The Feasibility Studies for Radical-Induced Decomposition and

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The Feasibility Studies for Radical-Induced Decomposition and Demetalation of Metalloporphyrins by Ultrasonication Shih Pin Tu and Teh Fu Yen* Department of Civil and Environmental Engineering University of Southern California, Los Angeles, California 90089 Received March 7, 2000. Revised Manuscript Received August 21, 2000

A feasibility study of the decomposition and demetalation of metalloporphyrins by ultrasonic irradiation process in the presence of radical-induced reactions is presented in this paper. Two representative model compounds, NiTPP and VOTPP, were investigated in this ultrasonic process on the laboratory scale. The extent of the decomposition was determined by UV-vis, while the metal analysis was measured by ICP/MS. In the initial investigation, the decomposition of metalloporphyrins, which were dissolved in different solvent-water mixtures, was performed under the ultrasonication process. Among these solvents, the chlorinated-type solvents (e.g., chloroform and dichloromethane) achieved a higher efficiency because they generated more oxidizing species under sonication at 20 kHz frequency. Other additives such as surfactant and hydrogen peroxide, which affect the decomposition efficiency, were also investigated. Under optimal condition, the decomposition efficiency reached about 90% in 1 h for both model compounds. An oxidative intermediate existed for both metalloporphyrins under ultrasonication. The decomposition reaction rates of these two compounds followed pseudo-first-order in reactant concentration and were inhibited by initial feed concentration. The dependence of the rate constants on the different initial concentrations could be determined by the LangmuirHinshelwood equation.

Introduction An increase in energy consumption has initiated the search for an alternative method for the refining of petroleum.1-6 To increase the efficiency of refining processes, demetalation of heavy crudes is necessary. Currently, catalytic thermal cracking is the most effective method for industrial refining practices in converting heavier oils into lighter and value-added products. However, this application has become less successful due to catalyst deactivation by metal complexes. Nearly all of these heavy oils contain a high proportion of contaminants. Metal is one of the major contaminants, usually ranging from 1 to 10 000 ppm.7-8 Among these trace metals, vanadium and nickel are the most abun* Corresponding author. (1) Yen, T. F. Upgrading through Cavitation and Surfactant, Proceedings of the 15th World Petroleum Congress, Forum 17; John Wiley: London, 1997. (2) Yen, T. F. Correlation between Heavy Crude Sources and Types and Their Refining and Upgrading Methods, Proceedings of the 7th UNITAR International Conference on Heavy Crude and Tar Sands; Petroleum Industry Press: Beijing, China, 1998; Vol. 2, pp 2137-2144. (3) Yen, T. F. Environmental Aspects of Petroleum Production, Transportation, Refining and Marketing, Proceedings of the 3rd International Petroleum Conference; Indian Oil Corp. Ltd.: New Delhi, India, 1999. (4) Yen, T. F. Future Sources of Heavy Crude and their Production and Upgrading Technology, Preprints; American Chemical Society Division of Fuel Chemistry, American Chemical Society: Washington, DC, 1999; Vol. 44, pp 76-79. (5) Yen, T. F. Energy 1999, 24, 41-42. (6) Lykhterova, N. M.; Lunin, V. V. Khim. Tekhnol. Topl. Masel (Moscow) 1998, 6, 3-5. (7) Yen, T. F. The Role of Trace Metals in Petroleum; Ann Arbor Science: Ann Arbor, MI, 1975.

dant in native petroleum.7 These metals are mostly in forms of metal-porphyrins and metal-nonporphyrins, which are present in native petroleum, especially in the heavy fractions of crude oil. Metal-porphyrins are typical metal complexes of porphyrin ligands. Metal-nonporphyrins can be metal complexes of tetradentate mixed ligands, highly aromatic porphin chelates, or porphyrin decomposed ligands that have lost the physical properties of typical porphyrins due to interrupted conjugation. The percentage of metal complexes as metalloporphyrin forms ranges from 5% to 34% for many crudes.7,9 Three types of metalloporphyrins found in petroleum are DPEP type, Etio type, and Rhodo type.10-11 These metal contaminants are also the major constituents of environmental pollutants emitted from oilfired power plants and the cause of the corrosion of equipment and poisoning of processing catalysts. Metalloporphyrins, especially when the metals are vanadium or nickel, are extremely stable and are environmentally recalcitrant. They exist in petroleum through geological ages and are referred to as geological biomarkers. The release of vanadium and nickel from (8) Speight, J. G. The Desulfurization of Heavy Oils and Residua; Marcel Dekker: New York, 1981. (9) Dean, R. A.; Whitehead, E. V. Proceedings of the Sixth World Petroleum Congress, Section V; Frankfurt/Main, 1963; World Petroleum Congress: London, 1963; Paper 9. (10) Baker, E. W.; Yen, T. F.; Dickie, J. P.; Rhodes, R. E.; Clark, L. F. J. Am. Chem. Soc. 1967, 89, 3631-3639. (11) Reynolds, J. G. In Asphaltenes and Asphalts, 2, Developments in Petroleum Science; T. F. Yen, T. F., G. V. Chilingarian, G. V., Eds.; Elsevier Science Publisher: Amsterdam, 2000; Chapter 3, pp 29-58.

10.1021/ef000044t CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000

Decomposition and Demetalation of Metalloporphyrins

metalloporphyrins requires a very strong acid, such as methanesulfonic acid or trifluoro acetic acid. A strong acid not only removes metal from porphyrinic ligand, but also produces a free base porphyrin where two hydrogen ions displace one central metal atom. Although a variety of methods have been investigated with the intention to remove metals from the macrocyclic porphyrin ligands,12-15 yet no industrial pretreatment process of demetallization is available. The chemical assisted ultrasound method has been applied to the degradation and decomposition for a great variety of organic substances.16-22 In principle, this is a most useful method for petroleum upgrading and refining.23-42 However, ultrasound approach for decomposition of metallo compounds in petroleum has not (12) Sugihara, J. W.; Garvey, R. G. Anal. Chem. 1964, 36, 2374. (13) Kimberlin, C. N.; Ellert, H. G.; Adams, C. E.; Hamner, G. P. U.S. Patent 3,203,892, 1965. (14) Eisner, U.; Harding, M. J. C. J. Chem. Soc. 1964, 4089. (15) Tomita, F.; Yokota, A.; Saito, K.; Yamamoto, C.; Abe, A. Pseudomonas for Removal of Porphyrins from Fossil Fuel. Jpn. Kokai Tokkyo Koho 1997, 8 95-342983, 1995. (16) Chen, J. R.; Xu, X. W.; Lee, A. S.; Yen, T. F. Environ. Technol. 1990, 11, 829-836. (17) Park, J. K.; Yen, T. F. In In-situ and On-site Bioreclamation; Battelle Press: Columbus, OH, 1995; Vol. 7 (Bioremediation of Recalcitrant Organics), pp 31-39. (18) Shiu, F. J. Y.; Yang, E. C. Y.; Yen, T. F. Energy Sources 1997, 19, 833-843. (19) Chang, H.-L.; Yen, T. F. Ultrasound Degradation of MTBE and TBA. Paper presented at the 217th ACS Meeting, Division of Colloid and Surface Chemistry, 1999. (20) Joseph, J. M.; Destaillats, H.; Hung, H.-M.; Hoffmann, M. R. J. Phys. Chem. A 2000, 104, 301-307. (21) Destaillats, H.; Hung, H.-M.; Hoffmann, M. R. Environ. Sci. Technol. 2000, 24, 311-317. (22) Chang, H.-L.; Yen, T. F. An Improved Chemical-Assisted Ultrasound Treatment for MTBE. Paper presented at the 2nd International Conference on Chlorinated and Recalcitrant Compounds, Monterey, CA, 2000. (23) Kuo, J.; Sadeghi, K.; Palmer, R. B.; Sadeghi, M.; Yen, T. F.; Jang, L. K. A New Extraction Technology for Tar Sand Production, Proceedings, Future of Heavy Crude and Tar Sands, 3rd International Conference; UN Institute of Technology and Research: Long Beach, 1985; Vol. 1, pp 452-465; also in the 3rd UNITAR/UNDP International Conference on Heavy Crude and Tar Sands; AOSTRA: Edmonton, Alberta 1988; Vol. 70, pp 739-747. (24) Lee, A. S.; Sadeghi, M.-A.; Yen, T. F. The Role of Peroxides Relating with Asphaltic Oil in Micelle Inversion Process, Proceedings, 21st Intersociety Energy Conversion Engineering Conference; American Chemical Society: Washington, DC, 1986; Vol. 3, pp 257-261. (25) Kuo, J. F.; Sadeghi, K.; Jang, L. K.; Sadeghi, M.-A.; Yen, T. F. Appl. Phys. Comm. 1986, 6, 205-212. (26) Lee, A. S.; Xu, X. W.; Yen, T. F. Paper No. 135. Preprints, UNITAR/UNDP 4th International Conference on Heavy Crude and Tar Sands; Alberta Oil Sands Technology and Research Authority: Edmonton, Canada, 1988; pp 135-1 to 135-9; also Proceedings of the 4th UNITAR/UNDP International Conference on Heavy Crude and Tar Sands; Extraction, Upgradings, Transportation; Alberta Oil Sands Technology and Research Authority: Edmonton, Canada, 1989; Vol. 5, pp 109-116, 145-146. (27) Sadeghi, M.-A.; Sadeghi, K.; Kuo, J. F.; Jang, L. K.; Yen, T. F. U.S. Patent 4,765,885, 1988. (28) Sadeghi, K. M.; Sadeghi, M.-A.; Momeni, D.; Yen, T. F. Advances in Oil-Field Chemistry: Enhanced Recovery and Production Stimulation. ACS Symp. Ser. 1989, 396, 393-409. (29) Sadeghi, K. M.; Sadeghi, M. A.; Chilingarian, G. V.; Yen, T. F. Geol. Nefti Gaza (Moscow) 1988, 8, 53-57. Also appeared in Khim Tekhnol. Topl. Masel (Moscow) 1988, 8, 24-28; English Translation in Chem. Technol. Fuels Oils 1989, 1-2, 3-11. (30) Sadeghi, K. M.; Sadeghi, M.-A.; Kuo, J. F.; Jang, L.-K.; Yen, T. F. Energy Sources 1990, 12,, 147-160. (31) Sadeghi, M.-A.; Sadeghi, K. M.; Kuo, J.-F.; Jang, L.-K.; Yen, T. F. U.S. Patent, 4, 891, 131, 1990. (32) Sadeghi, K. M.; Sadeghi, M.-A.; Blazquez, M.; Yen, T. F. An. Quim. (Madrid) 1990, 86, 175-181. (33) Esfandiari, R. S.; Sloss, J. M.; Sadeghi, K. M.; Yen, T. F. Fuel Sc. Technol. Int. 1991, 9, 537-548. (34) Sadeghi, K. M.; Sadeghi, M.-A.; Yen, T. F. Energy Fuels 1990, 4, 604-608. (35) Sadeghi, M. A.; Sadeghi, K. M.; Kuo, J. F.; Jang, L. K.; Yen, T. F. U.S. Patent 5017, 281 1991.

Energy & Fuels, Vol. 14, No. 6, 2000 1169

been investigated. This forms the thrust of present study. In brief, the method is based on the production of hydroxyl free radicals from the cavitation centers formed by an aqueous organic emulsion system under ultrasound. The use of additives such as hydrogen peroxide (initiator of hydroxyl free radicals) and surfactant (builder of orderly structures of micelles and vesicles) can enhance the efficiency of reactions greatly due to the principle of membrane-mimetic chemistry.53 Radical generation can occur in three different regions through ultrasonic cavitation.54 The first region involves the interior collapsing cavitation bubbles, where extreme temperature and pressure exist transiently. In this region, free radicals are generated from water molecules. The second region is the interfacial region between the collapsing bubbles and the bulk solutions where thermal dissociation and radical-induced reaction exist simultaneously.55 Surfactants behave as phase transfer agents between the oxidants in aqueous phase and the porphyrins in organic solvent phase. In the third region is the formation of ions and radical ions. In this paper, the feasibility of ultrasonic irradiation was applied to investigate its effectiveness for the decomposition of metalloporphyrins. This method is based on the considerations of a mild noncatalytic operation. A preliminary study of the mechanism and kinetics for the decomposition reactions is also included. Experimental Section Chemicals. Two model compounds of metalloporphyrins used in this ultrasonic decomposition and demetalation investigation were vanadyl tetraphenyl porphyrin (VOTPP, 97% (36) Yen, T. F.; Lin, J. R. Heavy Crude and Tar Sands? Hydrocarbons for the 21st Century; Upgrading, Government and Environment, UNITAR: New York, 1991, Vol. 4, pp 75-81; Discussions, 1991, Vol. 5, pp 174-175, errata p 213. (37) Sadeghi, M. A.; Sadeghi, K. M.; Kuo, J. F.; Jang, L. J.; Yen, T. F. Can. Patent 1283, 879, 1991. (38) Lian, H.; Yen, T. F. Asphaltenes: Fundamentals and Applications; Plenum Press: New York, 1996; pp 177-189. (39) Sadeghi, K. M.; Sadeghi, M. A.; Kuo, J. F.; Jang, L. K. Jang; Lin, J. R.; Yen, T. F. Chem. Eng. Comm. 1992, 117, 191-203. (40) Lin, J. R.; Yen, T. F. Energy Fuels 1993, 7, 111-118. (41) Lin, J. R.; Park, J. K.; Yen, T. F. Asphaltene Particles in Fossil Fuel Exploration, Recovery, Refining and Production Process; Plenum Press: New York, 1993; pp 91-100. (42) Sadeghi, K. M.; Lin, J. R.; Yen, T. F. Energy Sources 1994, 16, 439-449. (43) Agrawal, R.; Wei, J. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 505-514. (44) Suslick, K. S. Ultrasound: Its Chemical, Physical and Biological Effects; VCH: New York, 1988. (45) Nahor, G. S.; Neta, P.; Hambright, P.; Robinson, L. R. J. Phys. Chem. 1991, 95, 4415-4418. (46) Renner, M. W.; Buchler, J. W. J. Phys. Chem. 1995, 99, 80458049. (47) Seth, J.; Palaniappan, V.; Bocian, D. F. Inorg. Chem. 1995, 34, 201-2206. (48) Bonnett, R.; Dimsdale, M. J. J. Chem. Soc., Perkin Trans 1972, 1, 2540. (49) Mason, T. J.; Lorimer, J. P. Sonochemistry: Theory, Application and Use of Ultrasound in Chemistry; Ellis Horwood Ltd.: Chichester, UK, 1988. (50) Alippi, A.; Cataldo, F.; Galbato, A. Ultrasonics 1992, 30, 3, 148151. (51) Orzechowska, G. E.; Poziomek, E. J.; Hodge, V. F.; Engelmann, W. H. Environ. Sci. Technol. 1995, 29, 1373-1379. (52) Chen, J. R. Ph. D. dissertation, The Feasibility Studies of Radical-Induced Dehalogenation Processes: Distribution of Chloroform, PBB and PCB; University of Southern California: Los Angeles, CA, 1998. (53) Yen, T. F.; Gilbert, R. D.; Fendler, J. H. Membrane Mimetic Chemistry and Its Applications; Plenum Press: New York, 1994. (54) Henglein, A. Ultrasonics 1987, 25, 6-16. (55) Krishna, C. M.; Lion, Y.; Kondo, T.; Riesz, P. J. Phy. Chem. 1987, 91, 5847-5850.

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Table 1: The Properties and Structures of NiTPP and VOTPP, Including UV-vis Absorptions

purity) and nickel tetraphenyl porphyrin (NiTPP, 97% purity), which were purchased from Strem Chemical, Inc. (Newburyport, MA). The structures of VOTPP and NiTPP and some of their properties, which include UV-vis spectra, are listed in Table 1. These compounds usually stay in organic solvent phase but can well partition in micelles in an aqueous suspension. All of the other chemicals were of high purity and commercially available. Sample Suspensions. Sample suspension of VOTPP and NiTPP for different solvents was prepared as follows. A stock solution of organic phase of 1000 mg/L solution was prepared by adding 0.25 mg of model compound to a given solvent until reaching to a total volume of 250 mL. Since a significant fraction of these metalloporphyrin compounds were easily oxidized into nonporphyrin forms,43 argon gas at 5 psig was introduced to the solutions to maintain an inert environment. The mixture was stirred for 2 h, and then filtered through 5 µm filter papers to remove all the undissolved model compounds. The filter papers were then weighed to obtain the solubility limit of each of the model compounds for the required solvents. Next, the aqueous phase was prepared by using deionized and distilled (D/D) water of pH∼6.8. The ratio of the organic phase to the aqueous phase was always 1:10 (v/v). Both additives (surfactant and hydrogen peroxide) were added to the required D/D water phase in such a manner that 1:10 (v/v) was maintained. In the study of the surfactant the maximum concentration used was Span 20, 0.20% (v/v) in water; 30% hydrogen peroxide, 3.68% (v/v) in water. The volume concentrations were adjusted to the total volume of water used. In all experiments, initial concentrations were prepared only for organic solvents in which either VOTPP or NiTPP was dissolved. For example, 5 mL of chloroform solution containing 500 mg/L of the VOTPP was mixed into 50 mL of D/D water phase containing the given quantities of surfactant and hydrogen peroxide (v/v) in a 150 mL flask. Similarly, other set of experiments were conducted in this manner except the initial experiment was used to search for the best solvent. For most of the experiments both the surfactant, Span 20 (0.05%, v/v) and the hydrogen peroxide (2.7% v/v), adjusted to the water phase, were used. Ultrasound Irradiation. The 150 mL flask containing the solution then was inserted to the ultrasonic processor assembly. The horn probe of ultrasound was directly immersed into the reaction vessel at the same depth (about 1 cm) for every run. The ultrasonic irradiation was performed with an ultrasonic processor VCX-600 (Sonics & Materials, Inc.) at 60% of amplitude and at constant frequency of 20 kHz. A stream of 3 psig of argon gas was introduced into the sonication reactor

to achieve a higher decomposition efficiency.44 The temperature of mixture solution was maintained constant at 20 °C by circulating cooling water throughout all of the reactions. For each run, at least three replicates of each sample and blank were sonicated and measured. Analytical Methods. After ultrasonic reactions, chloroform was used to extract the unreacted metalloporphyrins using separation funnel with shaking. Two layers appeared after a short period of time; the chloroform with metalloporphyrins remained in the bottom layer. After 20 min, the CHCl3 layer in the separation funnel was collected and analyzed by a Hewlett-Packard 8452A spectrophotometer to determine the change in concentration with time. Table 1 also summarizes a UV-visible absorption feature of free metal parent porphyrin, tetraphenyl porphyrin (TPP), NiTPP, and VOTPP. Prior to analysis, anhydrous sodium sulfate powder was used to absorb the water content to avoid interference with UV-visible analysis. For metal analysis, 0.1 M NaOH and 0.18 M of EDTA (kept solution in pH ) 11) were added to the solutions prior to the extraction with chloroform. Following separation, the aqueous phase in the upper layer was collected for metal analysis by a Hewlett-Packard L500 ICP-MS.

Results and Discussion Solvents Effect. As shown in Table 1, both NiTPP and VOTPP are in crystalline form at room temperature. To test the feasibility of the decomposition for these two target compounds under ultrasonic irradiation, five solvents, dioxane, pyridine, toluene, dichloromethane, and chloroform were selected as dissolvers and the resulting suspensions with D/D water were irradiated by ultrasound. On the basis of initial investigations, two chlorine types (chloroform and dichloromethane) were more efficient as solvents than the other three in this ultrasonic decomposition process because they contained more oxidizing species with chloroform being the best. Since the primary chemical effect of ultrasound is acoustic cavitation, then the type of solvent used for cavitation reaction is important.19-22 For example, when chloroform (or dichloromethane) is used as a solvent and emulsified in D/D water, the hydroxyl radicals (OH•) and some other radicals will be generated from water molecules and chloroform under thermal dissociation by ultrasonic cavitation. The possible mechanisms of free radical reactions involved in


Figure 1. Effect of H2O2 on decomposition efficiency of targets in chloroform by ultrasound. (0.05% of Span20; 40 min sonication time at 20°C)

water/chloroform mixture under ultrasonic irradiation are described below in Equations 1-5.49-51 The generation of HCl causes OH•, Cl•, CCl3•, and Cl2 to serve as strong oxidants to efficiently oxidize metalloporphyrins in a low pH environment. )))

H2O 98 OH•+ H• )))

CHCl3 98 Cl• + CHCl2• )))

CHCl3 98 CCl3• + H• •

•

(1) (2a) (2b)

CHCl3+ OH f CCl3 + H2O

(3a)

CHCl3+ OH• f CHCl2• + HOCl

(3b)

CHCl3+ Cl• f CCl3• + HCl

(4)

•

2 Cl f Cl2

(5)

Additives Effect. Both hydrogen peroxide and surfactant are the additives to the cavitation reaction. The use of hydrogen peroxide has a beneficial effect. Hydroxyl radicals are known to be powerful and efficient chemical oxidants as well as having a greater oneelectron oxidation potential (approximately 2.7 V) in a low pH environment. Hence, hydroxyl radicals are extremely reactive and have a high potential to break carbon-chlorine and carbon-carbon double bonds.17 Hydroxyl radicals can be generated directly from water molecules (eq 1) or from additional hydrogen peroxide in ultrasonic cavitation system. As the experimental data point out in Figure 1, the addition of hydrogen peroxide enhances the decomposition efficiencies of target compounds either due to a higher concentration of hydroxyl radicals, or because of indirect reactions with more chloroform decomposition products. As indicated in Figure 1 the 2.7% (v/v of water) of hydrogen peroxide can help to increase the decomposition efficiency up to 8% and 10% for VOTPP and NiTPP, respectively. A decrease in efficiencies may be due to the scavenging effect of hydroxyl radicals themselves. Surfactant (Span 20) was used to reduce the interfacial tension and to enhance the transport between the two phases (chloroform and water). Surfactant can also rearrange the vesicles and help the micelles to disperse. Thus, the free radicals are easier to transfer from the


Figure 2. UV-vis absorption spectra of (a) pure NiTPP, and two runs collected during (b) 10 min and (c) 20 min of sonication.

water phase to organic solvent phase and to stay in the micelles.53 In this investigation, the decomposition efficiency similar to the plot of Figure 1 has indicated that it can be enhanced to about 15% by adding 0.05% (v/v of water) of Span 20 into this system. Similarly, the surfactant behaves as a phase transfer agent facilitating organometallics for transport to the micelles Overall Disappearance of NiTPP and VOTPP. The overall disappearance of NiTPP is shown in Figure 2, which displays an intensive absorption peak at a wavelength of 526 nm for pure NiTPP (see also Table 1) in chloroform and for the ones collected after 10 and 20 min irradiation time by ultrasound (see Figure 2a). The latter two samples (see Figure 2b,c) show the new peak at 622 nm, which is equivalent to the intermediate produced by the oxidation of NiTPP.45 In Figure 3, which shows the absorbance-time profiles for NiTPP with initial concentration of 100 mg/L, the quantity of intermediate increased to a maximum (at about 20 min) and then decreased during sonication time. This intermediate is most likely a π-cation radical (TPP•)+ as previously suggested.45-47 Cation radicals have been shown to be intermediates in reactions of metalloporphyrins with hydrogen peroxide.48 Furthermore, similar experimental trends were found in the decomposition of VOTPP with an intensive absorption peak at a wavelength of 548 nm for pure VOTPP (see Table 1) and a new peak at 650 nm during sonication process. Clearly in this case a π-cation radical has also been formed. Optimal Conditions. During the experimental work, the following optimum conditions for the removal of metals from metalloporphyrin complex were found. Most runs followed the conditions described below: solvent type MTPP concentration organic solvent water phase ratio span 20 content hydrogen peroxide content reaction temperature ultrasound intensity ultrasound frequency argon gas

chloroform 100-1000 mg/L (initial concentration) 1:10 (v:v) 0.05% (v:v of water) 2.7% (v:v of water) 20°C (inside the reactor) 60 W/cm2 20 kHz 3 psig

Metal Release. After sonication reaction, 0.02 M of EDTA (at pH ) 11) was introduced into the system to recover metals of any chemical form in the aqueous phase. Table 2 gives the mass balance of metal for NiTPP and VOTPP after the maximum reaction time. The results, which are shown in the last two columns,

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Figure 3. Absorbance-time profiles for NiTPP and intermediate during ultrasonic irradiation under optimal condition, 100 mg/L of initial concentration. Table 2 The Mass Balance of Metal for NiTPP and VOTPP Runs after Ultrasonic Process under Optimal Condition samples

metal content before sonicationa (mg) (1)

metal content in water phase after sonicationb (mg) (2)

metal recoveryc (%)(3)

sample compound decomposedd(%) (4)

NiTPP VOTPP

0.0516 0.0408

0.0463 0.0352

89.7 86.3

92.9 88.8

a 100 mg/L of sample in chloroform was analyzed by ICP/MS. bSonication time is 60 min for NiTPP and 80 min for VOTPP, analyzed by ICP/MS by EDTA recovery from aqueous phase. cCalculated from columns 1 and 2 obtained by ICP/MS. dMeasured by UV-vis (for comparison).

indicate that the percentages of the total metal recovery approximate to decomposition efficiencies within approximately 3% of experimental errors. It is believed that the macrocyclic ring in metalloporphyrin’s structure can be cleaved and metal can be completely released under ultrasonic process with chloroform/water emulsion. In this manner, metals can be recovered. Preliminary Kinetic Analysis. On the basis of the above observations and the studies of sonolytic decomposition of chloroform,49-52 a hypothetical mechanism (eq 6) may account for the ultrasonic decomposition of metalloporphyrin (MTPP). Through the formation of an intermediate complex, MTPP can be oxidized by oxidants, which are produced from chloroform/water by ultrasound. This cation radical intermediate can then be disintegrated into colorless products. At the same time, metal is released from tetrapyrrol’s center following the disappearance of the intermediate.

MTPP + oxidants 9 8I9 8P+M k k 1

2

(6)

where MTPP is metalloporphyrin, k1 is rate constant of disappearance of MTPP, k2 is rate constant of further decomposition of intermediate, I is oxidative intermediate, P is colorless products, and M is metal. The kinetics of the disappearance of metalloporphyrins can be determined as:

cation, in the presence of excess chloroform, it can be assumed that the concentrations of oxidants remain essentially constant throughout the reaction. Therefore, eq 7 can be reduced as follows:

d[MTPP] ) -k'1 [MTPP]m dt

(8)

Figure 4 shows the first-order plots of representative kinetic runs for NiTPP samples with varied initial concentrations of NiTPP from 100 to 500 mg/L at optimal conditions as described. The initial pseudo-firstorder rate constants (k′1) for target compounds are given in Table 3. It is observed that at higher concentrations reaction rate is retarded. For reactions carried out under membrane-mimetic conditions, the target compounds often are converted by surfactant to be on the micellar surface by adsorption process. The ensuing surface reaction is simply an oxidation similar to Langmuir-Hinshelwood type.56

-

dC KLHkLHC ) dt 1 + kLHC

(9)

(7)

where C is feed concentration, t is time, KLH is the intrinsic rate constant, and kLH is the adsorption coefficient. Note in eq 9, at low C kLHC , 1, the rate is proportional to the concentration of targeting compounds. By contrast at large C, kLHC . 1 and the rate

Although chloroform can be decomposed by ultrasoni-

(56) Steinfeld, J. I.; Francisco, J. S.; Hase, W. L., Chemical Kinetics and Dynamics, 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 1999.

d[MTPP] ) -k1[MTPP]m[oxidants]n dt



Figure 4. Pseudo-first-order plot for the decompostion of NiTPP under ultrasonic irradiation at optimal condition.

Figure 5. Rate constant analysis of VOTPP by Langmuir-Hinshelwood kinetics with reactant inhibition model and kLH ) 0.004. Table 3: Initial Pseudo-first-order Rate Constants for Several Initial Concentrations of Targets samples NiTPP VOTPP

k′1(hr-1) at various initial concentrations (mg/L) 100 2.670 1.620

200 2.232 1.368

300 1.662 1.128

500 1.380 0.846

Table 4: The Rate Parameters from Equation 9 samples

KLH ppm (hr)-1

kLH (ppm)-1

KLHkLH (hr)-1

deviation (%)

VOTPP NiTPP

472.8 647.9

0.004 0.0046

1.891 2.980

3.88 2.62

becomes independent of C. This reactant inhibition in membrane-mimetic fashion can also fit the Eley-Rideal model. Figure 5 shows that the rate constants of VOTPP can be easily modified to be independent of the initial concentration and fit the Langmuir-Hinshelwood kinetic model. The resultant values of targeting compounds for KLH and kLH are given in Table 4 by the

integration of eq 9.

C ln + kLH(C - Co) ) -KLHkLHt Co

(10)

Furthermore, verification of ultrasonic decomposition of metalloporphyrin is a stepwise sequential process, which can be made evident. For example, eq 6 can be expressed as:

d[MTPP] ) -k1[MTPP] dt

(11)

d[I] ) k1[MTPP] - k2[I] dt

(12)

d[M] ) k2[I] dt

(13)

Hence, 100 mg/L concentration under optimum conditions was carried out for both VOTPP and NiTPP. After solving the consecutive reaction of Equations 11-13 and

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Figure 6. Comparison of laboratory observed data with model predicted values in ultrasonic decomposition of VOTPP. I is intermediate; M is metal. k1 and k2 are expressed in min-1.

Figure 7. Possible reaction pathways for decompositions of metalloporphyrin in radical-induced ultrasonication. (us, ultrasonication; DP, dipyrrolic compounds; HC, hydrocarbons)

fitting the following computed lines to the experimental laboratory data, one obtains a fairly good comparison (Figure 6) using eqs 14-16.

[MTPP] ) [MTPP]0 exp(-k1t) [I] )

(14)

k1 [MTPP]0[exp(-k1t) - exp(-k2t)] (15) k2 - k1

[

[M] )[MTPP]0 1 +

k1 exp(-k2t) (k2 - k1)

-

]

k2 exp(-k1t) (k2 - k1)

(16)

In Figure 6, the reaction rate of first step (k1) is lower

than that of the second step (k2). The overall reaction velocity is controlled by the first step and it becomes rate limiting. From the studies so far, a tentative reaction pathway for the degradation by ultrasound can be outlined in Figure 7. Because of the uniqueness of chloroform, the generation of free radicals such as Cl•, CCl3•, and CCl3O2• may serve as strong oxidants in the k1 step of eq 6. Sonolytic decomposition of chloroform forms hypochlorous acid, HCl and CO2. In the future, the use of chloroform will be reduced to a minimal quantity in the emulsion. The results of the experiments indicate that the cavitation process by ultrasound is quite effective


for the decomposition of metalloporphyrins in chloroform/ water systems. The additions of Span 20 and hydrogen peroxide in adequate amounts can increase the decomposition efficiency, whereas an excess amount of hydrogen peroxide has a retardation effect. Metals released from these metallic compounds are also observed as occurring by consecutive reactions with an oxidation reaction intermediate. The rates of disappearance of metalloporphyrins are pseudo-first-order in concentrations of MTPP and inhibited by initial feed concentrations of MTPP. The current feasibility study can be


extended to the decomposition of other metal complexes such as copper, nickel, and iron phthalocyanine in the pigment industry. Acknowledgment. The authors are grateful to the reviewers for this paper. Thanks are due to Dr. J. G. Reynolds for his help in the improvement of this paper. The technical assistance of Rochelle Wong, Patricia Menjivar, and Dawood Momeni is also greatly appreciated. EF000044T

The Feasibility Studies for Radical-Induced Decomposition and

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