Environ. Sci. Technol. 2005, 39, 3990-3998
Adsorption of Single-Ring Organic Compounds to Wood Charcoals Prepared under Different Thermochemical Conditions DONGQIANG ZHU, SEOKJOON KWON, AND JOSEPH J. PIGNATELLO* Department of Soil and Water, Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106, New Haven, Connecticut 06504-1106
Environmental black carbon (BC) is believed to be an important adsorbent of organic pollutants. In this study, we examined the effects of changes in surface properties and adsorbate structure. A series of apolar compounds (cyclohexane, 1,2-dichlorobenzene, 1,4-xylene, 1,2,3,5tetramethylbenzene, 1,3,5-triethylbenzene) and a series of polar compounds (o-cresol, 4-nitrotoluene, 2,4-dinitrotoluene, and 2,4,6-trinitrotoluene) were sorbed from aqueous solution to maple wood char prepared under five thermochemical conditions. Two chars were prepared in air at 340 °C (C340) and 400 °C (C400). A subsample of C400 was treated with H2 in the presence of a supported Pt catalyst at 500 °C (C400-H) to remove surface O. Another was treated under N2 at 500 °C (C400-N) to serve as a control for C400-H. The reduced C400-H was further oxidized in air at 340 °C to reintroduce O (C400-H-A). The five chars vary in O content (26.1, 22.3, 4.2, 20.8, and 18.6 wt %, respectively) but show only minor differences in surface area and pore size distribution on the basis of N2 and CO2 adsorption analysis. These chars provide a basis for rationalizing sorption intensity as a function of sorbate molecular structure and surface chemistry. The following conclusions were drawn. (1) Polar interactions with surface O functional groups are not a significant driving force for adsorption. (2) When isotherms are adjusted for solute hydrophobicity (nhexdecane-water partition coefficient), sorption intensity of the polar compounds is greater than that of the apolar compounds, possibly because of π-π EDA interactions of the polar compounds with the basal plane of the graphene sheets. (3) The largest test compounds show steric exclusion from a portion of the adsorption space available to the other compounds. (4) Removal of O functionality by hydrogenation enhances sorption intensity of polar and apolar compounds, alike by reducing competitive adsorption by water molecules.
Introduction Environmental black carbon (BC) refers to carbonaceous residues of incomplete burning of fossil fuels and biomass (soot and charcoal or char) (1, 2). BC tends to sorb organic compounds more strongly and nonlinearly than other forms of natural organic matter (e.g., humic acids) (3-8), especially * Corresponding author phone: (203)974-8518; fax: (203)974-8502; e-mail:
[email protected]. 3990
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 11, 2005
at low solute concentration, and so may play an important role in the fate of organic pollutants, depending on its abundance in a particular environment. For example, Yang and Sheng (9) showed that enhanced sorption of pesticides in soil can occur from BC formed by the burning of crop residues. Cornelissen et al. (10) found that soot C in sediments at levels of 3.2-20% of total organic C dominated sorption of native phenanthrene at phenanthrene concentrations below about 10-5 of its water solubility. Mechanisms controlling the sorption of organic compounds to BCs at the molecular level are still unclear in many respects. The structure of chars is believed to be related to that of activated carbon (AC), which is manufactured in quantity for water and air purification. As currently understood, this structure consists of short stacks of small graphite-like sheets arranged in a highly disordered fashion to form a poorly interconnected micropore network (1, 2, 11-13). Individual sheets consist of a graphene (polycyclic aromatic) basal plane rimmed with functional groups, the kind and degree of which depend on the thermochemical conditions of char formation. Since chars are ordinarily formed in air, the functionalities are mainly O-containing groups, such as -OH, -CO2H, -O-, dO, -CHO, and the like. These groups potentially can undergo specific or nonspecific physisorption interactions with sorbing molecules. In addition, pH-dependent Coulombic interactions with charged molecules are possible. Naturally occurring BCs vary significantly in carbon source, temperature, and partial pressure of O2 during formation (14); hence, they are likely to vary considerably in O content and distribution of functionality. Although O functionalities of ACs have been found to play an important role in sorption of nonionic compounds, there is no consensus on the underlying molecular mechanisms governing this role. Coughlin and Ezra (15) proposed that O functional groups withdraw π electrons from the graphene surface, thereby decreasing dispersive (van der Waals) interactions of the surface with the adsorbate. Mattson et al. (16) proposed a donor-acceptor interaction (formally, nfπ), where an electron pair of surface carbonyl O acts as donor and the aromatic ring as acceptor. Other investigators hypothesize that O functionality preferentially sorbs water molecules that competitively block sorption of organic solutes (17-24) or sterically prevent their penetration into micropore space (21). Given the structural similarities between BCs and ACs, it is reasonable to hypothesize that the surface chemistry of BCs also plays an important role in sorptive interactions with pollutants. In previous reports (25, 26), we provided evidence for steric exclusion and for π-π electron donor-acceptor (EDA) interactions between nitroaromatic compounds (πacceptors) and the graphene basal plane on a char and on graphite as a model sorbent. In the present study, we sorbed a series of polar and apolar single-ring compounds varying in polarity, polarizability, and molecular size in aqueous solution to wood charcoals prepared under five different thermochemical conditions. We examined changes in surface area, pore size distribution, and surface functionality as possible reasons for the observed difference in sorption among the charcoals. The degree of surface oxidation can vary widely between combustionderived soots on one hand and biomass-derived chars on the other and can vary among chars depending on the concentration of dioxygen present during formation. To compare sorption intensity meaningfully, we normalize for hydrophobic driving forces by factoring solute concentration by the n-hexadecane-water partition coefficient (25, 27). 10.1021/es050129e CCC: $30.25
2005 American Chemical Society Published on Web 04/20/2005
TABLE 1. Elemental Analysis, Total Surface Area (TSA), Cumulative Surface Area (CSA), and Cumulative Pore Volume of Charcoals Prepared under Different Thermochemical Conditions CO2 GCMC analysis % by weight charcoal
preparation conditions
340 °C air, 2 hr 400 °C air, 2 h 400 °C air, 2 h, followed by 500 °C/H2, 3 h C400-N 400 °C air, 2 h, followed by 500 °C/N2, 3 h C400-H-A 400 °C air, 2 h, followed by 500 °C/H2, 3 h, and 340 °C/air, 2 hr C340 C400 C400-H
C
H
O
N
N2 BET N2 NLDFT CSA cumulative pore volume, cm3/g TSA,a CSA
