A Brief Review of Formaldehyde Removal through Activated Carbon Adsorption

Enzymatic Cleanup of Formaldehyde in Aqueous Solutions

Numerous methods have been developed to address gaseous formaldehyde pollution, but most of them cannot be applied directly to eliminate the pollution of formaldehyde in aqueous solutions. Formaldehyde in aqueous solutions can be leached from formaldehyde-containing solid materials (e. g., food, wood, clothes, resins) and absorbed from gaseous formaldehyde by water. Here we developed an enzymatic cleanup technique - the reconstitution of an enzyme cocktail consisting of three coenzyme-free oxidoreductases (i. e., formaldehyde dismutase, methanol oxidase, and formate oxidase) and catalase for the complete oxidation of formaldehyde. This enzyme cocktail catalyzed the reaction of formaldehyde and ambient dioxygen into carbon dioxide (CO2) and water, which was demonstrated by the stable isotope tracer technique. Significant levels of formaldehyde were detected from aqueous solutions leached from the squid, pomfret, fabric, and curtain in the market. When this enzyme cocktail was applied to treat the leachates of contaminated samples above, formaldehyde was eliminated with degradation ratios of up to 100 %. This enzymatic cleanup technique, featuring excellent biosafety (for example, degradable catalysts and non-immunogenicity), independence from light, high degradation ratios, and no special equipment required, could be widely used to treat contaminated food, drinking water, and formaldehyde-containing leachate.

The Photocatalytic Efficacy of Potassium Hydroxide-Based Modification of Titanium Dioxide in the Oxidative Destruction of Gaseous Formaldehyde

Formaldehyde (FA) is a carcinogenic oxygenated volatile organic compound and a key constituent of indoor air pollution. Photocatalytic oxidation (PCO) is a promising strategy for managing FA in indoor environments. Here, the PCO of FA in indoor air has been investigated by modifying TiO2 with KOH (KOH/TiO2: expressed as KT-x, where x = the molar concentration of KOH from 0.1-2 m). The KOH treatment achieves superior FA removal performance of KT-0.1 over TiO2 (e.g., space-time yield: 1.78E-03 versus 9.91E-04 molecules photon-1 mg-1 and dynamic clean air delivery rate: 10 versus 5.43 L mg-1 min-1). Such differences in photocatalytic activity reflect an enhancement in charge separation, molecular oxygen activation (on the photocatalyst surface), and electron mobility facilitated by the presence of surface hydroxyl groups and potassium on the TiO2 surface. On the KT-x surface, the PCO of FA proceeds through several reactive intermediates (e.g., formate and dioxymethylene). According to density functional theory, the PCO of FA by KT-x is promoted by the synergistic combination of oxygen vacancies and potassium impurities on the TiO2 surface. This research offers valuable insights into the development of cost-effective photocatalysts with enhanced PCO performance.

Soy protein selectively accumulates formaldehyde

Soy protein (SP) is easily obtained from defatted soybeans that have had soybean oil removed. Therefore, the materials consisting of soy protein are not only environmentally benign but also sustainable materials. We prepared the SP - GPTMS composite materials by mixing the SP and a silane coupling reagent, 3-glycidoxypropyltrimethoxysilane (GPTMS), and demonstrated the accumulation of various aldehydes, such as formaldehyde (HAld), acetaldehyde (AcAld), butyl aldehyde (BuAld), and benzaldehyde (BnAld), by the SP - GPTMS composite materials. As a result, when the composite materials were incubated in an aqueous multi-component solution containing four aldehydes, these materials effectively accumulated the aldehydes. The accumulated amounts of the aldehydes were BnAld < BuAld < AcAld < HAld and the amount of HAld was three times higher than that of BnAld, which had the lowest accumulated amount. These results suggested that the SP - GPTMS composite materials indicated a molecular selectivity for HAld. In addition, the accumulated amounts of HAld further increased under acidic conditions. Furthermore, according to the IR measurements, the HAld-accumulated SP - GPTMS composite materials showed the formation of Schiff base bonds. Therefore, the molecular selectivity of HAld in the SP - GPTMS composite material was due to the high electrophilicity of HAld and the low steric hindrance.

Tuning strategies of MIL metal organic frameworks for adsorptive removal of formaldehyde in air

Materials Institute Lavoisier (MIL) metal organic frameworks (MOFs) are known for their potential to adsorb gaseous organic pollutants. This study explores the synergistic effects between the selection of central metals (e.g., titanium, iron, and aluminum) and the incorporation of -NH2 groups in terms of adsorption efficiency against gaseous formaldehyde (FA). A group of the pristine MIL MOFs is synthesized using three different metals (i.e., titanium, iron, and aluminum) and terephthalic acid along with their NH2 derivatives using 2-aminoterephthalic acid. Among the pristine forms, MIL-125(Ti) achieves the highest FA adsorption capacity (Q) of 26.96 mg g-1 and a partition coefficient (PC) of 0.0898 mol kg-1 Pa-1. Further, amination significantly improves the FA adsorption potential of NH2-MIL-125(Ti) with a Q value of 91.22 mg g-1 (PC = 0.3038 mol kg-1 Pa-1). In situ diffuse reflectance infrared Fourier-transform spectroscopy reveals that the FA adsorption of plain MILs should be governed primarily by physisorption. In contrast, FA adsorption of NH2-MILs appears to be regulated by both physisorption and chemisorption, while the latter being affected mainly through FA-NH2 interactions (Schiff base reactions). These findings provide valuable insights into the utility of aminated MIL sorbents, possibly toward the efficient management of indoor air quality.

Preparation and characterization of cellulose nanofibril/chitosan aerogels with high-adsorbability and sensitive indication for indoor free formaldehyde

The toxic volatile organic compounds (VOCs), especially formaldehyde (FA), released from decoration materials pose a great threat to human health. In this study, formaldehyde adsorption performance of the specially formulated nanocellulose/chitosan aerogel (CNFCA) was investigated in simulated atmosphere. The physicochemical property of the composite aerogel was characterized, which had a large specific surface area (153.67 m2/g), a rough surface and an ultra-thin and porous structure. The composite aerogel showed excellent adsorption capacity for the formaldehyde, its theoretical maximum adsorption capacity was as high as 83.89 mg/g, and the adsorption process was more in accordance with the pseudo-second-order kinetics. The chromogenic reaction between the 4-amino-3-benzo-5-mercapto-1,2,4-triazolium (AHMT) and CNFCA was found that the color of the composite aerogel was depended on the free formaldehyde concentration. Based on this phenomenon, a colorimetric card was proposed and built to detection the formaldehyde in the atmosphere. Moreover, the adsorption mechanism research was found that the CNFCA with a multilayer structure belonged to physicochemical complex adsorption.

Doping of porous carbons with sulfur and nitrogen markedly enhances their surface activity for formaldehyde removal

The surfaces of phosphoric acid activated carbon, referred to as CG, and steam activated one, referred to as SX, were modified through an introduction of S- and N- groups originated from thiourea. The prepared samples were used for formaldehyde removal at room temperature. Heating at 450, 600 and 950 °C altered both surface chemistry and porosity. The extents of these modifications depended on the type of carbon. Using thiourea as the modifier resulted in an incorporation of significant amounts of nitrogen and sulfur to the carbon matrices. Their speciation depended on the heat treatment conditions. The activity of samples heated at 450 °C was governed by amine groups of thiourea retained on the surface. A further heat treatment converted gradually amine nitrogen into pyridines/pyrroles and quaternary nitrogen, shifting the adsorption mechanism to rather specific interactions than a direct chemical reactivity. Carbons with few times less nitrogen than in their amine-modified counterparts, but in quaternary form and with the small amount of sulfur in thiophenic configurations, regardless the origin, worked as very efficient adsorbents of HCHO. Due to the modification of the carbon matrix electronic structure, resulting in a positive charge on carbon atoms in the vicinity of the heteroatoms incorporated to carbon rings, the density of specific adsorption centers on the surface in larger pores was significantly higher than that in ultramicropores. This markedly contributed to efficient utilization of pores/surface, where heteroatom can exist and where otherwise the dispersive adsorptions forces would be weak, for HCHO removal at ambient conditions.

Diaminopropane-appended activated carbons for the adsorptive removal of gaseous formaldehyde using a portable indoor air purification unit

It is of significant practical interest to develop high-performance air purifier (AP) for removing carcinogenic volatile organic compounds present ubiquitously in indoor air (e.g., formaldehyde (FA)). In this regard, a portable AP system was designed by loading honeycomb ceramic filters with diaminopropane (DAP)-appended activated carbon (AC). The maximum removal efficiencies (REs) of AP loaded with 10, 20, 30, and 50 %-DAP/AC were 26.2, 28, 88.3, and 89.4 %, respectively, against 5 ppm FA (at 160 L min-1). Hence, the 30 % DAP unit was used mainly in this work. The removal efficiency of 30 %-DAP/AC (160 L min-1), when tested against 2 ppm FA, decreased from 90.3 to 73.2 % with an increase in relative humidity from 0 to 60 %. The performance of the AP unit, when assessed kinetically in terms of the clean air delivery rate (CADR), reached as high as 10.2 L min-1 at the flow rate of 160 L min-1. Isotherm analysis further demonstrated the complex multilayered adsorption behavior of FA. Based on the density functional theory (DFT) simulation, the superiority of DAP/AC for FA adsorption can be attributed to the synergy of covalent (chemisorption) and non-covalent (pore filling and film diffusion) interactions.

Spontaneous Combustion Characteristics of Activated Carbon Modified via Liquid Phase Impregnation during Drying

Spontaneous combustion characteristics are important issues for the safe operation of the wet-modified activated carbon drying process. The spontaneous combustion characteristics of activated carbon modified via liquid phase impregnation were fully investigated in this study. The modified activated carbon was prepared using columnar activated carbon and 4-amino-1,2-butanediol solution. Physical properties and surface functional group analyses were performed for activated carbon before and after modification. The ignition temperature of activated carbon before and after modification was then characterized using the methods of GB/T20450-2006, thermogravimetry-derivative thermogravimetry (TG-DTG), and TG-mass spectrometry (TG-MS). At the same time, the activation energy of activated carbon before and after modification was calculated by using thermodynamic analysis. Furthermore, a new self-designed test platform was introduced to investigate the spontaneous combustion characteristics of wet-modified activated carbon under the drying temperatures of 150, 175, 180, and 210 °C. The results show that the specific surface area of Brunauer, Emmett, and Teller (BET) is decreased by 368 m2·g-1, the total volume of pore size is decreased by 0.17 cm3·g-1, and the content of oxygen-containing functional groups is decreased by 0.071 mmol/g compared with row activated carbon. The ignition temperatures of the sample before modification characterized by the three methods are 483, 596, and 599 °C, respectively. The ignition temperatures of the sample after modification are 489, 607, and 611 °C, respectively. The activation energy of the modified activated carbon is increased by 35 kJ/mol compared to the original activated carbon. It is concluded that the temperature that triggers the modified activated carbon combustion during the drying process is between 175 and 180 °C, and the heat is mainly gathered at the longitudinal center of the combustion chamber through the investigation of spontaneous combustion experiments. The results in this study can contribute to safe production to prevent combustion in the process of modifying activated carbon during the drying process.

Effective Removal of Acetaldehyde Using Piperazine/Nitric Acid Co-Impregnated Bead-Type Activated Carbon

Acetaldehyde (CH₃CHO) in the atmosphere is associated with adverse health effects. Among the various options for use in removing CH₃CHO, adsorption is often employed because of its convenient application and economical processes, particularly when using activated carbon. In previous studies, the surface of activated carbon has been modified with amines to remove CH₃CHO from the atmosphere via adsorption. However, these materials are toxic and can have harmful effects on humans when the modified activated carbon is used in air-purifier filters. Therefore, in this study, a customized bead-type activated carbon (BAC) with surface modification options via amination was evaluated for removing CH₃CHO. Various amounts of non-toxic piperazine or piperazine/nitric acid were used in amination. Chemical and physical analyses of the surface-modified BAC samples were performed using Brunauer-Emmett-Teller measurements, elemental analyses, and Fourier transform infrared and X-ray photoelectron spectroscopy. The chemical structures on the surfaces of the modified BACs were analyzed in detail using X-ray absorption spectroscopy. The amine and carboxylic acid groups on the surfaces of the modified BACs are critical in CH₃CHO adsorption. Notably, piperazine amination decreased the pore size and volume of the modified BAC, but piperazine/nitric acid impregnation maintained the pore size and volume of the modified BAC. In terms of CH₃CHO adsorption, piperazine/nitric acid impregnation resulted in a superior performance, with greater chemical adsorption. The linkages between the amine and carboxylic acid groups may function differently in piperazine amination and piperazine/nitric acid treatment.

Facile Mesoporous Hollow Silica Synthesis for Formaldehyde Adsorption

Formaldehyde emitted from household products is classified as a hazardous substance that can adversely affect human health. Recently, various studies related to adsorption materials for reducing formaldehyde have been widely reported. In this study, mesoporous and mesoporous hollow silicas with amine functional groups introduced were utilized as adsorption materials for formaldehyde. Formaldehyde adsorption characteristics of mesoporous and mesoporous hollow silicas having well-developed pores were compared based on their synthesis methods-with or without a calcination process. Mesoporous hollow silica synthesized through a non-calcination process had the best formaldehyde adsorption characteristics, followed by mesoporous hollow silica synthesized through a calcination process and mesoporous silica. This is because a hollow structure has better adsorption properties than mesoporous silica due to large internal pores. The specific surface area of mesoporous hollow silica synthesized without a calcination process was also higher than that synthesized with a calcination process, leading to a better adsorption performance. This research suggests a facile synthetic method of mesoporous hollow silica and confirms its noticeable potential as a support for the adsorption of harmful gases.