Zinc Oxide-Catalysed Photo-Oxidative Degradation of Chlorophenols

Chlorophenols are priority pollutants that must be eradicated from the environment owing to the severity of their toxicity and resistance to traditional treatment. Photocatalytic oxidation is an advanced oxidation method which has proven reliability to eliminate persistent pollutants from air and water. The activity of zinc oxide for pollutant removal by photocatalytic oxidation has been well established. In this work the photocatalytic transformation of 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol in irradiated ZnO suspensions at 299 K was studied. The effect of operating parameters such as catalyst and concentration doses on the decomposition rate of these para-chlorinated compounds has been investigated and optimised. It was discovered that the optimum feed concentration for the phenolic compounds is 50 mg L-1. The optimum amount of ZnO was determined for the degradation of 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol which decreased as with increasing of chlorine substituent. For 4-chlorophenol degradation the first clearer description of the effect of doses using response surface was reported. Kinetic profiles on the decomposition of chlorophenols over ZnO were consistent with pseudo-zeroeth order rate scheme. For 2,4-dichlorophenol and 2,4,6- trichlorophenol the decomposition was slow at the short irradiation time. It was found that the degradability of chlorophenols increased as the number of ringchlorine increased. The effect of pH on the destruction rate was found to be influenced by chlorophenol adsorption and dissociation equilibrium. The effect of different anions on the rate of chlorophenol degradation was evaluated by utilising sodium salts as additives. Except for 4-chlorophenol it was found that, inorganic anion additives such as SO4 2-, S2O8 2- and Cl- demonstrated inhibition to the decomposition rate of chlorophenol. HPO4 2- was found to show strongest inhibition and could even hamper the degradation of 4-chlorophenol. The progression of intermediates during the mineralisation of chlorophenols was chromatographed on high performance liquid chromatograph (HPLC). The structure elucidation of pathway products en route to mineralisation of chlorophenols was performed by the combined gas chromatography-mass spectrometry (GC-MS) and HPLC methods. The study disclosed some hitherto unreported intermediates of photocatalytic decomposition of 4-chlorophenol and 2,4-dichlorophenol. Catechol was detected as new intermediate of 4-chlorophenol degradation. Similarly, 4- hydroxybenzaldehyde, benzoquinone and 4-chlorophenol are for the first time reported for 2,4-dichlorophenol degradation. The work also revealed the intermediates of 2,4,6-trichlorophenol which have not been in literature. It is highlighted herein the mechanism of formation of all pathway intermediates

Comparative analysis of ZnO-catalyzed photo-oxidation of p-chlorophenols

The present study compares for the first time the photocatalytic oxidation of three p-chlorophenols (4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol) in irradiated ZnO suspensions. The effect of operating parameters such as catalyst and concentration doses on the decomposition rate of these p-chlorinated compounds has been studied and optimized. The optimal feed concentration for each of the chlorinated phenolic compounds is 50 mg/L whereas the ZnO doses decreased as the number of chlorine substituent is increased. Kinetic profiles on the decomposition of chlorophenols over ZnO agreed with the pseudo-zeroeth order rate scheme with rate constants following the order 2,4,6-trichlorophenol > 2,4-dichlorophenol > 4-chlorophenol. The validity of the pseudo zero order model could be linked to the initial doses of the chlorophenols used vis-à-vis the catalyst. The study revealed stable intermediates of photocatalytic chlorophenol transformation by high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) technique. A combined mechanism is given to account for the photocatalytic destruction of the chlorophenols

Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: A review of fundamentals, progress and problems

Even though heterogeneous photocatalysis appeared in many forms, photodegradation of organic pollutants has recently been the most widely investigated. By far, titania has played a much larger role in this scenario compared to other semiconductor photocatalysts due to its cost effectiveness, inert nature and photostability. Extensive literature analysis has shown many possibilities of improving the efficiency of photodecomposition over titania by combining the photoprocess with either physical or chemical operations. The resulting combined processes revealed a flexible line of action for wastewater treatment technologies. The choice of treatment method usually depends upon the composition of the wastewater. However, a lot more is needed from engineering design and modelling for successful application of the laboratory scale techniques to large-scale operation. The present review paper seeks to offer an overview of the dramatic trend in the use of the TiO2 photocatalyst for remediation and decontamination of wastewater, report the recent work done, important achievements and problems

Adsorption of Aqueous Using Granular Adsorbents from Accanthospermum hispendum DC

Granular activated carbons have been important adsorbents for the decontamination of aqueous environmental contaminants. Acanthospermum hispidum weed represents a ready available source of low-cost adsorbents in sub-Saharan Africa that has barely been paid attention. The effects of pH, contact time, concentration, adsorbent dosage, particle size and temperature on the adsorptive removal of Pb (II) from aqueous solutions over activated carbon granules from the thorns of Accanthospermum hispindum (AC-T) were for the first time investigated and compared with those of the leaves (L), the sodium hydroxide modified thorns (NaOH-T) and regular thorns (T) of this plant. These adsorbents were characterised by the surface charge analysis, scanning electron microscopy (SEM) and the Attenuated Total Reflectance Fourier Transform infrared (ATR FTIR) spectroscopy. The SEM revealed a wafer-like appearance for the AC-T with a large distribution of open pores. The adsorption data of lead uptake onto the adsorbents were examined using two pseudo-order kinetic schemes and three isotherm models. To fully understand the adsorption capacities of the adsorbents, batch desorption recoveries were studied. The FTIR depicted the various functionalities responsible for the adsorption. Adsorption over AC-T was found to agree with pseudo second-order kinetic scheme, the Langmuir and Freundlich isotherm. This material exhibited the highest adsorption capacity. The order of reusability of the adsorbents is T < AC-T < NaOH-T

Adsorption of aqueous Cd(II) and Pb(II) on activated carbon nanopores prepared by chemical activation of doum palm shell

Non-uniformly sized activated carbons were derived from doum palm shell, a new precursor, by carbonization in air and activation using KOH, NaOH and ZnCl2. The activated carbon fibres were characterised by X-ray diffraction, N2 adsorption-desorption, scanning electron microscopy, particle size analysis and evaluated for Cd(II) and Pb(II) removal. The 40-50 nm size, less graphitic, mesoporous NaOH activated carbon yielded high adsorption efficiency, pointing largely to the influence surface area. The performance of the KOH based activated carbon was arguably explained for the first time in terms of crystallinity. The efficiencies of the mesoporous ZnCl2-formulated activated carbon diminished due to the presence of larger particles. Batch adsorption of divalent metals revealed dependence on adsorbent dose, agitation time, pH and adsorbate concentrations with high adsorption efficiencies at optimum operating parameters. The equilibrium profiles fitted Langmuir and Freundlich isotherms, and kinetics favoured pseudo-second order model. The study demonstrated the practicability of the removal of alarming levels of cadmium and lead ions from industrial effluents

Photocatalytic degradation of 2,4-dichlorophenol in irradiated aqueous ZnO suspension.

This paper focuses on the destruction of aqueous 2,4-dichorophenol in ZnO suspension irradiated by low wattage UV light at 299 K. The operating variables studied include initial 2,4-dichlorophenol concentration, photocatalyst doses and pH. At 1.5 g l-1 feed concentration of ZnO and 50 mg l-1 initial 2,4-dichlorophenol level, a complete degradation was achieved in 180 min. The decomposition kinetics with respect to 2,4-dichlorophenol approximates pseudo zero-order with rate constant peaking at 0.38 mg l-1 min-1. High performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) detected benzoquinone, 2-chlorohydroquinone, 4-chlorophenol, 3,5-dichlorocatechol, hydroquinone, 4-hydroxybenzaldehyde and phenol during the ZnO-assisted photodegradation of 2,4-dichlorophenol among which some pathway products are disclosed for the first time. The reaction mechanism accounting for the degradation pathway intermediates is proposed. Inorganic anion additives such as S2O8 2-, SO4 2-, Cland HPO4 2- manifested inhibition against 2,4-dichlorophenol removal

Photocatalytic removal of 2,4,6-trichlorophenol from water exploiting commercial ZnO powder.

2,4,6-trichlorophenol is an important water pollutant owing to the severity of its toxicity. The aqueous phase photocatalytic oxidation of 2,4,6-trichlorophenol over ZnO was investigated as a potential method for the abatement of this pollutant. The effects of operating parameters such as initial ZnO doses and substrate concentration on the removal of 2,4,6-trichlorophenol were studied and optimised at 0.75 g L− 1 and 50 mg L− 1, respectively. The photocatalytic system afforded the highest degradation efficiency at neutral pH. The decomposition of 2,4,6-trichlorophenol by the photoprocess agreed satisfactorily with pseudo zero-order kinetic model. The effect of the presence of SO42−, S2O82−, HPO42− and Cl− on the 2,4,6-trichlorophenol removal rate was for the first time revealed. Some hitherto unreported pathway intermediates of ZnO-assisted 2,4,6-trichlorophenol degradation were recorded using gas chromatography–mass spectrometry (GC–MS) and high performance liquid chromatography (HPLC). A tentative reaction mechanism for the formation of these intermediates was proposed

Photocatalytic treatment of 4-chlorophenol in aqueous ZnO suspensions : intermediates, influence of dosage and inorganic anions.

The photocatalytically driven removal of eco-persistent 4-chlorophenol from water using ZnO is reported here. Kinetic dependence of transformation rate on operating variables such as initial 4-chlorophenol concentration and photocatalyst doses was investigated. A complete degradation of 4-chlorophenol at 50 mg L−1 levels was realised in 3 h. Analytical profiles on 4-chlorophenol transformation were consistent with the best-line fit of the pseudo zero-order kinetics. The addition of small amounts of inorganic anions as SO42−, HPO4−, S2O82− and Cl− revealed two anion types: active site blockers and rate enhancers. Fortunately, Cl− and SO42− commonly encountered in contaminated waters enhanced the rate of 4-chlorophenol degradation. The reaction intermediates and route to 4-chlorophenol mineralisation were elucidated by combined RP-HPLC and GC–MS methods. In addition to previously reported pathway products of 4-chlorophenol photo-oxidation catechol was detected. A radical mechanism involving o-hydroxylation is proposed to account for the formation of catechol

Sol-gel synthesis of Fe₂O₃-doped TiO₂ for optimized photocatalytic degradation of 2,4- dichlorophenoxyacetic acid

Fe₂O₃-doped and undoped TiO₂ catalysts were synthesized by sol-gel method and used to optimize the photocatalytic degradation of 2,4-Dichlorophenoxyacetic acid (2,4-D). The catalysts produced were dominated by the tetragonal, crystalline anatase TiO₂ cell structure. The 0.05wt% Fe₂O₃-doped TiO₂ catalyst exhibited higher photocatalytic activity than that of undoped TiO₂ but its performance decline with increase Fe₂O₃ content due to possible increase of recombination centers. Photocatalytic degradation of 2,4-DA was optimized by response surface methodology. The highest 2,4-DA degradation (48%) was obtained when 1.0 g of 0.05wt% Fe₂O₃-doped TiO₂ is used to degrade 10 ppm of 2,4-DA at pH 4

Forecasting of global solar radiation using anfis and armax techniques

Procurement of measuring device, maintenance cost coupled with calibration of the instrument contributed to the difficulty in forecasting of global solar radiation in underdeveloped countries. Most of the available regressional and mathematical models do not capture well the behavior of the global solar radiation. This paper presents the comparison of Adaptive Neuro Fuzzy Inference System (ANFIS) and Autoregressive Moving Average with eXogenous term (ARMAX) in forecasting global solar radiation. Full-Scale (experimental) data of Nigerian metrological agency, Sultan Abubakar III international airport Sokoto was used to validate the models. The simulation results demonstrated that the ANFIS model having achieved MAPE of 5.34% outperformed the ARMAX model. The ANFIS could be a valuable tool for forecasting the global solar radiation