Removal of volatile organic compounds by cryogenic condensation followed by adsorption

Removal of volatile organic compounds (VOCs) from gaseous effluents by cryogenic condensation and adsorption has been studied. Mathematical models have been developed to predict the extent of removal of a binary mixture of VOCs in air by these two methods under a wide range of operating conditions. The model results are verified with the published work. A model parametric study carried out in this work suggests that if the concentrations of VOCs in the effluent stream vary over a wide range, condensation followed by adsorption is an effective technique to control the emissions. Condensation is found to be suitable if the VOCs emission levels are high (>1%). On the other hand, if the emission levels are low i.e. parts per millions (ppm) or sub ppm, adsorption is a preferred technique for removing the VOCs from the effluent stream. The model results in this work have significance from the perspective of understanding the mechanism of removal of VOCs by these two methods, determining the key operating parameters that control the removal process and also, defining an effective VOC control strategy.     

Kinetic study for photocatalytic degradation of volatile organic compounds in air using thin film TiO2 photocatalyst

The present paper examined the kinetics of photocatalytic degradation of volatile organic compounds (VOCs) including gaseous trichloroethylene (TCE), acetone, methanol and toluene. Variable parameters were initial concentration of VOCs, water vapor content and photon flux of ultra-violet (UV) light. A batch photo-reactor was specifically designed for this work. The photocatalytic degradation rate increased with increasing the initial concentration of VOCs, but maintained almost constant beyond a certain concentration. It matched well with the Langmuir–Hinshelwood (L–H) kinetic model. For the influence of water vapor in a gas phase photocatalytic degradation rate, there was an optimum concentration of water vapor in the degradation of TCE and methanol. And, water vapor enhanced the photocatalytic degradation rate of toluene, whereas it inhibited that of acetone. As for the effect of photon flux, it was found that photocatalytic degradation occurs in two regimes with respect to photon flux.     

Performance of a membrane-catalyst for photocatalytic oxidation of volatile organic compounds

The performance of a hybrid Sil-1/nanostructured anatase TiO₂ membrane-catalyst and a catalytic titanium silicalite-1 (TS-1) membrane was evaluated for gas-phase photocatalytic oxidation (PCO) of trichloroethylene. The membrane-catalyst outperformed the catalytic plate coated with a similar amount of nanostructured TiO₂ catalyst. However, the activity of the catalytic TS-membrane for PCO is low due to the insufficient number of active titanium sites in the TS-1 zeolite.     

Hierarchical pore structure of activated carbon fabricated by CO2/microwave for volatile organic compounds adsorption

An activated carbon pore-expanding technique was achieved through innovative reactivation by CO_2/microwave.The original and modified activated carbons were characterized by nitrogen adsorption–desorption,scanning electron microscopy,transmission electron microcopy,and Fourier transform infrared spectroscopy.The mesopore volume increased from 0.122 cm~3·g~(-1) to 0.270 cm~3·g~(-1),and a hierarchical pore structure was formed.A gradual decrease in the phenolic hydroxyl and carboxyl groups on the surface of activated carbon enhanced the surface inertia of granular activated carbon(GAC).The toluene desorption rate of the modified sample increased by 8.81% compared with that of the original GAC.Adsorption isotherm fittings revealed that the Langmuir model was applicable for the original and modified activated carbons.The isosteric adsorption heat of toluene on the activated carbon decreased by approximately 50%,which endowed the modified sample with excellent stability in application.The modified samples showed an enhanced desorption performance of toluene,thereby opening a way to extend the cycle life and improve the economic performance of carbon adsorbent in practical engineering applications.     

Response surface optimization of the photocatalytic degradation of atenolol using immobilized graphene-TiO2 composite

Photocatalytic processes using semiconductors have been widely explored due to their fascinating benefits in environmental remediation. In this study, a four-factor three-level Box-Benkhen design (BBD) was employed to assess the photocatalytic degradation of atenolol (ATL) using immobilized graphene-TiO₂ as a photocatalyst. The four variables that were considered in the BBD model were the photocatalyst concentration (10%-20%), pH (4-9), ATL concentration (10-30 mg/L), and light intensity (60-260 W/m²). A monolithic-type swirl-flow reactor, which allowed the immobilization of the photocatalyst, was employed in a semi-batch system to study the photocatalytic degradation kinetics of ATL. The optimum conditions where the highest rate constant (0.667 min⁻¹) was observed were graphene-TiO₂ concentration of 10%, pH of 6.5, ATL concentration of 30 mg/L, and light intensity of 160 W/m². The developed model well predicted the observed values indicated by a high R² of 0.897. Reaction rate constants obtained herein using graphene-TiO₂ in immobilized form were compared with slurry system and TiO₂.     

Evaluations of physico-chemical properties of TiO2/clinoptilolite synthesized via three methods on photocatalytic degradation of crystal violet

Different preparation routes for Ti O_2-supported natural and synthetic clinoptilolite(Ti O_2/CP) composites were thoroughly investigated on the basis of sol–gel, hydrothermal, and in-situ hydrothermal methods.The micro-structural features and physicochemical properties of resultant Ti O_2/CPs were characterized via X-ray diffraction patterns, scanning(transmission) electron microscope images, Fourier transform infrared spectra, inductively coupled plasma-optical emission spectrometry methods, BET-isotherms,UV–visible spectra, and surface charge potential distributions. The results showed that in-situ hydrothermal method led to well dispersions of loaded-Ti O_2 particles on the surface of leaf-like CP, while obviously aggregated Ti O_2 on a relatively distorted structure of CP was obtained using sol–gel and hydrothermal methods. Their adsorptive and photocatalytic efficiencies for removal of crystal violet(CV) dye in aqueous solution were also explored under UV-irradiations. The results demonstrated that Ti O_2/CPs synthesized via sol–gel and in-situ hydrothermal methods presented the excellent performances with 98%removal efficiencies as compare to the bare commercial Ti O_2 which achieved 53% removal of CV dye.While, the in-situ hydrothermally synthesized Ti O_2/CPs were the best due to their moderate energy cost,highest adsorption capacities and removal efficiencies. Particularly, the synthetic CPs as supports exhibited synergetic photocatalytic activities for the degradation of CV dye, which is attributed to their high surface areas, better adsorption capability, and fine dispersion of Ti O_2 particles. Adsorption and degradation kinetics of CV dye were found to follow the pseudo-second and pseudo-first order models,respectively.     

Boosting photogenerated carriers for organic pollutant degradation via in-situ constructing atom-to-atom TiO2/ZrTiO4 heterointerface

Constructing favorable heterointerface to facilitate photogenerated carrier transport is an effective way to improve the photocatalytic activities of semiconductor catalysts. However, the fabrication of atomic-level interface remains a challenge due to the difficulties in constructing intimate interface contact. Herein, a unique atom-to-atom TiO₂/ZrTiO₄ heterointerface with the potential formation of Ti–O–Ti(Zr) bonds between TiO₂ and ZrTiO₄ was constructed through the in-situ formation of the two components using a one-step sol-gel method. The resulting photocatalyst exhibited superior photocatalytic activity and high stability, i.e., about 97% of Rhodamine B was degraded after visible light irradiation for 90 min for three consecutive photodegradation cycles. By further analyses such as refined X-ray diffractometer (XRD), X-ray photoelectron spectrometer (XPS), Raman microscope (Raman), photoluminescence (PL), and photoelectrochemical spectra (TP and EIS), it revealed that the atom-to-atom heterointerface of TiO₂/ZrTiO₄ with stronger interaction forces would greatly accelerate the charge separation as well as provide generous defect sites, boosting the number of photogenerated carriers available at reaction sites, thus contribute to the improvement of photocatalytic performance. The present work demonstrates a new approach of in-situ construction for interface engineering TiO₂ based catalyst, providing design guidelines for heterointerface photocatalytic structures.     

Synthesis and characterisation of organo-silica hydrophobic clay heterostructures for volatile organic compounds removal

Mesostructured materials belonging to a new class of solid acids known as porous clay heterostructures (PCHs) have been prepared by chemical modification of a natural clay, by using a cationic surfactant, a neutral amine, and an equimolar mixture of bis(triethoxysilyl)benzene (BTEB) and tetraethyl orthosilicate (TEOS). The effect of different polymerisation times of the silica sources and of the hydrocarbon chain length of the neutral amine was studied. The materials retained their layered structure after the formation of stable pillars by the polymerisation of hydrolysed TEOS and BTEB. All materials were characterised by low temperature nitrogen adsorption isotherms, ¹³C CP MAS, ²⁹Si MAS and CP MAS NMR spectroscopy, thermal analyses and infrared spectroscopy. The specific surface BET areas of the materials were in the range 550–800 m² g⁻¹ and the corresponding microporous volume were near 0.2–0.3 cm³ g⁻¹. The reduction of the reaction time from 12 to 4 h avoids the extra-gallery polymerisation, contributing for a larger specific surface area. The increase of two carbon atoms in the neutral amine chain does not show much effect on the available surface area.

These materials were very effective as adsorbents of volatile organic compounds (VOCs), according to tests on methanol, methyl ethyl ketone, toluene and trichloroethylene. The water adsorption isotherms proved the hydrophobicity of the materials, suggesting their capabilities for VOC adsorption in the presence of water.     

Estimation of adsorption energies using the physical characteristics of activated carbons and the molecular properties of volatile organic compounds

Adsorption of volatile organic compounds (VOCs) by granular activated carbons (GACs) is a highly exothermic process and leads to temperature rises, which may reduce the separation efficiency. This study points out the significant characteristics of VOCs and GACs on adsorption energies. Adsorption energies were measured for a wide variety of VOCs, representative of different chemical groups, using 8 different commercial GACs with different porous structures. Afterwards a statistical analysis was applied to the experimental database thus obtained, which enabled one to pinpoint the most significant variables, linked to either VOC molecular properties or the intrinsic characteristics of GACs. Two statistical models have been tested: multi linear regression (MLR) and neuronal networks, and their efficiencies were compared in terms of prediction skill. The best results have been obtained from the MLR approach, which discriminated five different properties of the system. These variables were the polarisability, the heat of vaporization, the ionization potential and the surface tension for adsorbates and the mean micropore radius for GACs. The MLR model enabled one to compute integral adsorption enthalpies with a precision of around 10% and to draw conclusions on the dominant adsorption mechanisms.     

Adsorption of chlorinated volatile organic compounds in a fixed bed of activated carbon

Adsorption isotherms of dichloromethane and 1,1,2-trichloro-1,2,2-trifluoroethane on an activated carbon pellet, Norit B4, were studied. For these chemicals, the Sips equation gave the best fit for the single component adsorption isotherm. The adsorption affinity on activated carbon was greater for dichloromethane than that of 1,1,2-trichloro-1,2,2-trifluoroethane. An experimental and theoretical study was made for the adsorption of dichloromethane and 1,1,2-trichloro-1,2,2-trifluoroethane in a fixed bed. Experimental results were used to examine the effect of operation variables, such as feed concentration, flow rate and bed height. Intraparticle diffusion was able to be explained by a surface diffusion mechanism. An adsorption model based on the linear driving force approximation (LDFA) was found to be applicable to fit the experimental data.     

Liquid and vapor phase adsorption of chlorinated volatile organic compounds on hydrophobic molecular sieves

This work focused on the potential application of various hydrophobic molecular sieves for the sorption of four model chlorinated volatile organic compounds (CVOCs, i.e., chloroform, trichloroethylene, tetrachloroethylene, and carbon tetrachloride) from dilute liquid water streams.

Results obtained thus far have shown that silicalite-1 has a high affinity for these CVOCs, higher in fact than Centaur^® activated carbon, used as a benchmark in this study. Loading results for trichloroethylene from both liquid and vapor phase indicated that the liquid phase did not penetrate the pores of silicalite-1, while the solution did penetrate the pores of Centaur^® and a dealuminated NaY used for comparison.

Finally, three definitions from the literature for the “hydrophobicity” of molecular sieves were considered. An alternative definition for hydrophobicity is introduced here, which is easy to determine and is based on water retention.     

Gas-solid adsorption of selected volatile organic compounds on titanium dioxide Degussa P25

This paper focuses on the adsorption of gaseous trichloroethylene, toluene and chlorobenzene on the photocatalyst TiO₂ Degussa P25. An optimized EPICS (Equilibrium Partitioning In Closed Systems) methodology was used to study equilibrium partitioning. For the three compounds investigated, equilibrium adsorption was reached within of incubation. Adsorption isotherms, determined at a temperature (T) of and relative humidities (RH) of 0.0% and 57.8% were found to be linear (R²>0.993,n=5), indicating that no monolayer surface coverage was reached in the concentration interval studied ). Within the linear part of the isotherm, the influence of both relative humidity and temperature was investigated in a systematic way and discussed from a thermodynamic point of view. Data analysis resulted in a double linear regression for 22% ?RH?90% and . The equilibrium adsorption coefficient represents the equilibrium concentration ratio and ΔU_ads is the internal energy of adsorption . At RH=0.0%, experimental K values were a factor 5-10 higher than those expected from the regression equation, indicating that another adsorption mechanism becomes important below monolayer surface coverage of TiO₂ by water vapour molecules. Since surface interactions are of primary importance in photocatalytic reactions, this paper contributes to a better understanding of the basic mechanisms of TiO₂ mediated heterogeneous photocatalysis and is an interesting tool for developing optimized mathematical models.     

Adsorption of volatile organic compounds by means of activated carbon fibre-based monoliths

Activated carbon fibre monoliths (ACFMs) were prepared from the rejects of polymeric fibres (Nomex™). These were carbonised, agglomerated with a phenolic resin and steam activated at burnoff degrees between 0 and 40%. Adsorption experiments with n-butane at 30 °C show that, at high adsorbate concentrations, the amount adsorbed is a function of pore volume, but at low concentrations this mainly depends on pore size distribution. The porosity of Nomex-based ACFMs is formed by narrow micropores, which permit higher amounts of vapour to be adsorbed in low concentrations compared to monoliths prepared from different commercial activated fibres and a commercial granular activated carbon, which exhibits wider pores. The agglomeration of Nomex-fibres to form ACFMs does not cause any loss in adsorption properties with respect to non-agglomerated activated fibres. From the adsorption experiments of different vapours on a Nomex-based ACFM (40% burnoff) it was found that at high concentrations (p/p_o=1) the adsorbed volume was independent of the nature of the adsorbate and depended only on pore volume. However, at low vapor concentrations (p/p_o=0.004), the amount adsorbed depended on the adsorbate being well correlated to the molecular parachor and the polarizability of the adsorbates     

低温等离子体协同催化降解挥发性有机物的研究进展

低温等离子体协同催化技术在挥发性有机物（VOCs）治理中因具有反应高效、反应条件温和、设备简易等优点而受到广泛的研究和应用。文章介绍了低温等离子体协同催化降解VOCs的基本原理、技术研究进展,简述了低温等离子体的高反应活性在与催化剂的高反应选择性结合后所产生的协同作用,二者的结合不但提高了VOCs的降解效率、减少有害副产物生成,还弥补了单一使用低温等离子体技术的高能耗、副产物多的缺陷。此外,分析了低温等离子体与催化剂的联合方式及特点、低温等离子体与催化剂之间的相互作用和影响以及低温等离子体联合不同类型催化剂的协同原理。指出了研究中对完整机理分析的欠缺以及应用过程中对中间过程监测分析的困难,这也是低温等离子体协同催化降解挥发性有机物研究中的重要内容。     

纳米TiO2对水中对羟基苯甲酸的吸附及光催化降解

以优质纯H粉末TiO2为光催化剂,研究了水体中对羟基苯甲酸在纳米TiO2颗粒上的吸附行为,并研究了对羟基苯甲酸的二氧化钛光催化降解效果。结果表明:TiO2对对羟基苯甲酸的吸附作用明显依赖于水溶液的pH,对羟基苯甲酸的光催化降解效果与其在催化剂表面的吸附行为密切相关,提高吸附速率,对羟基苯甲酸的去除率也随之提高。     

VOCs在分子筛上吸附性能的研究进展

简单介绍了一些常见分子筛的构型及特点,讨论了不同构型分子筛对各种VOCs废气吸附效果的研究进展。总结了各类分子筛对VOCs的吸附特性,发现立方构型的分子筛对VOCs废气的吸附效果最佳。小结了影响分子筛吸附性能的因素,发现分子筛的比表面积、孔容、硅铝比、表面官能团等自身性质都会对吸附VOCs废气的性能产生一定的影响。另外,分子筛的吸附容量还与吸附时的进气流速、入口气体浓度、吸附剂床层高度等工况有关。最后,对现有研究做了总结与展望,为分子筛吸附VOCs的合理应用提供了理论支持。     

Construction of quantum-scale catalytic regions on anatase TiO2 nanoparticles by loading TiO2 quantum dots for the photocatalytic degradation of VOCs

We constructed quantum-scale catalytic regions on the surfaces of TiO₂ nanoparticles by loading TiO₂ quantum dots (QDs) with a two-step method. The removal rate and mineralization efficiency of toluene were measured and then used in evaluating the oxidation performance of the prepared samples. A home-built atmospheric surface photovoltage spectrometer and X-ray photoelectron spectrometer were used in analyzing band alignment across the interface between TiO₂ QD and TiO₂ particle and the transfer of charge carriers at the surface. Results showed that an upward band bending formed from the TiO₂ particle to the TiO₂ QD and promoted the accumulation of holes at the QD side. Moreover, the QD and surrounding substrate TiO₂ formed a quantum-scale catalytic region, improving the reaction probability of electron-hole pairs and corresponding intermediates. The mineralization efficiency of toluene in QD-loaded TiO₂ reached 95.8%. The synthetic method is green and simple, showing potential in scale production and industrial application.