Environ. Sci. Technol. 2005, 39, 2033-2041
Characterization of Aromatic Compound Sorptive Interactions with Black Carbon (Charcoal) Assisted by Graphite as a Model DONGQIANG ZHU AND JOSEPH J. PIGNATELLO* Department of Soil and Water, The Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106, New Haven, Connecticut 06504
Molecular interactions controlling the sorption of pollutants to environmental black carbons (soot, charcoal) are not well-resolved. Sorption of a series of aromatic compounds was studied to wood charcoal and nonporous graphite powder as a model adsorbent. Issues of concern were the possible involvement of π-π electron donor-acceptor (EDA) interactions of electron-poor and electron-rich solutes with the graphene (polycyclic aromatic) surface and size exclusion effects. Sorption of π-acceptors, benzonitrile (BNTL), 4-nitrotoluene (MNT), 2,4-dinitrotoluene (DNT), and 2,4,6trinitrotoluene (TNT), and to a lesser extent π-donor solutes, naphthalene (NAPH) and phenanthrene (PHEN), was greater than predicted by hydrophobic driving forces in accord with their acceptor or donor strength. Hydrophobic effects were estimated using a concentration-dependent free energy relationship between adsorption and partitioning into an inert solvent (n-hexadecane or benzene) for a nondonor/non-acceptor calibration set (benzene and chlorinated and methylated benzenes). Molecular complexation between acceptors and model graphene donors, NAPH, PHEN, and pyrene (PYR), in chloroform and benzene was tracked by ring-current induced upfield shifts in the 1H NMR spectrum and by charge-transfer bands in the UV/ visible spectrum. The EDA component of graphite-water adsorption for the acceptors correlated with the NMRdetermined complexation constant with the model donors in chloroform, which, in turn, correlated with π-acceptor strength (TNT > DNT > MNT > BNTL) and π-donor strength (PYR > PHEN > NAPH). Charcoal-graphite isotherms calculated from charcoal-water and graphite-water isotherms indicated molecular sieving effects on charcoal for tetrasubstituted benzenes (tetramethylbenzenes and TNT) and some trisubstituted benzenes (1,3,5-trichlorobenzene, possibly DNT). When steric effects are taken into account, the order in adsorption among acceptors was qualitatively similar for graphite and charcoal. The results suggest π-π EDA interactions of the acceptorssand possibly donors, although the calibration set may underestimate the hydrophobic effect for fused ring systemsswith both graphite and charcoal surfaces. For graphite, it is postulated that π-acceptors interact with electron-rich regions of * Corresponding author telephone: (203)974-8518; fax: (203)9748502; e-mail:
[email protected]. 10.1021/es0491376 CCC: $30.25 Published on Web 02/18/2005
2005 American Chemical Society
the basal plane near edges and defects and that π-donors interact with electron-depleted regions further away. A similar mechanism may operate on the charcoal but would be modified by the (mostly) electron-withdrawing effects of O functionality on the edges of graphene sheets.
Introduction Environmental black carbon (BC) refers to products of incomplete combustion (soot and charcoal, or char) of fossil fuels and biomass that enter the environment (1, 2). BC particles tend to sorb hydrophobic organic compounds more strongly on an organic carbon basis than macromolecular forms of natural organic matter (e.g., humic substances) (35) and thus may play an important role in the fate of pollutants in soils and aquatic sediments depending on their abundance in a given locale. BC particles present in aerosols pose a threat to human health when inhaled, possibly due to the presence of adsorbed compounds such as polycyclic aromatic hydrocarbons (PAHs) and other toxic compounds (6). This study focuses on adsorption of aromatic compounds to wood charcoal. The structure of BC (1, 2, 7-9) consists primarily of short stacks of graphene (polycyclic aromatic) sheets arranged in a highly disordered fashion to create a microporous network. The sheets are composed of several to several tens of fused rings and are oxidized along the edges. Molecular interactions controlling the sorption of organic compounds to BC particles are incompletely understood. Exactly how aromatic compounds interact with the graphene planes and edge functionality is not clear. In addition, since most of the surface area of BCs exists in pores DNT > MNT ∼ BNTL. These results indicate that specific π-π EDA interactions may occur between the π-acceptors and the graphite surface. Equation 9 can be rearranged to solve for the π-π EDA force: e,EDA e,EDA Ggr,i - aS(q)GS,i ) -RT[ln Kgr-W,i(q) - aS(q) ln KSW,i - bS(q)] (13) e,EDA e,EDA A range of values for -[Ggr,i - aS(q)GS,i ] is obtained depending on concentration. For benzene the range is (in kJ
FIGURE 2. Values of aS, bS, and R2 from regression using equation ln Kgr-W,i(q) ) aS(q) ln KSW,i + bS(q) on 9 compounds of benzene and chlorinated/methylated benzenes (set 3), where Kgr-W,i is apparent distribution coefficient for sorption of compound i to graphite, and KSW,i is the reference solvent-water partition coefficient. (a) Benzene. (b) n-Hexadecane. Standard deviations of selected data points for aS and bS are shown. The regressions for data sets 1 and 2 are as follows: aB ) 0.009 ln q + 1.68; bB ) -1.07 ln q - 8.56 (set 1: BEN + methylbenzenes) aB ) -0.016 ln q + 1.66; bB ) -0.916 ln q - 8.33 (set 2: BEN + chlorobenzenes) aH ) 0.006 ln q + 1.42; bH ) -1.03 ln q - 4.92 (set 1: BEN + methylbenzenes) aH ) -0.010 ln q + 1.25; bH ) -0.972 ln q - 4.07 (set 2: BEN + chlorobenzenes) mol-1) 6.9-7.9 for BNTL, 7.2-8.1 for MNT, 10-11 for DNT, and 13-20 for TNT. For hexadecane it is 11-12 for BNTL, 12-13 for MNT, 19-20 for DNT, and 20-26 for TNT. (The estimated uncertainly is (2-3 kJ/mol based on standard deviations of aS and bS and an assumed 10% error in Kd.) NAPH and PHEN are also enhanced with respect to the HEI in each case, more so when S ) H (Figure 3). This indicates the possibility that PAHs interact in other way(s) with the graphene surface besides dispersion forces modeled by the methyl- and chloro-substituted benzenes. The range e,EDA e,EDA of -[Ggr,i - aS(q)GS,i ] (kJ mol-1) (or equivalent in some other driving force) is 0.6-2.6 for NAPH and 1.2-6.1 for PHEN when benzene is the reference solvent; it is 3.9-5.8 for NAPH and 6.4-11 for PHEN when n-hexadecane is the reference solvent. Sorption to Charcoal. Charcoal-water sorption isotherms are shown in Figure 4a,b. As has been found in other studies of char materials (e.g., refs 30 and 36), neither the Freundlich nor the Langmuir model fits well to the isotherms. While MNT, DNT, and BNTL show the strongest sorption, the order in adsorption intensity is difficult to interpret without considering solvophobic effects and, in this case, steric effects. Sorption to char shows molecular sieving effects absent for nonporous graphite. This is because contrary to expectation based on molecular size and values of KHW,i and KBW,i: (i) 1,2,3,5-TeMB and 1,2,4,5-TeMB sorb weaker than 1,2-XYL and 1,2,4-TMB; (ii) 1,3,5-TCB and 1,2,4,5-TeCB sorb weaker than 1,2,4-TCB; (iii) 1,2,4-TMB and 1,4-XY sorb about equally; and (iv) 1,3,5-TCB sorbs weaker than 1,2,4-TCB.
FIGURE 3. Sorption isotherms on graphite normalized for hydrophobic effects. C′S is defined in text. (a) Benzene as reference solvent (CB′ ). (b) n-Hexadecane as reference solvent (C′H). The solid line is the hydrophobic effects isotherm (HEI) defined in text. For each graph, compounds in upper box correspond to set 3 used to calibrate hydrophobic effects.
FIGURE 4. Sorption isotherms for all compounds on charcoal. Abbreviations are given in Table 1. The same approach for normalizing hydrophobic effects in sorption to graphite may be used for charcoal. However, since we could not apply a universal sorption model, we neglected the concentration dependence of aS and bS. Instead, aS and bS were obtained by regression of pooled q-CW points to find the best fitting plane, defined by ln q ) ln CW + aS ln KSW,i + bS. Considering only those compounds for which there is no obvious steric effect (BEN, TOL, 1,4-XYL, and 1,2-DCB) yields: aB ) 1.00 ( 0.08 and bB ) 1.2 ( 0.2; aH ) 1.00 ( 0.08 and bH ) 1.1 ( 0.2. The normalized isotherms (q vs C′S) are VOL. 39, NO. 7, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Charcoal-graphite sorption isotherms on a unit surface area basis. The three solid lines represent qch ) 0.33qgr, qch ) qgr, and qch ) 3qgr.
FIGURE 5. Sorption isotherms on charcoal normalized for hydrophobic effects. (a) Benzene as reference solvent (C′B). (b) nHexadecane as reference solvent (C′H). C′B and C′H are defined in text. For each graph, compounds in upper box correspond to the set used to calibrate hydrophobic effects, and these compounds are represented by open symbols. plotted in Figure 5a,b. The theoretical hydrophobic effect isotherm (HEI) is not calculated explicitly, but it can be considered to lie in the band occupied by all white symbols. In graphs not shown: (i) adding the bulkier 1,2,4-TMB and 1,2,4-TCB to the hydrophobic effects set does not change a and b values much (aB ) 1.00 ( 0.06 and bB ) 1.2 ( 0.2; aH ) 1.00 ( 0.06 and bH ) 0.9 ( 0.1); (ii) including BEN plus all methylated and chlorinated benzenes in the set results in very poor convergence. Benzene and n-hexadecane serve equally well as a reference partition solvent. In both cases, values of aS are smaller for char than for graphite. Although unlikely in view of Sander and Pignatello (36), it is possible that the set of compounds used for calculating aS may itself be subject to systematic steric effects, which would partially offset hydrophobic effects. Figure 5 reveals several trends: (i) Tetra-substituted benzenes (1,2,3,5-TeMB, 1,2,4,5TeMB, 1,2,4,5-TCB) and trisubstituted benzenes (i.e., 1,3,5TCB) in which the substituents straddle the c2 axis are displaced below the HEI band, consistent with size-exclusion. 1,2,4-TCB and 1,2,4-TMB show borderline behavior. (ii) Sorption of π-acceptors (BNTL, MNT, DNT, and TNT) is enhanced relative to the HEI band. Unlike graphite, there is no correlation of enhancement with π-acceptor ability. This can be explained by steric effects. It can be inferred that without steric effects TNT, and to a lesser extent DNT, would show stronger sorption than observed. (iii) The NAPH isotherm is displaced slightly upward of the HEI. This leads to an ambiguous interpretation. Either NAPH undergoes a special interaction with the surface (potentially including π-π) that is partially canceled by size exclusion, or NAPH is subject only to hydrophobic effects and behaves sterically like a disubstituted benzene. The isotherm of PHEN was not constructed on char. Figure 6 compares adsorption to charcoal and graphite on a unit surface area basis. For each experimental adsorbed concentration of a given solute on charcoal (qch), the corresponding adsorbed concentration on graphite (qgr was calculated using the Freundlich parameters; Table 1). Lines 2038
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representing charcoal-graphite distribution coefficients Kch-gr () qch/qgr) of 0.33, 1.0, and 3.0 are displayed. It is seen that the bulky, tetra-substituted benzenes, regardless of EDA ability (1,2,3,5-TeMB, 1,2,4,5-TeMB, and TNT) exhibit the lowest Kch-gr values ( NAPH > PHEN) character.
Acknowledgments This project was funded by a grant (99-35107-7816) from the NRICGP (CSREES, U.S. Department of Agriculture).
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Supporting Information Available Tables listing the values of chemical shifts and equilbrium constants for association of acceptors with model donor compounds in chloroform and benzene and chemical shifts of different control (i.e., non-donor/acceptor) compounds in the same solvents; figures showing the chemical shifts of acceptors as a function of donor concentration in chloroform, and a figure showing sorbed concentration on graphite vs normalized concentration in organic reference solvents assuming aB and aH are 1. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review June 8, 2004. Revised manuscript received November 29, 2004. Accepted January 5, 2005. ES0491376
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