Effects of activated carbon characteristics on the simultaneous adsorption of aqueous organic micropollutants and natural organic matter
Introduction
Activated carbon is frequently used for the removal of organic micropollutants from drinking water. However, selecting an effective activated carbon for a given treatment objective remains a challenge because the combined effects of physical and chemical adsorbent characteristics on the adsorption of trace organic compounds in the presence of natural organic matter (NOM) are not well understood. NOM adsorption that precedes the adsorption of micropollutants (“NOM preloading”) (e.g., Carter and Weber, 1994; Kilduff et al., 1998; Knappe et al., 1999; Speth, 1991; Summers et al., 1989) or that occurs concurrently with the adsorption of micropollutants (e.g., Gillogly et al., 1998; Knappe et al., 1998; Najm et al., 1991; Newcombe et al., 1997, Newcombe et al., 2002) lowers the micropollutant adsorption capacity of activated carbon relative to that measured in the absence of NOM.
When micropollutants adsorb concurrently with NOM, as is typical in adsorption processes involving powdered activated carbon (PAC), the magnitude of the decrease in adsorption capacity is dependent on the initial concentration of the micropollutant relative to that of the competing NOM (Knappe et al., 1998; Najm et al., 1991). The initial concentration effect is particularly important in drinking water treatment applications, where the concentration of NOM (mg/L level) is about 3–6 orders of magnitude larger than the concentration of most micropollutants (μg/L level for many pesticides and chlorinated hydrocarbons to ng/L level for many taste-and-odor-causing compounds and endocrine disruptors). The nature of the NOM (molecular size distribution, chemical composition) also affects the extent of competition (Kilduff et al., 1998; Newcombe et al., 1997, Newcombe et al., 2002; Bernazeau et al., 1996). When NOM and micropollutants adsorb concurrently, NOM constituents with molecular sizes similar to that of the targeted micropollutant cause the largest decrease in adsorption capacity by competing directly with the micropollutant for adsorption sites (Newcombe et al., 1997, Newcombe et al., 2002; Li et al., 2003a). In contrast, NOM constituents of larger molecular size have little effect on the adsorption capacity of small organic micropollutants (Newcombe et al., 1997, Newcombe et al., 2002; Li et al., 2003a). Similar observations with respect to NOM molecular size have been made for activated carbons preloaded with NOM (Kilduff et al., 1998; Li et al., 2003a, Li et al., 2003b). It should be noted, however, that larger NOM constituents, when preloaded on activated carbons, can greatly decrease the rate of micropollutant adsorption (Li et al., 2003a, Li et al., 2003b).
In terms of physical activated carbon characteristics, NOM effects on micropollutant adsorption capacity are more severe for activated carbons that primarily possess small micropores (∼<10 Å in width) than for activated carbons that have a broader pore size distribution that includes larger micropores and small mesopores (Newcombe et al., 2002; Li et al., 2003a, Li et al., 2003b; Pelekani and Snoeyink, 1999, Pelekani and Snoeyink, 2000, Pelekani and Snoeyink, 2001; Ebie et al., 2001). While a broader pore size distribution tends to lessen the NOM impact in terms of a percent decrease from the single-solute micropollutant adsorption capacity, it does not necessarily mean that the carbon with the broader pore size distribution has the higher micropollutant adsorption capacity in the presence of NOM. For example, Newcombe et al. (2002) showed that the percent decrease in 2-methylisoborneol (MIB) adsorption capacity was largest for two coconut-shell-based activated carbons while it was smallest for a chemically activated wood-based activated carbon. Nonetheless, the MIB adsorption capacity, which was primarily related to the volume of pores with widths between 10 and 12 Å, was lowest for the wood-based carbon because its pore volume in the 10–12 Å range was lower than that of the coconut-shell-based activated carbons (Newcombe et al., 2002).
In terms of chemical activated carbon characteristics, no studies to date have systematically evaluated adsorbent surface chemistry effects on the simultaneous adsorption of micropollutants and NOM. However, Kilduff et al. (2002) evaluated the effects of adsorbent surface chemistry on TCE adsorption capacity for activated carbons that were preloaded with NOM. The percent decrease in TCE adsorption capacity resulting from NOM preloading became smaller with increasing surface acidity of coal-based activated carbons. However, while the effect of NOM preloading became smaller, the single-solute TCE adsorption capacity also decreased with increasing surface acidity, and the latter effect dominated over the former. Therefore, the net result was that the most hydrophobic adsorbent, i.e. the activated carbon with the lowest surface acidity, exhibited the largest TCE adsorption capacity following NOM preloading (Kilduff et al., 2002). In contrast to the results obtained for coal-based activated carbon, decreasing the surface acidity of wood-based activated carbons did not increase the effect of NOM preloading on the percent reduction in TCE adsorption capacity (Kilduff et al., 2002). The adsorbent with the lowest surface acidity among the wood-based activated carbons was again the most effective for TCE adsorption both in the presence and absence of preloaded NOM (Kilduff et al., 2002).
As illustrated above, the combined effects of activated carbon pore structure and surface chemistry on the simultaneous adsorption of micropollutants and NOM have not been studied in a systematic manner. Thus, the objectives of this research were to (1) identify the effects of activated carbon pore structure and surface chemistry on the adsorption of TCE and MTBE in the presence of co-adsorbing NOM and (2) identify simple descriptors of activated carbon characteristics that facilitate the selection of suitable adsorbents for the removal of organic contaminants from waters containing NOM.
Section snippets
Solvents
Single-solute isotherm experiments were conducted in ultrapure laboratory water (Raleigh, NC tap water treated by reverse osmosis, ion exchange, and granular activated carbon adsorption, resistance ⩾14.85 MΩ/cm). Ultrapure water was amended with a 1 mM phosphate buffer (0.5 mM Na2HPO4·H2O and 0.5 mM NaH2PO4) to maintain a pH of 7.2. Furthermore, 100 mg/L sodium azide were added to eliminate aerobic biological activity.
Micropollutant adsorption in the presence of NOM was evaluated with Sacramento-San
Results and discussion
To evaluate the effects of activated carbon pore structure and surface chemistry on micropollutant adsorption capacities in the presence of co-adsorbing NOM, MTBE and TCE isotherm data were collected in Sacramento-San Joaquin Delta water (SJDW) for each of the 15 adsorbents. The effect of co-adsorbing NOM was evaluated for each adsorbent by comparing its micropollutant isotherm in ultrapure water to that in SJDW. Fig. 1 illustrates such a comparison for TCE adsorption isotherm data collected
Conclusion
To evaluate pore structure and surface chemistry effects on the adsorption of TCE and MTBE from aqueous solution, a matrix of ACFs with three activation levels and four surface chemistry levels was studied along with three commercially available GACs. The following conclusions were drawn from the relationships obtained between the micropollutant adsorption isotherm data in the presence of co-adsorbing NOM and the adsorbent characteristics:
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The volume of micropores with widths corresponding to
Acknowledgments
The authors would like to thank the American Water Works Association Research Foundation (AWWARF) for funding this research and the National Science Foundation (NSF) for supporting Patricia Quinlivan through a Graduate Research Fellowship. Additionally, we would like to thank the Contra Costa Water District for supplying SJDW. Furthermore, our gratitude is extended to Nippon Kynol, Inc. for donating the ACF samples, and to Dr. Yoshihiko Matsui at Gifu University for obtaining and shipping the
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