Environ. Sci. Technol. 1999, 33, 3217-3224
Role of Granular Activated Carbon Surface Chemistry on the Adsorption of Organic Compounds. 1. Priority Pollutants TANJU KARANFIL* Department of Environmental Engineering and Science, Clemson University, 342 Computer Court, Anderson, South Carolina 29625 JAMES E. KILDUFF Department of Environmental and Energy Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180
Uptake of two synthetic organic contaminants (SOCs), trichloroethylene and trichlorobenzene, by one coal-based and one wood-based granular activated carbon (GAC), modified using liquid-phase oxidation (HNO3) and heat treatment in an inert atmosphere (N2), and by several different as-received GACs was compared. Carbons were characterized by elemental analysis, surface area and pore size distribution, water vapor adsorption, acid-base adsorption characteristics measured using the Boehm technique, and a mass titration/pH equilibration method to determine the pHpzc. The results of isotherm experiments with the surface-treated coal- and wood-based carbons indicated that carbon surface acidity played an important role on the adsorption of hydrophobic SOCs. It was found that increasing surface acidity increased the polarity of the surface and reduced adsorption of hydrophobic SOCs by GAC. However, no significant trend was evident for as-received carbons; their behavior differed significantly from surface-treated carbons. The Boehm characterization technique did not appear to be a robust predictor of surface reactivity of as-received carbons toward lowmolecular weight hydrophobic target compounds. However, the Boehm method was useful for correlating the reactivity of carbon surfaces precleaned by acid-washing and heat treatment, and subsequently modified with a single oxidant, as done in this study.
Introduction Granular activated carbon (GAC) adsorption is a versatile technology and particularly suited for removing both regulated synthetic organic chemicals (SOCs) and dissolved naturally occurring organic materials (NOMs) from water and wastewaters. SOCs are generally of concern for reasons relating directly to human health and NOMs are considered to be the major precursor to disinfection byproducts (DBPs) formed during drinking water treatment operations. The United States Environmental Protection Agency (USEPA) has designated GAC adsorption as a “best available technology” for removing both SOCs and NOMs (1-3). Activated carbon is a microporous adsorbent that can be produced from a variety of carbonaceous materials, including * Corresponding author phone: (864) 656-1005; fax: (864) 6560672; e-mail:
[email protected]. 10.1021/es981016g CCC: $18.00 Published on Web 08/12/1999
1999 American Chemical Society
wood, coal, lignin, coconut shells, and sugar. Its unique adsorption properties result from its high-surface area, micropores, and broad range of surface functional groups. The structure of activated carbon is comprised of carbon atoms that are ordered in parallel stacks of hexagonal layers, extensively cross-linked and tetrahedrally bonded. Several heteroatoms, including oxygen, hydrogen, nitrogen, and others, can be found in the carbon matrix, in the form of single atoms and/or functional groups. They are chemically connected to the carbon atoms with unsaturated valences that are located at the edges of graphite basal planes (4-5). Oxygen is the dominant heteroatom in the carbon matrix, and the presence of functional groups, such as carboxyl, carbonyl, phenols, enols, lactones, and quinones, has been postulated (6). Surface functional groups influence adsorption properties and reactivities of activated carbons. Several techniques, including heat treatment, oxidation, amination, and impregnation with various inorganic compounds, are available to modify activated carbons. These modifications may change surface reactivity as well as structural and chemical properties of the carbon, which can be characterized using various methods, as described in detail elsewhere (6). Our current understanding indicates that adsorption of organic compounds by activated carbon is controlled by two major interactions (7-25): First, physical interactions include size exclusion and microporosity effects. Size exclusion may control access of molecules to finer carbon pores where the majority of the surface area for adsorption is located. Its impact is primarily a function of the accessible adsorbent surface area, which is governed by the relative size distributions of the carbon pores and the target molecules. This is an especially important phenomenon for mixtures of organic macromolecules, such as NOMs (as compared to small molecular weight SOCs), and its significance will be discussed in detail in the companion paper (26). Although size exclusion reduces adsorptive uptake of macromolecules, the microporous nature of activated carbons has a positive impact on the adsorption of small molecules. With all other factors being equal, and assuming the adsorbate and the carbon surface are chemically compatible, it is likely that sorption energy is greater in micropores. As the pore width approaches to adsorbate dimensions, multiple contact points on the adsorbent surface become possible and surface forces overlap. Therefore, increasing microporosity is expected to increase the adsorption of low-molecular weight molecules. Second, chemical interactions involve the chemical nature of the surface, the adsorbate, and the solvent. They can be significant for both small and large organic compounds. Hydrophobic interactions relate primarily to the compatibility between the adsorbate and the solvent. In addition to adsorption by nonspecific dispersion forces, adsorbate may specifically interact with the carbon surface, including basal plane electrons, unpaired electrons located on the edges of terminated basal planes, and surface functional groups. Such groups can influence the polarity of the surface and its interaction with the solvent. Furthermore, such sorption mechanisms may be influenced by the composition of background water for ionizable adsorbates; for example, electrostatic interactions can be influenced by pH and ionic strength. Despite voluminous literature on adsorption of organic compounds by activated carbon, there is still much to learn about the mechanisms of chemical interactions occurring on the carbon surfaces. The need for mechanistic information is reflected in the empirical nature of several isotherm models, design tools, and mathematical models. Chemical interacVOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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tions between organic molecules and carbon surfaces can be significant and, in some cases, may overwhelm physical interactions. These interactions are a function of three factors: molecular structure of the target compound, surface chemistry of activated carbon, and solution chemistry. For example, π-π dispersion interactions have been reported to be dominant in the adsorption of aromatic compounds, whereas electrostatic interactions appears to be important for the adsorption of aliphatic anions (13-15). We previously examined adsorption of several well-characterized organic macromolecules by a single GAC and showed that the chemical composition of the surface can significantly impact the uptake (11). In the present work, we undertook a systematic investigation to further explore the role of carbon surface chemistry on the adsorption of two regulated hydrophobic pollutants and several NOMs. A good understanding of the role of GAC surface chemistry on adsorbate uptake is critical in the selection, design, and production of novel sorbents for removal of SOCs and NOMs from drinking water supplies.
Hypothesis and Objectives The hypothesis guiding this study is that the sorption affinity for the surface of an adsorbate depends, in part, on the physical characteristics of the carbon pore structure, the chemical characteristics of activated carbon surfaces and the chemical structure of the sorbate. Surface acidity was the surface property chosen for detailed investigation because it has been implicated as a factor controlling uptake (13-15, 18-25). Increases in surface acidity may reduce the catalytic activity of the surface, increase selectivity for water, modify the charge distribution on graphitic basal planes, change the adsorbate configuration, or produce some combination of these effects. It has been postulated that increasing the number of polar oxygen molecules within the carbon matrix or oxygen-containing surface functional groups increases the polarity of carbon surfaces and, therefore, their selectivity for water. It has been further postulated that adsorbed water clusters may block carbon pores and reduce sorption capacity for hydrophobic compounds (18-19). Several studies have documented that oxidized surfaces exhibit reduced uptake of some SOCs (13, 20-24). However, other studies have suggested enhanced uptake of polar adsorbates by oxidized carbons (25). No study to date has examined the impact of surface oxidation on the uptake of natural organic materials or provided a comparison of the relative impact of surface oxidation on the uptake of NOMs and low-molecular weight hydrophobic compounds. The specific objectives of this project were to: (1) elucidate how carbon pore structure and surface properties influence the uptake of synthetic organic compounds and natural (macromolecular) materials; (2) identify adsorbent properties that optimize the uptake of SOCs and NOMs; (3) evaluate whether a simple surface characterization technique, the Boehm method, can be used as a reliable predictor of the carbon surface reactivity toward SOCs and NOMs. This paper will address the above objectives with respect to the adsorption of two SOCs, trichloroethylene (TCE) and 1,2,4 trichlorobenzene (TCB). It will also include the results of carbon characterization experiments. The impact of surface acidity on the adsorption of NOMs will be examined in a companion paper (26).
Materials and Methods Sorbents. The surfaces of a thermally activated coal-based carbon (Calgon F400) and an acid-activated wood-based carbon (Westvaco WVB) were modified using liquid-phase oxidation (HNO3) and heat treatment in an inert atmosphere (N2). These carbons were chosen for surface treatment 3218
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because of significant differences in their pore structures and raw materials. The goal was to prepare carbons having different surface chemistry (i.e., different type and surface density of acidic surface functional groups), while maintaining the same surface area and pore structure. Therefore, the role of carbon surface chemistry on adsorption could be systematically investigated. Activated carbons were used as received from the manufacturers without any crushing. Initially, carbons were sonicated and rinsed with distilled and deionized water. Subsequently, they were pretreated to remove ash components and alkaline impurities using 2 N HCl in a Soxhlet extractor for 42 h. Acid-washed (AW) carbon samples were heat-treated under a nitrogen atmosphere at 1000 C for 24 h (designated by HT 1000) to remove oxygencontaining functionality. A portion of the heat-treated carbons were oxidized in aqueous solutions of 70% HNO3 at different temperatures and for different reaction times (designated by OX followed by time in hours and temperature C, e.g., OX2/70) to create new surface carboxyl, lactonic, and phenolic groups and thus increase surface acidity. A portion of oxidized carbons were subsequently heat-treated at 650 C for 24 h (designated by HT 650) to remove strongly acidic and other CO2 evolving groups in an attempt to produce carbons with different types of oxygen-containing functional groups, to create different functional group density, or both. Wood is much less resistant to high pressure, temperature, and oxidation conditions than coal material. Therefore, less vigorous treatment techniques were employed for the woodthan the coal-carbon, while attempting to modify the surface chemistry in order to protect the physical integrity of the carbons. A total of 10 carbons with different surface characteristics were prepared. In this research, we also examined a series of carbons as-received from different manufacturers, produced from different raw materials, and activated to different degrees. One group of adsorbents (Calgon) was manufactured from bituminous coal and included the F400, WPLL, FS100, and BPL carbons. A new batch of F400 was used in this phase of the work. A second group of adsorbents was manufactured from wood, and included the MICRO, MESO and MACRO carbons (Westvaco). These are research adsorbents whose names signify their relative pore-size distributions. Surface-treated and as-received carbons were characterized by: (i) elemental analysis; (ii) surface area and pore size distribution; (iii) water vapor adsorption; (iv) acid-base adsorption characteristics, measured by the Boehm technique (27); (v) a mass titration/pH equilibration method to determine the pHpzc, the pH at which the total net surface charge of GAC is zero (28). The Boehm method is a selective neutralization of surface acidic groups of varying strengths using bases that have conjugate acids with a wide range of acid dissociation constants (pKa). It is often assumed that NaHCO3 (pKa ) 6.37) uptake corresponds to strong carboxylic acidity, while Na2CO3 (pKa ) 10.25) further reacts with weak carboxylic and f-lactonic functionality, and NaOH (pKa ) 15.74) further reacts with phenolic acidity (28). For modified carbons, due to the limited amount of sample present, the pHpzc values were predicted using a correlation developed between pHpzc and a number of carboxylic groups present on several activated carbons (29). The details of the experimental protocols for carbon preparation and characterization can be found elsewhere (30-31). Sorbates. TCE and TCB were selected as model SOCs. TCE is a widespread contaminant, designated as a priority pollutant by the USEPA, and it is a regulated compound under the Safe Drinking Water Act. TCB is also a regulated priority pollutant, is more hydrophobic (log Kow ) 4.00) than TCE (log Kow ) 2.42), and adsorbs more strongly. Adsorption Isotherms. Isotherm experiments were conducted using the completely mixed batch reactor (CMBR)
bottle-point method as described in detail elsewhere (32). As-received or surface-treated carbons were equilibrated with TCE or TCB solutions in 250 mL amber glass bottles (headspace free) that were kept well-mixed for a period of three weeks on a rotary tumbler, a time sufficient to reach equilibrium as demonstrated by preliminary rate studies. After the equilibration period, reactors were sampled and analyzed after hexane extraction by gas chromatography using electron-capture detection (Model 5890, Hewlett Packard, Palo Alto, California, USA) that was calibrated by using external standards. Precision for TCE and TCB analysis, including the extraction step, was 3% or better. All isotherms were conducted in the presence of a 0.01 M phosphate buffer, at pH 7, and room temperature of 21 ( 3 °C. Isotherm Modeling. The classical Freundlich isotherm equation, which has been used in many adsorption studies reported in the literature, was selected to model adsorption data for TCE and TCB:
qe ) KFCen where qe is the amount adsorbed at equilibrium, Ce is the equilibrium solution phase concentration, KF is the Freundlich parameter for a heterogeneous adsorbent, and the exponential term, n, is related to the magnitude of the adsorption driving force and to the adsorbent site energy distribution. Linear geometric mean functional regression of the log-transformed experimental data was used to determine the parameters log KF and n. Confidence intervals (95%) were also determined for each parameter based on the regression of log-transformed data. Since isotherm data were collected over a wide range of concentration and displayed some curvature on log-log coordinates, Freundlich model parameters were fit to the low-concentration data (i.e., Ce e 150 µg/L) where the model provided an accurate description of the data. The Freundlich isotherm can be expressed on a mass or surface area basis, and in this study, both forms were used. The use of surface area normalized isotherms was necessary to compare the uptake by surface-treated carbons with those of as-received carbons having different surface areas and pore size distributions. For surface area normalization, it was assumed that the entire BET surface area would be accessible for TCE and TCB adsorption. This assumption was necessary since no detailed information was available for surface area distribution in the micropore regions (pore widths
