Role of Granular Activated Carbon Surface Chemistry on the

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Environ. Sci. Technol. 1999, 33, 3225-3233

Role of Granular Activated Carbon Surface Chemistry on the Adsorption of Organic Compounds. 2. Natural Organic Matter TANJU KARANFIL* AND MEHMET KITIS Department of Environmental Engineering and Science, Clemson University, 342 Computer Court Anderson, South Carolina 29625 JAMES E. KILDUFF AND ANDREW WIGTON Department of Environmental and Energy Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180

The role of granular activated carbon (GAC) surface chemistry on the adsorption of four model dissolved organic material (DOM) isolates and four surface water natural organic material (NOM) samples was investigated by using (1) ten carbons prepared by modifying the surfaces of a coalbased and a wood-based carbon and (2) seven different as-received GACs. Because changes in the pore structure resulting from surface treatment were small, while changes in the surface chemistry were significant, the impact on the DOM and NOM uptake by surface-treated carbons was systematically linked to the changes in the carbon surface chemistry alone. For the surface-treated coal-based carbons, there was (1) no significant capacity difference between acid-washed and heat-treated carbon samples, (2) oxidation of the carbon surface significantly decreased the uptake, and (3) the capacity was partially restored by subsequent heat treatment of the oxidized surfaces. A decreasing uptake with increasing surface acidity was evident, and the effects of surface acidity on uptake were qualitatively similar to the two SOCs studied in Part 1 of this series. The experiments with asreceived coal-based carbons exhibited the same behavior; however, the reactivity of modified and as-received carbons for DOM and NOM uptake was significantly different. For the wood-based carbon, the impact of surface treatment on adsorption of DOMs and NOM was surprisingly minimal or absent. This finding was in contrast to the effects of surface acidity on uptake of the two SOCs studied in Part 1 in this series. Overall, the reactivity of carbon surfaces to DOM and NOM uptake depended on the raw material type, activation conditions and surface treatment.

Introduction Naturally occurring organic materials (NOMs) in dissolved, colloidal, or particulate forms are ubiquitous in surface and groundwaters. The dissolved and colloidal forms (i.e., DOMs, those constituents passing a 0.45 micron filter) are the most problematic and undesirable fractions of NOM with regard * Corresponding author: phone: (864) 656-1005; fax: (864) 6560672; e-mail: [email protected]. 10.1021/es9810179 CCC: $18.00 Published on Web 08/11/1999

 1999 American Chemical Society

to water treatment and supply. DOM cannot be completely removed from water using conventional treatment processes. It can bind or complex synthetic organic chemicals (SOCs) and/or toxic metals and carry them through treatment facilities and distribution systems. DOM can serve as a substrate for bacterial growth in distribution systems and constitutes the major precursor of disinfection byproducts (DBPs) formed during water treatment (1). The United States Environmental Protection Agency (USEPA) is planning to impose more stringent standards on DBPs because of their potential risk to public health. As a result, granular activated carbon (GAC) adsorption has been designated as a “best available technology” for DOM removal (2-4). In most current practical applications, GAC fixed-bed adsorbers are designed to remove dissolved pollutants, such as SOCs, from water. The presence of DOM in water has been found to significantly reduce the performance and capacity of GAC adsorbers for target SOCs (5-10). The reduction in GAC efficiency may be caused by several factors (8-16): (1) DOM is generally present in natural waters at much higher concentrations than target pollutants; (2) DOM may compete with pollutants by several mechanisms, including direct competition for adsorption sites and pore blockage, and (3) DOM does not desorb readily because of its high molecular weight and ability to bind at multiple sites. Although simultaneously meeting multiple objectives in a single treatment process may be difficult, understanding the interactions between DOM and GAC is essential to optimize their removal from water, to minimize their impact on the removal of the target pollutants, or both. Size exclusion and electrostatic interactions are the two major interactions that control the extent of DOM adsorption by GAC. The average size of DOM molecules have been reported between 4 and 40 Å (17-20), whereas the bulk of GAC surface area is commonly located in the pores with widths smaller than 20 Å (i.e., micropores) (21, 22). Although there are numerous uncertainties regarding the precise determination of DOM molecular size, the available data clearly indicate that some fraction of DOM will not be able to access finer carbon pores and will be prevented from fully employing the large surface area present for adsorption. Previous research has provided experimental evidence indicating the importance of size exclusion effects (23-27). Preferential adsorption of low molecular weight fractions of humic acid solutions have been reported (28-30). Karanfil et al. (31) showed that adsorption of chemically homogeneous polystyrene sulfonate fractions and different size fractions of three humic acids obtained through ultrafiltration decreases with increasing molecular weight (above a MW of about 1300 g/mol). For adsorption on ion-exchange resins, Fu and Symons (32) found that for a given hydrophobicity and structure, resin effectiveness was directly related to adsorbent pore size. Some studies listed above (e.g., preferential adsorption studies conducted with a single adsorbent) conclusively document the importance of size exclusion phenomena. However, other studies attempted to draw conclusions about the effects of carbon pore structure by comparing uptake by different adsorbents. It is difficult to interpret these effects on the basis of size exclusion alone, because chemical interactions between organic molecules and carbon surfaces can be significant, and in some cases, may overwhelm physical interactions (22). For example, we previously reported that GAC adsorption of two larger humic acids was higher than the uptake of two smaller fulvic acids (31). While this result might be expected on the basis of Traube’s Rule, and is consistent with the observation of others researchers VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Physicochemical Characteristics of Model Adsorbates apparent weight average MWa carboxylic aciditya phenolic aciditya aromaticitya free liquid average b 2 (g/mol of PSS) (meq/g DOC) (meq/g DOC) (%) diffusivity (m /s) radiusc (Å)

DOMs polymaleic acid (PMA) laurentian fulvic acid (LaFA) laurentian humic acid (LaHA) aldrich humic acid (AHA) a From Karanfil et al., (31). Stokes-Einstein equation.

b

2601 2402 3982 4006

13.0 11.7 8.9 7.9

3.2 9.3 5.7 3.6

23.0

10-10

16

28.2 33.4 57.7

10-10 1.32 × 10-10 1.32 × 10-10

16 21 21

Predicted using Free Liquid Diffusivity vs PSS MW correlation reported by Cornel et al. (18). c Calculated using the

TABLE 2. Compositional Characteristics of Natural Raw Watersa parameters

unit

Charleston WTP

Myrtle Beach WTP

Tomhannock Res.

Rensselaer County

DOC UV 254 SUVA alkalinity total hardness pH

(mg/L) (abs) (L/mg org-C‚m) (mg CaCO3/L) (mg CaCO3/L)

4.75 0.150 3.16 66 27 7.8

14 0.541 3.86 94 40 7.8

3.00 0.074 2.49 35 76 7.2

5.67 0.158 2.79 N/D N/D 7.4

a All samples were filtered through 0.8 and 0.45 µm Gelman Supor Pro filter before the experiments. Values reported are the average of triplicate measurements. N/D: not determined.

(33-34), it is in opposition to many of the previously cited studies if interpreted on the basis of size alone. A plausible explanation for these findings is that the fulvic acids studied were more hydrophilic and hence more soluble in water relative to humic acids; therefore, they exhibited a lower extent of adsorption despite their smaller size and greater ability to access adsorbent surfaces. The hydrophilic or hydrophobic nature of humic and fulvic acids derives in part from the functional groups within their structure; because these groups can interact with functional groups on the GAC surface, both play important roles in DOM adsorption. In addition, background water chemistry may also affect adsorption of DOM due to the amphoteric nature of these functional groups. Despite the potential importance of carbon surface functional groups, to date, there is no report in the literature regarding to the role of GAC surface chemistry on DOM adsorption. This is partly because it is not possible to control both the effects of pore structure and surface chemistry by using only as-received carbons, as documented in the previous experimental investigations.

Objectives and Approach Our objective in this research was to systematically investigate the role of carbon surface chemistry and pore structure on DOM adsorption. The approach taken was to examine DOM uptake by 10 surface-treated GACs prepared from one coalbased (microporous) and one wood-based (mesoporous) carbon. These carbons provide an excellent opportunity for this study because their pore structure was relatively unchanged by surface modification, while the surface chemistry was modified significantly, as documented in the companion paper (22). Therefore, the impact on the DOM uptake can be systematically linked to the changes in the carbon surface chemistry alone. In addition, DOM uptake by surface-treated carbons was compared with uptake by several well-characterized as-received activated carbons. Furthermore, the significance of pore structure was evaluated by comparing the pore size distribution of treated and asreceived carbons with the molecular size (or size distribution) of the DOM. This approach allowed us to examine the reactivities of carbons prepared from different raw materials, activated in different ways and exposed to different treatment conditions. Isolated and well-characterized model DOMs 3226

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were used to provide insight to the effects of DOM molecular properties on adsorption by different surfaces. In addition, experiments were conducted using four natural surface waters to further verify the findings with the isolated DOMs.

Materials and Methods Sorbents. In this research, a total of 10 carbons prepared by modifying the surfaces of a thermally activated coal-based (Calgon F400) and an acid-activated wood-based carbon (Westvaco WVB) were used. The treatment techniques, sequences, and the impact on the carbon pore structure and surface chemistry all were described in detail in the companion paper (22). In addition, four GACs manufactured from bituminous coal (F400, WPLL, FS100, and BPL) and three GACs manufactured from wood (MICRO, MESO, and MACRO) were used as-received from the suppliers. Sorbates. Model DOMs included polymaleic acid (PMA), a fulvic acid surrogate, natural soil humic (LaHA) and fulvic acids (LaFA) extracted from Laurentian soil, and Aldrich humic acid (AHA) purified to remove ash components. Physicochemical characteristics of these compounds have been reported in one of our previous publications (31) and a summary of the important characteristics with respect to this work is provided in Table 1. Raw water samples were collected from the influents of Charleston and Myrtle Beach, SC, drinking water treatment plants, from the Tomhannock reservoir, the water supply for the City of Troy, NY, and from a stream draining a rural agricultural watershed in Rensselaer County, NY. The compositional properties of these waters are given in Table 2. Adsorption Isotherms. Isotherm experiments were conducted using the completely mixed batch reactor (CMBR) bottle-point method as described in detail elsewhere (33). In brief, a DOM aqueous solution was equilibrated with activated carbon for one month in well-mixed 250 mL batch reactors under oxic conditions. After equilibration, an aliquot of solution was filtered (0.45 µm Supor, Gelman Sciences, Ann Arbor, MI) and analyzed for UV absorbance (DMS 200, Varian Optical, Victoria, Australia) and dissolved organic carbon (DOC) concentration (TOC-500 and TOC-5000, Shimadzu Corp., Japan). All isotherms were conducted in the presence of a 0.01 M phosphate buffer, at pH 7 and room temperature of 21 ( 3 °C, except isotherms with raw water

TABLE 3. Accessible Surface Areas for NOM Adsorption and Normalized Freundlich Affinity Parameters PMA (d ) 32 Å)a

LaFA (d ) 32 Å)a

LaHA (d ) 42 Å)a

adsorbent

total surface area

KF c

accessible surface areab

KF d

KFc

accessible surface areab

KFd

KFc

accessible surface areab

KFd

WPLL FS100 BPL F400

294 751 1200 948

0.362 3.332 8.344 7.592

1.3 15 32 23

0.317 0.509 0.679 0.748

0.396 2.646 7.131 5.777

1.3 15 32 23

0.351 0.414 0.628 0.656

0.119 2.836 2.139 2.800

1 9 17 14

0.119 0.791 0.547 0.612

a Calculated average hydrodynamic diameter using the Stokes-Einstein Equation. b Area available for adsorption of humic or fulvic acid on a carbon and used in the normalization of isotherms, m2/g. c (mg DOC/g GAC)1-n. d (mg DOC/m2 GAC)1-n.

samples were conducted after filtration (0.45 µm Supor) without any pH or buffer adjustment. Isotherm Modeling. The shapes of NOM isotherms (i.e., concave upward on arithmetic coordinates) are characteristic of sorption from multicomponent solutions of compounds having different sorption affinities. These isotherms are expressed as a function of a lumped concentration parameter (e.g., total organic carbon), and they depend on both GAC dose and initial mixture concentration. Therefore, the adsorption behavior of such NOM mixtures cannot be uniquely described using the traditional form of the Freundlich isotherm, which is a function of the liquid-phase equilibrium concentration only. However, a unique mixture isotherm, linear on log-log coordinates, can be obtained when uptake is plotted as a function of the nonadsorbed organic matter per unit carbon mass (Ce/Do). This modified form of the Freundlich isotherm has been used by several researchers to model commercial, natural humic materials and NOM adsorption from natural waters (28-31, 35-36):

qe ) KF (Ce/Do)n where qe is the amount adsorbed at equilibrium (normalized to adsorbent mass) Ce is the equilibrium solution phase concentration, KF is the Freundlich 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. Linear geometric mean functional regression of the log-transformed experimental data was used to determine the parameters log KF and n. Confidence intervals (95%) were also determined for each parameter based on the regression of logtransformed data. In this research, modified Freundlich isotherm approach was used to model DOM isotherms both on a mass and surface area basis. The use of surface area normalized isotherms was necessary to compare DOM uptake by surface-treated carbons with uptake by as-received carbons having different surface areas and pore size distributions. Given the reported average sizes of aquatic NOMs and the measured pore size distribution of activated carbons, it is evident that only a fraction of the total surface area will be available for adsorption. The accessible area for each DOM on each GAC surface was calculated using the following approach. First, the average sizes of the model DOMs were estimated through the use of weight-average molecular weight (MW) determined by size exclusion chromatography (SEC) (31), free liquid diffusivity data reported by Cornel et al. (18), and the StokesEinstein relationship. The SEC system used for MW measurements was calibrated with monodisperse polystrene sulfonate (PSS) polymers, therefore, DOM molecular weights were expressed using the unit “g/mol as PSS” (Table 2). Free liquid diffusivities of model organic matter used in this study were estimated using the Cornel et al. (18) data, which was obtained under conditions identical to those in our SEC measurements. They were then used as an input to the Stokes-Einstein relationship to calculate an apparent average

hydrodynamic radius. Values of 16 and 21 Å were calculated for fulvic and humic acids, respectively, used in this research. The impact of background water chemistry on the size of DOM must also be considered. It has been proposed that NOM molecules have a larger molecular radii (extended configuration) with decreasing ionic strength (38, 39). Free liquid diffusivity values were measured in solutions having an ionic strength of 0.10 M; therefore, at the ionic strength of our isotherm experiments (i.e., 0.01 M as NaCl), DOM molecules are expected to have larger sizes than the calculated radii of 16 and 21 Å. In addition, adsorbed macromolecules will decrease the pore widths. Therefore, we assumed that surfaces in pores less than twice the calculated molecular size in a 0.10 M ionic strength solution were not accessible. This selected two-to-one ratio (rNOM:rPORES) is consistent with the approach previously used by Summers and Roberts (29). From the pore size distribution data, the surface area in pores having widths larger than 64 and 84 Å was determined for each carbon and used to normalize the isotherms of fulvic and humic acids, respectively. The areas used in the isotherm normalization are shown under the accessible surface area column in Table 3. It is, however, important to remember that humic and fulvic acids are polydisperse mixtures, and it is practically impossible to measure a precise accessible surface area on a carbon surface by a DOM. Therefore, our size calculations and surface area normalization procedure described above constitute a simple approach based on reasonable assumptions.

Results and Discussion Adsorption of DOM by Surface Modified GACs. Isotherm experiments were conducted for PMA, LaFA, and AHA using the surface-treated coal-based and wood-based carbons. For surface-treated coal-based carbon (F400), the results are presented in Figure 1 and the dose and surface-areanormalized Freundlich isotherm parameters are given in Table 4. Since the physical integrity of the coal-based carbon was not affected by the surface treatment (22), the role of surface chemistry on the adsorption of each DOM sample was analyzed by comparing the isotherm results on a mass basis. Initial acid-washing of the carbons was done to remove alkaline impurities and ash components. These impurities have been shown to increase humic adsorption by GAC (40) and may also contribute to some catalytic activity of the carbon surface (41). Although acid-washing reduced the ash content of F400 from 6.3 to 4.7%, it did not affect adsorption of the three DOMs tested (Figure 1). Apparently, acid extractable metal impurities on the coal-based carbon surface did not play a significant role on the DOM uptake. On a mass basis, uptakes of both PMA and LaFA by the untreated and acid-washed carbons were similar. One plausible explanation is that these two macromolecules have a similar size and a similar amount of carboxylic type acidity in their structure (Table 1). However, uptake of AHA was significantly lower as compared to PMA and LaFA (Table 4). Several possible reasons are apparent. First, as a result of size exclusion, the larger AHA molecules may have been prevented from VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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reaching surfaces accessible to PMA and LaFA. Second, specific interactions (attractive forces) between charged functional groups and the positively charged GAC surface would be expected to be greater for PMA and LaFA, which have higher carboxylic-type acidity. Finally, PMA and LaFA have been shown to exhibit greater oxygen sensitivity than AHA (37). Heat treatment of the acid-washed F400 carbon (F400 HT 1000) completely removed the low degree of total surface acidity initially present; however, it did not affect the DOM uptake significantly (Figure 1). Surface oxidation after the heat treatment increased the surface acidity, while decreasing HCl uptake of the carbon. The density of strong acid functionalities increased with increasing oxidation temperature and reaction time (22). Surface oxidation decreased the adsorption of all DOM samples as reflected in a significant reduction in Freundlich affinity parameter KF. However, the impact was more significant, on both a mass and surface area basis, for PMA and LaFA in comparison to AHA (Table 4). Given the smaller predicted sizes of PMA and LaFA, one

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KFb

KFc

PMA

nd

6.043 0.938 0.367 (7.052-5.178) (1.094-0.803) (0.438-0.296) F400 AW 4.645 0.903 0.450 (5.137-4.201) (0.999-0.817) (0.512-0.388) F400 HT 1000 5.293 0.817 0.362 (6.919-4.050) (1.068-0.625) (0.484-0.240) F400 OX 2/70 2.699 0.277 0.242 (3.150-2.312) (0.323-0.238) (0.313-0.171) F400 OX 9/70 1.848 0.171 0.203 (2.262-1.510) (0.210-0.140) (0.275-0.131) F400 OX 2/70 HT 650 3.206 0.528 0.373 (4.140-2.482) (0.682-0.409) (0.471-0.274) F400 OX 9/70 HT 650 1.969 0.414 0.503 (2.394-1.619) (0.504-0.341) (0.579-0.427) WVB AW 1.270 0.041 0.326 (1.809-0.891) (0.058-0.029) (0.455-0.199) WVB HT 1000 1.75 0.117 0.417 (2.001-1.530) (0.134-0.102) (0.468-0.365) WVB OX 2/50 1.813 0.110 0.395 (2.067-1.569) (0.125-0.095) (0.437-0.353) WVB OX 2/70 2.222 0.050 0.179 (2.509-1.967) (0.058-0.045) (0.230-0.127) a Values in the parantheses for K and n are the 95% confidence intervals. b (mg DOC/g F

F400 untreated

modified carbons

d

0.303 (0.341-0.265) 0.291 (0.354-0.228) 0.352 (0.448-0.255) 0.427 (0.471-0.382) 0.350 (0.412-0.288) 0.361 (0.403-0.318) 0.426 (0.507-0.344) 0.495 (0.554-0.436) 0.316 (0.355-0.278) 0.307 (0.320-0.293) 0.208 (0.265-0.150)

nd

0.639 (0.696-0.586) 0.524 (0.623-0.440) N/D 0.428 (0.503-0.266) 0.358 (0.417-0.307) 0.502 (0.586-0.431) 0.549 (0.580-0.520) 0.105 (0.123-0.090) 0.099 (0.111-0.089) 0.144 (0.161-0.129) 0.152 (0.159-0.145)

N/D 2.201 (2.586-1.367) 1.653 (1.927-1.418) 2.665 (3.109-2.285) 2.325 (2.456-2.202) 1.250 (1.459-1.071) 1.366 (1.523-1.225) 1.404 (1.568-1.257) 1.223 (1.283-1.167)

KFc

AHA

3.095 (3.373-2.840) 2.706 (3.221-2.273)

KFb

dimensionless. N/D, not determined.

0.873 (0.970-0.787) 0.759 (0.906-0.637) 0.771 (0.914-0.651) 0.493 (0.551-0.440) 0.267 (0.328-0.218) 0.690 (0.780-0.610) 0.510 (0.603-0.431) 0.103 (0.124-0.086) 0.105 (0.118-0.094) 0.096 (0.099-0.092) 0.065 (0.077-0.055)

KFc

LaFA

GAC)1-n. c (mg DOC/m2 GAC)1-n.

6.802 (7.550-6.128) 6.266 (7.474-5.255) 5.138 (6.091-4.334) 2.752 (3.079-2.460) 1.861 (2.280-1.518) 4.335 (4.904-3.832) 3.084 (3.650-2.606) 1.352 (1.625-1.125) 2.503 (2.809-2.231) 2.372 (2.463-2.285) 2.485 (2.968-2.081)

KFb

TABLE 4. Dose and Surface-Area-Normalized Freundlich Isotherm Coefficients for Model and Isolated NOMs on Modified Carbonsa

FIGURE 1. Adsorption of PMA, LaFA, and AHA by modified and untreated coal-based carbons. N/D 0.392 (0.453-0.331) 0.425 (0.482-0.368) 0.358 (0.407-0.309) 0.492 (0.509-0.474) 0.515 (0.512-0.468) 0.436 (0.482-0.390) 0.509 (0.549-0.470) 0.547 (0.564-0.550)

0.402 (0.440-0.365) 0.383 (0.449-0.316)

nd

plausible explanation for this difference is that the impact of changes in the carbon surface chemistry on DOM adsorption increases as the ability of molecules to access (and interact with) adsorbent surfaces increases. If water clusters form around polar functional groups on the surface, the reduction in effective pore size may be more significant in smaller pores. In addition, at the pH of the isotherm experiments (i.e., pH 7), PMA and LaFA are expected to carry a higher net negative charge density than AHA due to their higher carboxylic-type of acidity and smaller size. The carbon surfaces also become more negatively charged with increasing surface acidity, as indicated by lower pHpzc values (22). Therefore, it is also possible that the more significant impact on the adsorption of PMA and LaFA resulted from an increase in the intensity of repulsive forces between the adsorbates and the GAC surface. On a surface area basis, uptake of AHA was less impacted by surface oxidation than either PMA or LaFA. Uptake of the different NOM samples by the most oxidized carbon (i.e., OX 9/70) is more strongly correlated with the carboxylic-type acidity of the macromolecules than with the molecular size, demonstrating the important role of chemical interactions in controlling the DOM adsorption. Subsequent heat treatment of oxidized carbons at 650 °C reduced the total surface acidity of by about a factor of 3, primarily by reducing the density of strongly acidic groups on the surface. The impact on DOM adsorption was to partially restore the reduction in uptake that occurred as a result of surface oxidation. Here, too, the increase was more significant for PMA and LaFA than AHA (Table 4), consistent with the observed impacts of surface oxidation. The applicability of the findings with model DOMs to NOMs in natural waters was investigated by conducting isotherm experiments using selected modified GACs and raw water samples collected from the influents of Charleston and Myrtle Beach, SC, drinking water treatment plants (WTPs), from the Tomhannock reservoir, the water supply for the City of Troy, NY, and from a stream draining a rural agricultural watershed in Rensselaer County, NY. Representative results from these experiments are presented in Figure 2 and the dose-normalized Freundlich parameters for all isotherms are given in Table 5. The trends are consistent with those observed with model DOMs: (1) no significant capacity difference was observed between acid-washed and heat-treated carbons samples; (2) oxidation of the carbon surface significantly decreased the uptake; (3) the capacity was partially restored by subsequent heat treatment of the oxidized surfaces. Although no isolation and characterization of surface water NOMs was performed, the similarity of isotherm trends to those for PMA and LaFA suggests that these natural waters are composed of relatively small molecular weight NOM molecules. For natural water samples, the slopes of the isotherms were in the range of 0.5-0.7, except Myrtle Beach NOM, which exhibited a steeper adsorption pattern. The order of NOM uptake by both untreated and oxidized carbons appeared to correlate well with specific ultraviolet absorbance (SUVA), except Myrtle Beach NOM, which had the highest SUVA. Because the results of coal-based carbons indicated that surface acidity appears to have a significant effect on DOM uptake, it was hypothesized that uptake of wood-based carbon (WVB) should increase upon reduction in its high level of initial surface acidity. Furthermore, given the mesoporous nature of wood-based carbon and its higher surface area, its overall DOM capacity was expected to exceed the lower surface area and more microporous coal-based carbon (F400). Heat treatment reduced the total surface acidity of the wood-based carbon by about 50%; however, the impact on the NOM adsorption was surprisingly minimal (e.g., PMA) or absent (Figure 3). Subsequent oxidation of the heat-treated carbon doubled the total surface acidity and

FIGURE 2. Adsorption of Charleston and Myrtle Beach NOM by modified coal-based carbon. tripled the density of strongly carboxylic groups on the carbon surface. However, the impact on the DOM adsorption was still not significant. The isotherm results for the NOM samples obtained from Charleston and Myrtle Beach WTPs were consistent with those of model DOMs (Data not shown). The significant impact of surface oxidation on the DOM uptake by the coal-based carbon and the absence of such an impact for the oxidized wood-based carbon suggests two possibilities: (1) the type of acidity created on the two carbon surfaces under similar surface treatment conditions and their reactivities toward DOM adsorption may be quite different and may depend on the type of raw material (consistent with the significant difference observed for the two synthetic organic compounds reported in the companion paper (22)); (2) other differences in surface chemistry exist and may contribute to the observed trends for wood-carbons, but such differences cannot be characterized using the methods employed in this study. The first hypothesis was investigated by comparing correlations between surface-area-normalized Freundlich affinity parameters and surface acidity characterized using Boehm method. In terms of the impact on the isotherm parameters, a clear trend was found for the coalbased carbon: the Freundlich affinity parameter KF decreased with increasing total surface acidity (Figure 4). In contrast, the trend for the wood-based carbon was much weaker. The shape of pore size distribution is such that any choice of minimum pore size would give similar comparison between coal- and wood-based carbons. Furthermore, surface treatment did not change significantly pore size distribution for either coal- or wood-based carbons. Therefore, the trends shown in Figure 4 are not sensitive to the choice of pore size used to calculate accessible surface area. Comparison of the trends for modified coal- with woodbased carbons also indicated that the reactivity of carbon VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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a

5.969 (6.236-5.714) 5.316 (6.437-4.391) 2.022 (2.317-1.765) 3.416 (3.861-3.022) 1.754 (2.245-1.369) 3.164 (3.339-2.998) 2.165 (2.326-2.014) b

0.345 (0.419-0.271) 0.370 (0.452-0.287) 0.544 (0.591-0.497) 0.442 (0.497-0.387) 0.465 (0.547-0.382) 0.314 (0.380-0.247) 0.439 (0.510-0.369)

nc

N/D N/D N/D

N/D N/D N/D

nc 0.643 (0.728-0.557) 0.651 (0.767-0.535) 0.606 (0.670-0.543) 0.568 (0.625-0.512)

Tomhannock

3.375 (3.808-2.992) 3.314 (3.904-2.812) 2.348 (2.599-2.121) 3.228 (3.519-2.961)

KF

b

(mg DOC/g GAC)1-n. c Dimensionless. N/D, not determined.

10.695 (12.132-9.428) 11.247 (13.054-9.690) 3.342 (3.779-2.955) 6.316 (7.147-5.560) 3.693 (4.604-2.962) 5.906 (7.037-4.957) 3.491 (4.179-2.916)

0.563 (0.609-0.517) 0.725 (0.971-0.479) 0.731 (0.835-0.627) 0.680 (0.507-0.344) 0.639 (0.808-0.470) 0.503 (0.546-0.460) 0.543 (0.593-0.493)

Myrtle Beach NOM

KF

b

nc

Charleston NOM

Values in the parantheses for KF and n are the 95% confidence intervals.

WVB OX 2/70

WVB HT 1000

WVB AW

F400 OX 9/70 HT 650

F400 OX 9/70

F400 HT 1000

F400 AW

modified carbons

KFb

TABLE 5. Dose-Normalized Freundlich Isotherm Coefficients for NOMs in Natural Waters on Modified Carbonsa

FIGURE 3. Adsorption of PMA, LaFA, and AHA by modified woodbased carbons.

FIGURE 4. The impact of carbon surface acidity on DOM uptake by surface-treated carbons. KF*, Surface-area-normalized Freundlich isotherm parameter.

N/D

N/D

N/D

nc

N/D

N/D

N/D

0.632 (0.702-0.562) 0.656 (0.707-0.605) 0.672 (0.702-0.642) 0.660 (0.701-0.618)

Rensselaer County

4.159 (4.571-3.784) 4.004 (4.282-3.744) 2.240 (2.297-2.184) 3.151 (3.325-2.987)

KF

b

N/D Dimensionless. N/D, not determined.

N/D N/D

N/D N/D N/D

N/D

0.535 (0.609-0.460) 0.419 (0.462-0.376) 0.631 (0.782-0.480) 0.656 (0.954-0.357) 0.794 (0.845-0.746) 0.761 (0.826-0.701) 0.722 (0.803-0.649) 0.052 (0.111-0.024)

N/D N/D

(mg DOC/g GAC)1-n. c (mg DOC/m2 GAC)1-n.

N/D

N/D

N/D

0.104 (0.122-0.089) 0.141 (0.157-0.126) 0.738 (1.043-0.523)

b

Values in the parantheses for KF and n are the 95% confidence intervals.

N/D

N/D

1.117 (1.211-1.022) 1.049 (1.144-0.954) 0.722 (0.852-0.592) N/D

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a

MACRO

0.087 (0.114-0.066) 0.174 (0.221-0.137) 0.538 (0.742-0.390) MESO

WPLL

BPL

FS100

Wood-based MICRO

5.777 (5.931-5.627) 2.646 (2.765-2.532) 7.131 (7.618-6.675) 0.396 (0.439-0.357) 0.261 (0.290-0.231) 0.306 (0.332-0.281) 0.276 (0.300-0.251) 0.497 (0.558-0.453) 0.773 (0.821-0.728) 0.534 (0.555-0.516) 0.694 (0.739-0.651) 0.101 (0.112-0.091) 7.592 (8.111-7.106) 3.332 (3.563-3.117) 8.344 (8.856-7.861) 0.362 (0.400-0.327) Coal-based F400

KF KF KF carbon

0.676 (0.692-0.661) 0.434 (0.443-0.425) 0.643 (0.692-0.597) 0.123 (0.135-0.112)

d

0.832 (0.940-0.725) 0.867 (0.932-0.802) 0.508 (0.657-0.360)

KF

2.800 (3.560-2.203) 2.836 (3.363-1.360) 2.139 (3.255-2.471) 0.119 (0.348-0.040) 0.306 (0.317-0.295) 0.315 (0.331-0.299) 0.299 (0.327-0.271) 0.538 (0.598-0.478)

nd KF c

LaHA

b

nd c

LaFA

b

nd PMA

KFc b

TABLE 6. Dose- and Surface-Area-Normalized Freundlich Isotherm Coefficients for NOMs on As-Received Carbonsa

surfaces for DOM was different at the same surface acidity and depended on raw material type, consistent with the trends observed for low molecular weight priority pollutants, TCE and TCB (22). In addition, no distinct correlation was found with a particular functional group as determined using Boehm method; in general KF was observed to decrease with an increase in each type of acidic group (data not shown). Therefore, it appears that the surface chemistry of carbons governs the uptake in a way that cannot be precisely quantified using the classical Boehm method. The impact of surface treatment on the Freundlich exponential coefficient, n, was less consistent than the effects on KF. For the woodbased carbon (WVB), the n-value decreased upon heat treatment for all NOM materials, except one (PMA). For the coal-based carbon (F400), n either stayed the same (PMA) or went up. A reduction in n corresponds to the creation of a more energetically heterogeneous surface. For the more oxidized WVB, it seems likely that the removal of oxygencontaining groups by heat treatment created more highenergy sites, increasing surface heterogeneity in that direction. Reasons for increases in n upon heat treatment for the F400 are not readily apparent but may have resulted from a widening of high-energy micropores. In most (but not all) cases, surface oxidation following heat treatment appears to increase the value of n. It is possible that the surface becomes more uniform as a result of the loss or blockage of high energy sites. Increases in surface heterogeneity upon oxidation (decreases in n) could be explained by the creation of reactive functional groups on the surface. This behavior was seen with PMA uptake by both F400 and WVB carbons, and LaFA by the WVB carbon. Adsorption of DOM by As-Received GACs. Adsorption of DOM by as-received GACs was investigated by conducting isotherm experiments for PMA, LaFA, and LaHA. For the coal-based carbons, the extent of DOM adsorption (on a mass basis) was found to be inversely correlated with the microporosity of carbons: BPL g F400 > FS100 > WPLL (Table 6). Adsorption increased with increasing surface area in the mesopore- and macropore-size range (i.e., greater than 20 Å width). Carbon surface chemical properties also appear to influence the adsorption. On the basis of pore structure alone, the capacity of F400 should be more similar to the FS100 than the BPL carbon. However, for PMA and LaFA, the capacity of the F400 on a mass basis was more comparable to the BPL, and significantly higher than that of FS100. For LaHA, similar adsorption uptakes were observed on all three carbons. The role of carbon surface chemistry on the DOM adsorption was examined by comparing isotherms normalized on an accessible-surface-area basis (Figure 5). The DOM uptakes of F400 and BPL carbons having the lowest total acidity and basicity were slightly higher than FS100, except for LaHA. WPLL, with the highest total acidity and basicity, had the lowest capacity for all DOMs tested. These trends are consistent with the notion that carbon surface functional groups play a role on the DOM uptake. Correlations, similar to the ones observed with the surface-treated carbons, were found between surface area normalized KF constant and carbon surface acidity (data not shown); however, no distinct correlation was apparent with a particular functional group. Although the trend observed for the acidic functional groups is consistent with that reported by Summers and Roberts, the trend for the basic groups is weak and contradictory. Summers and Roberts (29) reported a strong correlation between Freundlich KF constants of the surface-areanormalized isotherms and the HCl consumption and pHPZC of carbons. The difference between these two results can be explained, in part, by the different relationships between surface acidity and surface basicity (as measured by the Boehm technique) for different carbon source materials and

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FIGURE 5. Surface-area-normalized isotherms of PMA, LaFA, and LaHA by as-received coal-based carbons. methods of activation. For the group of carbons studied by Summers (29), surface acidity and HCl consumption were inversely correlated, while for the group studied in this research, the opposite was true. Furthermore, for some carbons, these trends can be changed by pretreatment techniques. It would be expected that the uptake of organic acids would be increased by increasing surface basicity, other factors being equal. Therefore, we view the correlation between decreasing uptake with increasing surface acidity as most reliable. For the wood-based as-received carbons, the extent of adsorption (on a mass basis) increased with increasing average pore radius (Table 6). While detailed pore size distributions were not measured for these carbons, and accessible surface areas could not be calculated, it is likely that MESO and MACRO carbons exhibit higher uptake than the MICRO carbon because of the increase in the percentage of surface area in pores in the mesopore- and macroporesize range. Surface area in pores having widths greater than 3232

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100 Å may contribute to the greater extent of adsorption exhibited by the MACRO carbon vs MESO carbon. Surface charge apparently does not account for the difference, because all of the wood-based carbons have similar surface acidity, and the MACRO carbon has a lower pHPZC (22). Uptake appears to increase with decreasing basicity, but the changes in basicity are too small to explain the changes in uptake; the trend with basicity is probably seen because basicity correlates with pore size for these carbons. Surface charge clearly plays a role when comparing the extent of adsorption on the basis of raw material type. On the basis of pore structure alone, both the MESO and MACRO carbons would be expected to exhibit a greater extent of adsorption than all coal-based carbons, and the MICRO carbon would be expected a greater capacity than both the FS100 and WPLL carbons. The significantly lower extent of adsorption observed for all three DOMs tested can only be explained on the basis of surface chemistry. The adsorbent surface of the wood-based carbon is negatively charged at pH 7, the pH of the experiment. Therefore, both the surface and the DOMs are negatively charged; the repulsive forces developed as a result of like-charge repulsion may cause a reduction in the extent of adsorption. This explanation is consistent with the results of surface-treated carbons and the findings of Summers and Roberts (29). Comparison of the trends between as-received and surface-treated coal carbons did not result in a strong correlation between the reactivity of carbon surface and the surface acidity despite the same raw material type (data not shown). While general trends between surface acidity and DOM uptake are suggested, and in case of surface-treated carbons, such trends are more clear, the Boehm method does not appear to reflect the changes in carbon surface reactivity resulting from surface treatment. For example, the as-received F400 carbon and the F400 OX 2/70 H6 modified sample have similar total acidity (22). The latter is less microporous and has less strong carboxylic group density compared to the former. On the basis of these characteristics, one would expect the F400 OX 2/70 H6 sample to exhibit higher (or at least similar) capacity. On the contrary, uptake of PMA and LaFA by the as-received F400 (Table 6) was significantly higher than that of the F400 OX 2/70 H6 (Table 4). Therefore, it seems that the acidic groups created by surface oxidation are not similar in nature to those functionalities initially present on the as-received carbon, and the Boehm technique does not reflect this difference. However, the technique seems useful for characterizing the changes in the surface chemistry resulting from surface treatments similar to those employed in this research. Recently developed techniques for predicting surface acidityconstant distributions may provide more quantitative information to predict surface reactivity of carbons for DOM uptake (42) however, the evaluation of such techniques was not within the scope of this research. Overall, the results of this study indicate that the capacity of a GAC for a DOM is a function of two major factors: (1) carbon pore size vs DOM molecular size; (2) carbon surface acidity vs DOM chemical composition. Adsorbent capacity is expected to increase with decreasing DOM size as long as both the surface and sorbate macromolecules are chemically compatible. Repulsive forces between strongly acidic functionalities (such as carboxylic groups) on the GAC surface and within the DOM structure may significantly reduce adsorption capacity. Our results indicate that NOM removal can be maximized by selecting GACs with minimal surface acidity and pores widths in the range of 20-100 Å (e.g., more mesoporous). However, the optimum pore size distribution for a given NOM will depend on its molecular size distribution, a property that is system specific. It appears that the trends observed with surface-treated and as-received coal carbons

are in qualitative agreement. However, the reactivity of carbon surfaces to DOM uptake depends on the raw material type, activation conditions and surface treatment. The Boehm technique is not sensitive enough to provide a precise distribution of surface proton binding affinity; therefore, it cannot be used as a quantitative predictor of reactivity of different carbons for DOM adsorption. However, the technique was useful for characterizing the changes in the surface reactivity resulting from surface treatments similar to those employed in this research.

Acknowledgments This research was funded in part by the Clemson University Research Grants (Grant: 1-30-0919-51-4455). Partial support for J. Kilduff and A. Wigton from the Kodak Corporation is gratefully acknowledged.

Literature Cited (1) Disinfection By-Products in Water Treatment: The Chemistry of Their Formation and Control; Minear, R. A., Amy, G. L., Eds.; Lewis Publishers: Boca Raton, FL, 1996. (2) Safe Drinking Water Act Amendments of 1986, PL-99-339. (3) Pontius, F. W. J.sAm. Water Works Assoc. 1996, 88 (3), 36. (4) Pontius, F. W. J.sAm. Water Works Assoc. 1996, 88 (8), 16. (5) Smith, E. H.; Tseng, S.; Weber. W. J., Jr. Environ. Prog. 1987, 6, 18. (6) Sonthmeir, H.; Crittenden, J. C.; Summers, R. S. Activated Carbon for Water Treatment, 2nd ed.; DVGW-Forschungsstelle, FRG, 1988. (7) Summers, R. S.; Haist, B.; Koehler, J.; Ritz, J.; Zimmer, G.; Sonthmeir, H. J.sAm. Water Works Assoc. 1989, 81, 66. (8) Carter, M. C.; Weber, W. J., Jr.; Olmstead, K. P. J.sAm. Water Works Assoc. 1992, 84, 81. (9) Carter, M. C.; Weber, W. J., Jr. Environ. Sci. Technol. 1994, 28, 614. (10) Knappe, D. R.; Snoeyink, V. L.; Roche, P.; Prados, M. J.; Bourbigot, M.-M. Water Res. 1997, 31, 2899. (11) Weber, W. J., Jr.; Pirbazari, M. J.sAm. Water Works Assoc. 1982, 74, 203. (12) Pirbazari, M.; Weber, W. J., Jr. J. Environ. Eng. Div., ASCE 1984, 110, 656. (13) Smith, E. H.; Weber. W. J., Jr. Environ. Sci. Technol. 1988, 22, 313. (14) Speth, T. F.; Miltner, R. J. J.sAm. Water Works Assoc. 1989, 81, 141. (15) Carter, M. C. Ph.D. Thesis, University of Michigan, Ann Arbor, 1993. (16) Narbaitz, R.; Benedek, A. J. Environ. Eng. Div., ASCE 1994, 120, 1400. (17) Thurman, E. M.; Wershaw, R. L.; Malcolm, R. L.; Pinckney, D. J. Org. Geochem. 1982, 4, 27.

(18) Cornel, P. K.; Summers, R. S.; Roberts, P. V. J. Colloid Interface Sci. 1986, 110, 149. (19) Cameron, R. S.; Thornton, B. K.; Swift, R. S.; Posner, A. M. J. Soil Sci. 1972, 23, 394. (20) Aiken, G. R.; Malcolm, R. L. Geochim. Cosmochim. Acta 1987, 51, 2177. (21) Carter, M. C.; Weber, W. J., Jr.; Olmstead, K. P. J.sAm. Water Works Assoc. 1992, 8, 81. (22) Karanfil, T.; Kilduff, J. E. Environ. Sci. Technol. 1999, 33, 32173224. (23) Lee, M.; Snoeyink, V. L.; Crittenden, J. J.sAm. Water Works Assoc. 1981, 73, 440. (24) Cannon, F. S.; Roberts, P. V. J. Environ. Eng. Div., ASCE 1982, 118, 766. (25) Weber, W. J., Jr.; Voice, T. C.; Jodellah, A. J.sAm. Water Works Assoc. 1983, 75, 612. (26) Ogino, K.; Yukihiro, K.; Minoura, T.; Agui, W.; Abe, M. J. Colloid Interface Sci. 1988, 121, 161. (27) Starek, J.; Zukal, A.; Rathousky, J. Carbon 1994, 32, 207. (28) Summers, R. S.; Roberts, P. V. J. Colloid Interface Sci. 1988, 122, 367. (29) Summers, R. S.; Roberts, P. V. J. Colloid Interface Sci. 1988, 122, 382. (30) Kilduff, J. E.; Karanfil, T.; Chin, Y.-P.; Weber, J. W., Jr. Environ. Sci. Technol. 1996, 30, 1336. (31) Karanfil, T.; Kilduff, J. E.; Schlautman, M. A.; Weber, J. W. Jr. Environ. Sci. Technol. 1996, 30, 2187. (32) Fu, P. L. K.; Symons, J. M. J.sAm. Water Works Assoc. 1990, 82, 70. (33) Karanfil, T.; Kilduff, J. E.; Schlautman, M. A.; Weber, J. W. Jr. Water Res. 1998, 32, 154. (34) McCreary, J. J.; Snoeyink, V. L. Water Res. 1980, 14, 151. (35) Summers, R. S. Ph.D. Thesis, Stanford University, Palo Alto, CA, 1986. (36) Harrington, G. W.; DiGiano, F. A. J.sAm. Water Works Assoc. 1989, 81, 93. (37) Karanfil, T.; Schlautman, M. A.; Kilduff, J. E.; Weber, J. W., Jr. Environ. Sci. Technol. 1996, 30, 2195. (38) Kilduff, J. E.; Weber, J. W., Jr. Environ. Sci. Technol. 1992, 26, 569. (39) Ghosh, K.; Schnitzer, M. Soil Sci. 1980, 129, 266. (40) Randtke, S. J.; Snoeyink, V. L. J.sAm. Water Works Assoc. 1983, 75, 406. (41) Grant, T. M.; King, J. C. Ind. Eng. Chem. Res. 1990, 29, 264. (42) Jagiello, J.; Bandozs, T.; Putreya, K.; Schwarz, J. A. J. Colloid Interface Sci. 1995, 172, 341.

Received for review October 1, 1998. Revised manuscript received March 16, 1999. Accepted June 25, 1999. ES9810179

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