Environ. Sci. Technol. 2002, 36, 3432-3438
Pesticide Adsorption by Granular Activated Carbon Adsorbers. 2. Effects of Pesticide and Natural Organic Matter Characteristics on Pesticide Breakthrough Curves YOSHIHIKO MATSUI* Department of Civil Engineering, Gifu University, Yanagido 1-1, Gifu 501-1193 Japan DETLEF R. U. KNAPPE Department of Civil Engineering, North Carolina State University, Campus Box 7908, Raleigh, North Carolina 27695-7908 KENJIRO IWAKI AND HIRONOBU OHIRA Department of Civil Engineering, Gifu University, Yanagido 1-1, Gifu 501-1193 Japan
The principal objective of this study was to elucidate mechanisms by which NOM affects the adsorption of a nonpolar (simazine) and a polar (asulam) herbicide on activated carbon. Experiments were carried out in microcolumns that were continuously fed solutions containing NOM with different molecular weight (MW) distributions and intermittently solutions containing the same NOM plus simazine or asulam. The MW distributions of a groundwater NOM were altered by coagulation and ultrafiltration, which resulted in the preferential removal of high-MW, UV260-absorbing NOM. At a given NOM loading, the simazine removal efficiency was higher in the column that was preloaded with raw groundwater than in columns receiving coagulated or ultrafiltered water. In contrast, the asulam removal efficiency was similar for all three NOM solutions at a given NOM loading. Therefore, the results suggested that low-MW, UV260-absorbing NOM molecules competed directly with strongly adsorbing pesticides, such as simazine, for adsorption sites. For more weakly adsorbing pesticides, such as asulam, direct competition for adsorption sites originated not only from the strongly adsorbing, low-MW NOM, but also from more weakly adsorbing, higher-MW NOM. Consequently, the competing NOM fraction increases as the adsorbability of the SOC decreases, a result that was confirmed by adsorption data for additional pesticides of similar size. However, a smaller pesticide competed more effectively for adsorption sites than a larger pesticide of similar polarity, suggesting that the concentration of competing NOM decreases as the MW of the SOC decreases.
Introduction Granular activated carbon (GAC) adsorbers are frequently used to remove disinfection-byproduct precursors, taste* Corresponding author phone: +81-58-293-2429; fax: +81-58230-1891; e-mail:
[email protected]. 3432
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 15, 2002
and-odor compounds, and synthetic organic chemicals (SOCs), such as pesticides, from drinking water sources. One difficulty in the design and operation of GAC adsorbers is to model the effects of natural organic matter (NOM) on the adsorption of SOCs. NOM is a complex mixture of organic compounds derived from soil (primarily fulvic acids) and biological activity inside a water body (e.g. polysaccharides and proteins), and the physical and chemical properties of NOM change both seasonally and geographically (e.g. refs 1-3). Both NOM (4-6) and SOC (7, 8) characteristics determine the extent to which NOM adsorption decreases SOC adsorption capacities and rates. Accordingly, it is important to obtain a clearer understanding of how both NOM and SOC properties affect SOC breakthrough curves. With respect to NOM characteristics, preloaded fulvic acid reduced the remaining GAC adsorption capacity for Rhodamine B more severely than preloaded humic acid at the same total organic carbon (TOC) loading (9). Given that fulvic acids tend to be smaller than humic acids (e.g. ref 10), the data of Sontheimer et al. (9) provided an early indication that SOC removal efficiencies are compromised to a greater extent by the adsorption of smaller NOM molecules. Upon fractionating NOM by ultrafiltration (UF), both simultaneous loading (4, 5) and NOM preloading studies (6) substantiated that the smallest NOM size fractions caused the largest decreases in micropollutant adsorption capacities. The smallest size fraction of the aquatic NOM studied by Newcombe et al. (4, 5) was the permeate of a 500 Da UF membrane while the smallest size fraction of the soil humic acid studied by Kilduff et al. (6) was the permeate of a 3 kDa UF membrane. Furthermore, the preloading effect on the remaining SOC adsorption capacity was independent of the initial molecular weight (MW) distribution of the NOM as long as the mass of the
