Investigation of the Synergistic Effect of Sonolysis and Photocatalysis of Titanium Dioxide for Organic Dye Degradation

Chitosan-sunflower meal biochar hydrogel incorporated with green synthesized NiO nanoparticles for enhanced catalytic reduction of anthropogenic water pollutants

Anthropogenic activities have been one of the crucial driving factors for water pollution globally, thereby warranting a sustainable strategy for its redressal. In this study, we have developed a hydrogel-biochar nanocomposite for catalytic reduction of water pollutants. To begin with, green synthesis of nickel oxide nanoparticles (NiO NPs) was accomplished from waste kinnow peel extract via the environmentally benign microwave method. The formation of NiO NPs was affirmed from different analytical techniques namely ultraviolet-visible (UV-Vis), Fourier transform infrared (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy-dispersive spectroscopy (EDS). The FESEM images revealed spherical nature of NiO NPs. The average particle size was found to be 15.61 nm from XRD data. A novel hydrogel-biochar nanocomposite comprising the green NiO NPs, sunflower meal biochar and chitosan was prepared (Cs-biochar@ NiO) and explored as a nanocatalyst towards catalytic reduction of pollutants such as 4-nitrophenol, potassium hexacyanoferrate (III) and organic dyes methyl orange (MO), Congo red (CR), methylene blue (MB) in the presence of a reducing agent, i.e. NaBH4. Under optimized conditions, the reduction reactions were completed by 120 s and 60 s for 4-NP and potassium hexacyanoferrate (III) respectively and the rate constants were estimated to be 0.044 s-1 and 0.110 s-1. The rate of reduction was found to be faster for the dyes and the respective rate constants were 0.213 s-1 for MO, 0.213 s-1 for CR and 0.135 s-1 for MB. The assessment of the nanocatalyst in the reduction of binary dye systems depicted its selectivity towards the anionic dyes CR and MO. The nanocatalyst displayed effective reduction of dyes in real-water samples collected from different sources. Taken altogether, this study validates the design of sustainable hydrogel-biochar nanocatalyst for the efficient reduction of hazardous anthropogenic water pollutants.

Simultaneous degradation of contaminants of emerging concern and disinfection by solar photoelectro-Fenton process at circumneutral pH in a solar electrochemical raceway pond reactor

Simultaneous contaminants of emerging concern (CECs) removal and wild microorganisms' inactivation was evaluated by applying solar photoelectro-Fenton (SPEF) process in actual secondary effluent collected from a real municipal wastewater treatment plant (MWWTP). 20 L of a mixture of four CECs was used as model pollutants (200 μg/L of acetaminophen, caffeine, sulfamethazine, and sulfamethoxazole each one). The SPEF process was carried out on fully sunny days, at circumneutral pH using the complex Fe3+-EDDS, in a solar electrochemical - raceway pond reactor (SEC-RPR). Initially, the optimal conditions for CECs degradation were determined using a response surface model based on current density, iron complex concentration and Fe3+-EDDS addition time (to allow previous accumulation of H2O2) as model inputs. A current density of 24.6 mA/cm2, a Fe3+-EDDS complex concentration of 0.089 mM and 3.8 min of previous H2O2 accumulation were the resulting optimum conditions that were afterwards applied for the simultaneous degradation of the CECs synthetic mixture and wild microorganisms inactivation in actual secondary effluent. About 85% CECs removal and complete E. coli inactivation were achieved in 30 min, approximately, while E. faecalis and total coliforms could be inactivated under detection limit in 60 min and 75 min, respectively.

Carbonaceous CoCr LDH nanocomposite as a light-responsive sonocatalyst for treatment of a plasticizer-containing water

The carbonous-based nanocomposites of CoCr layered double hydroxide (LDH) with graphene oxide (GO) and reduced graphene oxide (rGO) were prepared. The successful synthesis of the CoCr LDH in hydrotalcite crystalline structure was deduced from the pattern obtained from X-ray diffraction, and the chemical composition of its surface was checked by X-ray photoelectron spectroscopy. The prosperous decorating of LDH on the sheets of rGO and GO was authenticated by the energy dispersive X-ray spectroscopy analysis and micrographs of scanning electron and transmission electron microscopy. The photo-assisted sonocatalytic activity of the prepared nanocomposites was appraised for the decomposition of dimethyl phthalate (DMP) as a plasticizer. The highest decomposition efficiency of 100% was obtained in the existence of CoCr LDH/rGO nanocomposite (0.5 g/L) during 20 min of reaction time via photo-assisted sonocatalysis. The rGO improved the catalytic activity of the CoCr LDH by increasing the specific surface area from 1.2 m²/g to 4.5 m²/g and reducing the band gap from 1.7 eV to 1.3 eV. Moreover, the results of the colony-forming unit method endorsed antibacterial property improvement of the CoCr LDH via hybridizing with rGO. The results of this research provide an optimistic perspective for applying carbonous-based nanocomposites of CoCr LDH as a novel catalyst with antibacterial properties in photo-assisted sonocatalytic processes.

Sonophotocatalysis with Photoactive Nanomaterials for Wastewater Treatment and Bacteria Disinfection

Sonophotocatalysis is described as a combination of two individual processes of photocatalysis and sonocatalysis. It has proven to be highly promising in degrading dissolved contaminants in wastewaters as well as bacteria disinfection applications. It eliminates some of the main disadvantages observed in each individual technique such as high costs, sluggish activity, and prolonged reaction times. The review has accomplished a critical analysis of sonophotocatalytic reaction mechanisms and the effect of the nanostructured catalyst and process modification techniques on the sonophotocatalytic performance. The synergistic effect between the mentioned processes, reactor design, and the electrical energy consumption has been discussed due to their importance when implementing this novel technology in practical applications, such as real industrial or municipal wastewater treatment plants. The utilization of sonophotocatalysis in disinfection and inactivation of bacteria has also been reviewed. In addition, we further suggest improvements to promote this technology from the lab-scale to large-scale applications. We hope this up-to-date review will advance future research in this field and push this technology toward widespread adoption and commercialization.

An ultrasound/O3 and UV/O3 process for atrazine manufacturing wastewater treatment: a multiple scale experimental study

An ultraviolet (UV) and ultrasound (US) enhanced ozonation method were developed to investigate their efficiency on the removal of atrazine and chemical oxygen demand (COD) in authentic atrazine manufacturing wastewater. The bench-scale tests suggested a positive effect of UV and US on the degradation of atrazine within a limited energy range. The pilot-scale flow-through system was further tested by using response surface methodology. The results showed that O₃ and its interaction with UV promoted the degradation of both COD and atrazine while its interaction with US inhibited the removal of COD but promoted the removal of atrazine. The optimal removal rate of atrazine (96.9%) was achieved in the condition of 6.86 W/L UV, 1.96 g/L·h O₃ and 294 W/L US. Chloride ions hindered the atrazine degradation, but the generated free chlorine radicals were still able to react with atrazine. In terms of energy-effectiveness, the configuration of 14.7 W/L UV and 1.96 g/L·h O₃ is the best option, which have the electrical energy per order of 181.6 kWh/m³ for atrazine and 0.13 kWh/g COD. These method and findings could be helpful in the development of energy-efficient advanced oxidation processes in treating wastewater with high salinity and COD loadings.

Floating Carbon-Doped TiO2 Photocatalyst with Metallic Underlayers Investigation for Polluted Water Treatment under Visible-Light Irradiation

In the current study, we analysed the influence of metallic underlayers on carbon-doped TiO2 films for RhB decomposition and Salmonella typhimurium inactivation under visible-light irradiation. All the experiments were divided into two parts. First, layered M/C-doped-TiO2 film structures (M = Ni, Nb, Cu) were prepared by magnetron sputtering technique on borosilicate glass substrates in the two-step deposition process. The influence of metal underlayer on the formation of the carbon-doped TiO2 films was characterised by X-ray diffractometer, scanning electron microscope, and atomic force microscope. The comparison between the visible-light assisted photocatalytic activity of M/C-doped TiO2 structures was performed by the photocatalytic bleaching tests of Rhodamine B dye aqueous solution. The best photocatalytic performance was observed for Ni/C-doped-TiO2 film combination. During the second part of the study, the Ni/C-doped-TiO2 film combination was deposited on high-density polyethylene beads which were selected as a floating substrate. The morphology and surface chemical analyses of the floating photocatalyst were performed. The viability and membrane permeability of Salmonella typhimurium were tested in cycling experiments under UV-B and visible-light irradiation. Three consecutive photocatalytic treatments of fresh bacteria suspensions with the same set of floating photocatalyst showed promising results, as after the third 1 h-long treatment bacteria viability was still reduced by 90% and 50% for UV-B and visible-light irradiation, respectively. The membrane permeability and ethidium fluorescence results suggest that Ni underlayer might have direct and indirect effect on the bacteria inactivation process. Additionally, relatively low loss of the photocatalyst efficiency suggests that floating C-doped TiO2 photocatalyst with the Ni underlayer might be seen as the possible solution for the used photocatalyst recovery issue.

Oxidative decomposition and mineralization of caffeine by advanced oxidation processes: The effect of hybridization

The study consists of a detailed investigation of the degradability of the emerging water contaminant-caffeine by homogeneous and heterogeneous Advanced Oxidation Processes (AOP's), estimation of a synergy index for each hybrid operation thereof, and proposing the most plausible reaction mechanisms that are consistent with the experimental data. It also encompasses evaluation of the effect of the water matrix represented by carbonate species and humic acids, as strong scavengers of hydroxyl radicals. The results showed that single AOP's such as sonolysis (577 kHz) and photolysis with H2O2 provided complete caffeine elimination, but they were insufficient for the mineralization of the compound. Hybrid AOP's were considerably more effective, particularly when operated at a heterogeneous mode using commercial TiO2. The most effective hybrid process was UV-H2O2/TiO2, which provided more than 75% TOC decay at the minimum test doses of the reagent and catalyst. While the addition of ultrasound to the process significantly increased the rate of caffeine decomposition, it reduced the overall degradation of the compound to 64% in terms of TOC decay. The antagonistic effect was attributed to the formation of excess H2O2, and the presence of cavity clouds and/or high density layers that inhibited the transmission of UV light. The effect of natural water ingredients was found to reduce the reaction rates, signifying the major contribution of hydroxyl radicals to the destruction of caffeine. The proposed reaction mechanisms based on OH radical attack and the calculated energy barriers were in good agreement with the experimentally detected reaction byproducts.

Photocatalytic and Sonocatalytic Degradation of EDTA and Rhodamine B over Ti0 and Ti@TiO2 Nanoparticles

Herein, we report a comparative study of photocatalytic (Xe-lamp) and sonocatalytic (345 kHz power ultrasound) degradation of Ethylenediaminetetraacetic acid (EDTA) and Rhodamine B (RhB) in the presence of Ti0 and Ti@TiO2 core-shell nanoparticles (NPs). Ti@TiO2 NPs have been obtained by sonohydrothermal treatment (20 kHz, 200 °C) of commercially available Ti0 NPs in pure water. The obtained material is composed of quasi-spherical Ti0 particles (30–150 nm) coated by 5–15 nm crystals of anatase. In contrast to pristine TiO2, the Ti@TiO2 NPs exhibit the extend photo response from UV to NIR light region due to the light absorption by nonplasmonic Ti core. EDTA can be oxidized effectively by photocatalysis in the presence of Ti@TiO2 NPs. By contrast, air passivated Ti0 nanoparticles was found to be inactive in the photocatalytic process for both EDTA and RhB. Photocatalytic degradation of EDTA over Ti@TiO2 NPs exhibits strong photothermal effect, which has been attributed to the higher yield of oxidizing radicals produced by light at higher bulk temperature. The efficiency of RhB photocatalytic degradation depends strongly on RhB concentration. At [RhB] ≥ 1 × 10−3 M, its photocatalytic degradation is not feasible due to a strong self-absorption. At lower concentrations, RhB photocatalytic degradation is observed, but at lower efficiency compared to EDTA. We found that the efficient sonochemical degradation of RhB does not require the presence of any catalysts. For both processes, EDTA and RhB, sonochemical and photocatalytic processes are more effective in the presence of Ar/O2 gas mixture compared to pure Ar. The obtained results suggest that the choice of the optimal technology for organic pollutants degradation can be determined by their optical and complexing properties.

Editorial Catalysts: Special Issue on Recent Advances in TiO2 Photocatalysts

The development of civilization and the massive use of traditional energy sources has led to progressive environmental degradation that requires immediate action [...]

Construction of S-doped MgO coupled with g-C3N4 nanocomposites with enhanced photocatalytic activity under visible light irradiation

In the present work, a series of photocatalysts in which S-MgO (SM) was coupled with g-C3N4 (CN) were synthesized and characterized in detail using various characterization techniques.

Modeling and Optimization of Pollutants Removal during Simultaneous Adsorption onto Activated Carbon with Advanced Oxidation in Aqueous Environment

The paper presents the results of studies on the modeling and optimization of organic pollutant removal from an aqueous solution in the course of simultaneous adsorption onto activated carbons with varied physical characteristics and oxidation using H₂O₂. The methodology for determining the models used for predicting the sorption and catalytic parameters in the process was presented. The analysis of the influence of the sorption and catalytic parameters of activated carbons as well as the oxidizer dose on the removal dynamics of organic dyes-phenol red and crystal violet-was carried out based on the designated empirical models. The obtained results confirm the influence of specific surface area (S) of the activated carbon and oxidizer dose on the values of the reaction rate constants related to the removal of pollutants from the solution in a simultaneous process. It was observed that the lower the specific surface area of carbon (S), the greater the influence of the oxidizer on the removal of pollutants from the solution. The proposed model, used for optimization of parameters in a simultaneous process, enables to analyze the effect of selected sorbents as well as the type and dose of the applied oxidizer on the pollutant removal efficiency. The practical application of models will enable to optimize the selection of a sorbent and oxidizer used simultaneously for a given group of pollutants and thus reduce the process costs.

A Comparative Study on Oxidation of Acidic Red 18 by Persulfate with Ferrous and Ferric Ions

Ferrous and ferric salts were tested for the persulfate activation (PS/Fe2+ and PS/Fe3+) and the oxidation of Acid Red 18 (AR18). A complete removal was attained after 90 min in both PS/Fe2+ and PS/Fe3+ processes with the persulfate concentration of 6 mM. High concentrations of PS, Fe2+, and Fe3+ promoted the AR18 degradation in both processes and the optimized pH were 3 and 3.3 for PS/Fe2+ and PS/Fe3+ processes, respectively. The mechanism of PS activation by Fe3+ was also investigated. It was found that hydroxyl radical (HO•) and sulfate radical (SO4−•) were formed and acted as dominating radicals in both processes. It is also deduced that Fe recycle offers Fe2+ for PS activation in PS/Fe3+ process to produce HO• and SO4−•. The less radical side reactions lead to a higher contribution of HO• and SO4−• on AR18 degradation in PS/Fe3+ process.