Kian Mun Lee

Recent developments of zinc oxide based photocatalyst in water treatment technology: A review.

Today, a major issue about water pollution is the residual dyes from different sources (e.g., textile industries, paper and pulp industries, dye and dye intermediates industries, pharmaceutical industries, tannery and craft bleaching industries, etc.), and a wide variety of persistent organic pollutants have been introduced into our natural water resources or wastewater treatment systems. In fact, it is highly toxic and hazardous to the living organism; thus, the removal of these organic contaminants prior to discharge into the environment is essential. Varieties of techniques have been employed to degrade those organic contaminants and advanced heterogeneous photocatalysis involving zinc oxide (ZnO) photocatalyst appears to be one of the most promising technology. In recent years, ZnO photocatalyst have attracted much attention due to their extraordinary characteristics. The high efficiency of ZnO photocatalyst in heterogeneous photocatalysis reaction requires a suitable architecture that minimizes electron loss during excitation state and maximizes photon absorption. In order to further improve the immigration of photo-induced charge carriers during excitation state, considerable effort has to be exerted to further improve the heterogeneous photocatalysis under UV/visible/solar illumination. Lately, interesting and unique features of metal doping or binary oxide photocatalyst system have gained much attention and became favourite research matter among various groups of scientists. It was noted that the properties of this metal doping or binary oxide photocatalyst system primarily depend on the nature of the preparation method and the role of optimum dopants content incorporated into the ZnO photocatalyst. Therefore, this paper presents a critical review of recent achievements in the modification of ZnO photocatalyst for organic contaminants degradation.

Simple Response Surface Methodology: Investigation on Advance Photocatalytic Oxidation of 4-Chlorophenoxyacetic Acid Using UV-Active ZnO Photocatalyst

The performance of advance photocatalytic degradation of 4-chlorophenoxyacetic acid (4-CPA) strongly depends on photocatalyst dosage, initial concentration and initial pH. In the present study, a simple response surface methodology (RSM) was applied to investigate the interaction between these three independent factors. Thus, the photocatalytic degradation of 4-CPA in aqueous medium assisted by ultraviolet-active ZnO photocatalyst was systematically investigated. This study aims to determine the optimum processing parameters to maximize 4-CPA degradation. Based on the results obtained, it was found that a maximum of 91% of 4-CPA was successfully degraded under optimal conditions (0.02 g ZnO dosage, 20.00 mg/L of 4-CPA and pH 7.71). All the experimental data showed good agreement with the predicted results obtained from statistical analysis.

Advanced Chemical Reduction of Reduced Graphene Oxide and Its Photocatalytic Activity in Degrading Reactive Black 5

Textile industries consume large volumes of water for dye processing, leading to undesirable toxic dyes in water bodies. Dyestuffs are harmful to human health and aquatic life, and such illnesses as cholera, dysentery, hepatitis A, and hinder the photosynthetic activity of aquatic plants. To overcome this environmental problem, the advanced oxidation process is a promising technique to mineralize a wide range of dyes in water systems. In this work, reduced graphene oxide (rGO) was prepared via an advanced chemical reduction route, and its photocatalytic activity was tested by photodegrading Reactive Black 5 (RB5) dye in aqueous solution. rGO was synthesized by dispersing the graphite oxide into the water to form a graphene oxide (GO) solution followed by the addition of hydrazine. Graphite oxide was prepared using a modified Hummers’ method by using potassium permanganate and concentrated sulphuric acid. The resulted rGO nanoparticles were characterized using ultraviolet-visible spectrophotometry (UV-Vis), X-ray powder diffraction (XRD), Raman, and Scanning Electron Microscopy (SEM) to further investigate their chemical properties. A characteristic peak of rGO-48 h (275 cm−1) was observed in the UV spectrum. Further, the appearance of a broad peak (002), centred at 2θ = 24.1°, in XRD showing that graphene oxide was reduced to rGO. Based on our results, it was found that the resulted rGO-48 h nanoparticles achieved 49% photodecolorization of RB5 under UV irradiation at pH 3 in 60 min. This was attributed to the high and efficient electron transport behaviors of rGO between aromatic regions of rGO and RB5 molecules.

Mechanism and kinetics study for photocatalytic oxidation degradation: a case study for phenoxyacetic acid organic pollutant

Photocatalysis is a rapidly expanding technology for wastewater treatment, including a wide range of organic pollutants. Thus, understanding the kinetics and mechanism of the photocatalytic oxidation (PCO) for degradation of phenoxyacetic acid (PAA) is an indispensable component of risk assessment. In this study, we demonstrated that the central composite design (CCD) coupled with response surface methodology (RSM) was successfully employed to probe the kinetics and mechanism of PCO degradation for PAA using an efficient zinc oxide (ZnO) photocatalyst. In our current case study, four independent factors such as ZnO dosage, initial concentration of PAA, solution pH, and reaction time on the PCO degradation for PAA were examined in detail. Based on our results obtained from RSM analyses, an efficient pathway leading to the high degradation rate (>90%) was applying 0.4 g/L of ZnO dosage with 16mg/L of concentration of PAA at pH 6.73 for 40 minutes. The experimental results were fitted well with the derived response model with R2 = 0.9922. This study offers a cost-effective way for probing our global environmental water pollution issue.

Effect of reduced graphene oxide-hybridized ZnO thin films on the photoinactivation of Staphylococcus aureus and Salmonella enterica serovar Typhi

The immobilization of photocatalyst nanoparticles on a solid substrate is an important aspect for improved post-treatment separation and photocatalyst reactor design. In this study, we report the simple preparation of reduced graphene oxide (rGO)-hybridized zinc oxide (ZnO) thin films using a one-step electrochemical deposition, and investigated the effect of rGO-hybridization on the photoinactivation efficiency of ZnO thin films towards Staphylococcus aureus (S. aureus) and Salmonella enterica serovar Typhi (S. Typhi) as target bacterial pathogens. Field-emission scanning electron microscopy (FESEM) revealed the formation of geometric, hexagonal flakes of ZnO on the ITO glass substrate, as well as the incorporation of rGO with ZnO in the rGO/ZnO thin film. Raman spectroscopy indicated the successful incorporation of rGO with ZnO during the electrodeposition process. Photoluminescence (PL) spectroscopy indicates that rGO hybridization with ZnO increases the amount of oxygen vacancies, evidenced by the shift of visible PL peak at 650 to 500nm. The photoinactivation experiments showed that the thin films were able to reduce the bacterial cell density of Staph. aureus and S. Typhi from an initial concentration of approximately 10(8) to 10(3)CFU/mL within 15min. The rGO/ZnO thin film increased the photoinactivation rate for S. aureus (log[N/No]) from -5.1 (ZnO) to -5.9. In contrast, the application of rGO/ZnO thin film towards the photoinactivation of S. Typhi did not improve its photoinactivation rate, compared to the ZnO thin film. We may summarise that (1) rGO/ZnO was effective to accelerate the photoinactivation of S. aureus but showed no difference to improve the photoinactivation of S. Typhi, in comparison to the performance of ZnO thin films, and (2) the photoinactivation in the presence of ZnO and rGO/ZnO was by ROS damage to the extracellular wall.

Polymers as Water Disinfectants

Today, microbial infection appeared as one of the most critical environmental pollutions from our water stream. Indeed, the rising of public awareness for water pollution and water security has urged both researchers and industries to develop cost-effective antimicrobial polymer system. Although a range of polymers have antimicrobial properties, the most frequently studied polymer for water disinfection is chitosan. It offers several advantages, including biodegradable, non-toxic in nature, biocompatible and inexpensive, as compared to other low molecular weight antimicrobial polymers. In general, low molecular weight antimicrobial agents suffer several disadvantages, such as toxicity to the environment and short-term antimicrobial ability. Moreover, using chitosan biopolymer could enhance the efficacy of some existing antimicrobial agents and antifungal agents and minimize the environmental problems. In this chapter, the brief introduction of chitosan as well as modified chitosan on the development of water disinfection is extensively discussed. In particular, this chapter discusses the physicochemical properties of chitosan and different synthesis approaches for chitosan.

One-step solvothermal synthesis of rGO/TiO2 nanocomposite for efficient solar photocatalytic degradation of Methylene Blue dye

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Polymeric Nanocomposites for Visible-Light-Induced Photocatalysis

TiO2 photocatalysts have been applied in treating wide range of organic contaminants, ranging from dye effluents to persistent organic pollutants. Discharging of those contaminants has polluted our natural water resources and reduced the quality and quantity of the clean water for our daily usage. Although TiO2 photocatalysts show high removal efficiency towards most of the pollutants, the fast recombination rate and large bandgap impede its practical use under visible light irradiation. Considerable efforts have been employed to immobilize TiO2 onto different substrates, particularly on polymer owing to their highly abundance and low cost. This chapter highlighted the various types of polymer-supported TiO2 photocatalyst in degrading organic pollutants.

Stability of tungsten oxide nanotubes film for improving photocatalytic oxidation reaction

ABSTRACT Well-aligned anodic tungsten oxide (WO3) nanotubes with lengths approaching 600 nm was successfully synthesised via electrochemical anodisation of tungsten (W) film at 40 V in a bath with electrolyte (pH 3) consisted of 1 M of sodium sulphate (Na2SO4) and 0.7 wt-% ammonium fluoride (NH4F) for 15 min. It was found that the production of dense compact oxide layer on pure W film could be explained with high concentration of H+ ions accelerated the hydrolysis ability on the W surface to form thick WO3 layer under acidic condition (pH: 3). The photocatalytic activity performance was increased by ≈15% for the dense WO3 nanostructures film as compared to the thin and irregular WO3 nanostructures film because of the high active surface area to absorb more photons from solar irradiation for triggering the charge carriers separation and then improvement of internal and external diffusion of the reactants.

Fabrication of TiO2 Nanoparticles on Large-Area Graphene Oxide Sheets as Promising Photocatalytic Material

Nowadays, the discharge of synthetic dyes effluents into water system has created serious water contamination problems which could gravely impact the quality of human life. Photocatalytic oxidation (PCO) by TiO2 has shown superior performance on towards the remediation of various organic pollutants. In this study, two dimensional (2D) graphene oxide (GO) sheets supported titanium dioxide (TiO2) nanoparticles were successfully synthesized by a simple and efficient sol-gel method using titanium isopropoxide (TTIP) and GO as the precursor. The graphite powder was first oxidized into hydrophilic GO using modified Hummer’s method followed by the addition of titanium solution. The resulting nanocomposites were then applied in the PCO of methyl orange under UV irradiation. The TiO2 nanoparticles were spherical in shape, with a particle size of 20-30 nm were well-deposited on the GO sheets. The presence of anatase phase in GO/TiO2 was confirmed by the XRD studies. The nanocomposites exhibited enhanced photocatalytic activity than TiO2, probably due to the addition of graphene oxide which retard the electron-hole recombination in the hybrid material.