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Ind. Eng. Chem. Res. 1996, 35, 2024-2031
An Investigation of the Mechanisms of Chemical Regeneration of Activated Carbon Chi-Cheng Leng and Neville G. Pinto* Department of Chemical Engineering, 697 Rhodes Hall, University of Cincinnati, Cincinnati, Ohio 45221-0171
The mechanisms underlying chemical regeneration of activated carbon loaded with physisorbed organics have been explored. Using four aromatic adsorbates and five chemical regenerants in batch equilibrium and regeneration experiments, it has been shown that solubility of the organics and surface characteristics of the adsorbent under regeneration conditions strongly influence the regeneration efficiency. Solubility in the regenerant has been enhanced either by the addition of micelles or by the generation of soluble forms of the adsorbate. In general, the latter technique is more effective. It has been demonstrated that soluble forms of the adsorbate can be obtained by pH control or the use of reactive regenerants. pH can also be used to control surface charge characteristics and, consequently, adsorbate affinity for the adsorbent. For adsorbates that are difficult to solubilize or are very strongly bound to the surface, reduced affinity for the adsorbent through a change in surface charge characteristics can be used to improve regeneration efficiencies. Introduction Activated carbon has been widely used to remove a range of organic compounds from both industrial and municipal waste waters. After the exhaustion of its adsorption capacity, activated carbon must be replaced with fresh carbon or it must be regenerated. For powdered activated carbon (PAC), inexpensive disposal methods and a low initial cost have led to the predominant use of fresh carbon. For granular activated carbon (GAC), a high initial cost mandates regeneration. It has been estimated that the cost of carbon regeneration, for a commercial GAC contractor, is around 75% of the operating and maintenance cost (EPA, 1989). Therefore, there is a considerable impetus to develop economical regeneration processes. The most common regeneration technique used is thermal regeneration, in which adsorbates are desorbed by means of volatilization and oxidation at high temperature. However, 5-10% of the carbon is usually lost by attrition, excessive burnoff, and washout during each cycle (Guymont, 1980). Chemical regeneration is an alternative to thermal regeneration. It has a number of significant advantages. Among these are the following: (1) It can be done in situ, thus unloading, transporting, and repacking of the adsorbent are eliminated. (2) The loss of carbon resulting from thermal desorption is eliminated. (3) Recovery of valuable adsorbates is possible. (4) With proper subsequent treatments such as distillation, chemical regenerants can be reused (Cooney et al., 1983). The most commonly used regenerants can be categorized into two groups: organic solvents or inorganic chemicals. It has been found that organic regenerants with solubilizing powers are more effective than inorganic regenerants with oxidizing powers. A detailed investigation on the regeneration of activated carbon (Martin and Ng, 1984) examined the effects of the molecular size, structure, and properties of the adsorbates on the regeneration of GAC. It was concluded that * Author to whom correspondence should be addressed. Fax: (513) 556-3473. Phone: (513) 556-2761. Email: npinto@ alpha.che.uc.edu.
S0888-5885(95)00576-8 CCC: $12.00
the efficacy of the organic regenerants within a group, i.e., carboxylic acids, amines, and chloromethanes, decreased as their molecular weight increased. It is likely that the smaller regenerants can penetrate further into the micropores and, therefore, displace the sorbates more effectively. Martin and Ng (1987) also used acetone, methanol, ethanol, formic acid, and acetic acid to regenerate carbon exhausted by commercial humic acid. While methanol and acetone were found to be of limited use, formic acid and acetic acid were effective regenerants. Sutikno and Himmelstein (1983) used acetone to regenerate carbon exhausted by phenol and developed a mathematical model to characterize the desorption of phenol. Chatzopoulos et al. (1993) used an external mass-transfer homogeneous surface diffusion model (HSDM) to describe the adsorption and desorption of toluene in the aqueous phase and successfully predicted toluene desorption rates under various operating conditions. Among inorganic regenerants, sodium hydroxide has been found to be effective in the regeneration of GAC. It has been shown that the desorption of phenol with 4% aqueous solution of sodium hydroxide was commercially effective (Himmelstein et al., 1973). The main mechanism of regeneration was postulated to be the formation of sodium phenate, which is easily desorbed from the surface of carbon. It was also concluded that the high pH arising from NaOH modified the polarity of surface oxides, thus reducing the force of attraction between phenol and GAC. In studying regeneration mechanisms on activated carbon, it is important to remember that adsorption can be either by physisorption or chemisorption. For example, recent research on the adsorption of phenol on activated carbon has shown that the presence of dissolved oxygen in the adsorbate solution induces polymerization reactions on the surface that enhances the adsorptive capacity of activated carbon for phenolic compounds (Vidic and Suidan, 1990; Vidic et al., 1991, 1993, 1994a,b; Nakhla et al., 1992, 1994; Grant and King, 1990). Compared with physical adsorption, chemical adsorption makes regeneration more difficult and reduces the life of activated carbon. Nakhla et al. (1994) have shown that the phenol extraction efficiency offered © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 6, 1996 2025
by solvents for the anoxic condition was about 80%, but it was only 25% under oxic conditions. Similar experiments conducted by Vidic and Suidan (1991) observed between 85-95% and 12-35% regeneration efficiencies for the anoxic and oxic experiments, respectively, depending on the sorption capacity. Grant and King (1990) reported regeneration efficiencies around 70% and 25% for the anoxic and oxic isotherms, respectively. Mange and Walker (1986) also reported that the chemisorbed phenol cannot be easily removed by solvents. Since the focus of this study is the mechanisms of chemical regeneration of physisorbed organic compounds, it is essential to suppress chemisorption. We have recently shown that the anoxic and oxic adsorption equilibrium characteristics of phenolic compounds on Kureha MP carbon are identical (Leng and Pinto, 1995a). This implies that surface polymerization is absent in this case. It is for this reason that MP carbon was selected for this work. Micelles are aggregates of amphiphilic molecules (surfactants) that possess well-defined regions of hydrophobic and hydrophilic character. When the concentration of amphiphilic molecules in solution increases above a characteristic value known as the critical micelle concentration (cmc), these molecules associate to form relatively well-defined regions known as micelles. The Kraft point (KP) for a micelle is that temperature below which the solubility of the surfactant is less than the cmc. Thus, micelles cannot exist below this temperature. Based on the nature of the polar head group of the amphiphile, micelles can be categorized an anionic, cationic, nonionic, or zwitterionic. The potential of aqueous micelle solutions in the regeneration of GAC lies in their ability to solubilize hydrophobic compounds that are otherwise insoluble in water. Solubilization effects of micelles can enhance or degrade a separation. The increase in solubility of hydrocarbon compounds results from hydrophobic interaction with the exposed hydrocarbon chains of the micelle, from electrostatic interaction with the head groups, or from a combination of both (Cove et al., 1987). A literature survey on chemical regeneration indicates that, though there have been a large number of studies on chemical regeneration, there is no clear understanding of the underlying mechanisms. Moreover, despite their ability to enhance the solubility of organics in aqueous media, micelles have, to our knowledge, not been applied to the regeneration of activated carbon. In this paper, we report the results of a study to investigate the applicability of micelles to carbon regeneration. Furthermore, the results of an experimental investigation to better understand the mechanisms of chemical regeneration are also presented. Experimental Methods and Procedures Materials/Analytical Methods. Kureha MP spherical activated carbon supplied by Kureha Chemical Co. (New York) was used for all the experiments. The carbon was sieved (U.S. Mesh No. 30 × 40) before it was used. Pretreatment of the carbon involved boiling in deionized water for 1 h, followed by drying at 110 °C for 24 h. The treated carbon was stored in a dessicator. Phenol, aniline, benzoic acid, and nitrobenzene were chosen as the test adsorbates. All of the adsorbates used were of analytical grade and were purchased from Sigma Chemical Co. (St. Louis), and all the solutions were prepared with deionized water.
Figure 1. Variation of light absorbance with solution concentration of aniline.
Concentrations of adsorbates in both aqueous solution and methanol were measured using a Shimadzu UV 1604 UV/vis spectrophotometer. The UV wavelengths used for phenol, aniline, benzoic acid, and nitrobenzene were 270, 286, 250, and 290 nm, respectively. Solubility Measurements. The procedure used for solubility measurements was essentially the same as the one described by Klevens (1950). For the solid organics, phenol and benzoic acid, excess amounts of solute, more than required to saturate the solutions, were added, and the systems were thoroughly mixed until equilibrium was reached at room temperature. Triplicate samples were prepared for each solute concentration. Preliminary experiments indicated that 1 h was sufficient for equilibration. Once equilibrated, the samples were centrifuged at 5000 rpm for 1 h to separate the undissolved solute. A measured amount of the supernatant was then carefully withdrawn with a pipet and diluted to bring the solute concentration into the detectable range. Analyses of concentrations in the diluted samples were conducted using the UV/vis spectrophotometer, and the solubility was backcalculated from this concentration. For the liquid materials, aniline and nitrobenzene, appropriate amounts of solute were added to the solvent, and the systems were thoroughly mixed until equilibrium was reached at room temperature. Preliminary experiments indicated that 1 h was sufficient for equilibration. The absorbance of the solutions at 800 nm was measured using the spectrophotometer. By plotting the absorbance versus the solute concentration, the solubility was determined by the point at which absorbance increased sharply with concentration. A typical result is shown in Figure 1. Adsorption Isotherm Measurements. The equilibrium isotherms for phenol, aniline, benzoic acid, and nitrobenzene from an aqueous solution on MP carbon were determined at 23 ( 1 °C using a bottle-point method. Different amounts of carbon were weighed and added into 15 bottles (36 mL). A total of 35 mL of the organic solution was added to the bottles. Each set of bottles included two bottles without activated carbon, to serve as blanks to examine for sorbate volatilization and adsorption onto the wall. The bottles were covered with parafilm and caps and shaken at 1000 rpm for 120 h; preliminary experiments indicated that adsorption equilibrium was reached within 120 h. Upon equilibration, solutions were sampled with a 0.45 µm Acrodisc
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syringe filter (Gelman Science) and analyzed on the UV-vis spectrophotometer. Since Kureha carbon is very hard, the amount of residual fines in solution was found to be insignificant for UV absorbance measurements, when filtered as described above. The complete isotherms were calculated using appropriate mass balances. Regeneration Efficiency Measurements. The batch desorption experiments were carried out in three steps: (1) adsorption of organic compounds on MP carbon; (2) regeneration with selected regenerants; (3) extraction with 35 mL of methanol. The adsorption for organic compounds from an aqueous solution on MP 30/40 was conducted at 23 ( 1 °C using a bottle-point procedure. In order to avoid any volatilization of the organic, no head-space was left in the bottle. Approximately 0.25 g of MP carbon was placed in a 36 mL bottle, and then 35 mL of the organic solution was added. The concentrations of phenol, aniline, benzoic acid, and nitrobenzene used were 2000, 500, 300, and 400 ppm, respectively. Using the procedure described earlier (Adsorption Isotherm Measurements section), the phases were equilibrated and the amount adsorbed was determined. The equilibrium solution was then decanted, and the organically loaded carbons were transferred to a new bottle. The carbon was washed with about 100 mL of deionized water prior to the addition of 20 mL of the regenerant of interest. The regenerant solution was mixed with the carbon in a shaker at 1000 rpm for 24 h, after which it was decanted. Following the regeneration step, the carbons were transferred into a new bottle. Around 200 mL of deionized water was added to wash out the regenerants and organics from the pores of the MP carbon. After decanting the wash water, 35 mL of methanol was added into each bottle to extract phenol, aniline, benzoic acid, or nitrobenzene. The bottles were shaken for 120 h on a shaker at 1000 rpm. Preliminary studies showed that this period of time was sufficient to ensure equilibrium between the MP carbon and methanol phases. The addition of methanol as a final step in the protocol is necessary to enable the determination of the amount desorbed during regeneration. Because all the regenerant solutions are strong UV absorbers, it is not possible to determine the concentration of the adsorbate in the solution at the end of the desorption step directly. One approach would be to use a separation technique, such as chromatography, to separate the sorbent from the regenerant. However, this is a relatively tedious process. A much simpler method is to use a solvent with a known extraction efficiency as a basis. Thus, in the above case, if the extraction efficiency of methanol is known as a function of the amount adsorbed, the amount of adsorbate in the methanol at the end of step 3 in the above protocol can be used to calculate the amount of adsorbate left in the adsorbent by the regenerant at the end of step 2. It is clear that, if this approach is to work, the extraction efficiency of the selected solvent must be correlated accurately to the adsorbate concentration. Methanol Extraction Efficiency. Methanol was selected as the extractant for step 3 in the above method because it is a strong solvent for all the adsorbates of interest. Preliminary studies have shown that the residual regenerants of step 2 did not interfere with the UV absorbance of adsorbates at the end of step 3.
Figure 2. Correlation data for extraction efficiency of methanol: (O) phenol, A0 ) 45 mg, kd ) 3.92, nd ) 0.12, R2 ) 0.90; (0) aniline, A0 ) 9 mg, kd ) 3.83, nd ) 0.18, R2 ) 0.95; (4) benzoic acid, A0 ) 9 mg, kd ) 3.35, nd ) 0.51, R2 ) 0.98; (3) nitrobenzene, A0 ) 4 mg, kd ) 2.26, nd ) 0.35, R2 ) 0.95.
The extraction efficiency experiments were conducted by repeating the procedure described in the Regeneration Efficiency Measurements section, but without step 2; i.e., the adsorption of organics on MP carbon was followed by extraction with 35 mL of methanol. For each adsorbate, 12 samples of approximately 0.25 g of MP carbon were used, and the adsorption step (step 1) covered the following solution concentration ranges: phenol, 100-2000 ppm; aniline, 100-500 ppm; benzoic acid, 100-300 ppm; nitrobenzene, 100-400 ppm. On the basis of these experiments, the methanol extraction efficiency data in Figure 2 were obtained. It was found that these data were well correlated for all the adsorbates with the equation:
EP ) kd(AM)nd
(1)
where EP, the extraction efficiency, is the ratio of the amount of adsorbate in the methanol after step 3 to the amount of adsorbate on the carbon after step 2. The best-fit values of kd and nd for each of the adsorbates are also shown in Figure 2. Once the correlations for EP were available, the regeneration efficiency for each regenerant was calculated from:
(
Re ) 1 -
)
AM × 100% A0EP
(2)
where AM is the amount of adsorbate in the methanol phase after step 3, and A0 is the amount of adsorbate on activated carbon after step 1. Results and Discussion The regenerants and adsorbates used in this work were selected to systematically probe the mechanisms of chemical regeneration. Five types of regenerants in combination with four aromatic adsorbates were used to investigate the effects of solubility, pH, and specific chemical interactions on desorption. Water, methanol, and micellar solutions were used mainly to determine the importance of the solubility effect and pH. Hydro-
Ind. Eng. Chem. Res., Vol. 35, No. 6, 1996 2027 Table 1. Regeneration Efficiencies of Micellar Solutions regeneration efficiency (%) adsorbate
water
2 cmc SDS
4 cmc SDS
8 cmc SDS
10 cmc CTAB
20 cmc CTAB
30 cmc CTAB
methanol
phenol aniline benzoic acid nitrobenzene solution pH
6.2 3.6 0.24 0.15 5
22.9 39.4 62.1 11.3 9.53
29.7 39.8 66.1 6.6 9.92
34.7 36.6 58.3 3.2 10.64
12.4 31.4 37.6 3.3 9.34
21.9 33.5 35.9 2.0 9.82
30.1 27.4 35.1 1.3 10.46
88.1 70.5 80.5 13.2 8.68
Table 2. Characteristics of SDS and CTAB
a
name
abbreviation
cmc (M)
Na
radius
KP (°C)
sodium dodecyl sulfate cetyltrimethylammonium bromide
SDS CTAB
0.0081 0.00092
62 61
25 48
16 22
N ) aggregation number, the preferred number of monomers per micelle.
Table 3. Solubility Enhancement with SDS and CTAB organic compound
water
phenol aniline benzoic acid nitrobenzene
8.5 (8.7)a 3.5 (3.6) 0.25 (0.27) 0.2 (0.19)
solubility (g/100 g of solution) 2 cmc SDS 10 cmc CTAB methanola 11.4 4.3 0.48 0.27
9.5 4.1 0.46 0.32
infinity infinity 46 very soluble
a Data taken from Perry’s Chemical Engineering Book, 6th ed. (1984).
chloric acid, formic acid, and sodium hydroxide were used to study the pH effect and chemical interactions. Micellar Solutions. It is well-known that water is a very weak regenerant for activated carbon (Pahl et al., 1973), due to the relatively low solubility of organics in this solvent. Shown in Table 1 are regeneration efficiencies of water obtained in this work for the four aromatics of interest. Clearly, water is a poor regenerant, with a less than 10% regeneration efficiency in the best case. This is consistent with the water solubility of the organics. It is therefore reasonable to expect that, if the solubility can be enhanced, the regeneration efficiency will be improved. One way of increasing the solubility of organics in water is to add surfactants and form micelles in solution. This was attempted with two micellar solutions: SDS and CTAB. Important micellar properties of the two surfactants are summarized in Table 2 (McIntire, 1990), and experimentally obtained solubility data are shown in Table 3. Clearly, the addition of surfactants increases the solubility of all the organics significantly. Thus, it is to be expected that, if solubility plays a role, regeneration efficiencies of micellar solutions will be significantly higher than that of water. Both micellar solutions were found to have a strong effect on regeneration efficiency for all the adsorbates studied, increasing the efficiency relative to water by at least a factor of 5 in all cases and by as much as 250 in one case. While these increases are significant, they are for the most part too low to be of practical use. Clearly, much larger enhancements in solubility are necessary to obtain suitably high regeneration efficiencies. This is evident from the regeneration efficiency and solubility data for methanol, which are also shown in Tables 1 and 3, respectively. For example, the high solubilities of phenol and aniline give regeneration efficiencies for these adsorbates in the 70-90% range, versus the 10-40% obtained with micellar solutions. While solubility plays an important part in the regeneration process, a closer examination of the data indicates that it is by no means the only factor affecting regeneration with micellar solutions. Shown in Table 1 are the experimentally observed pH values for each
of the regenerant solutions. It is well-known that regenerant solution pH can strongly influence desorption by changing the charge characteristics of the adsorbate and adsorbent surface (Snoeyink et al., 1969). Two of the adsorbates selected, aniline and nitrobenzene, are weak bases. The conjugate acid for nitrobenzene has a pKa value of -11 (Allinger et al., 1971), and aniline has a pKa value of 27 (Roberts et al., 1971). This implies that, for the pH range covered by the conditions shown in Table 1, these adsorbates are predominantly in their neutral form, and the variation in pH with concentration of the micelles does no affect their charge characteristics. Thus, the only influence of pH for these adsorbates can be through changes in the adsorbent surface characteristics. As pH increases, it is known that the carbon surface shows some negative charge characteristics (Snoeyink et al., 1969). Thus, for nitrobenzene and aniline, as the micelle concentration increases, which corresponds to an increase in pH, the combined effect of a more negatively charged adsorbent surface and increased solubility should result in a higher regeneration efficiency. However, exactly the opposite is observed for nitrobenzene. Also, for aniline, while there is an increase from the lowest to the intermediate micelle concentration in both cases, the regeneration efficiency is the lowest at the highest micelle concentration. Clearly, something other than solubility and pH also influences the regeneration characteristics. An explanation is obtained from the adsorption characteristics of the surfactants. Berthod et al. (1986) have studied adsorption of SDS and CTAB on silica. They concluded that the amount adsorbed decreases as the polarity of the stationary phase increases and increases slightly with an increase in the concentration of the surfactant in the solution phase. Since activated carbon is more hydrophobic than silica, it is to be expected that there is significant adsorption of SDS and CTAB on this adsorbent. Furthermore, since the surfactant molecules are generally larger (25-50 Å) than the adsorbent pores, 75% of the pore volume of MP 30/40 is smaller than 10 Å, adsorption will lead to blockage of the pores, restricting access between the external solution and internal adsorbent area, which will reduce the effectiveness of the regenerant. The behavior of benzoic acid is also consistent with this explanation. Benzoic acid has a pKa of 4.20 (Lide, 1992), and for the pH range of Table 1 this adsorbate is predominantly in the highly soluble, anionic form C6H5COO-. Consequently, the solubility of benzoic acid will increase with increasing micelle concentration. In addition, desorption is favored by repulsive interactions between the anion and negative charges that exist on
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Figure 3. Adsorption isotherms on Kureha 30/40 at 23 °C.
the carbon surface at high pH. All of these factors point toward an increase in the regeneration efficiency at higher micelle concentrations. Yet, as can be seen (Table 1), this is not what is experimentally observed. Once again, at higher micelle concentrations, significant pore blockage by the surfactant results in lower regeneration efficiencies. The data for phenol (Table 1) appear to contradict the pore blockage explanation, since the regeneration efficiency increases monotonically with increasing micelle concentration. However, for phenol the situation is different, since it has a pKa of 9.99 (Lide, 1992), which falls inside the pH span in Table 1. Because of this, the fraction of phenol that is ionized changes significantly with pH. Thus, the effects of increased pH, enhanced desorption, and solubility due to increased micellar concentration dominate, leading to improved regeneration efficiencies despite increased pore blockage. The partial or complete exclusion of the micelles from the internal pores of the carbon is further supported by a comparison of the regeneration efficiencies of the two micelles (Table 1). In all cases, CTAB gives lower regeneration efficiencies than SDS. This can be explained by the degree of blockage of micelles, since SDS is half the size of CTAB (Table 2), which may make it more effective in solubilizing organics from porous adsorbents. Because of the number of mechanisms that play a role in regeneration with micellar solutions, it is not surprising that there is considerable variability in the regeneration efficiency for different adsorbates. For example, at 8 cmc SDS the range observed (Table 1) is from 3% to almost 60%. Furthermore, these efficiencies do not correlate directly to the adsorption affinities; i.e., the adsorbate with the highest adsorption affinity does not give the lowest regeneration efficiency. Shown in Figure 3 are the adsorption isotherms of the adsorbates on Kureha MP 30/40. It can be seen that benzoic acid adsorbs strongly, yet is most easily removed by the regenerant. Aniline and phenol, which are adsorbed less strongly, are removed less efficiently. The reason for this appears to be in the mechanism by which solubility is enhanced. For aniline, over all conditions used, the molecule is predominantly neutral. Thus, dissolution by the micelle is the mechanism for solubility enhancement. For benzoic acid, in contrast, enhanced solubility is due to almost complete ionization. This is supported by data
reported elsewhere (Leng and Pinto, 1995b) on the effects of ionization on adsorption isotherms; it has been shown that benzoic acid adsorbs much more strongly at pH 3, where it is neutral, than at pH 7 or 11.6. Similar observations were also made for phenol, for which the adsorption was much stronger at pH 7 than at pH 11.6. This and the regeneration data suggest that, at the lowest micelle concentrations (2 cmc for SDS and 10 cmc for CTAB), phenol is only partially ionized, and dissolution in the micelle is the predominant mechanism. At higher micelle concentrations, ionization becomes more significant, though, at even the highest micelle concentration, the ionization of phenol is not complete and the regeneration efficiencies are lower than for benzoic acid. For nitrobenzene, which like aniline is neutral over the pH range, the mechanism is also dissolution in the micelle. However, because nitrobenzene is very strongly bound to the adsorbent, regeneration efficiencies are very low. These results indicate that the micelles are more effective in enhancing solubility through ionization than dissolution. This is probably the case because the micelles are, in great part, size excluded from the adsorbent. Effects of pH. The effects of pH on regeneration efficiency were further investigated by using hydrochloric acid, formic acid, and sodium hydroxide as regenerants. Shown in Table 4 are the regeneration efficiencies obtained with these solutions and the pH in each case. In order to isolate pH effects from reaction effects, such as acid-base reactions, only data for adsorbates that do not react with the regenerants are discussed in this section; regeneration efficiency data shown in bold type involve a reaction and are discussed in the next section. The data on the regeneration of benzoic acid and nitrobenzene with acidic solutions illustrate the underlying mechanisms most clearly. As expected, since the pH is now below the pKa, the regeneration efficiency for both species is well below that observed with micellar solutions. What is interesting, however, is that, as the pH falls from 0.91 for 5% HCl to 0.45 for 30% HCl, the regeneration efficiency increases for both species. Furthermore, at the lower pH of -1.22 with formic acid, the regeneration efficiencies for benzoic acid and nitrobenzene increase sharply. Finally, on comparison of these efficiencies with that for water with a pH ≈ 5 (Table 1), it is clear that strongly acidic solutions are more effective for regeneration than water. This behavior suggests that the pH can affect the characteristics of the activated carbon surface significantly. In strongly acidic solutions, the surface has a significantly positive charge character due to protolysis of acid functional groups (Snoeyink et al., 1969), and this drives the desorption of neutral organic molecules. The regeneration data for aniline and nitrobenzene with alkaline NaOH solutions (Table 4) substantiate this explanation. Very alkaline solutions are seen to give higher regeneration efficiencies than water for these adsorbates. Since the organic species are predominantly in the neutral form in both cases, this indicates that the negative charge character of the surface at high pH drives the desorption of the molecules in alkaline solution. The important role that surface charge can play is best illustrated by the desorption of nitrobenzene. This species has a very strong affinity for carbon (Figure 3) and is hard to remove, as is clear from the micelle, methanol, and water data presented. However, it appears that the desorption of this adsorbate is sensitive
Ind. Eng. Chem. Res., Vol. 35, No. 6, 1996 2029 Table 4. Regeneration Efficiencies of Acids and Basesa regeneration efficiency (%) adsorbate
5% HCl
30% HCl
formic acid
0.5% NaOH
1% NaOH
2% NaOH
4% NaOH
phenol aniline benzoic acid nitrobenzene solution pH
9.8 99.5 20.0 7.5 0.91
12.9 99.8 22.0 11.8 0.45
91.5 99.9 75.7 11.8 -1.22
69.3 34.2 78.0 7.4 12.67
71.9 34.2 78.0 7.4 12.80
71.4 34.0 76.3 9.3 12.98
71.3 33.1 75.8 10.6 13.08
a
Values in bold signify possible reactions.
Table 5. Summary of Reactions between Regenerants and Adsorbates NaOH
HCl
HCOOH
C6H5OH + NaOH w C6H5O-Na+ + H2O C6H5COOH + NaOH w C6H5COO-Na+ + H2O C6H5NH2 + NaOH w no reaction C6H5NO2 + NaOH w no reaction C6H5OH + HCl w no reaction C6H5COOH + HCl w no reaction C6H5NH2 + HCl w C6H5NH3+ C6H5NO2 + HCl w no reaction C6H5OH + HCOOH w C6H5OH2+ C6H5COOH + HCOOH w no reaction C6H5NH2 + HCOOH w C6H5NH3+ C6H5NO2 + HCOOH w no reaction
to the surface charge, be it positive or negative. For example, increases in pH under strongly alkaline conditions lead to increases in regeneration efficiency (Table 4). Also, under the very acidic conditions produced by formic acid, the regeneration efficiency is remarkably higher than any obtained through solubility enhancements. Reactive Regenerants. The acid and base regenerants can react with some adsorbates as shown in Table 5. Shown in Table 4, in bold type, are the corresponding regeneration efficiencies obtained with these reactive regenerants. In general, the reactive regenerants give good to excellent regeneration efficiencies. Phenol and benzoic acid react with NaOH to give the soluble salts C6H5O-Na+ and C6H5COO-Na+, respectively. It is interesting to note that in both cases the regeneration efficiency drops steadily with an increase in NaOH concentration, and 1% NaOH is more effective than the 4% NaOH recommended in a previous study (Himmelstein et al., 1973). This dependence on NaOH concentration can be attributed to higher adsorption of OH- at higher NaOH concentrations, which hinders the desorption process (Martin and Ng, 1984). For hydrochloric acid and formic acid, the high regeneration efficiency for aniline can be attributed to the formation of C6H5NH3+ (Morrison, 1987). The reaction is initiated by electron-withdrawing inductive effects, in which the lone pair of electrons on nitrogen is partly delocalized into the aromatic ring. Phenol, in contrast, does not react with HCl because the basicity of -OH is much smaller than that of -NH2. However, in the much more acidic environment of formic acid, the electron-withdrawing inductive effect between the -OH group on phenol and H+ is favored, resulting in the formation of C6H5OH2+, which facilitates desorption. The very strong influence that a reactive regenerant can have is illustrated clearly by comparing the low regeneration efficiency of phenol with HCl and the very high value obtained with formic acid. Mixed Regenerants. Mixed regenerants offer the possibility of synergistically combining multiple mechanisms to enhance regeneration efficiencies. In order to explore this possibility, the effectiveness of NaOHmicelle mixtures was studied.
Figure 4. Effect of SDS concentration on regeneration efficiency of 2% NaOH solution.
Figure 4 shows the effects of the concentration of SDS in a 2% NaOH regenerant, for carbons exhausted with phenol, benzoic acid, and nitrobenzene. Since the micelle has very little influence on the pH of these very basic solutions, the pH is essentially that of a 2% aqueous NaOH solution (approximately 13). Thus, the addition of the micelle will only affect desorption through a change in solubility and pore blockage. In general, the addition of SDS has a relatively small effect on the regeneration. The efficiency for phenol is most strongly influenced, increasing by between 5 and 8%. The efficiency for benzoic acid is less affected, increasing by 2-3%. Nitrobenzene is regenerated with the same efficiency as with a 2% NaOH solution. These results are consistent with expectations. For phenol and benzoic acid, desorption is mainly through a chemical reaction that generates a water-soluble species. Thus, the enhanced solubility for water-insoluble organics in the external solution is secondary. For nitrobenzene, it has been observed earlier that desorption is primarily driven by a decreased affinity at the carbon surface. Since the carbon surface characteristics are essentially the same as for a 2% NaOH solution, with SDS being effectively excluded from the internal surface, the regeneration efficiency is unchanged. Figure 5 shows the regeneration efficiencies obtained by adding CTAB to 2% and 4% NaOH solutions. Once again, the pH is essentially unaffected by the addition of CTAB. The regeneration efficiencies for phenol and benzoic acid increase by 5-10%, depending on the concentration of CTAB. For benzoic acid, a weak maximum is observed at an intermediate concentration. This suggests that two factors, solubility and pore blockage, have a part in the regeneration. As the concentration of the micelle increases, pore blockage by
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Figure 7. Effect of NaOH on regeneration efficiency of methanol. Figure 5. Effect of CTAB concentration on regeneration efficiency of 2% and 4% NaOH solutions.
Figure 6. Effect of time on regeneration efficiency.
CTAB adsorption increases, negating, to a smaller or larger extent, the effect of enhanced solubility. The influence of pore blockage is most evident for nitrobenzene, for which desorption is actually suppressed by the addition of CTAB. In this case, as has been observed earlier, since the enhancement in solubility has a negligible effect on regeneration, the influence of pore blockage becomes immediately evident. A comparison of the regeneration efficiencies of 2% NaOH-CTAB and 4% NaOH-CTAB solutions (Figure 5) provides additional support for the influence of OHadsorption on regeneration. It is observed that the 2% NaOH solution generally gives higher regeneration efficiencies than 4% NaOH. This is consistent with the argument made earlier that increased adsorption of OH- on the carbon surface can hinder regeneration. Additional insight on desorption with NaOH-micelle regenerants is gained by studying the kinetics of regeneration. Figure 6 compares the regeneration efficiencies of 2% NaOH and 2% NaOH-30 cmc CTAB solutions as a function of regeneration time for benzoic acid and phenol. From these data it is seen that the two regenerant solutions have essentially identical kinetic characteristics, except at very long regeneration times, where the higher efficiency of the solution with CTAB becomes evident. This suggests that water-
insoluble forms of the adsorbates take longest to desorb, and their desorption is influenced by solubility in the external solution. 2% NaOH in methanol was also evaluated as a mixed regenerant for activated carbon. Shown in Figure 7 are regeneration efficiencies obtained in comparison to pure methanol. It is seen that in each case there is a substantial increase in the regeneration efficiency. For phenol, this can be attributed partially to increased ionization at higher pH, since 2% NaOH increases the pH of the methanol solution from 8.68 to 13.58, and partly to the acid-base reaction with NaOH. Since benzoic acid is essentially completely ionized at the pH of methanol, the large increase in regeneration efficiency is likely to be due to the formation of a soluble salt from the acid-base reaction with NaOH. For aniline and nitrobenzene, both compounds are already in their neutral form at pH ) 8.68. Thus, the increase in regeneration efficiency is rooted in the surface charge character of the carbon surface at the substantially increased pH. Conclusions It has been demonstrated that chemical regeneration of activated carbon is dependent primarily on the solubility of the adsorbate in the regenerant solution, relative adsorption capacity, and the charge characteristics of the adsorbent surface. Solubility of organic adsorbates in aqueous solutions can be enhanced by controlling the pH, by the addition of micelles, or by the use of reactive regenerants that produce soluble products. Solubility enhancement through the addition of micelles is less effective than pH control or reactive regeneration, since micelles are sterically excluded from the pores of the adsorbent. Thus, their effect is limited to the external solution; regenerants that enhance solubility in the pore liquid are found to be more effective. Surfactants also tend to adsorb on the activated carbon, leading to pore blockage that hinders desorption. Enhancing solubility through pH control or the addition of reactive regenerants can be very effective for obtaining high regeneration efficiencies. For selection between these two techniques, the acid-base characteristics of the adsorbate and its potential reactivity should be considered. For example, it has been shown for aniline that regeneration with basic solutions is only
Ind. Eng. Chem. Res., Vol. 35, No. 6, 1996 2031
partially successful because of the limitation of solubility. However, since aniline reacts with HCl to form a soluble salt, essentially complete regeneration is possible with this regenerant. When a compound such as nitrobenzene is adsorbed very strongly on activated carbon and/or is difficult to transform into a soluble form, controlling the charge characteristics on the carbon surface can be effective for regeneration. This can be achieved with extreme pH values. Very low pH will result in protonation of the surface, and hydrophobic molecules will tend to desorb under these extreme conditions. For example, it was demonstrated that carbon loaded with nitrobenzene was very resistant to chemical regeneration, except when a very acidic solution was used. Nomenclature A0 ) amount of adsorbates adsorbed on activated carbon after step 1, mg AM ) amount of adsorbates in the methanol phase after step 3, mg EP ) ratio of the amount of adsorbates in methanol after step 3 to the amount of adsorbates adsorbed on carbon after step 2, dimensionless kd ) coefficients of eq 1, (1/mg)nd nd ) coefficient of eq 1, dimensionless
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Received for review September 18, 1995 Accepted April 4, 1996X IE950576A
X Abstract published in Advance ACS Abstracts, May 1, 1996.
