Environ. Sci. Technol. 1988, 22, 919-924
74-82-8; ethane, 74-84-0; methyl viologen, 1910-42-5. 14L 12
Literature Cited Brugger, P. A.; Cuendet, P.; Gratzel, M. J. Am. Chem. SOC. 1981, 103, 2923. Gratzel, M. Energy Resources through Photochemistry and Catalysis; Academic: New York, 1983, and references cited
therein. Tan, C. K.; Newberry, V.; Webb, T. R.; McAuliffe, C. A. J . Chem. SOC.,Dalton Trans. 1987, 1299. Tan, C. K.; Wang, T. C. Enuiron. Sci. Technol. 1987,21, 508. Gratzel,M.; Kalyanasundaram,K.; Kiwi, J. Struct. Bonding (Berlin) 1982, 49, 37. Meinel, A. B.; Meinel, M. P. Applied Solar Energy-An Introduction; Addison-Wesley: Reading, MA, 1979; p 42. Wang, T. C.; Lenahan, R. A. Bull. Environ. Contam. Toxicol. 1984, 32, 429. Burmaster, D. E. Environment 1982, 24, 6. Stroller, P. Time 1985, October, 76. U.S.Environmental Protection Agency Fed. Regist. 1979, 44, 233. Ace Glass Incorporated Catalog 900; Ace Glass Inc.: Vineland, NJ, 1984; p 216.
Time (hours1
Figure 5. R@uction of carbon tetrachloride (100 ppm, 13.0 pmol) to methane and ethane in the water photolysis system during exposure to sunlight (solar energy = 0.90 cai cm-* min-', air temperature = 16-26 OC): (0)carbon tetrachlorlde; (0)chloroform; (0)methylene chloride; (A) methane; (A)ethane.
Acknowledgments We thank C. R. Hendry for technical assistance in analyzing the samples. We are indebted to T. R. Webb for technical discussions. Registry No. EDTA, 60-00-4; TCE, 79-01-6; CC14, 56-23-5; HzO, 7732-18-5;Pt, 7440-06-4; Ru(bipy):+, 15158-62-0;methane,
Received for review July 13,1987. Revised manuscript received December 9,1987. Accepted January 27,1988. This study was funded by Harbor Branch Oceanographic Institution.
Adsorption and Desorption of Dinitrotoluene on Activated Carbon Patience C. Ho*lt and C. Stuart Daw$
Chemistry Division and Engineering Technology Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 3783 1-6201 The adsorption of 2,4-dinitrotoluene (DNT) in aqueous solution by two commercial activated carbons and desorption of DNT from these adsorbents by solvent extraction were studied. Both carbons proved to be excellent adsorbents for the removal of DNT from aqueous solution. The solvents tested for extracting the adsorbed DNT were water, acetone, methanol, and mixtures involving these solvents. Acetone and methanol are both effective solvents for the removal of DNT from spent carbons, while acetone is the most effective solvent on removal of chemicals (produced during the adsorption) other than DNT. Analysis of the solvent extracts revealed that in the solution, besides DNT, at least six other chemicals were present. Four of the chemicals were identified as 2,4-dinitrobenzyl alcohol, 2,4-dinitrobenzaldehyde, 2,4-dinitrobenzoic acid, and 2,4-dinitrobenzoate. The presence of these compounds suggests that after DNT adsorption chemical reactions, such as oxidation, occurred. Introduction The manufacturing and handling operations in military munitions facilities generate a significant volume of aqueous effluents contaminated with nitrated organics, such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and 1,3,5-trinitrohexahydro-1,3,5-triazine (RDX). Release of these compounds into the surrounding environment constitutes an environmental hazard. Chemistry Division.
* Engineering Technology Division. 0013-936X188/0922-0919$01.50/0
To date, the technique most frequently used to clean nitro-organic-contaminated waste water in military munition plants is by adsorption on granular activated carbon. When the activated carbon becomes exhausted, the only disposal option currently available is landfilling at an approved hazardous waste disposal site. This practice is costly and is becoming increadngly unacceptable as public pressure grows to develop alternative technologies for hazardous waste disposal. Regeneration and reuse of the spent carbon is being considered by the U.S. Army as one possible method for reducing reliance on land disposal. While the carbon must still ultimately be disposed of or destroyed when it can no longer be regenerated, extending the useful life of the carbon directly reduces the final waste volume. During the years, pilot plant tests have been conducted on regeneration of chemically contaminated carbons (excludes explosive-contaminated carbons) by thermal process. Other methods such as chemical, solvent, and biological techniques are being tested only on isolated cases and mostly on laboratory scale. Regeneration of explosive-saturated carbon by thermal (I, 2) and by solvent (3-6) processes is still in laboratory experimental stages. The explosives involved in most cases were largely TNT and RDX (1-6). All of these technologies involve the common concerns of minimizing personnel exposure to explosive and toxic hazards while at the same time maximizing the efficiency of the carbon regeneration. A key part of addressing these concerns is the accurate measurement and control of the nitrobody loading on the carbon. When actual carbon analyses are incomplete or
0 1988 American Chemical Society
Environ. Sci. Technoi., Vol. 22, No. 8, 1988 919
unavailable, some basis must still be provided for estimating the magnitude and variability of the contaminant species. In this paper we present the adsorption/desorption (by solvent extraction) behavior of 2,4-dinitrotoluene (DNT), which is a principal nitrobody contaminant from TNT manufacturing plants (such as Radford Army Ammunition Plant) and is also the least studied of the major waste water contaminants. The commercial activated carbons (Filtrasorb 300 and Filtrasorb 400) produced by Calgon Corp. were evaluated because they are representatives of the majority of the granular activated carbon used for munitions waste water treatment. The primary objectives of this study are (1)to provide basic information needed for the development of chemical analysis procedures for explosive-contaminated carbon, (2) to provide a basis for estimating the maximum levels of DNT contamination on spent carbons, (3) to provide a basis for estimating the maximum regeneration possible for DNT-contaminated carbon by means of solvent extraction, and (4) to provide basic information that could be useful in developing new, more cost-effective carbon regeneration processes. In order to understand the mechanism of adsorption/ desorption of DNT on carbon, a preliminary analysis of the chemicals other than DNT desorbed from the spent carbon is also discussed.
Experimental Section Materials. 2,4-Dinitrotoluene (Aldrich, 97%) was purified as described earlier (7) to a purity greater than 99 % . Methanol (Fisher, HPLC grade) and acetone (Fisher, certified ACS) were used as received without further purification. Water was deionized and double-distilled. Granular bituminous coal charcoals, Filtrasorb FS 300 (0.8-10 mm) and FS 400 (0.55-0.75 mm) from Calgon and a powdered FS 300 ground by us (90-212 pm), were used as adsorbents. According to the manufacturing information, the total surface area (N2,BET method) of FS 300 and FS 400 are 950-1050 and 1050-1200 m2/g, respectively; the pore volume of the former is 0.85 cm3/g and that of the latter is 0.94 cm3/g (8). The surface area and pore volume of powdered FS 300 (ground by us) were not measured. The granular carbons were washed several times with distilled water to remove the fines, and then dried at 105 "C for 3 days before using. The powdered FS 300 was not washed before use. Adsorption and Desorption Procedures. (A) Adsorption. The equilibrium adsorptive capacity (9) of the virgin (granular or powdered) and regenerated carbons were measured by batch isotherm tests. Known amounts (by weight) of carbon (virgin and regenerated) (10-50 mg) and aqueous DNT solution (10-35 g; concentration, 25-120 ppm) were placed in small glass bottles (ca. 40 mL) with glass stoppers and shaken with an Edberbach shaker for 5 days (testing results have shown that equilibration can be reached in 1day of shaking). After equilibration, the samples were centrifuged, and the aqueous phase was removed and analyzed by either HPLC or UV. The solid phase then was dried in air on filter paper for 4 days before regeneration, which will be discussed later. The results of adsorption obtained this way are presented as Freundlich isotherms and shown in Figures 1-3. In addition, large amounts of DNT-loaded carbon were produced for desorption experiments (column, Soxhlet, and batch extraction) by passing saturated DNT aqueous solution (flow rate, ca. 4 mL/min) through a small glass column (0.5 cm i.d., 8-10 cm high) packed with virgin carbon. The carbon was retained with a porous Teflon plug and some glass wool on the bottom of the column. The spent carbon was 920
Environ. Sci. Technol., Vol. 22, No. 8, 1988
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recovered by filtration through removal of the equilibrated aqueous DNT solution. After being rinsed with a small amount of water, the spent carbons were dried on filter paper in air for at least 3 days. Typically, the spent carbon prepared in this manner contained about 30-40% (by weight) DNT. (B) Desorption. Desorption experiments were conducted by using column, batch, and Soxhlet procedures. The solvents used for regeneration were water, methanol, acetone, and mixtures of methanol/water and acetone/ water. In the batch procedure, the experiments were carried out as follows: (1)Data of cycle 3, which were shown in Figures 1 and 2, were obtained by placing 10-50 mg of spent carbon
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produced in cycle 2 (second adsorption) in 40-mL glass bottles with glass stoppers and by shaking with 27-60 g of acetone for 5 days. After equilibrium, the sample was centrifuged, the liquid phase was then removed, and the regenerated carbon was dried in air before being used for further adsorption/desorption cycles. The definition of cycles in this study is as follows: cycle 1, adsorption on virgin carbon; cycle 2, first desorption and second adsorption; cycle 3, second desorption and third adsorption. It should be noted that the regenerated carbons used in cycle 2s for second adsorption of Figures 1 and 2 were produced by Soxhlet extraction with acetone. (2) Data presented in Figure 4a were obtained by placing 20-100 mg of spent carbon (prepared by column adsorption) in glass bottles, and samples were extracted several times each with 10-50 g of various solvents. Because of the low solubility of DNT in water (the solubility of DNT in water vs temperature is given in Figure 5 ) each sample was equilibrated with water for 7 days. Extraction was done either by shaking at room temperature or by placing the samples in a water bath and stirring them occasionally at 70 "C. After equilibration, the liquid phase was removed and the solid phase was dried in air before extracting again with new solvent. The percentage of cumulative amount of DNT removed from spent carbon at room temperature and at 70 "C vs g of solvent/g of spent carbon is plotted in Figure 4a. When methanol and acetone were used as solvent, the procedures were carried out the same way except the extracting time was reduced to 3 days because equilibration can be reached in about 1 day with these
organic solvents. The results are also given in Figure 4a. In the column procedure, a known amount of spent carbon (0.2-1.8 g) was packed in a small glass column (0.5 cm i.d., 0.5-3.7 cm high) and eluted with solvent at a flow rate of 3-5 mL/min. Because of the relatively large carbon particle size and the relatively small size of column, the problem of channeling probably cannot be avoided. The results are given in Figure 4b. In the Soxhlet extraction, ca. 100 mg of spent carbon was refluxed with 300 mL of solvent. Each sample was extracted for 1-3 days depending upon the solvent used. The amount of DNT removed from spent carbon by Soxhlet extraction with various solvents is given in Table I. The extracts resulting from the desorption procedures were analyzed by UV, HPLC, and GC/MS. Extracts of virgin uncontaminated carbon were used as the control. Analytical Methods. The concentration of DNT in solution was analyzed either by a high-performanceliquid chromatograph (HPLC) equipped with a Varian VariChrom UV-vis detector (reverse-phasecolumn 25 cm long, 4.6 mm i.d., packed with Alltech C18) or by a Perkin-Elmer 559 UV-vis spectrophotometer. The instruments were calibrated with standard solutions of DNT in water. Chemicals desorbed from the spent carbons were characterized as follows: after desorption, the solution was filtered, the solvent was removed, and the residue was dried under nitrogen for several hours. The dried residue was then silylated with N-methyl-N-(trimethylsily1)trifluoroacetamide (MSTFA) (Pierce) before GC/MS analysis. The GC/MS analysis was done by using a Hewlett-Packard HP 5995A gas chromatograph/mass spectrometer system fitted with a 30-m Durabond I narrow-bond fused silica column (J. W. Scientific). The column temperature was programmed from 90 (1-min hold) to 300 OC with an increase of 10 OC/min.
Results a n d Discussion Adsorption Isotherms for Virgin and Regenerated Carbon. The equilibrium adsorptive capacities of DNT on granular FS 300, FS 400, powdered FS 300, and reEnviron. Sci. Technol., Vol. 22, No. 8, 1988 921
generated spent FS 300 and FS 400 were determined by batch method and interpreted by the Freundlich isotherm ( I O , 2 1 ) . Although the adsorption is believed to be heterogeneous, when treating dilute solutions, the empirical Freundlich equation can well serve the purpose on the evaluation of these activated carbons. Thus the DNT adsorption isotherms can be expressed as
X / M = kC,lfn
1
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(1)
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(2)
where X / M is the amount of DNT (mg) adsorbed per gram of carbon at equilibrium, X is the amount of DNT (mg) adsorbed by carbon, M is the weight (g) of carbon used, Cfis the concentration (ppm) of aqueous phase at equilibrium, and k and n are constants for a given adsorption system. For the DNT concentration in this study, eq 2 appears to hold true, and a log-log plot of X / M vs Cfgives a straight line with a slope of l / n and the intercept k at Cf= 1ppm. The k value represents the equilibrium adsorptive capacity of carbon ( X / M ) in mg of DNT/g of carbon at an equilibrium concentration of 1.0 ppm of DNT. The slope l / n gives an estimate of the adsorption interaction energy, with smaller n values reflecting a lower energy (6, I O ) . Figures 1 and 2 show the linear leastsquares best fit Freundlich isotherms for each of the two granular virgin carbons and the corresponding acetoneregenerated carbons, respectively. Figure 3 shows the Freundlich isotherm for the powdered FS 300. From the intercept values (X/W at Cf= 1 ppm, the equilibrium adsorptive capacities of virgin FS 300, virgin FS 400, and virgin powdered FS 300 are 0.21,0.30, and 0.25 g of DNT/g of C, respectively. The corresponding isotherm slopes are 0.171, 0.223, and 0.333 (n = 5.83, 4.49, and 3-33),respectively. Granular and powder virgin FS 300 can remove roughly about the same amounts of DNT from aqueous solution, while granular virgin FS 400 adsorbed slightly more DNT than FS 300. After DNT adsorption, the spent granular carbons were regenerated by acetone extraction. Three adsoptions and two desorptions were performed on each carbon (cycle 1, adsorption on virgin carbon; cycle 2, first desorption and second adsorption; cycle 3, second desorption and third adsorption). Desorption was accomplished by Soxhlet method in cycle 2 and batch method (shaken 5 days at room temperature) in cycle 3. Figures 1 and 2 show that the equilibrium adsorptive capacities of FS 300 and FS 400 after Soxhlet extraction are nearly as great as the virgin carbon, whereas the capacities after batch extraction are drastically decreased. These decreases of capacities in cycle 3 as shown in both Figures 1 and 2 are probably expected. Soxhlet extraction is a method involving multiple extraction, while the batch extraction applied in cycle 3 is a one-time extraction. The amount of DNT that can be removed from the spent carbon is proportional to the volume of solvent used for extraction. The relative difference of these two extraction methods is further demonstrated in Figure 6. The Freundlich isotherms of cycle 2s in both Figure 6a and b are reproduced from Figures 1 and 2. The Freundlich isotherms of cycle 2 were produced by treating spent carbons (spent FS 300 and FS 400) of the same origins as in cycle 2s with one-time batch extraction (shaken with acetone at room temperature for 5 days). The regenerated carbons were then equilibrated with aqueous DNT solution (shaken 5 days at room temperature). It can be seen that the Soxhlet-extracted carbon exhibited at least a 40% better recovery of adsorptive capacity than the batch-extracted 922
Environ. Sci. Technol., Vol. 22,
No. 8, 1988
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carbon. Adsorption/desorption cycle beyond 3 was not attempted in this study. Desorption with Various Solvents. DWT adsorbed onto granular FS 300 and FS 400 was extracted with water, acetone, methanol, and their mixture by column, batch, and Soxhlet procedures separately. Desorption of DNT from spent powdered FS 300 was not performed in this study. The desorption effectiveness is highly dependent on the solubility of DNT and other chemicals (produced in carbon after adsorption) in a particular solvent and the volume of solvent used. Chemical reactions occurring inside carbons will be discussed later. Table I compares the percentage of DNT that can be removed from spent carbons in the first cycle by various solvents and solvent systems with the Soxhlet process. Both acetone and methanol are better solvents than water. Figure 4a and b illustrate the effects of solvent/spent carbon weight ratio (g of solventjg of spent carbon) and temperature on the percentage of DNT removed by batch and column extraction. On the basis of Figure 4a the cumulative desorption (batch method) of DNT in water increases linearly with the water/carbon ratio and also increases significantly as temperature is increased. Two sets of data are shown for each of the desorption of DNT from spent FS 400 at room temperature and at 70 OC. The slopes of each of the two sets of data of spent FS 400, at room temperature and at 70 "C, are approximately equal to each other, which presumably indicates that the extraction efficiency is consistent. The magnitude of the temperature effect on extractability is supported by Figure 5, which shows the increase of DNT solubility in water with temperature (at ca. 70 "C, DNT melts and a two-phase liquid system is formed). Despite the temperature effect,
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DNT extraction with water is rather inefficient. At 70 "C, a solvent/carbon ratio of 3000 extracts only about 25% of the adsorbed DNT. Much greater extraction efficiencies were obtained by using either methanol or acetone as the batch-extracting solvent. Figure 4a shows that most the DNT can be removed with relatively small amounts of these organic solvents and that the cumulative percentage of DNT removed increases rapidly with increasing solventlcarbon ratio. At constant temperature and amount of solvent, methanol and acetone definitely prove to be much better extracting solvents than water. Figure 4b shows the extraction efficiencies of acetone and methanol by the column process. The flow rate of column treatment was in the range 3-5 mL/min, and it was found that the influence of flow rate on desorption was not too apparent. Because of the relatively large carbon particle size and the relatively small size of column, the problem of channeling cannot be avoided. In the column treatment, the desorption proceeded at a very rapid initial rate. Beyond a certain percentage of removal, the desorption efficiency started+to decrease and 100% removal was approached asymptotically. By using water as a solvent, the column desorption procedure was not completely conducted. This is because in a column packed with spent FS 400 of the same origin as used in Figure 4b, it was found that at a solvent/carbon ratio of 1439, only 0.48% DNT was recovered from the column. With all three solvents and extracting procedures tested, it seems that more DNT could be removed from spent FS 400 than from spent FS 300. This is presumably because the adsorption energy onto FS 400 is slightly lower than FS 300 (12). The efficiency of desorption of DNT from carbon by acetone and by methanol is comparable by comparison of the spent FS 300 and FS 400 extracting by the Soxhlet procedure (Table I) and by batch method (Figure 4a). However, acetone is a more powerful solvent than methanol when applied to column extracting procedures to remove DNT from spent FS 300 (Figure 4b). Acetone is also a more efficient solvent than methanol on the removal of chemicals other than DNT from spent carbon. Examples are demonstrated in Figure 7. Figure 7a is a HPLC spectrum for a methanol extract of spent FS 400 (first cycle, column method) and Figure 7b is a HPLC spectrum for a subsequent batch acetone extract of the previous methanol-extracted sample. These spectra indicate that most of the DNT was removed by the initial methanol extraction; chemicals other than DNT (DNT derivatives)
were largely removed by acetone. Analysis of Desorption Extracts and Evidence of Chemical Reactions. As mentioned previously, the HPLC analysis of extracts (Figure 7) revealed the presence of DNT derivatives. Also, when spent FS 300 and FS 400 were extracted by water, methanol, or acetone in the batch, column, or Soxhlet method, all extracts were yellow in color. Some of the derivatives are probably responsible for the color changes (7). Panels a-c of Figure 8 show the HPLC spectra of water, methanol, and acetone extracts, respectively, of DNT-contaminated FS 400 (first extraction, batch method). The number of DNT derivatives and their relative abundance in the extracts depend on the solvent system used and the number of adsorption/desorption cycles the carbon has experienced. Acetone is the most efficient for DNT derivative extraction, followed by methanol and then water. When water was the solvent, the abundance of products (including DNT) increased with increasing temperature. In our experiments, there were typically three to six distinct derivatives appearing in the extracts for carbon that had only undergone one adsorption cycle, while there were as many as eight in acetone extracts from second and third cycles. After characterization by GC/MS and comparison with the spectra of authentic compounds (if available), a major compound that was found in all three extracts was confirmed as 2,4-dinitrobenzoic acid. 2,4-Dinitrobenzaldehyde also appeared in all three extracts, although its level was rather low in water. Other compounds that could be identified were 2,4-dinitrobenzyl alcohol ( 7 ) in methanol and acetone extracts and methyl 2,4-dinitrobenzoate (mass spectrum, base ion peak at m./e 195 and molecular ion peak at m / e 226) in acetone extract. The rest of the other compounds that were present have not yet been identified due to limited information. For background information, virgin FS 300 and FS 400 were also extracted in the same manner with acetone, and none of these compounds were found. It has been known for some time that activated carbons are highly porous materials with a large surface area containing various chemical reactivity groups such as radicals, carboxyl groups, and oxides (5, 13, 14). It is also known that adsorption of certain species, such as organic chemicals, from solution by carbon and other adsorbents is a relatively complex phenomenon; it depends on the nature of solute-solvent interactions in the solution phase and in the interfacial region, as well as on their interactions with the adsorbent (10). The high chemical reactivity of carbon surface will further complicate the problem. When organic compounds in aqueous solutions are treated with activated carbon, not only will they be heterogeneously adsorbed, but two kinds of adsorption will occur, i.e., physical and chemical adsorption. Physical adsorption is reversible, nonspecific, and the adsorbed substance can be completely desorbed. On the other hand, chemical adsorption is Environ. Sci. Technol., Vol. 22, No. 8 , 1988
923
specific (specific sites are involved) and is presumably at least partially irreversible if the adsorption is achieved by the formation of high molecular weight materials such as polymers between the adsorbate (here is DNT) and surface chemical groups of the carbon (adsorbent) because complexes formed this way tend to be more strongly adsorbed than low molecular weight compounds. However, in some cases, if the chemically reactive sites serve as carriers of specific chemical reactions, for instance as catalysts, and the products produced are not high molecular weight species, such as 2,4-dinitrobenzoic acid and other DNT derivatives found in the solvent extracts in this study, then of the chemical reaction products formed, most can be removed from the carbon (adsorbent) by solvent extraction with solvents in which these products are dissolved in. The presence of the above-mentionedderivatives suggests that after DNT adsorption by carbon, chemical reactions occurred, and the presence of compounds such as 2,4-dinitrobenzyl alcohol, 2,4-dinitrobenzaldehyde,and 2,4-dinitrobenzoic acid proved that one of the reactions is a side-chain (the methyl group) oxidation (7). More studies are needed before a definite conclusion of the reaction mechanisms can be drawn. From the data shown in Table I and Figure 4, one can see that DNT-contaminated carbons can be highly regenerated by solvent extraction with solvents such as methanol, acetone, and their mixtures with water. Although the total amount of organic compounds removed from the carbons cannot be quantified, it is certain that the amounts of organic compounds removed from these carbons were higher than the data shown in Table I and Figure 4, which only represent the amount of DNT removed. The relatively high percentage of organic compounds that could be desorbed from the spent carbons indicated that the amounts of high molecular weight compounds which did not dissolve in methanol and acetone were not high.
Conclusions The efficiency of removal of DNT from aqueous solution by Calgon FS 300 and FS 400 are different, with FS 400 being generally more adsorptive. Grinding FS 300 increase its equilibrium adsorptive capacity, but the capacity still remains less than that of FS 400. For an aqueous-phase DNT concentration of 100 ppm, it is possible to achieve equilibrium DNT loadings on FS 400 m high as 800 mg of DNT/g of dry carbon. Such high loadings could have significant impact on impact on handling safety. Regeneration of DNT-contaminated carbon by solvent extraction with solvents such as acetone is an attractive and promising process. Some of the adsorbed DNT is converted to its derivatives. The presence of derivatives of 2,4-dinitrobenzyl alcohol, 2,4-dinitrobenzaldehyde,and 2,4-dinitrobenzoic
924
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acid suggests the occurrence of side-chain oxidation.
Acknowledgments We express our thanks to Calgon Carbon Corp., Pittsburgh, PA, for supplying the activated carbons. Registry No. DNT, 121-14-2; carbon, 7440-44-0; water, 7732-18-5;acetone, 67-64-1; methanol, 67-56-1; 2,4-dinitrobenzoic acid, 610-30-0; 2,4-dinitrobenzaldehyde,528-75-6; 2,4-dinitrobenzyl alcohol, 4836-66-2; methyl 2,4-dinitrobenzoate, 18959-17-6.
Literature Cited (1) Eskelund, G. R.; Wu, K. K.; Rosenblatt, D. H.; Davis, G. T.; Demek, M. M.; Dennis, W. H., Jr. A Laboratory Study of Carbon Adsorption for Elimination of Nitrobody Waste from Army Ammunition Plants; Picatinny Arsenal: Dover, NJ, 1973; AMCMS 4932.05.4114. (2) Leeper, J. E.; Layne, W. S.; Wahl, R. M.; Tash, J. E.; Jackson, B., Jr.; Santoe, J. S. Thermal Regeneration of Explosive Laden Carbon i n a Rotary Calciner; LCWSL US.Army Armament Research and Development Command: Dover, NJ, 1977; AD-E400032. (3) Demek, M. M.; Davis, G. T.; Richmond, J. A.; Sommer, H. Z.; Rosenblatt, D. H.; Eskelund, G. R. Studies on the Regeneration of Activated Carbon for Removal of a-TNT from Waste Waters; Edgewood Arsenal: Aberdeen Proving Ground, MD, 1974; EC-TR-74008. (4) Castorina, T. C.; Haberman, J.; Sharma, J. Charcoal Regeneration; LCWSL U S . Army Armament Research and Development Command: Dover, NJ, 1977; Part I, ADA048757. (5) Haberman, J.; Castorha, T.; Semel, S.; Kramer, H. Charcoal Regeneration; LCWSL U.S. Army Armament Research and Development Command: Dover, NJ, 1980; Part 11, ADA087216. (6) Haberman, J. Charcoal Regeneration; LCWSL U.S. Army Armament Research and Development Command: Part IV, AD-A130002. (7) Ho, I?. C. Environ. Sci. Technol. 1986, 20, 260-267. (8) Bulletin 20-68b; Calgon Carbon Corp.: Pittsburgh, PA, 1986. (9) Standard Definitions of Terms Relating to Activated Carbon;American National Standards Institute: New York, 1976; ANSI/ASTM D 2652-76. (10) Freundlich, H. Colloid & Capillary Chemistry;translated by Hatfield, H. S.; Methuen: London, 1926. (11) Adameon, A. W. Physical Chemistry of Surfaces, 4th ed.; Wiley: New York, 1982. (12) Rollor, M. A,; Suidan, M. T.; Cross, W. H.; Vargo, S. A. J. Enuiron. Eng. Diu. (Am.SOC.Civ. Eng.) 1982,108, No. EE6, 1361-1377. (13) Bonnet, J. B. Carbon 1968, 6, 161-176. (14) Szwarc, M. J. Polym. Sci. 1956, 19, 589-590.
Received for review August 7,1987. Accepted February 11,1988. This research was sponsored by the U.S.Army Toxic and Hazardous Materials Agency under Interagency Agreement 40-1755-86. Oak Ridge National Laboratory is operated by Martin Marietta Energy Systems, Inc., for the US.Department of Energy under Contract DE-AC05-840R21400.
