The presence of emerging contaminants such as pharmaceuticals in natural waters has raised increasing concern due to their frequent appearance and persistence in the aquatic ecosystem and the threat ...to health and safety of aquatic life, even at trace concentrations. Conventional water treatment processes are known to be generally inadequate for the elimination of these persistent contaminants. Therefore, the use of advanced oxidation processes (AOPs) which are able to efficiently oxidize organic pollutants has attracted a great amount of attention. The main limitation of AOPs lies in their high operating costs associated with the consumption of energy and chemicals. Fenton-based processes, which utilize nontoxic and common reagents and potentially can exploit solar energy, will considerably reduce the removal cost of recalcitrant contaminants. The disadvantages of homogeneous Fenton processes, such as the generation of high amounts of iron-containing sludge and limited operational range of pH, have prompted much attention to the use of heterogeneous Fenton processes. In this review, the impacts of some controlling parameters including the H2O2 and catalyst dosage, solution pH, initial contaminants concentrations, temperature, type of catalyst, intensity of irradiation, reaction time and feeding mode on the removal efficiencies of hetero/homogeneous Fenton processes are discussed. In addition, the combination of Fenton-type processes with biological systems as the pre/post treatment stages in pilot-scale operations is considered. The reported experimental results obtained by using Fenton and photo-Fenton processes for the elimination of pharmaceutical contaminants are also compiled and evaluated.
•Safe, environmentally-benign and relatively cheap reagents of Fenton-type reactions.•Comprehensive review of controlling parameters on the Fenton-type reactions.•Narrow pH range and iron-containing sludge as the homogenous Fenton limitations.•The slow reaction kinetics is the major drawback of heterogeneous Fenton reaction.•Recent strategies to address these limitations are presented.
Display omitted
•Applicable objective functions and their relations at the level of district.•Optimization of distributed integration into districts.•Optimal network configuration and component ...selection to design a district.•Optimal management, operation and planning and subsystem building blocks.•Widely-used optimization tools for district heating and cooling.
Modelling, simulation and optimization of an isolated building separated from the district in which they operate is no longer of interest as a view point of improved efficiency, economic benefits and exploitation of renewable energy resources. Instead, district energy systems have the capacity to obtain several benefits, regarding the practical, environmental and safety by taking advantage of large poly-generation energy conversion technologies. The use of optimization techniques to design such high-efficient systems is strongly motivated by minimizing of the cost for the required infrastructures, minimizing emission, and maximizing the generation or efficiency but is particularly challenging because of the technical characteristics and the size of the real world applications. In this paper, different types of optimization problems, constraints and techniques as well as the optimization tools used in district energy systems are discussed.
Display omitted
•The limitations of the TiO2-based photocatalyst for photocatalytic degradation of VOCs are reviewed.•The approaches for modifying TiO2-based photocatalyst to improve the performance ...of photodegradation of VOCs are discussed.•Applications of TiO2 and modified-TiO2 photocatalysts for VOCs removal in the gas phase are summarized.•Fundamental of metal and non-metal doped TiO2, co-doped TiO2, and composite TiO2 with other semiconductors are considered.•The effects of key controlling parameters on PCO efficiency are reviewed.
Heterogeneous photocatalytic oxidation process (PCO) is a promising technology for removing indoor volatile organic compounds (VOCs) contaminants. Titanium dioxide (TiO2) has been regarded as the most suitable photocatalyst for its cost effectiveness, high stability and great capability to degrade various VOCs. However, no TiO2-based photocatalysts completely satisfy all practical requirements given photoexcited charge carriers’ short lifetime and a wide band gap requiring ultraviolet (UV) radiation. Strategies for improving TiO2 photocatalyst activities by doping with different metal and/or non-metal ions and by coupling with other semiconductors have been examined and reported. These techniques can improve PCO performance through the following mechanisms: i) by introducing an electron capturing level in the band gap that would generate some defects in the TiO2 lattice and help capture charge carriers; ii) by slowing down the charge carrier recombination rate and increasing VOCs degradation. This paper reports the outcomes of a comprehensive literature review of TiO2 modification techniques that include approaches for overcoming the inherent TiO2 limitations and improving the photocatalytic degradation of VOCs. Accordingly, it focuses on the recent development of modified-TiO2 used for degrading gas phase pollutants in ambient conditions. Modification techniques, such as metal and non-metal doping, co-doping, and the heterojunction of TiO2 with other semiconductors, are reviewed. A brief introduction on the basics of photocatalysis and the effects of controlling parameters is presented, followed by a discussion about TiO2 photocatalyst modification for gas phase applications. The reported experimental results obtained with PCO for eliminating VOCs are also compiled and evaluated.
•In situ growth of g-C3N4 on hierarchical magnetic ZnO obtained nanocomposite.•Operating parameters were optimized by response surface methodology.•Partial mineralization and high loss of ...antibacterial activity follow SMX removal.•Degradation pathways include cleavage of S-N bound and sulfone moiety abstraction.•Hierarchical photocatalysts are easily separated by external magnetic force.
The degradation of sulfamethoxazole (SMX) by a synthesized hierarchical magnetic zinc oxide based composite ZnO@g-C3N4 (FZG) was examined. Hierarchical FZG was synthesized by using Fe3O4 nanoparticle as the magnetic core and urea as the precursor for in situ growth of g-C3N4 on the surface of petal-like ZnO. The effect of catalyst dosage (0.4-0.8 g/L), solution pH (3–11) and airflow rate (0.5-2.5 L/min) on the SMX removal efficiency and the optimization of process was studied by response surface methodology (RSM) based on central composite design (CCD). The obtained RSM model with R2 = 0.9896 showed a satisfactory correlation between the predicted values and experimental results of SMX removal. Under the optimum conditions, i.e. 0.65 g/L photocatalyst concentration, pH = 5.6 and airflow rate = 1.89 L/min, 90.4% SMX removal was achieved after 60 min reaction. The first-order kinetic rate constant for SMX removal by using FZG was 0.0384 min−1 while the rate constant by commercial ZnO was 0.0165 min−1. Moreover, under the optimum conditions, about 64% COD removal and 45% TOC removal and a considerable reduction in toxicity were observed. The analysis of generated intermediates during the photocatalytic degradation of SMX was conducted by LC-HR-MS/MS method and a degradation pathway was proposed.
Photocatalytic oxidation (PCO) is a well-known technology for air purification and has been extensively studied for removal of many air pollutants. Titanium dioxide (TiO2) is the most investigated ...photocatalyst in the field of environmental remediation owed to its chemical stability, non-toxicity, and suitable positions of valence and conduction bands. Various preparation techniques including sol-gel, flame hydrolysis, water-in-oil microemulsion, chemical vapour deposition, solvothermal, and hydrothermal have been employed to obtain TiO2 materials. Hydro-/Solvothermal (HST) synthesis, focus of the present work, can be defined as a preparation method in which crystal growth occurs in a solvent at relatively low temperature (<200 °C) and above atmospheric pressure. This paper aims to provide a comprehensive and critical review of current knowledge regarding the application of HST synthesis for fabrication of TiO2 nanostructures for indoor air purification. TiO2 nanostructures are categorized from the morphological standpoint (e.g. nanoparticles, nanotubes, nanosheets, and hierarchically porous) and discussed in detail. The influence of preparation parameters including hydrothermal time, temperature, pH of the reaction medium, solvent, and calcination temperature on physical, chemical, and optical properties of TiO2 is reviewed. Considering the complex interplay among catalyst properties, a special emphasis is placed on elucidating the interconnection between various photocatalyst features and their impacts on photocatalytic activity.
Display omitted
•Highly efficient TiO2 photocatalysts can be synthesized via hydrothermal method.•Morphology and physico-chemical properties of TiO2 govern the performance of PCO air cleaners.•Photocatalyst features can be finely tuned during hydrothermal synthesis to optimize the activity.•Modifying TiO2 with graphene oxide or dopants boost photocatalytic reactions efficiency.•Hierarchically porous TiO2 materials offer superior adsorption and light utilization properties.
The presence of antibiotics in water bodies has received increasing attention since they are continuously introduced and detected in the environment and may cause unpredictable environmental hazards ...and risks. The photocatalytic degradation of sulfamethoxazole (SMX) by ZnO in the presence of fluoride ions (F-ZnO) was evaluated. The effects of operating parameters on the efficiency of SMX removal were investigated by using response surface methodology (RSM). Under the optimum condition, i.e. photocatalyst dosage = 1.48 g/L, pH 4.7, airflow rate = 2.5 L/min and the concentration of fluoride ions = 2.505 mM, about 97% SMX removal was achieved by F-ZnO after 30 min of reaction. The mechanism of reactions, COD removal efficiency and reaction kinetics were also investigated under optimum operating conditions. In addition, about 85% COD reduction was obtained after 90 min photocatalytic reaction. The pseudo-first-order kinetics rate constants for the photodegradation of SMX were found to be 0.099, 0.058 and 0.048 min−1 by F-ZnO, ZnO and TiO2 (P25), respectively. The figure-of-merit electrical energy per order (EEO) was used for estimating the electrical energy efficiency, which was shown to be considerably lower than the energy consumption for the reported research on removal of SMX by photocatalytic degradation under UV irradiation. Toxicity assays were conducted by measuring the inhibition percentage (PI) towards E. coli bacteria strain and by agar well diffusion method. The results showed that after 30 min of reaction, the toxicity of the treated solutions by all photocatalysts fell within the non-toxic range; however, the reduction in toxicity by F-ZnO was faster than those by ZnO and P25. Despite the positive effects of surface fluorination of ZnO on the SMX and COD removal and reaction kinetics, its lower stability compared to ZnO and P25 in the repeated experiments gave rise to some doubts about its performance from a practical point of view.
•RSM was used for process optimization and parameter impact analysis.•F-ZnO showed high efficiency for elimination and mineralization of contaminants.•SMX and COD removal were achieved by a low-energy photoreactor.•Mechanism, kinetics, stability and toxicity assessments were investigated.•Surface fluorination had a negative effect on photostability of ZnO.
Urban Heat Island (UHI) has significant impacts on the buildings energy consumption and outdoor air quality (OAQ). Various approaches, including observation and simulation techniques, have been ...proposed to understand the causes of UHI formation and to find the corresponding mitigation strategies. However, the causes of UHI are not the same in different climates or city features. Thus, general conclusion cannot be made based on limited monitoring data.
With recent progress in computational tools, simulation methods have been used to study UHI. These approaches, however, are also not able to cover all the phenomena that simultaneously contribute to the formation of UHI. The shortcomings are mostly attributed to the weakness of the theories and computational cost.
This paper presents a review of the techniques used to study UHI. The abilities and limitations of each approach for the investigation of UHI mitigation and prediction are discussed. Treatment of important parameters including latent, sensible, storage, and anthropogenic heat in addition to treatment of radiation, effect of trees and pond, and boundary condition to simulate UHI is also presented. Finally, this paper discusses the application of integration approach as a future opportunity.
•Adsorption isotherms of toluene and MEK on various TiO2 photocatalysts were obtained.•Influence of key features of TiO2 photocatalyst on the adsorption process was highlighted.•FTIR analysis was ...conducted to study the nature and distribution of surface OH groups.•Effect of relative humidity on the Langmuir adsorption parameters has been investigated.
Adsorption of pollutants onto photocatalyst surface plays a critical role in the efficacy of photocatalytic oxidation (PCO) technology for air purification applications. In this article, gas phase adsorption of toluene and methyl ethyl ketone (MEK) was studied using a small-scale single-pass continuous flow system. Equilibrium adsorption isotherms for toluene and MEK on six commercially available TiO2 photocatalysts (P25, PC500, PC105, UV100, PC-S7, and S5-300A) coated on nickel foam substrates were determined. The selected photocatalysts cover a wide range of important photocatalytic properties including surface area, porosity, acidity, and population of surface hydroxyl groups. The TiO2 coated filters were characterized by SEM technique to evaluate the quality of coating and homogeneity of titania powder on the support material. Furthermore, a thorough Fourier transform infrared spectroscopy (FTIR) was performed to investigate the nature of surface hydroxyl (OH) groups on various photocatalysts. Using the adsorption isotherm data, Langmuir model adsorption coefficients for toluene and MEK on various photocatalysts and humidity levels have been determined. Additionally, the impacts of challenge compound polarity and air relative humidity (RH) on the adsorption efficiency and Langmuir constants were assessed. The adsorption capacity for MEK (a highly polar compound) was much higher than that of toluene (a non-polar compound) on all photocatalysts, regardless of the humidity content. The results vividly showed that the relative humidity negatively affects the adsorption capacity for both MEK and toluene, however to different extents due to the differences in water solubility and polarity. FTIR characterization of MEK/toluene saturated titania samples revealed that isolated OH groups (both terminal (Ti-OH) and bridged (Ti-OH-Ti)) serve as highly active adsorption sites for volatile organic compound (VOC) molecules.
•The effects of catalyst and ECs concentration, catalyst structure, pH, water matrix, presence of H2O2 and air bubbling on the photo-degradation efficiency have been reviewed.•ZnO is an effective ...catalyst for harvesting a wide range of wavelengths that emerging water contaminants can be degraded by ZnO in photocatlytic processes.•ZnO has the advantage of low cost, nontoxicity, chemical stability and reusability as a catalyst in photo-degradation.•Synergistic effects are reported by combining ultrasound and photo-degradation.
The detection of pharmaceuticals and endocrine disrupting compounds (EDCs), known as emerging contaminants (ECs), in the environment has attracted growing concern due to their toxicity and potential hazard to the ecosystems and humans. These contaminants are consumed at high quantities worldwide and they are released deliberately or accidentally into the water resources. The conventional treatment technologies that use biological processes cannot effectively remove these contaminants. Therefore, the development of efficient and sustainable removal methods for these emerging contaminants is essential. Photocatalytic removal of emerging contaminants by using zinc oxide catalyst (ZnO) is a promising process due to the unique characteristics of this catalyst such as absorption of a larger fraction of the solar spectrum, wide band gap, biocompatibility, non-toxicity and low cost. Recently, a considerable effort has been made to improve the photocatalytic performance of ZnO by doping with elements, optimizing preparation methods, and using nano-ZnO. In addition, the efficiency of photocatalytic degradation processes have been shown to be dependent on the pH of solution, type and amount of ZnO, concentration of contaminants, presence of natural organic matter (NOM) and electron acceptor. This paper presents a review of parameters which affect the performance of photocatalytic degradation processes by using the ZnO catalyst, and discusses methods for the modification of zinc oxide structure.
Photocatalytic oxidation (PCO) has a great potential to eliminate various gaseous pollutants even at low concentrations. Numerous studies have been carried out to improve the effectiveness and ...performance of this technology. In addition, the development of appropriate models can enhance the understanding of reactor performance and the evaluation the intrinsic kinetic parameters that enable the scale up or re-design of more efficient large-scale photocatalytic reactors. This paper reviews recent research works on mathematical modeling of gas phase photocatalytic reactors and analyses different key factors that can enhance pollutants decomposition performance. First, the fundamental of the photocatalytic oxidation process and degradation reaction mechanism are briefly described. Then, to study kinetics of reaction, this paper focuses on Langmuir-Hinshelwood equation, which is by far the most common kinetic model that takes both adsorption and reaction processes into account. Moreover, an overall mass balance that contains advection, diffusion, and reaction rate terms, is analyzed to obtain a comprehensive mathematical model. In the end, the influence of key operating parameters (e.g. flow rate, catalyst surface area and porosity, and catalyst thickness) on the photocatalytic process and removal efficiency of the reactor are discussed.
•Comprehensive literature review on mathematical modeling of photocatalytic oxidation of VOCs in the air is presented.•Fundamental of PCO process including mass transfer and degradation reaction mechanism are reviewed.•Analysis of key operating parameters affecting on PCO performance and on mathematical model is conducted.•A mass balance containing advection, diffusion and reaction rate equations can provide detailed modeling of PCO reactor.
PDF
