Activated carbon fibers for efficient VOC removal from diluted streams: the role of surface functionalities

High-efficiency removal of benzene vapor using activated carbon from Althaea officinalis L. biomass as a lignocellulosic precursor

Benzene is a primary air pollutant commonly found widespread in the indoor environment. It has always been a research focus on the environment due to the causes of significant human health concerns. It has been widely utilized in the synthesis of solvent production, which can rarely be found in high concentrations in outdoor air or high amounts in indoor air, depending on its sources. It is aimed to remove different initial benzene concentrations (from 5 to 1500 ppm) with the production of activated carbon as an excellent adsorbent with a high surface area to be used in these situations. Lignocellulosic wastes have great potential for activated carbon for their advantages (abundant, recycled, and low-cost materials, etc.). This study aimed to evaluate biowaste material for activated carbon production from Althaea officinalis L. biomass by chemical activation (H₂SO₄, LiOH, and ZnCl₂) at temperatures between 500 and 900 °C. Newly developed powdered activated carbons (Ao-ACs) are also tabulated as Ao-AC1-45 for easy reference. Benzene vapor was collected into Tenax TA® tubes by automatic thermal desorption in conjunction with a capillary gas chromatography-mass spectrometry (TD-GC/MS). The significant surface area and production yield of Ao-ACs were obtained at 1424 m²/g (Ao-AC43) and up to 40.32%, respectively. The maximum gas-phase benzene adsorption capacity was 140 mg/g at 270 min. This research has focused on adsorption gas-phase benzene removal onto Ao-ACs as a low-cost adsorbent from the Althaea officinalis L. biomass. Conspicuously, more study is needed to perform the enhanced adsorption of airborne pollutants capacity with inexpensive activated carbon from waste biomass materials.

Enhanced removal efficiency of toluene over activated carbon under visible light

Toluene removal rates using activated carbon (AC) at various relative humidity (RH) levels (0%, 30%, 60%) were compared under dark and visible-light conditions. Light exposure significantly increased toluene-removal efficiency independent of RH. When AC was pre-treated with an optimal concentration of HNO₃, its toluene-removal efficiency was enhanced further with light, an effect that can be attributed to increased surface-area and porosity. Fourier-transform infrared analysis confirmed that exposure of HNO₃-modified AC to light induced partial oxidation of toluene. Within visible-light range (380-650 nm), shorter wavelengths were more effective for toluene-removal compared with longer wavelengths. This suggests that hydroxyl groups formed on AC-surface under light strongly interact with aromatic rings of toluene, allowing for greater uptake of toluene. Moreover, AC can sustain its photo-activity when mixed with cement and cured, suggesting its potential applications in air-purifying building materials. An efficient and practical method for regeneration of spent AC is also demonstrated.

Air oxidation coupling NH3 treatment of biomass derived hierarchical porous biochar for enhanced toluene removal

In this study, hierarchical porous biochar was prepared from poplar sawdust by air oxidation coupling with NH₃ treatment for the removal of toluene. The results showed that the mesopore volume of the sample with air oxidation (PS‒O₂) increased significantly to 0.263 cm³/g from the blank sample (PS, 0.053 cm³/g). This could be attributed to the selective removal of the lignin carbon by air oxidation to develop mesopores in biochar. Following further NH₃ treatment (PS‒O₂‒NH₃), the basic surface chemistry on biochar was improved due to increased basic N-containing groups and decreased acidic O-containing groups, together with the micropore volume also increased to 0.231 cm³/g from 0.186 cm³/g of PS‒O₂. The formation mechanism of hierarchical porous structure of biochar was also discussed. The adsorption capacity of PS‒O₂‒NH₃ for toluene reached 218.4 mg/g at the initial concentration of 820 mg/m³, which was 383.2% higher than that of PS. The adsorption isotherm study indicated that the adsorption process of toluene was monolayered and the maximal adsorption capacity of PS‒O₂‒NH₃ for toluene could reach as high as 476.2 mg/g. The results demonstrated that air oxidation coupling NH₃ treatment is a highly effective method for the preparation of hierarchical porous biochar for enhancing toluene adsorption performance.

Synergetic effect between adsorption and photodegradation on rGO/TiO2/ACF composites for dynamic toluene gaseous removal

The toluene poses a serious threat to the atmospheric environment and human health. Herein, the reduced graphene oxide (rGO)/TiO₂ immobilized on the activated carbon fiber (ACF) are fabricated by ultrasonic assisted sol-gel impregnation method to photodegrade dynamic toluene. Characterizations of rGO/TiO₂/ACF composites reveal that the majority of graphene oxide (GO) is reduced to rGO and rGO/TiO₂ is evenly loaded onto the ACF surface in the form of a smooth film. Furthermore, the photoelectrochemical experiments demonstrate both rGO and ACF can enhance significantly the separation efficiency of electron-hole pairs. The maximum removal efficiency of rGO/TiO₂/ACF-0.75% can be up to 85% under ultraviolet irradiation. The rGO/TiO₂/ACF exhibits more excellent adsorption and photodegradation activity for dynamic toluene than both rGO/TiO₂ and ACF due to the synergetic effect rather than a simple linear combination of the rGO/TiO₂ and ACF for toluene conversion. The possible photodegradation pathway is proposed according to intermediates measured by GC-MS, and adsorption coupling photocatalytic mechanisms are discussed.

The contribution of oxygen-containing functional groups to the gas-phase adsorption of volatile organic compounds with different polarities onto lignin-derived activated carbon fibers

Lignin-based activated carbon fibers (LCFK) were prepared by electrospinning method and evaluated in adsorption of volatile organic compounds (VOCs). Batch adsorption experiments for various component were carried out in a fixed-bed reactor. The molecular polarity of VOCs plays a pivotal role in the monocomponent dynamic adsorption. As a result, the adsorption capacity of toluene was larger than that of methanol or acetone. In the various multicomponent atmospheres (without water), the components interact with each other and competitive adsorption phenomenon occurs, resulting in the adsorption capacity of each component decreased significantly. Also, the samples before and after adsorption were characterized via Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and Boehm titration. The results reveal that methanol and acetone, controlled by physical adsorption, prefer to be adsorbed on polar groups on the surface of LCFK through the dipole-dipole interactions (i.e., van der Waals' forces). Differently, the adsorption of toluene onto LCFK was controlled by physical and chemical processes, and the lactone groups have a positive contribution to the adsorption of toluene. It was also observed that water vapor can enhance the negative effect on the adsorption of VOCs, especially for toluene. The results from this study will be valuable for explaining the mechanisms of competitive adsorption among each component in the various multicomponent atmospheres and understanding the contribution of chemical functional groups on the surface of LCFK in the adsorption process.

Catalytic oxidation of VOCs over Mn/TiO2/activated carbon under 185 nm VUV irradiation

Volatile organic compounds (VOCs) are regarded as the major contributors to air pollution, and should be strictly regulated. Photocatalytic oxidation (PCO) is of great interest for the removal of VOCs owing to its strong oxidation capability. However, its application is greatly limited by catalytic deactivation. Vacuum Ultraviolet (VUV) irradiation provides a novel way to improve the photocatalytic activity while much O₃ will be generated which may cause secondary pollution. In this study, a multi-functional catalyst of Mn/TiO₂/activated carbon (AC) was developed to eliminate and utilize O₃, as well as enhance catalytic oxidation of VOC degradation via ozone-assisted catalytic oxidation (OZCO). The results indicate that Mn modified TiO₂/AC (i.e. 0.1%Mn/20%TiO₂/AC) achieved a toluene removal efficiency of nearly 86% with 100% elimination rate of O₃. With the help of Mn/TiO₂/AC catalyst, O₃ was catalytically decomposed and transformed into active species of O (¹D) and OH, thus enhancing toluene removal. The combination of VUV irradiation with multi-functional catalyst provides a novel and efficient way for the degradation of VOCs.

Behaviors and kinetics of toluene adsorption-desorption on activated carbons with varying pore structure

This work was undertaken to investigate the behaviors and kinetics of toluene adsorption and desorption on activated carbons with varying pore structure. Five kinds of activated carbon from different raw materials were selected. Adsorption isotherms and breakthrough curves for toluene were measured. Langmuir and Freundlich equations were fitted to the equilibrium data, and the Freundlich equation was more suitable for simulating toluene adsorption. The process consisted of monolayer, multilayer and partial active site adsorption types. The effect of the pore structure of the activated carbons on toluene adsorption capacity was investigated. The quasi-first-order model was more suitable for describing the process than the quasi-second-order model. The adsorption data was also modeled by the internal particle diffusion model and it was found that the adsorption process could be divided into three stages. In the external surface adsorption process, the rate depended on the specific surface area. During the particle diffusion stage, pore structure and volume were the main factors affecting adsorption rate. In the final equilibrium stage, the rate was determined by the ratio of meso- and macro-pores to total pore volume. The rate over the whole adsorption process was dominated by the toluene concentration. The desorption behavior of toluene on activated carbons was investigated, and the process was divided into heat and mass transfer parts corresponding to emission and diffusion mechanisms, respectively. Physical adsorption played the main role during the adsorption process.

Toluene and acetaldehyde removal from air on to graphene-based adsorbents with microsized pores

Volatile organic compound (VOC) gases can cause harm to the human body with exposure over the long term even at very low concentrations (ppmv levels); thus, effective absorbents for VOC gas removal are an important issue. In this study, accordingly, graphene-based adsorbents with microsized pores were used as adsorbents to remove toluene and acetaldehyde gases at low concentrations (30ppm). Sufficient amounts of the adsorbents were prepared for use on filters and were loaded uniformly at 0.1-0.5g on a 50×50mm² area, to evaluate their adsorption features with low gas concentrations. The morphology and chemical composition of the adsorbents were characterized using scanning electron microscopy, N₂ adsorption-desorption isotherms, X-ray photoelectron spectroscopy, and Raman spectroscopy. Microwave irradiation and heat treatment near 800°C under KOH activation resulted in enlargement of the pristine graphene surface and its specific surface area; maximum volume capacities of 3510m³/g and 630m³/g were observed for toluene and acetaldehyde gas. The high removal efficiency for toluene (98%) versus acetaldehyde (30%) gas was attributed to π-π interactions between the pristine graphene surface and toluene molecules.

Adsorption properties of regenerative materials for removal of low concentration of toluene

A specific type of material, activated carbon fiber (ACF), was modified by SiO₂, and the final products ACF-x were obtained as ACF-12.5, ACF-20, ACF-40, and ACF-80 according to different dosages of tetraethoxysilane (TEOS). The modified material on the ACF surface had a significant and smooth cover layer with low content of silica from scanning electron microscope (SEM) image. The modified ACF-x showed the stronger hydrophobicity, thermal stability, and adsorption capacity, which had almost no effect in the presence of water vapor and no destruction in multiple cycles. ACF-20 was proven as the most efficient adsorbent in humid conditions. The dual-function system composed of the regenerative adsorbents and the combustion catalyst would be efficient in consecutive toluene adsorption/oxidation cycles, in which the combustion catalyst was prepared by the displacement reaction of H₂PtCl₆ with foam Ni. Therefore, the adsorption/catalytic oxidation could be a promising technique in the indoor air purification, especially in the case of very low volatile organic compound (VOC; toluene) concentration levels.

IMPLICATIONS

Exploring highly effective adsorptive materials with less expensive costs becomes an urgent issue in the indoor air protection. ACF-20 modified by SiO₂ with Pt/Ni catalysts shows stronger hydrophobicity, thermal stability, and adsorption capacity. This dual-function system composed of the regenerative materials and the combustion catalyst would be a promising technique in the indoor air purification, especially in the case of removal of very low concentration of toluene.