358 research outputs found
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
Gender Differentials in the Adoption of Drought-Tolerant Maize Varieties among Farmers in the Northern Region of Ghana
This study examined gender differences in farmers’ adoption of drought-tolerant maize (DTM) varieties in northern region of Ghana. Specifically, it described the socio-economic characteristics of the farmers by gender; determined their rate of adoption of the DTM varieties; and analyzed the factors affecting the rate of adoption of the varieties. The study was conducted in the Northern region of Ghana. Simple random sampling technique was used to obtain data from 600 respondents, comprising 551 males and 49 females. Data were collected with the use of a survey questionnaire that contained questions on respondents’ socio-economic characteristics such as age, years of schooling, household size, farm size, access to credit, level of awareness and adoption of DTM varieties and extension contact, among others. Data were analyzed using descriptive statistics and probit regression model. The results for the entire respondents showed average values of 49.75 years for age, 10.8 for household size, with a significant difference (p<0.05) between male and female farmers for each of the variables. Average farm size of 3.38 ha, with a significant difference (p<0.01) between male and female farmers for the variables. The adoption rate of DTM was 13.3% in the entire sample with a significant difference (p<0.05) between male (13.4%) and female farmers (12.2%). The determinants of adoption of DTM varieties for both male and female farmers were age of the household head, education, source of seed, and level of awareness, specific to male farmers was educational status of the respondents while farm size was specific to female famers. It was concluded that policy strategies aimed at improving the uptake of DTM varieties must consider equality in male and female farmers’ access to resources such as credit, labour and land. Keywords: Gender, Drought-tolerant, Maize varieties, Adoption, Ghana. DOI: 10.7176/JBAH/11-20-04 Publication date:October 31st 202
Threats and challenges of smart grids deployments - a developing nations’ perspective
Considerable efforts in huge investments are being made to achieve a resilient Smart Grids (SGs) deployment for the improvement of power delivery scheme. Unsurprisingly, many developing nations are making slow progress to the achievement of this feat, which is set to revolutionize the power industry, own to several deployment and security issues. Studying these threats and challenges from both technical and non-technical view, this paper presents a highlight and assessment of each of the identified challenges. These challenges are assessed by exposing the security and deployment related threats while suggesting ways of tackling these challenges with prominence to developing nations. Although, a brief highlight, this review will help key actors in the region to identify the related challenges and it’s a guide to sustainable deployments of SGs in developing nations
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
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