On the capture of ultralow-level benzene in indoor environments: Experiments, modeling and molecular simulation

https://doi.org/10.1016/j.seppur.2020.117306Get rights and content

Highlights

  • High-resolution equilibrium data of benzene adsorption is analyzed with molecular simulation results.

  • CMK-3 has a greater specific adsorptive capacity than ACF at ultralow concentrations.

  • The fundamental differences between benzene and water in terms of adsorption are revealed.

  • Keeping capillary condensation of water from occurring in adsorbents is critical for real application.

Abstract

Benzene is one of the major indoor air pollutants, according to the World Health Organization (WHO), and understanding its adsorption and the difference between the adsorption behavior of benzene and water are essential in developing high-performance, water-resistant adsorbents. This paper presents a comprehensive study of benzene adsorption in carbon nano-porous materials by combining high-resolution measurement, modeling and molecular simulation. High-resolution isotherms and isosteric heats of benzene adsorption in two materials, microporous activated carbon fiber (ACF) and micro-mesoporous CMK-3 were provided. It is found that CMK-3 has higher specific adsorption capacity than ACF at very low pressures (<10−3 P/P0), making it a promising adsorbent for benzene capture in indoor environments. The Dubinin – Radushkevich model were used to fit the isotherms over the low-pressure regions. Furthermore, the experimental isotherm and the isosteric heat were analyzed with molecular simulation results, and the emphasis is placed on the behavior at ultralow pressures of benzene, typically found in indoor environments. Suggestion on the evaluation and management of adsorbents in terms of minimizing the impacts of ambient moisture were provided.

Introduction

Benzene is one of most well-known indoor air pollutants, and human exposure to an environment of benzene concentration greater than 50 ppm triggers neurological symptoms such as headache, asthenia, and nausea [1]. However, such high concentration of benzene is rare under normal conditions, and it is the long-term exposure to low-level benzene in indoor environment [2] that is more concerning due to its numerous sources and genotoxicity (i.e., there is not a level of exposure that does not invoke a carcinogenic response). While outdoor benzene mainly originates from the traffic, the indoor sources are more abundant, including: (1) building materials and furniture [3] and (2) human activities, such as using consumer products, cooking with fossil fuel and smoking [4], [5]. Benzene, like formaldehyde, is classified by the International Agency Research on Cancer (IARC) as a known human carcinogen (Group I) [6]. Due to its genotoxicity, no safe level of exposure for benzene can be recommended according to the World Health Organization (WHO) [7], and therefore from the practical standpoint benzene concentration in indoor environments should be kept as low as possible. This is different from the practice with formaldehyde, where the 0.1 mg/m3 (ca. 80 ppb, 30-minute average) guideline is considered to prevent short-term irritations and long-term carcinogenic effects [7]. As a result, benzene is one of the major contributors to the volatile organic compounds (VOCs) that pose high cancer risk (>10−6) across the world [8], [9], [10].

Adsorption by carbonaceous adsorbents is one of the most widely used technologies to combat gaseous pollutants [11]. Benzene adsorption on various carbonaceous materials, including activated carbon [12], [13], [14], ordered mesoporous carbon (OMC) [15] and microporous biocarbon [16], [17] have been reported in recent years. However, none of those studies have reported high-resolution isotherms and adsorption heats at very low pressures. The isotherm provides the equilibrium adsorptive capacity for a given pressure (or concentration) and an adsorbent exhibiting larger capacity at high concentrations does not necessarily have larger capacity at low concentrations, compared to the other adsorbents [18], [19]. Therefore, evaluation of the isotherm over the ultralow pressure region is necessary for adsorbents designed for indoor air purification where the concentrations of gaseous pollutants are generally very low. Furthermore, the lack of equilibrium data at low concentrations is the one the major obstacles for improving the breakthrough modeling of gaseous filters [20]. The heat of adsorption is another important variable because it is the energy required for the regeneration of adsorbents [21]; Sidheswaran et al. demonstrated that using activated carbon fiber (ACF) filters with a cyclic regeneration process in the HVAC system could save 35–50% of the energy required for air conditioning by cutting the ventilation rate [22].

Ambient moisture plays a major role in the failure indoor air filters. Developing water-resistant adsorbents for indoor air pollutants demands a profound understanding the fundamental differences between the mechanisms of benzene and water adsorption in porous carbon. To this end, molecular simulation is a powerful tool compared with the conventional modeling. In this paper, we aim to provide a better understanding of how benzene is captured in carbon nanopore, particularly at ambient concentrations, and how benzene differs from water in their adsorption mechanism by combining experiments, modeling and molecular simulation.

Section snippets

Materials

The microporous activated carbon fiber (ACF) and the ordered mesoporous CMK-3 were selected as the adsorbents. Nitrogen adsorption at 77.6 K was conducted with the 3Flex Physisorption apparatus (Micrometrics Instrument Corporation, USA) for characterizing the textural properties.

Isotherm

The high-resolution adsorption isotherms of benzene at 298 K and 288 K were measured on the same 3Flex instrument.

Isosteric heat

The Clausius–Clapeyron (CC) equation was used to calculate the isosteric heat at a given loading N, based

Characterization

Fig. 2a shows the isotherms of nitrogen adsorption at 77.6 K for CMK-3 and ACF. The International Union of Pure and Applied Chemistry (IUPAC) has identified six types of isotherms of gas/solid adsorption, each of which broadly describes how adsorption occurs as a function of pressure [38]. CMK-3 has the Type IV isotherm with an H1 hysteresis loop, which is indicative of the presence of mesopores. This is confirmed by the pore size distribution (PSD) shown in Fig. 2b, obtained with the non-local

Conclusions

We presented high-resolution isotherms and isosteric heats of benzene with a special focus on the low-pressure region for two materials: ACF and CMK-3. The micro-/mesoporous CMK-3 has higher specific adsorption capacity than the microporous ACF at <10−3 P/P0, suggesting CMK-3 a promising adsorbent for benzene capture in ambient/indoor environment.

From the analysis of the isosteric heats obtained from experiments and molecular simulation, we have shown that benzene is not sensitive to the

Acknowledgements

The research was financially supported by The National Key Research and Development Program of China, “The Study of Formation and Control of Atmospheric Pollution” (No. 2017YFC0211500).

References (47)

  • K. Vikrant et al.

    Adsorption performance of standard biochar materials against volatile organic compounds in air: A case study using benzene and methyl ethyl ketone

    Chem. Eng. J.

    (2020)
  • J.E. Szulejko et al.

    Seeking the most powerful and practical real-world sorbents for gaseous benzene as a representative volatile organic compound based on performance metrics

    Sep. Purif. Technol.

    (2019)
  • J.E. Szulejko et al.

    Is the maximum adsorption capacity obtained at high VOC pressures (>1000 Pa) really meaningful in real-world applications for the sorptive removal of VOCs under ambient conditions (<1 Pa)?

    Sep. Purif. Technol.

    (2019)
  • M.A. Sidheswaran et al.

    Energy efficient indoor VOC air cleaning with activated carbon fiber (ACF) filters

    Build. Environ.

    (2012)
  • C.D. Wick et al.

    Temperature effects on the retention of n-alkanes and arenes in helium–squalane gas–liquid chromatography: experiment and molecular simulation

    J. Chromatogr. A

    (2002)
  • L. Liu et al.

    Development of averaged solid–fluid potential energies for layers and solids of various geometries and dimensionality

    Adsorption

    (2018)
  • D. Pacilé et al.

    Electronic properties and atomic structure of graphene oxide membranes

    Carbon

    (2011)
  • L. Liu et al.

    On the mechanism of water adsorption in carbon micropores – a molecular simulation study

    Chem. Eng. J.

    (2019)
  • L. Liu et al.

    Formaldehyde adsorption in carbon nanopores – new insights from molecular simulation

    Chem. Eng. J.

    (2019)
  • V.K. Saini et al.

    How the adsorption properties get changed when going from SBA-15 to its CMK-3 carbon replica

    Sep. Purif. Technol.

    (2010)
  • L. Liu et al.

    Water adsorption on carbon – a review

    Adv. Colloid Interface Sci.

    (2017)
  • X. Li et al.

    Hydrophobic modified activated carbon using PDMS for the adsorption of VOCs in humid condition

    Sep. Purif. Technol.

    (2020)
  • M. Nakamura et al.

    Equilibration-time and pore-width dependent hysteresis of water adsorption isotherm on hydrophobic microporous carbons

    Carbon

    (2010)
  • Cited by (11)

    • New Insights into the Capture of Low-level Gaseous Pollutants in Indoor Environment by Carbonaceous Materials: Effects of Functional Groups, Pore Size, and Presence of Moist

      2022, Separation and Purification Technology
      Citation Excerpt :

      It requires a profound understanding of how water and the pollutants adsorb at the macroscopic level (e.g., isotherms, isosteric heat) and more so at the microscopic level (compressibility and density distributions). The isotherm of benzene adsorption in carbon is of Type I while that of water is of Type V, as they reflect two distinct mechanisms with benzene adsorption on the graphene surface via van der Waals (vdW) interactions while water on the FGs via strong electrostatic interactions (hydrogen bonding because of the small size of hydrogen atoms, allowing the closer approach of partial charges of opposite signs) [40,41]. Polar pollutants such as formaldehyde and ammonia give isotherms that are intermediate between that of benzene and that of water because they are attracted to both FGs and graphene surfaces, for which the extent of these attractions depend on the relative strengths between the vdW interaction and the electrostatic interaction [42].

    • In-situ activated ultramicroporous carbon materials derived from waste biomass for CO<inf>2</inf> capture and benzene adsorption

      2022, Biomass and Bioenergy
      Citation Excerpt :

      Benzene is one of the most common indoor environmental pollutants. Long-term exposure to an environment with a benzene concentration of more than 50 ppm can cause neurological symptoms such as nausea, asthenia, headache and asthenia [57–60]. As shown in Fig. 10a, the adsorption isotherm of MPCs to benzene is type I, indicating that their adsorption mechanism is filled with micropores [26], because the pore size of MPC is mainly concentrated around 0.6 nm.

    • Thermocatalytic oxidation of gaseous benzene by a titanium dioxide supported platinum catalyst

      2022, Chemical Engineering Journal
      Citation Excerpt :

      A good number of VOCs have been identified to actively participate in atmospheric photochemical reactions (e.g., the formation of hazardous secondary aerosols and smog) and in the depletion of the ozone layer [4,5]. Among VOCs, aromatic hydrocarbons (e.g., benzene) are ubiquitous in the indoor as well as outdoor air [1,6]. Benzene is of particular interest as it is emitted in large quantities from diverse sources (e.g., petroleum refining, organic chemical production, textile dying/printing, leather manufacturing, pharmaceutical production, pesticide manufacturing, adhesive production, ink manufacturing, automobile spray coating, and electronic equipment manufacturing) [3,7,8].

    View all citing articles on Scopus
    View full text