Environ. Sci. Technol. 2006, 40, 1321-1327
Exploring Molecular Sieve Capabilities of Activated Carbon Fibers to Reduce the Impact of NOM Preloading on Trichloroethylene Adsorption TANJU KARANFIL,* SEYED A. DASTGHEIB, AND DINA MAULDIN Department of Environmental Engineering and Science, Clemson University, 342 Computer Court, Anderson, South Carolina 29625
Adsorption of trichloroethylene (TCE) by two activated carbon fibers (ACFs) and two granular activated carbons (GACs) preloaded with hydrophobic and transphilic fractions of natural organic matter (NOM) was examined. ACF10, the most microporous activated carbon used in this study, had over 90% of its pore volume in pores smaller than 10 Å. It also had the highest volume in pores 5-8 Å, which is the optimum pore size region for TCE adsorption, among the four activated carbons. Adsorption of NOM fractions by ACF10 was, in general, negligible. Therefore, ACF10, functioning as a molecular sieve during preloading, exhibited the least NOM uptake for each fraction, and subsequently the highest TCE adsorption. The other three sorbents had wider pore size distributions, including high volumes in pores larger than 10 Å, where NOM molecules can adsorb. As a result, they showed a higher degree of uptake for all NOM fractions, and subsequently lower adsorption capacities for TCE, as compared to ACF10. The results obtained in this study showed that understanding the interplay between the optimum pore size region for the adsorption of target synthetic organic contaminant (SOC) and the pore size region for the adsorption of NOM molecules is important for controlling NOM-SOC competitions. Experiments with different NOM fractions indicated that the degree of NOM loading is important in terms of preloading effects; however the way that the carbon pores are filled and loaded by different NOM fractions can be different and may create an additional negative impact on TCE adsorption.
Introduction Granular activated carbon (GAC) adsorption is widely employed to remove various synthetic organic contaminants (SOCs) from water supplies (1, 2). However, naturally occurring dissolved organic matter (NOM), which is ubiquitous in natural waters, reduces adsorption capacity and kinetics of SOCs by activated carbons (3-26). Pore blockage and site competition are the two mechanisms responsible for this reduction caused by NOM. In a fixed-bed adsorber, the mass-transfer zone (MTZ) of NOM components moves * Corresponding author phone: (864) 656-1005; fax: (864) 6560672; e-mail:
[email protected]. 10.1021/es051285o CCC: $33.50 Published on Web 01/20/2006
2006 American Chemical Society
ahead of the MTZ of the target SOC. As a result, fresh GAC in the lower layers of the adsorber is continuously preloaded or fouled by NOM components prior to SOC adsorption, thereby reducing the operational life and impairing performance of the GAC adsorber for the target SOC. Activated carbon fibers (ACFs) have been developed within the past four decades from various organic precursors (e.g., polyacrylonitrile, cellulose, pitch) (27). ACFs have some superior physicochemical characteristics as compared to conventional activated carbons. The pore size distribution of ACF, which can be controlled during production, is uniform, and the distance of transitional pores from large mesopores at the surface to micropores is short. These characteristics provide important advantages in both liquid and gas-phase applications: (1) By carefully selecting pore size distribution of ACFs with respect to the dimensions of the target compound, it may be possible to use the fibers as molecular sieves. (2) As the pore width approaches the target compound molecular dimensions, multiple contact points between the adsorbate and adsorbent surface become possible, and surface forces overlap. As a result, higher adsorption capacities are obtained. (3) Short transitional pore regions result in increased sorption kinetics for the target compound. Despite these advantages, ACFs are not used today in water treatment applications primarily due to their high cost. However, understanding the capabilities of these materials is important to assess their potential applications, especially with their increasing use in many fields and anticipated decreasing cost of production. In terms of the impact of NOM on SOC adsorption during typical water treatment applications, two major factors are important to consider: (1) optimum micropore region for the target SOC adsorption, and (2) the pore size region for the adsorption of NOM components. The optimum micropore size region for the SOC adsorption depends on the molecular dimensions of the target compound. For example, during single-solute adsorption studies it has been proposed that the important pore size region for adsorption of trichloroethylene (molecular dimensions: 6.6 × 6.2 × 3.6 Å) is 5-8 or 7-9 Å (28, 29), while for atrazine (11.5 × 10.9 × 6.7 Å) and methyl tertiary-butyl ether (MTBE) (7.5 × 6.0 × 6.5 Å) pore size regions of 8-20 and 9-13 Å, respectively, were reported to be important (17, 26) (see Supporting Information for molecular simulations of TCE, atrazine, and MTBE). It should be noted that different “optimum” pore size ranges proposed for adsorption of the same target compound by different research groups probably relates to the variability in obtaining micropore size distribution of a particular carbon (30) and different ways to describe molecular dimensions. Recent studies with microporous ACFs also showed that pores smaller than 10 Å are not accessible for the majority of NOM molecules (17, 26, 31, 32). This is in agreement with the approximate diameters of aquatic NOMs reported in the range of 10 to 17 Å in several studies, as compiled by Moore et al. (33). Therefore, it appears that 10 Å represents an important cutoff value for developing strategies to minimize the impact of NOM on SOC adsorption, which can be classified into two categories, as discussed in the following paragraphs. If the pore size region 50 Å)) in addition to primary micropores (i.e., 10 Å is the optimum region for the SOC adsorption (e.g., atrazine), a molecular sieve approach, as discussed above, will not be effective since both NOM components and SOC can access the same pore region. Therefore, both the pore size distribution and pore volume in the secondary micropores and mesopores will become important to reduce the NOM-SOC competition. For example, Pelekani and Snoeyink (16) identified the 8-20 Å region as the optimum region for atrazine adsorption that needs to be maximized to reduce the NOM-SOC competition. They showed that activated carbons with large secondary micropore volumes were more resistant to preloading effects despite their higher NOM uptakes. One experimental evidence for the proposed categorization above can be seen in a recent study of Quinlivan et al. (26). These researchers examined adsorption of TCE and MTBE by three microporous ACF under simultaneous adsorption conditions. ACF10, the fiber that had the majority of its surface area in pores 4 L/mg-m) indicates hydrophobic nature of DOM (34). Therefore, the third objective of the study was to examine the role of different NOM components in the preloading effects for both activated carbon fibers and granular activated carbons.
Experimental Section Activated Carbons and Their Characterization. Two ACFs (ACF10 and ACF20H) and two GACs (F400 and Macro) were used in this study. Two phenol formaldehyde-based activated carbon fibers (American Kynol, Inc.) are microporous materials with relatively uniform but different pore size distributions. F400 (Calgon Corp.) is a microporous, coalbased, and steam-activated GAC, whereas Macro (Westvaco 1322
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Inc.) is a mesoporous, wood-based, and phosphoric acidactivated carbon. The two virgin GACs were initially heattreated under helium at 900 °C for 2 h (shown by code “He”) because it has been shown that the presence of surface acidity has a detrimental impact on the TCE adsorption due to water cluster formation on the activated carbon surface (29, 35). ACF10 and ACF20H were used without any further treatment. Various characterization methods have been used to determine physical and chemical characteristics of activated carbons. These methods included the following: (i) pHPZC and total HCl and NaOH uptake for determination of activated carbon surface acidity/basicity; (ii) X-ray photoelectron spectroscopy (XPS) for the surface elemental analysis; and (iii) surface area and pore size distribution using nitrogen isotherms. Details of these methods have been described elsewhere (31). NOM. NOM samples were collected from influents of Charleston and Spartanburg drinking water treatment plants in South Carolina using a reverse osmosis (RO) system, as described elsewhere (36). Mass balance calculations showed that the NOM recoveries during RO isolation were over 90%, indicating that most of the NOM from each source was captured. The SUVA254 values of Charleston and Spartanburg samples were 4.4 and 2.7 L/mg-m, respectively. Therefore, the two NOM used in the study had significantly different degrees of aromatic contents. Collected NOMs were fractionated to hydrophobic (HPO, the fraction captured on the XAD-8 resin) and transphilic (TPH, the fraction captured on the XAD-4 resin) fractions according to a resin fractionation method developed by USGS researchers with some modifications (37). The hydrophilic fraction of both waters that passed both resins was not used due to limitation of its available mass for the overall experimental matrix. The HPO and TPH fractions were characterized by SUVA254 and the weight average molecular weight (Mw) obtained from size exclusion chromatography (SEC) measurements as follows: Charleston (HPO): SUVA254 ) 3.5 L/mg-m, Mw ) 1908 Da Charleston (TPH): SUVA254 ) 2.5 L/mg-m, Mw ) 1554 Da Spartanburg (HPO): SUVA254 ) 2.8 L/mg-m, Mw ) 1341 Da
Spartanburg (TPH): SUVA254 ) 1.8 L/mg-m, Mw ) 1202 Da The analytical methods used for DOC, UV254, and SEC measurements are described in detail elsewhere (32, 38). TCE Adsorption Isotherms and NOM Preloading Experiments. Constant-dose aqueous phase isotherm experiments for a wide range of TCE initial concentrations were performed for both single-solute and NOM preloading experiments. For single-solute experiments, 10 mg of activated carbons was equilibrated with various TCE concentrations in 250 mL amber glass bottles (headspace free) for two weeks on a rotary tumbler. For GACs, particles between the U.S. standard sieve sizes of 30 and 40 (i.e., 600 and 425 µm) were used. Preliminary kinetic experiments showed that two weeks time was sufficient to reach equilibrium for both GACs and ACFs. For preloading experiments, 10 mg of activated carbons was initially contacted with 20 mg DOC/L of a NOM fraction in 250 mL bottles for two weeks. A relatively high NOM concentration to activated carbon dose ratio (i.e., 500 mg DOC/g GAC) was used in these experiments to evaluate the preloading resistance potential of different activated carbons. After the two weeks preloading period, predetermined amounts (
