(13) Wong, J. B.; Ranz, W. E.; Johnstone, H. F. J. Appl. Phys. 1956, 27, 161. (14) Landahl, H. D.; Herrmann, R. G. J. CoEloid Interface Sci. 1949, 4,103. (15) Wong, J. B.; Johnstone, H. F. 1953, Urbana, IL, Engineering Experimental Station, University of Illinois, Technical Report 11. (16) Sehmel, G. A.; Hodgson, W. H. ERDA Symp. Ser. 1976, No. 38, 399. (17) Sehmel, G. A.; Hodgson, W. H. Richland, WA, Jan 1978, Battelle Pacific Northwest Laboratories Report PNL-SA-6721. (18) Miller, J. “Dry Deposition of Particles to Cylindrical Plant Fi-
bers”; Department of Civil Engineering, Carnegie-Mellon University, Technical Report, May 1980. _ -Wells, _ _ A. C.; Chamberlain, A. C. Br. J . Appl. Phys. 1967, 18, (19) 17Y3.
(20) Chamberlain, A. C. Proc. R. SOC.London, Ser. A 1966, 296, 45. (21) Chamberlain, A. C.; Chadwick, R. C. Ann. Appl. Biol. 1972,71, 141. Received for review February 19,1980. Accepted October 9,1980. This work was supported in part by EPA grant no. V-0502-NTEX and MPC grant no. 79-5.
Investigationsof Solvent-Regenerable Carbon-Sulfur Surface Compounds for Phenol Removal in a Packed Column Chin H. Chang’t and David W. Savage Exxon Research and Engineering Company, P.O. Box 45, Linden, New Jersey 07036
w Carbon-sulfur compounds, C,S, have been studied as solvent-regenerable adsorbents for the removal of phenol from aqueous solution in a continuous column packed with granular adsorbent. The C,S material studied was prepared easily by reacting poly(viny1idene fluoride) charcoal with sulfur dioxide a t 600 “C. Results of adsorption-regeneration cycling experiments at room temperature show that C,S has a virgin capacity of 8.1 lb/ft3 with an influent concentration of 2000 ppm a t a flow rate of -10 bed volumes/h. Regeneration of the adsorbent with 2.0 bed volumes of 2-propanol recovers 90% of its capacity. Parallel studies on a conventional activated carbon, Filtrasorb 300, under similar experimental conditions show that the activated carbon has an initial capacity of 5.6 lb/ft3 and its capacity falls gradually to a value of 4.1 lb/ft3 after several cycles. The difference in regenerability between C,S and the activated carbon is attributed to the difference in surface groups. Introduction Recently, we have investigated carbon-sulfur surface nonstoichiometric compounds (C, S) as adsorbents for the removal of aromatic compounds from aqueous solutions ( I ) . Equilibrium isotherms showed that C, S materials have similar or higher capacities for model compounds such as naphthalene and phenol than that of Filtrasorb 300, a conventional activated carbon. Kinetic studies showed that the rate of adsorption for C,S is as fast as, if not faster than, that of the activated carbon. However, energies of adsorption for the novel C,S material were found to be lower than that for the conventional activated carbon. The difference in binding energy (heat of desorption) between an aromatic compound and these two different adsorbents can be attributed to differences in surface groups existing on the adsorbents. Amorphous structures of C,S materials and activated carbons are quite similar. However, instead of carbon-oxygen surface groups which have been detected on conventional activated carbons (2,3),we found that C,S materials contain carbon-sulfur surface complexes ( 4 ) . Although the exact mechanism of the adsorption of organic compounds from aqueous solution onto activated carbon is not well established, Mattson and his co-workers suggested as a contributing mechanism that aromatic compounds are adsorbed on actiAddress correspondence to Corporate Research-Science Laboratories, Exxon Research and Engineering Co.. P. 0. Box 45. Linden. New Jersey 07036. +
0013-936X/81/0915-0201$01 .OO/O
@ 1981 American Chemical Society
vated carbon by a donor-acceptor complex mechanism involving carbonyl oxygens of the carbon surface acting as the electron donor and the aromatic ring of the adsorbate acting as the acceptor (5).A C-S group is less polar than a C-0 group because of the smaller electronegativity of sulfur compared to oxygen. Thus a C-S group would not bind an aromatic compound as strongly as a C-0 group does. Spectroscopic investigations of naphthalene-adsorbent complexes on C, S and Filtrasorb 300 support the above speculations ( I ) . Activated carbon is an effective adsorbent for the treatment of potable and industrial wastewaters. However, the treatment applicability of the adsorbent may be limited by an economical regeneration process. For most applications, thermal regeneration of activated carbon is the method chosen because of the high binding energies between the organics and the adsorbent. The thermal regeneration leads to the destruction of the adsorbate, the loss of carbon, and the lowering of the carbon capacity for subsequent cycles. With the increasing cost of chemical raw materials, a nondestructive regeneration process which can recover these adsorbates becomes economically more attractive. It has been known for some time that organic adsorbates can be partially desorbed from conventional activated carbons by liquid solvents. In a method of establishing the drinkingwater standard for organic compounds, organics are adsorbed by granular activated carbon and they are extracted from dried carbon by chloroform and ethyl alcohol (6,7).However, carefully defined experiments by Pahl and his co-workers (8) showed that the rate of desorption of phenol from activated carbon by a number of liquid solvents is slow and the percent of adsorbate removal is also low. Slow desorption rate requires long contact time, and incomplete regeneration decreases the adsorbent capacity for subsequent adsorption cycles. Both limitations are detrimental for a practical water-treating process. Most recently, a solvent-regeneration process which recovers valuable adsorbates has been demonstrated based on a class of polymeric adsorbents, Amberlite (9). I t has been speculated that aromatics are removed with these polymers by physical adsorption; van der Waals’ forces bind the aromatics to the macroreticular resin structure with binding energies of several kcal/mol for dichlorophenols (IO) and 12 kcal/mol for phenol (11).Because of the extremely low binding energies, these phenols can be easily recovered by solvent regeneration of the resin. Although favorable economics of the solvent-regeneration process with the polymeric adsorbents for practical phenol removals from wastewater has been shown (9),the use of these Volume 15, Number 2, February 1981 201
adsorbent materials is not without limitations. The polymeric adsorbent has a slow rate of adsorption as a result of the low binding energy between the organic adsorbate and the adsorbent. Consequently, the loading capacity of the adsorbent depends strongly on the flow rate of the waste stream (12). Furthermore, the adsorbent capacity is low at low adsorbate concentrations in the influent solution. At a flow rate of 2-3 adsorbent bed volumes/h, Amberlite XAD-4 has a capacity of less than 3 Ib of phenol/ft3 of the resin a t an influent concentration of 2000 ppm (9). The capacity increases to -6.4 Ib/ft3 at a phenol concentration of 1%.In a process for the treatment of waste stream with low phenol concentration, low capacity requires frequent regeneration of the resin and therefore a high regeneration cost. In our previous studies using naphthalene as a model organic adsorbate (Z), we have demonstrated that C,S materials have a capacity for the adsorbate similar to that of the conventional activated carbon while the binding energy is significantly lower (13).With a hinding energy that is significantly lower than that of activated carbon yet greater than that of polymeric adsorbents, C,S materials could offer a better balance than either activated carbon or resin adsorbent between dissolved aromatic cleanup capacity and solvent regenerability. This paper presents an extention of our earlier studies on C,S. The removal of phenol from aqueous solution by C,S and the regeneration of the spent adsorbent with organic solvents in acontinuous, down-flow column will be described. Parallel studies were made on Filtrasorb 300 and Ambersorb XE-340, which is a carbonized synthetic resin whose chemical composition is intermediate between that of activated carbon and polymeric adsorbents (14). Experimental Section The C,S material was prepared by reacting poly(viny1idene fluoride) (PVDF) charcoal with sulfur dioxide a t 600 "C in a rotating reactor which is described elsewhere ( 4 ) .The PVDF charcoal was prepared by carbonizing PVDF (Aldrich Chemical Co.) in nitrogen. The conventional activated carbon, Filtrasorb 300, was obtained from Calgon Corp., and Ambersorh XE-340 was obtained from Rohm and Haas Co. Both adsorbents were used without any further chemical treatment. Samples of C,S and Filtrasorb 300 were sieved through US mesh No. 60 (250pm) and retained at No. 80 (180pm) for C,S and No. 100 (150pm) for Filtrasorb 300. The particle sizes for Ambersorb XE-340 are between 300 to 850 pm, and they were used as received. The surface area and the total pore volume of these adsorbents were determined by measuring the adsorption and desorption isotherms of nitrogen at ll K. These were carried out with a Micromeritics DigiSorh 2500 unit (manufactured by the Micromeritics Instrumental Corp.). The carbon and sulfur contents of these materials were analyzed by rapidly burning the material in a stream of oxygen and separating and determining the combustion products by a gas chromatograph using the oxygen stream as a carrier gas. Aqueous solutions of phenol a t a concentration of 2000 ppm were prepared hy dissolving 4.000 g of phenol (MCB, Reagent, ACS) in a 2000-mL volumetric flask. Standard solutions of various concentrations for calibrating the fluorometer used in monitoring the phenol concentration during the column experiment were prepared from a 2000-ppm solution by dilution. In order to study the solvent regeneration of spent adsorbents, we also prepared standard solutions of phenol in 2-propanol (Aldrich Chemical Co., spectrophotometric grade) in a similar procedure. A schematic diagram of the experimental setup for the column experiment is shown in Figure 1. The column used in this investigation was made from a Pyrex tube of 1-cm i.d. A 202
Environmental Science & Technology
RESNOR
ADSORBENT C O L U M N ADSORBENT SINTERED GLASS FILTER
P R E C r
~~
FLOW CELL
EFFLUENT RECEIVER
SPECTROFLUOROMETER
Figure 1. Schematic diagram of the experimental setup for column experiments.
Table 1. Some Properties of Adsorbents Studied for Phenol Removal told pore
w
0
150-250
923
3.4
300-850
459
0.50
27.0
180-250
1230
0.72
adsarbant
r t %
Filtrasorb 300 Ambersorb XE-340 C,S (PVDF)
particle
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0.51
piece of sintered glass (Ace, C porosity) was fused into the column as a filter a t the downstream end. A column length of 12 em was used throughout this investigation. The adsorbent of various particle sizes was packed into the column, and the column was shaken so that the maximum amount of the adsorbent was packed. The column was maintained at 25 f 2 OC during adsorption. However, the entire column could be heated with a heating tape. The influent solution was pumped from the reservoir into the column by a minipump (Valcor Molel104A 44B). Pyrex glass, stainless steel, or Teflon was used for the construction of the column system so that the adsorption of the adsorbate onto the system was minimal. The phenol concentration in the effluent solution was monitored with a JY3 spectrofluorometer (Instruments SA, Inc.). After the breakthrough, the concentration of phenol increased to the level where the fluorescence intensity was not linear with respect to the concentration. Samples of 10 mL each were collected and diluted for accurate determination of the effluent concentration. Before the regeneration of the adsorbent, the column was drained of remaining aqueous solution by pumping air through the column. The organic solvent was pumped into the column containing spent adsorbent. Samples of 10 mL each were collected and diluted for the analysis of phenol in the solvent. After the adsorbent was regenerated, the column was washed with -100 mL of distilled water a t room temperature before the influent solution was introduced again for the subsequent adsorption. The concentration of phenol was determined by measuring the fluorescence signal of phenol withihe spectrofluorometer. The excitation wavelength was chosen so that the interference from the Raman signal of the solvent was minimized and a
high fluorescence efficiency was maintained. During this study, the phenol solution was excited at 274 nm from a 150-W high-pressure xenon arc lamp. A linear correlation between the phenol concentration and the fluorescence intensity a t 294.8 nm can be maintained from several ppb to several ppm before the self-quenching of phenol becomes important.
Results and Discussion Some properties of the adsorbent materials used in the present study are shown in Table I for comparison. The conventional activated carbon, Filtrasorb 300, contains only oxygen surface groups as evidenced from the carbon and sulfur analyses which showed no detectable amount of sulfur. However, Ambersorb XE-340 contains a small weight percent of sulfur (3.4 wt %) besides carbon and possibly oxygen and hydrogen. Dry samples of C,S (PVDF) (26.8 g) and Filtrasorb 300 (23.9 g) were packed individually in the 1-cm i.d. X 72-cm column which has a bed volume of 56.6 mL. The influent phenol solution of 2000 ppm was pumped into the column, and the effluent solution was collected and its volume measured after the solution passed through the spectrofluorometer for the monitoring of phehol concentration (see Figure 1).Results of typical column experiments on these adsorbents are shown in Figure 2, where the solid line is for C,S and the broken line is for Filtrasorb 300. The effluent phenol concentrations are plotted as a function of the total volume of influent treated, which is expressed in terms of bed volumes as shown in Figure 2. Data shown are for the first-cycle adsorption. During all of the column experiments, efforts were made to control the flow rate at 10 mL/min, which corresponds to 10.6 bed volumesh. However, the flow rate was slower initially because of the slow evolution of gases from pores of the adsorbent. With this relatively high flow rate, phenol breakthroughs were found to be quite sharp for both C,S and Filtrasorb 300. Table I1 summarizes results of column experiments on a number of adsorbents for the removal of phenol from water solutions. In order to compare our results with those obtained from other laboratories (9,15),we use different units for the flow rate and the phenol capacity as shown in the table. Furthermore, phenol breakthrough concentrations used for calculating the adsorbent capacity are also listed. Because of sharp breakthroughs observed in this study for C,S and Filtrasorb 300, phenol capacity is relatively constant when one uses either 1- or 2000-ppm breakthrough concentration for computation. However, in the case of the carbonaceous adsorbent (Ambersorb XE-348), the phenol capacity is considerably lower a t low breakthrough concentration (Table 11).
In addition, the phenol capacity of Amberlite XAD-4 is quite adversely affected by high flow rates (12). Although the flow rate chosen for this investigation (3.1 gpm/ft2) is almost 3 times that used by Model1 and his coworkers (15),the phenol capacity of Filtrasorb 300 measured in this study is in excellent agreement with the value obtained by this MIT group for a breakthrough concentration of -2000 ppm. This indicates that, with the relatively high flow rate used in this study, the adsorption of phenol by the activated carbon is not limited by either the diffusion process or the rate of adsorption. Besides Amoco GX-31, which has a surface area of -2-2.5 times that of the other adsorbents, C,S has the highest phenol capacity as shown in Table 11. In addition to its high capacity, C,S was found to be regenerated thermally a t a milder temperature than that required for Filtrasorb 300 regeneration. Kinetic studies on the solvent regeneration of spent adsorbents with low phenol loadings (0.04 g/g) showed that nearly 100%of phenol on C,S was desorbed in 2 h while only 45% was removed from Filtrasorb 300 under similar experimental conditions ( 1 ) .In the present column experiments, spent adsorbents were also regenerated in the column with 2-propanol. Good solvents for phenol include acetone, methanol, and dilute aqueous strong inorganic bases (9).However, 2-propanol was chosen because acetone quenches the fluorescence signal of phenol and methanol usually contains several ppm of aromatic impurities which affect the determination of the phenol concentration. Figure 3 illustrates the solvent regeneration of both C,S and Filtrasorb 300 in the packed column. For a flow rate of -10
Z
Q c
I I 2000-COLUMN:
1
I
I
1
I
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I
1500-
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I
Z
I
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Table II. Phenol Capacities of Various Adsorbents at Room Temperatures adsorbent
Influent soh phenol concn, ppm
Amberlite XAD-4 Ambersorb XE-348 Ambersorb XE-348 Anthrasorb CC 1230 Amoco GX-3 1 Filtrasorb 300 Filtrasorb 300
2000 2000 2380 2380 2300 2380 2000
C,S(PVDF)-II
2000
a
PH
