Electroanalysis of Crude Oil and Petroleum-Based Fuel for Trace Metals

Energy & Fuels 2007, 21, 295-302

295

Electroanalysis of Crude Oil and Petroleum-Based Fuel for Trace Metals: Evaluation of Different Microwave-Assisted Sample Decompositions and Stripping Techniques Rodrigo A. A. Munoz,† Paulo R. M. Correia,‡ Angerson N. Nascimento,† Cı´ntia S. Silva,† Pedro V. Oliveira,† and Lucio Angnes*,† Departamento de Quı´mica Fundamental, Instituto de Quı´mica, UniVersidade de Sa˜ o Paulo, AV. Prof. Lineu Prestes, 748-05508-900 Sa˜ o Paulo SP, Brazil, and Escola de Artes, Cieˆ ncias e Humanidades, UniVersidade de Sa˜ o Paulo, AV. Arlindo Bettio, 1000-03828-000 Sa˜ o Paulo SP, Brazil ReceiVed August 18, 2006. ReVised Manuscript ReceiVed October 26, 2006

Electroanalytical stripping techniques have been well-used for trace-metal determinations, because of their remarkable sensitivity and selectivity. However, when these techniques are applied for organic materials, such as crude oil and petroleum-based fuels, the samples must be decomposed. This paper evaluates the use of different microwave ovens for the decomposition of crude oil and diesel fuel to determine the content of copper, lead, mercury, and zinc in the digestates. A focused-microwave (FM) oven using H2SO4/HNO3/H2O2, operated at atmospheric pressure, and a closed-vessel microwave (CVM) oven using HNO3/H2O2, operated under pressure in a vessel, were evaluated. Square-wave stripping voltammetry (SWSV) and stripping chronopotentiometry (SCP) at gold film electrodes were applied for copper, lead, and mercury. Potentiometric stripping analysis (PSA) at mercury film electrodes was applied for copper, lead, and zinc. SWSV was more affected by residual organic matter, especially for lead determination. SCP presented higher sensitivity for copper and mercury at gold electrodes. PSA at mercury electrodes was preferred for lead and zinc determination. Better detection limits were attained for FM-digested solutions, after 0.8-1.0 g of sample can be digested, in contrast to the low quantities (0.10-0.25 g) used when pressurized vessels were explored. Nevertheless, the loss of mercury was verified when samples were decomposed in the FM oven.

1. Introduction Trace-metal determinations in crude oils and petroleum products are important in the oil industry, and their demand has increased. Crude oil contains several metals as organometallic complexes,1 and information on these trace elements is important to geological (crude oil origin) and environmental studies (anthropogenic emission of petroleum-based fuels).2 The poisoning of catalysts in the petroleum cracking process and the corrosion of equipment because of the presence of trace metals in oils also has been widely reported.1-5 The mercury content in crude oil is of capital importance for petroleum industries, because the metal can deposit in the equipment, which consequently prejudices maintenance operations6,7 and can cause severe injuries to exposed workers.8 Moreover, because environmental agencies do not control the mercury emisson in the * Author to whom correspondence should be addressed. E-mail address: [email protected]. † Departamento de Quı´mica Fundamental, Instituto de Quı´mica, Universidade de Sa˜o Paulo. ‡ Escola de Artes, Cie ˆ ncias e Humanidades, Universidade de Sa˜o Paulo. (1) Milner, O. I.; Glass, J. R.; Kirchner, J. P.; Yurick, A. N. Anal. Chem. 1952, 24, 1728-1732. (2) Al-Swaidan, H. M. Talanta 1996, 43, 1313-1319. (3) Doukkali, A.; Saoiabi, A.; Zrineh, A.; Hamad, M.; Ferhat, M.; Barbe, J. M.; Guilard, R. Fuel 2002, 81, 467-472. (4) Khuhawar, M. Y.; Lanjwani, S. N. Talanta 1996, 43, 767-770. (5) Hansen, J.; Hodgkins, C. R. Anal. Chem. 1958, 30, 368-372. (6) Wilhelm, S. M.; Liang, L.; Kirchgessner, D. Energy Fuels 2006, 20, 180-186. (7) Liang, L.; Horvat, M.; Fajon, V.; Prosenc, N.; Li, H.; Pang, P. Energy Fuels 2003, 17, 1175-1179. (8) Wilhelm, S. M.; Bloom, N. Fuel Process. Technol. 2000, 63, 1-27.

atmosphere, the metal content in petroleum-based fuels needs to be monitored. Molecular absorption spectrophotometry,1 atomic absorption spectrometry (AAS),9,10 inductively coupled plasma-optical emission spectrometry (ICP-OES),11 inductively coupled plasma-mass spectrometry (ICP-MS),2,12 high-performance liquid chromatography (HPLC),4 gas chromatography (GC),13 and X-ray fluorescence spectroscopy (XFS)14 methods have been reported for the determination of metals in crude oils. Electroanalytical techniques were applied for trace-metal determination in crude oils only after the sample decomposition.15 The digestion procedure is necessary to release the metals from the sample matrix and to eliminate the organic matter, which interferes with electroanalysis.15 The traditional digestion procedures (dry ashing and wet digestion) are time-consuming and are more susceptive to external contamination and loss of volatile elements.16 The use of a microwave oven to improve and accelerate the decomposition process has been increas(9) Langmyhr, F. J.; Aadalen, U. Anal. Chim. Acta 1980, 115, 365368. (10) Runnels, J. H.; Merryfield, R.; Fisher, H. B. Anal. Chem. 1975, 47, 1258-1263. (11) Fabbe, J. L.; Ruschak, M. L. Anal. Chem. 1985, 57, 1853-1863. (12) Lord, C. J. Anal. Chem. 1991, 63, 1594-1599. (13) Dilli, S., Patsalides, E. Anal. Chim. Acta 1981, 128, 109-119. (14) Vilhunen, J. K.; Bohlen, A.; Schmeling, M.; Klockenkamper, R.; Klockow, D. Spectrochim. Acta B 1997, 52, 953-959. (15) Tamrakar, P. K.; Pitre, K. S. Indian J. Chem. A 2000, 39, 779783. (16) Turunen, M.; Peraniemi, S.; Ahlgren, M.; Westerholm, H. Anal. Chim. Acta 1995, 311, 85-91.

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296 Energy & Fuels, Vol. 21, No. 1, 2007

ing.17,18 Microwave-assisted methods for the decomposition of crude oil and related products were described through the application of closed-vessel microwave (CVM) ovens equipped with pressurized vessels19-23 and focused-microwave (FM) radiation at atmospheric pressure.24-29 However, only two microwave digestion methods of these studies were applied for electroanalysis: both works use mercury22 or mercury film electrodes for trace-metal determinations.29 The electroanalytical technique usually applied for trace-metal determination in several matrices is the anodic stripping voltammetry (ASV), because of its remarkable sensitivity and selectivity.30 The reason for few electroanalytical methods for crude oil analysis is the required efficiency of the sample decomposition. The incomplete decomposition of organic samples with HNO3 generates organic nitro-compounds and other species, which are electroactive and can interfere with ASV determinations.31-33 To overcome the interferences from organic matter, Jagner developed the potentiometric stripping analysis (PSA) method.34 The preconcentration step, which involves the application of a reduction potential to accumulate the analyte on the electrode surface, is similar to the ASV technique. The stripping step (reoxidizing of the electrodeposited element) is performed by a chemical oxidant that is present in the electrolyte. Because no current is applied to reoxidize the analyte, PSA is less succeptible to interference due to organic matter.34 Alternatively, for a chemical oxidant, a constant current (of hundreds of nanoamperes) can be applied in the stripping step, which characterizes the constant-current stripping potentiometry, which is also called stripping chronopotentiometry (SCP). In addition to PSA, SCP was successfully applied for the analysis of samples that contained a significant amount of organics such as biological fluids, by just diluting the samples in an adequate electrolyte.35-38 (17) Jin, Q. H.; Liang, F.; Zhang, H. Q.; Zhao, L. W.; Huan, Y. F.; Song, D. Q. Trends Anal. Chem. 1999, 18, 479-484. (18) Luque-Garcı´a, J. L.; de Luque, de Castro, M. D. Trends Anal. Chem. 2003, 22, 90-98. (19) Bettinelli, M.; Spezia, S.; Baroni, U.; Bizarri, G. J. Anal. Atom. Spectrom. 1995, 10, 555-560. (20) Wondimu, T.; Goessler, W.; Irgolic, K. J. Fresenius J. Anal. Chem. 2000, 367, 35-42. (21) Sanz-Segundo, C.; Hernandez-Artiga, M. P.; de Cisneros, J. L.; Bellido-Milla, D.; Naranjo-Rodriguez, I. Mikrochim. Acta 1999, 132, 8994. (22) Sanz-Segundo, C.; de Cisneros, J. L.; Hernandez-Artiga, M. P.; Naranjo-Rodriguez, I.; Bellido-Milla, D. Bull. Electrochem. 2002, 18, 165172. (23) Liu, J. H.; Sturgeon, R. E.; Willie, S. N. Analyst 1995, 120, 19051909. (24) No´brega, J. A.; Costa, L. M.; Santos, D. M.; Analytica 2002, 1, 32-37. (25) Costa, L. M.; Silva, F. V.; Gouveia, S. T.; Nogueira, A. R. A.; No´brega, J. A. Spectrochim. Acta B 2001, 56, 1981-1985. (26) No´brega, J. A.; Trevizan, L. C.; Arau´jo, G. C. L.; Nogueira, A. R. A. Spectrochim. Acta B 2002, 557, 1855-1876. (27) Sanna, G.; Pilo, M. I.; Piu, P. C.; Tapparo, A.; Seeber, R. Anal. Chim. Acta 2000, 415, 165-173. (28) Costa, L. M.; Silva, F. V.; Gouveia, S. T.; Nogueira, A. R. A.; No´brega, J. A. Spectrochim. Acta B 2001, 56, 1981-1985. (29) Munoz, R. A. A.; Silva, C. S.; Correia, P. R. M.; Oliveira, P. V.; Angnes, L. Microchim. Acta 2005, 149, 199-204. (30) Wang, J. Analytical Electrochemistry; VCH: New York, 1994; pp 44-53. (31) Kopanica, M.; Stara, V. Electroanalysis 1991, 3, 925-928. (32) Danielsson, L.; Jagner, D.; Josefson, M.; Weterlund, S. Anal. Chim. Acta 1981, 127, 147-156. (33) Mader, P.; Szakova, J.; Curdova, E. Talanta 1996, 43, 521-534. (34) Jagner, D.; Granelli, A. Anal. Chim. Acta 1976, 83, 19-26. (35) Munoz, R. A. A.; Angnes, L. Microchem. J. 2004, 77, 157-162. (36) Ostapczuk, P. Clin. Chem. 1992, 38, 1995-2001. (37) Huiliang, H.; Jagner, D.; Renman, L. Anal. Chim. Acta 1987, 202, 117-122. (38) Jagner, D.; Aren, K. Anal. Chim. Acta 1982, 141, 157-162.

Munoz et al.

The purpose of the present work is to compare the use of a focused microwave (FM) oven and a closed-vessel microwave (CVM) oven for the acid decomposition of crude oil and diesel fuel. The performance of both decomposition methods was evaluated through the application of square-wave stripping voltammetry (SWSV) and SCP at gold film electrodes for the determination of copper, lead, and mercury in the digested samples. PSA at mercury film electrodes was used for the determination of copper, lead, and zinc. 2. Experimental Section 2.1. Reagents, Samples, and Materials. Copper, mercury, lead, and zinc reference solutions were prepared from Titrisol standard solutions (Merck, Darmstadt, Germany). High-purity deionized water (R g 18 MΩ) that was obtained from a Milli Q water purification system (Millipore, Bedford, MA) was used to prepare all solutions and samples. Analytical-grade nitric (65%, m/v) and sulfuric acids (97%, m/v), hydrogen peroxide (30%, m/v), sodium chloride, sodium acetate and ammonium monohydrogen phosphate (Merck, Darmstadt, Germany) were used without further purification. Diesel fuel samples were obtained at local gas stations. Brazilian crude oil samples that had been collected by Petrobras Research Center were analyzed. All samples were stored in polyethylene bottles at room temperature. The Multi-Element Oil-Based Standard (Alfa Aesar, MA) reference material, which contains 100 µg/g of copper, lead, and other elements, was analyzed in the same manner. All the glassy and plastic materials used for preparation and storage of the samples and solutions were previously decontaminated with nitric acid (10% v/v) for 24 h. The digested samples were stored in polyethylene vessels at 4 °C. 2.2. Instrumentation. An FM-assisted oven (CEM Star System 2, Matthews, NC) with two cavities and a closed-vessel microwave oven Multiwave 3000 (Anton Paar, Graz, Austria) were used for sample digestions. Electrochemical measurements were performed with an Autolab model PGSTAT20 potentiostat (EcoChemie, Utrecht, The Netherlands). A model ZEEnit 60 (AnalytikjenaAG, Jena, Germany) graphite furnace/atomic absorption spectrometer was used to evaluate the results obtained using SWSV and SCP. Determination of total organic content was performed in an Elemental Analyzer 2400 CHN (PerkinElmer). 2.3. Electrochemical Cell and Electrodes. The electrochemical cell was easily constructed, using 10-mL graduated polypropylene cylinders. These flasks were cut in order to work with 4-mL electrochemical cells (interior volume). A Teflon cover was constructed with the adequate measurements to fit firmly at the top of these flasks, and the three electrodes were positioned into holes exactly designated onto this cover. The low price of these volumetric flasks allowed us to purchase many units and replace them after each analysis. The determination of copper, lead, and mercury in the digested samples was performed using thin film gold electrodes manufactured from compact disks (Mitsui Gold Standard), using the procedure developed in our group.39-46 The electrode area was approximately 5 mm2, delimited with enamel. (39) Augelli, M. A.; Munoz, R. A. A.; Richter, E. M.; Cantagallo, M. I.; Angnes, L. Food Chem. 2007, 101, 579-584. (40) Augelli, M. A.; Munoz, R. A. A.; Richter, E. M.; Gouveia, A.; Angnes, L. Electroanalysis 2005, 17, 755-761. (41) Richter, E. M.; Pedrotti, J. J.; Angnes, L. Electroanalysis 2003, 15, 1871-1877. (42) Richter, E. M.; Augelli, M. A.; Magarotto, S.; Angnes, L. Electroanalysis 2001, 13, 760-764.

Electroanalysis of Oils and Fuels for Trace Metals

Energy & Fuels, Vol. 21, No. 1, 2007 297

Table 1. Heating Program for Focused Microwave (FM) and Closed-Vessel Microwave (CVM) Decompositions of Crude Oil and Diesel Fuel Samples Focused Microwave (FM) Decomposition

step

ramp time (min)

hold time (min)

temp, T (oC)

1 2 3 4 5 6

5 2 2 10 5 0a

0 3 5 3 3 7

100 110 130 180 220 220

Closed-Vessel Microwave (CVM) Decomposition ramp time (min)

hold time (min)

temp, T (oC)

5 5 10

2 5 35 20

80 140 190 b

a Aliquots of 1 mL of H O were added automatically for 7 min during 2 2 the sixth step of the FM decomposition. b Cooling temperature and twicepowerful exhaustion, because of the need for fast cooling of the vessels.

A thin mercury film that was plated onto a glassy carbon electrode (the working electrode) was used to determine the copper, lead, and zinc content. The mercury thin-layer film was obtained through application of a voltage of -800 mV for 5 min in a solution that contained 1 mmol/L mercury(II) and 100 mmol/L HCl. The reference and auxiliary electrodes were a miniaturized Ag/AgCl(sat) wire47 and a platinum wire, respectively. 2.4. Procedure. 2.4.1. Focused-Microwave (FM) Digestion. The digestion of lubricating oils in the FM oven was previously described.29 A similar program was adapted for diesel fuel and crude oil digestions. This program consisted of transferring an aliquot of 1 mL (0.85 g) of the sample to the microwave vessel and adding 10 mL of (both) concentrated sulfuric and nitric acids. Hydrogen peroxide (30% m/v) was added during the last heating program step, as listed in Table 1. H2O2 quantities of 10 and 20 mL were necessary for the complete decomposition of diesel fuel and crude oil, respectively. The digested samples were stored at 4 °C in 50-mL polyethylene vessels until analysis. 2.4.2. Closed-Vessel Microwave (CVM) Digestion. A new method for the digestion of crude oil and diesel fuel in the CVM oven was developed, in which 100 mg of sample was transferred to each vessel plus 4 mL of HNO3, 2 mL of H2O2, and 2 mL of H2O. The heating program that is listed in Table 1 then was performed. The digested samples were stored at 4 °C in 15-mL polyethylene flasks until analysis. 2.4.3. Electroanalytical Measurements. Square-wave voltammetry (SWSV) was applied for the simultaneous determination of copper, lead, and mercury in the digested samples, whereas SCP was only used for the determination of copper and mercury, and both SWSV and SCP were used at gold electrodes. The SWSV and SCP parameters are presented in Table 2. Aliquots of 200 µL of digested sample were diluted 10 times (pH ∼1) in the electrochemical cell with a solution that contained 20 mmol/L NaCl. PSA at mercury film electrodes was utilized for the determination of the lead, copper, and zinc content in the digested samples. The dissolved oxygen of these solutions was used as the reoxidizing agent in the stripping step. Aliquots of 100 µL (43) Munoz, R. A. A.; Matos, R. C.; Angnes, L. J. Pharm. Sci. 2001, 90, 1972-1977. (44) Munoz, R. A. A.; Matos, R. C.; Angnes, L. Talanta 2001, 55, 855860. (45) Richter, E. M.; Augelli, M. A.; Kume, G. H.; Mioshi, R. N.; Angnes, L. Fresenius J. Anal. Chem. 2000, 366, 444-448. (46) Angnes, L.; Richter, E. M.; Augelli, M. A.; Kume, G. H. Anal. Chem. 2000, 72, 5503-5506. (47) Pedrotti, J. J.; Angnes, L.; Gutz, I. G. R. Electroanalysis 1996, 8, 673-675.

of digested sample were diluted with 2 mL of 0.1 mol/L CH3COONa (final pH of ∼1). The copper and lead contents then were determined in the first scan. After that, the addition of 500 µL of 0.6 mol/L NaOH was necessary to elevate the pH of the solution to pH ∼5 and then zinc determination was performed. Table 2 presents the PSA parameters for the first scan (Cu and Pb) and the second scan (Zn). 2.4.4. Graphite Furnace Atomic Absorption Spectrometry Determinations of the Digested Samples. The determinations of lead, copper, and zinc in graphite furnace atomic absorption spectrometry (GFAAS) were performed as previously described in the literature.29 For copper and lead determination, an aliquot of 5 µL of chemical modifier [20 µg of (NH4)2HPO4] was coinjected with 20 µL of sample into the graphite-tube atomizer. For zinc determination, an aliquot of 20 µL of sample was just introduced into the graphite tube without chemical modifier. Analytical curves with a standard solution of 1 vol % HNO3 were obtained for calibration purposes. The digested solutions were diluted 5-10 times before analysis. 3. Results and Discussion 3.1. Comparison between the Microwave-Assisted Decomposition Procedures. Although the FM heating program time was shorter (45 min), the CVM oven is able to decompose 16 samples at a time (82 min) and, consequently, its throughtput is 4 times higher than that for the FM oven. After the fifth step of the FM program, a carbonaceous residue was formed on the bottom of the digested flask, which was finally decomposed by the programmable addition of H2O2. A larger volume of H2O2 was needed for crude oil, because of its higher carbon content. The use of H2SO4 allowed us to attain a temperature of 220 °C, because HNO3 alone was not able to decompose hydrocarbon molecules at atmospheric pressure and such temperatures, because of its low boiling temperature (120.5 °C). Between 120.5 °C and 220 °C, H2SO4 was the main responsible for the oxidation of organic matrix. The FM oven has a transparent screen, which facilitated visual examination of the sample and, consequently, helps the development of the heating program for sample decomposition. Moreover, infrared sensors monitored the temperature of each digestion vessel in real time. The CVM oven did not offer the possibility of observing the digestion process. On the other hand, is more efficient, because of the high temperature and pressure of the closed vessel. The CVM procedure required smaller volumes of HNO3 and H2O2 for digestion of crude oil and diesel fuel samples. The resultant solutions from the CVM oven were clear yellow, whereas clear colorless solutions were obtained from the FM (atmospheric pressure) decomposition. In both cases, the final solutions presented residual carbon contents that were lower than the detection limit (