Photocatalytic Wastewater Treatment Research Articles

An Enthusiastic Glance in to the Visible Responsive Photocatalysts for Energy Production and Pollutant Removal, with Special Emphasis on Titania

As a consequence of the rapid growth of industry, major problems are created related to energy and environment. Sunlight being one of the most potential alternative source of energy, the development of efficient solar-energy storage systems is an important subject in the fields of science and technology. Here we have reviewed and summarized some of the recent reports on visible responsive photocatalysts. In this review, the influence of various metal oxide photocatalysts on energy production and pollutant removal are presented with special emphasis on titania based photocatalysts. The photoactivity of titania for various pollutant degradation, modified titania (TiO2) systems, their physical and chemical characteristics, and so forth, are described in detail at this juncture. Different methods used to enhance the visible light absorption of TiO2, like doping with metals and nonmetals, coupling with other metal oxides, and so forth, have been discussed. Various applications of photocatalysts including photocatalytic treatment of waste water, pesticide degradation and water splitting to produce hydrogen are summarized. The development of photocatalysts that function under visible light for the efficient utilization of sunlight is an area of current interest and thus the different methods of preparation for the visible active photocatalysts are also explored.

Synthetic trends for BiVO4 photocatalysts: Molybdenum substitution vs. TiO2 and SnO2 heterojunctions

The influence of hydrothermal synthesis, thermal post-treatment at 500°C and Mo content on a series of Bi1−x/3V1−xMoxO4 (0.02<x<0.15) photocatalysts was investigated for methylene blue (MB) degradation and O2 evolution. Mo incorporation stabilizes the tetragonal scheelite phase of BiVO4, thereby inducing a small blue shift of the band gap. This adverse effect can be partially compensated by incorporation of lower Mo amounts (up to ca. 4%), which stabilizes the particle morphology upon calcination. Both BiVO4@TiO2 and BiVO4:Mo@TiO2 heterojunction composites with TiO2 particle coatings in the 10nm range were newly synthesized and displayed promising photocatalytic performance in MB degradation. Comparative studies with SnO2 heterojunctions revealed a superior influence of TiO2 deposition over SnO2 coating on both BiVO4 and Bi1−x/3V1−xMoxO4 oxide substrates. Different synthetic guidelines for BiVO4-based solid solutions and heterojunctions in photocatalytic wastewater treatment and water oxidation are discussed.

Preparation of poly(vinyl alcohol)/poly(acrylic acid)/TiO 2/carbon nanotube composite nanofibers and their photobleaching properties

The composite nanofibers of poly(vinyl alcohol) (PVA)/poly(acrylic acid) (PAAc)/titanium(IV) oxide (TiO 2) were prepared by electrospinning for a novel photocatalytic treatment of waste water. To improve the photoelectronic properties of PVA/PAAc/TiO 2 composite nanofibers, carbon nanotubes (CNTs) were used as an additive. The TiO 2 and CNTs were immobilized in the PVA/PAAc hydrogels as electrospun nanofibers for an easier recovery after the wastewater treatment. The improved efficiency of pollutant dye removal was observed at pH 10 due to the pH-sensitive swelling behavior of the PVA/PAAc/TiO 2/CNTs composite nanofibers. The photocatalytic activity of TiO 2 was improved noticeably by applying electric field to the CNTs-embedded composite nanofibers.

Performance of TiO 2 photocatalyst supported on silica beads for purification of wastewater after absorption of reflow exhaust gas

The photocatalytic treatment of wastewater containing organic compounds in the exhaust gas from a reflow furnace is studied in a laboratory-scale batch recirculation type reactor system with a packed column of TiO 2/SiO 2 beads (TiO 2 supported on SiO 2 beads). A comparison is made between the photocatalytic activities of the TiO 2/SiO 2 beads under irradiations with UV light emitted from 6 W blacklight blue fluorescent lamps (BL) and germicidal lamps (GL). When the wastewater is treated over 24 h, the conversions on the TOC concentration base are 73 and 90% with UV irradiations from BL and GL, respectively, indicating that GL is more useful for the present photocatalytic treatment. Quantitative and qualitative analyses of the treated wastewater by GC/MS reveal that the photocatalytic reactor system can satisfactorily decompose not only aromatic but also aliphatic compounds into harmless substances (CO 2, H 2O and other mineral ions). Although several intermediates are present in the treated wastewater, their concentrations are very low. The TiO 2/SiO 2 photocatalyst can retain more than 80% of the original activity even after repeated use of three times.

A review of TiO2 nanoparticles

Climate change and the consumption of non-renewable resources are considered as the greatest problems facing humankind. Because of this, photocatalysis research has been rapidly expanding. TiO2 nanoparticles have been extensively investigated for photocatalytic applications including the decomposition of organic compounds and production of H2 as a fuel using solar energy. This article reviews the structure and electronic properties of TiO2, compares TiO2 with other common semiconductors used for photocatalytic applications and clarifies the advantages of using TiO2 nanoparticles. TiO2 is considered close to an ideal semiconductor for photocatalysis but possesses certain limitations such as poor absorption of visible radiation and rapid recombination of photogenerated electron/hole pairs. In this review article, various methods used to enhance the photocatalytic characteristics of TiO2 including dye sensitization, doping, coupling and capping are discussed. Environmental and energy applications of TiO2, including photocatalytic treatment of wastewater, pesticide degradation and water splitting to produce hydrogen have been summarized.

Photodegradation of commercial dye, Procion Blue HERD from real textile wastewater using nanocatalysts

The aim of the study is to degrade reactive dye, Procion Blue HERD (PBH) in the presence of artificial UV light and nanophotocatalyst and to investigate the effectiveness of semiconductor photocatalytic treatment of real textile wastewater. For degradation of PBH, batch experiments were carried out by irradiating the aqueous solution of dye, in the presence of different semiconductors such as titanium dioxide (TiO 2) and zinc oxide (ZnO) in the presence of UV light. The rate of decolorization was estimated from residual concentration spectrophotometrically. The effect of process parameters viz. pH (2–10), initial concentration of dye (10-100 ppm), and photocatalyst loading (0.5–2 g/l) on decolorization of dye has also been assessed. The experimental results indicated that 100% decolorization of dye occurred with ZnO at pH 7 and with TiO 2 at pH 4. Optimum value of photocatalyst dose was found to be 1 g L − 1 for ZnO and 1.5 g L − 1 for TiO 2. The rate constants for degradation of PBH using ZnO and TiO 2 were calculated to be 0.077 and0 .075 min − 1 , respectively. The performance of the photocatalytic system indicated that zinc oxide appears to be a suitable alternative to TiO 2 in the presence of UV light at neutral pH. Finally, experiments using real textile wastewater were also carried out in order to examine the effectiveness of the method to a more complex substrate.

Application of Solar Photocatalytic Treatment to Industrial Wastewater from a Chrome Plating Unit

The present study is targeted on the feasibility of photocatalytic treatment of industrial wastewater containing metal ions. Photoreductive deposition of metal ions using solar energy irradiated TiO2, has been investigated in presence of citric acid as a hole scavenger. Treatment studies have also been performed at pH values 2, 4, 7, 8, and 10 to determine optimum reaction conditions. Results showed that at the pH value of 2, 100% reduction was achieved for Cr(VI) in a minimum time period. A maximum removal of 95.5% was achieved for Ni(II) at the pH value of 10.

Determination of key operating conditions for the photocatalytic treatment of olive mill wastewaters

The TiO 2-mediated photocatalytic treatment of olive mill wastewater (OMW) was investigated in a batch type, laboratory scale photoreactor. UV-A irradiation was provided by a 400 W, high pressure mercury lamp and Degussa P25 TiO 2 was used as the catalyst. A factorial design approach was used to study the effect of various operating conditions such as initial organic loading, TiO 2 loading, pH, contact time and the addition of hydrogen peroxide on the conversion of COD and total phenols and experimental models describing the respective removals were developed. Effluent decolorization as well as process efficiency in terms of energy consumption were also evaluated. Finally, the acute ecotoxicity of OMW samples prior to and after photocatalytic treatment was assessed.

A combined photocatalytic and electrochemical treatment of wastewater containing propylene glycol methyl ether and metal ions

Simulated wastewater containing Ni 2+, Zn 2+ and propylene glycol methyl ether (PGME), a solvent in polymer solution used in metal coating, was treated by a combined electrochemical and photocatalytic technique. The effect of electrode spacing and corrugated electrodes on the removal of Ni 2+ and Zn 2+ by electrodeposition was investigated. The solution pH and temperature were kept at 6.0 and 25 °C. The metal removal was almost doubled when the electrode spacing was decreased from 6.4 to 1.3 cm. A further 40% increase was obtained with the corrugated cathode at the electrode spacing of 1.3 cm. A 96% removal of Ni 2+ and Zn 2+ was obtained after 48 h of treatment at a liquid flux of 0.0334 m 3 m −2 s −1. For the combined electrochemical–photocatalytic treatment in a rotary reactor, using immobilized TiO 2 at a liquid flux of 0.0148 m 3 m −2 s −1, metal concentrations decreased about 35% after 48 h of treatment. Lower metal removal could be attributed to the lower liquid flux and the higher liquid volume of 2.4 times that used in the sole electrochemical treatment. However, the PGME removal of 6.9 mg cm −2 was obtained. This was much higher than the removal amounts of 4.5 mg cm −2 and 1.9 mg cm −2 for the sole photocatalytic method and the sole electrochemical treatment, respectively.

Solar photo-catalytic treatment of textile wastewater for biodegradability enhancement

The objective of our study was to increase the biodegradability of actual industrial (textile) wastewater using solar photo catalysis. Live industry effluent was taken and characterised. The results showed that wastewater is non-biodegradable in nature besides having an intense colour. The effluent was treated using titanium dioxide as a photo catalyst in a shallow pond slurry type reactor. The results showed 96% colour removal and 92% COD reduction, along with enhancement in the biodegradability of the effluent from 0.3 to 0.6 at optimised reaction conditions (i.e., 0.1% catalyst loading, pH of 5.5 and an oxidant conc. of 1 ml/200 ml) and a solar UV intensity of 26 W/m².

Photocatalytic treatment of black table olive processing wastewater

The photocatalytic treatment of an effluent from black table olive processing over TiO 2 suspensions was investigated. The study focused on the effect of various operating parameters on the treatment efficiency including initial organic load, catalyst type, concentration and reuse, and addition of hydrogen peroxide. Initial organic load, expressed in terms of chemical oxygen demand (COD), was studied in the range 1–8 g/L, anatase TiO 2 concentrations in the range 0.25–2 g/L and H 2O 2 concentrations in the range 0.025–0.15 g/L. Treatment efficiency, which was assessed in terms of COD, total phenols, aromatics and color reduction, generally increased with decreasing initial COD and increasing contact time, catalyst and H 2O 2 concentrations; however, for H 2O 2 there was a maximum dosage above which performance deteriorated. Depending on the conditions employed, nearly complete decoloration (>90%) could be achieved, while mineralization never exceeded 50%. Shake-flask tests with non-acclimated activated sludge showed that both the original and photocatalyzed effluents were degradable aerobically with the biodegradation rate of the original effluent being three times greater than the oxidized one. On the other hand, photocatalytic oxidation of the original effluent was at least two orders of magnitude faster than its biological oxidation to achieve comparable levels of degradation.

Photocatalytic Treatment of Wastewater Contaminated with Organic Waste and Copper Ions from the Semiconductor Industry

Simultaneous decontamination of organic compounds and copper ions in the wastewater discharged from semiconductor manufacturing facilities by a UV/TiO2 photocatalytic degradation was investigated in this study. Two organic compounds of ethyl lactate and phenol and copper ions were studied due to their common applications in various wafer fabrication processes and higher toxicities. The optimal TiO2 dosage and initial pH for the photocatalytic oxidation of ethyl lactate and phenol were 0.1 g/L and 3.0−4.0. Photocatalytic degradation under these optimal conditions was able to simultaneously mineralize those solvents and remove copper(II) in the synthetic wastewater. Under anoxic condition, oxygen inhibited copper reduction and copper(II) ions were removed through precipitation. Under anaerobic conditions, the removal rate of copper(II) ions and the rate of reduction of the organic solvents content were lower than that in the aerobic condition.

Photocatalytic treatment of wastewater from cottonseed processing: Effect of operating conditions, aerobic biodegradability and ecotoxicity

The heterogeneous and homogeneous UV-A photocatalytic treatment of an acidic waste stream originating from cottonseed processing was investigated. Several TiO 2 samples were screened in heterogeneous slurry experiments and a commercially available TiO 2 anatase (Millennium PC 500) was found to be considerably more active than the rest. The effect of key operating conditions such as effluent dilution (COD from 800 to 8000 mg/L), Millennium TiO 2 loading (from 100 to 1500 mg/L), solution pH (from 2.6 to 10) and the water matrix (presence of effluent solids and dissolved oxygen) on treatment efficiency was also studied. In general, degradation decreased with increasing effluent concentration, increasing solution pH from acidic to neutral and alkaline conditions and in the absence of oxygen serving as electron acceptor. Experiments with 500 mg/L TiO 2 for 240 min led to nearly complete mineralization at 800 mg/L initial COD but higher catalyst loadings suppressed degradation. Homogeneous experiments with 100 mg/L Fe 2+ or Fe 3+ were also performed leading to about 50% COD reduction; this increased to about 65% when H 2O 2 was added, i.e. simulating a Fenton reaction. The original, un-treated effluent was very toxic to marine bacteria Vibrio fischeri and very slowly biodegradable aerobically as assessed by shake-flask tests. Nonetheless, a 10-fold dilution (i.e. 800 mg/L COD) eliminated completely ecotoxicity and improved substantially biodegradation rates. On the other hand, photocatalytically pre-treated samples were less biodegradable than the un-treated one. Deep photocatalytic oxidation resulted in decreased ecotoxicity to V. fischeri, while moderate photocatalytic oxidation increased toxicity probably due to the formation of toxic intermediates.

Photocatalytic Treatment of RDX Wastewater with Nano-Sized Titanium Dioxide (5 pp)

The polynitramines, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), are important military explosives and regulated toxic hazardous compounds. Production, testing and use of the compounds has resulted in numerous acres of contaminated soils and groundwater near many munitions facilities. Economical and efficient methods for treatment of wastewater and cleanup of soils or groundwater containing RDX and HMX are needed. This study focuses on the photocatalytic treatment of RDX wastewater with nano-sized titanium dioxide (nano-TiO2) under simulated sunlight, whose intensity and wavelength are similar to that of the real sunlight in Xi'an at noon. The objective is to determine the potential for RDX destruction with nano-TiO2 in aqueous solution. An activated carbon fiber (ACF) cloth-loaded with nano-TiO2 was put into the RDX containing solution, and the concentration of RDX was measured (by HPLC-UV) at regular time intervals under simulated sunlight. The RDX degradation percentage of the photocatalytic process is higher than that of Fenton oxidation before 80 min, equivalent after 80 min, and it reaches 95% or above after 120 min. The nano-TiO2 catalyst can be used repeatedly. The photocatalytic degradation kinetics of RDX under simulated sunlight can be described by a first-order reaction kinetics equation. The possible degradation mechanism of RDX was presented and the degradation performance was compared with that of biological method. It was demonstrated that the degradation of RDX wastewater is very effective with nano-TiO2 as the photocatalytic catalyst under simulated sunlight. The efficiency of the nano-TiO2 catalyst for RDX degradation under simulated sunlight is nearly identical to that of Fenton oxidation. To date, a number of catalysts show poor absorption and utilization of sunlight, and still need ultraviolet light irradiation during wastewater degradation. The nano-TiO2 used in the described experiments features very good degradation of RDX under simulated sunlight, and the manufacturing costs are rather low (around 10 Euro/m2). Moreover, the degradation efficiency is higher compared to that of the biological method. This method exhibits great potential for practical applications owing to its easiness and low cost. If it can be applied extensively, the efficiency of wastewater treatment will be enhanced greatly.

Photocatalytic treatment of wastewater from paper mill by TiO 2 loaded on activated carbon fibers

Titanium dioxide was loaded on activated carbon fibers (ACF) with different surface areas by using the residue after epoxy resin being calcined as the link between TiO 2 and ACF, and their surface properties and photodegradations of wastewater from paper mill have been investigated. TiO 2 powder distributed on ACF in the form of small clusters, and its surface element state changed after UV irradiation. After TiO 2 was loaded on ACF, the surface area of supported catalysts (TiO 2/ACF) decreased, but the pore structure of ACF remained well. The photodegradation results showed that an increase in surface area favored high COD removal for paper mill effluents; The best COD removal was obtained only by using TiO 2/ACF2 with moderate surface area and pore structure for organic molecules from the effluents, and its photocatalytic activity was higher than suspended P25 and without any decline when used repeatedly.

An optical fiber monolith reactor for photocatalytic wastewater treatment

AbstractA two‐dimensional (2‐D) mathematical model to describe the photocatalytic degradation of aqueous‐phase organic compounds in a multichannel optical fiber monolith reactor (OFMR) was developed and validated by experimental results. The model incorporated an empirical radiation field submodel, an annular flow dynamics submodel, and a Langmuir–Hinshelwood kinetics submodel. Parameters for the radiation field submodel were derived experimentally. The rate constant in the model accounted for both surface adsorption rate and intrinsic reaction rate. The model presented may be used for the design and optimization of the OFMR. Satisfactory agreement between the model predictions and experimental results over a range of experimental conditions was observed. The effects of the reactor geometry including the fiber diameter, the ratio of the fiber diameter, and the channel diameter and the reactor length were investigated. Higher efficiency of degradation was observed for the reactor with the larger fiber and smaller ratio of the fiber diameter to the channel diameter at the expense of a lower liquid throughput. The reactor efficiency was higher in the upflow mode than in the downflow mode for the water stream. © 2006 American Institute of Chemical Engineers AIChE J, 2006

A single TiO2-coated side-glowing optical fiber for photocatalytic wastewater treatment

By means of TiO2-layer-on-SiO2-layer, anatase TiO2 was deposited on novel side-glowing optical fibers, which can provide side UV radiation along the whole fiber length. FE-SEM images show that the double layers adhered well to the side-glowing optical fiber, and the TiO2 coating was homogeneous and smooth. The decomposition reaction of reactive brilliant red dye X-3B on a single TiO2-coated side-glowing optical fiber indicated that the side-scattering UV light intensity was strong enough for photocatalytic oxidation reaction. Therefore, TiO2-coated side-glowing optical fibers open up a new way to use the optical fiber reactor in photocatalytic wastewater treatment.

Development of an optical fiber monolith reactor for photocatalytic wastewater Treatment

A photocatalytic reactor, which employs a ceramic multi-channel monolith as a support for TiO2 and bare quartz fibers inserted inside the monolithic channels as both a light-transmitting conductor and a support for TiO2, was constructed and tested for water treatment by investigating the photocatalytic degradation of o-dichlorobenzene (DCB) and phenanthrene (PHE). This configuration provides a higher surface area for catalyst coating per unit reactor volume compared to the continuous annular reactor (CAR) and optical fiber reactor (OFR). The light distribution profile inside each cell of the monolith is analyzed. Exponential decay of light was observed in propagation along the quartz fiber core and penetration into the TiO2 film. Optimum thickness of TiO2 layer on the optical fiber was found to be ≈ 0.4 μm in this study. The kinetics of DCB and PHE degradation were pseudo-first order. The effect of the water flow velocity was investigated and showed that the operation was in the mass transfer control regime. Overall rate constants were extracted from the experimental data; and these were then used to calculate the apparent quantum efficiency of photocatalytic degradation. Greater apparent quantum efficiency was observed for the optical fiber monolithic reactor (OFMR) compared with that of the CAR.

A single TiO2-coated side-glowing optical fiber for photocatalytic wastewater treatment

A single TiO2-coated side-glowing optical fiber for photocatalytic wastewater treatment

TiO 2 photocatalysts deposited on fiber substrates by liquid phase deposition

Hybrid organic–inorganic fibers and carbon fibers were coated with TiO 2 by liquid phase deposition (LPD). Continuous films were easily accessible on untreated carbon fibers after short mineralization times, whereas the density of nucleation sites on the hybrid substrates was increased by hydroxylation. The photocatalytic activity of TiO 2 powder prepared by LPD was investigated and the material showed its best performance after heat treatment at 300–400 °C. Due to their better thermal stability at these temperatures, carbon fiber filaments were predominantly used as substrates for LPD TiO 2. The photocatalytic activity of the coated fibers per illuminated surface area was found to be higher than that of LPD particles and even higher than that of commercial TiO 2 powders normally used in suspension for photocatalytic wastewater treatment.