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