Environ. Sci. Technol. 2001, 35, 2844-2848
Removal of Chemical Contaminants from Water to below USEPA MCL Using Fiber Glass Supported Activated Carbon Filters ZHONGREN YUE, CHRISTIAN MANGUN, AND JAMES ECONOMY* Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801 PATRICIA KEMME, DONALD CROPEK, AND STEPHEN MALONEY CN-E, USACERL, Champaign, Illinois 61826-9005
A new, low-cost fiber glass supported activated carbon (FGAC) filter has been prepared that displays enhanced adsorption characteristics for the removal of BTEX (benzene, toluene, ethylbenzene, and p-xylene) from water to below maximum contaminant level (MCL) regulated by USEPA and two chemical warfare agent simulants (diisopropylmethyl phosphonate and 2-chloroethyl ethyl sulfide) to barely detectable levels. Breakthrough curves for both the FGAC filter and a commercially available granular activated carbon (GAC) filter containing equal weights of adsorbent show that the FGAC filter has greatly improved kinetics of adsorption over the GAC filter for all six chemical contaminants. Benzene breakthrough curves showed the FGAC filter effluent to contain less than one ppb as compared to several parts per million in the GAC filter effluent. This was 2 orders of magnitude better than the GAC and represents a major advance in generating good water quality for the military as well as the general public. The FGAC filter showed a much lower pressure drop and could be completely regenerated at least 6 times by heating to 190 °C under vacuum.
Introduction Problems associated with removal of chemical contaminants from water have become issues of great concern, especially to the military as well as the general public in their pursuit of clean drinking water. Benzene, toluene, ethylbenzene, and xylene (BTEX), ubiquitous contaminants, are present in ground and surface water due to petroleum product releases from leaking underground storage tanks. BTEX and the potential contamination of chemical warfare agents (CWA) may appear in water supplies within a hostile war zone. In response to this problem, USEPA designated BTEX as priority chemical contaminants that need to be reduced to a very low level in drinking water (1). The military and general public are also looking for a portable water purification system for use during camping and for soldiers in the field. The three most common technologies used to remove contaminants from water are biological, chemical, or physical * Corresponding author phone: 217-333-9260. Fax: 217-333-2736. E-mail:
[email protected]. 2844
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treatment. Biological degradation is one of the most costeffective approaches to remove BTEX from water, but there are several factors that affect the rate and extent of BTEX degradation including pollutant concentration, active biomass concentration, temperature, pH, and availability of nutrients and electron acceptors (2). The approach introduces new problems such as secondary pollution from remaining nutrients and a risk of microbial contamination (3, 4). Chemical treatment, such as ozone disinfection, is being used in many drinking water plants in the United States and has been found to be rapid and efficient for decomposing chemical contaminants in water. However, maintaining these reaction conditions is difficult, and some chemically decomposed byproducts remain in the water. Hence, biological or chemical treatments are not particularly reliable, especially for making a portable water purification system. In such cases, physical treatment methods may offer an acceptable alternative. Traditionally, commercially available granular activated carbon (GAC) has been widely used to remove chemical contaminants from water (1, 5-6). Unfortunately, they suffer from a number of drawbacks that include slow adsorption kinetics, poor selectivity, and the need for expensive containment systems. To address some of these disadvantages, activated carbon fibers (ACFs) were developed (7) that offered a number of advantages over GAC including greatly improved contact efficiency with the media leading to greater rates of adsorption. Unfortunately, the ACFs are very expensive (∼$100/lb) and are not very durable during handling. To address the limitations of current ACFs, Economy et al. (8) has recently devised a new low cost route that consists of coating glass fiber textiles with a phenolic resin and then activating the coating to produce a microporous structure. Such fibers appear to be cost competitive to the GACs ($1-2/lb). The wear resistance of these new fibers is 20× better than the commercially available ACFs (9). The excellent wear characteristics are a direct result of the carbon coating protecting the glass fiber surface. The results of gas adsorption showed that by tailoring the pore surface chemistry these new fibers display far superior performance over existing activated carbons (9-11). The adsorption isotherms of organic contaminants from water using tailored FGACs showed that one of the FGACs, activated with CO2/H2O at 800 °C, has a higher percent removal of BTEX, DIMP, and HM than the other FGACs, and such FGAC had higher adsorption capacities than the GAC on a carbon basis; however, on an overall weight basis (including the glass substrate), the FGAC was inferior (12). In the present study, FGAC filters were designed using a low cost newly developed process to explore the kinetics of adsorption of organic contaminants from water. The breakthrough experiments were done to compare the removal of the BTEX and the chemical warfare simulants dissopropylmethyl phosphonate (DIMP) and half mustard (HM) from water by using FGAC and GAC filters. The pressure drops of both FGAC and GAC filters were measured. The regeneration of the FGAC filter was investigated.
Materials and Methods Materials. The FGAC filter assembly cartridges were prepared as shown in Figure 1, by first forming a roll of phenolic impregnated glass mat, followed by curing (heat-setting) in air at 150-170 °C for 6 h and activation with CO2/H2O at 800 °C for several hours. Posttreatment of FGAC filter assembly cartridges was also used to modify the pore chemistry by oxidizing the filter assembly cartridges with HNO3 at room temperature for 30 min, followed by heat-treating in N2 at 10.1021/es001858r CCC: $20.00
2001 American Chemical Society Published on Web 06/02/2001
FIGURE 1. The preparation of FGAC filter assembly cartridge. 600 °C for 30 min. The phenolic precursor was impregnated on a fiberglass mat consisting of 13 µm diameter fibers with 10% submicron fiber dispersed throughout. This was specially prepared by Johns Manville and had 55 wt % phenolic resin content. A second coating step was also employed prior to activation to increase phenolic resin content and the resulting carbon content on the glass by dip coating the heated sample into a solution containing 100 mL of ethyl alcohol, 15 g of phenolic resin and 1 g of hexamethylenetetramine. The various activation conditions used to produce FGAC samples are delineated in Table 1. The FGAC filter was then assembled as part of the above cartridge, which included a glass tubing with two-ends sealed, two plastic connectors, and Parafilm wrapped around the FGAC filter. The schematic drawing of FGAC filters is shown in Figure 2 A. As comparison, the GAC filter containing equal weight of adsorbent, shown in Figure 2B, was constructed from plastic tubing with a thin glass mat at the bottom to hold GAC in place and loose glass fibers at the top to take up free space. GAC Filtrasorb 200 was employed which is designed by Calgon Carbon Corporation for the removal of taste and odor compounds and dissolved organic compounds in potable water treatment. It can be used to treat surface and groundwater sources for the production of drinking water. Its iodine number is 850 mg/g (Min.) and effective size is 0.55-0.75 mm. The six organic materials were obtained from Aldrich (GR grade) including the four BTEX solvents, diisopropylmethyl phosphonate (DIMP) and chloroethylethyl sulfide (half mustard (HM)). The chemical structures for BTEX and the two CWA simulants are shown in Figure 3. Methods. All the filters were put into water, and the air bubbles inside the filter were driven out under vacuum prior to use. The experimental apparatus for breakthrough tests contains a stock with pre-prepared solution and a cartridge pump. Both the FGAC filter and GAC filter containing equal weights of adsorbent were run at the same time. A flow rate of 50 mL/min of contaminant solution was controlled with a cartridge pump and passed through the two filters. The solution was taken every 20 min for effluent and 60 min for influent. Two samples were applied for each point. For BTEX, the water was first analyzed by liquid chromatography (LC). When the concentration of BTEX was below the limit of detection for HPLC, the purge and trap (P & T) method with gas chromatographic/mass spectrometric (GCMS) equipment was employed to detect very low concentrations. For DIMP and HM contaminant measurements, they were first extracted from water with CH2Cl2 for
6 h, then the extracted solution was injected into the GCMS system to detect the concentration level. HPLC was carried out using a Waters LC Module 1 with Millennium 32 version 3.105 software (Waters Corp.). The analytical column was a Supelco ABZ+ (Supelco), 15 cm × 4.6 mm × 5 mm, with UV/Vis absorption detection (Waters). The GCMS was a Hewlett-Packard (HP) 5890 GC/5970 MS controlled by HP ChemStation version B.02.04 (HP). This system was also used in the purge and trap (P & T) experiments with a Tekmar ALS 2016 sampler and LSC 2000 controller (Tekmar). The analytical column was a DB-5 capillary column (J & W Scientific), with a 5% phenylmethyl siloxane stationary phase, 30 m × 0.25 mm × 0.25 mm film thickness. The regeneration of the FGAC filter was carried out under vacuum at 190 °C for different times after peeling off the Parafilm around the FGAC filter and taking off the two plastic connectors. After regeneration, the FGAC filter was reassembled and tested again. The pressure drops through the filters were obtained by pumping water into the filter oriented horizontally and measuring the difference between inlet and outlet pressure. The measurements of pressure drops were done before and after breakthrough experiments. Carbon content on glass was measured using Hi-Res TA Instruments 2950 thermogravimetric analyzer (TGA) by burning off the coating in air. Analysis of surface area, pore size distribution & pore volume was carried out with an Autosorb-1 controlled by Autosorb-1 for windows 1.19 software (Quantachrome Corp.).
Results and Discussion Characterization of FGAC Filters and GAC Filters. In this study, three kinds of FGAC filter assembly cartridges were prepared, as shown in Table 1. FGAC-SL had a lower carbon content of 32 wt %, FGAC-SS had a higher carbon content of 40-50 wt % due to secondary coating, and FGAC-SS* was obtained by posttreating FGAC-SS with HNO3 at room temperature for 30 min and then heat-treating in N2 at 600 °C for 30 min. All the three FGACs have lower specific surface area of 483-700 m2/g, based on total materials (glass + carbon coating), but higher specific surface area of 1361-1505 m2/ g, if based only on carbon coating, as compared with a specific surface area of 770 m2/g for GAC. The physical characteristics of all filters are listed in Table 2. Filter-SL, which was made from 13 g of FGAC-SL, has a larger diameter and cross sectional area than filter-GAC containing equal weight of GAC. Filter-SS was made from FGAC-SS with high carbon content. Its length is the same as filter-SL, but its diameter is smaller than filter -SL. The weight of each filter-SS was a little different, varying from 9.4 to 10 g, due to difficulties in mass control of the secondary coating. The cross sectional area of filter-SS is close to filter-GAC. The order of their bulk densities is filter-GAC > filter-SS > filterSL. Breakthrough Curves of BTEX, DIMP, and HM. On the basis of USEPA drinking water regulations, the maximum contaminant level (MCL) of BTEX for permissible drinking water is 0.005, 2, 0.7, and 10 ppm, respectively (1). The maximum contaminant level goal (MCLG) for benzene is 0 ppm. Therefore, the difficulty and key point for removal of BTEX is to remove benzene from contaminated water to MCL and even to MCLG. Another important factor for removal of BTEX from water in these experiments is the selection of influent concentration of BTEX. In this study, the influent concentration was selected according to the equilibrium adsorption value of BTEX onto the FGAC and a convenient time needed to run until the breakthrough point. In each case, the influent selected was much higher than MCL. For benzene, the influent was 3000 VOL. 35, NO. 13, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Preparation Conditions for FGAC Filter Assembly Cartridges activation of FGAC cartridges FGAC-SL FGAC-SS FGAC-SS* GAC a
secondary coating no yes yes
T (°C)
t (min)
gases
800 800 800
300 600 600
CO2/H2O CO2/H2O CO2/H2O
surface area
posttreatment
carbon content (wt %)
m2/g of carbon
m2/g of adsorbent
32.1 40-50 41
1505 ∼1400 1361 777
483 560-700 558 777
no no yesa
The sample FGAC-SS was further posttreated with HNO3 at room temperature for 30 min and then heat-treated in N2 at 600 °C for 30 min.
TABLE 2. Physical Characteristics of Filters for Breakthrough Profile Experiments on FGAC Filter-SL, FGAC Filter-SS, and GAC Filter filters assembly cartridges
FIGURE 2. Schematic drawings of FGAC and GAC filters.
FIGURE 3. Chemical structures of compounds tested. times higher than its MCL. Each influent selected was close to real data of BTEX contaminated groundwater from deep monitoring wells (13). The breakthrough experiments on BTEX were performed with two continuous up-flow filters containing equal weight of either FGAC or GAC. Breakthrough curves for BTEX from both FGAC and GAC filters are shown in Figure 4. Two important differences can be noted: (i) In all cases, an immediate low concentration breakthrough is noted for GAC 2846
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Filter-SL vs Filter GAC FGAC-SL
GAC
Filter-SS vs Filter GAC FGAC-SS
carbon coating (wt %) 32 adsorbent size (µm) 13 + coating bed mass (g) 13 length of bed (mm) 152 diameter of bed (mm) 23.75 core diameter (mm) 8.8 cross sectional area 382.19 (mm2) volume (cm3) 58.09
23.23
30.18
bulk density (g/cm3)
0.559
0.311, 0.331
0.224
40-50 550-750 13 + coating 13 9.4-10 120 152 15.7 17.8 8.0 193.59 198.58
GAC 550-750 9.4-10 87-92 15.7 193.59 16.8417.81 0.558, 0.561
filters due to short circuiting through the interstitial sites of the bed. Using benzene as an example, the first effluent concentration recorded is 147 ppb for the GAC-filter in contrast to below 1 ppb for the FGAC filter-SL (see insert curve within Figure 4B, where the effluent concentration of benzene was measured by P & T). This was about 2 orders of magnitude better than GAC. Since the maximum contaminant level (MCL) for benzene is 5 ppb, the danger level is immediately exceeded when using the GAC filter. (ii) After the initial breakthrough, the effluent concentration for GAC filters steadily climbs while FGAC filters stay level (close to zero) until a sharp breakthrough curve occurs. This is indicative of fast adsorption and a very short mass transfer zone giving FGAC filters an added advantage under dynamic conditions. This indicates that FGAC filters have good contact efficiency resulting in fast adsorption kinetics and high carbon utilization. This is in contrast to the GAC where the much longer diffusion path lengths through the mesopores to the buried micropores results in premature breakthrough. According to USEPA drinking water regulations, the effective adsorption time or bed volume fed should be taken into account up to breakthrough MCL. Comparative studies showed that for very low MCL, such as benzene (0.005 ppm) and ethylbenzene (0.7 ppm), FGAC filter-SL with lower carbon content is much better than the GAC filter due to a fast adsorption and a very short mass transfer zone of the FGAC filter. But for high MCL, such as toluene (2 ppm) and xylene (10 ppm), it was found that the effective adsorption time until breakthrough MCL was longer on the GAC filter than on the FGAC filter, even though the effluent concentration on the GAC filter is much higher than on the FGAC filter. One of the reasons is that the total adsorption capacities of the GAC filter is higher than on the FGAC filter, due to more weight of effective carbon adsorbent for the GAC. On the other hand, the GAC has greater adsorption ability at high concentration than at low concentration. In such cases, FGAC filter-SS, with higher carbon content, was employed due to its higher total adsorption capacity. The results showed that for toluene breakthrough, the bed volume fed before
FIGURE 5. Breakthrough curves for DIMP and HM from both FGAC and GAC filters.
FIGURE 6. Benzene breakthrough curves on regenerated filter-SL.
FIGURE 4. Breakthrough curves for benzene (B); toluene (T); ethylbenzene (E); and xylene (X) from both FGAC and GAC filters. breakthrough MCL is a little larger on FGAC filter-SS than on the GAC filter, but for xylene, the GAC filter is still better than FGAC filter-SS due to a very high MCL. It should be noted that if the effluent is higher than 2 ppm, xylene might be detected by its odor. The results for DIMP and HM breakthrough shown in Figure 5 give similar trends. That is, an immediate low concentration breakthrough is noted for GAC filters, while the FGAC filters stay level (close to zero) until a sharp breakthrough occurs. For DIMP breakthrough, it was found that FGAC filter-SS* was better than other FGAC filters, such as FGAC filter-SL and FGAC filter-SS. This can be explained due to a molecular sieving effect whereby the DIMP molecule is too large to enter the microporous structure. However, posttreated assembly cartridge FGAC-SS* may increase its micropore size and affinity with tailored pore surface
chemistry (14-16). This indicates that FGAC filters should be better than GAC filters for removal of DIMP and HM from water to a drinking water level. Unfortunately, no data for MCL was available on these two molecules. Regeneration of the FGAC Filter. Regeneration is very important for a water purification system. Since the removal of benzene is a key point for BTEX adsorption, the circuiting regeneration-breakthrough test of FGAC filter-SL for benzene was repeated 6 times, and the results are shown in Figure 6. After the first benzene breakthrough, FGAC filter-SL was regenerated under vacuum (1 in. Hg at 190 °C) then held for 48 h. After which, the breakthrough tests were repeated. Finally, the results showed that the FGAC filter-SL could be completely regenerated at least 6 times by heating to 190 °C under vacuum. Pressures Drops of Filters. The pressure drops of all filters were measured before and after breakthrough experiments, and the results are shown in Figure 7. It can be seen that the pressure drops of all filters increased with increasing flow rate. It was found that in all cases, the pressure drops VOL. 35, NO. 13, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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50 mL/min, the initial pressure drop was lower than 0.5 psi.
Acknowledgments We thank the Defense Advanced Research Project Agency/ DSO for funding this research. We also appreciate the free samples from Johns Manville.
Literature Cited
FIGURE 7. Pressure drops through FGAC filter-SL, FGAC filter-SS, and GAC filter-GAC as a function of flow rate. measured before this breakthrough test were lower than those measured after the breakthrough test. Presumably, during the breakthrough test, some air (or BTEX) bubbles were formed inside the filters and (or) some carbon powder blocked the flow path, leading to an increasing pressure drop during testing. Thus, the pressure drops were plotted as an area due to this variation. Comparative studies showed the pressure drop through FGAC filter-SL (13 g) was much lower than that through the GAC filter (13 g) as shown in Figure 7A, probably because of a bigger cross section area and lower bulk density of FGAC filter-SL. But for FGAC filter-SS (see Figure 7B), its pressure drop was much higher than FGAC filter-SL (Figure 7A) due to its higher carbon content, decreased cross section area and improved bulk density, and even higher than the GAC filter (Figure 7B) containing equal weight of adsorbent with almost equal cross section area, probably due to a longer bed of FGAC filter-SS. However, the measured pressure drops of the FGAC filter are lower than expected due to the spiral wound nature of the FGAC filter assembly cartridge. For example, at the tested flow rate of
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(1) American Water Works Association: Water Quality and Treatment: A Handbook of Community Water Supplies, 4th ed., McGraw-Hill, Inc., 1990, p 106. (2) Alvarez, P. J. J.; Cronkhite, L. A.; Hunt, C. S. Environ. Sci. Technol. 1998, 32 (4), 509-515. (3) Lemoine, D.; Jouenne, T.; Junter, G. A. Appl. Microbial Biot. 1991, 36 (2), 257-264. (4) Bowers, E. J.; Crowe, P. B. J. AWWA 1988, 80 (9), 82-91. (5) Noll, K. E.; Gounaris, V.; Hou, W. S. Adsorption Technologies for Air and Water Pollution Control; Lewis publishers: Chelea, MI, 1994. (6) Giffin, S. D.; Davis, A. P. J. Environ. Eng. 1998, 124 (10), 921931. (7) Economy, J.; Lin, R. Y. Appl. Polym. Symp. 1976, 29, 199-212. (8) Economy, J.; Daley, M., US patent 5834114 1998. (9) Daley, M. A.; Mangun, C. L.; Economy, J. Air & Water Management Association, 89th annual meeting, Nashville, TN, June, 1996. (10) Economy, J.; Mangun, C. L. Proc. ERDEC Sci. Conf. Chem. Biol. Def. Res. 1999, 365-371. (11) Economy, J.; Mangun, C. L. Macromol. Symp. 1999, 143, 75-79. (12) Mangun, C. L.; Yue, Z. R.; Economy, J.; Maloney, S.; Kemme, P.; Cropek, D. Chem. Mater. 2001, in press. (13) Kelley, C. A.; Hammer, B. T. Environ. Sci. Technol. 1997, 31 (9), 2469-2472. (14) Dimotakis, E. D.; Cal, M. P.; Economy, J.; Rood, M. J.; Larson, S. M. Environ. Sci. Technol. 1995, 29 (7), 1876-1880. (15) Daley, M. A.; Mangun, C. L.; DeBarr, J. A.; Riha, S.; Lizzio, A. A.; Donnals, G. L.; Economy, J. Carbon 1997, 35 (3), 411-417. (16) Economy, J.; Foster, K.; Andreopoulos, A. G.; Jung, H. Chemtech 1992, 22(10), 597-603.
Received for review November 8, 2000. Revised manuscript received April 6, 2001. Accepted April 17, 2001. ES001858R
