrode has been synthesized by simple electrochemical anodization method and characterized by several physicochemical techniques. In particular, the PC, EC and PEC activity of newly synthesized Cs/TiO2NTs electrodes has been investigated using against the degradation of 4-CG. The effect of operating parameters like Cs concentration, electrolyte concentration, external current and pH on degradation efficacy has been examined. PEC oxidation using Cs/TiO2NTs lead to 92% degradation of 4-CG in 6 h of solar light irradiation under optimized conditions (2.5 mM Cs, 160 mg L-1 Na2SO4, 0.03 A current and pH 3). A comparative assessment between PEC, PC and EC process manifested that PEC process has been most efficient when compared with other two processes and Cs/TiO2NTs exhibited higher PEC activity than bare-TiO2 electrodes in terms of degradation and mineralization of organic pollutant. The generation of •OH radicals has been found to be highest in PEC when compared to EC and PC process. Possible intermediates/byproducts have been identified by GC-MS technique and a corresponding tentative degradation pathway has been proposed. Cytotoxicity study showed that PEC has potential to detoxify 4-CG. Hence, combination of TiO2 electrodes decorated with Cs metal can act as a highly efficient photoelectrode for the degradation of hazardous pollutants. An attempt has been made to synthesize Au/TiO2NTs electrode for the effect degradation of 4-CG. Anodization technique has been adopted for the preparation gold loaded TiO2 nanotubes electrode. The synthesized electrode has been characterized by FE-SEM and EDS, XRD, UV-vis DRS and Raman measurements. 4-CG degradation experiments have been performed and optimum process parameters have been determined (0.15 mM Au concentration, 0.08 g/L electrolyte concentration, 0.03 A external current and pH 3). The presence of generated •OH radicals has been detected and their concentration with respect to treatment time has been determined. The intermediates formed during the degradation process have also been identified. A novel Ag co-doped GO/TiO2 nanotube photoelectrode has been synthesized by anodic oxidation method to assess its PEC, PC and PEC activity against the degradation of PCP. The nanotube electrode has been characterized by XPS, XRD, along with different spectroscopic and microscopic techniques, enabling to ensure the synthesis of planned photoeletrode material. The effect of applied current, pH, Na2SO4 concentration and Ag loading on the removal of PCP has been studied which resulted in high degradation (87%) and mineralization (78%) efficacy after 25 min of short treatment time. Moreover, it exhibited enhanced activity when compared to TiO2 and GO/TiO2 nanotube electrodes. The contribution of different reactive species in PEC process has been studied by conducting experiments in the presence of scavengers and assessment of •OH in the degradation process has been performed. The photoelectrode exhibited good reusability properties. Degradation products have also been identified using LC-MS technique. This study demonstrates that the combination of semiconductor and graphene oxide decorated with Ag nanoparticles can act as a highly efficient photoelectrodes for removing biorecalcitrant compounds. Finally, the PEC degradation potential of all the synthesized electrodes have been assessed for the treatment of simulated P & P mill effluent. The simulated effluent has been prepared by diluting the real effluent of P & P mill collected after the bleaching section of P & P mill with tap water. During the experiments, the conductivity and pH of the effluent has not been altered. All the experiments have been performed under the naturally available solar light. The prepared effluent has been characterized before and after treatment by measuring its chemical oxygen demand (COD), total organic carbon (TOC), total solids (TS), chloride, sulphate, nitrate, nitrite and phosphate concentrations. All the electrodes exhibited satisfactory performance in treating the simulated effluent however, the co-doped Ag-GO/TiO2 nanotube electrode has been observed to be best among all the other synthesized electrodes. All the synthesized nanotubes electrodes exhibited significant degradation efficiency towards the biorecalcitrant compounds and simulated P & P mill effluents. Cs/TiO2 and Au TiO2 nanotube electrodes exhibited considerable degradation of 4-CG while, GO/TiO2 and Ag-GO/TiO2 nanotube electrodes showed the significant degradation of PCP. The role of reactive species in the degradation process has been studied and observed that each reactive species has significant effect in regulating the PEC degradation. The simulated effluent was subjected to PEC treatment using all the synthesized electrodes and analyzed by measuring its COD, TOC, chloride, sulphate, nitrate, nitrite, phosphate and total solids concentration.