Article pubs.acs.org/est
π+−π Interactions between (Hetero)aromatic Amine Cations and the Graphitic Surfaces of Pyrogenic Carbonaceous Materials Feng Xiao and Joseph J. Pignatello* Department of Environmental Sciences, The Connecticut Agricultural Experimental Station, 123 Huntington Street, P.O. Box 1106, New Haven, Connecticut 06504-1106, United States S Supporting Information *
ABSTRACT: Many organic compounds of environmental concern contain amine groups that are positively charged at environmental pH. Here we present evidence that (hetero)aromatic amine cations can act as π acceptors in forming π+−π electron donor−acceptor (EDA) interactions with the π electron-rich, polyaromatic surface of pyrogenic carbonaceous materials (PCMs) (i.e., biochar, black carbon, and graphene). The π+−π EDA interactions combine a cation−π force with a π−π EDA force resulting from charge polarization of the ring’s quadrupole. Adsorption on a biochar and reference adsorbent graphite was conducted of triazine herbicides, substituted anilines, heterocyclic aromatic amines, and other amines whose charge is insulated from the aromatic ring. When normalized for the hydrophobic effect, the adsorption increased with decreasing pH as the amines became ionized, even on graphite that had no significant fixed or variable charge. The cationic π acceptor (quinolinium ion) was competitively displaced more effectively by the π acceptor 2,4-dinitrobenzene than by the π donor naphthalene. The maximum electrostatic potential of organocations computed with density functional theory was found to be a strong predictor of the π+−π EDA interaction. The π+−π EDA interaction was disfavored by electropositive alkyl substituents and by charge delocalization into additional rings. Amines whose charge was insulated from the ring fell far out of the correlation (more positive free energy of adsorption). Identifying and characterizing this novel π+−π EDA interaction on PCMs will help in predicting the fate of organocations in both natural and engineered systems.
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INTRODUCTION Pyrogenic carbonaceous materials (PCMs) such as chars from vegetation fires, charcoal produced for fuel, biochar produced for use as a soil amendment and other purposes,1 atmospheric soot particles,2 and activated carbons, are typically strong adsorbent of organic compounds compared to other materials. Released intentionally or unintentionally into the environment, PCMs can contribute greatly to the total sorption of chemicals in soils and sediments,3−5 and thus alter the bioavailability, leachability and off-site transport of contaminants3−6 and intentionally applied chemicals such as insecticides, fungicides, and herbicides.7,8 Strong adsorbency is designed into the function of activated carbons for their use in water purification9,10 and treatment of polluted sediments and soils.11−13 Many herbicides, veterinary pharmaceuticals, personal care products, endocrine disrupting compounds, dyes, and other compounds of environmental concern are ionic or may become ionized at environmental pH. Compared to neutral compounds,3,5,14−16 the conceptual and quantitative models for interactions of organic ions with PCMs are far less developed.17,18 The conventional conceptual model for ionic organic species holds that they may undergo Coulombic attraction/repulsion at charged sites on the adsorbent in © XXXX American Chemical Society
addition to the weak forces available to uncharged molecules, including London-van der Waals, H-bonding, and the hydrophobic effect (solute exclusion from water). We focus here on identification and quantification of a novel surface interaction of organocations not involving Coulombic attractive/repulsive forces that may contribute to their adsorption by PCMs. An important structural feature of PCMs is the polyaromatic sheet. The fused-ring size, degree of functionalization along the edges, and degree of sheet stacking in a given PCM body vary with heating conditions. An analogy can be drawn between the polyaromatic sheets of PCMs and the hexagonal sp2-carbon (“graphene”) sheets that make up the surface of graphite, graphene materials, and carbon nanotubes. Previous studies have indicated that exposed πelectron rich polyaromatic surfaces of PCMs may interact with π-electron poor solutes, for example, cyano- or nitroaromatic compounds, to form π−π electron donor−acceptor (EDA) complexes.16,19,20 A major contributor to the ground-state energy of the π−π EDA interaction is electrostatic forces Received: September 2, 2014 Revised: December 17, 2014 Accepted: December 24, 2014
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DOI: 10.1021/es5043029 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology Table 1. Relevant Properties of 12 (Hetero-)Aromatic Amines in This Study
a
pH 7−11.5 that is at least two pH units higher than the pKa value; ionic strength, 0.1 M (NaCl). bpH 2 (HCl); ionic strength, 0.1 M (NaCl). Water solubility. dAssumed identical to benzylamine. The values of pKa and water solubility are from Syracuse Research Corporation,39 except for pKa1 and pKa2 of 1,10-phenanthroline, which were cited in Wijnja et al.25 The experimental values of K0ow are from Syracuse Research Corporation,39 except for prometon, pyridine, quinoline, 1,10-phenanthroline, and 1,2,3,4-tetrahydro-1-naphthylamine, which were determined here. c
of cation−π and π−π EDA interactions. This interaction may be termed π+−π EDA.23,24 Wijnja et al.25 observed complexation between mono- or diprotonated 1,10-phenanthroline cations and the polycyclic aromatic hydrocarbon, phenanthrene in water, and proposed that complexation is due to π+−π EDA interactions. At low pH the veterinary antibiotic sulfamethazine exhibited sorption behavior on a biochar consistent with π+−π EDA interactions of the protonated p-aminosulfonamide ring and the polyaromatic surfaces of the biochar.17 To our knowledge, however, no systematic studies of π+−π EDA interactions between aromatic cations and PCMs have been published. One objective of this study was to evaluate π+−π EDA interactions for a set of aromatic and heteroaromatic amines of industrial and agricultural importance with graphitic surfaces
between σ−π quadrupoles of the interacting ring systems.16 The strength of the interaction depends on the difference in their quadrupole moments, which are affected by ring substituents that donate or withdraw electron density, especially through the π system, and the polarizability of the donor π cloud. The strongest π donors are nonpolar polyaromatic rings. If the π acceptor entity is positively charged, it may also undergo cation−π interactions with a π donor entity. Cation-π interactions involve attraction of a cation with the uncharged π system electron cloud. Metal cation-π interactions are well-known,21 and a cation−π interaction between tetraalkyl ammonium cation and polycyclic aromatic hydrocarbons has also been proposed. 22 The interaction of an organocation whose charge is located within the ring or delocalized into the ring can undergo a combination B
DOI: 10.1021/es5043029 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
graphite (see Figure S1 in the Supporting Information (SI) section). The adsorption equilibration time was chosen on the basis of separate kinetic tests. A time of 60 days seemed sufficient for adsorption by biochar, and 48 h was sufficient for adsorption of both the neutral and cationic amines on graphite. While these times do not guarantee complete equilibrium, we believe they come close and that the conclusions would be unchanged by using longer times. Controls without adsorbents showed no significant abiotic loss of these amines during the experiments. After adsorption, the aqueous phase was sampled and microfiltered (0.45 μm nylon filter). Adsorption to the filter was negligible. Concentrations of organic compounds were determined within 2 days by high-performance liquid chromatography (HPLC) using external standards (see SI for details). Adsorption isotherms were fit to the Freundlich model:
through a combination of experiments that assess adsorption− pH profiles and competitive adsorption with other solutes. A second objective was to identify probes for free energy parametrization that can be useful for establishing quantitative structure−property relationships. A set of 12 organic amines was investigated, including three triazine herbicides (ametryn, prometon, and terbutryn), a series of aromatic amines (aniline, N-methylaniline, N,N-dimethylaniline), and a series of heteroaromatic amines (pyridine, quinoline, 1,10-phenanthroline). Triazine herbicides are among the most heavily used herbicides in the U.S., and are frequently detected in streams and groundwater.26,27 The other amines in this set are used mainly as chemical intermediates for a variety of industrial, commercial, and agronomic products, and several of them (e.g., aniline, N,N-dimethylaniline, N-methylaniline, pyridine, and quinoline) have been found in the natural environment.28−33 We also employ two reference amines (benzylamine and 1,2,3,4-tetrahydro-1-naphthylamine) whose charged N is not conjugated with the ring, and two neutral compounds (naphthalene and 2,4-dinitrotoluene) to probe the mechanistic hypothesis through competitive effects. Two PCM adsorbents were selected: biochar and graphite. Biochar is produced from biomass waste of agricultural, forest or urban origin in reactors operating under conditions of low oxygen and moderate temperature (e.g., 300−700 °C). Biochar has attracted attention as a potentially beneficial soil amendment and a means to sequester carbon.1 Graphite is a nonfunctionalized, nonporous carbonaceous material that serves as a useful reference adsorbent because it offers a graphene surface without complications resulting from pore-filling/size exclusion and dipole−dipole interactions.16,20 Single or few-layer graphitic carbons (graphene and carbon nanotubes) are finding applications in many areas of engineering and technology.34,35
Cs = KFCw n
(1)
where Cs and Cw are the adsorbed and aqueous-phase concentrations, respectively; n is the Freundlich exponent providing an indication of isotherm nonlinearity; and KF (mg1−n Ln kg−1) is the Freundlich adsorption coefficient. The parameters were obtained by nonlinear least-squares regression weighted by the dependent variable. The observed (concentration-dependent) distribution ratio Kd (L/kg) is defined as the adsorbed-to-solution concentration ratio:
Kd =
Cs Cw
(2)
KF Cn−1 w .
The Kd is related to KF by Kd = The Kow was used as an index of a compound’s hydrophobicity. The values of K+ow and K0ow for the positively charged and neutral species, respectively, were determined at 20 °C by a U.S. EPA shake-flask method.38 The pH of the aqueous phase was adjusted to 2 with HCl for K+ow values, and to pH 7−11.5 (at least 2 units above the pKa value) for K0owvalues. The ionic strength of the aqueous phase was adjusted to 0.1 M with NaCl in all cases. The pH-dependent value, Kow(pH), was calculated by a speciation model:
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EXPERIMENTAL SECTION The test chemicals were purchased from Sigma-Aldrich or Fisher Scientific at the highest purity available. Relevant properties are listed in Table 1. The K0ow and K+ow are the octanol−water partition coefficient of the neutral and cationic species (chloride as the counterion), respectively. The values of K+ow and some K0ow were determined in this study (see below). Biochar was made anoxically from maple wood shavings at a final heat treatment temperature of 400 °C and time of 2 h, as described previously.36 It contains 74.8% C, 4.0% H, 18.6% O,
