Adsorption of low-concentration VOCs on various adsorbents: Correlating partition coefficient with surface energy of adsorbent

Enhancing removal performance of ortho xylene by adding polydimethylsiloxane into two-stage biofilter

A two-stage biofilter was built, augmented with polydimethylsiloxane (PDMS), to enhance the degradation of ortho xylene (o-xylene), and evaluate the feasibility of different PDMS concentrations for improving the removal effect. The results showed that PDMS concentration of 0.50 % significantly enhanced the purification efficiency and mineralization rate of o-xylene to 85(±1)% and 81 %, respectively. Simultaneously, the surface tension of the circulating liquid was reduced by 31.91 mN/m. Furthermore, the polysaccharide concentration of biofilters were increased by 6.90 mg/g and 7.38 mg/g, respectively, while the protein concentration was enhanced by 7.98 mg/g and 9.29 mg/g, respectively. It is worth noting that Sphingomonas and Sphingobium emerged as the dominant bacterial genera after intensification. Fusarium and Cladosporium became the predominant fungal genera in BTF1 and BTF2, respectively. Therefore, the two-stage biofilter containing bacteria and fungi combined with the addition of PDMS can effectively improve the degradation effect.

Interior and Exterior Surface Modification of Zr-Based Metal-Organic Frameworks for Trace Benzene Removal

The emission of volatile organic compounds (VOCs) significantly contributes to air pollution and poses a serious threat to human health. Benzene, one of the most toxic VOCs, is difficult for the human body to metabolize and is classified as a Group 1 carcinogen. The development of efficient adsorbents for removing trace amounts of benzene from ambient air is thus of great importance. In this work, we studied the benzene adsorption properties of four Zr-based metal-organic frameworks (Zr-MOFs) through static volumetric and dynamic breakthrough experiments. Two previously reported Zr-MOFs, BUT-12 and STA-26, were prepared with a tritopic carboxylic acid ligand (H3L1) functionalized with three methyl groups, and STA-26 is a 2-fold interpenetrated network of BUT-12. Two new isoreticular Zr-MOFs, BUT-12-Et and STA-26-Et, were synthesized using a similar ligand, H3L2, where the methyl groups are replaced with ethyl groups. There are mesopores in BUT-12 and BUT-12-Et and micropores in STA-26 and STA-26-Et. The four Zr-MOFs all showed high stability in liquid water and acidic aqueous solutions. The microporous STA-26 and STA-26-Et showed much higher benzene uptakes than mesoporous BUT-12 and BUT-12-Et at room temperature under low pressures. Particularly, the benzene adsorption capacity of STA-26-Et was high up to 2.21 mmol/g at P/P0 = 0.001 (P0 = 12.78 kPa), higher than those of the other three Zr-MOFs and most reported solid adsorbents. Breakthrough experiments confirmed that STA-26-Et could effectively capture trace benzene (10 ppm) from dry air; however, its benzene capture capacity was reduced by 90% under humid conditions (RH = 50%). Coating of the crystals of STA-26-Et with polydimethylsiloxane (PDMS) increased the hydrophobicity of the exterior MOF surfaces, leading to a more than 2-fold improvement in its benzene capture capacity in the breakthrough experiment under humid condition. PDMS coating of STA-26-Et likely slowed down the water adsorption process, and thus, the adsorbent afforded more efficient capture of benzene. This work demonstrates that modifying both the interior and exterior surfaces of MOFs can effectively enhance their performance in capturing trace benzene from ambient air, even under humid conditions. This finding is meaningful for the development of new adsorbents for effective air purification applications.

Evaluation of the adsorption performance and thermal treatment-associated regeneration of adsorbents for formaldehyde removal

Indoor air pollution remains a major concern, with formaldehyde (HCHO) a primary contributor due to its long emission period and associated health risks, including skin allergies, coughing, and bronchitis. This study evaluated the adsorption performance and economic efficiency of various adsorbents (biochar, activated carbon, zeolites A, X, and Y) selected for HCHO removal. The impact of thermal treatment on adsorbent regeneration was also assessed. The experimental apparatus featured an adsorption column and HCHO concentration meter with an electrochemical sensor designed for adsorption analysis. Zeolite X exhibited the highest adsorption performance, followed by zeolite A, zeolite Y, activated carbon, and biochar. All adsorbents displayed increased HCHO removal rates with an extended length/diameter (L/D) ratio of the adsorption column. Zeolite A demonstrated the highest economic efficiency, followed by zeolite X, activated carbon, zeolite Y, and biochar. Higher L/D ratios improved economic efficiency and prolonged the replacement cycle (the optimal timing for adsorbent replacement to maintain high adsorption performance). Sensitivity analysis of adsorbent regeneration under varying thermal treatment conditions (150, 120, and 80°C) and durations (60, 45, and 30 min) revealed minimal changes in adsorption efficiency (±3%). The results indicated the potential of adsorbent regeneration under energy-efficient thermal treatment conditions (80°C, 30 min). In conclusion, this study underscores the importance of a comprehensive assessment, considering factors such as adsorption performance, replacement cycle, economic efficiency, and regeneration performance for the selection of optimal adsorbents for HCHO adsorption and removal.Implications: This study underscores the importance of adsorption technology for the removal of formaldehyde and similar volatile organic compounds (VOCs), highlighting the potential of alternative adsorbents, such as environmentally friendly biochar, in addition to traditional strategies, such as activated carbon and zeolites. Our findings demonstrate the feasibility of adsorbent regeneration under energy-efficient thermal treatment conditions. These results hold promise for improving indoor air quality, reducing environmental pollutants, and enhancing responses to air contaminants like fine dust and VOCs.

Catalytic degradation of benzene at room temperature over FeN4O2 sites embedded in porous carbon

Benzene and its aromatic derivatives are typical volatile organic compounds for indoor and outdoor air pollution, harmful to human health and the environment. It has been considered extremely difficult to break down benzene rings at ambient conditions without external energy input, due to the extraordinary stability of the aromatic structure. Here, we show one such solution that can thoroughly degrade benzene to basically water and carbon dioxide at 25 °C in air using atomically dispersed Fe in N-doped porous carbon, with almost 100% benzene conversion. Further experimental studies combined with molecular simulations reveal the mechanism of this catalytic reaction. Hydroxyl radicals (·OH) evolved on the atomically dispersed FeN4O2 catalytic centers were found responsible for initiating and completing the oxidation of benzene. This work provides a new chemistry to degrade aromatics at ambient conditions and also a pathway to generate active ·OH oxidant for generic remediation of organic pollutants.

Swellable Array Strategy Based on Designed Flexible Double Hypercross-linked Polymers for Synergistic Adsorption of Toluene and Formaldehyde

High-capacity adsorption and removal of complex volatile organic compounds (VOCs) from real-world environments is a tough challenge for researchers. Herein, a swellable array adsorption strategy was proposed to realize the synergistic adsorption of toluene and formaldehyde on the flexible double hypercross-linked polymers (FD-HCPs). FD-HCPs exhibited multiple adsorption sites awarded by a hydrophobic benzene ring/pyrrole ring and a hydrophilic hydroxyl structural unit. The array benzene ring, hydroxyl, and pyrrole N sites in FD-HCPs effectively captured toluene and formaldehyde molecules through π-π conjugation and electrostatic interaction and weakened their mutual competitive adsorption. Interestingly, the strong binding force of toluene molecules to the skeleton deformed the pore structure of FD-HCPs and generated new adsorption microenvironments for the other adsorbate. This behavior significantly improved the adsorption capacity of FD-HCPs for toluene and formaldehyde by 20% under multiple VOCs. Moreover, the pyrrole group in FD-HCPs greatly hindered H₂O molecule diffusion in the pore, thus efficiently weakening the competitive adsorption of H₂O toward VOCs. These fascinating properties enabled FD-HCPs to achieve synergistic adsorption for multicomponent VOC vapor under a highly humid environment and overcame single-species VOC adsorption properties on state-of-the-art porous adsorbents. This work provides the practical feasibility of synergistic adsorption to remove complex VOCs in real-world environments.

Valorization and characterization of bio-oil from Salvadora persica seed for air pollutant adsorption

Salvadora persica (SP) is an important medicinal plant. Numerous articles have been conducted on the leaf, the roots, and the stem of the plant, but there is little information about the seed. Thus, the present work tries to identify the chemical composition of SP seed bio-oil and investigates its use as an adsorbent for cyclohexane removal. This study extracted bio-oil from seeds using different polar and non-polar organic solvents. Two techniques have been used to determine the chemical composition of the bio-oil extracted: FTIR and GC-MS. Results show that the extracted bio-oil presented 13 new major organic bio-compounds in n-hexane and ethanol SP seed extracts. Moreover, the analytical results showed that the two extracts are complex and contained thiocyanic acid, benzene, 3-pyridine carboxaldehyde, benzyl nitrile, ethyl tridecanoate, ethyl oleate, and dodecanoic acid ethyl ester. Additionally, each technique of analysis showed that the extracted bio-oils from SP seeds are rich in non-polar compounds. Indeed, the major fatty acids obtained are pentadecylic acid, myristic acid, lauric acid, oleic acid, margaric acid, and tricosanoic acid. This work provides guidelines for identifying these compounds, among others, and offers a platform for using SP seeds as a herbal alternative for various chemical, industrial, and medical applications. Furthermore, the capacity of SP extracts for air pollution treatment, namely, the removal of cyclohexane in batch mode, was investigated. Results showed that cyclohexane adsorption could be a chemical process involving both monolayer and multilayer adsorption mechanisms. The pores and the grooves on the surface of the SP bio-oil extract helped in adsorbing the cyclohexane with an outstanding maximum removal capacity of about 674.23 mg/g and 735.75 mg/g, respectively, for the ethanol and hexane SP extracts, which is superior to many other recent adsorbents.

Using Waste Tire-Derived Particles to Remove Benzene and n-Hexane by Dynamic and Static Adsorption

Scrap tire rubber particles were used and evaluated to adsorb some gaseous volatile organic compounds (VOCs), such as benzene and n-hexane. The results present that the adsorption capacities were 0.18 and 0.072 mg/g for n-hexane and benzene, respectively, in the static adsorption mode; the effective adsorption may be attributed to the carbon black of the tire. The adsorption process is in accordance with the Freundlich isothermal model and Lagergren pseudo-first-order kinetic equation. Correspondingly, the adsorption process is multilayer adsorption analyzed by the intramolecular diffusion model. In the dynamic adsorption mode, the maximum adsorption efficiencies of n-hexane and benzene were 80.7 and 81%, respectively, at flow velocities of 0.1 L/min n-hexane and 0.2 L/min benzene.

Adsorption of acetone, ethyl acetate and toluene by beta zeolite/diatomite composites: preparation, characterization and adsorbability

The hierarchical porous composites (Beta/Dt) were prepared by secondary growth method using natural diatomite and beta zeolite. Moreover, XRD, SEM, and BET characterize the composite's composition, surface structure, and pore structure. The adsorbability of Beta/Dt was evaluated by adsorption of three common volatile organic compounds (VOCs) of the printing industry: acetone, ethyl acetate, and toluene. The results show that under the optimum preparation condition, the adsorption capacities of the three VOCs on Beta/Dt were about 3.5 times those of pure beta zeolite and 4.7-35.3 times those of diatomite, respectively. It indicates the synergistic adsorption effect between beta zeolite and diatomite. The superior adsorption capacity of Beta/Dt can be attributed to the suitable micropore size, the increase of the diffusion channels, and the chemical adsorption on modification diatomite. The adsorption of acetone, ethyl acetate, and toluene on Beta/Dt conformed to the pseudo-second-order kinetic model. In contrast, adsorption isotherms conformed to the Langmuir model, meaning that both physical and chemical adsorption occurred simultaneously during the adsorption process, and the adsorption belonged to the monolayer adsorption. The chemical adsorption mechanism can be ascribed to the nucleophilic reaction between the three VOCs (acetone, ethyl acetate, and toluene) and Beta/Dt with positive charges resulting from the modification diatomite. Furthermore, the composite could still keep more than 90% of the adsorption capacity of the original adsorbent after five regeneration cycles.

Comparative studies on the VOC sorption performances over hierarchical and conventional ZSM-5 zeolites

The sorption behaviors of hexane, toluene and mesitylene as probe volatile organic compounds (VOCs) over hierarchical and conventional zeolite ZSM-5 were investigated by a series of experiments, such as dynamic adsorption, temperature-programmed desorption and cycle adsorption tests. The results showed that hierarchical ZSM-5 exhibited better adsorption capacity for toluene and mesitylene, better diffusion of VOCs and superior cycle adsorption efficiency. As we believe, these findings will offer valuable information for the development of zeolite based adsorbents for VOC elimination or recycling.

Triple combination of natural microbial action, etching, and gas foaming to synthesize hierarchical porous carbon for efficient adsorption of VOCs

Fungi residue, vinasse, and biogas residue differ from general biomass waste due to natural microbial action. Microbial fermentation helps create natural channels for the permeation of activators and produces proteins for natural nitrogen doping. Inspired by these advantages on porous carbon synthesis, this study adopted dual activators of KOH and KHCO₃ to synthesize porous carbon with different pore ratios for efficient adsorption of volatile organic compounds (VOCs). The fungi residue possessed the least lignin due to the most severe microbial action, contributing to the best pore structures after activation. The etching effect from potassium compounds and gas foaming from the carbonate decomposition contributed to creating hierarchical porous carbon with ultra-high surface area, ca. 1536.8-2326.5 m²/g. However, KHCO₃ addition also caused nitrogen erosion, such that lower adsorption capacity was attained even with a higher surface area when the mass ratio of KOH/KHCO₃ decreased from 2.5:0.5 to 2:1. The maximum adsorption capacities of chlorobenzene (CB) and benzene (PhH) reached 594.0 and 394.3 mg/g, respectively. Pore structure variations after adsorption were evaluated by freeze treatment to discover the adsorption mechanism. The surface area after CB and PhH adsorption decreased 40.3% and 34.5%, respectively. Most of the mesopores might transform into micropores due to the mono/multilayer stacking of adsorbates. The VOC adsorption kinetics were simulated by the Pseudo-first- and -second-order models and Y-N model. This paper provides a new approach for high-value biomass waste utilization after microbial action to synthesize efficient adsorbents for VOCs.

Adsorption performance and kinetic study of hierarchical porous Fe-based MOFs for toluene removal

In light of the promising merits of large surface area, uniform pore size, and tunable functional groups, metal-organic frameworks (MOFs) have great potential to be utilized for adsorbing volatile organic compounds (VOCs). In this study, three Fe-based MOFs, MIL-100(Fe), MIL-101(Fe), and MIL-53(Fe), were synthesized systematically and used to adsorb a typical VOC, toluene. Static adsorption, dynamic breakthrough curves, and adsorption kinetics were conducted to assess the adsorption performance. Additionally, the surface functional groups, pore structure, and morphology were systematically characterized by means of XRD, SEM, XPS, FTIR and N₂ adsorption-desorption analyses to reveal the cause of the difference in adsorption of these Fe-based MOFs. The results revealed that the maximum equilibrium adsorption capacity of 663 mg/g was achieved by MIL-100(Fe) with the highest specific surface area and pore volume. The dynamic adsorption of toluene on MIL-100(Fe) was in accordance with the pseudo-first order kinetic model and the Langmuir isothermal model. The formed π-π stacking interaction between organic ligands and the benzene ring in the MIL-100(Fe) cluster is the primary adsorption mechanism based on XPS analysis. Moreover, MIL-100(Fe) was easily regenerated via microwave irradiation with a negligible adsorption capacity decrease after three cycles. This work highlights the feasibility of hierarchical porous Fe-based MOFs as toluene adsorbents and promotes the application of MOFs in the field of pollution control.

Mechanistic insights into and modeling the effects of relative humidity on low-concentration VOCs adsorption on hyper-cross-linked polymeric resin by inverse gas chromatography

Water vapor is very common in contaminated streams, which has a great influence on the adsorption of low-concentration volatile organic compounds (VOCs) due to the competition between water and VOCs. Understanding adsorption mechanisms and predicting adsorption of VOCs under different relative humidity (RH) are of great importance to design effective adsorption unit. In this study, we comprehensively investigated the effects of RH on the surface properties of hyper-cross-linked polymeric resin (HPR) and adsorption of 18 VOCs at low concentration on HPR under five levels of RH using inverse gas chromatography (IGC). Further, a promising RH-dependent poly-parameter linear free energy relationships (PP-LFERs) model was developed. It was found that water vapor caused the decrease of surface free energy (γ_s^t) of HPR due to the occupation of active sites by water molecules, resulting in the decrease of adsorption partition coefficients (K). Moreover, the γ_s^t could accurately quantify the effects of RH on the surface properties of HPR. Therefore, the RH-dependent PP-LFERs model was established by correlating RH and γ_s^t. The developed model overcame the limited predictive ability of existing models only under a specific RH level, and excellently predicted the lnK values of VOCs (R² = 0.944, RMSEt = 0.36 and RMSEv = 0.47) under various RH.