Room Temperature Sonolysis-Based Advanced Oxidation Process for

Anal. Chem. 2000, 72, 4979-4984

Room Temperature Sonolysis-Based Advanced Oxidation Process for Degradation of Organomercurials: Application to Determination of Inorganic and Total Mercury in Waters by Flow Injection-Cold Vapor Atomic Absorption Spectrometry J. L. Capelo, I. Lavilla, and C. Bendicho*

Facultad de Ciencias (Quı´mica), Departamento de Quı´mica Analı´tica y Alimentaria, Universidad de Vigo, As Lagoas-Marcosende s/n, 36200 Vigo, Spain

A new oxidation method based on room-temperature ultrasonic irradiation (sonolysis) is proposed for conversion of organomercurials into inorganic mercury and subsequent determination by flow injection-cold vapor atomic absorption spectrometry. This advanced oxidation process eliminates the need for chemical oxidants, high temperature, and pressure for degradation of organomercurials so that total mercury can be determined with sodium tetrahydroborate(III) or tin(II) chloride as reducing agents. Complete oxidations can be accomplished within 3 min, using a 40% sonication amplitude (100 W nominal power) provided by a probe ultrasonic device (20 kHz frequency) and a 1 mol L-1 HCl liquid medium. The presence of HCl was seen to be necessary for fast oxidation of organomercurials, in contrast to other chemical oxidants such as H2O2 or HNO3 which yielded incomplete oxidation. Further advantages of the proposed method over existing methods which are currently employed for oxidation prior to total Hg determination are the removal of hazardous wastes and the decreased risk of Hg losses by volatilization. Oxidation kinetics indicated a pseudofirst-order reaction with apparent rate constants (k) of 3.2 × 10-2 and 1.6 × 10-2 s-1 for methylmercury and phenylmercury, respectively. Oxidation experiments in the presence of foreign substances acting as OH radical scavengers showed a tolerance at least up to a concentration of 1000 mg L-1. Likewise, model wastewaters with chemical oxygen demand of up to 1000 mg L-1 could be processed without diminishing the oxidation efficiency. The method was applied to determination of inorganic and total mercury in simulated wastewaters and spiked environmental waters in combination with selective reduction. Accurate determination of Hg in environmental samples is recognized to be an important issue for assessing the environmental quality as a consequence of the toxicity of this element * Corresponding author. E-mail: [email protected] 10.1021/ac000470b CCC: $19.00 Published on Web 09/19/2000

© 2000 American Chemical Society

and its ability to bioacumulate in living organisms.1 Mercury can be present in environmental samples as inorganic mercury or organomercurials. Determination of total Hg can be performed by cold vapor atomic absorption spectrometry (CVAAS) with good sensitivity and selectivity, the only requirement being that mercury to be present as inorganic mercury (Hg2+) so that it can be effectively reduced to elemental mercury (Hg0).2 Organomercurials are not reduced to Hg0 by SnCl2/HCl, which usually is the recommended reducing agent. On the other hand, the use of a more energic reducing agent than SnCl2/HCl such as sodium tetrahydroborate (III) does not provide the same sensitivity for all mercury species.3,4 Thus, methyl-mercury is converted into CH3HgH which must be previously decomposed to measure the atomic absorption signal.5-7 Therefore, Hg determination requires a prior oxidation of the organic mercury to yield inorganic mercury (i.e., wet oxidation or dry ashing). So far, methods available for oxidation of mercury compounds include the use of strong acids, oxidants, high temperatures, UV irradiation, and ozone. Most of methods demand concentrated oxidants (e.g., H2O2, KMnO4, K2Cr2O7, K2S2O8) or acids (e.g., HCl, H2SO4, HNO3) in off-line or on-line procedures.8-23 These methods (1) Tessier, A.; Turner, D. R. Metal Speciation and bioavailability in aquatic systems; John Wiley and Sons: Chichester, UK, 1995. (2) Welz, B.; Sperling, M. Atomic Absorption Spectrometry, 3rd ed.; Wiley-VCH: Weinheim, Germany, 1999. (3) Baxter, D. C.; Frech, W. Anal. Chim. Acta 1990, 236, 377-384. (4) Oda, C. E.; Ingle, J. D. Anal. Chem. 1981, 53, 2305-2309. (5) Puk, R.; Weber, J. H. Anal. Chim. Acta 1994, 292, 175-183. (6) Tseng, C. M.; De Diego, A.; Martin, F. M.; Amouroux, D.; Donard, O. F. X. J. Anal. At. Spectrom. 1997, 12, 743-750. (7) Tseng, C. M.; De Diego, A.; Pinaly, H.; Amouroux, D.; Donard, O. F. X. J. Anal. At. Spectrom. 1998, 13, 755-764. (8) Hanna, C. P.; Mcintosh, S. A. At. Spectrosc. 1995, 16, 106-114. (9) Welz, B.; Tsalev, D. L.; Sperling, M. Anal. Chim. Acta 1992, 261, 91-103. (10) Tsalev, D. L.; Sperling, M.; Welz, B. Analyst (Cambridge, U.K.) 1992, 117, 1735-1741. (11) Tsalev, D. L.; Sperling, M.; Welz, B. Analyst (Cambridge, U.K.) 1992, 117, 1729-1733. (12) Guo, T.; Baasner, J. Talanta 1993, 40, 1927-1936. (13) Welz, B.; He, Y.; Sperling, M. Talanta 1993, 40, 1917-1926. (14) Guo, T.; Baasner, J. J. Autom. Chem. 1996, 18, 217-220. (15) Hanna, C. P.; Tyson, J. F.; Mcintosh, S. A. Anal. Chem. 1993, 65, 653656.

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for mercury oxidation are potentially a source of sample contamination, analyte losses by volatilization, and additionally, they involve the use of concentrated and corrosive reagents that can originate hazardous laboratory wastes. During recent years, several investigations have addressed the development of advanced oxidation processes (AOPs) which are characterized by production of the hydroxyl radical (OH) as a primary oxidant.24 This oxidizing radical can be generated by chemical reaction between Fe(II) and H2O2 (i.e., Fenton’s reagent), providing an effective oxidant power under relatively mild conditions of pH and temperature for determination of organomercurials in environmental water and urine samples.25 An increased interest has been focused on methods for generation of the OH radical without addition of reagents. Water treatment by ultrasonic irradiation (sonolysis) appears to be an effective method for the decomposition of organic pollutants present in water, mainly volatile and hydrophobic substances, as a result of the acoustic cavitation induced in the liquid medium. The extreme temperatures and pressures generated during cavitation result in solute thermolysis as well as the formation of hydroxyl radical and hydrogen peroxide.26,27 Ultrasound has been used for oxidation of volatile and semivolatile species such as alcohols, phenols, chlorinated hydrocarbons, parathion, hydrogen sulfide, and carbon tetrachloride. 28-33 The mechanisms that contribute to oxidation are direct combustion within the gas phase of the collapsing cavitation bubbles, combustion of solutes in the hot interfacial zones of cavitation bubbles, and oxidation in the bulk solution by the radicals generated (e.g., OH). The production of oxidizing species during sonolysis of aqueous solutions begins with the thermolysis of water as follows24

H2O f OH + H Most of the OH and H radicals formed recombine to yield H2O. The OH radicals can self-react to form hydrogen peroxide (16) Guo, T.; Baasner, J.; Gradl, M.; Kistner, A. Anal. Chim. Acta 1996, 320, 171-176. (17) Guo, T.; Baasner, J. Anal. Chim. Acta 1993, 278, 189-196. (18) Birnie, S. E. J. Autom. Chem. 1988, 10, 140-143. (19) Bloxham, M. J.; Hill, S. J.; Worsfold, P. J. J. Anal. At. Spectrom. 1996, 11, 511-514. (20) Lamble, K. J.; Hill, S. J. J. Anal. At. Spectrom. 1996, 11, 1099-1103. (21) Rio-Segade, S.; Bendicho, C. Spectrochim. Acta, Part B 1999, 54, 11291139. (22) Sasaki, K.; Pacey, G. E. Talanta 1999, 50, 175-181. (23) Tao, G.; Willie, S. N.; Sturgeon, R. E. Analyst (Cambridge, U.K.) 1998, 123, 1215-1218. (24) Hua, I.; Hoffmann, M. R. Environ. Sci. Technol. 1997, 31, 2237-2243. (25) Ping, L.; Dasgupta, P. K. Anal. Chem. 1989, 61, 1230-1235. (26) Mason, T. J.; Lorimer, J. P. Ultrasound: Theory, Applications and Uses of Ultrasound in Chemistry; Ellis Horwood: Chichester, U.K., 1988. (27) Mason, T. J. Practical Sonochemistry: User’s Guide to Applications in Chemistry and Chemical Engineering; Ellis Horwood: Chichester, U. K., 1991. (28) Hua, I.; Hoffmann, M. R. Environ. Sci. Technol. 1996, 30, 864-871. (29) Petrier, C.; Micolle, M.; Merlin, G.; Luche, J. L.; Reverdy, G. Environ. Sci. Technol. 1992, 26, 1639-1642. (30) Cheung, H. M.; Bhatnagar, A.; Jansen, G. Environ. Sci. Technol. 1991, 25, 1510-1512. (31) Kotronaru, A.; Mills, G.; Hoffmann, M. R. Environ. Sci. Technol. 1992, 26, 1460-1462. (32) Kotronaru, A.; Mills, G.; Hoffmann, M. R. Environ. Sci. Technol. 1992, 26, 2420-2428. (33) Kotronaru, A.; Mills, G.; Hoffmann, M. R. J. Phys. Chem. 1991, 95, 36303638.

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2OH f H2O2 To the best of our knowledge, ultrasonic irradiation has not been attempted for oxidation of organomercurials. This AOP should be an attractive alternative to oxidation methods on the basis of the action of concentrated chemicals (i.e., acids or oxidants) combined with heating, with the inherent risk of contamination or losses by volatilization. In this work, a new oxidation method based on room temperature ultrasonic irradiation is proposed for degradation of organomercurials and subsequent mercury determination by flow injection cold vapor atomic absorption spectrometry. Parameters such as sonication time, composition of the liquid medium, vibrational amplitude of the probe, and the presence of foreign substances such salts and organic matter are fully investigated. Sodium tetrahydroborate(III) is used for total Hg determination after conversion of all mercury species into inorganic mercury by sonolysis. The use of a selective reducing agent such as tin(II) chloride in hydrochloric acid medium is tried for the separate determination of inorganic mercury and total mercury. EXPERIMENTAL SECTION Apparatus. The flow injection system used for cold vapor generation consisted of a four channel Gilson (Villiers le bel, France) Minipuls 3M peristaltic pump, a Perkin-Elmer (U ¨ berlingen, Germany) membrane gas-liquid separator, a four-way Rheodyne (Supelco, Bellefonte, PA) injection valve with a 500-µL loop, and a Fisher and Porter (Warminster, PA) flow meter (25180 mL min-1 N2). Tygon tubing of different internal diameters was used for carrying the reducing agent, carrier solution, and waste solution. Optimum conditions for cold vapor generation using NaBH4 as a reducing agent were established as follows: a 0.3% mass/v NaBH4 solution stabilized in 1% mass/v NaOH; a 1 mL/min NaBH4 flow rate; a 1% mass/mass HCl solution used as carrier; a 10 mL/min flow rate of the carrier solution. Optimum conditions for the application of SnCl2 as a reducing agent were as follows: a 10% mass/v SnCl2 solution in 1% mass/mass HCl used as reducing stream; a 1 mL/min flow rate of SnCl2 solution; a 0.01% mass/mass HCl solution used as carrier; a 12 mL/min flow rate of the carrier solution. In both cases, a 80 mL/min flow rate of carrier gas (N2) was used. A 100 W, 20 kHz VC-100 ultrasonic processor (Sonics and Materials) equipped with a 6-mm titanium microtip was used. Ultrasonic vibrations at the probe tip were fixed at any desired amplitude level using a power setting in the 10-100% range. The ultrasonic processor was enclosed inside a sound-proof chamber during operation. Mercury absorbance was measured with a Unicam (Cambridge, UK) Atomic Absorption Spectrometer model solar 939 equipped with a Unicam EC90 electrically heated quartz tube furnace. The quartz furnace was kept at 200 °C during operation. A mercury hollow-cathode lamp operated at 6 mA was used as a radiation source. The mercury analytical line at 253.7 nm and a slit width of 0.5 nm were used for measurements. Reagents. All chemicals were of analytical-reagent-grade. Tin(II) chloride (0.53 mol L-1) used as reducing agent was prepared by dissolving the appropriate mass of tin(II) chloride dihydrate (Panreac, Barcelona, Spain) in concentrated hydrochloric acid and diluting with ultrapure water.

Sodium tetrahydroborate(III) (Merck, Darmstadt, Germany) was prepared fresh daily by dissolving the solid in 0.25 mol L-1 sodium hydroxide solution (Carlo Erba, Milan, Italy). Diluted hydrochloric acid was used as carrier. An inorganic mercury stock standard solution (1000 mg L-1) was prepared from mercury chloride (Carlo Erba). A methylmercury stock standard solution (100 mg L-1) was prepared from methyl-mercury chloride (Riedel-de Ha¨en, Seelze, Germany) by dissolving the appropriate amount of the solid and making up to volume with ultrapure water. Stock standard solutions (100 mg L-1) of phenylmercury and p-tholylmercury were prepared from the corresponding chloride salts (Riedel-de Ha¨en) by dissolving the appropriate amount of the solid in methanol (5% v/v methanol for phenylmercury and pure methanol for p-tolylmercury) and making up to volume with ultrapure water. All stock standard solutions were stored in a refrigerator at 4 °C and protected from light. Working standard solutions were prepared just before use by appropriate dilution of the stock standard solutions. The interference study was carried out by dissolving the appropriate amounts of the following compounds: manganese(II) nitrate 4-hydrate (Panreac); magnesium nitrate hexahydrate (Merck); iron(III) nitrate (Merck); potassium chloride (Prolabo, Fontenay, France); sodium chloride (Panreac); calcium carbonate (Aldrich, Milwaukee, WI). The influence of each compound was determined at four concentration levels, i.e., 10, 100, 1000, and 10 000 mg L-1. Simulated waste effluents spiked with different mercury compounds were prepared by dissolving the appropriate amounts of solid oxalic acid (Merck) or potassium hydrogen phthalate (Panreac) to produce a chemical oxygen demand (COD) of up to 1000 mg L-1. Oxidation Procedure. Five milliliters of sample containing 1 mol L-1 HCl was transferred into a polyethylene tube (50 mL capacity) and sonicated by means of a probe ultrasonic processor for 3 min at a 40% sonication amplitude (100 W nominal power). The oxidation vessel was inserted in an ice bath so that the temperature did not exceed 25 °C during sonolysis. Oxidations were performed just before measurement by FI-CVAAS. RESULTS AND DISCUSSION Influence of the Liquid Medium Composition on Ultrasonic Oxidation of MeHg+. The reduction reactions of Hg2+ and MeHg+ with sodium tetrahydroborate are the following5-7

Hg2+ + 2NaBH4 + 6H2O f Hg0 + 7H2 + 2H3BO3 + 2Na+ MgHg+ + NaBH4 + 3H2O f MeHgH + 3H2 + H3BO3 + Na+

For effective Hg determination as MeHgH, the quartz tube furnace should be heated so that the MeHgH can be decomposed into Hg0. On the contrary, several workers have found the direct reduction of MeHg+ to Hg0 to some extent under certain conditions.34-36 However, obtaining the same sensitivity with the (34) Sazarnini, C.; Sacchero, G.; Aceto, M.; Abollino, O.; Mentasti, E. J. Chromatogr. 1992, 626, 151-157.

use of sodium tetrahydroborate for inorganic and organic mercury which is necessary for calibration with Hg(II) standards is difficult to achieve. Solutions containing Hg species were prepared just prior to use, since storing conditions of organomercurials in solution are critical, the following variables being cited in the literature: container material, acid concentration and its oxidation potential, history and pretreatment of containers, temperature and influence of light.37 Thus, some workers have reported the photolytic degradation of MeHg+.38 On the other hand, some investigations have demonstrated the high stability of MeHg+, this compound even being partly oxidized under drastic conditions (i.e., oxidizing acids, 200 °C temperature, pressure digestion).38 The addition of HNO3 and K2Cr2O7 to prevent loss in Hg2+ at parts per billion concentrations can cause partial decomposition of organomercurials.4 The authors have found that calibration standards of methyl and phenylmercury, stabilized in a 0.25% mass/v KMnO4 + 15% mass/mass HNO3 + 0.12% mass/v NaCl-NH2OH‚HCl solution and allowed to stand for a time before measurement, provided the same sensitivity as Hg2+.39 As consequence, no stabilization was employed for the test solutions. Preliminary studies were performed to evaluate the influence of the medium composition on degradation of MeHg+ by ultrasonic irradiation. In these experiments, the peak absorbances produced by the cold vapor generation from Hg2+ and MeHg+ solutions of 20 µg L-1 (Hg) were compared under three pretreatment conditions, i.e., without ultrasonic irradiation, with ultrasonic irradiation at room temperature (i.e., 25 °C), and with ultrasonic irradiation in a heated medium (i.e., 60 °C). Results shown in Table 1 indicated that, under typical reaction conditions in the manifold used for cold vapor generation, sensitivity for MeHg+ is usually below 20% in comparison with that achieved for Hg2+ when solutions are not ultrasonically irradiated, as can be observed from the recovery values (i.e., the ratio between the peak absorbance obtained with MeHg+ and Hg2+). Heating of the solution should be avoided owing to the decreased cavitation that occurs with increasing temperature and the enhanced volatilization risk of Hg species. Sonication of solutions containing Hg2+ or MeHg+ (60% amplitude, 2-min sonication time) was attempted after addition of diluted acids (HCl and HNO3), acid mixtures and acids combined with H2O2. The lowest Hg recovery (