Modeling and simulations of the removal of formaldehyde using silver nano-particles attached to granular activated carbon

Adsorptivity and kinetics for low concentration of gaseous formaldehyde on bamboo-based activated carbon loaded with ammonium acetate particles

The highly promising formaldehyde (HCHO)-removing materials are essential for eliminating interior pollution to safeguard the public's health with increasing indoor HCHO contamination situations being recorded on a global scale. In the paper, bamboo charcoal (BC) was activated with boric acid to prepare bamboo-based activated carbon (BAC), and then impregnated with ammonium acetate solution to successfully develop porous adsorbent with ammonium acetate particles (N/BAC), which was applied to remove low concentration of HCHO at room temperature. The adsorption performance for HCHO was systematically investigated while the surface chemical properties and microstructure of the as-prepared adsorbents were described and analyzed. The specific surface area, total pore volume and microporous volume of N/BAC sample were 240.09 m²/g, 0.27 cm³/g and 0.12 cm³/g, which increased by 42.40 m²/g, 0.15 cm³/g and 0.03 cm³/g compared with BAC sample, respectively. The specific surface area and the microporous volume, as well as the content of oxygen- and nitrogen-containing functional groups of N/BAC sample were augmented by contrast with other samples, and numerous ammonium acetate particles were present on the surface. Precisely because of this, the N/BAC sample exhibited a high removal rate of 98.89%, which was 18.38% greater than that of BAC sample. A superior correlation coefficient (0.9999) from the experimental values of the kinetics and the fitted values of the pseudo-second-order kinetic model demonstrated that the adsorption process of HCHO on N/BAC sample was physical-chemical combined adsorption. The adsorption of HCHO on N/BAC sample was investigated under different humidity, and the results showed that the adsorbent yet had excellent adsorption capacity (87.93%) under RH 75%. Moreover, the N/BAC sample was renewable, and the removal rate still reached 82.81% after five cycles of regeneration. Therefore, the as-prepared adsorbent is an effective, economical and sustainable material, and could be used to remove HCHO from real contaminated indoor air.

Thermal-Driven Formation of Silver Clusters Inside Na/Li FAUY Zeolites for Formaldehyde Detection

In this research, the LiY zeolite was firstly synthesized by using NaY as the parent zeolite; thereafter, the LiYAg and NaYAg zeolites created for formaldehyde gas detection were prepared with further Ag⁺-Li⁺/Na⁺ exchange and a mild thermal treatment at 300 °C to promote the formation of luminescent Ag CLs. The spectra experimental results indicated that Ag CLs showed stronger and blue-shifted emissions in LiYAg compared with in NaYAg, and the emission intensity of Ag CLs in both zeolites monotonously decreased when exposed to increasing formaldehyde gas content. Moreover, the linear dependence of the Ag CLs' emission intensity variation on formaldehyde content indicated a reliable method for fast and sensitive formaldehyde detection. According to the XPS, UV-vis absorption, and N₂ adsorption-desorption isotherm studies, the formaldehyde-gas-induced luminescence quenching of Ag CLs is due to the formation of Ag₂O and Ag NPs, in which the higher content of Ag⁺/Ag⁰ redox couples in LiYAg and larger surface area of NaYAg benefit the precise detection of formaldehyde gas in low- and high-content ranges, respectively. Furthermore, the blue-shifted peak position and widened FWHM of Ag CLs can also be used for the indication of formaldehyde gas and the detection limit of NaYAg and LiYAg, which both meet with the standards of the WHO and OSHA.

Efficient removal of formaldehyde by polyethyleneimine modified activated carbon in a fixed bed

Polyethyleneimine modified activated carbon (PEI-AC) was prepared through a treatment of immersion, and used for the adsorption of formaldehyde. The adsorption capacity of formaldehyde by unmodified AC is 190.1 mg g^-1, and the adsorption capacity of formaldehyde can reach to 317.6 mg g^-1 after 10 g L^-1 of PEI modified, being about 1.67 times than unmodified activated carbon (AC: 191.2 mg g^-1). And the 10 g L^-1 of PEI modified AC (PAC-30) has the highest adsorption capacity of formaldehyde, reached to 650 mg g^-1, with an increasing magnitude of 240% in comparison with that without modified AC. This is mainly due to changes in the pore structure and surface functional groups after modification. However, as the PEI concentration increases, the adsorption performance is inhibited. Through kinetic studies, it was found that all adsorption curves follow the second-order kinetics, and the breakthrough curves follow the Boltzmann model, and the adsorption process can also be described by the intraparticle diffusion model.

High-performance materials for effective sorptive removal of formaldehyde in air

As formaldehyde (FA) is well-known for its carcinogenic potential, various techniques for its removal have been developed based on recovery (e.g., adsorption/absorption and condensation) or destructive treatment (e.g., incineration and thermal/ catalytic oxidation). Among them, adsorption has been one of the most preferable options due to its low price and simplicity. In this review, we summarize state-of-the-art knowledge about adsorption mechanisms with respect to its key controlling variables (e.g., surface chemical properties of adsorbent, temperature, and relative humidity) and adsorption performance of materials with particular emphasis on advanced materials (e.g., carbon nanotubes, metal-organic frameworks, graphene oxides, and porous organic polymers) and their modified forms in comparison with conventional sorbents (e.g., AC and zeolite). However, it is yet difficult to assess the adsorption capacity of each material on a parallel basis because adsorption experiments of each material were conducted under different conditions (e.g., large differences in the initial loading concentrations). The partition coefficient (PC) was employed for evaluating adsorption performance between different materials at an equivalent level to overcome the limitation based on adsorption capacity concept. For instance, among the list of the surveyed materials, the highest PC was recorded by γ-CD-MOF-K (31.2 mol kg^-1 Pa^-1). This study should offer valuable insights into the selection and development of outstanding materials for the sorptive removal of FA.

Advanced nanonetwork-structured carbon materials for high-performance formaldehyde capture

Facile design and construction of advanced materials for eliminating the indoor formaldehyde pollution is still a great challenge but very desirable to provide clean air for human life. Herein, we report a high-performance formaldehyde adsorbent, i.e., a new type of nanonetwork-structured carbon (NNSC) with a hollow nanosphere as network unit by developing a facile, efficient and post-treatment-free strategy. The NNSCs can be easily obtained by a simple carbonization of a mixture, in which natural wheat husk and Teflon are used as carbon precursor and biotemplate-in-situ-remover, respectively. The as-constructed NNSC exhibits a unique three-dimensional interconnected micro-, meso- and macroporous nanonetwork. Benefiting from such a valuable hollow nanosphere-interconnected network structure, the NNSCs show surprising formaldehyde gas adsorption properties including super-high storage capacity, ultrafast adsorption rate and efficient adsorptively active surface. Remarkably, their specific adsorption capacity and maximum adsorption rate are as high as 120.3 mg g^-1 m^-3 and 44.6 mg g^-1 m^-3 h^-1, which make 18-fold and 41-fold enhancement when compared to activated carbon commercially used for formaldehyde adsorption, respectively. This work highlights an efficient solution to develop high-performance formaldehyde adsorbents by facile and rational construction of novel porous structure, simultaneously to provide a new avenue to high-value advanced materials for challenging environmental issue.

Characterization and adsorption capacity of potassium permanganate used to modify activated carbon filter media for indoor formaldehyde removal

This study examined the effect of potassium permanganate (KMnO₄)-modified activated carbon for formaldehyde removal under different face velocities and different initial formaldehyde concentrations in building environment. We chose the coconut shell activated carbon due to their high density and purity. Moreover, they have a clear environmental advantage over coal-based carbons, particularly in terms of acidification potential. The chemical properties were characterized by FTIR to show the functional groups, EDS to calculate each component of their energy bands to know how the ratio is. Also, the morphology of the surface was examined with scanning electron microscopy (SEM). The BET determines specific surface area, pore size, and pore volume. It was found that where the initial formaldehyde concentration and the face velocity are low, adsorption capacity is high. The adsorption isotherms of formaldehyde on modified activated carbon are well fitted by both Langmuir and Freundlich equations. The rate parameter for the pseudo-first-order model, pseudo-second-order model, and intraparticle diffusion model was compared. The correlation coefficient of pseudo-second-order kinetic model (0.999 > R² > 0.9548) is higher than the coefficient of pseudo-first-order kinetic model (0.5785 < R² < 0.8755) and intraparticle diffusion model (0.9752 < R² < 0.9898). Thus, pseudo-second-order kinetic model is more apposite to discuss the adsorption kinetic in this test, and the overall rate of the modified activated carbon adsorption process appears to be influenced by more than one step that is both the intraparticle diffusion model and membrane diffusion.

A New Generation of Surface Active Carbon Textiles As Reactive Adsorbents of Indoor Formaldehyde

Highly porous carbon textiles were modified by impregnation with urea, thiourea, dicyandiamide, or penicillin G, followed by heat treatment at 800 °C. This resulted in an incorporation of nitrogen or nitrogen and sulfur heteroatoms in various configurations to the carbon surface. The volume of pores and, especially, ultramicropores was also affected to various extents. The modified textiles were then used as adsorbents of formaldehyde (1 ppmv) in dynamic conditions. The modifications applied significantly improved the adsorptive performance. For the majority of samples, formaldehyde adsorption resulted in a decrease in the volume of ultramicropores. The enhancement in the adsorption was linked not only to the physical adsorption of formaldehyde in small pores but also to its reactivity with sulfonic groups and amines present on the surface. Water on the surface and in challenge gas decreased the adsorptive performance owing to the competition with formaldehyde for polar centers. The results collected show that the S- and N-modified textiles can work as efficient media for indoor formaldehyde removal.

An in-situ thermally regenerated air purifier for indoor formaldehyde removal

Formaldehyde is a common indoor pollutant that is an irritant and has been classified as carcinogen to humans. Adsorption technology is safe and stable and removes formaldehyde efficiently, but its short life span and low adsorption capacity limit its indoor application. To overcome these limitations, we propose an in-situ thermally regenerated air purifier (TRAP) which self-regenerates as needed. This purifier has four working modes: cleaning mode, regeneration mode, exhaust mode, and outdoor air in-take mode, all of which are operated by valve switching. We developed a real-scale TRAP prototype with activated carbon as adsorbent. The experimental testing showed that the regeneration ratios for formaldehyde of TRAP were greater than 90% during 5 cycles of adsorption-regeneration and that through the 5 cycles, there was no damage to the adsorption material as confirmed by scanning electron microscope (SEM) and Brunauer-Emmett-Teller (BET) tests. The total energy consumption by the prototype for purifying 1000 m³ indoor air was 0.26 kWh. This in-situ thermal-regeneration method can recover the purifier's adsorption ability through at least five cycles.

Multifunctional hybrid porous filters with hierarchical structures for simultaneous removal of indoor VOCs, dusts and microorganisms

Air purification often requires multiple layers of filters with different functions to remove various air pollutants, which lead to high pressure drop, high air flow path and frequent filter replacement. In this work, a novel multifunctional Ag@MWCNTs/Al₂O₃ hybrid filter with a depth-type hierarchical structure for simultaneous removal of fine particles, microorganisms and VOCs was designed and fabricated. The novel hybrid air filter showed leading air purification performances to date, achieving 82.24% degradation of formaldehyde at room temperature, 99.99% formaldehyde degradation at 55 °C and complete retention of indoor airborne microorganisms. The complete particle retention rate (100% retention) based on the most penetrating particle size (MPPS, aerodynamic diameter ≦300 nm) of Ag@MWCNTs/Al₂O₃ was achieved at an only 35.60% pressure drop compared with the pristine Al₂O₃ filter, leading to the highest quality factor (Q_f) ever reported. Furthermore, the Ag@MWCNTs/Al₂O₃ hybrid filter showed excellent water repellency (water contact angle of 139.6 ± 2.9°), prolonging the service life of the filters and improving the air purification efficiency. The novel Ag@MWCNTs/Al₂O₃ hybrid filter exhibits remarkable air purification performance in both laboratory synthetic conditions as well as in the "real world" and shows great promise as an effective single replacement for multiple layers of air purifying filters.

Utilization of nSiO2, fly ash, and nSiO 2/fly ash composite for the remediation of triphenyltin (TPT) from contaminated seawater

The removal of triphenyltin chloride from contaminated simulated seawater with adsorption method was discussed. The adsorbents used are fly ash, nSiO2, and nSiO2/fly ash composite. The results showed that the adsorption of the adsorbents increases with increase in the adsorbent dose, contact time, pH, stirring speed, initial TPT concentration, and decreased with increase in temperature. The adsorption fitted well with the Freundlich isotherm, showing that the adsorbent and TPT combined with function groups and the adsorption kinetics followed the pseudo-second-order kinetic model. The thermodynamic parameters were also evaluated. Optimal conditions for the adsorption of TPT from simulated seawater were applied to TPT removal from natural seawater. A higher removal efficiency of TPT (>99%) was obtained for the nSiO2/fly ash composite but not for fly ash and nSiO2.

Nanostructural Features of Silver Nanoparticles Powder Synthesized through Concurrent Formation of the Nanosized Particles of Both Starch and Silver

Green innovative strategy was developed to accomplish silver nanoparticles formation of starch-silver nanoparticles (St-AgNPs) in the powder form. Thus, St-AgNPs were synthesized through concurrent formation of the nanosized particles of both starch and silver. The alkali dissolved starch acts as reducing agent for silver ions and as stabilizing agent for the formed AgNPs. The chemical reduction process occurred in water bath under high-speed homogenizer. After completion of the reaction, the colloidal solution of AgNPs coated with alkali dissolved starch was cooled and precipitated using ethanol. The powder precipitate was collected by centrifugation, then washed, and dried; St-AgNPs powder was characterized using state-of-the-art facilities including UV-vis spectroscopy, Transmission Electron Microscopy (TEM), particle size analyzer (PS), Polydispersity index (PdI), Zeta potential (ZP), XRD, FT-IR, EDX, and TGA. TEM and XRD indicate that the average size of pure AgNPs does not exceed 20 nm with spherical shape and high concentration of AgNPs (30000 ppm). The results obtained from TGA indicates that the higher thermal stability of starch coated AgNPS than that of starch nanoparticles alone. In addition to the data obtained from EDX which reveals the presence of AgNPs and the data obtained from particle size analyzer and zeta potential determination indicate that the good uniformity and the highly stability of St-AgNPs).