Effect of Pore-Blocking Background Compounds on the Kinetics of

Water with Systems, University of Illinois at. Urbana-Champaign, 205 North Mathews Avenue, Urbana,. Illinois 61801, and Nanyang Environment and Water...
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Environ. Sci. Technol. 2008, 42, 4825–4830

Effect of Pore-Blocking Background Compounds on the Kinetics of Trace Organic Contaminant Desorption from Activated Carbon P R I S C I L L A C . T O , †,‡ B E N I T O J . M A R I Ñ A S , * ,‡ V E R N O N L . S N O E Y I N K , †,‡ A N D WUN JERN NG§ Division of Environmental Science and Engineering, Faculty of Engineering, National University of Singapore, Singapore 117576, Department of Civil and Environmental Engineering and Center of Advanced Materials for the Purification of Water with Systems, University of Illinois at Urbana-Champaign, 205 North Mathews Avenue, Urbana, Illinois 61801, and Nanyang Environment and Water Research Institute (NEWRI), 50 Nanyang Ave, Block N1-B3b-29, Singapore 639798

Received January 8, 2008. Revised manuscript received April 1, 2008. Accepted April 3, 2008.

This study examined the effect of pore-blocking (PB) background organic matter, which is known to hinder adsorption kinetics, on the rate of trace contaminant desorption. Adsorption, displaced desorption (DD) and nondisplaced desorption (NDD) kinetic tests were performed using powdered activated carbon (PAC) that was preloaded with natural organic matter (NOM). Since the NOM contained both strongly competing (SC) and PB components, the proposed model separated the contributions of the SC and PB NOM to the overall diffusion coefficient of the target contaminant. By factoring out the SC NOM contribution, which increases the overall diffusion coefficient, it was found that the relationship used to model the effect of PB NOM on adsorption kinetics could also describe desorption kinetics. The results highlighted the substantial influence of competitive SC NOM on the kinetics of adsorption and desorption. SC NOM competition aids contaminant removal by offsetting the undesirable effects of pore blocking on adsorption kinetics. However, for desorption events, PB NOM serves a practical benefit of reducing the rate of release of adsorbed micropollutants, while SC NOM counters that gain by both displacing contaminants and accelerating their diffusion.

Introduction Activated carbon adsorption is a common treatment technology for the removal of natural water contaminants such as organic matter causing tastes and odors, organic matter producing disinfection byproducts, as well as a wide range of synthetic organic compounds such as disinfection byproduct and herbicides. It is well-known that natural organic matter (NOM) will have an adverse effect on contaminant * Corresponding author phone: +1-217-333-6961; fax: +1-217333-6968; e-mail: [email protected]. † National University of Singapore. ‡ University of Illinois at Urbana-Champaign. § Nanyang Environment and Water Research Institute (NEWRI). 10.1021/es800058s CCC: $40.75

Published on Web 05/28/2008

 2008 American Chemical Society

removal efficiency by activated carbon adsorption. However, one aspect that has received little investigation is the significance of NOM during the desorption of micropollutants from activated carbon. Desorption of trace contaminants from activated carbon can be described by two broad categories: nondisplaced desorption (NDD) and displaced desorption (DD). NDD refers to release of adsorbed contaminants to restore equilibrium in the event of a decrease in the bulk solution contaminant concentration. DD occurs when a competing molecule displaces the adsorbed contaminant thereby decreasing the carbon’s capacity for the contaminant. Desorption is generally not a problem in practice as long as the effluent concentration remains below regulatory limits or treatment goals. However, high transient concentrations of strongly competing (SC) compounds in adsorbers having a carbon residence time greater than the hydraulic residence time can cause undesirable contaminant concentrations in the adsorber effluent because of desorption. Contaminant desorption can also occur when the adsorber influent concentration of the pollutant drops. More subtly, in a granular activated carbon (GAC) packed bed adsorber, the continual displacement of more weakly adsorbing contaminants by more strongly adsorbing SC organic matter in the adsorber influent can result in the eventual release of a high concentration of a contaminant if the adsorber is not removed from service at the end of its useful life. Several studies have addressed the ability of large molecular weight NOM to constrict or block activated carbon pores, thus slowing down or even preventing the diffusion of smaller compounds into those pores (1–5). Partial or complete pore blockage by pore-blocking (PB) NOM is likewise expected to influence trace compound desorption. The work of Ding et al. (6) and Schideman et al. (7) emphasized the importance of the mechanism of pore blockage. Using a three-component (trace compound, SC NOM and PB NOM) modeling approach to predict trace compound removal in a powdered activated carbon-ultrafiltration (PAC-UF) reactor (6) and a GAC packed bed reactor (7), both studies showed that if the intraparticle pore-blocking mechanism was excluded, predictions of reactor performance were significantly overestimated. Inclusion of the decrease in adsorption kinetics due to pore blockage considerably improved predictions. Contaminant desorption events have often been observed in activated carbon systems (8–10), but with little understanding of the mechanisms involved, it is difficult to recommend control or mitigation strategies. In a previous study (11), it was demonstrated that the adsorption and desorption kinetics of a trace contaminant increased as surface coverage of SC matter increased, suggesting that the surface competition provoked a shift from diffusion of the target molecule along carbon’s surfaces to diffusion that involved more movement through pore spaces. The purpose of the present study is to further develop the understanding of desorption by investigating the effect of pore-blocking (PB) background organic matter on trace contaminant desorption kinetics.

Materials and Methods Water. Milli-Q ultrapure water (Millipore, Billerica, MA) was adjusted to pH 7 with 1 mM phosphate buffer. A 0.01% w/w concentration of sodium azide was maintained to prevent biological activity. Adsorbates. Radio-labeled atrazine (Syngenta Crop Production AG, Basel, Switzerland) was used as the target trace VOL. 42, NO. 13, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Molecular weight distribution of Suwannee River NOM; Inset shows calibration curve. contaminant. The atrazine was received in acetone and had a specific radioactivity of 52.25 µCi/mg. The acetone was removed by a direct stream of nitrogen gas until only solid atrazine remained, which was then dissolved in ultrapure water. The solution was stored at 4 °C when not in use. Analysis of the atrazine solution by high performance size exclusion chromatography, or HPSEC, (Shimadzu HPLC, Tokyo, Japan) showed negligible residual of acetone. The column used was PL Aquagel-OH 30 8 µm (Polymer Laboratories, Amherst, MA). The initial radiochemical and chemical purities were 98.6 and 99.4%, respectively, as reported by the manufacturer. Periodic checks of atrazine chemical purity were determined by area distribution using HPLC with the Eclipse XDB-C18 column (Agilent, Santa Clara, CA). After two, seven and 14 months, chemical purity measured 99.2, 98.0, and 98.0%. HPLC analysis of the atrazine in solution on day 20 of the atrazine-NOM competitive isotherm revealed no apparent degradation of atrazine during the experiment. Suwannee River NOM (International Humic Substances Society, St. Paul, MN), isolated by reverse osmosis, was received in dry form, and then dissolved in ultrapure water. The solution was filtered through a 0.45 µm nylon filter to remove suspended solids, and sodium azide at 0.01% w/w was added to prevent biological degradation of the NOM during storage. The final concentration of the stock solution was 190.7 mg/L as organic carbon, which was diluted with ultrapure water to achieve the desired NOM concentration. The stock solution was stored at 4 °C when not in use. 1,4-Dichlorobenzene, or p-DCB (Sigma, St. Louis, MO), was used as a surrogate SC compound to supplement the SC NOM. Solid p-DCB was dissolved in methanol to a concentration of 10 000 mg/L and stored at 4 °C when not in use. Adsorbent. A commercial PAC, Norit SA-UF (NORIT France, S.a.r.l., Le Blanc Mesnil Cedex, France) was dried at 105 °C overnight, cooled in a desiccator, then weighed. Relevant properties of this carbon can be found elsewhere (5). Molecular Weight Distribution (MWD) of NOM. Figure 1 displays the MWD of the prepared solution of Suwannee River NOM characterized by HPSEC (Shimadzu HPLC, Tokyo, Japan). The separation was performed at 40 °C using the PL Aquagel-OH 30 8 µm column (Polymer Laboratories, Amherst, MA). The mobile phase was prepared with ultrapure water, buffered with 0.004 M phosphate buffer and 0.096 M NaCl to pH 6.8. The flow rate was 1 mL/min. UV absorbance was measured at 254 nm. A molecular weight calibration was performed using polystyrene sulfonate standards of 208, 1100, 4480, 6430, 13 000, and 32 000 Da (Polysciences Inc., Warrington, PA; Sigma, St. Louis, MO). The number-averaged and weight-averaged molecular weight were calculated to be MWn ) 3701 Da and MWw ) 9602 Da. Analytical Methods. All samples were filtered through a 0.45 µm nylon syringe filter to remove the PAC. For atrazine analysis, samples of 2.5 mL were collected and 18 mL of liquid scintillation cocktail (Ecoscint, National Diagnostics, Inc. Atlanta, GA) was added. Radioactivity was measured 4826

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using a liquid scintillation counter (Beckman LS6500, Fullerton, CA). Samples of 16 mL were collected for NOM analysis. NOM concentrations were determined using nonpurgeable organic carbon (NPOC) combustion oxidation analysis (Shimadzu TOC-Vcsh, Tokyo, Japan), and were expressed as mg/L of NPOC. Repeatability of NPOC measurements were within ( 4.6%. During the DD tests, duplicate samples of 1.6 mL were collected for p-DCB analysis. Immediately, 0.4 mL of hexane was added and the autosampler vial was capped. The hexane phase of each sample was analyzed in triplicate using GC-MS (Shimadzu GC-2010 and GCMS-QP2010, Tokyo, Japan). Batch Isotherm Tests. Bottle-point isotherms were conducted for single-solute atrazine adsorption (C0,Atrazine ) 100 µg/L) and for three initial conditions of competitive adsorption (C0,Atrazine ) 100 µg/L + C0,NOM ) 4 mg/L; C0,Atrazine ) 100 µg/L + C0,NOM ) 4 mg/L + C0,p-DCB ) 2 mg/L; C0,Atrazine ) 500 µg/L + C0,NOM ) 4 mg/L). Solute concentrations were measured on day 7, and again on day 14 to confirm that equilibrium was reached by day 7. NOM adsorption isotherms were also determined for C0,NOM ) 3.6 and 5.3 mg/L. Freundlich equilibrium parameters (K, 1/n) were used to describe the adsorption equilibrium for each solute. Batch Atrazine Adsorption Kinetic Tests with Preloaded PB NOM. Fresh PAC of 2, 4, 8, and 12 mg/L were preloaded for seven days with NOM (C0,NOM ) 4 mg/L). The PAC concentrations were chosen to yield a range of preadsorbed NOM concentrations. The NOM concentration was measured on day 7 to check for equilibrium. On day 8, atrazine was spiked into each bottle to achieve an initial concentration of 100 µg/L. Atrazine concentration was then measured over four hours. Batch Atrazine Adsorption Kinetic Tests with Simultaneously Adsorbed PB NOM. Fresh PAC of 6 mg/L was simultaneously contacted with 100 µg/L of atrazine and 4 mg/L of NOM. Atrazine and NOM concentrations were monitored over four hours. Batch DD Kinetic Tests with Preloaded PB NOM. Both DD and NDD kinetic tests in the presence of preloaded PB NOM were conducted for comparison with adsorption tests. Fresh PAC of 2, 4, 8, and 12 mg/L was first preloaded with atrazine (C0,Atrazine ) 105 or 84.6 µg/L). On day 4, NOM was added to each bottle to obtain an initial concentration of 4 mg/L. This sequential adsorption mimics the situation in which influent PB NOM adsorbs after uptake of the target contaminant. Atrazine and NOM concentrations were measured on day 11 to check for equilibrium. Finally, p-DCB was added on day 12 to displace atrazine, and atrazine desorption was monitored over 500 min. NOM and p-DCB concentrations were also measured during this period. Batch DD Kinetic Tests in the Absence of Preloaded PB NOM. Fresh PAC of 4 mg/L was first preloaded with atrazine (C0,Atrazine ) 84.6 µg/L). Atrazine concentration was measured on day 11 to check for equilibrium. On day 12, p-DCB was added to displace atrazine, and atrazine and p-DCB concentrations were measured over 500 min. Batch NDD Kinetic Tests with Preloaded PB NOM. Fresh PAC of 2, 4, 8, and 12 mg/L was first preloaded with atrazine (C0,Atrazine ) 420 µg/L). On day 4, NOM was added to each bottle to achieve an initial concentration of 4 mg/L. Atrazine and NOM concentrations were measured on day 11 to check for equilibrium. On day 12, each reactor system’s volume was expanded from the starting volume (750, 450, 200, and 150 mL, respectively) to 2250 mL. The dilution solution for each reactor contained no atrazine, but only a NOM concentration matching the appropriate equilibrium solution concentration of NOM for that reactor. This was intended to promote atrazine desorption by creating a reverse concentration gradient of atrazine while preventing PB NOM desorption by keeping NOM concentration constant. Atrazine

FIGURE 2. Atrazine adsorption kinetics in the presence of preloaded or simultaneously adsorbed (S) NOM; Legend shows PAC doses.

FIGURE 3. Atrazine kinetics for displaced desorption by p-DCB, in the presence of preloaded NOM or in the absence of NOM (A); Legend shows PAC doses.

FIGURE 4. Atrazine kinetics for nondisplaced desorption by system volume expansion, in the presence of preloaded NOM or in the absence of NOM (A); Legend shows PAC doses. desorption and NOM concentration was monitored for about 3000 min, and a final sample was taken on day 7 after commencing desorption. Batch NDD Kinetic Tests in the Absence of Preloaded PB NOM. Fresh PAC of 12.3 mg/L was first preloaded with atrazine (C0,Atrazine ) 511 µg/L). Atrazine concentration was measured on day 11 to check for equilibrium. On day 12, the reactor system’s volume was expanded from 185 to 2250 mL with buffered ultrapure water containing no atrazine. Atrazine desorption was monitored for about 3000 min.

Results and Discussion The results from the atrazine adsorption, DD and NDD kinetic tests in the presence of preadsorbed PB NOM are shown in Figures 2, 3, and 4, respectively. Each figure also includes one data set showing atrazine adsorption or desorption kinetics without pore blockage (simultaneous adsorption of NOM in Figure 2, or absence of NOM in Figures 3 and 4). A duplicate data set for Cc ) 4 mg/L is included in Figure 3 to illustrate experimental reproducibility. The Freundlich equilibrium parameters (K, 1/n) determined from independent

adsorption isotherm tests for the different experimental conditions are listed in Table 1. These parameters were used as inputs for the homogeneous surface diffusion model (HSDM) (12) to solve for the best-fitting diffusion coefficient (Ds) for each kinetic data set. The HSDM assumes instantaneous equilibrium at the external surface of the particle, surface homogeneity for uniform adsorption, negligible bulk or film diffusion, physical adsorption as the only reaction, and Ds independent of concentration. The decrease in intraparticle diffusivity resulting from preloaded PB NOM restricting access to pores was expected to be more pronounced that the increase in diffusivity resulting from SC NOM, so a net decrease in atrazine Ds was expected. The equilibrium parameter, K, for the NDD tests could not be derived from isotherms because upon volume expansion, new NOM was added so that the concentration of the equilibrated NOM solution would remain constant. This was intended to minimize desorption of the equilibrated PB NOM. However, since NOM is a complex mixture, preferential adsorption of the more adsorbable SC NOM molecules would be expected. A reequilibration of NOM molecules likely took place, along with adsorption of new SC NOM, thus causing a change in competitive equilibrium. Hence, K was determined for each NDD kinetic experiment from the atrazine concentration on day 7 after the start of desorption, under the assumption that equilibrium was attained by day 7. Although this was not confirmed for NDD in the present study, atrazine adsorption on PAC has been reported to equilibrate by day 7 in spite of pore blocking by polystyrene sulfonate, a surrogate for PB NOM (5). However, the 1/n parameter determined from the independent isotherms (1/n ) 0.319) was applied to model the NDD kinetic tests. This is consistent with the observation in many adsorption studies that in the presence of a competing solute, a change in initial concentration of the target component would result in a nearly parallel isotherm for the target compound. In the current study, a simple modeling analysis revealed that the fitted Ds was not sensitive to small changes in 1/n. A similar feature in the HSDM fittings of the data from both adsorption and desorption with preloaded PB NOM was the initial underestimation and subsequent overestimation of sorption kinetics (Figures 2-4). This suggests that although a constant Ds is a reasonable simplification, the diffusion coefficient may in fact decrease over time. One possibility is that the nonhomogeneity of activated carbon gives rise to bottleneck regions of pore blockage, perhaps near the micropore entrances. As such, sites on the external surface of the particle or internal sites that are not blocked by PB NOM could be more quickly accessed, followed by slower diffusion to reach inner sites. In terms of desorption, atrazine likely desorbs from less blocked pores initially, while the atrazine in narrower, blocked pores desorbs at a slower rate. Since a constant Ds provides a reasonably good fit for kinetic tests without preloaded PB NOM, constant Ds may be a more appropriate assumption for solute adsorption without pore blocking competition. Isolating the Pore Blocking Component of Ds. A previous study showed that surface competition by strongly competing (SC) compounds led to increased adsorption/desorption kinetics of a target trace compound (11). Since NOM is made up of both SC and PB compounds, it was necessary to separate the SC NOM effect in order to singly examine the effect of the PB NOM fraction on desorption kinetics. Assuming that the effects of SC and PB NOM on atrazine kinetics are distinct but concurrently active, the diffusion coefficient can be represented as having SC and PB contributions as follows: Ds ) Ds,0

fSC fPB

VOL. 42, NO. 13, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

(1)

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TABLE 1. Pseudo-Single-Solute Freundlich Equilibrium Parameters Used for HSDM Modeling; Atrazine-only and NOM-only Parameters Given As Reference relevant test

C0, Atrazine µg/L

atrazine-only NOM-only adsorption DD NDD

C0,NOM mg/L

C0,p-DCB mg/L

100 3.6, 5.3 4.0 4.0 varies*

100 105 varies*

2.0

K (µg/mg) (µg/L)-1/n

1/n (-)

24.4 0.0233 (mg/mg)(mg/L)-1/n 9.75 3.44 determined from day 7 end point

0.404 1.22 0.363 0.347 0.319

* Each NDD reactor starts with the same initial atrazine and NOM concentration before dilution. The equivalent initial concentration after dilution varies because it depends on total volume, and the dilution factor for each reactor was different.

TABLE 2. Experimental Conditions, Measured Parameters, and Calculated Values To Isolate SC and PB NOM Effect on Overall Diffusion Coefficient

Ads

initialCc mg/L

C0, Atrazine µg/L

C0,NOM mg C/L

qNOM µg C/mg PAC

2 4 8 9.9 6

99.5 99.5 99.5 99.5 102

4.1 4.1 4.1 4.1 4.0

112 105 95.6 93.3 0

DD

2 4 8 12 4

105 105 105 84.6 84.6

4.0 4.0 4.0 4.0 4.0

120 111 105 83.3 0

NDD

2 4 8 12 12.3

137a 82.2a 36.6a 27.4a 42.0b

4.0 4.0 4.0 4.0 0

118 110 102 93.5 0

C0,p-DCB mg/L

qEBCc µg/mg PAC

qEBC µg C/mg PAC

Ds cm2/min

fSC (-)

Ds,SC cm2/min

fPB (-)

Ds,PB cm2/min

168 101 47.5 36.5 74.1

84.0 50.5 23.8 18.3 37.1

2.9e-12 8.7e-12 2.4e-11 3.0e-11 1.3e-10

4.5 3.1 2.0 1.8 2.5

2.2e-10 1.5e-10 9.6e-11 8.5e-11 1.2e-10

74.5 17.1 4.0 2.8 0.9

6.4e-13 2.8e-12 1.2e-11 1.7e-11 5.1e-11

2.0 2.0 2.0 2.0 2.0

548 466 337 245 250

274 233 169 123 125

4.1e-11 5.2e-11 7.0e-11 3.6e-10 2.2e-10

12.4 10.7 8.0 6.1 6.2

6.0e-10 5.1e-10 3.9e-10 2.9e-10 3.0e-10

14.5 9.9 5.5 0.8 1.4

3.3e-12 4.9e-12 8.7e-12 5.9e-11 3.6e-11

421 299 201 177 0

211 150 101 88.5 0

5.2e-12 7.8e-12 6.2e-12 5.7e-12 1.5e-11

9.8 7.2 5.2 4.7 1.0

4.7e-10 3.5e-10 2.5e-10 2.3e-10 4.8e-11

90.2 44.5 40.2 39.5 3.2

5.3e-13 1.1e-12 1.2e-12 1.2e-12 1.5e-11

a Equivalent C0,Atrazine given the initial preloaded concentration of C0, Atrazine ) 420 µg/L before volume expansion. Equivalent C0,Atrazine given the initial preloaded concentration of C0, Atrazine ) 511 µg/L before volume expansion. c qEBC determined for postdesorption equilibrium. For DD, qEBC includes p-DCB contribution. Multiply qEBC by 50% to express in terms of µg C/mg PAC. b

where Ds,0 is the atrazine single-solute surface diffusion coefficient, fSC is called the “SC factor”, and fPB the “PB factor”. The SC and PB factors describe the factor by which the surface diffusion coefficient of atrazine would increase or decrease due to the surface competition by the respective NOM fractions. The following terms can also be defined to express the diffusion coefficient which isolates either the SC or PB NOM effect: Ds,SC ) Ds,0fSC Ds,PB ) fSC )

(

Ds,0 Ds ) fPB fSC

A × qEBC +1 Ds,0

(2) (3)

)

(4)

where qEBC is the adsorbed concentration of the equivalent background compound (EBC) (13), representative of the surface coverage of the SC NOM component. The atrazineonly diffusion coefficient for the given PAC, Ds,0, is 4.8e-11 cm2/min, and the parameter describing the rate of Ds increase due to the EBC, A, is assumed to be the same for SC NOM as that previously determined for p-DCB, or A ) 1.0e-12 (cm2/min)(µg/mg PAC)-1 (11). The origin of eq 4 and details for qEBC calculation can be found in ref 11. For the present adsorption and desorption experiments, the fitted Ds and calculated parameters, including the PB-only diffusion coefficient Ds,PB are listed in Table 2. 4828

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An important assumption in determining qEBC in eq 4 is that the SC NOM component is adsorbed before atrazine diffuses along the surface into or out of the carbon, and thus is able to influence surface diffusion of the trace compound. In the adsorption and DD kinetic tests, the experimental design ensured the preadsorption of NOM. Although the adsorption of p-DCB, also acting as additional SC matter in the DD tests with preloaded PB NOM, was started simultaneously to atrazine desorption, the diffusion of adsorbing p-DCB was found to be several times faster than that of atrazine desorption. This was consistent with what was reported for experiments performed in the absence of PB NOM (11). Consequently, the adsorption of p-DCB was essentially completed before any significant atrazine desorption had taken place. For NDD tests with preloaded NOM, it is unknown how much of the new SC NOM added during volume expansion adsorbed before atrazine desorption. Regardless, further analysis revealed that the calculated Ds,PB did not change significantly whether using qEBC determined from equilibrium before or after desorption, demonstrating that the additional EBC had only a small impact on NDD kinetics. Determining Surface Coverage of PB NOM. It was assumed that since the NOM used was primarily composed of large molecular weight molecules (Figure 1) and since the true mass of SC NOM adsorbed cannot be quantified in isolation, the total NOM surface concentration qNOM was a reasonable approximation for the adsorbed concentration

FIGURE 5. Adsorption, displaced, and nondisplaced desorption kinetics (a) without consideration for SC NOM effect and (b) with consideration for SC NOM effect. of PB NOM. Further, since the adsorption, DD and NDD experiments had the same initial concentration of NOM (C0 ) 4.0-4.1 mg/L), the amount of PB surface coverage is assumed to be consistent for a given mass of total NOM adsorbed. This assumption allows comparability to be maintained across adsorption, DD and NDD tests. The qNOM values listed in Table 2 were calculated with eq 5 using the Freundlich coefficients for NOM adsorption equilibrium (Table 1) and Ceq,NOM which was measured immediately before the start of atrazine adsorption or desorption. qNOM ) K × Ceq,NOM1⁄n qNOM )

(C0,NOM - Ceq,NOM) Cc

(5) (6)

Although direct measurements of C0,NOM and Ceq,NOM were taken, eq 5 rather than eq 6 was used to minimize the error in determining PB NOM surface coverage. Equation 6 was highly sensitive to any inaccuracy of Ceq,NOM due to the NPOC analysis. Note that the adsorbed EBC concentration, qEBC (in µg C/mg PAC) exceeded the total NOM adsorbed, qNOM, in DD and several NDD tests. In the DD tests, this is because qEBC includes the p-DCB which acts as additional SC matter, but which does not contribute to the mass of PB matter. In the case of the NDD tests, it is physically impossible for qEBC to be greater than qNOM, indicating that the small fraction of SC NOM is actually a very strong competitor for atrazine sites, with competing power comparable to a much higher concentration of an EBC with the same K and 1/n parameters as atrazine. One reason that NOM might compete more strongly in the NDD tests is that the higher initial atrazine concentration promotes some atrazine adsorption in pores of wider diameters, so a larger proportion of NOM is able to directly compete for atrazine adsorption sites. Effect of PB NOM on Atrazine Desorption Kinetics. Figure 5 shows the Ds values obtained from fitting of the various kinetic curves in Figures 2-4 and the calculated Ds,PB values (eqs 2–4) versus the corresponding adsorbed concentration of PB NOM. Figure 5a shows a general trend of decreasing diffusion coefficient with increasing surface loading of PB NOM, consistent with past studies showing the same behavior for atrazine adsorption in the presence of various poreblocking compounds and NOM from different natural water sources (5, 6, 14, 15). Notice that the atrazine diffusion coefficients for the DD experiments exceeded those for adsorption tests with similar qNOM, revealing that p-DCB used in the DD experiments provided stronger direct competition than the SC NOM. When the SC effect of increasing the diffusion coefficient is factored out, the diffusion coefficient Ds,PB decreases as

qNOM increases, and the dependence of the diffusion coefficient on qNOM converges for adsorption, DD and NDD experiments (Figure 5b). This brings out the importance of a phenomenon that has not been given attention in past adsorption studies, that is, that the SC NOM effect accelerates the rate of atrazine diffusion. This effect is especially significant when SC competition is strong, characterized by p-DCB in the DD tests. In this case, the effect of SC NOM to accelerate atrazine diffusion beneficially counters the effect of PB NOM to decrease diffusion rate. On the contrary, when there is less SC competition, the pore-blocking effect is more pronounced, and Ds,PB has a value closer to that of Ds. The strength of SC NOM versus PB NOM to influence atrazine kinetics is captured well by comparing the magnitudes of fPB and fSC in Table 2. A further conclusion is that DD and NDD differ only in the degree of the SC kinetic effect. Since trace compound displacement is induced by SC NOM, DD kinetics were faster because greater SC competition (in this case, p-DCB) was present. The adsorption points in Figure 5b can be fit with the modeling method of Li et al. (15), with slight amendments to account for SC NOM effect of kinetics as follows: Ds,PB )

{

Ds,0 qNOM e qcr 1 ;f ) exp[β(qNOM - qcr)] qNOM > qcr fPB PB

}

(7)

As seen in Figure 5b, the equation modeling the adsorption data can also satisfactorily describe Ds,PB of DD and NDD. Thus, for the same activated carbon, PB NOM source and trace compound, the modeling parameters for adsorption can be applied to predict the impact of PB NOM on desorption kinetics. For the current adsorption results, the critical surface loading above which pore blocking takes place, qcr, is 87.8 µg/mg PAC and the rate at which Ds,PB decreases exponentially, β, is 0.173 mg/µg. The physical significance of the critical surface loading is explained and explored by Li et al. (15). When making predictions for desorption, note that eq 7 can only predict Ds,PB, the PB NOM contribution to the diffusion coefficient. The next step would be to incorporate the effect of SC NOM on the diffusion coefficient (eq 4) into eq 1 to predict the overall desorption diffusion coefficient, Ds. Isolating the PB effect on kinetics reveals that the onset of pore blockage is actually at a lower surface loading than previously calculated without considering the kinetic enhancement effect of SC NOM. Practically, this means that the SC NOM benefits contaminant adsorption by countering early symptoms of pore blockage. In contrast, if the trace contaminant has already adsorbed, pore blocking is favorable for preventing or delaying desorption. In that case, SC NOM acts as a detriment by hastening contaminant diffusion. VOL. 42, NO. 13, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Desorption of trace contaminants from activated carbon is a realistic concern for applications having long carbon retention times allowing for SC NOM buildup, or for influent sources containing substantial concentrations of SC NOM. In particular, high amounts of adsorbed SC NOM impair micropollutant removal by occupying valuable carbon capacity, displacing contaminants, and accelerating their diffusion during desorption. Incidents of NDD may be less damaging than DD events because of the slower desorption kinetics associated with less SC competition. However, in reality, NDD and DD events will often occur in combination. Finally, if activated carbon is nearly saturated with a target contaminant, the accumulation of PB NOM is actually desirable for hindering SC NOM uptake and micropollutant release. Further research on desorption could incorporate factors of adsorbability, pore size distribution, different contaminant and NOM properties, and time effects to develop a more complete picture of the implications of contaminantNOM interactions in activated carbon treatment processes.

Acknowledgments This work was supported by the National Science Foundation Graduate Research Fellowship program, the WaterCAMPWS, a Science and Technology Center of Advanced Materials for the Purification of Water with Systems under NSF agreement number CTS-0120978, and the National University of Singapore (NUS). We also thank NUS for providing the use of laboratory facilities and Syngenta Crop Production AG (Basel, Switzerland) for the donation of radio-labeled atrazine and thoughtful comments.

Literature Cited (1) Carter, M. C.; Weber, W. J., Jr.; Olmstead, K. P. Effects of background dissolved organic matter on TCE adsorption by GAC. J. Am. Water Works Assoc. 1992, 84 (8), 81–91. (2) Smith, E. H.; Tseng, S.-K.; Weber, W. J., Jr. Modeling the adsorption of target compounds by GAC in the presence of background dissolved organic matter. Environ. Prog. 1987, 6 (2), 18–25. (3) Kilduff, J. E.; Karanfil, T.; Weber, W. J., Jr. TCE adsorption by GAC preloaded with humic substances. J. Am. Water Works Assoc 1998, 90 (5), 76–89.

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