Relative importance of multiple mechanisms in sorption of organic compounds by multiwalled carbon nanotubes

Abstract

Sorption of hydrophobic organic compounds (HOCs) (phenanthrene, lindane and atrazine) by original and OH-functionalized multiwalled carbon nanotubes (F-MWCNTs) was examined. Functionalization of MWCNTs with hydrophilic moieties greatly reduced their ability to sorb HOCs. The surface area (SA) and sum of meso- and macropore volumes of MWCNTs were governing characteristics that influenced their affinity and capacity for sorption of HOCs. Molecular size of HOCs markedly influenced their volume sorption capacity (Q₀) by the original MWCNTs, and the differences in Q₀ values between various HOCs were less for F-MWCNTs, than for the original MWCNTs. The introduced hydroxyl groups may have reduced accessibility of a large portion of sites in meso- and macropores of MWCNTs that were originally available for smaller HOCs, but not for those with larger molecular size. Sorption of HOCs by MWCNTs was dominantly controlled by hydrophobic interaction regardless of their chemical structure. K_ow-normalized sorption coefficients (K_d/K_ow) of the tested compounds for all MWCNTs followed the order: atrazine > phenanthrene > lindane, implying that π–π interactions enhanced sorption of aromatics by MWCNTs. The enhancement was more pronounced for chemicals with activating groups and larger molecular size relative to those with smaller molecular size and without any substituent.

Introduction

Since their discovery, carbon nanotubes (CNTs) have been proposed for numerous potential applications because of their outstanding physiochemical properties. With increasing industrial production and use of CNTs, a large quantity of them will inevitably be released into the environment. Due to the large surface area (SA) of CNTs, they have been proposed as a promising adsorbent to remove hydrophobic organic compounds (HOCs) from the environment [1]. Since sorption is a critical factor that affects transport, fate, bioavailability and persistence of HOCs in the environment, a better understanding of the underlying interaction mechanisms between HOCs and CNTs is of importance for probing environmental applications of CNTs and the environmental behaviors of both CNTs and HOCs.

It has been documented that hydrophobicity of HOCs is an important factor affecting their sorption by CNTs [2]. A recent study, however, reported that although nitrobenzene was much less hydrophobic than benzene, toluene, and chlorobenzene, it had much higher sorption by multiwalled CNTs (MWCNTs) [3]. Another review reports the inability to establish a unifying relationship between K_d values of HOCs by CNTs and their K_ow[4]. Furthermore, the hexadecane-water distribution coefficient (K_HW)-normalized K_d values of various HOCs by CNTs differed over three orders of magnitude [5]. Other studies indicate the important roles of π–π interactions between aromatics and graphene sheets of CNTs or black carbons in HOC sorption [6], [7], [8], [9]. The role of hydrogen bonding in sorption of polar compounds by CNTs and activated carbons was also emphasized in previous studies [10], [11]. However, up to now, direct evidence for the relative importance of various mechanisms in HOC sorption by CNTs is scarce. As reported, molecular size of HOCs greatly influenced their sorption by black and activated carbons primarily due to the molecular sieve effect [12], but the relationship between molecular size of HOCs and their sorption by CNTs is largely unclear.

Functionalization of CNTs can happen by incidental exposure to strong oxidizing agents (e.g., O₃ or OH radicals) after they enter the environment [13], [14]. Also, introduction of hydrophilic moieties to surfaces of CNTs broadens their usefulness and number of potential practical applications due to dispersion enhancement. However, the introduced polar functionalities reduced accessibility of HOC molecules to CNT surfaces, thus decreasing their sorption [15]. In another study, the introduced O-containing moieties increased sorption of p-xylene by CNTs, but decreased o-xylene sorption by CNTs [16]. These inconsistent findings showed that impact of CNT functionalization on its sorption for HOCs remains unclear.

The key objectives of this work were to: (1) provide direct evidence for the relative importance of multiple mechanisms in HOC sorption by CNTs; (2) examine the influences of introduced hydrophilic moieties on CNT sorption of HOCs; and (3) probe the roles of molecular size and specific interactions (e.g., hydrogen bonding) of HOCs in their sorption by CNTs. To achieve our aims, sorption of HOCs with strikingly different physicochemical properties by original and F-MWCNTs was examined. Results of this work will help systematically understand the underlying mechanisms controlling sorption of HOCs by CNTs.