Mechanisms of aromatic molecule - Oxygen-containing functional group interactions on carbonaceous material surfaces

Highlights

• OFGs on different sites of CMs had different effects on aromatic compound adsorption.

• Adsorption was correlated negatively with mesoporous OFGs and positively with external OFGs.

• Aromatic molecular planes were parallel to the graphene sheets of CMs.

• The π-π EDA was the main force for aromatic adsorption on graphene sheets of CMs.

Abstract

Surface oxygen-containing functional groups (OFGs) at different sites of carbonaceous materials showed different effects on the normalized monolayer adsorption capacity (Q_BET/A) obtained from the modified BET model. The OFGs on mesoporous surfaces inhibited the adsorption via the water competition, whereas those on the external surface promoted the adsorption due to the enhanced hydrophobic driving force and electrostatic forces, as analyzed from the adsorption molar free energy. Multiple linear relationships were established between the monolayer adsorption capacity Q_BET/A and the amounts of OFGs on mesoporous and the external surfaces ([O]_meso and [O]_external, respectively). The properties of aromatic adsorbate compounds, the polar area radio of aromatic molecule to water (PA_ad/w), and the log K_ow together influenced the inhibition or promotion effects of OFGs. These results would allow predictions of adsorption behavior of aromatic compounds on carbonaceous materials on the basis of OFGs parameters. Theoretical calculations and simulations projected the configuration of aromatic molecules being parallel to the graphene sheets of carbonaceous materials. The symmetry-adapted perturbation theory (SAPT) energy decomposition showed that the electrostatic forces intensified with the increase of adsorbate polarity. These analyses revealed that the electrostatic forces were enhanced in the presence of OFGs and that the π-π EDA (electron donor-acceptor) was the main force.

Introduction

The mass production and use of carbonaceous materials (CMs, such as activated carbon and carbon nanotubes) inevitably lead to their emission or leakage to the environment. Agricultural activities (such as straw burning) and deliberated or natural forest fires generate CMs (such as biochars) directly in the natural environment (Goldberg, 1985). These manufactured or natural CMs have unique pore structures and pore-size distributions with large surface areas, and have strong abilities to adsorb organic molecules (Apul et al., 2013; Brooks et al., 2012; Chen et al., 2007, 2008a, 2008b; Kong et al., 2011). Their profound influences on the fate, transport and bioavailability of environmental organic pollutants cannot be ignored (Mauter and Elimelech, 2008; Yang and Sheng, 2003). Under different production, synthesis, purification and pyrolysis conditions, oxygen-containing functional groups (OFGs) are inevitably introduced into CMs, and these polar OFGs alter the adsorption of organic compounds on CMs (Wu et al., 2012; Zhu et al., 2005).

The influence of OFGs on the adsorption capacity of CMs is always one of the research focuses. In engineering applications, adsorption capacity affects the removal efficiency of CMs for pollutants. It also affects the re-release and migration of adsorbed pollutants on CMs in the environment. However, up to now, there is no unified understanding of adsorption capacity of CMs. Due to different measurement standards of adsorption capacity, different conclusions may be produced on the adsorption capacity of the same carbon material. In general, there are two different ways to evaluate adsorption capacity: (1) the comparison of adsorbed amounts of compounds under a given liquid phase equilibrium concentration (Chun et al., 2004; Li et al., 2016); (2) based on the fitting models with different adsorption isotherms, the so-called maximum adsorption capacity can be obtained by regression (Jantunen et al., 2010; Wu et al., 2012; Yang et al., 2016b, 2018; Yu et al., 2017) (such as the maximum adsorption capacity Q₀ based on the Langmuir or the DA model, etc.). Both of the two methods have certain limitations. For the former, due to the nonlinearity of adsorption isotherms, the amounts of adsorbed compounds on the same two CMs corresponding to different adsorption equilibrium concentration may have opposite orders of quantity. For the latter, the actual adsorption condition and the preconditions corresponding to the fitting model are not completely consistent, so the maximum adsorption capacity obtained by regression fitting of different models often deviates from the true value. The so-called maximum adsorption capacity is usually a comprehensive reflection of different physical and chemical properties of the adsorbent (such as pore structure and functional groups), and it is not accurate to discuss the influence of OFGs on adsorption capacity solely based on this comprehensive index. The only thing agreed upon was that the effect of surface area on adsorption capacity can be ruled out by surface-area-normalized capacity.

The adsorption capacity obtained by different measurement standards have mostly revealed that the introduction of OFGs inhibited the adsorption of organic compounds due to the competition of water molecules (Li et al., 2002; Wu et al., 2012; Zhu et al., 2005). However, these studies are mostly based on the surface adsorption characteristics of CMs with complex pore structures. A number of studies have shown that the influence of active groups at different sites on adsorption is different (Li et al., 2018; Liu et al., 2020; Simaioforidou et al., 2019; Weinberger et al., 2016). The active groups such as OFGs on the surfaces of mesoporous materials or non-porous materials may have promoting or inhibiting effects on adsorption. Therefore, an important issue in this field is the connection between the adsorption capacity by rational measurement and the precise positions of OFGs on carbon surfaces.

Besides the OFGs of CMs, the physiochemical properties of organic compounds may also play crucial roles in the adsorption. For example, the relative adsorption of polar nitrobenzene with respect to non-polar benzene is promoted by the OFGs on the biochar surface (Chun et al., 2004). The adsorption affinity of organic compounds on carbon nanotubes containing OFGs is linearly related to their solvation parameters, and the water competition effect on more polar compounds is less (Li et al., 2016). The adsorption capacity of organic compounds on biochar obtained under different pyrolysis conditions is affected by their molecular sizes (Yang et al., 2016b, 2018).

In this paper, we describe the adsorption of eight polar or nonpolar aromatic compounds on a series of porous and non-porous CMs with increasing amounts of OFGs. The surface area-normalized monolayer adsorption capacities were analyzed within the framework of the modified BET model applied in aqueous phase (Andreu et al., 2007). The effect of OFGs and their distribution on mesoporous and the external carbon surfaces on adsorption capacity were characterized by multiple linear regressions. Benzene and chlorobenzene were used as probe adsorbates for analysis of molar adsorption free energy influenced by OFGs. Theoretical calculations were conducted to resolve the dominant adsorption interaction and configuration in the presence of OFGs.