Environ. Sci. Technol. 2007, 41, 7888-7894
Approaches To Mitigate the Impact of Dissolved Organic Matter on the Adsorption of Synthetic Organic Contaminants by Porous Carbonaceous Sorbents YANPING GUO, ABHISHEK YADAV, AND TANJU KARANFIL* Department of Environmental Engineering and Earth Sciences, Clemson University, 342 Computer Court, Anderson, South Carolina 29625
Adsorption of trichloroethylene (TCE) and atrazine, two synthetic organic contaminants (SOCs) having different optimum adsorption pore regions, by four activated carbons and an activated carbon fiber (ACF) was examined. The selected adsorbents had a wide range of pore size distributions but similar surface acidity and hydrophobicity. Single solute and preloading (with a dissolved organic matter (DOM)) isotherms were performed. Single solute adsorption results showed that (i) the adsorbents having higher amounts of pores with sizes about the dimensions of the adsorbate molecules exhibited higher uptakes, (ii) there were some pore structure characteristics, which were not completely captured by pore size distribution analysis, that also affected the adsorption, and (iii) the BET surface area and total pore volume were not the primary factors controlling the adsorption of SOCs. The preloading isotherm results showed that for TCE adsorbing primarily in pores 10 Å, respectively (6, 9, 12, 14). On the other hand, our recent work on trichloroethylene (TCE) adsorption showed that the most microporous carbon fiber (i.e., the fiber having a pore volume mainly in the micropores of 20 Å. This was just about the total volume needed for the adsorbed atrazine molecules. As a result, OLC showed the second most significant reduction in the atrazine uptake after ACF10. The other three carbons, CRC, F400He, and HD4000ST, showed similar uptakes under the two preloading conditions tested (Figure 3). One common characteristic of these carbons is that they all had similar pore volume in the 10-20 Å region (Table 1). However, it should be noted that CRC showed significantly lower DOM uptake than F400He and HD4000ST because CRC is more microporous than the other two sorbents (Figure 1). Therefore, one possible explanation for the similar uptakes of these activated carbons under preloading conditions is that the amount of DOM adsorbed per unit volume in the 10-20 Å pore size region was comparable at each preloading condition for these carbons, resulting in similar percent decreases in the available adsorption sites and thus showing similar atrazine uptakes. It should also be noted that in single solute adsorption experiments, CRC, F400He, and HD4000ST showed similar atrazine adsorptions despite the difference in their pore size distribution, confirming the importance of the 10-20 Å region for atrazine adsorption. Similar uptakes of these carbons under preloading conditions suggest that the major competition between atrazine and DOM components was likely occurring in the 10-20 Å pore size region. The percent reduction in atrazine uptake increased with decreasing equilibrium concentration (Table 3). Therefore, an increase of n and decrease of KF was obtained when the Freundlich isotherm equation was applied (Table 2). Carter et al. (5, 25) interpreted this type of isotherm pattern as an indication of site competition. This is in agreement with the overlapping adsorption regions of atrazine and some DOM components observed in this study and with a previous report that some DOM components were able to replace the preloaded atrazine from activated carbons (6). Practical Implications. The isotherm results obtained in this study showed that for TCE, the SOC molecule adsorbing primarily in pores
