A kinetic and equilibrium study of competitive adsorption between atrazine and Congo red dye on activated carbon: the importance of pore size distribution

PERGAMON

Carbon 39 (2001) 25–37

A kinetic and equilibrium study of competitive adsorption between atrazine and Congo red dye on activated carbon: the importance of pore size distribution Costas Pelekani a , *, Vernon L. Snoeyink b a

b

Australian Water Quality Center, Private Mail Bag 3, Salisbury, SA 5108 Australia Department of Civil and Environmental Engineering, University of Illinois, 205 North Mathews Avenue, Urbana, IL 61801, USA Received 15 August 1999; accepted 20 February 2000

Abstract A series of phenolic resin-based microporous activated carbon fibers (ACF) with different micropore size distributions were used to assess the role of pore size distribution (PSD) in the mechanism of competitive adsorption between the organic micropollutant, atrazine, and a compound larger in size, Congo red dye (CR). Batch kinetic and equilibrium experiments with the CR / atrazine system consisted of single-solute, simultaneous adsorption, CR preloading followed by atrazine contact, and atrazine preloading followed by CR contact. Based on the previous pore characterization studies and the PSD, two types of pore structures were proposed: telescopic pores and branched pores. With the telescopic pore structure, evidence is presented to support a transition from surface pore blockage to pore constriction (without loss of atrazine capacity) to direct competition for adsorption sites, with increasing average micropore size. With the branched pore structure (micropores branching off from mesopores), direct competition for adsorption sites in a fraction of the large micropores and pore constriction and pore blockage of smaller micropores were found to be important. The kinetics of adsorption was found to be important in determining the impact of simultaneous adsorption, while CR surface coverage and preloading time were the key factors controlling the impact of preloading on atrazine adsorption.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Activated carbon; Carbon fibers; C. Adsorption; D. Microporosity

1. Introduction An understanding of activated carbon adsorption processes requires knowledge of adsorbate–adsorbent properties, including their respective solution chemistries and pore size distribution. This knowledge is invaluable in selecting a carbon adsorbent for a particular treatment application, and applying it in an effective manner. In drinking water treatment, some applications of activated carbon include: the control of disinfection by-product precursors (DBP) which are present as part of the natural organic matter (NOM) mixture present in all waters; the

*Corresponding author. Tel.: 161-882-590-369; fax: 161882-590-228. E-mail address: [email protected] (C. Pelekani).

removal of synthetic organic chemicals including pesticides, herbicides and industrial waste products; taste and odor compounds which are derived from algal metabolites and bacteria; and algal toxins. Of the important factors that influence the removal of trace compounds in the presence of NOM, the role of pore size distribution in relation to the size of the contaminant molecule and the molecular size distribution of NOM has not been extensively studied. Using conventional activated carbons, Newcombe et al. [1] found that low molecular weight NOM fractions consistently had a more deleterious impact on adsorption of the taste and odor compound, 2-methylisoborneol (MIB), than the larger NOM fractions. They proposed that direct competition between MIB and NOM compounds of similar size dominated, with minimal pore blockage by the larger NOM components. Pelekani and Snoeyink [2] used an activated carbon fiber (ACF) containing only primary micropores (pore width less than

0008-6223 / 01 / $ – see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00078-6

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C. Pelekani, V.L. Snoeyink / Carbon 39 (2001) 25 – 37

˚ to assess the role of pore size in the mechanism of 8 A) competition between atrazine and groundwater NOM. It was shown that the NOM could not adsorb in the primary micropores and competed by surface pore blockage. Increasing the volume of secondary micropores (pore width ˚ resulted in a large improvement in atrazine 8–20 A) adsorption capacity, and a shift to direct competition for adsorption sites in these larger micropores. Sakoda et al. [3] assessed the feasibility of using ACFs in column filtration mode to remove trihalomethanes, including chloroform (CHCl 3 ), from drinking water containing NOM. One ACF with micropores of width less ˚ and another ACF containing both micropores than 14 A, ˚ were used. With the strictly and mesopores (,60 A) microporous ACF, a 15% reduction in CHCl 3 capacity was obtained, with respect to CHCl 3 adsorption in organic-free water. However, with the mesoporous ACF, the relative decrease was 70%. It was proposed that NOM adsorbed in the mesoporous ACF not only reduced the number of adsorption sites for CHCl 3 , but also blocked their passage into smaller micropores. The authors provided no information about the molecular size distribution of the NOM present in the drinking water and the relative adsorption capacities of the two ACFs for NOM. In developing a conceptual model of competitive adsorption between small organic contaminants and other compounds, based on pore size, the molecular size heterogeneity of NOM can make it difficult to ascertain the specific impacts of molecules of different size in pores of varying size. This can be overcome using molecular probes, compounds of known size and shape in competitive adsorption studies. Pelekani and Snoeyink [4] used five ACF adsorbents with different micropore size distributions to assess the impact of pore size on the competition mechanism between the target organic micropollutant, atrazine, and methylene blue dye, a compound of similar size. It was shown that these compounds directly competed for adsorption sites in the accessible micropore region. When only primary micropores were present, overlapping pore wall potentials in these small pores resulted in strong binding of atrazine, with subsequent desorption being a very slow process.

Therefore, for short contact times (e.g. 7 days) adsorption in these pores was essentially irreversible. In larger secondary micropores, this phenomenon was not observed and displacement of preloaded atrazine by methylene blue was a relatively fast process. In this study, using the same microporous ACFs, the impact of increasing the size of the competing compound on the competitive adsorption mechanism in different size pores was evaluated by a series of batch kinetic and equilibrium experiments. Specifically, Congo red (CR), an anionic dye with a molecular weight of 651 (cf. 284 for methylene blue), was used. CR is much larger than methylene blue, and is possibly more representative of the molecular size of small NOM molecules present in drinking water supplies.

2. Experimental

2.1. Materials 2.1.1. Organic-free water Deionized-distilled water (DDW) with a dissolved organic carbon concentration of less than 0.2 mg / l was used to prepare all solutions. 2.1.2. Adsorbates The target micropollutant was atrazine, a selective preemergent herbicide that is widely used in North America and Europe. Congo red (CR), an anionic dye, much larger in size than atrazine was selected as the competing adsorbate. The structural formulae of atrazine and CR are shown in Fig. 1. 14 C-labeled atrazine (Novartis, Greensboro, NC) was utilized due to ease of analysis and the small sample volumes required. CR was received 97% pure (Aldrich, Milwaukee, WI). Table 1 compares selected chemical properties of CR and atrazine. Molecular dimension data were obtained using ChemSketch 3.5 (Advanced Chemistry Development, Toronto, Canada). 2.1.3. Activated carbon fiber adsorbents (ACF) Four microporous phenolic resin-based ACFs with

Fig. 1. Molecular structures of atrazine (left) and Congo red dye (right).

C. Pelekani, V.L. Snoeyink / Carbon 39 (2001) 25 – 37 Table 1 Selected chemical properties of Congo red dye and atrazine Property

Atrazine

CR

Molecular weight (g / mol) Molar volume (cm 3 / mol) ˚ Width (A) ˚ Depth (A) ˚ Thickness (A)

215.68 169.8 9.6 8.4 |3

650.73 a 422.9 26.2 7.4 4.3

a

Does not include associated sodium ions.

increasing degrees of activation, designated ACF-10, ACF15, ACF-20 and ACF-25 were used (Nippon Kynol, Japan). They were received as twilled-weave fabrics. A mesoporous ACF was produced by further activating ACF25 in a bench-scale tubular reactor furnace (Lindberg; Model 54232). A 60:40 steam:nitrogen gas mixture was used (1 l / min at 1 atm and 23C), with a furnace temperature of 8508C. One-gram samples of ACF-25 were exposed for at least 8 h, resulting in a yield of only 4–6%. Preliminary experiments showed that very high burnoffs were required to produce significant mesoporosity. This adsorbent was designated MESO. The carbons were dried at 1058C and stored in a desiccator to minimize moisture contact. In all experiments, the ACFs were cut into small pieces, except for MESO ACF, which was extremely friable and rapidly broke into short fiber lengths during contact.

2.2. Methods 2.2.1. Atrazine analysis 14 C-Atrazine was quantified by liquid scintillation [5]. This was achieved by mixing 2.5-ml aliquots of 0.22 mm filtered sample with 18 ml of scintillation cocktail (Ecoscint, National Diagnostics, Manville, NJ), and measuring the resulting fluorescence in a liquid scintillation counter (Tri-Carb Model 1600 CA, Packard Instrument, Downers Grove, IL). The specific activity (38.7–56.3 mCi / mg) of the 14 C-atrazine yielded a detection limit of |0.05 mg / l without sample pre-concentration.

27

was chosen to ensure equilibration at the low relative partial pressures (,10 25 ) which is critical for the analysis of the micropore region [6]. The isotherm data were used to calculate the BET surface area, micropore volume and pore size distributions of the ACFs.

2.2.4. Adsorption isotherms Adsorption isotherms were performed at pH 7.0 using the bottle-point technique [7]. Due to the acid–base character of the CR dye, all solutions were buffered with 1 mM phosphate buffer. At pH 7.0, CR was completely ionized (red color). The pKa of CR is |5.5. In the simultaneous adsorption experiments, the CR concentration used was |8 mM (5.2 mg / l). The atrazine concentration was |50 mg / l (0.23 mM). Samples for analysis were taken after 7 days and 30 days contact time. In the CR preloading experiments, the adsorbents were exposed to CR for 7 days, filtered and contacted with single-solute atrazine. Atrazine samples were taken after an additional 7 days and 30 days of contact time. Additional preloading tests were performed with MESO ACF (see Section 3.7). In the atrazine preloading experiments, the adsorbents were loaded with single-solute atrazine using an initial concentration of 400–500 mg / l. After 7 days of contact, the bottles were sampled and the atrazine-loaded ACF-10 samples were filtered and contacted with single-solute CR solution (8 mM) for an additional 7 days. 2.2.5. Batch kinetic tests A jar test apparatus (Six Paddle Stirrer Model 7790-400, Phipps & Bird, Richmond, VA) and modified square jars 2 (B-KER , Phipps & Bird) were used to assess the short term adsorption kinetics (3 days) of atrazine for various competitive adsorption modes. A liquid volume of 2 l was used for all tests. The ACFs were soaked overnight in 10 ml of DDW, to ensure complete wetting. In the preloading tests, the carbon was filtered after the initial loading step prior to contact with the adsorbate of interest.

3. Results and discussion

2.2.2. Congo red ( CR) analysis CR was analyzed using a Beckman DU 7500 diode array spectrophotometer (Beckman Instruments, Fullerton, CA), with detection at 497 nm. The detection limit was |0.5 mM. 2.2.3. Characterization of ACF physical properties A Coulter Omnisorp 100 (Hialeah, FL) was used for the volumetric measurement of the nitrogen adsorption isotherms at 77 K. The ACFs were outgassed at 2008C under vacuum for 36 h prior to analysis. The nitrogen adsorption experiments were performed in static mode using a mass flow controller programmed to provide a fixed dose of nitrogen to the sample container. This mode of operation

3.1. Physical adsorbent characterization Table 2 summarizes the micropore size, pore volume and surface area data. As the level of activation increases, there is an increase in the BET surface area, micropore volume and average pore size. The pore size distributions (PSDs) were calculated using the 3-D discrete PSD model developed by Sun et al. [8], and are shown in Fig. 2. ACF-10 contains mainly primary micropores, with a pore ˚ ACF-15, ACF-20 and ACF-25 are size range of 6–9 A. ˚ with ACF-25 having a shifted to larger pore sizes (7–9 A), ˚ range. MESO small volume of micropores in the 14–20 A ACF has a bimodal PSD with a large volume of micro-

C. Pelekani, V.L. Snoeyink / Carbon 39 (2001) 25 – 37

28 Table 2 Physical properties of ACFs Property 2

BET surface area (m / g) Micropore volume (cm 3 / g)a Total pore volume (cm 3 / g)a Fraction microporosity (%)a Mesopore area (m 2 / g)b ˚ a Average micropore width (A) a ˚ Average pore width (A) a

b

ACF-10

ACF-15

ACF-20

ACF-25

MESO

877 0.307 0.307 100 16.2 7.1 7.1

1518 0.631 0.631 100 24.9 7.8 7.8

1615 0.686 0.686 100 29.5 7.8 7.8

1918 0.884 0.893 99 48.4 8.9 9.7

2999 1.474 2.127 69.3 414 10.6 18.6

Using 3-D PSD model.

S

S DD

24.6164 Applying BJH model: t 5 ]]]]] P 0.0340 2 log ] Po

0.95 0.4292

[15] SMESO (m 2 / g) 5

dV (P/P ) E ]]] . t ads

o

0.40

Fig. 2. 3-D model PSDs for the ACFs.

˚ range, and a significant volume of pores in the 8–10 A ˚ range. The degree of microporosity pores in the 14–54 A is almost 100% for ACF-10→ACF-25, decreasing to 69% for MESO ACF, which has a large mesopore volume. Daley et al. [9] showed, via scanning tunneling microscopy experiments, the telescopic pore structure of ACF10, ACF-15, ACF-20 and ACF-25. They also found that within the bulk fiber the pores formed an interconnected network. This is consistent with the narrow pore size distributions of these materials. In contrast, a different pore structure is proposed for MESO ACF due to the bimodal nature of its PSD. Rather than telescopic pores, a branched

Fig. 3. Proposed pore structures.

pore tree structure exists, with small micropores branching off from large micropores or mesopores. A comparison of these pore structures is shown in Fig. 3. Kawabuchi et al. [10] also proposed this transition in the pore structure of ACFs with increasing activation, via chemical vapor deposition experiments. It should be pointed out that the proposed structures are only snapshots within the bulk fibers. The adsorbents still possess an interconnected pore network, otherwise the large pore volumes attainable with activated carbons could not exist.

3.2. Single-solute Congo red adsorption Fig. 4 illustrates the 7-day single-solute CR adsorption isotherms. As the average pore size, total micropore volume and total mesopore surface area increase, there is a consistent increase in CR adsorption capacity. ACF-10 had a very low CR capacity (2 mmol / g) while MESO ACF showed extremely high capacity (|800 mmol / g). ACF-15, ACF-20 and ACF-25 showed intermediate adsorption capacities of 30, 50 and 220 mmol / g, respectively. The maximum CR loadings (mmol / g) were converted to cm 3 / g using a molar volume of 422.9 cm 3 / mol for the CR molecule (from Table 1), and plotted against pore volume

C. Pelekani, V.L. Snoeyink / Carbon 39 (2001) 25 – 37

29

Fig. 4. Single-solute CR isotherms for ACFs at pH 7.0.

in different size ranges to determine which region gave the best correlation. An excellent correlation was obtained ˚ range was considered when pore volume in the 8.5–9.5 A (Fig. 5). This is not to say that CR did not adsorb in larger pores. Apparently, the concentration of CR used was not sufficient to fill the larger micropores and mesopores of MESO ACF. Although not measured, surface chemistry may play a role in CR adsorption, particularly since the anionic form of the dye was used in adsorption experiments. Krupa and Cannon [11] found that CR adsorption on activated carbon at pH 8–10 was strongly correlated ˚ confirming with adsorption in pore sizes from 14–475 A, that CR can adsorb in large micropores and mesopores. In another study, Kasaoka et al. [12] determined that the ˚ If the critical adsorption pore size for CR was 22.6 A. ˚ lower limit of 14 A is strictly true, then ACF-10, ACF-15 and ACF-20 should yield no CR adsorption, which clearly is not the case. The depth and width of the ionized CR ˚ and 26.2 A, ˚ respectively (Table 1). The molecule are 7.4 A critical adsorption pore dimension is expected to be a contribution of both of these dimensions, with the large

width possibly being the limiting factor since it will result in steric hindrance inside the narrow micropores. The experimental results show that part of the CR molecule can adsorb in small secondary micropores. These studies by Krupa and Cannon, and Kasaoka et al. used very high CR concentrations: 200 mg / l and 2000 mg / l, respectively. These values are 10–100 times higher than those used in this study. In addition, a short contact time of 3 days was used. These experimental conditions place emphasis on adsorption in the larger pores, instead of focusing on the small micropores.

3.3. Single-solute atrazine adsorption The Freundlich adsorption parameters for the 7-day single-solute atrazine isotherms are summarized in Table 3. The data are valid for residual concentrations less than 100 mg / l. It was found that atrazine adsorption correlated ˚ at high well with pore volume in pores larger than 7.5 A (.1000 mg / l) residual concentrations [4]. At low solution concentrations, the volume of small micropores and pos-

Fig. 5. Correlating CR adsorption with ACF pore size.

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Table 3 Freundlich parameters for atrazine adsorption on ACFs ACF

K (mg / g)(l / mg)1 / n

1 /n

10 15 20 25 MESO

3.9 22.3 22.7 13.2 6.7

0.15 1.02 0.97 0.63 0.66

sibly surface chemistry influenced atrazine adsorption, leading to lower capacities for ACF-25 and MESO ACF than ACF-15 and ACF-20. Atrazine adsorption capacity was much higher than CR capacity on all ACFs, consistent with its smaller size and ability to access correspondingly smaller micropores. Three competitive adsorption modes were used to elucidate the mechanism of competition, including: • simultaneous adsorption of atrazine and CR; • adsorption of CR followed by atrazine contact (CR preloading); • adsorption of atrazine followed sequentially by DDW and CR contact (atrazine preloading).

3.4. Competitive adsorption experiments on ACF-10 Fig. 6 compares the effect of simultaneous adsorption and CR preloading on atrazine capacity on ACF-10, which contained the smallest micropores. From the single-solute experiments the adsorption capacity for atrazine was 25 times greater than for CR, indicative of the smaller size of the atrazine molecule. Both modes of contact reduced atrazine capacity by more than one order of magnitude. Pelekani and Snoeyink [4] showed a similar reduction in capacity using a smaller competing adsorbate, methylene blue, even though it had a single-solute capacity 30 times

higher than CR. The degree of direct site competition between methylene blue and atrazine would be much greater than that between CR and atrazine. The larger CR molecules adsorb on the outer surface of the fiber, and in the large micropores and mesopores (if any) that are very near the fiber surface. This results in pore blockage and a reduction in size (constriction) of the larger pores. Relatively few molecules of CR can thus prevent the adsorption of many atrazine molecules. The similar results for the simultaneous adsorption and CR preloading experiments support the concept of pore blockage at the fiber surface. Surface pore blockage is a relatively fast process compared to diffusion within micropores because of the short distance the CR molecule must travel. Although the atrazine preload experiment followed by CR contact was not performed with ACF-10, Pelekani and Snoeyink [4] showed that methylene blue was not able to displace any significant quantity of atrazine after 7 days, because the strong adsorption potentials in small micropores resulted in very slow desorption. With a much larger molecule, such as CR, which cannot access the same micropores, negligible displacement of atrazine is expected after even long contact times. Figs. 7 and 8 show an increase in atrazine adsorption at 30 days, for both the simultaneous adsorption and CR preload experiments. However, the data are still well below the single-solute isotherm; this indicates that CR has both a kinetic effect and an equilibrium effect. The capacity reduction is due to complete pore blockage of a fraction of the pores, whereas the reduction in the rate of atrazine adsorption is likely due to hindered diffusion because of a reduction in pore size. An important observation is that the 30-day CR preload capacity is higher than the 30-day simultaneous adsorption capacity. The reason for this behavior is that in the CR preload experiment, the preloaded carbon was contacted with single-solute atrazine, and thus desorption of pore-blocking molecules occurred that was not observed in the simultaneous adsorption experiment.

Fig. 6. Effect of simultaneous adsorption and CR preloading on atrazine adsorption capacity on ACF-10 (7 days contact).

C. Pelekani, V.L. Snoeyink / Carbon 39 (2001) 25 – 37

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Fig. 7. Effect of contact time on atrazine adsorption on ACF-10 under simultaneous adsorption conditions.

Fig. 8. Effect of contact time on atrazine adsorption on ACF-10 under CR preload conditions.

3.5. Competitive adsorption on ACF-15 and ACF-20 Fig. 9 shows the effect of simultaneous adsorption and CR preloading on atrazine adsorption capacity with ACF15. The initial CR concentration was 8 mM (5.2 mg / l) and the initial atrazine concentration was 50 mg / l (0.23 mM).

For carbon doses greater than 3 mg / l, the 7-day competitive adsorption data lie essentially on the single-solute isotherm line, indicating no competitive effect due to the presence of CR. At lower carbon doses, a significant kinetic effect was observed at 7 days (data point at 23 mg / l), which shifted to 8 mg / l at 30 days. Two-day batch

Fig. 9. Effect of contact time on atrazine capacity for simultaneous CR / atrazine adsorption on ACF-15.

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C. Pelekani, V.L. Snoeyink / Carbon 39 (2001) 25 – 37

Fig. 10. Atrazine adsorption kinetics on ACF-15: impact of CR under simultaneous adsorption.

kinetic tests (Fig. 10) showed that the adsorption rate of atrazine in the presence of CR is less than in DDW, even at relative high carbon doses of 22 mg / l. The result indicates that CR constricts micropores without complete pore blockage, since the single-solute capacity is still attained. The hypothesis of micropore constriction is reasonable in light of the larger average micropore size of ACF-15 (7.8 ˚ versus 7.1 A ˚ for ACF-10: see Table 2). In ACF-10, the A micropores were too small for CR to access resulting in pore blockage. Widening of these pores greatly increased CR adsorption. Preloading ACF-15 with CR (Fig. 11) yielded similar results to simultaneous adsorption. At relatively high carbon doses (.6 mg / l), the competitive adsorption data lie on the single-solute curve. At low carbon doses (1–2 mg / l), the 7-day atrazine capacity was more deleteriously affected than simultaneous adsorption. At 30 days, there was a significant increase in atrazine uptake by the carbon in the low dose samples, but the data were well below the single-solute isotherm. The reduced rate is consistent with micropore constriction and the lower capacity is in agreement with a small amount of micropore blockage.

The impact of preloading ACF-15 with atrazine, followed by exposure to single-solute CR solution is shown in Fig. 12. After the initial 7-day contact with atrazine, the carbon was removed from the atrazine solution and placed in a CR solution without atrazine for an additional 7 days. Measurement of desorbed atrazine showed that the data lie to the left of the single-solute isotherm, indicating insufficient desorption of atrazine to re-equilibrate with fresh deionized-distilled water (DDW). More atrazine was desorbed when atrazine-preloaded carbon was placed in fresh DDW without CR contact. The result suggests that micropore constriction by CR reduced the desorption rate of atrazine. The fact that the data do not lie to the right of the single-solute isotherm indicates that CR cannot displace atrazine from ACF-15. ACF-20 has a similar micropore size distribution to ACF-15, but has a slightly larger pore volume (0.63 versus 0.69 cm 3 / g). Since these carbons are similar and the results are similar, the data are not shown. As with ACF15, kinetic limitations were only observed at low carbon doses (,2 mg / l). These data shifted to the single-solute isotherm (simultaneous adsorption test) after 30 days,

Fig. 11. Effect of contact time on atrazine capacity for CR preload conditions on ACF-15.

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Fig. 12. Atrazine preload (ACF-15) followed by 7 days of contact with 8 mM CR.

confirming micropore constriction without pore blockage. Batch kinetic tests showed much faster adsorption of atrazine than with ACF-15, indicating the presence of some larger micropores to facilitate transport into the small micropores. Although the presence of CR reduced the atrazine uptake rate, the impact was less than with ACF15, and the single-solute capacity was attained after 48 h. Preloading with CR reduced the rate of adsorption for low carbon doses (,2 mg / l) at 7 days. However, unlike ACF-15, the data shifted to the single-solute isotherm at 30 days, indicating no loss in atrazine capacity and thus the absence of pore blockage. This confirms the presence of slightly larger micropores, where CR will adsorb and result in pore constriction without pore blockage, even at high surface coverage. Preloading with atrazine followed by single-solute CR contact showed very similar results to ACF-15, indicating that CR could not displace pre-adsorbed atrazine.

3.6. Competitive adsorption on ACF-25 The 7-day simultaneous adsorption and CR preload adsorption isotherms are shown in Fig. 13. Simultaneous adsorption resulted in a small reduction in atrazine ad-

sorption, while preloading with CR yielded a larger impact, with a 30% reduction in atrazine capacity (evaluated at 1 mg / l). The 30-day simultaneous adsorption data were the same as the 7-day data, contrary to the results obtained with ACF-10, ACF-15 and ACF-20. This behavior is consistent with the presence of a significant ˚ volume of larger secondary micropores in the 14–20 A range in ACF-25, in which the larger CR molecules preferentially adsorb (even though it has been shown that it can adsorb in smaller pores), without restricting access of atrazine to the small micropores. Batch kinetic tests (Fig. 14) showed that the atrazine uptake rate in the presence of 8 mM CR was the same as in DDW, providing evidence for the absence of micropore constriction with ACF-25. This is an important result because the CR adsorption capacity on ACF-25 was seven times and five times greater than for ACF-15 and ACF-20, respectively. This is associated with the increase in pore volume in secondary micropores. In comparison with ACF-10, which showed a significant competitive effect associated with surface pore blockage, the competitive effect has been substantially reduced and pore blockage minimized by shifting the pore size distribution to larger sizes (e.g. ACF-25). This highlights the importance of having a wide distribution of micropores to

Fig. 13. Adsorption isotherms for atrazine on ACF-25.

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34

Fig. 14. Effect of CR on atrazine adsorption kinetics on ACF-25 under simultaneous adsorption conditions.

reduce competitive adsorption effects with small organic compounds. Table 4 summarizes the results of an atrazine preload experiment with ACF-25 followed by contact with singlesolute CR solution. The amount of atrazine displaced was related to the surface coverage of atrazine, with more atrazine being displaced at high coverage (e.g. 2.3% at 1458 mmol / g). At low surface coverage, atrazine preferentially adsorbs in small micropores that cannot be accessed by CR, so atrazine displacement is low. At high coverage, atrazine fills both the small and large micropores. CR is able to effectively compete with atrazine adsorbed in the larger micropores, resulting in an increase in the level of displacement at high loadings. In the competitive adsorption experiments, most of the data lie in the 1–10 mg / g loading range (4.6–46 mmol / g), which is much less than the lowest loading tested in the atrazine preload test (168 mmol / g). This is important because Table 4 would indicate that negligible displacement of atrazine should occur if all the atrazine is adsorbed in micropores too small for CR to access. Carter et al. [13] proposed that the slope of an isotherm (adsorbent preloaded with the competing species) provided information about adsorption site heterogeneity for the target compound. Fig. 13 shows that preloading with CR did not alter the site heterogeneity for atrazine adsorption; the slope is identical to the single-solute isotherm. This is consistent with CR not being able to access the smallest micropores due to steric hindrance. Direct competition for

adsorption sites is therefore less important and pore blockage must be predominant. However, this is not consistent with the mechanisms established with ACF-10, ACF-15 and ACF-20. The pore size distribution of ACF25 in Fig. 2 indicates a small volume of micropores in the ˚ range, with the absence of micropores of size 14–20 A ˚ This is a strong indication that a small fraction of 9–14 A. the telescopic pores was converted to a branched pore ˚ branching structure with the smaller micropores (7–9 A) off from the larger micropores. With this structure, some CR molecules could effectively block pores leading off from the larger micropores. This mechanism would be consistent with the uniform reduction in atrazine adsorption capacity that is observed over a wide concentration range. Direct competition for adsorption sites would only become important at higher loadings. The small reduction in capacity observed in the simultaneous adsorption isotherm is probably also due to a pore blocking effect. The impact is less because the small atrazine molecules diffuse much faster into the micropore region than the larger CR molecules, so more atrazine is able to penetrate into the primary adsorption space before some of the micropores are completely blocked. The 30% reduction in atrazine capacity (relative to single-solute) under CR preloading conditions was not observed with ACF-15 and ACF-20. It is possible to explain this result using the transition in pore structure. Preloading with CR would allow these molecules to block the entrances to small micropores branching off from larger pores.

Table 4 Summary of ACF-25 atrazine preload (CR contact) experimental results Carbon dose (mg / l)

Atrazine loading (mmol / g)

Atrazine displaced by CR (mM)

CR removed from solution (mM)

Atrazine displacement by CR (%)

0.91 2.2 6.29

1457.9 228.98 161.55

0.298 0.0368 0.00313

0.61 1.99 4.1

2.28 1.46 0.088

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35

Fig. 15. Simultaneous adsorption and CR preload isotherms for atrazine on MESO ACF.

3.7. Competitive adsorption on MESO ACF Fig. 15 compares the simultaneous adsorption and CR preload adsorption isotherm results for the MESO ACF. The initial atrazine and CR concentrations used in the simultaneous adsorption test were 50 mg / l and 5.2 mg / l, respectively. At 7 days, relative to the single-solute isotherm, simultaneous adsorption yielded a small reduction in atrazine capacity. Only a small increase in atrazine adsorption was measured after 70 days, indicating 7 days was sufficient to attain equilibrium. Direct competition for adsorption sites in some of the secondary micropores is the most plausible mechanism to explain the small capacity ˚ range, reduction. Most of the micropores lie in the 8–10 A ˚ size range for ACF-25. The greater compared to the 7–9 A overlap in adsorption pore regions between atrazine and CR increases the probability of direct competition for sites in a fraction of these pores. The low concentration of atrazine used results in preferential adsorption at high energy sites (i.e. the smallest accessible micropores). Direct competition for sites would increase at higher atrazine surface coverages, due to co-adsorption of CR in larger micropores. Two types of preloading tests were conducted. In the first mode, MESO ACF was exposed to a CR solution (initial concentration of 50 mM (32.5 mg / l)) for 70 days

prior to contact with single-solute atrazine (initial concentration of 50 mg / l). In the second mode, the initial CR concentration was 8 mM and the contact time was 7 days prior to atrazine exposure. The 70-day CR preload isotherm yielded a tremendous impact on atrazine adsorption, with negligible increase in adsorption capacity from 7 days to 70 days atrazine contact. This is in contrast to the 7-day CR preload isotherm (7 days atrazine contact), which showed similar behavior to the simultaneous adsorption isotherm except for the lowest carbon doses where observable deviation of the isotherm from linearity was obtained. Although different initial CR concentrations were used in the two preloading experiments, the variation in carbon dose resulted in similar CR loadings. With this similarity, the effect of loading time can be evaluated. Table 5 compares the mass removal of atrazine for different preloading times. Relative capacity reductions of 5–8 times were observed with increasing preloading time. It is hypothesized that CR may not be at true equilibrium after 7 days, and is continuing to diffuse into smaller micropores. The 70-day preload results strongly support slow diffusion of the bulky CR molecules into smaller micropores, reducing both the concentration of high energy sites for atrazine via adsorption at those sites and pore blockage of the small micropores. This has important implications for water treatment plants that use granular activated

Table 5 Effect of CR loading time on atrazine removal on MESO ACF Adsorbed CR (7 days loading) (mmol / g)

Adsorbed CR (70 days loading) (mmol / g)

Mass of atrazine removed (Exp. 1) (mg / g)a

Mass of atrazine removed (Exp. 2) (mg / g)a

Atrazine mass removal ratio (Exp. 1 / Exp. 2)

727 444 276 205

783 427 310 234

4.29 2.88 1.83 1.37

0.55 0.40 0.33 0.24

7.8 7.2 5.5 5.7

a

Initial atrazine concentration55461 mg / l; 7 days atrazine contact.

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C. Pelekani, V.L. Snoeyink / Carbon 39 (2001) 25 – 37

carbon (GAC) filtration systems for periodic taste and odor control. The presence of natural organic molecules not removed by pretreatment processes, which are of the appropriate size and shape, can reduce the ability of the GAC filter to remove specific micropollutants because of the slow diffusion of these molecules deep into the micropore structure, resulting in loss of high energy sites and pore blockage. The non-linearity of the CR preload isotherms at high residual atrazine concentrations indicates pore blockage. The impact is much greater than with ACF-25. MESO ACF is much more highly activated than ACF-25, and it is hypothesized that a greater fraction of this carbon was converted from the telescopic pore structure of these phenolic resin-based ACFs, previously established by Daley et al. [9], to a branched pore structure. This is consistent with the bimodal pore size distribution of MESO ACF (Fig. 2), with a significant pore volume in ˚ Table 2 shows that mesopores of width less than 50 A. mesopores contribute 31% of the total pore volume of MESO ACF, compared to 1% for ACF-25. With this pore structure, CR adsorbed in mesopores could block or constrict micropores in much the same way as was observed for ACF-10 / 15 / 20, especially at high loadings. The importance of this is that it provides some insight into the competition mechanisms observed on conventional activated carbons, which typically have a pore tree structure [14]. To confirm the diffusion of CR molecules into smaller micropores with increasing contact time, the 70-day simultaneous adsorption samples were filtered and the carbon contacted with fresh DDW for an additional 7 days. Only 3–7% of the atrazine that was equilibrated in the 70-day system desorbed. This is a strong indication of CR hindering atrazine desorption because of its deep penetration into the micropore structure. Preloading MESO ACF with single-solute atrazine for 7 days, followed by contact with single-solute CR solution

(8 mM) for an additional 7 days, provided evidence to support direct competition for adsorption sites in part of the secondary micropore region (Fig. 16). Consistent with the ACF-25 results, more atrazine was displaced with increasing loading. However, the dominant micropore size ˚ compared with 7–9 A ˚ for range in MESO ACF is 8–10 A, ACF-25. Therefore, direct competition for adsorption sites is more likely to occur on MESO ACF due to the greater overlap in adsorption pore regions of the competing species. This is in contrast to ACF-25 where pore blockage was proposed at the low residual atrazine concentrations. This is an important distinction because it highlights the sensitivity of the competition mechanism to pore size.

4. Conclusions Using five activated carbon fiber adsorbents with different micropore size distributions the importance of pore size in determining the mechanism of competition between the small organic compound, atrazine, and a much larger competing adsorbate, Congo red dye (CR), was evaluated. This was accomplished via kinetic and equilibrium experiments, including single-solute and simultaneous adsorption isotherms, and various modes of adsorbate preloading experiments. The following competitive adsorption mechanisms were identified. • When only primary micropores are present (i.e. ACF10), the inability of CR to access these small pores resulted in surface pore blockage and a consequent large reduction in atrazine adsorption capacity. • Increasing the micropore width to include small secondary micropores (ACF-15 and ACF-20) resulted in an elimination of any effect of CR on adsorption capacity. However, CR was found to have a significant kinetic effect, due to micropore constriction. The kinet-

Fig. 16. Displacement effect of CR on atrazine preloaded MESO ACF.

C. Pelekani, V.L. Snoeyink / Carbon 39 (2001) 25 – 37

ic effect was most pronounced at low carbon doses (,2 mg / l), which corresponds to high CR surface coverage; the higher the surface coverage, the slower the rate of adsorption. • Increasing the pore volume and shifting the micropore size distribution so as to include a significant volume of ˚ range (i.e. ACFsecondary micropores in the 8–20 A 25) resulted in a low degree of competition by CR. No kinetic limitations were observed indicating elimination of micropore constriction. Displacement of preloaded atrazine by CR increased with increasing atrazine surface coverage, consistent with direct competition for adsorption sites in larger micropores, but could not explain competition at low atrazine coverage. Conversion of a small fraction of the telescopic micropores to a pore-tree structure, with the smaller micropores leading off from larger micropores apparently resulted in complete pore blockage. • A very high degree of activation (i.e. MESO ACF) apparently resulted in a much greater conversion to a branched pore structure with small micropores branching off from large micropores and mesopores, without ˚ range to serve as transition any pores in the 10–14 A pores. Although simultaneous adsorption and CR preloading showed small impacts on atrazine adsorption at low residual concentration, significant impacts from preloading were observed at high CR coverage during preloading. The change in pore structure resulted in micropore constriction and pore blockage. • Increasing the CR preload contact time greatly reduced atrazine adsorption, via slow diffusion into smaller micropores where loss of site heterogeneity and pore blockage may result.

Acknowledgements Thanks to Professor Yoshihiko Matsui (Gifu University, Japan) who provided the base ACF material from Nippon Kynol, to produce the MESO ACF. The authors would also like to thank the University of Adelaide and the Australian–American Education Foundation who were the primary funding sources for Costas Pelekani’s doctoral studies through the George Murray and Fulbright Postgraduate scholarships, respectively.

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