Peroxymonosulfate activation with an α-MnO2/Mn2O3/Mn3O4 hybrid system: parametric optimization and oxidative degradation of organic dye.

The present study proposed the synthesis of low-toxicity and eco-friendly spherically shaped manganese oxides (α-MnO2, Mn2O3, and Mn3O4) by using the chemical precipitation method. The unique variable oxidation states and different structural diversity of manganese-based materials have a strong effect on fast electron transfer reactions. XRD, SEM, and BET analyses were used to confirm the structure morphology, higher surface area, and excellent porosity. The catalytic activity of as-prepared manganese oxides (MnOx) was investigated for the rhodamine B (RhB) organic pollutant with peroxymonosulfate (PMS) activation under the condition of control pH. In acidic conditions (pH = 3), complete RhB degradation and 90% total organic carbon (TOC) reduction were attained in 60 min. The effects of operating parameters such as solution pH, PMS loading, catalyst dosage, and dye concentration on RhB removal reduction were also tested. The different oxidation states of MnOx promote the oxidative-reductive reaction under acidic conditions and enhance the SO4•-/•OH radical formation during the treatment, whereas the higher surface area offers sufficient absorption sites for interaction of the catalyst with pollutants. A scavenger experiment was used to investigate the generation of more reactive species that participate in dye degradation. The effect of inorganic anions on divalent metal ions that genuinely occur in water bodies was also studied. Additionally, separation and mass analysis were used to investigate the RhB dye degradation mechanism at optimum conditions based on the intermediate's identification. Repeatability tests confirmed that MnOx showed superb catalytic performance on its removal trend.

Contribution of electrolyte in parametric optimization of perfluorooctanoic acid during electro-oxidation: Active chlorinated and sulfonated by-products formation and distribution.

The present study investigated the roles of peroxydisulfate (PDS) radicals and sulfate radicals (SO4•-) that formed from sulfate (SO42-) during electrochemical oxidation of perfluorooctanoic acid (PFOA). The effect of operating parameters such as different types of electrolytes (NaCl, NaClO4, and Na2SO4), initial pH, current density, dose of electrolyte, and initial concentration of PFOA using electrochemical oxidation for perfluorooctanoic acid (PFOA) decomposition study was investigated. A difference in the removal efficiency with different electrolytes (i.e., Cl-, ClO4-, and SO42-) illustrated an increasing effect of electrooxidation of PFOA in the order of ClO4- < Cl- < SO42-, which suggested that •OH induced oxidation and direct e- transfer reaction continued to play a crucial role in oxidation of PFOA. At the optimum treatment condition of j = 225.2 Am-2, Na2SO4 concentration = 1.5 gL-1, [PFOA]o = 50 mgL-1 and initial pH = 3.8 maximum PFOA removal of 92% and TOC removal of 80% was investigated at 240 min. The formation of three shorter-chain perfluorocarboxylates (i.e., perfluoroheptanoic acid (PFHpA), perfluorohexanoic acid (PFHxA), and perfluoropentanoic acid (PFPeA) and formate (HCOO-) ions were detected as by-products of PFOA electro-oxidation, showing that the C-C bond first broken in C7F15 and then mineralized into CO2, and fluoride ion (F-). The fluorine recovery as F- ions and the organic fluorine as the shorter-chain by-products were also obtained. The degradation kinetic has also been studied using the nth-order kinetic model.

Development of Bi2WO6 and Bi2O3 - ZnO heterostructure for enhanced photocatalytic mineralization of Bisphenol A.

Present study proposed the synthesis of mixed p-type and n-type nanocomposite heterostructures by co-precipitation method. The as-synthesized heterostructures were characterized through different characterization techniques. The as-synthesized Bi2WO6 and Bi2O3-ZnO heterostructures were tested as photocatalysts during the photodegradation of Bisphenol A (BPA). The Bi2O3-ZnO heterostructure nanocomposite was found to be a more effective photocatalyst than Bi2WO6. The effect of operating parameters including catalytic dose (0.02-0.15 gL-1), initial BPA concentration (5-20 mgL-1), temperature change (5-20 °C) and solution pH changes (4, 5, 7, and 8) were evaluated with Bi2O3-ZnO under UV-light irradiation by selecting a 300 W Xe lamp. More than 90% BPA was degraded with 0.15 gL-1 Bi2O3-ZnO, keeping 1.0 mM H2O2 concentration fixed in 250 mL of reaction suspension. The HPLC and GC-MS were used to detect the reaction intermediates and final products. A plausible degradation pathway was proposed on the basis of the identification of reaction intermediates. Repeatability test analysis confirmed that the as-synthesized catalyst showed superb catalytic performance on its removal trend. The kinetics of degradation of BPA were well fitted by the power laws model. With the order of reaction being 0.6, 0.9, 1.2, and 1.3 for different operating parameters, i.e., catalyst dose, initial pH, temperature, and initial BPA concentration.

Microbial peroxide producing cell mediated lignin valorization.

Lignin is one of the most abundant naturally occurring polymers and can produce value-added products such as vanillin and p-coumaric acid. In the current work, in-situ depolymerization of lignin for its valorization in a microbial peroxide-producing cell (MPPC) system was performed. It is an electrochemical cell that requires no external energy to produce H2O2 for the advanced oxidation process. Lignin in the MPPC system undergoes oxidative depolymerization to generate value-added products. The maximum open-circuit voltage (OCV) was 1.143 V, the current density and power densities were 14 mA/cm2 and 13 mW/cm2, respectively, along with the production of 26 mM of H2O2. The degradation of signature linkages such as ß-ß bond and ß-0-4 bond were analyzed and confirmed using FTIR spectroscopy. Vanillin, p-coumaric acid, ferulic acid, etc. were generated during depolymerization and were detected using LC-QTOF-MS.

Ultrasound-assisted electrochemical treatment of cosmetic industry wastewater: Mechanistic and detoxification analysis.

This study aims to investigate the mineralization of cosmetics producing industrial wastewater (CW) using sono-electrochemical (US-EC) treatments. The influence of operating parameters such as current density (j), electrolyte (Na2SO4) concentration (m), initial pH (pHo), and ultrasonic power was investigated using Ti/RuO2 dimensionally stable electrodes. The results demonstrated 80.9% chemical oxygen demand (COD) removal efficiency, 433.5 kWh (kg COD removed)-1 of specific energy consumption at the optimum conditions of P = 100 W, j = 213 A m-2, pHo= 7.6 (natural pH), and m = 1.5 g L-1. With the application of ultrasound, COD removal efficiency increases from 60.2% to 80.9%, with a synergistic effect of 1.1. Kinetics study analysis confirms that mineralization follows the nth order kinetics model. In the presence of ultrasound, the performance of electrochemical treatment gets enhanced due to higher electron transfer, the enhanced production of •OH radicals, and sulfate radicals (SO4•-). The pathway for the degradation of the compound was suggested by quadrupole time of flight mass spectroscopy (QToF-MS). The operating cost of the process was also evaluated to establish the applicability of the US-EC process at the industrial scale.

Binary electrochemical mineralization of heterocyclic nitrogenous compounds: parametric optimization using Taguchi method and mineralization mechanism.

The main objective of the present work was to understand the interactive behaviour of various operating parameters including concentration of pollutants during binary electrochemical mineralization of the two nitrogenous heterocyclic pollutants in the aqueous solution. Indole and pyrrole were selected as pollutants, whereas Pt/Ti was selected as anode and cathode. The effects of different operating parameters like current density, solution conductivity, initial concentration of the pollutants and time were studied. Taguchi method was used to optimize these operating parameters for obtaining the ultimate rate of degradation for the nitrogenous compounds. There were basically two responses, i.e. chemical oxygen demand (COD) degradation and specific energy consumption. These responses were maximized and minimized, respectively. At the optimum condition, removal efficiencies of pyrrole, indole and COD were found to be 46.1%, 62.4% and 61.4%, respectively. The optimum value of specific energy consumption was found to be 159.5 kWh per kg COD removed. Possible mineralization pathways are also proposed on the basis of the identified intermediates by gas chromatography coupled with mass spectroscopy. The operating cost was also calculated for the binary lab-scale treatment of the indole and pyrrole and compared with reported cost analysis for the electrochemical treatment.

Mechanistic insight into ultrasound-induced enhancement of electrochemical oxidation of ofloxacin: Multi-response optimization and cost analysis.

In this paper, a hybrid advanced oxidation process of sonoelectrochemical, in which ultrasound and electrochemical are applied simultaneously, has been used for the degradation of ofloxacin (bio-recalcitrant pharmaceutical pollutant). Response surface methodology based central composite design was applied to understand the parametric effects of ultrasonic power, current density, initial pH, and electrolyte dose. Enhanced ofloxacin degradation was obtained using sonoelectrochemical (≈95%) process in comparison to the electrochemical (≈60.6%) and sonolysis alone (≈7.2%) after 120 min treatment time. Multi-response optimization was used so as to maximize COD removal (70.12%) and minimize specific energy consumption (11.92 kWh (g COD removed)-1)at the optimized parametric condition of pH = 6.3 (natural pH), ultrasonic power = 54 W, current density = 213 A m-2, and Na2SO4 electrolyte dose = 2.0 g L-1. It was revealed that •OH radicals contribute major to the ofloxacin degradation reaction among the other oxidizing agents. Degradation of the ofloxacin followed pseudo-first-order kinetics with a higher reaction rate, which confirmed the synergistic effect of 34% between ultrasound and electrochemical approaches. The degradation pathway of ofloxacin removal was elucidated at optimum condition by the temporal evolution of the intermediate compounds and final products using gas chromatography coupled with mass spectroscopy (GC-MS), liquid chromatography-mass spectroscopy (LC-MS), high-resolution mass spectroscopy (HR-MS), and Fourier transform infrared spectroscopy (FTIR). Atomic force microscopy (AFM) and field emission scanning electron microscope (FE-SEM) coupled with energy dispersed X-ray (EDX) were used to determine the morphology of electrodes. Operational cost analysis was done based on the reactor employed in the present study.

Physical features of ultrasound assisted enzymatic degradation of recalcitrant organic pollutants.

This work has attempted to provide answer to the interaction of sonolysis and enzymatic treatment on degradation of recalcitrant dyes in a combined treatment. The model system comprises of two dyes, acid red and malachite green as model pollutants, along with horseradish peroxidase as a model enzyme and ultrasound of 20 kHz frequency. A dual approach of coupling experimental results with simulations of cavitation bubble dynamics has been adopted. Utilization of oxidation potential of horseradish peroxidase has been found to be a function of convection level in the medium. Cavitation phenomenon is found to have an adverse effect on enzyme action due to generation of high amplitude shock waves, which denature the enzyme. Degradation of dye at high static pressure increases due to absence of cavitation and high energy interaction (or collisions) between enzyme and dye molecules, which are beneficial towards enzymatic oxidation of the latter. High intensity convection generated by ultrasound also obviates need for an external shielding agent such as PEG that prevents attachment of the phenoxy radicals to enzyme that blocks the active sites of the enzyme.