Environ. Sci. Technol. 2002, 36, 3426-3431
Pesticide Adsorption by Granular Activated Carbon Adsorbers. 1. Effect of Natural Organic Matter Preloading on Removal Rates and Model Simplification 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 RYUICHI TAKAGI Department of Civil Engineering, Gifu University, Yanagido 1-1, Gifu 501-1193 Japan
The adsorptive removal of periodic spikes of the trace synthetic organic chemicals (SOCs) simazine and asulam from water containing natural organic matter (NOM) was studied in pilot-scale granular activated carbon (GAC) adsorbers over a period of nearly 3 years. The SOC removal percentage obtained at any preloading time and bed depth was independent of the liquid-phase SOC concentration, and equations derived from the ideal adsorbed solution theory and a pore surface diffusion model validated this observation. The pseudo-steady-state SOC removal rate, (∂C/∂z), at each preloading time and bed depth was therefore first order with respect to the liquid-phase SOC concentration, C. Furthermore, the removal modulus, k, in the resulting SOC removal rate expression was a reflection of the solidphase concentration of the NOM fraction that interfered with the adsorption of SOCs. Analysis of the removal modulus values indicated that the mass transfer zone of the NOM fraction competing with asulam traveled more rapidly through the GAC adsorber than that competing with simazine. Given the similar molecular sizes of the targeted SOCs, this result was primarily explained by differences in SOC adsorbabilities, where the more weakly adsorbing asulam was less capable of displacing preloaded NOM. Consequently, the NOM fraction competing with asulam constituted a larger percentage of the total NOM than that competing with simazine.
Introduction Granular activated carbon (GAC) adsorption is the best available technology for the control of many agrochemicals and other synthetic organic chemicals (SOCs) in drinking water. The design and operation of a GAC adsorber remain complicated, however, because natural organic matter (NOM) adsorption (or NOM fouling) affects the effectiveness of GAC * Corresponding author phone: +81-58-293-2429; fax: +81-58230-189; e-mail:
[email protected]. 3426
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 15, 2002
adsorbers by decreasing both the adsorption capacity and the adsorption rate of trace SOCs (1, 2). Because of the unknown character of the SOC-competitive NOM, i.e., the NOM that adversely affects SOC adsorption by pore blockage, pore constriction, and/or direct competition for adsorption sites (3-5), a straightforward application of multisolute adsorption models remains difficult. Further studies are therefore required to elucidate reduction patterns in SOC adsorption rate and capacity with respect to NOM loading. If such patterns can be accurately described, they can be incorporated into modeling strategies that facilitate the design of GAC adsorbers and the determination of the remaining life of operating GAC adsorbers. Reduction patterns in SOC adsorption rate and capacity are a function of the SOC-competitive NOM loading, which changes with GAC service time and bed depth. For example, Summers et al. (6) observed that NOM preloading of an adsorber containing coal-based GAC decreased the trichloroethene (TCE) adsorption capacity, first rapidly and then more gradually, over the 25-week study period. After 4 weeks of preloading, the TCE adsorption capacity increased with increasing bed depth, a result that can be explained by a higher NOM loading and/or the presence of more strongly adsorbed NOM in the top reaches of the GAC adsorber. With increasing NOM loading (or GAC service time), the TCE capacity dependence on bed depth became less prominent, suggesting that the NOM loading and the character of the adsorbed NOM became more uniform across the GAC adsorber. Knappe et al. (2, 7) reported that preloaded NOM reduced the atrazine adsorption capacity uniformly over the 2.2-m depth of a GAC adsorber after only 8 weeks of operation, a time at which the TOC adsorption capacity of the woodbased GAC had been exhausted. Differences in the characteristics of the target SOC (polarity, molecular size), the NOM (polarity, charge, molecular size distribution), and the adsorbent (pore size distribution, surface charge, polarity) were all likely contributors to the differences between the results of Summers et al. (6) and Knappe et al. (2, 7). Regarding adsorbate polarity, Mallevialle et al. (8) observed that the GAC adsorption capacity for pyridine decreased more rapidly with NOM preloading time than that of phenol, a result that was attributed to the weaker adsorption of the more polar pyridine. The different effects of NOM loading (or GAC service time) on the reduction patterns of the pyridine and phenol adsorption capacities suggest that an adsorbed NOM fraction existed that could be displaced by phenol but not by pyridine. Especially at shorter operational times, when the adsorbed NOM was likely comprised of both polar and nonpolar components, the phenol-competitive NOM was not identical to the pyridine-competitive NOM. Consequently, a bulk NOM parameter such as the TOC loading at any given GAC service time or bed depth does not uniquely relate to the reductions in adsorption capacity and removal rate that can be expected for SOCs with different adsorbabilities. Despite the above-noted complexities, NOM adsorption on activated carbon appears to yield one surprisingly simple result with respect to the adsorption of trace SOCs from natural water: SOC removal percentages across a fixed bed adsorber become independent of the influent SOC concentration. For example, Matsui et al. (9) showed that the removal percentages of the pesticides simazine and asulam across a microcolumn containing pulverized GAC were independent of their influent concentrations but individually dependent on the NOM loading. These results were confirmed for the removal of 2-methylisoborneol (MIB) in laboratory-scale columns packed with partially spent GAC taken from 10.1021/es0113652 CCC: $22.00
2002 American Chemical Society Published on Web 07/02/2002
TABLE 1. Properties of Simazine and Asulam (41, 42) name
formula
mol wt, g/mol
aqueous solubility 5 mg/L at 20-22 °C 3.5 mg/L at 25 °C 4000 mg/L at 20-25 °C
simazine
C7H12ClN5
201.7
asulam
C8H10N2O4S
230.2
operating full-scale adsorbers (10). For trace SOCs in the presence of NOM, a direct proportionality also exists between the SOC adsorption capacity and the initial SOC concentration at a given powdered activated carbon (PAC) dose in batch reactors (11-13). This result implies that a linear equilibrium relationship exists between solid- and liquidphase SOC concentrations at a given NOM loading, which is accomplished with a given PAC dose (11, 14). The objectives of this study were (1) to verify both experimentally and theoretically that, in the presence of NOM, the trace SOC removal percentage across the depth of a pilotscale GAC adsorber is independent of the liquid-phase SOC concentration and (2) to utilize this result to develop a parameter capable of describing the depth profile of the SOCcompetitive NOM loading. A third objective of this study was to characterize the SOC-competitive NOM, which will be addressed in a companion paper (15). To evaluate the importance of SOC and activated carbon properties, experiments were conducted with two pesticides of different polarity (simazine and asulam) and two carbons of different polarity and pore structure (thermally activated coal-based GAC and chemically activated wood-based GAC).
Materials and Methods Water. Water containing NOM was obtained by diluting the effluent from a municipal wastewater treatment plant (Kakamigahara, Japan) with dechlorinated tap water (Gifu, Japan). The wastewater treatment plant employed an activated sludge process followed by ferric chloride-coagulation/ sedimentation/sand filtration. Total organic carbon (TOC) and ultraviolet absorbance at 260 nm (UV260) served as parameters for bulk NOM quantitation. The TOC and UV260 of the column influent and effluent were measured daily (TOC: Model 810, Sievers Instruments, Inc., Boulder, CO; UV260: Model UV-240, Shimadzu Co., Kyoto, Japan). Influent TOC and UV260 levels varied between 1.1 and 1.8 mg/L and 2.0-2.5 m-1, respectively. Adsorbates. The SOCs employed in this study were reagent grade simazine and asulam (Wako Pure Chemicals Industries, Ltd., Osaka, Japan). These herbicides were selected because they have similar molecular weights but different polarity as indicated by their aqueous solubilities and octanol-water partition coefficients (Table 1). A stock solution of each pesticide was prepared by the direct addition of the pesticide to the above-described water without the assistance of organic solvent. To remove nondissolved pesticide particles, the stock solution was filtered through a 0.2-µm membrane filter. For SOC-spike experiments, pesticide stock solutions were diluted with the above-described water to the desired influent concentration. High performance liquid chromatography (HPLC) with a reverse-phase partition column and UV detection was used for the analysis of simazine (column: Wakosil-II 5C-18-150, Wako Pure Chemicals Industries, Ltd., Osaka, Japan; eluent: CH3CN/H2O ) 70/30; HPLC system: 600E & 600F, Waters Co., Milford, MA) while high performance size-exclusion chromatography with UV detection was employed for the analysis of asulam (column: GL-W520, Hitachi Ltd., Tokyo, Japan; eluent: 0.02 M-KH2PO4 + 0.02 M-Na2HPO4; HPLC system: LC Module-1 plus, Waters Co., Milford, MA).
octanol/water partition coeff (log Kow) 2.5 0.23
Adsorbents. Thermally activated coal-based (Filtrasorb 400, Calgon Carbon Corporation, Pittsburgh, PA) and chemically activated wood-based (PICABIOL, PICA, Levallois, France) GACs were used for pilot column studies. In addition, crushed porous ceramic (NGK Insulators, Ltd., Nagoya, Japan, nominal pore size 15 µm) served as a nonadsorbing control. The GACs and the ceramic were sieved, and particle size fractions of 1.00-1.18 mm were selected. The absence of pesticide removal in columns packed with the porous ceramic verified that adsorbate removal did not occur as a result of biological activity in the pilot columns. Pilot Columns. GACs and crushed ceramic were packed into PVC columns (25-mm inside diameter). Sampling ports at 10-cm intervals permitted the determination of concentration depth profiles. GAC adsorbers were operated at a superficial velocity of 4.8 m/h for a period of 3 years. SOC spikes were added intermittently to the columns by switching the flow line of the SOC-free feed solution to a pesticidespiked feed solution. Samples were taken from the sampling ports at a slow flow rate (about 0.5 mL/min) to not disturb the column flow rate. Upon switching to pesticide-spiked water, reduced concentration profiles. (C/C0 at each bed depth increased for about 5 days, after which time a pseudosteady state was reached that represented the pesticide removal capability of the GAC column at a given preloading time and bed depth. A similar approach of evaluating the performance of NOM-preloaded GAC has been described in prior publications (9, 10).
Results and Discussion Effect of SOC Concentration. To determine the effect of the influent SOC concentration on SOC removal patterns by NOM-preloaded GAC, water containing simazine or asulam at different initial concentrations was applied to GAC adsorbers after different preloading times. Figures 1 and 2 show the resulting pseudo-steady-state depth profiles of reduced simazine and asulam concentrations (C/C0), respectively. For simazine, the 10-µg/L spike was introduced to the GAC columns from day 648 to day 654, and the 120µg/L spike was introduced to the GAC columns from day 690 to day 697. As these two experiments were conducted at similar preloading times, the NOM loading on a given GAC was assumed to be equal. For both GACs, agreement between C/C0 depth profiles at the two influent concentrations (Figure 1) proved that the simazine removal percentage was independent of the influent simazine concentration (or for that matter the liquid-phase simazine concentration at any point in the GAC adsorber) at a given NOM loading. Results for the more polar asulam confirmed this observation (Figure 2). Therefore, the data in Figures 1 and 2 show that neither adsorbent nor adsorbate characteristics affected the independence of the pesticide removal percentage from the influent pesticide concentration. Furthermore, the results obtained with pilot-scale adsorbers in this study are consistent with previous findings for smaller fixed-bed adsorbers as well as for other adsorbates, such as MIB (9, 10). The depth profile data in Figures 1 and 2 show that both simazine and asulam removals were greater in the adsorbers containing coal-based GAC. Both the presence of a more suitable pore volume and the lower polarity of the coalVOL. 36, NO. 15, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3427
FIGURE 1. Effect of influent simazine concentration on simazine removal by (a) coal-based GAC and (b) wood-based GAC. NOM preloading times for each initial simazine concentration are shown in the legend.
FIGURE 2. Effect of influent asulam concentration on asulam removal by (a) coal-based GAC and (b) wood-based GAC. NOM preloading times for each initial asulam concentration are shown in the legend. based GAC can explain the observed results. Prior studies have shown that the pore volume of chemically activated wood-based GACs that is suitable for the removal of trace SOCs from aqueous solutions is smaller than that of many thermally activated carbons (16-18). Furthermore, the relatively high oxygen-content and thus the greater polarity of the pore surfaces in chemically activated wood-based GACs leads to the enhanced adsorption of water. Water can adsorb by means of hydrogen bonds on oxygen-containing functional groups, and clustering of additional water molecules occurs around water molecules adsorbed at these sites (1926). Such water clusters adversely affect the adsorption of trace SOCs by (1) preventing pollutant access to nonpolar regions on the activated carbon surface, (2) reducing the interaction energy between the pollutant and the adsorbent surface, and/or (3) effectively blocking pollutant access to micropores (26-31). Between the two adsorbates, asulam was more poorly removed than simazine even though asulam tests were conducted after a shorter NOM-preloading time. Asulam adsorption on activated carbon is energetically less favored than that of simazine because asulam, a relatively polar adsorbate, associates more strongly with the solvent (water) than simazine, which is less polar. Furthermore, the weaker adsorbability of asulam implies that it is less capable of displacing preadsorbed NOM. Consequently, the fraction of preloaded NOM affecting the adsorption of asulam, i.e., the asulam-competitive NOM, differs from that affecting the adsorption of simazine, i.e., the simazine-competitive NOM, as discussed in more detail below and in the companion paper (15). The above tests for simazine and asulam were conducted with GACs that were preloaded with NOM. To evaluate whether the SOC removal percentage from NOM-containing water is also independent of the influent SOC concentration 3428
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 15, 2002
FIGURE 3. Effect of influent concentration on atrazine removal by virgin wood-based GAC. Materials and methods for this test have been described elsewhere (43). when starting with virgin activated carbon, the removal of atrazine was studied with a chemically activated wood-based GAC at initial concentrations of 2 and 10 µg/L (Figure 3). Agreement between the normalized atrazine breakthrough curves substantiated that the SOC removal percentage across a fixed bed adsorber is independent of the influent SOC concentration, even in the absence of preloaded NOM at the start of the test. Reconciliation between Experimental Data and Adsorption Models. The experimentally observed independence of the trace SOC removal percentage from the influent SOC concentration is not consistent with the nonlinearity of singlesolute isotherms. In single-solute systems, the adsorptive capacity is not linearly proportional to feed concentration; therefore, two different feed concentrations do not produce identical breakthrough curves when normalized by the feed concentration (e.g. refs 32 and 33). It will be shown, however, that the experimentally observed independence of the trace SOC removal percentage in the presence of NOM is reconcilable with equations derived from the ideal adsorbed
solution theory (IAST, e.g. refs 34 and 35) and a pore surface diffusion model (PSDM, e.g. ref 32). For batch adsorption systems evaluating the adsorption of trace SOCs in the presence of NOM, both experiments and an equation derived from the IAST revealed that the remaining SOC concentration at equilibrium, when expressed as a fraction of the initial concentration, c/c0, is independent of the initial SOC concentration at a given activated carbon dose (11, 13)
c 1 ) n c0 1 + C q (nK/n C NOM NOM qNOM)
(1)
where c and q are the liquid- and solid-phase concentrations of the SOC, respectively, cNOM and qNOM are the liquid- and solid-phase concentrations of the SOC-competitive NOM, respectively, c0 is the initial SOC concentration, n and K are Freundlich exponent and constant of the SOC, respectively, and nNOM refers to the Freundlich exponent of the SOCcompetitive NOM. By incorporating the mass balance for the SOC (c0 ) c + qCC)
q)
qNOM (nNOM qNOM/nK)n
c
q ) KSOC c
(3)
KSOC ) functionK(qNOM)
(4)
and
Since qNOM varies with time, bed depth, and radial position in a GAC particle, the value of KSOC, which is a function of qNOM, varies in the same manner. However, as previously shown for qNOM (11), KSOC is independent from the SOC concentration, provided that q , qNOM. When a PSDM is applied for describing SOC adsorption kinetics in a GAC bed, the governing equations are (e.g. ref 32)
∂q 1 ∂ ∂c ∂q 1 ∂ ) D r2 + 2 D Sr 2 ∂t Fr 2 ∂r P ∂r ∂r ∂r r DP
∂c ∂r
|
r )R
+ D SF
)
∂q ∂r
|
r )R
(
(
) kf C - c
∂C ∂C ∂2C 3kf (1 - ) +u - DZ 2 + C-c ∂t ∂z R ∂z
(
DP ) functionP(qNOM)
(8)
DS ) functionS(qNOM)
(9)
By substituting eq 3 into eqs 5-7 and dividing the resulting equations by the influent SOC concentration (C0), the PSDM can be expressed in the following manner:
KSOC
[
| |
) )
(6)
))0
(7)
r )R
r )R
(5)
where t is the service time, F is the apparent GAC particle density, r is the radial distance from the center of a GAC particle, DP is the pore diffusion coefficient, DS is the surface diffusion coefficient, kf is the film mass transfer coefficient, R is the GAC particle radius, c is the SOC concentration in a liquid-filled pore, C is the SOC concentration in bulk liquid, q is the solid-phase concentration of the SOC, is the porosity of the packed bed, u is the superficial velocity, z is the depth, and DZ is the dispersion coefficient. NOM loading reduces not only the equilibrium adsorption capacity but also the adsorption rate (1, 2). Lower SOC adsorption rates as a result of preloaded NOM therefore suggest that the intraparticle
] ]}
∂(c/C0) ∂(c/C0) ∂KSOC 1 ∂ + (c/C0) ) 2 D Pr 2 + ∂t ∂t ∂r Fr ∂r
{ [
∂(c/C0) ∂KSOC 1 ∂ + (c/C0) DSr 2 KSOC 2 ∂r ∂r ∂r r DP
∂(c/C0) ∂r
|
r )R
[
+ DSF KSOC
]|
∂(c/C0) ∂KSOC + (c/C0) ∂r ∂r
[
kf (C/C0) - (c/C0)
(2)
Thus, a linear equilibrium relationship exists between the solid- and liquid-phase SOC concentrations at a given NOM loading as long as the solid-phase SOC concentration is very small compared to the SOC-competitive NOM loading (14). The trace SOC adsorption isotherm can therefore be described by a linear model in which the proportionality coefficient (KSOC) is a function of the solid-phase concentration of the SOC-competitive NOM (qNOM); i.e.
(
diffusivities (DP and DS) in the PSDM vary as a function of the SOC-competitive NOM loading (qNOM); i.e.
∂(C/C0) ∂2(C/C0) ∂(C/C0) + +u - D Z ∂t ∂z ∂z2 3kf (1 - ) (C/C0) - (c/C0) R
[
|
r )R
| ]
r )R
(10)
r )R
)
(11)
] ) 0 (12)
Equations 10-12 illustrate that the governing equations for fixed bed adsorption can be normalized with respect to the influent concentration and rewritten in terms of the dimensionless variables c/C0 and C/C0. The values of KSOC, DP, and DS are functions of the SOC-competitive NOM loading, which varies in a unique pattern, both temporally and spatially, for a given SOC, GAC, and influent NOM. Consequently, the solution to eqs 10-12 yields a unique breakthrough curve for a given SOC, GAC, and influent NOM that is independent from the influent SOC concentration, provided that the SOC is present at trace levels. This result would be obtained regardless of whether an SOC is continuously or periodically spiked into the column influent as long as the solid-phase SOC concentration is very small compared to the SOC-competitive NOM loading. Hence, the experimental observation that the removal percentage of a trace SOC in the presence of NOM was independent of the SOC concentration at a given GAC service time is consistent with existing models describing SOC adsorption equilibria and kinetics in the presence of NOM. Indirect Determination of the SOC-Competitive NOM Loading. At a given NOM preloading time, the results presented in Figures 1 and 2 showed that the reduced trace SOC concentration profile (C/C0) is independent of the bulkliquid SOC concentration C at any point in the GAC adsorber. Thus, the differentiation of ln(C/C0) with respect to the filter depth (z), i.e., ∂(lnC)/∂z is also independent from C, as are ln(C/C0) and C/C0. Therefore,
(
-
)
∂(lnC) ∂z
qNOM
)k
(13)
where the constant (k) was termed the removal modulus. For a given SOC, GAC, and influent NOM, the change of the removal modulus (k) value is controlled by the solid-phase concentration of the SOC-competitive NOM (qNOM), which varies with bed depth and GAC service time; i.e.
k ) functionk (qNOM) VOL. 36, NO. 15, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
(14) 9
3429
FIGURE 4. Removal modulus values for (a) simazine adsorption by coal-based GAC, (b) simazine adsorption by wood-based GAC, and (c) asulam adsorption by coal-based GAC.
FIGURE 5. NOM removal in pilot-scale adsorbers. Therefore, the value of k reflects an indirect measure of the SOC-competitive NOM loading on the GAC at a given service time and bed depth. When the bulk-liquid SOC concentration decreases slowly with bed depth, i.e., k is small, the SOC-competitive NOM loading is large. Analogously, when the bulk-liquid SOC concentration decreases rapidly with bed depth, i.e., k is large, the SOC-competitive NOM loading is small. Furthermore, at a given qNOM, eq 13 suggests that the SOC removal rate, ∂C/∂z, is first order with respect to C
∂C ) -kC ∂z
(15)
To estimate removal modulus values, and thus indirectly the SOC-competitive NOM loadings, the finite difference, ∆(lnC)/∆z, was computed from the experimental data at different bed depths and preloading times. Figure 4 compares removal modulus values in the upper and lower adsorber reaches for simazine and asulam after different preloading times. For a simazine spike on days 354-356, Figure 4a shows that pseudo-steady-state simazine removals by coal-based GAC yielded a smaller removal modulus value in the upper section (z ) 22-42 cm) of the GAC bed than in the lower section (z ) 52-72 cm). Consequently, the results suggest that the simazine-competitive NOM loading was larger in the upper section of the GAC column. During this period, TOC and UV260 removals by the adsorber containing coalbased GAC were approximately 60 and 50%, respectively (Figure 5), indicating that a NOM mass transfer zone existed in the bed and that the NOM loadings varied quantitatively (and most likely also qualitatively) with bed depth. On days 440-443, the removal modulus values for both the upper and lower sections of the column had become smaller than on days 354-356, which suggests that the simazinecompetitive NOM loading increased during the additional preloading time. While the difference in removal modulus values between the upper and lower sections remained distinct, it became smaller, suggesting that the simazine3430
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 15, 2002
competitive NOM loading became more uniform over the bed depth. On days 695-697, the difference between the upper and lower sections was noticeably smaller than at earlier preloading times, suggesting that the depth-distribution of the simazine-competitive NOM had become more uniform. This result is also supported by the increased TOC and UV260 values in the column effluent during this period (Figure 5). On days 1010-1012, the removal modulus values for the upper and lower column sections as well as the difference between them had decreased even further (Figure 4a). Thus, results over the study period of nearly 3 years showed that the simazine-competitive NOM loading on the coal-based GAC continued to increase throughout the depth of the GAC adsorber and that it became more uniform with time. The removal modulus values describing simazine adsorption by wood-based GAC (Figure 4b) were smaller than those describing simazine adsorption by coal-based GAC (Figure 4a) at each GAC service time. This observation was primarily attributed to the lower simazine adsorption capacity of the more polar wood-based GAC. The difference between removal modulus values for the upper (z ) 27-47 cm) and lower (z ) 57-77 cm) sections of the column was still apparent; however, the difference was less marked than in the adsorber containing coal-based GAC, even early in the test period. Compared to the adsorber containing coal-based GAC, the simazine-competitive NOM was thus more uniformly distributed over the depth of the adsorber containing wood-based GAC. The latter result is confirmed by the earlier breakthrough of NOM, as measured by both TOC and UV260, in the adsorber containing wood-based GAC (Figure 5). These results explain in part why the remaining TCE adsorption capacity of a thermally activated coal-based GAC depended on bed depth, even after a preloading time of 25 weeks (6), while the remaining atrazine adsorption capacity of a chemically activated wood-based GAC showed no dependence on bed depth after only 8 weeks of preloading (2, 7). The removal modulus values describing asulam adsorption by coal-based GAC (Figure 4c) were smaller than those describing simazine adsorption by the same GAC (Figure 4a). This result was primarily attributed to the lower adsorbability of the more polar asulam. Asulam removal modulus values were lower for the upper section (z ) 22-42 cm) of the bed than for the lower section (z ) 52-72 cm) at all tested preloading times, but differences between the upper and lower sections were smaller than those for simazine at comparable preloading times. Consequently, the asulamcompetitive NOM was distributed more uniformly throughout the GAC bed than the simazine-competitive NOM. In other words, the mass transfer zone of the asulam-competitive NOM was more dispersed and traveled more rapidly through the GAC bed than that of the simazine-competitive NOM. This trend concurs with minicolumn results describing
the effect of NOM preloading time on the remaining adsorption capacities of phenol and pyridine, which showed a more rapid capacity decrease for the more polar pyridine than for the less polar phenol (8). The results of the current study illustrate that the simazine-competitive NOM was not equal to the asulam-competitive NOM. The manner in which the adsorption of a given NOM affects the removal of trace SOCs is therefore SOC-specific and governed by such SOC properties as polarity and molecular size. Given the similar molecular weights of simazine and asulam (Table 1), differences between the simazine- and asulam-competitive NOM were primarily a result of the lesser ability of the more weakly adsorbing asulam to displace preloaded NOM. Consequently, the asulam-competitive NOM fraction was most likely a larger percentage of the total NOM than the simazine-competitive NOM fraction. Regarding competition mechanisms, it is almost certain that a fraction of the preloaded NOM reduced the adsorption capacities of the studied SOCs by pore blockage, especially after long GAC service times when the NOM loading was large (e.g., refs 36-40). However, observing differences between the simazine- and asulam-competitive NOM fractions suggests that some NOM fractions continued to compete directly with the SOCs for adsorption sites over the entire study period. These results concur with the interpretation of a prior study (39), in which pore blockage and the resulting surface area reduction of a highly preloaded activated carbon accounted for only a fraction of the observed SOC capacity decrease when the SOC loading was low, as was the case in this study. Overall, the results of this study showed that the presence of preloaded NOM simplifies the estimation of the GAC bed life by eliminating the dependence of the achievable SOC removal percentage on the liquid-phase SOC concentration at any preloading time and bed depth. Consequently, the SOC removal rate at a given preloading time and bed depth is first order with respect to the liquid-phase SOC concentration. The introduced SOC removal modulus in the firstorder rate expression illustrated that the SOC-competitive NOM loading for a given influent NOM is dependent on both GAC and SOC characteristics. Further studies are therefore needed to correlate removal modulus values with SOC, GAC, and NOM properties before a more general adsorption model can be presented.
Acknowledgments The authors express their appreciation to Mitsutaka Uematsu for his assistance in the experiments. The authors would also like to thank Lars B. Reutergardh for reviewing this manuscript. The pulverized ceramic and PICABIOL were obtained from NGK Insulators, Ltd. and Kubota Co., respectively. The results of this study do not reflect the views of the companies and no official endorsement should be inferred.
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32)
(33) (34) (35) (36) (37) (38) (39)
Literature Cited (1) Carter, M. C.; Weber, W. J., Jr. Environ. Sci. Technol. 1994, 28, 614-623. (2) Knappe, D. R. U.; Snoeyink, V. L.; Roche, P.; Prados, M. J.; Bourbigot, M.-M. J. Am. Water Works Assoc. 1999, 91(10), 97109. (3) Pelekani, C.; Snoeyink, V. L. Water Res. 1999, 33(5), 1209-1219. (4) Pelekani, C.; Snoeyink, V. L. Carbon 2000, 38, 1423-1436. (5) Pelekani, C.; Snoeyink, V. L. Carbon 2001, 39, 25-37. (6) Summers, R. S.; Haist, B. H.; Koehler, J.; Ritz, J.; Zimmer, G.; Sontheimer, H. J. Am. Water Works Assoc. 1989, 81(5), 66-74. (7) Knappe, D. R. U.; Snoeyink, V. L.; Matsui, Y.; Prados, M. J.; Bourbigot, M.-M. Water Supply 1994, 14(2), 1-14. (8) Mallevialle, J.; Fiessinger, F.; Maloney, S. W.; Charles, P. In Proceedings of the AWWA Seminar on Nonspecific Organic
(40) (41) (42) (43)
Analysis for Water Treatment Process Control and Monitoring; American Water Works Association: Denver, CO, 1984; pp 149174. Matsui, Y.; Kamei, T.; Kawase, E.; Snoeyink, V. L.; Tambo, N. J. Am. Water Works Assoc. 1994, 86(9), 91-102. Gillogly, T. E. T.; Snoeyink, V. L.; Vogel, J. C.; Wilson, C. M.; Royal, E. P. J. Am. Water Works Assoc. 1999, 91(8), 98-110. Knappe, D. R. U.; Matsui, Y.; Snoeyink, V. L.; Roche, P.; Prados, M. J.; Bourbigot, M.-M. Environ. Sci. Technol. 1998, 32, 16941698. Gillogly, T. E. T.; Snoeyink, V. L.; Elarde, J. R.; Wilson, C. M.; Royal, E. P. J. Am. Water Works Assoc. 1998, 90(1), 98-108. Graham, M. R.; Summers, R. S.; Simpson, M. R.; Macleod, B. W. Water Res. 2000, 34, 2291-2300. Yuasa, A. In Influence and removal of organics in drinking water; Mallevialle, J., Suffet, I. H., Chan, U. S., Eds.; Lewis Publishers: Boca Raton, FL, 1992; pp 19-34. Matsui, Y.; Knappe, D. R. U.; Iwaki, K.; Ohira, H. Environ. Sci. Technol. 2002, 16, 3432-3438. Newcombe, G. In Encyclopedia of Colloid and Interface Science; Hubbard, A., Ed.; Marcel Dekker: New York, in press. Newcombe, G.; Morrison, J.; Hepplewhite, C.; Knappe, D. R. U. Carbon, in press. Li, L.; Quinlivan, P. A.; Knappe, D. R. U. Carbon, in press. Puri, B. R. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, pp 191-282. Dubinin, M. M.; Zaverina, E. D.; Serpinsky, V. V. J. Am. Chem. Soc. 1955, 77, 1760-1766. Barton, S. S.; Evans, M. J. B.; Holland, J.; Koresh, J. E. Carbon 1984, 22, 265-272. Pan, D.; Jaroniec, M. Langmuir 1996, 12, 3657-3665. Mu ¨ ller, E. A.; Rull, L. F.; Vega, L. F.; Gubbins, K. E. J. Phys. Chem. 1996, 100, 1189-1196. Mu ¨ ller, E. A.; Gubbins, K. E. Carbon 1998, 36, 1433-1438. McCallum, C. L.; Bandosz, T. J.; McGrother, S. C.; Mu¨ller, E. A.; Gubbins, K. E. Langmuir 1999, 15, 533-544. Mu ¨ ller, E. A.; Hung, F. R.; Gubbins, K. E. Langmuir 2000, 16, 5418-5424. Coughlin, R. W.; Ezra, F. S. Environ. Sci. Technol. 1968, 2, 291297. Mahajan, O. P.; Moreno-Castilla, C.; Walker, P. L., Jr. Sep. Sci. Technol. 1980, 15, 1733-1752. Kaneko, Y.; Abe, M.; Ogino, K. Colloids Surfaces 1989, 37, 211222. Pendleton, P.; Wong, S. H.; Schumann, R.; Levay, G.; Denoyel, R.; Rouquerol, J. Carbon 1997, 35, 1141-1149. Franz, M.; Arafat, H. A.; Pinto, N. G. Carbon 2000, 38, 18071819. Sontheimer, H.; Crittenden, J. C.; Summers, R. S. Activated Carbon Adsorption for Water Treatment, 2nd ed.; DVGW Forschungsstelle, Engler-Bunte-Institut, Universita¨t Karlsruhe: Karlsruhe, Germany, 1988. Suzuki, M. Adsorption Engineering; Kodansha-Elsevier: Tokyo, Japan, 1990. Myers, A. L.; Prausnitz, J. M. AIChE J. 1965, 11, 121-127. Radke, C. J.; Prausnitz, J. M. AIChE J. 1972, 18, 761-768. Carter, M. C.; Weber, W. J., Jr.; Olmstead, K. P. J. Am. Water Works Assoc. 1992, 84(8), 81-91. Carter, M. C.; Weber, W. J., Jr. Environ. Sci. Technol. 1994, 28, 614-623. Kilduff, J. E.; Karanfil, T.; Weber, W. J., Jr. J. Colloid Interface Sci. 1998, 205, 271-279. Kilduff, J. E.; Karanfil, T.; Weber, W. J., Jr. J. Colloid Interface Sci. 1998, 205, 280-289. Kilduff, J. E.; Karanfil, T.; Weber, W. J., Jr. J. Am. Water Works Assoc. 1998, 90(5), 76-89. Verschueren, K. Handbook of Environmental Data on Organic Chemicals; Van Nostrand Reinhold: New York, U.S.A., 1996. U.S. EPA Office of Drinking Water Health Advisories. Drinking Water Health Advisories: Pesticides; Lewis Publishers: Chelsea, MI, 1989. Knappe, D. R. U.; Snoeyink, V. L.; Roche, P.; Prados, M. J.; Bourbigot, M.-M. Water Res. 1997, 31, 2899-2909.
Received for review October 15, 2001. Revised manuscript received May 6, 2002. Accepted May 24, 2002. ES0113652
VOL. 36, NO. 15, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3431
