Regeneration by Wet Oxidation of an Activated Carbon Saturated with

Electrochemical regeneration of phenol-saturated activated carbon – proposal of a reactor. Odivan Zanella , Denise Bilibio , Wagner Luiz Priamo , Is...
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Ind. Eng. Chem. Res. 2002, 41, 1344-1351

Regeneration by Wet Oxidation of an Activated Carbon Saturated with p-Nitrophenol Juan F. Gonza´ lez,* Jose´ M. Encinar, Antonio Ramiro, and Eduardo Sabio Departamento de Ingenierı´a Quı´mica y Energe´ tica, Universidad de Extremadura, Avda Elvas S/N, 06071 Badajoz, Spain

The regeneration of a commercial activated carbon (AC) was studied using the wet air oxidation (WAO) process in the temperature range 150-180 °C and an oxygen partial pressure of 0.718 MPa. p-Nitrophenol (PNF) was used as the adsorbate. First, the oxidation process of PNF was studied. In the absence of AC, it was independent of the initial concentration of PNF at the conditions used in this work. The temperature and oxygen partial pressure exerted positive effects on the oxidation process. The activation energy of the PNF oxidation process and the reaction order with respect to oxygen were 134.8 kJ mol-1 and 0.55, respectively. The desorption process of PNF from the saturated AC was studied in the aforementioned temperature range. It was found to occur during the heating period, with an equilibrium being reached. The regeneration process was studied at these same temperatures, yielding an activation energy for PNF oxidation of 99.1 kJ mol-1. The isotherms of the virgin and regenerated AC and the phenol, methylene blue, and iodine indices indicated that the WAO regeneration produced notable modifications in the porous structure of the carbon, diminishing its adsorption capacity. Nevertheless, at a regeneration temperature of 170 °C, 80% of the adsorption capacity of AC can be recovered. Introduction The contamination of natural liquid effluents due to such diverse factors as the uncontrolled disposal of urban and industrial wastes, increasing use of herbicides and pesticides, etc., is of growing concern for current society. Many of these pollutants are organic substances (phenols, simazine, atrazine, etc.) that are potential health risks. One particularly interesting solution to this problem is the retention of these pollutants on activated carbon (AC). After the exhaustion of its adsorption capacity, AC can either be replaced or regenerated, and indeed its high cost makes regeneration an attractive possibility. Various regeneration processes have been proposed.1,2 The most common is thermal regeneration, but the installation costs are very high and 5-10% of the carbon is lost by attrition, burnoff, and washout during each cycle.3 Another is chemical regeneration. It has been established that this technique’s regeneration efficiency depends on the solubility of the physisorbed organics and on the surface characteristics of the adsorbent under regeneration conditions.4 The regeneration of AC by wet air oxidation (WAO) is a process in which the saturated AC is subject in water to air or oxygen at 0.1-1 MPa oxygen partial pressure and a temperature of 130-250 °C.5 This technique has been widely studied in recent years. There have been descriptions of the selective oxidation of different adsorbates and the regeneration of the adsorption capacity of ACs by WAO,5-9 including studies of the following individual processes: (1) the retention of an adsorbate by the virgin AC via the determination of the adsorption isotherms, (2) desorption of the * To whom correspondence should be addressed. Phone: (34) 924-289619. Fax: (34) 924-289601. E-mail: [email protected].

adsorbate from the AC into the water, (3) oxidation of the adsorbate in water by dissolved oxygen, and (4) determination of the adsorption by the regenerated AC. The aqueous-phase oxidation of an organic compound takes place via free radicals, and there is an induction period followed by a rapid reaction phase during which most of the organic compound is destroyed.5,10-15 In a study of the desorption kinetics of phenol on AC,16 it was found that, when the samples are thermally regenerated, the rate of desorption and the chemical species desorbed depend on the treatment prior to heating. The adsorption properties of AC can also be changed by the process by alterations in its porous structure or by the presence of oxygen-containing functional groups on the adsorbing surface.5,17,18 These aspects referred to WAO processes, and their possible applications have been discussed recently in an excellent and extensive review: 19 the WAO process is studied on pure compounds, and the mechanism, kinetics, and structure oxidizability are reported. Also, the industrial applications of the WAO, including spent carbon regeneration, are discussed, and recommendations and suggestions for further research are given. Although a WAO process could be applied to directly remove some of the aforementioned pollutants from wastewater, in most applications, WAO is not used as a complete treatment method but only as a pretreatment step where the wastewater is rendered nontoxic and the chemical oxygen demand is reduced sufficiently for the biological treatment to become applicable as the final treatment.19 This is related to the very high capital cost of a WAO system, which also depends on the flow, oxygen demand of the effluent, severity of the oxidation conditions, and construction material required.19 The combination of a carbon adsorption and biological treatment followed by regeneration of spent carbon by WAO can handle industrial wastes which are too dilute for

10.1021/ie010474m CCC: $22.00 © 2002 American Chemical Society Published on Web 02/02/2002

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Figure 1. Experimental setup for the regeneration and oxidation processes.

economic processing by WAO and are nonbiodegradable.20 The adsorption of the pollutants onto AC before the WAO treatment reduces considerably the flow of water to treat because the pollutant is concentrated on AC. In this way, the operating and reactor costs of WAO systems can be reduced. With these antecedents, the objectives of this present work were as follows: (1) to study the overall WAO process divided into the above four processes with the aim of optimizing the regeneration conditions of a commercial active carbon saturated with p-nitrophenol (PNF); (2) to study the influence of the operating variables on wet oxidation and desorption processes of PNF; (3) to determine the adsorption isotherms and phenol, methylene blue, and iodine indices to characterize the porous structure of activated and regenerated carbons. Experimental Methods and Procedures Materials. In this work, a commercial AC was used as the adsorbent (Darco, 20-40 mesh, granulated). The granules were dried at 110 °C for 12 h and stored in a desiccator at room temperature. Analytical-grade PNF (Sigma; purity > 99%) was used as the adsorbate. Oxygen of 99.999% purity (N-50) was used for the oxidation experiments. Ultrapure water was used in all of the experiments. Phenol Oxidation. The oxidation of PNF was carried out in a 0.5 L Eningen Berghof 7412 autoclave, covered by Teflon and equipped with a heating and temperature control system, inlet for the oxygen feed, system for condensation, and a collection of liquid samples, pressure gauge, and magnetic stirrer system. Figure 1 shows a diagram of the setup. The liquid sample line and thermocouple were immersed in the solution. The autoclave was loaded with 433 mL of the PNF solution at the desired concentration, and the system was preheated to the required temperature for the run. Oxygen was then injected into the autoclave until the desired oxygen partial pressure was reached in addition to the self-generated steam pressure. This time was taken as time zero for the reaction. The pressure of the autoclave was maintained after sampling by addition of oxygen throughout the process. Samples were ex-

tracted periodically and analyzed to determine the PNF content using a Waters 486 high-performance liquid chromatograph (HPLC) equipped with an UV detector and an injector connected to a 15 × 0.4 cm Tracer Analytical Spherisorb column with 5 µm packing. Desorption of PNF. The desorption experiments were carried out in the autoclave described above. For this, 0.5 g of AC saturated with PNF and 433 mL of ultrapure water were placed in the autoclave. The amount of PNF adsorbed on the saturated AC was 0.178 g/g of carbon (horizontal point of the adsorption isotherm at 20 °C). The temperature range used in these experiments was 150-180 °C. The moment the desired temperature was reached in the autoclave was taken as time zero for a run. A 5 µm Millipore SV filter was used to remove the AC particles from the samples, and their PNF content was assayed by HPLC. Regeneration of AC. The WAO regeneration experiments were carried out in the autoclave described above. The runs were performed at temperatures of 150-180 °C and an oxygen partial pressure of 0.718 MPa. The PNF loading in the saturated AC was 0.178 g/g of carbon. In all of the experiments, 0.5 g of saturated AC and 433 mL of ultrapure water were placed in the reactor and were subjected for a period of 3.5-6.5 h to the aforementioned conditions of temperature and pressure. Adsorption Isotherms. The isotherms of virgin AC and the WAO regenerated carbon were obtained by the immersion method. For this, 50 mg of the adsorbent, previously dried at 110 °C, and 10 mL of the PNF solution at the desired concentrations were put into test tubes. The concentration range used was 5-1400 ppm. The stoppered tubes were placed in a rotary mixer in a Selecta Hotcold-M isothermal chamber for 2 days to reach equilibrium at 20 ( 0.1 °C. The PNF content of the samples was assayed by UV spectrophotometry (UV 7500 DINKO spectrophotometer). The PNF content on the carbon was determined by mass balance, and the Langmuir adsorption isotherms were calculated. Phenol, Methylene Blue, and Iodine Indices. The phenol, methylene blue, and iodine indices of the carbons were determined using standard methods. These indices allow one to evaluate qualitatively the porous structure of the carbons. The phenol index is the number of milligrams of phenol adsorbed from an aqueous solution per gram of AC and is related to the carbon’s capacity to adsorb gases and odors, i.e., to the distribution of micropores in the carbon.21 The methylene blue index is defined as the number of milliliters of a standard solution of methylene blue discolored per 0.1 g of AC (on a dry basis) and is related to the mesopore distribution in the carbon.22 The concentrations of both solutions were measured by UV spectrophotometry. The iodine index is defined as the number of milligrams of iodine adsorbed in an aqueous solution per gram of AC when the residual concentration of iodine in the filtrate is 0.02 N and is related to the micropore distribution in the carbon.23 This determination was carried out by titration with sodium thiosulfate. Oxidation of AC in the Absence of PNF. An oxidation experiment with virgin AC at 180 °C and an oxygen partial pressure of 0.718 MPa was performed to evaluate the effect of oxygen on AC in the absence of PNF. For this purpose, after the oxidation process, we performed a trial to reproduce a given point of the horizontal section of the 20 °C isotherm of virgin AC.

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Figure 2. Profiles of the adimensional concentration of PNF versus time. Influence of the initial concentration of PNF on the oxidation process (T ) 170 °C, PO2 ) 0.718 MPa, stirring speed ) 600 rpm).

Figure 3. Profiles of the adimensional concentration of PNF versus time. Influence of the temperature on the oxidation process ([PNF]0 ) 0.2 g L-1, PO2 ) 0.718 MPa, stirring speed ) 600 rpm).

For the virgin AC, this point corresponded to 0.178 g of PNF/g of carbon. In this trial, 0.435 g of virgin AC and 0.0826 g of PNF were diluted up to 150 mL in a flask with ultrapure water and the stoppered flask was placed in rotary mixer of the isothermal chamber at 20 °C for 2 days. The same procedure was followed for the oxidized AC in the absence of PNF. Results and Discussion Wet Oxidation Kinetics of PNF. The kinetics of wet oxidation of PNF in the absence of AC was studied first. Four sets of experiments were carried out by varying the initial concentration of PNF (0.1-0.2 g L-1), stirring speed (300-900 rpm), temperature (150-180 °C), and oxygen partial pressure (0.3-0.9 MPa). The overall oxidation process is governed by two steps: (1) mass transfer of oxygen from the gas phase to the liquid phase; (2) reaction between the dissolved oxygen and phenol. The resistance due to the first step was eliminated by manipulating the stirring speed. The initial PNF concentration had no influence on the oxidation rate because the curves practically coincide, as can be seen in Figure 2. A PNF concentration of 0.2 g L-1 was, therefore, chosen to study the rest of the variables. Upon variation of the stirring speed, it was found that the rate of oxidation remained unchanged when the stirring speed was increased beyond 600 rpm. Similar results have been reported by other workers.5,12 The temperature and oxygen partial pressure exerted positive effects on the oxidation rate (Figures 3 and 4). As can be seen, the uncatalyzed homogeneous oxidation of PNF in the aqueous phase involves an induction period followed by a fast reaction period. This is typical of a free-radical mechanism. During the induction period, little of the PNF was oxidized. Most of the PNF was oxidized in the fast reaction period once a critical concentration of free radicals had been reached in the medium. These results agree with those observed by other workers in the wet oxidation of phenol and substituted phenols.5,11,12,15,19 We applied a kinetic model of first order with respect to PNF and nth order with respect to oxygen. The disappearance rate of PNF can be expressed by

-d[PNF]/dt ) kPO2n[PNF]

(1)

where [PNF] is the PNF concentration, t is time, k is

Figure 4. Profiles of the adimensional concentration of PNF versus time. Influence of the oxygen partial pressure on the oxidation process ([PNF]0 ) 0.2 g L-1, T ) 170 °C, stirring speed ) 600 rpm).

the apparent kinetic constant, PO2 is the oxygen partial pressure, and n is the reaction order with respect to oxygen. Integration gives

-ln([PNF]/[PNF]0) ) k′t

(2)

where [PNF]0 is the initial PNF concentration and k′ is kPO2n. A plot of the left-hand side of eq 2 versus time would result in a straight line of slope k′. Equation 2 was applied to the fast reaction period for the four sets mentioned above. As an example, Figure 5 is a plot of eq 2 for the set where the temperature was varied, and Table 1 lists the results obtained in the least-squares fit for the four sets. The values of the correlation coefficients are very good, showing that the model agrees well with the experimental data. One can see that the slope is practically constant with increasing [PNF]0 and stirring speed but increases with increasing temperature and oxygen partial pressure. The values of k′ where the temperature was varied can be expressed as an Arrhenius function:

k′ ) k0 exp(-Ea/RT)

(3)

with an activation energy, Ea, of 134.8 kJ mol-1, an Arrhenius preexponential factor, k0, of 8.73 × 1013 min-1, and a correlation coefficient of 0.99. The high value of the activation energy indicates that the WAO

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Figure 5. Plot of eq 2. Influence of the temperature. Table 1. Kinetic Constants and Correlation Coefficients: Oxidation of PNF [PNF]0, g L-1

stirring speed, rpm

T, °C

PO2, MPa

k′, min-1

correlation coefficient (r2)

0.1 0.15 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

600 600 600 600 600 600 300 900 600 600 600

170 170 170 150 160 180 180 180 170 170 170

0.718 0.718 0.718 0.718 0.718 0.718 0.718 0.718 0.3 0.5 0.9

0.0164 0.0165 0.0162 0.002 0.0038 0.0229 0.0220 0.0227 0.0105 0.0134 0.0195

0.99 0.99 1 0.99 0.99 0.99 0.99 0.99 1 0.99 0.99

process was governed by chemical reactions. Other workers have reported values of the activation energy of around 93 kJ mol-1 for the wet oxidation of phenol5,11,14 and 48-190 kJ mol-1 for the wet oxidation of phenol and substituted phenols.12 Mishra et al. (1995)19 report values of activation energies of phenol and substituted phenols obtained by other workers in the range 20.5-238.7 kJ mol-1. The rate constants in Table 1 where the oxygen partial pressure was varied give the reaction order with respect to oxygenbecause

k′ ) kPO2n

(4)

ln k′ ) ln k + n ln PO2

(5)

and hence

A plot of the left-hand side of eq 5 versus ln PO2 would result in a straight line of slope n. Equation 5 was applied to the rate constants, giving a reaction order of 0.55 and a correlation coefficient of 0.99. Similar results have been reported for the WAO of phenol,5,11,24 although other authors have reported a first-order reaction.14,25 Mishra et al. (1995)19 in their paper report values obtained by different workers for the reaction order with respect to oxygen in the range 0.5-1.5. Desorption Process. The desorption process was carried out under the aforementioned conditions in the absence of oxygen. Figure 6 shows the influence of the temperature on the temporal evolution of the PNF concentration in the liquid phase. One immediately sees that the desorption process takes place during heating because for a given experiment the PNF concentration in the aqueous phase remains constant over time. Also,

Figure 6. Profiles of the concentration of PNF versus time. Influence of the temperature on the desorption process.

an increase in the temperature increases the PNF concentration in the aqueous phase. However, given that desorption is an equilibrium process, the total degree of desorption is not reached. If all of the PNF had passed into solution, a PNF concentration of 134 × 10-5 mol L-1 would had been obtained, whereas the best results were obtained for 180 °C with a PNF concentration of 77.2 × 10-5 mol L-1. WAO regeneration, in removing PNF by oxidation from the solution, will have the ultimate effect of removing it from AC without drastically affecting the carbon itself. Regeneration of AC by Wet Oxidation. The WAO regeneration of AC was carried out under the conditions described above. This process occurs in the bulk of the liquid,5 and the following sequence of steps occurs: (1) desorption of PNF from AC and mass transfer from the interior to the exterior of the AC particle (intraparticle diffusion); (2) mass transfer from the external surface of the particle to the bulk of the liquid (film diffusion); (3) mass transfer of oxygen from the gas to the liquid bulk; (4) oxidation of PNF in the liquid bulk. In this work, only steps 1 and 4 were studied because the objective was to recuperate the AC’s retention capacity. However, the regeneration process involves a very complex triphase reaction in which it is difficult to minimize the limitations of mass transfer. The effect of the presence of AC on the values of the mass-transfer coefficients in this work should be negligible. Some workers26,27 have reported that solid particles may affect the volumetric mass-transfer rate (kLa) only if the particle size is less than 5 µm. The particle size used in this work was 20-40 mesh (0.850.4 mm). Moreover, other workers28 have confirmed that the effective interface area remains practically unaffected when the solid loading in the liquid bulk is less than 20% (v/v). The solid loading in the liquid bulk in this work was 0.24% (v/v). These experimental conditions thus confirm the scant influence of AC on the mass-transfer coefficients. Figure 7 shows the temporal evolution of the PNF concentration in solution for the four temperatures used in this work. As can be observed, the temperature exerts a positive effect on the disappearance rate of PNF. However, between 170 and 180 °C the effect is smaller, probably indicating that there might be an optimal rate of regeneration at about 170 °C. Henry’s law, that an increase in the temperature reduces the solubility of oxygen in the liquid phase, also would explain the

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Figure 8. Adsorption isotherms of PNF on the carbons at 20 °C. Figure 7. Profiles of the adimensional concentration of PNF versus time. Influence of the temperature on the regeneration process. Table 2. Kinetic Constants and Correlation Coefficients: Regeneration of AC by WAO correlation correlation T, °C k′, min-1 coefficient (r2) T, °C k′, min-1 coefficient (r2) 150 160

0.0054 0.012

0.99 0.99

170 180

0.0192 0.0203

0.98 0.98

results. Moreover, it would be pointless to raise the temperature from 170 to 180 °C so as to eliminate a small amount of PNF in the solution, in view of the reduction that the adsorption capacity of AC could undergo. This will be looked at in more detail in the following section, where the results of the adsorption of virgin and regenerated AC will be described. As mentioned above, the regeneration process is very complex because it depends on many subprocesses that can influence the overall rate. However, the experimental data of the curves of Figure 7 fit eq 2 well. Table 2 shows the results of the fits. One can see the positive effect of the temperature on the apparent rate constant, k′, but this effect is smaller between 170 and 180 °C. Comparing the values of k′ of Tables 1 and 2, one observes that the values of k′ obtained in the regeneration process are slightly greater, except for 180 °C. This suggests that the regeneration process is governed by chemical reactions. Expressing the values of k′ as an Arrhenius function (eq 3), we obtained an activation energy, Ea, of 99.1 kJ mol-1 and an Arrhenius preexponential factor, k0, of 9.63 × 109 min-1. There was a reduction of 26.4% in the activation energy with respect to that obtained in the oxidation of PNF in the absence of AC. This may be because desorption of PNF from AC into the liquid could influence the overall regeneration process with respect to oxidation, where PNF is in the liquid bulk at all times. Adsorption Isotherms and Texture Parameters. To quantify the loss of adsorption capacity of the AC due to the regeneration process, the retention isotherms at 20 °C were obtained and the data were fitted to Langmuir and Brunauer-Emmett-Teller (BET) models. The phenol, methylene blue, and iodine indices give a qualitative picture of the distribution of pores in the carbon. Figure 8 shows the adsorption isotherms of PNF at 20 °C for virgin AC and regenerated carbons at 160, 170, and 180 °C, where Qe is the amount of solute adsorbed per kilogram of adsorbent and Ce is the concentration of solute in the solution. The adsorption isotherm of

PNF on virgin AC is of the H type according to the classification of Giles et al.29 This type of isotherm corresponds to the adsorbent having a great affinity for PNF and, therefore, to low values of Ce. The retention occurs rapidly, with the plots appearing almost vertical at the beginning. This great affinity may be related to surfaces with well-developed porosity or to the solute retention taking place according to a chemisorption process. At higher values of Ce, a plateau is reached that may reflect the surface of the solid being covered completely by PNF molecules. The curves have no apparent shoulder, and hence there is no evidence for any multilayer retention in the interval of values of Ce studied in this work. Figure 8 also shows that the regeneration process allows a major percentage of the adsorbent capacity of the virgin AC to be recovered. However, while an increase in the regeneration temperature from 160 to 170 °C has a positive effect on the adsorption capacity of the regenerated carbon, from 170 to 180 °C, the effect is negative. This probably reflects the competition between two effects: (1) a better elimination of the adsorbed solute, liberating more active sites; (2) a possible attack at the active sites by oxygen, modifying the functional groups on those sites. Indeed, some workers attribute the loss of activity of carbon at temperatures g185 °C to the formation of such oxygen-containing functional groups on the adsorbing surface of the carbon.5,17-19 This would explain the results obtained in this work because at temperatures 170 °C (180 °C) the second effect predominates, diminishing the affinity and adsorbent capacity of the regenerated carbon. We had confirmation of this by means of an oxidation experiment with virgin AC in the absence of PNF at 180 °C and an oxygen partial pressure of 0.718 MPa, as mentioned above. When the same operations are performed for virgin AC and oxidized carbon in the absence of PNF, the retention of PNF in this latter case was 0.172 g of PNF/g of carbon. This result is supporting evidence that oxygen attacks the active sites of the oxidized carbon, modifying their functional groups and thereby reducing the carbon’s adsorption capacity. The isotherms of the regenerated carbons (see Figure 8) show a certain transition toward L-type isotherms in the Giles classification. This would agree with the decrease in the affinity of the carbons for the PNF solute. Carbons with L-type isotherms are considered to have an excess of acid groups on their surfaces.30 In the present case, the oxidative regeneration would have produced carboxylic groups on the carbon surface. The

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activities in the surface of the adsorbent. For dilute solutions, the activities aw and ac can be replaced by molar fractions. Also, the activities in the surface can be considered as proportional to the adsorbed amount. Therefore, eq 7 can also be expressed as

K)

Qe 55.5Ce(Qm - Qe)

(8)

where 55.5 is the number of water moles per kilogram of solution. Equations 6 and 8 give

b ) 55.5K

(9)

and K is related to the Gibbs free energy by the expression Figure 9. Langmuir fit of the isotherm data.

K ) exp(-∆GADS0/RT)

Table 3. Fit of the Langmuir Equation sample

Q m, (mol g-1) × 104

b, L mol-1

correlation coefficient (r2)

virgin AC RC at 160 °C RC at 170 °C RC at 180 °C

14.2 10.6 11.6 10.1

26834.3 8597.0 15302.1 10113.3

0.996 0.994 0.997 0.996

a

RC ) regenerated carbon.

Table 4 lists the results obtained for Qm, b, K, and ∆G for the virgin AC and the three regenerated carbons (RC) and the percentage of recovery of these parameters with respect to the virgin AC. The results for Qm show the following order of regeneration:

RC (170 °C) > RC (160 °C) > RC (180 °C)

monolayer formation for these carbons is similar to that of the virgin AC case, described above. These results may be compared with those of Mundale et al. (1991).5 They used an AC which was saturated with 0.165 g of phenol/g of carbon (a retention similar to that obtained in the present work), and the loss in activity at 185 °C was 20-30%. They described this degeneration as being due to surface oxidation. At a temperature of 150 °C and an oxygen partial pressure of 1 MPa, there was minimal reduction in the activity, and they concluded that these should be the optimal operating conditions for the regeneration process. The experimental data were fitted by a Langmuir equation

Ce Ce 1 + ) Qe Qmb Qm

(6)

which is valid when the affinity of the adsorbent for the solute is much larger than that for the solvent and the solution is very dilute. In this equation, Qm is the amount of solute necessary for the formation of the monolayer and b is a constant related to the energy of the adsorption process. A plot of the left-hand side of eq 6 versus Ce allows Qm and b to be determined. Figure 9 shows these plots for virgin AC and for regenerated carbons at 160, 170, and 180 °C. Table 3 lists the results. Figure 9 shows how well the Langmuir equation fits the experimental data. This confirms that the retention of PNF takes place through the formation of a monolayer. The equilibrium constant (K) and the variation of the Gibbs free energy for adsorption from solution can be calculated from the b values. The equilibrium constant can expressed as

K)

(10)

awacS awSac

(7)

where aw and ac are the activities of water and the solute in solution and awS and acS are the respective

However, the results for b and, therefore, for K yield a different order:

RC (170 °C) > RC (180 °C) > RC (160 °C) These results suggest that the regeneration temperature had a more marked effect on the affinity for the solute than on the maximum adsorption capacity. This implies a decline in the slope of the initial section of the isotherm; i.e., higher values of Ce are necessary to retain a given amount of solute. This would correspond to isotherm shapes closer to the L type than to the H type. The results obtained for ∆G indicate that the PNF adsorption process is energetically favored, so that the order of regeneration with respect to virgin AC is the same as that for K. The BET model was also applied to the experimental data. The corresponding linear expression for the adsorption isotherm of a nonelectrolytic solute can be written as

Ce Qe(Cs - Ce)

)

B - 1 Ce 1 + Qm B QmB Cs

(11)

where B is a constant related to the energy of the adsorption process and Cs is the solubility of the solute in the solvent at the working temperature. Table 5 lists the results of fitting eq 11 to the experimental data. The value of Cs was calculated from the horizontal section of the PNF solubility curve at 20 °C. The result was 16 g L-1. The values of the correlation coefficients indicate that the fits were very good. Also, the results obtained with this model agree with those obtained with the Langmuir model. From the values of Qm obtained in the two models, it is possible to determine an apparent specific surface area by means of solute retention using the expression

S ) NQmA

(12)

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Table 4. Parameters Obtained by Langmuir Fit and Percentage of Recovery Relative to Virgin AC sample

Qm, (mol g-1) × 104

% recovery

b, L mol-1

K

% recovery

∆G, kJ mol-1

% recovery

virgin AC RC 160 °C RC 170 °C RC 180 °C

14.2 10.6 11.6 10.1

100 74.6 81.7 71.1

26834.3 8597.0 15302.1 10113.3

481.8 154.9 275.7 182.2

100 32.1 57.2 37.8

-15.06 -12.29 -13.69 -12.69

100 81.6 90.9 84.3

Table 5. Fit of the BET Equation sample

Q m, (mol g-1) × 104

B

correlation coefficient (r2)

virgin AC RC at 160 °C RC at 170 °C RC at 180 °C

14 10.4 11.4 9.8

22.9 7.4 13.1 8.8

0.996 0.995 0.997 0.997

Table 6. Specific Surface Area Obtained from the Retention of PNF on the Carbon at 20 °C sample

S L, m2 g-1

SBET, m2 g-1

virgin AC RC at 160 °C

450 337

443 329

sample

SL, m2 g-1

SBET, m2 g-1

RC at 170 °C RC at 180 °C

368 319

361 311

Table 7. Phenol, Methylene Blue, and Iodine Indices of the Carbons index sample

phenol

methylene blue

iodine

virgin AC RC at 160 °C RC at 170 °C RC at 180 °C

94.8 69.2 73.2 71.6

108.9 108.3 109 108.6

508.1 362.7 435.6 437.5

where N is Avogadro’s number and A is the area of the solute molecule (52.5 Å2 for the case of PNF). Table 6 gives the results, where SL and SBET represent the specific surface areas obtained from Qm according to the Langmuir and BET models, respectively. Although these specific surface areas evaluated from Qm can differ from the surface area really accessible to the solute, they are sufficient to give an idea of the porous structure of the carbon. One sees that the values obtained from the two models are very similar. The results confirm that the regeneration process modified the internal porous structure of the carbon and that the optimum regeneration temperature in this work was 170 °C. Finally, the phenol, methylene blue, and iodine indices determined for the four carbons give an idea of how the carbon’s porous structure is modified by WAO regeneration. Table 7 lists these results. One can see that the highest regeneration temperature corresponds to a decrease in the phenol index. This reduction could be due to some micropores admitting PNF molecules which have not been destroyed in the oxidation process. The best results for this index were with a regeneration temperature of 170 °C. The methylene blue index remained practically unchanged by the oxidation process. This may be because mesopores are feeder pores and allow the molecules of PNF and oxygen to circulate without their structure being altered. The iodine index is lower in the regenerated carbons than in the virgin AC. However, it increases slightly with increasing regeneration temperature. This may reflect the destruction of micropores, with a concomitant increase in the number of macropores, due to oxidation of the carbon at the higher temperature. In light of the carbon loss results within the first regeneration cycle, it is necessary to study for how many

cycles a given batch of carbon might be used. This will be dealt with in future work because it is a question that engineers would like to have resolved in designing equipment for this type of process. Some workers31 have reported results of run cycles: utilizing phenol as the sorbate and powdered AC (PAC) as the adsorbent, they obtained the oxidation of 5.62% in the PAC after the third cycle. Conclusions (1) The WAO regeneration of an AC saturated with PNF is a complex process involving several mass transport and oxidation reactions. The reaction conditions used in this work minimized the mass transports as far as possible. The kinetics of the PNF oxidation were found to be independent of the initial PNF concentration, with an activation energy and reaction order with respect to oxygen of 134.8 kJ mol-1 and 0.55, respectively. (2) The desorption process in the absence of AC takes place during the heating period of the autoclave and is favored by increasing the temperature. (3) The regeneration process was studied at the same temperatures as the oxidation process. We obtained an activation energy of 99.1 kJ mol-1. The difference with the value of 134.8 kJ mol-1 reflects the effect of the desorption rate of PNF from AC. (4) The retention isotherms of virgin and regenerated ACs at 20 °C were determined and gave excellent fits to the Langmuir and BET models. A certain transition toward L-type isotherms was observed for the regenerated carbons, implying an excess of acid groups on the surface. (5) The parameters obtained from applying the aforementioned models, Qm, K, ∆G, and S, were compared for the four carbons. While all regenerated carbons showed a reduced adsorption capacity, the best results were obtained at a regeneration temperature of 170 °C, with 80% adsorption capacity relative to virgin AC. (6) The phenol, methylene blue, and iodine indices showed that the regeneration process modifies the porous structure of the carbons. The best results were again obtained at a regeneration temperature of 170 °C. Acknowledgment The authors express their gratitude to the Comision de Ciencia y Tecnologı´a of Spain for the financial support received to perform this study, through Project AMB97/0339. Literature Cited (1) Loven, A. W. Activated carbonsPerspective on carbon regeneration. Chem. Eng. Prog. 1973, 69 (11), 56. (2) Perrich, J. R. CRC Handbook of activated carbon adsorption for wastewater treatment; CRC Press Inc.: Boca Raton, FL, 1981. (3) Guymont, F. J. The effect of capital and operation costs on GAC Adsorption System Design. In Activated carbon adsorption of organics from the aqueous phase; McGuire, M. J., Suffet, I. H., Eds.; Ann Arbor Science: Ann Arbor, MI, 1980; Vol. 2.

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Received for review May 29, 2001 Revised manuscript received November 15, 2001 Accepted November 21, 2001 IE010474M