Separation and Purification Technology (v.67, #2)

Editorial by Guohua Chen (121).

Guest editorial by Ping Gao; Xijun Hu (122).

Continuous electrodeionization for removal and recovery of Cr(VI) from wastewater by Yunqing Xing; Xueming Chen; Peidong Yao; Dahui Wang (123-126).
Continuous electrodeionization (CEDI) was investigated for removal and recovery of Cr(VI) from synthetic wastewater. Different anion exchange resins, including the gel strong-base resin, gel weak-base resin, macro-porous strong-base resin, and macro-porous weak-base resin, were examined. The gel strong-base resin was found to be the best one for use in CEDI. It was shown that CEDI could remove and recover Cr(VI) effectively from wastewater. After treatment, Cr(VI) concentration was reduced from initial 40–100 mg/L to 0.09–0.49 mg/L, and a pure chromic acid solution containing Cr(VI) as high as 6300 mg/L Cr(VI) was obtained. The current efficiency and energy consumption was 16.1–18.8%, and 4.1–7.3 kWh/mol Cr(VI), respectively.
Keywords: CEDI; Efficacy; Chromic acid; Resins selection; Current efficiency; Energy consumption;

Perchlorate and nitrate are contemporary contaminants in drinking water. In most perchlorate containing surface or ground waters, nitrate was also present at significant concentrations. The present study investigated the removal of perchlorate and nitrate at the Ti–water interface by indirect electrochemical reduction process. Results indicated that perchlorate and nitrate could be reduced readily at the surface of a Ti anode simultaneously. Upon the application of an anodic current at the Ti electrode, Ti species such as Ti(III) or Ti(II) were generated. These multivalent Ti species are strong reducing agent that can reduce perchlorate and nitrate ion. The dominant end product of the indirect electrochemical reduction was chloride or nitrite. However, when these two anions were co-existing, the nitrite concentration was negligible. The reduction for both anions followed typical Langmuir–Hinshelwood kinetics; zero-order and first-order reaction at high and low concentrations, respectively. The first-order reaction rate constant was in the range of 10−5  s−1. Perchlorate and nitrate at initial concentration of 200 and 1000 ppm individually were reduced to final concentration of <20 and <200 ppb, respectively, over a short reaction time of 6–8 h. The reaction mechanism was studied with the aid of surface analysis of the solid end products (e.g., TiO2 particles) remaining on the Ti electrode using XPS and SEM/EDX. It was observed that doping of chlorine and nitrogen atoms onto the solid TiO2 particles contributed partially to imbalance of the total Cl and N mass in the system. Additionally, formation of volatile chlorine and hypochlorite species brought mass imbalance to total chlorine.
Keywords: Indirect electrochemical reduction; Ti electrode; Perchlorate; Nitrate; Chlorine-doping; Nitrogen-doping;

Fabrication and photo-electrocatalytic properties of highly oriented titania nanotube arrays with {1 0 1} crystal face by Yang Hou; Xinyong Li; Ping Liu; Xuejun Zou; Guohua Chen; Po-Lock Yue (135-140).
Highly oriented titania nanotube (TN) arrays with {1 0 1} crystal face were prepared on the surface of titanium substrate by liquid chemical deposition method. The obtained titania samples were characterized by X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM), BET measurements, diffuse reflectance spectroscopy (DRS), and electron paramagnetic resonance (EPR) spin trap technique. Results indicate that the nanotubes grown in this study are well-aligned and organized into high-density uniform arrays. The typical dimensions of the hollow tube are ∼90 nm in outer diameter, ∼60 nm in inner diameter, ∼15 nm in wall thickness and ∼300 nm in height. The BET surface area of the obtained titania nanotube is 275 m2/g. The EPR signals reveal that hydroxyl radical (•OH) species can be produced in the nanotube system, and no •OH radical EPR signals were detected under dark and/or nanotube non-existing situations. The catalytic degradation of a textile azo dye, acid orange 7 (AO7), in aqueous solution with titania nanotube arrays electrode was carried out using photo-electrocatalytic (PEC) process, comparing with electrochemical process (EP) and photocatalytic (PC) process. A significant photo-electrochemical synergetic effect was observed, which is due to the efficient charge separation and transfer at the surface-interface of titania nanotube arrays. The kinetic constant of PEC degradation of AO7 using TN electrode was 48.7%, which is higher than that using P-25 TiO2 film and TiO2 particular film electrode. The enhanced degradation rate of AO7 using titania nanotube arrays could be attributed to their larger specific surface area and nanotubular structure with preferred reactive crystal face, which would increase their absorption capacity to the targeted substrates and the rate of surface-interface charge transfers in titania nanotube semiconductor redox systems.
Keywords: Highly oriented; Crystal face; Titania nanotubes; Photo-electrocatalytic; Hydroxyl radical;

Kinetic and mechanistic investigation of the ozonolysis of 2,4-xylidine (2,4-dimethyl-aniline) in acidic aqueous solution by Amilcar Machulek; Eliso Gogritchiani; José E.F. Moraes; Frank H. Quina; André M. Braun; Esther Oliveros (141-148).
The ozonolysis of 2,4-xylidine (2,4-dimethyl-aniline) in acidic aqueous solution was investigated by determining the major reaction products and their evolution as a function of the reaction time and their dependence on the pH of the reaction system. 2,4-Dimethyl-nitrobenzene and 2,4-dimethyl-phenol were found to be primary reaction products; their formation might be explained by electron transfer and substitution reactions. 2,4-Dimethyl-phenol was further oxidized yielding 2,4-dimethyl- and/or 4,6-dimethyl-resorcinol by electrophilic addition of HO• radicals. The best fitting phenomenological kinetic model and the good convergence of calculated and experimentally determined rate constants imply two additional competitive pathways of substrate oxidation: (i) electrophilic addition of HO• radicals and fast subsequent substitution would also yield the resorcinol derivatives. (ii) Substrate and isolated products are thought to be oxidized by hydrogen abstraction at the benzylic sites, but the corresponding products (alcohols, aldehydes, and carboxylic acids) could not be identified. Fe(II) was added to probe for the presence of H2O2, but had no or only a minor effect on the kinetics of the ozonolysis.
Keywords: Ozonolysis; 2,4-Xylidine (2,4-dimethyl-aniline); Kinetics; Mechanisms; Fenton reaction;

Preparation of microfibrous entrapped activated carbon composite by Huiping Zhang; Lianlian Gao; Xijun Hu (149-151).
A kind of microfibrous entrapped activated carbon composites were prepared by the wet lay-up paper-making process followed by sintering in N2 at 950 °C for 30 min. SEM and ASAP 2020 apparatus were used to characterize the structure and test the adsorption isotherm. SEM results indicated that the junctures of the ceramic fiber were completely welded together to cause a sinter-locked three-dimensional network, thereby entrapping micron-sized activated carbon. The porous structure was analyzed with BET equations, H–K equation and BJH theory. Results showed that the specific surface area was 428 m2/g, the total pore volume was 0.217 m3/g, the total micropore volume was 0.0935 m3/g which was 43.1% of the total pore volume, the mesopore volume was 40.9% and the macropore volume was 16.0%. Compared between microfibrous composite and primary activated carbon, the surface areas and the pore structure were almost the same, but the pore volumes all decreased.
Keywords: Microfibrous; Activated carbon; Adsorption isotherm; Pore size distribution;

A mesoporous TiO2−x N x photocatalyst prepared by sonication pretreatment and in situ pyrolysis by Guisheng Li; Jimmy C. Yu; Dieqing Zhang; Xianluo Hu; Woon Ming Lau (152-157).
A novel method for preparing a visible-light-driven mesoporous TiO2−x N x photocatalyst has been developed. It involves the in situ pyrolysis of the product from a chelation reaction under sonication between TiCl4 and ethylenediamine in an ethanol solution of the triblock copolymer F127. The as-prepared photocatalysts exhibit very strong photoactivity in the photocatalytic oxidation of methylene blue under irradiation in the visible spectral region. The samples were characterized by spectroscopic techniques including ultraviolet–visible light reflectance (UV–vis), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effects of ultrasound on the physicochemical properties and photoactivity of mesoporous TiO2−x N x are discussed based on the characterization results.
Keywords: Titanium dioxide; Nitrogen doping; Pyrolysis; Chelation; Sonication;

Modeling of a pilot-scale trickle bed reactor for the catalytic oxidation of phenol by Qiang Wu; Xijun Hu; Po Lock Yue; Jian Feng; Xi Chen; Huiping Zhang; Shizhang Qiao (158-165).
A mathematical model was developed to simulate the catalytic wet air oxidation (CWAO) of aqueous phenol in a trickle bed reactor (TBR). Both ‘axial dispersion’ and ‘plug flow’ models were proposed. ‘Steady-state’ mass transfers across different phases inside the reactor have all been considered in parallel with oxidation reactions catalyzed by heterogeneous copper catalyst supported on activated carbon. The changes in the concentrations of oxygen and phenol in various phases were thus depicted as a function of bed length. In order to validate the accuracy of the established TBR model, a series of experiments on phenol oxidation were performed on a pilot-scale TBR containing 5.6 l of catalysts. The model was found able to give satisfactory predictions for nearly half of all the runs. The discrepancies between the experimental and modeling results were investigated for the less promising runs. It was also noticed that similar simulation results could be attained from ‘axial dispersion’ model against ‘plug flow’ model. Following the discussion on the changes of phenol and oxygen concentrations in the various phases, it is finally concluded that the performance of the TBR of this study depends largely on gas-to-liquid mass transfer process. Further suggestions with regards to reactor optimization are also proposed on the basis of experimental outcome.
Keywords: Modeling; Trickle bed reactor; Mass transfer; Wet air oxidation; Copper/activated carbon catalyst;

Separation of acid-dyes mixture by bamboo derived active carbon by L.S. Chan; W.H. Cheung; S.J. Allen; G. McKay (166-172).
In the present study, the activated carbon is produced using phosphoric acid treatment of the waste bamboo scaffolding and activated at either 400 or 600 °C. The effect of acid to bamboo ratio (Xp) up to 2.4 has been studied. The BET surface area increased with increasing Xp and activating temperature. BET surface area up to 2500 m2/g carbon has been produced. In order to simulate effluent treatment from textile industry, the produced carbon was tested for its dye adsorption capacities. Two acid dyes with different molecular sizes were used, namely Acid Yellow 117 (AY117) and Acid Blue 25 (AB25). In a single component system, it was found that dye with smaller molecular size, AB25, was readily adsorbed onto the carbon while the larger size dye, AY117, showed little adsorption. As a result, it is possible to tailor-make the carbon for the adsorption of dye mixtures in industrial applications, especially textile dyeing, i.e. molecular sieve effect. A binary AY117–AB25 mixture was used to test the possibility of the molecular sieve effect. Furthermore, experimental results were fitted to equilibrium isotherm models, Langmuir, Freundlich and Sips for the single component system. For the binary component system, extended single-component equilibrium isotherm models were used to predict the experimental data.
Keywords: Activated carbon; Acid dye; Bamboo scaffolding; Adsorption;

Physical properties, stability, and photocatalytic activity of transparent TiO2/SiO2 films by Julien Bennani; Ralf Dillert; Thorsten M. Gesing; Detlef Bahnemann (173-179).
Titania/silica composite films were prepared by dip coating of glass substrates in alcoholic suspensions of nanocrystalline anatase particles. Amorphous silica was used as a binder with the aim of producing stable and resistant transparent coatings on glass. It was found that transparency, stability, thickness and photocatalytic activity of the anatase/silica films were strongly affected by changes in the viscosity of coating suspensions. High viscosities lead to thick films, which were opaque and instable. The thickest stable and transparent film had an average thickness of 518 nm. Interference measurements were found to be useful tools to assess the relative film thickness. Moreover, the refractive index of the anatase/silica coatings was obtained by relating the interference pattern with the film thickness obtained by scanning electron microscopy measurements. Methylene blue degradation was conducted to characterise the photocatalytic activity of films and was compared with that of commercially available Pilkington Active™ glass.
Keywords: Titania–silica transparent films; Photocatalysis; Superhydrophilicity; Easy-to-clean coating; Sol–gel chemistry;

Removal of anthraquinone reactive dye from wastewater by batch hydrolytic–aerobic recycling process by Haitao Wang; Qingbiao Li; Ning He; Yuanpeng Wang; Daohua Sun; Wenyao Shao; Kun Yang; Yinghua Lu (180-186).
The degradation of anthraquinone dye Reactive Blue 19 (RB19) of 600 mg/L in the synthetic wastewater was studied using a biofilm hydrolytic–aerobic recycling process (recycling process). The COD (chemical oxygen demand) and RB19 removal efficiency could be up to 91% and 90% in 24 h, respectively, with the recycling rate being 10 mL/min. When the recycling rates were 8 mL/min and 10 mL/min, the average COD and RB19 removal rate reached the largest values of 109.9 mg/L h and 23.6 mg/L h, respectively. The results also showed that RB19 removal rate in both the hydrolytic and aerobic reactor declined whereas COD removal rate in the aerobic reactor increased with increasing of recycling rate. With the recycling rate of 10 mL/min, VFA (volatile fatty acid) in the wastewater was of the lowest value and pH value was apparently higher than those with other recycling rates. The ratios of VFA/alkalinity were all less than 0.6 when the recycling rate was more than 5 mL/min, which indicated that the activity of hydrolytic microorganisms had not been inhibited and the recycling process maintained a stable condition. Results in this work shown that hydrolytic–aerobic recycling process successfully dealt with the issues of over-acidification and inhibition in the hydrolysis process and effectively enhanced the removal efficiencies of RB19 and COD.
Keywords: Removal; Reactive anthraquinone dye RB19; Biofilm; Recycling; Hydrolytic–aerobic;

Photocatalytic purifiers, which are in development for treating gaseous effluents at flow rates generally higher than several tens of m3  h−1, usually employ TiO2-coated materials either as planar or folded fibrous filters, or as honeycomb monoliths. Our primary objective was to compare the photocatalytic efficacy of two types of these materials using a ca. 0.4 m3 close-loop, air tight, photocatalytic reactor we built at EDF. This loop – through which air can be flowed at rates from 16 to 1800 m3 h−1, i.e. 0.11–12.25 m s−1 – includes a paralleliped (560 mm × 295 mm × 200 mm) into which alternate banks of lamps and TiO2 coated-materials (entrance area = 408 cm2) can be accommodated. The materials tested were (i) an Ahlstrom-supplied, thin, non-woven tissue dried at room temperature after impregnation with both Degussa TiO2 P-25 and colloidal SiO2 used as a binder and (ii) an aluminum honeycomb-shaped material we coated with Degussa TiO2 P-25 and dried at room temperature. U-shaped lamps emitting at 254 nm and having an electrical power of 18 or 35 W were employed. The geometries of the materials and the spatial arrangements of the lamps and materials provided an irradiance of the materials as high and homogeneous as possible according to our modeling [Catal. Today 122 (2007) 66–77]. Methanol was chosen as the test pollutant because it is easily mineralized, which minimized the inhibition of the photocatalytic activity by intermediate products. In the recirculation regime, the initial rates of methanol removal observed for the folded tissue were multiplied by at least four – depending on the numbers of materials and banks of three 35 W lamps – when the honeycomb material was utilized. Several factors can add to cause this high increase in efficacy: distinct shapes and TiO2 supports, differences in photons scattering, and the reduced accessibility of reactants to TiO2 because of SiO2. Regarding energy consumption, the use of two photocatalytic materials sandwiching one bank of three 35 W lamps instead of one material in-between two banks of three lamps led to about the same efficacy. The honeycomb material was also tested with toluene at an initial concentration corresponding to the same amount of carbon as when methanol was the pollutant. The removal rate and CO2 formation rate were lower for toluene than for methanol, which can be easily explained by a lower reactivity with respect to oxidation, a smaller adsorbed amount and a higher competition with the more numerous degradation intermediate products. That comparison illustrates the need of trials for every effluent to be treated. Considerable differences in pressure drop between the two materials clearly demonstrated another interest of using honeycomb shapes, at least for the materials and configurations investigated. Furthermore, a deactivation, together with a yellowing, was noticed at high irradiance for the Ahlstrom tissue, which included cellulose fibers. Folding the material enables one to employ lower irradiances for minimizing this effect, while maintaining the efficacy because of the increase in the material area in the reactor, as well as slightly lowering the pressure drop.
Keywords: Photocatalytic air purification; Methanol; Toluene; TiO2 coatings; Air filters;

Studies on adsorption of phenol and 4-nitrophenol on MgAl-mixed oxide derived from MgAl-layered double hydroxide by Shuangling Chen; Zhi Ping Xu; Qian Zhang; G.Q. Max Lu; Zheng Ping Hao; Shaomin Liu (194-200).
In this research, the removal of two phenols (4-nitrophenol and phenol) from aqueous solution was investigated using MgAl-mixed oxide. This oxide was prepared by calcining crystalline MgAl–CO3-layered double hydroxide (LDH) at 500 °C for 4 h. We found that it takes 10–12 h for adsorption of 4-nitrophenol to reach the equilibrium at room temperature while the equilibrium time is 20–25 h for phenol adsorption. The kinetic process of 4-nitrophenol adsorption seemingly follows the first-order reaction but phenol is adsorbed in a pseudo-second-order model. We also noted that the maximum adsorption amount of 4-nitrophenol by fitted three-parameter Langmuir–Freundlich isotherm is 367.8 mg/g, much higher than that of phenol (46.9 mg/g). The differences in the adsorption kinetics and dynamics have been related to the adsorption mechanism and adsorbate–adsorbent interactions. The reconstruction of MgAl-mixed oxide in aqueous solution incorporates 4-nitrophenolate into the interlayer. However, it is difficult to intercalate phenolate due to its weaker affinity for LDH in comparison with OH. In addition, adsorption of MgAl-mixed oxide for 4-nitrophenol and phenol is slightly affected by the initial pH, but considerably facilitated by increasing the adsorption temperature.
Keywords: Adsorption; Phenols; Layered double hydroxide (LDH); Reconstruction; Isotherm;

Ruthenium(II)-tris-bipyridine/titanium dioxide codoped zeolite Y photocatalyst: Performance optimization using 2,4-xylidine (1-amino-2,4-dimethyl-benzene) by Stefan H. Bossmann; Michael Wörner; Megh R. Pokhrel; Bodo Baumeister; Sabine Göb; André M. Braun (201-207).
The heterogeneous photocatalysts of the ruthenium(II)-tris-bipyridine/titanium dioxide/zeolite Y ([Ru(bpy)3]2+/TiO2/zeolite Y) family perform in a manner very similar to a “Fenton-catalyst” in the oxidative degradation of the model pollutant 2,4-xylidine (1-amino-2,4-dimethyl-benzene). Upon photoexcitation using visible light, an electron-transfer reaction to hydrogen peroxide is observed. The oxidation rate of 2,4-xylidine follows a distinct percolation behavior, which is dependent on the amount of titanium dioxide incorporated as nanoparticles into the zeolite Y's framework. The maximum of the photocatalytic degradation of 2,4-xylidine at pH 3.0 was found to be at 34.5 ± 0.5% TiO2 per weight. The optimization of the performance parameters of this particular [Ru(bpy)3]2+/TiO2/zeolite Y photocatalyst was undertaken in a pilot reactor (V  = 3.25 L), equipped with a medium pressure mercury lamp (TQ 718). Important factors in the apparent photocatalytic efficiency have been determined: (a) the availability of dissolved oxygen, (b) the substrate concentration and (c) the concentration of the dispersed photocatalyst. At higher starting concentrations of 2,4-xylidine (>200 mg C L−1), the photocatalytic oxidation process is hampered by parasitic light absorption by an azo-dye formed and, especially, by competitive adsorption of hydrogen peroxide and 2,4-xylidine at the nanoscopic TiO2-centers. It is noteworthy that 2,4-dimethylphenol and oxalic acid were the main reaction products, when 200 mg C L−1 of 2,4-xylidine were oxidatively degraded, whereas a multitude of reaction intermediates were detected when the starting concentration of 2,4-xylidine was 500 mg C L−1. Evidence is given that hydroxylamine is formed during the oxidation of 2,4-xylidine, which subsequently is either reduced to ammonium or generated from the disproportionation of hydroxylamine to ammonium and nitrogen. In the discussion section of this report, the observed reaction behavior is rationalized, based on the elementary chemical reactions occurring in the irradiated heterogeneous reaction mixture.
Keywords: “Fenton-like” oxidation reactions; Titanium dioxide nanoparticles; Heterogeneous photocatalysis; AOP; Zeolite Y-encapsulated ruthenium-tris-bipyridine; 2,4-Xylidine (1-amino-2,4-dimethyl-benzene);

We successfully synthesized porous co-polyimide membranes from a polycondensation reaction of an aromatic dianhydride of 3,3′,4,4′-biphenyltetracarboxylicdianhydride (SBPDA) with diamines of p-phenylenediamine (PPDA) and 4,4′-oxydianiline (ODA) by a wet phase inversion process. Factors affecting the mechanical strength of the membrane and porosity were discussed. By tuning the parameters, a spongy type porous membrane with high mechanical strength and high porosity was obtained. In addition, the porous co-polyimide membrane consisted of a porous top layer and a spongy type sub-structure. This material is suitable as the matrix of a multifunctional composite membrane of a direct methanol fuel cell (DMFC) by filling protonic electrolytes in the micropores of the matrix.
Keywords: Co-polyimide membrane; Porous structure; Wet phase inversion; Fuel cell membrane;

Photo Fenton discoloration and mineralization of high concentration Orange II (2 mM) was conducted by using three catalysts containing Fe in the presence of 2 × 8 W UVC light (254 nm) and 100 mM H2O2 at an initial solution pH of 3.0. The three catalysts are laponite clay based Fe nanocomposite (Fe-Lap-RD), betonite clay based Fe nanocomposite (Fe-B), and iron oxide hydrated (FeOOH). All of them showed a good photo catalytic activity in the discoloration of 2 mM Orange II. 100% discoloration was achieved within 120 min in the presence of 1.0 g Fe-Lap-RD/L while 100% discoloration needs 180 min reaction in the presence of 1.0 g Fe-B or FeOOH. In terms of discoloration, the efficiency of the catalysts follows the order: Fe-Lap-RD > Fe-B ≈ FeOOH. However, in the mineralization of 2 mM Orange II, only Fe-Lap-RD and Fe-B showed good photo catalytic activity while FeOOH showed poor photo catalytic activity after 300 min reaction. In the cases of 1.0 g Fe-Lap-RD/L and Fe-B, more than 95–98% TOC removal of 2 mM Orange II can be achieved while only 82% TOC removal was obtained in the presence of 1.0 g FeOOH/L. In terms of the final TOC removal, the efficiency of the three catalysts follows the order: Fe-Lap-RD > Fe-B > FeOOH. The efficiency of 1.0 g Fe-Lap-RD/L is similar to that of 10 mg Fe3+/L. The results revealed that using Fe-Lap-RD as a heterogeneous catalyst for the degradation of high concentration Orange II is successful. Furthermore, our results also illustrate that both homogeneous and heterogeneous photo Fenton reactions are responsible for the complete discoloration and mineralization of high concentration Orange II.
Keywords: Photo Fenton; Heterogeneous catalyst; Clay; Wastewater; High concentration;

An investigation on the adsorption of acid dyes on bentonite based composite adsorbent by Shizhang Qiao; Qiuhong Hu; Fouad Haghseresht; Xijun Hu; Gao Qing (Max) Lu (218-225).
We report the adsorption study of two commercial dyes, Acid Red 18 (AR18) and Acid Yellow 23 (AY23), onto bentonite based composite adsorbent from aqueous solution. The modified bentonite adsorbent was developed with the expectation to better adsorb anionic dyes. Single dye adsorption equilibrium was measured and the experimental data was analysed by the Langmuir, Freundlich and Redlich-Peterson isotherms. The results indicate that the Redlich-Peterson model provides the best correlation of the experimental data. The adsorption capacities estimated from the Langmuir model for AR18 and AY23 were at 69.8 and 75.4 mg/g, respectively. Binary dye adsorption was also investigated. The results reveal that the multicomponent nature of the dye wastewater introduces additional complications to the adsorption process. Many new factors need to be taken into account, including the interactions between the adsorbate molecules, the variation in adsorption affinity, and the fraction of adsorption sites being shared by each adsorbate.
Keywords: Clay; Adsorption; Dye; Remediation;

Kinetics rate model of the photocatalytic oxidation of trichloroethylene in air over TiO2 thin films by Gianluca Li Puma; Ignasi Salvadó-Estivill; Timothy N. Obee; Stephen O. Hay (226-232).
The photocatalytic oxidation of trichloroethylene (TCE) over a TiO2 thin film was investigated in a flow-through photocatalytic reactor. The effects of TCE concentration and water vapor concentration on the oxidation rates were investigated. Rate models based on variations of the Langmuir–Hinshelwood kinetic model were found to represent the results unsatisfactorily. Therefore, a general rate equation for the oxidation of TCE was derived from an elementary reaction mechanism of TCE photocatalytic oxidation over TiO2. The model, based on chlorine atom attack of the TCE molecule, yields a relatively complex equation including the explicit dependence on water vapor concentration, TCE concentration, photon flux and quantum yield. The rate equation yields half-order dependence on the photon flux at high photon fluxes and first-order dependence at low photon fluxes and reduces to first-order dependence on TCE and inverse dependence on water vapor under specific conditions. The kinetic parameters of the photocatalytic oxidation of TCE on TiO2 thin film were estimated by fitting the model to the experimental results. The approach demonstrated in this paper represents a more rational method of kinetic analysis than the mechanical adoption of a Langmuir–Hinshelwood type rate equation often reported in the literature.
Keywords: Photocatalysis; Titanium dioxide; Air pollution; Reaction kinetics; VOC;

A combined technique of photo-doping and MOCVD for the development of heterogeneous photo-Fenton catalyst by Frank Leung-Yuk Lam; Xijun Hu; Thomas Ming-Hung Lee; Kwong Yu Chan (233-237).
A novel technique combining photo-doping and metal-organic chemical vapor deposition (MOCVD) is used to synthesize heterogeneous iron (Fe) catalyst supported on TiO2/SiO2 substrate. The TiO2/SiO2 substrate is obtained through MOCVD method to disperse TiO2 onto silica gel and then iron is coated onto the substrate by photo-doping. The developed catalyst, Fe/TiO2/SiO2, is characterized by various techniques, including ICP-OES, XRD, XPS, and physisorption. It is found that the doped ferric ion is selectively deposited on the TiO2 layer of the catalyst surface without any significant change of the BET specific surface area. XPS spectrum further proves that the doped iron is in trivalent form and mainly distributed on the catalyst surface. The developed catalysts are evaluated in the photo-Fenton degradation on a dye pollutant, Orange II. Complete discoloration and 80% of mineralization can be achieved. A calcination step is required after the photo-doping in order to minimize the iron leaching during the photo-Fenton reaction.
Keywords: Photo-Fenton catalysts; Composites; Environment; Porous media; Remediation;

Synthesis of exfoliated CNT–metal–clay nanocomposite by chemical vapor deposition by Martin K.S. Li; Ping Gao; Po-Lock Yue; Xijun Hu (238-243).
A novel exfoliated carbon nanotube (CNT)–metal–clay composite was synthesized using catalyst decomposition of acetylene over nanoclay immobilized iron and platinum catalysts. The exfoliated nanoclay immobilized nano-sized catalysts, e.g. Pt or Fe, with diameters of a few nanometers was obtained via a metal organic chemical vapor deposition (MOCVD) process where the temperature of chemical vapor deposition (CVD) corresponds to an optimum decomposition temperature of organo-clay and a complete degradation of organic ligand of organic metal precursors. Counter diffusion and charge imbalance of nanoclay facilitated exfoliation of clay gallery and the uniform deposition and chemical bonding of the metal catalysts. Chemical bonding between metal catalysts and clay surfaces was verified using WAXD and transmission electron microscopy (TEM) analysis. The BET surface area and volume of the metal–clay were increased by 10 times and 3 times, respectively. The use of an organo-clay Cloisite 20A was the main course of clay exfoliation in the CVD process and the exfoliated structure was retained after the growth of MWNT as confirmed by WXRD analysis. Efforts were also made to prepare a nanocomposite membrane of MWNT–Pt–clay in Nafion for use in hydrogen fuel cells. The use of MWNT–Pt–clay in the Nafion membrane is believed to impart the membrane with simultaneously better mechanical strength, good self-humidifying performance and higher barrier resistance properties.
Keywords: Carbon nanotubes; Organo-clay nanocomposite; Nafion membrane;

Simultaneous photocatalytic removal of ammonium and nitrite in water using Ce3+–Ag+ modified TiO2 by Li-Fen Liu; Yang Zhang; Feng-Lin Yang; Guohua Chen; Jimmy C. Yu (244-248).
Simultaneous photocatalytic removal of both ammonium and nitrite from water via coupled oxidation and reduction for N2 formation was studied, using TiO2 loaded with different ratios of Ce3+ and Ag+. The catalysts were prepared using a sol–gel method with PEG or P123 as soft template. The catalysts with the highest activity (cerium and silver loading both at 0.1%) in N removal were characterized by TEM, XRD and UV–vis spectra. The crystalline TiO2 was mainly anatase type, its size averaged at 8.8–9.6 nm. Contents of Ce and Ag and ratios of ammonium to nitrite (NH4–N/NO2 –N) in the reaction solution affected either the catalyst activity or the nitrogen removal efficiency. The best removal rates were 69.9% NH4 +–N, simultaneously 59.9% NO2 –N, and 64.9% TN (total nitrogen) in annular reactor, after 2 h UV (254 nm, 20 W) irradiation when ratio of NH4–N/NO2 –N was 1. Using solar light, the same solution after 2 h reaction using the same Ce and Ag co-modified TiO2, removal of NH4–N, NO2–N and TN was 44.8%, 45.3% and 45.1% respectively using a tubular reactor.
Keywords: TiO2 photo-catalyst; Metal ion co-loading; Nitrogen removal;