A Review on the Design and Performance of Enzyme-Aided Catalysis of Carbon Dioxide in Membrane, Electrochemical Cell and Photocatalytic Reactors

Multi-enzyme cascade catalysis involved three types of dehydrogenase enzymes, namely, formate dehydrogenase (FDH), formaldehyde dehydrogenase (FaldDH), alcohol dehydrogenase (ADH), and an equimolar electron donor, nicotinamide adenine dinucleotide (NADH), assisting the reaction is an interesting pathway to reduce thermodynamically stable molecules of CO2 from the atmosphere. The biocatalytic sequence is interesting because it operates under mild reaction conditions (low temperature and pressure) and all the enzymes are highly selective, which allows the reaction to produce three basic chemicals (formic acid, formaldehyde, and methanol) in just one pot. There are various challenges, however, in applying the enzymatic conversion of CO2, namely, to obtain high productivity, increase reusability of the enzymes and cofactors, and to design a simple, facile, and efficient reactor setup that will sustain the multi-enzymatic cascade catalysis. This review reports on enzyme-aided reactor systems that support the reduction of CO2 to methanol. Such systems include enzyme membrane reactors, electrochemical cells, and photocatalytic reactor systems. Existing reactor setups are described, product yields and biocatalytic productivities are evaluated, and effective enzyme immobilization methods are discussed.

Influence of Operating Parameters on Photocatalytic Oxidation of 2,4-Dichlorofenol in Aqueous Solution by TiO2/Stainless Steel Photocatalytic Membrane

The influence of some operating parameters of an UV photocatalytic reactor with TiO2/stainless steel photocatalytic membrane on the photocatalytic oxidation of 2,4-dichlorophenol from aqueous solutions was studied in this paper. It was shown that the pH of the working solution substantially influences the photocatalytic degradation of the organic substrate, with the degradation efficiency increasing with decreasing the pH of the working solution by a maximum corresponding to pH 3. The rate constant of the photocatalytic oxidation process is about twice as high at pH 3 comparative with pH 7 for the same initial concentration of the organic substrate. The molar ratio of hydrogen peroxide/organic substrate also influences the photocatalytic oxidation process of the organic substrate. The results obtained in this paper highlight the fact that a stoichiometric molar ratio is favorable for the photocatalytic degradation of 2,4-dichlorophenol. It has also been shown that the initial concentration of the organic substrate influences the rate of photocatalytic degradation. It appears that the rate of photocatalytic degradation decreases with the increasing of initial concentration of 2,4-dichlorophenol.

Modified Composite Based on Magnetite and Polyvinyl Alcohol: Synthesis, Characterization, and Degradation Studies of the Methyl Orange Dye from Synthetic Wastewater

The goal of the present paper was to synthesize, characterize, and evaluate the performance of the modified composite based on magnetite (Fe3O4) and polyvinyl alcohol (PVA). The obtained composite was used to degrade Methyl Orange dye from synthetic wastewater by a laboratory photocatalytic reactor. Various parameters of the photodegradation process were tested: composite dosage, amount of hydrogen peroxide (H2O2), and pH. The composite was characterized by Fourier Transform Infrared (FTIR) Spectroscopy, X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). The degradation experiments indicated that the complete dye decolorization depended on the amount of H2O2. In addition, the H2O2 could accelerate Methyl Orange degradation to more highly oxidized intermediates in the presence of UV light (99.35%). The results suggested that the obtained modified composite could be used to treat wastewater containing various types of dyes.

Application of ZnO-Nd Nano-Photocatalyst for the Reactive Red 198 Dye Decolorization in the Falling-Film Photocatalytic Reactor

A large amount of Reactive red 198 (RR198) is released yearly into the environment. RR198 is toxic for human and aquatic creatures; therefore, it should be removed from wastewater before releasing into the environment. In this study, the nano ZnO-Nd -photo-catalyst for the first time was synthesized by the combustion method. First, the physical characteristics of the generated nano photocatalyst were evaluated using FESEM, XRD, Bandgap calculation, and FTIR analysis. Then, the ZnO-Nd nano-photocatalyst was suspended into the contaminated water with RR198 dye in a falling-film photocatalytic reactor. The effects of parameters such as the amount of H2O2, catalyst dose, pH, and initial concentration of dye were investigated during the experiments. Finally, the decolorization process with the falling-film photocatalytic reactor was optimized using response surface methodology (RSM). The physical characteristics showed that the average particle size of the synthesized ZnO-Nd was 40 nm. Doping ZnO with Nd reduced the photocatalyst energy bandgap by 14%. The results indicated that the optimum amount of catalyst dose and pH level was 0.1 g/L and 5, respectively. The simultaneous usage of H2O2 and ZnO-Nd with an H2O2/dye ratio of two increased dye removal performance by 90%. The results demonstrated that the developed equations can be applied to predict the performance of the falling-film photoreactor. This study showed that using the nano ZnO-Nd photocatalyst in a falling-film photocatalytic reactor under optimum operating conditions is an appropriate way to remove RR198 from water.