Environ. Sci. Technol. 1996, 30, 2195-2201
Adsorption of Organic Macromolecules by Granular Activated Carbon. 2. Influence of Dissolved Oxygen TANJU KARANFIL,† MARK A. SCHLAUTMAN,§ JAMES E. KILDUFF,‡ AND WALTER J. WEBER, JR.* Environmental and Water Resources Engineering Program, Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125
The influence of dissolved oxygen (DO) on the uptake of natural and synthetic macromolecular dissolved organic materials (DOMs) from aqueous solutions by granular activated carbon (GAC) was investigated. The sorption behaviors of six of the nine DOMs tested were enhanced by the presence of DO; these included polymaleic acid, peat humic acid, Leonardite humic acid, Suwannee River fulvic acid, and Laurentian fulvic and humic acids. The sorptions of Aldrich and soil humic acids and polystyrene sulfonate were not sensitive to the presence or absence of DO. Sensitivity of sorption to DO generally increased with decreasing molecular size, polydispersity, and aromaticity and with increasing acidity for the five natural DOMs. Oxygen-sensitive sorption of the low molecular weight fractions of peat humic acid, a slightly oxygen-sensitive DOM, was noticeably greater than that of the whole material. Sorption of the low molecular weight fractions of Aldrich humic acid, an oxygen-insensitive DOM, did not show any particular sensitivity to oxygen.
Introduction Granular activated carbon (GAC) adsorption is one of the best available technologies for removing target synthetic organic chemicals (SOCs) from polluted drinking water sources (1). However, the presence of background dissolved organic materials (DOMs) in natural waters generally has an adverse impact on GAC removal of SOCs, with the net * Author to whom correspondence should be addressed; telephone: (313) 764-2274; fax: (313) 763-2275; E-mail address: wjwjr@engin. umich.edu. † Present address: Department of Environmental Systems Engineering, L. G. Rich Environmental Research LaboratorysClemson Research Park, Clemson University, Clemson, SC 29634-0919. ‡ Present address: Department of Environmental and Energy Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. § Present address: Environmental, Ocean and Water Resources Division, Department of Civil Engineering, Texas A&M University, College Station, TX 77843-3136.
S0013-936X(95)00587-6 CCC: $12.00
1996 American Chemical Society
result being a significant decrease in the operational life of fixed-bed GAC adsorbers. The two most important factors responsible for decreased GAC performance are effective competition of DOM components with target SOCs for available adsorption sites and preloading of GAC by DOMs prior to SOC uptake (2-4). Scientists and engineers need to understand the multiple factors involved in the sorption of DOMs and their components by GAC to overcome these detrimental effects. Therefore, we have undertaken systematic investigations in our laboratories to elucidate the sorption characteristics of DOMs. Previous work has focused on characterizing the sorption of model and synthetic DOMs by GAC using size exclusion and ultrafiltration techniques (5) and investigating the role of solution chemistry on these characteristics (6). In the preceding paper, we examined the influence of DOM physicochemical properties on their sorption behaviors by GAC (2). In this paper, we address a more recently observed phenomenon, namely, the impact of dissolved oxygen (DO) on the sorption of DOMs by GAC.
Background Prior studies of DO effects on the uptake of various compounds by GAC have shown that this parameter is important for phenolic compounds (7-15). In these systems, GAC surfaces are thought to act as catalysts for the polymerization of phenolic compounds by oxidative coupling reactions. Conversely, the sorptions of chlorobenzene and most aliphatic compounds have been found to be insensitive to the presence of DO (10, 16). One exception to the latter case has been the recent report showing a DO effect for the sorption of cis-1,2-dichloroethene (17). Sorptions of DOMs from a lake (10), a river (18), and a municipal landfill leachate (16, 17) and of model and natural humic and fulvic acids (14) have shown varying degrees of sensitivity to the presence of DO, the effect generally being less significant than for simple phenolic compounds. In addition to reducing GAC sorption capacities for target SOCs, carbon preloading by DOMs may also “poison” the catalytic effects of clean GAC surfaces for subsequent SOC polymerization reactions. Chin et al. (19) reported inhibition of phenol coupling on GAC in column studies by syringic acid and also by relatively high concentrations of fulvic acids (over 30 mg of TOC/L). They postulated that inhibition of phenol coupling resulted from either free radical scavenging or from competition for active sites on the GAC surface. Karanfil et al. (14) examined the adsorption and oxidative coupling of o-cresol on virgin and preloaded GAC. The preloaded GAC had been previously exposed to an oxygen-sensitive DOM under both oxygenic and oxygen-free conditions before o-cresol uptake. Karanfil et al. (14) found that the two different preloading conditions had similar effects on subsequent o-cresol reactions. A decrease of 20% in adsorption and an additional 25% reduction in oxidative coupling was observed for o-cresol. The authors postulated that the extent of surface coverage may be the most important factor relative to the effects of preloading on the adsorption and polymerization of target SOCs. The results of Sorial et al. (16) are consistent with this hypothesis. They found that the presence of oxygen-
VOL. 30, NO. 7, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2195
sensitive DOMs in a solution matrix creates oxygendependent sorption for target compounds that are normally oxygen-insensitive. This would be expected because the presence of oxygen will create higher extents of loading for DOMs whose uptake is affected by oxygen.
Objective and Hypotheses The present work was undertaken to investigate the effects of DO on DOM sorption by GAC, there having been no prior systematic investigation of this potentially important phenomenon. Our primary objective was to elucidate those characteristics of DOMs responsible for such oxygen effects. Several hypotheses that guided this work are discussed in the following paragraphs. It has been observed that the lower molecular weight fractions of polydisperse mixtures of organic macromolecules are preferentially removed from solution by GAC (2, 20-22). Thus, we hypothesized that the molecular sizes of DOMs and/or their components can be important also with respect to oxygen effects. That is, because more of a GAC surface and its “catalytic active sites” become accessible as the size of a sorbate molecule decreases, it is likely that the impact of DO will be greater on the polymerization and enhanced sorption of smaller macromolecules. Once molecules enter GAC pores, chemical interactions between the solute and sorbent begin to control the sorption phenomena and the effects of oxygen. From a chemical perspective, oxygen sensitivity has been reported widely for the sorption of simple phenolic compounds and, most particularly, for those having few, if any, other functional groups on the parent molecule (8, 13, 23). Phenolic structures comprise approximately 20-40% of the total acidity of humic materials (24). Therefore, we hypothesized further that the phenolic content or aromaticity may also correlate with DO effects on the uptake of DOM fractions, particularly humic substances, by GAC. These two hypotheses combine to suggest that the size of a DOM macromolecule relative to a GAC pore physically controls its access to active catalytic sites, after which its chemical structure determines the magnitude of any oxygen effects.
Experimental Section Materials. (A) Natural and Synthetic DOMs. The nine natural and model organic macromolecules used in this study were polymaleic acid (PMA), Aldrich (AHA), peat (PHA), soil (SHA), and Leonardite (LeHA) humic acids, Suwannee River fulvic acid (SFRA), Laurentian fulvic and humic acids (LaFA and LaHA), and polystyrene sulfonate (PSS). The sources and physicochemical characteristics of these macromolecules and the methods employed for their characterization have been described in detail in the preceding paper (2). (B) GAC. A uniform size (80/100 U.S. standard mesh size) of Filtrasorb (F-400) granular activated carbon (Calgon) was used in the experiments. Physicochemical characteristics of this GAC and the methods employed for its characterization are described elsewhere (5, 25). Methods. (A) Analytical Techniques. All natural and model humic substances were quantified using TOC analysis and UV absorbance. An oxygen microprobe (YSI 530) having a detection limit of 0.1 mg/L was used to measure dissolved oxygen concentrations. Additional analytical details are provided elsewhere (2, 25). (B) Isotherm Experiments. Detailed procedures for the isotherm experiments have been reported elsewhere (2,
2196
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 7, 1996
14, 25). Briefly, all sorption experiments were conducted at pH 7 at room temperature (21 ( 2 °C). The solutions were buffered with 0.001 M Na2HPO4. The effects of DO on the sorption of natural and model DOMs were examined in bottle-point, variable-dose, sorption experiments using two different initial oxygen conditions. For the first condition, designated as oxic, a 60-mL air headspace was left in each isotherm bottle (Wheaton, 160 mL), providing approximately 18 mg of total initial oxygen in the bottles. For the second condition, designated as anoxic, bottles were completely filled in an anaerobic chamber with a N2-purged stock solution, thereby obtaining a maximum of 0.05 mg of initial DO in each bottle. (C) Modeling of Isotherm Results. A modified form of the Freundlich equation was used to model the results of oxic and anoxic DOM isotherms:
qe ) KF(Ce/D)n
(1)
where KF is the Freundlich affinity parameter for a heterodisperse system and the exponential term, n, is related to the magnitude of the adsorption driving force and to the distribution of the energy sites on the adsorbent (26). Linear geometric mean functional regression of the log-transformed experimental data was used to determine the parameters log KF and n (27). The geometric mean functional regression algorithm accounts for errors incurred in the measurement and calculation of the respective liquidand solid-phase equilibrium concentrations (27, 33, 34). Confidence intervals (95%) were determined for each parameter based on the regression of log-transformed data. The usefulness of this modified Freundlich equation has been shown previously for describing the sorption of heterogeneous humic substances and other natural DOMs by GAC for a wide range of adsorbent dosages, initial concentrations, ionic strengths, temperatures, and oxygenic conditions (2, 21, 22, 28).
Results and Discussion Oxygen Sensitivity of DOM Sorption. Oxygen effects on the sorption of natural and synthetic organic macromolecules were evaluated by measuring variable-dose oxic and anoxic isotherms. The results of these experiments indicated varying degrees of sensitivity to the presence of oxygen. For example, dissolved oxygen (DO) had a high impact on the sorption of PMA and LaFA, whereas smaller effects were found for LaHA, LeHA, SRFA, and PHA. No DO effects were detected for purified AHA, SHA, and PSS. Representative results from these experiments are shown for LaFA using the conventional and the dose-normalized form of the Freundlich isotherm (Figure 1). The modified form of the Freundlich equation (eq 1) was used to model the results of the oxic and anoxic DOM isotherms. The coefficients of the normalized Freundlich isotherms are listed in Table 1. Using this approach, the ratio of the Freundlich affinity parameters determined under oxic and anoxic conditions, KF,o/KF,a, can be used to compare the oxygen sensitivity of different macromolecules. For a complete evaluation, however, this analysis must be accompanied by a visual comparison of the overall isotherm data, particularly for the cases in which there are significant differences between the n values of oxic and anoxic isotherms for the same compounds. Our analysis showed that PMA’s sensitivity to the presence of DO was significantly different than the natural
TABLE 1
Dose-Normalized Freundlich Isotherm Coefficients for Natural and Synthetic Organic Macromoleculesa Oxic
Anoxic
DOM
KF,o (mg DOC/g GAC)1-n
n (-)
KF,a (mg DOC/g GAC)1-n
n (-)
KF,o/KF,a (-)
PMA
7.35 (7.67-7.04) 5.73 (5.95-5.55) 8.76 (9.90-7.76) 6.50 (6.99-6.05) 1.83 (1.86-1.80) 2.27 (2.39-2.16) 2.05 (2.16-1.95) 4.57 (4.71-4.44) 3.83 (4.50-3.27)
0.261 (0.280-0.242) 0.311 (0.326-0.296) 0.348 (0.398-0.298) 0.248 (0.276-0.220) 0.477 (0.485-0.469) 0.471 (0.487-0.456) 0.442 (0.456-0.427) 0.360 (0.374-0.347) 0.464 (0.500-0.428)
2.70 (2.89-2.52) 3.64 (3.79-3.51) 6.59 (6.90-6.29) 5.33 (6.11-4.65) 1.66 (1.78-1.55) 2.18 (2.31-2.05) 2.11 (2.20-2.02) 4.84 (5.26-4.45) 4.26 (4.48-4.04)
0.377 (0.400-0.353) 0.291 (0.309-0.273) 0.352 (0.366-0.357) 0.220 (0.259-0.181) 0.406 (0.431-0.380) 0.434 (0.454-0.414) 0.429 (0.444-0.415) 0.292 (0.322-0.263) 0.400 (0.416-0.385)
2.72
LaFA SRFA LaHA PHA LeHA SHA AHA PSS a
1.57 1.33 1.22 1.10 1.04 0.97 0.94 0.90
Values in parentheses for KF and n are the 95% confidence intervals.
FIGURE 2. Influence of molecular size (weight) on oxygen-enhanced DOM sorption. Filled and open circles show oxygen-sensitive and -insensitive humic substances, respectively. The dashed line suggests the general parametric trend. Values in parentheses are the DOM polydispersities.
FIGURE 1. Influence of dissolved oxygen on Laurentian fulvic acid sorption (LaFA). (a) Conventional Freundlich isotherms, (b) dosenormalized Freundlich isotherms. humic materials. For example, the value shown in Table 1 for the enhanced sorption of PMA is about two times higher than those for the natural humic materials. The high reactivity of PMA relative to the other DOMs most likely reflects a continuation of its original synthesis, enhanced by the GAC surface and the presence of oxygen. Because the effects of DO on PMA sorption apparently are
not representative of natural DOMs, PMA was excluded from the subsequent analyses. The synthetic organic macromolecule, PSS, was also not included in the following analyses because of its substantially different chemical structure relative to humic materials (2). The two oxygeninsensitive humic acids AHA and SHA were retained to provide a baseline for comparing oxygen sensitivity. However, AHA and SHA are distinguished from the other oxygen-sensitive humics by different symbols in the figures. Influence of DOM Macromolecular Size on their Oxygen-Sensitive Sorption. The influence of DOM molecular size on the oxygen-sensitive GAC sorption was evaluated by plotting the oxygen sensitivities KF,o/KF,a versus the weight-average molecular weights (Mw) as illustrated in Figure 2. It is apparent that the size of an organic macromolecule is important because the sorption of the smaller macromolecules is more sensitive to the presence of oxygen. The trend of the numbers in parentheses, the polydispersities of the macromolecules, in Figure 2 suggests that the more uniform and smaller macromolecules are more oxygen sensitive. This trend is highlighted by a plot
VOL. 30, NO. 7, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2197
TABLE 2
Dose-Normalized Freundlich Isotherms Coefficients for Organic Macromolecules and Their Smaller Than 3K Fractionsa oxic DOM PHA PHA < 3K fraction AHA AHA < 3K fraction a
anoxic
KF,o (mg DOC/g GAC)1-n
n (-)
KF,a (mg DOC/g GAC)1-n
n (-)
KF,o/KF,a (-)
1.83 (1.86-1.80) 5.09 (5.84-4.43) 4.57 (4.71-4.44) 11.38 (12.75-10.16)
0.477 (0.485-0.469) 0.706 (0.807-0.604) 0.360 (0.374-0.347) 0.425 (0.485-0.365)
1.66 (1.78-1.55) 3.39 (3.92-2.94) 4.84 (5.26-4.45) 11.12 (12.92-9.57)
0.406 (0.431-0.380) 0.760 (0.856-0.665) 0.292 (0.322-0.263) 0.440 (0.522-0.358)
1.10 1.50 0.94 1.02
Values in parentheses for KF and n are the 95% confidence intervals.
FIGURE 3. Influence of polydispersity on oxygen-enhanced DOM sorption. Filled and open circles show oxygen-sensitive and -insensitive humic substances, respectively. The solid line suggests the general parametric trend.
of oxygen sensitivities versus polydispersities (polydispersity ) Mw/Mn, where Mn and Mw are the number- and weightaverage molecular weights, respectively) as shown in Figure 3. However, polydispersity is not a primary factor controlling oxygen effects; instead, the relatively good trend observed in Figure 3 is an indirect effect of the close correlation between the polydispersities and molecular weights of the DOMs employed in this study. Given that lower molecular weight DOM components are preferentially removed from solution by GAC (e.g., ref 2) and that increases in oxygen effects are observed with decreasing molecular weights of the different organic macromolecules, we hypothesized that the different size fractions of a DOM may have different degrees of oxygen sensitivity, the expected trend being greater oxygen effects for the smaller size fractions. To investigate this hypothesis, oxic and anoxic isotherm experiments were conducted with the smallest components of PHA and AHA obtained by ultrafiltration fractionation. Figure 4 and Table 2 show that both the removal capacity of GAC and the impact of oxygen for PHA significantly increased for the
