Wet Air and Extractive Ozone Regeneration of 4-Chloro-2

Jun 8, 2004 - 06071 Badajoz, Spain, and Departamento de Ingenierı´a Quı´mica, Universidad de Castilla la Mancha,. Plaza de Manuel Meca s/n, 13400 ...
0 downloads 0 Views 149KB Size
Download PDF
Ind. Eng. Chem. Res. 2004, 43, 4159-4165

4159

Wet Air and Extractive Ozone Regeneration of 4-Chloro-2-methylphenoxyacetic Acid Saturated Activated Carbons F. J. Rivas,*,† F. J. Beltra´ n,† O. Gimeno,† and J. Frades‡ Departamento de Ingenierı´a Quı´mica y Energe´ tica, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain, and Departamento de Ingenierı´a Quı´mica, Universidad de Castilla la Mancha, Plaza de Manuel Meca s/n, 13400 Almade´ n, Ciudad Real, Spain

The wet air regeneration of some 4-chloro-2-methylphenoxyacetic acid (MCPA) exhausted activated carbons (ACs) has been investigated. The wet oxidation process of MCPA in the absence of ACs suggests the development of a a radical mechanism. Oxygen partial pressure has been shown not to affect the process, while pH exerts a negative influence. Temperature has a positive effect in the range of 160-210 °C. The use of a heterogeneous catalyst or radical promoters significantly enhances the oxidation process in terms of both MCPA removal and mineralization levels. Regenerated ACs undergo a deterioration in their adsorption capacity, achieving the best results when the regeneration stage is carried out in the presence of a radical promoter. Alteration of the microporous structure is likely the reason for such a deterioration. The use of the extractive ozonation process as the regenerating technology seems to be a promising alternative for microporous adsorbents. Experiments conducted by using commercial acetic acid as the solvent and ozone as the oxidizing agent have resulted in practically 100% recuperation of both the solvent and the AC adsorption capability. 1. Introduction Over the last 100 years, the agricultural community has routinely and consistently applied pesticides to control pests in order to increase crop yield. Some of these pesticides are persistent in the environment and thus may be present in the environment long after they have been applied. As a result, residues of a number of pesticides can be found in soils and water at levels that may pose a human health risk. Application rates, duration of use, and persistence are the major factors that contribute to the presence of residual pesticides in soil, surface, and groundwater at concentrations above permitted limits. Given the typically low concentration and low solubility of pesticides in contaminated waters, adsorption onto activated carbon (AC) is a feasible technology for the removal of these types of substances from a diluted aqueous matrix. However, this water treatment is a nondestructive process, and contaminants are concentrated in the solid phase but not converted to harmless or biodegradable compounds.1 The latter step is normally attained at the time of the AC regeneration step. Exhausted AC regeneration can be carried out in several ways, i.e., liquid-solid extraction, pH swing, supercritical fluid regeneration, thermal regeneration, etc.2 Thermal regeneration accomplished in the range of 8501000 °C is the preferred technology to reactivate the capacity of the adsorbent. However, it is economically feasible for large plants using more than 2 × 105 kg of granular AC/year.3 An emerging technology that uses milder conditions than the thermal regeneration is the wet air regeneration (WAR) completed in the aqueous phase by means * To whom correspondence should be addressed. Fax: 0034-924-289385. E-mail: [email protected]. † Universidad de Extremadura. ‡ Universidad de Castilla la Mancha.

of moderated temperatures and pressures.4,5 The advantages of this technique include low losses of carbon during treatment, reduced pollutant emission, reduced costs as compared to thermal regeneration, possibility of in situ regeneration, enhancement of oxidation reactions as the contaminant concentration is increased, etc.3,6 Moreover, the use of heterogeneous catalysts and/ or radical promoters allows for the implementation of milder regeneration operating conditions and the benefits associated with it. Therefore, in this study the WAR of three commercially available ACs was initially assessed. For this purpose, ACs were saturated with MCPA (4-chloro-2methylphenoxyacetic acid, CAS number 94-74-6), a phenoxyherbicide extensively used in agriculture to control annual and perennial weeds in cereals, grasslands, trees, and turf.7 Because the capability of the ACs to retain MCPA has been recently reported in a previous paper,7 the following stages were, therefore, conducted: (a) Confirmation of the wet air oxidation (WAO) for the effective removal of MCPA. For doing so, oxidation experiments of MCPA were carried out in the absence of AC. (b) Regeneration of exhausted ACs by simultaneous desorption-oxidation under WAO conditions. Noncatalytic, catalytic, and promoted experiments were completed. (c) Comparison of the adsorption efficiency between fresh and regenerated ACs. Because a significant loss in the adsorption capacity was experienced for ACs of microporous structure, an alternative regeneration technique was also investigated. The use of ozone in AC regeneration processes has also appeared in the past few years, with the process carried out at room temperature. In this case, the combination of liquid-solid extraction and ozone oxidation leads to advantages similar to those previously

10.1021/ie030756h CCC: $27.50 © 2004 American Chemical Society Published on Web 06/08/2004

4160 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 Table 1. Catalyst Physicochemical Properties

active metal loading size surface area apparent bulk density

Figure 1. (a) WAR reactor: 1, sampling port; 2, heat exchanger; 3, magnetic agitation; 4, injection port; 5, pressure gauge; 6, basket with turbines; 7, temperature controller; 8, thermocouple; 9, air cylinder; 10, furnace. (b) Ozonation setup: 1, oxygen cylinder; 2, ozone generator; 3, ozone gas analyzer; 4, cooling water; 5, reactor; 6, sampling port; 7, AC bed; 8, peristaltic pump; 9, rotameter.

reported for the WAR, with the main advantage of avoiding contact of ACs with oxidizing agents. The following experimental series were then conducted: (a) Confirmation of the ozonation in an organic solvent solution for the effective removal of MCPA. (b) Regeneration of AC in discontinuous steps (i.e., extraction + solvent recovery). (c) Regeneration of AC in a continuous mode (simultaneous extraction and oxidation). (d) Comparison of the adsorption efficiency between fresh and regenerated ACs. 2. Experimental Section WAR experiments were carried out in a 316 stainless steel Parr autoclave operated in batch mode. The experimental setup consisted of a cylindrical reactor of 0.6 L capacity equipped with a magnetic stirrer to homogenize the reaction mixture. A rotating basket inside the reactor was used to place the exhausted AC. In some cases, an injection cylinder was used to add a promoter (i.e., hydrogen peroxide) once the operating temperature and pressure were attained. Samples were steadily withdrawn through a sampling port refrigerated by means of a heat exchanger. A schematic drawing of the reactor is shown in Figure 1a. Ozone experiments were carried out in a 0.3 L reactor equipped with a recirculation system for continuous

Pt/γ-alumina (Johnson Matthey Chemicals)

CuO/AC Sofnocarb A21 (Molecular Products Ltd.)

0.5% (w/w) pellets, 3 mm 100 m2 g-1 1 g cm-3

5% (w/w) granular, 6-12 mesh 1000 m2 g-1 0.77 g cm-3

extraction-oxidation runs. Figure 1b illustrates the main parts of the experimental setup. The adsorbents used in this study were commercial ACs: Norit 0.8 supplied by Aldrich (U.K.) and Aquacarb 207C and Aquacarb 208A supplied by Waterlink Sutcliffe Carbons (U.K.). General characteristics given by the manufacturers are reported in a previous work.7 Briefly, ACs used present Brunauer-Emmett-Teller areas of around 1000-1200 m2 g-1 and alkaline pHs in the following order: Norit 0.8 > Aquacarb 207C > Aquacarb 208A. Bulk densities range from 0.4 g cm-3 for Norit 0.8 to roughly 0.5 g cm-3 for the other two absorbents studied. Before use, the ACs were washed with distilled water to remove any powdered particulate. Then, the ACs were dried overnight at 110 °C to eliminate moisture from the porous structure. MCPA, 95% purity, was supplied by Aldrich (U.K.). Aqueous solutions of MCPA were made by adding an excess of the pesticide to distilled water and stirring overnight. After saturation of water by MCPA, excess of the herbicide was removed by filtration through nylon membrane filters. The MCPA initial concentration was determined by comparison to standard solutions made in high-performance liquid chromatography (HPLC)grade methanol. Heterogeneous catalysts used were platinum supported onto alumina and copper oxide supported onto AC. The main physicochemical properties of both solids are displayed in Table 1. The concentration of MCPA in an aqueous solution was measured by HPLC by using a UV/vis detector set at 230 nm. The column used for the analysis was an Aqua 5 µm C18 200 Å (Phenomenex). The mobile phase composed of acetonitrile (40)/water (58.8)/acetic acid (1.2) was pumped at a flow rate of 1 mL min-1 under these conditions; MCPA showed a retention time of ca. 9.5 min. All samples were filtered through nylon membrane filters (0.2 µm) before injection. The total carbon (TC) and total organic carbon (TOC) were measured by means of a Dohrman DC-190 analyzer. The bottle point isotherm technique was employed to determine the equilibrium capacity of the ACs investigated for MCPA adsorption.7 3. Results and Discussion 3.1. WAO of MCPA. Some preliminary experiments were completed to determine the existence of nonoxidative reactions (i.e., hydrolysis), leading to the disappearance of the herbicide.8 Hence, some runs were carried out in an inert atmosphere of nitrogen (50 atm of total pressure) over the temperature range of 150210 °C. Experimental results showed no change in the MCPA remaining concentration throughout the reaction period, confirming, therefore, the absence of nonoxida-

Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4161

treatment, MCPA conversion increases from 1% experienced at 160 °C up to 14%, 67%, and 98% obtained at 180, 190, and 210 °C, respectively. From Figure 2, the presence of an induction period in the MCPA oxidation profiles that is typical of free-radical-mediated processes is also observed.10,11 The lag period decreased as the operating temperature is increased. A similar trend was also experienced for the case of the remaining TC left in the aqueous phase (see Figure 2b); nevertheless, by a comparison of both figures, it can be seen that the TC depletion rate is lower than the MCPA depletion rate, indicating the accumulation of intermediates in the reaction media which are more refractory than the parent compound. It has to be pointed out that, given the low pH used, TC and TOC measurements did coincide, so TC curves indicate the mineralization level achieved. A possible pathway of MCPA removal under WAO conditions would be the generation of hydroperoxyl radicals similarly to the WAO of phenol11 and photocatalysis of MCPA;12 in this case, oxygen would attack the CH2 group of the aromatic substituent to form an organic radical according to

Figure 2. (a) WAO of MCPA: influence of temperature. Experimental conditions: CMCPA0 ) 820 ppm; PT ) 5.0 MPa (air); pH ) 2.8. T (°C): O, 160; b, 170; 0, 180; 9, 190; 4, 210. (b) WAO of MCPA: influence of temperature on the mineralization level. Experimental conditions: CMCPAo ) 820 ppm; PT ) 5.0 MPa (air); pH ) 2.8. T (°C): O, 160; b, 170; 0, 180; 9, 190; 4, 210.

tive processes. These results are consistent with the recalcitrant nature of MCPA and its salt to hydrolysis.9 If the regeneration technique is to be effective, the process has to be able to destroy the contaminants adsorbed onto the AC surface. Therefore, MCPA oxidation under WAO conditions was investigated as a first approach of WAR applicability. Consequently, a series of experiments was undertaken by varying some of the most influential parameters. In a first attempt, the oxygen partial pressure effect in the reactor was studied (range: 30-70 atm of total pressure) by keeping constant the rest of the operating variables potentially affecting the process (i.e., T ) 200 °C and pH0 ) 2.8, nonbuffered). Experimental results (not shown) did show a negligible role played by this parameter. Therefore, it can be postulated that at the conditions investigated the reaction order corresponding to the oxygen concentration is zero, provided that this reagent is in excess. Similar results have been found for the WAO of other pesticides such as atrazine.8 As a consequence, the rest of the experiments were conducted under the same total pressure inside the reactor (i.e., 50 atm). Temperature is one of the most influential factors affecting the kinetics of wet oxidation processes. Thus, the following experimental series was aimed at assessing the effect of this parameter on the depletion profiles of MCPA. Runs were completed in the interval of temperatures from 160 to 210 °C and with an initial nonbuffered pH of the herbicide in solution of pH0 ) 2.8. Figure 2a depicts the dimensionless remaining concentration of MCPA against time for the aforementioned series. As observed from this figure, temperature exerts a significant positive influence on the conversion of the parent compound. For instance, after 100 min of

The observed induction period would be due to reaction [1], which can be considered as the intiation step in the radical mechanism. The organic radical, stabilized by different forms of resonance, may undergo an excision, losing the acetic group to form 4-chloro-2-methylphenol as indicated by Topalov and co-workers12 in the photocatalytic degradation of MCPA; also formation of 3-methyl-5-chlorocatechol has been reported to be one of the major MCPA metabolites.9 Even the organic radical might react with an oxygen molecule to eventually generate the corresponding organic peroxy radical. Formation of hydroxyl radicals would finally allow for the formation of dechlorinated byproducts; however, it is out of the scope of this work to go into detail about the actual mechanism. Further investigations were then conducted to check for the influence of the initial pH in the reaction bulk. Thus, circumneutral (pH ) 7) and alkaline operating conditions (pH ) 11) were used to carry out MCPA oxidation experiments at different temperatures. Contrary to the common oxidation behavior of ionized species being more reactive than molecular species, an increase in the pH led to a strong inhibition of the MCPA oxidation rate even for the highest temperature investigated (see Figure 3). Similar results were reported in a previous work10 for the WAO of phenol in the pH range from 5 to 12; only values of pH above 13

4162 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004

Figure 3. WAO of MCPA: influence of pH. Experimental conditions: CMCPA0 ) 820 ppm; PT ) 5.0 MPa (air); T ) 210 °C. pH: O, 2.8; b, 7.0; 0, 11.0.

led to a higher phenol conversion. Nevertheless, in this work the highest pH used was 11. Given the pKa of MCPA (3.1), pH values of 7 and 11 indicate that MCPA is totally dissociated, with the ionic form being the predominant species in solution. Either the initiating step in the mechanism could be catalyzed by the presence of protons in the media or the ionic form of MCPA is less reactive in a WAO environment than the corresponding neutral molecule can be different reasons to explain the experimental facts, although they are speculative in nature because no proof of it has been previously reported. Accordingly, mineralization of the organics present in the aqueous matrix also underwent a significant decrease with conversions lower than 15% after 100 min of treatment at 210 °C and pH ) 7. Given the radical nature of the mechanism involved in the oxidation of MCPA and taking into account the main objective of this work, i.e., AC regeneration, an attempt was made to moderate the reaction conditions for MCPA elimination. A decrease in the working temperature regeneration would prevent to some extent the modification of the AC surface structure, altering, therefore, its adsorption properties.13 Thus, some experiments were conducted at neutral pH and relatively lower temperature (180 °C). Runs were, therefore, conducted by adding two heterogeneous catalysts to the reaction media: a proprietary CuO catalyst supported on AC and a commercially available platinum catalyst supported on alumina. Besides Table 1, a deeper characterization of both catalysts can be found elsewhere.14,15 Also, an additional run was carried out by injection of a well-known free-radical promoter, for instance, hydrogen peroxide. Figure 4 depicts the experimental results obtained in these experiments. As is inferred from this figure, the platinum catalyst does not present any activity in terms of the MCPA degradation; however, the copper-based catalyst significantly accelerates the process as compared to the noncatalytic run. Despite the important positive effect observed by the usage of CuO/AC, it has to be pointed out that this catalyst showed some leaching even at the initial neutral pH used in this work (because nonbuffered solutions were used, the pH decreased to acidic values throughout the process). The presence of copper in the solution brings about an additional problem associated with its removal from water. To avoid toxic metals in the solution, hydrogen peroxide was used to initiate the radical mechanism. The presence of a radical promoter (hydrogen peroxide) at the beginning of the process leads to the total

Figure 4. WAO of MCPA: influence of the presence of heterogeneous catalyst and promoters. Experimental conditions: CMCPA0 ) 820 ppm; PT ) 5.0 MPa (air); pH ) 7.0; T ) 180 °C. Symbols: 2, no catalyst or promoters; O, no catalyst or promoters (210 °C and pH ) 2.8); b, no catalyst or promoters (pH ) 2.8); 0, Pt on alumina (2 g/L); 9, CuO on AC (2 g/L); ∆, CH2O2,0 ) 2.17 × 10-2 M.

Figure 5. WAO of MCPA: influence of the presence of heterogeneous catalyst and promoters on the mineralization level. Experimental conditions: as in Figure 4.

disappearance of the initial lag phase due to the rapid generation of radical species, mainly hydroxyl radicals.11 Moreover, Figure 5 shows that the TC abatement and MCPA conversion obtained with the CuO/AC catalyst and after the addition of hydrogen peroxide are much higher than those observed for the noncatalytic experiment carried out at 180 °C and with acidic pH. Also from Figures 4 and 5, it is observed that MCPA and TC do follow a similar trend when CuO/AC or H2O2 is added to the reaction media (especially when hydrogen peroxide is used), indicating the almost complete mineralization of the parent compound. Total oxidation of MCPA should lead to a better regeneration of the AC because reaction intermediate readsorption is avoided after the oxidation stage. 3.2. Wet Oxidative Regeneration. In a previous paper, the effect of WAO conditions on the fresh carbon was studied.7 In these preliminary experiments, a significant loss in the adsorption capacity for Norit 0.8 was experienced. Contrarily, WAO conditions exerted a positive influence on Aquacarb 207 and Aquacarb 208A when they were pretreated for 2 h. Norit 0.8 shows the highest fraction of microporosity of the three ACs studied, and, consequently, the loss of part of the microporous structure is likely the reason for the observed decrease in the adsorption capacity.5 Also, the reversibility of the adsorption process was assessed through desorption experiments of MCPA

Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4163

Figure 6. WAR of Norit 0.8: fresh and regenerated AC isotherm comparison. Adsorption experiment conditions: pH ) 2.8; T ) 25 °C. Symbols: 0, noncatalytic regeneration; b, CuO-AC (2 g/L) regeneration; O, H2O2 (2 × 10-2 M) regeneration.

Figure 7. WAR of Aquacarb 207C: fresh and regenerated AC isotherm comparison. Adsorption experiments conditions: pH ) 2.8; T ) 25 °C. Symbols: 0, noncatalytic regeneration; b, CuOAC (2 g/L) regeneration; O, H2O2 (2 × 10-2 M) regeneration.

carried out at high temperature and pressure. In the three ACs investigated, MCPA was partially desorbed from the adsorbents, giving the possibility of its oxidation both in the liquid bulk and on the AC surface.7 Spent carbons were subjected to wet oxidative regeneration at 180 °C under an air total pressure of 5.0 MPa. By considering the results obtained in the WAO of MCPA, the regeneration was also conducted in the presence of the CuO/AC catalyst or hydrogen peroxide. In all of the experiments carried out with the three ACs, MCPA could be detected at time zero (once the reactor reached the working temperature), although the concentration of the parent compound was always lower than the theoretical value if all of MCPA was desorbed from the ACs (i.e., approximately 8-15% of the maximum concentration). In any case, after roughly 1 h of reaction, no MCPA and TC could be measured in the samples withdrawn from the reactor. After the oxidation-regeneration process was completed, isotherm curves were once more calculated and compared to values obtained with fresh carbon. Hence, Figures 6-8 illustrate the percentage of adsorption capacity restored (as compared to fresh carbon) after the regeneration as a function of the liquid-phase equilibrium concentration. From these figures, it is inferred that the isotherm shape changes as compared to the profile obtained by using fresh carbon and, therefore, the recovery in the adsorption capacity depends on the zone of the isotherm analyzed, i.e., the

Figure 8. WAR of Aquacarb 208A: fresh and regenerated AC isotherm comparison. Adsorption experiments conditions: pH ) 2.8; T ) 25 °C. Symbols: 0, noncatalytic regeneration; b, CuOAC (2 g/L) regeneration; O, H2O2 (2 × 10-2 M) regeneration.

concentration of MCPA in equilibrium. The change of the isotherm shape after an oxidative AC regeneration has previously been reported.16,17 The main reasons for these exerimental facts are the alteration of the surface functional groups responsible for the adsorption process. Broadly speaking, how regeneration experiments carried out in the presence of hydrogen peroxide give the best results in terms of the recovery of the adsorption properties of the ACs studied is observed. These results are consistent with the higher mineralization level achieved when using the promoter. Contrarily, runs conducted in the absence of any catalyst or promoter led to a significant loss in the adsorption capability. The latter results are clearly observed for the particular case of Norit 0.8 with losses up to 75% in efficiency. As stated previously, among the three ACs studied, Norit 0.8 showed the highest fraction of micropores and a higher effect of WAO operating conditions was expected for this adsorbent. Consequently, it was decided to apply a different regeneration technology to Norit 0.8 by using low temperatures not affecting the surface structure of the AC. 3.3. Liquid-Liquid Extractive Ozone Regeneration. From the previous results, it can be deduced that WAR is a suitable regeneration technique for those ACs that do not present the higher fraction of pores in the microporous region. For those ACs belonging to the microporous solid type, a regeneration stage using milder conditions is required. In this sense, the combination of liquid-liquid extraction and further destruction of contaminants in the bulk of the extracting solvent can be envisaged as a suitable process. Thus, the use of a cheap and harmless solvent seems to be one of the key steps in the process. Also, the oxidant has to be able to reduce the contaminant load of the solvent without affecting its properties so the solvent can be reused. In this work the combination of commercial acetic acid as the organic solvent and ozone as the oxidant was chosen based on the following: (a) Commercial acetic acid (80%) is cheap and easy to handle. (b) The solvent does not present a particularly high toxicity so any amount of this solvent remaining inside the AC does not constitute a serious concern in water treatment. (c) Both MCPA and ozone are soluble in acetic acid. This property is remarkably interesting in terms of the kinetics of the process because ozone is not very soluble in water but it is soluble in acetic acid.

4164 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004

Figure 9. Two-step extraction (70 h + 70 h) of MCPA into commercial acetic acid. Experimental conditions: V ) 0.6 L; T ) 25 °C; 1 g of MCPA saturated Norit 0.8; 50 mL of acetic acid in each extraction. Symbols: O, fresh acetic acid; 2, regenerated acetic acid. Inset: ozonation of MCPA after extraction with acetic acid. Experimental conditions: V ) 0.1 L; T ) 20 °C, gas flow rate ) 40 L h-1, ozone inlet concentration ) 38 mg L-1.

(d) MCPA does react with ozone at a relatively high rate when dissolved in water, so it is assumed that a similar reactivity will be found in acetic acid. First of all, an experimental series was conducted to corroborate the last assumptions. Thus, Figure 9 shows the extraction capacity of commercial acetic acid in two successive stages by using 50 mL of solvent in each stage. From this figure, it can be seen that approximately 95% of adsorbed MCPA can be recovered by a discontinuous extraction in two steps. The extraction was conducted under mild agitation, the process likely controlled by external diffusion, so no conclusive statements can be made about the kinetics of the extraction process. Despite the operating conditions used, the extraction of MCPA mainly occurs in the first hours of the process, achieving equilibrium in roughly 10 h. Additionally, in an ozonation experiment conducted in acetic acid (experimental conditions: V ) 0.3 L; T ) 20 °C; gas flow rate ) 18 L h-1, CO3inlet ) 34.3 mg L-1, CMCPA0 ) 1621 mg L-1), it was demonstrated that the herbicide was effectively removed in just a few minutes (results not shown). On the basis of the previous results, the extractionozonation process was tested in two different contact patterns. Thus, in a first attempt, the extraction (AC regeneration) and further oxidation (solvent regeneration) were conducted separately, so ozone did not contact the AC. Additionally, a continuous extraction process was also completed by recirculation of acetic acid through a bed in which the exhausted AC Norit 0.8 was placed (for 3 h without bubbling ozone and by feeding a mixture of oxygen-ozone for 2 h more). In the first case, after extraction of MCPA with acetic acid, the AC was again used to remove MCPA in the aqueous solution. From Figure 10, how the regenerated carbon has recovered practically 100% of its adsorption capacity (up solid triangles) is observed. The contaminated acetic acid was subsequently subjected to an ozonation stage (inset of Figure 9) so MCPA could be eliminated; after this stage, acetic acid was used in a new cycle. Thus, Figure 9 shows that acetic acid can be effectively reused in a new extraction cycle without experiencing any loss in its extractive properties. Again the regenerated AC was used in a third cycle. As observed from Figure 10, after three reuses AC Norit 0.8 does not suffer any loss in the adsorption activity. Although just an equilibrium point has been used to

Figure 10. Adsorption of MCPA onto AC Norit 0.8. Experimental condictions: CMCPA0 ) 300 mg L-1 (average value); T ) 20 °C; V ) 0.6 L; AC mass ) 1 g. Symbols: O, fresh carbon; 2, one regeneration; 9, two regenerations; 4, one regeneration (continuous process).

Figure 11. Extraction-ozonation of MCPA in continuous mode. Experimental conditions: T ) 20 °C; V ) 0.2 L; AC mass ) 3 g; acetic acid flow rate ) 0.42 mL s-1; gas flow rate (from 180 min) ) 36 L h-1; ozone inlet concentration (from 180 min) ) 38 mg L-1. Dash-dotted line on top: maximum amount of MCPA in acetic acid.

check for the regeneration level achieved, because the solid was not subjected to extreme conditions (surface not affected), it is expected that the isotherm shape has not changed and the percentage of recovery is independent of the value of Ce. It was attempted to implement the previous two-step process in a combined process, so extraction and regeneration could be carried out in a unique experimental setup. For doing so, exhausted AC (3 g) was placed in a column (120 mm × 8.5 mm) and acetic acid pumped from the reactor through it at a flow rate of 0.42 mL s-1 for 3 h. Thereafter, an oxygen-ozone mixture was fed to the reactor while maintaining the recirculation of acetic acid through the carbon bed. Figure 11 shows the evolution concentration profile of MCPA for the continuous process. As is inferred from this figure, after 3 h 54% of the initially adsorbed MCPA has been extracted, a value comparable to the discontinuous extraction for the same time. Once ozone is fed, MCPA steadily disappears from the solution and the extraction-oxidation of the herbicide proceeds simultaneously. After the continuous regeneration process, the adsorbent was tested and the results were compared to those obtained by using fresh carbon. Figure 10 depicts the performance of the adsorption of MCPA onto Norit 0.8 with time and the corresponding comparison with values obtained with fresh carbon. As is observed from this figure, how the Norit 0.8 adsorption capacity has

Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4165

been practically restored it can be seen, experiencing results similar to those found in the two-stage process. 4. Conclusions From this study, the following conclusions can be drawn: (a) WAO of MCPA is a suitable oxidation technology to effectively remove the herbicide from water solutions. Under the experimental conditions investigated, temperature, oxygen partial pressure, and pH exert positive, null, and negative effects on the MCPA oxidation rate, respectively. (b) The use of the appropriate catalyst (i.e., copper oxide based catalysts) and/or the addition of radical promoters (i.e., hydrogen peroxide) have a positive influence on both the MCPA removal rate and mineralization level. (c) WAR of exhausted ACs under different operating conditions shows that hydrogen peroxide promoted regeneration is the best option in terms of restoring the adsorption capacity of the ACs. The loss of adsorption properties after the WAR process is intimately related to the microporous structure of the absorbent. In this sense, widening of the pore diameter by a combination of adjacent pores is likely the reason for the adsorption capability deterioration. (d) Extractive ozonation is presented as an alternative process for AC regeneration using milder conditions than WAR. For the particular case of MCPA, acetic acid is found to be a feasible extracting solvent because of its chemical properties, economic aspects, and low toxicity. (e) Experiments carried out in two different contact patterns (two-stage process and continuous process) have demonstrated the practically total restoration of AC adsorption properties. (f) This study represents a first approach to the extractive ozone regeneration. More work has to be done to investigate the optimum values of the main influencing parameters. Acknowledgment The authors thank the CICYT of Spain and European FEDER Funds for their economic support through Project PPQ2000/0412 Literature Cited (1) Polaert, I.; Wilhelm, A. M.; Delmas, H. Phenol wastewater treatment by a two-step adsorption-oxidation process on activated carbon. Chem. Eng. Sci. 2002, 57 (9), 1585.

(2) Shende, R. V.; Mahajani, V. V. Wet oxidative regeneration of activated carbon loaded with reactive dye. Waste Manage. 2002, 22, 73. (3) Matatov-Meytal, Y. I.; Sheintuch, M.; Shter, G. E.; Grader, G. S. Optimal temperatures for catalytic regeneration of activated carbon. Carbon 1997, 35 (10-11), 1527. (4) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet air oxidation. Ind. Eng. Chem. Res. 1995, 34, 2. (5) Salvador, F.; Jimenez, C. S. Effect of regeneration treatment with liquid water at high pressure and temperature on the characteristics of three commercial activated carbons Carbon 1999, 37 (4), 577. (6) Mundale, V. D.; Joglekar, H. S.; Kalam, A.; Joshi, J. B. Regeneration of spent activated carbon by wet air oxidation. Can. J. Chem. Eng. 1991, 69, 1149. (7) Gimeno, O.; Kolaczkowski, S. T.; Plucinski, P.; Rivas, F. J.; Alvarez, P. Removal of the herbicide MCPA by commercial activated carbons: equilibrium, kinetics and reversibility. Ind. Eng. Chem. Res. 2003, 42, 1076. (8) Rodriguez, E.; Rivas, F. J.; Beltra´n, F. J.; Alvarez, P. Wet peroxide degradation of atrazine. Chemosphere 2003, 54, 71-78. (9) http://pmep.cce.cornell.edu. (10) Kolaczkowski, S. T.; Beltran, F. J.; Mclurgh, D. B.; Rivas, F. J. Wet air oxidation of phenol: factors that may influence in global kinetics. Process Saf. Environ. Prot. 1997, 75, 257. (11) Rivas, F. J.; Kolaczkowski, S. T.; Beltra´n, F. J.; McLurgh, D. B. Development of a model for the wet air oxidation of phenol based on a free radical mechanism. Chem. Eng. Sci. 1998, 53, 2575. (12) Topalov, A.; Abramovic´, B.; Molna´r-Ga´bor, B.; Csana´di, J.; Arcsen, O. Photocatalytic oxidation of the herbicide (4-chloro-2methylpehnoxy)acetic acid (MCPA) over TiO2. J. Photochem. Photobiol. A 2001, 140, 249. (13) Gonza´lez, J. F.; Encinar, J. M.; Ramiro, A.; Sabio, E. Regeneration by Wet Oxidation of an activated carbon saturated with p-nitrophenol. Ind. Eng. Chem. Res. 2002, 41, 1344. (14) Rivas, F. J.; Kolaczkowski, S. T.; Beltra´n, F. J.; McLurgh, D. B. Degradation of maleic acid in a wet air oxidation environment in the presence and absence of a platinum catalyst. Appl. Catal. B 1999, 22, 279. (15) Alvarez, P. M.; McLurgh, D. B.; Plucinski, P. Copper oxide mounted on activated carbon as catalyst for wet air oxidation of aqueous phenol. 1. Kinetic and mechanistic approaches. Ind. Eng. Chem. Res. 2002, 41 (9), 2147. (16) Alvarez, P. M.; Beltra´n, F. J.; Serrano, V. G.; Jaramillo, J.; Rodriguez, E. Comparison between thermal and ozone regenerations of spent activated carbon exhausted with phenol. Water Res. 2004, in press. (17) Hayda, S.; Ferro Garcı´a, M. D.; Rivera-Utrilla, J.; Joly, J. P. Adsorption of p-nitrophenol on activated carbon with different oxidations. Carbon 2003, 41 (3), 387.

Received for review October 6, 2003 Revised manuscript received March 8, 2004 Accepted May 4, 2004 IE030756H