Published online 7 November 2005
Published in J Environ Qual 34:2328-2333 (2005)
DOI: 10.2134/jeq2005.0173
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Waste Management
HPLC-MSn to Investigate the Oxidative Destruction Pathway of Aromatic Sulfonate Wastes
Fabio Gosetti,
Valentina Gianotti,
Mauro Ravera and
Maria Carla Gennaro*
DISAV–Dipartimento di Scienze dell'Ambiente e della Vita, Università del Piemonte Orientale "A. Avogadro", Spalto Marengo 33-15100 Alessandria, Italy
* Corresponding author (gennaro{at}mfn.unipmn.it)
Received for publication May 9, 2005.
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ABSTRACT
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The article concerns the problem of destruction and remediation of industrial wastes containing aromatic sulfonates. The effects of an oxidation process induced by thermally activated persulfate in the degradation of 1,5-naphthalenedisulfonate (NDS) are investigated by the use of UV-Vis spectrophotometry, high performance liquid chromatography–mass spectrometry (HPLC-MS) and HPLC-MSn techniques. The results obtained indicated that thermal activation (80°C) of a solution containing NDS and sodium persulfate in a molar ratio equal to 1 leads to the cleavage of the naphthalene structure. Estimated chemical oxygen demand (COD) indicates that when the excess of persulfate is equal to 100, NDS undergoes a mineralization of the order of 90%. A degradation pathway undergone by NDS when thermally activated in the presence of persulfate is proposed.
Abbreviations: amu, atomic mass unit • API, atmospheric pressure ionization • COD, chemical oxygen demand • DEO, direct electrochemical oxidation • ESI, electrospray ionization • HPLC-MS, high performance liquid chromatography–mass spectrometry • MEO, mediated electrochemical oxidation • NDS, 1,5-naphthalendisulfonic acid disodium salt • TEA, triethylamine • TIC, total ion current
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INTRODUCTION
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THE SO-CALLED LAGOONS are open-air basins in which industrial wastes have been disposed a long time. Typically, lagoons contain aromatic sulfonates coming from sulfonation processes in the manufacture of dyes, surfactants, detergents, and pharmaceutics. The muddy wastes are characterized by high values of chemical oxygen demand (COD) and contain high concentrations of salts, typically sodium sulfate (Na2SO4). Therefore, aromatic sulfonates must be destroyed to prevent environmental contamination. No data are available about a direct toxicity for human health and environment coming from aromatic sulfonates. The threat to the environment is due to possible degradation processes of these chemicals, processes that can naturally occur after their disposal in the environment and that can lead, as reported by many authors, to the formation of toxic aromatic amines (Fogg and Summan, 1983; Garrigos et al., 2002; Lancaster and Lawrence, 1982, 1983; Lawrence et al., 1981; Richfield-Fratz and Bailey, 1987; Stavric et al., 1979). Sulfonate wastes often are effluents of dye industries and are intensily colored. Many remediation processes have been in the past employed essentially aimed to decolorise the wastes. But a process or a chemical reaction able to transform a chromophoric group present in the molecular structure of the dye into a nonchromophoric group does not exclude the formation of toxic intermediates or products (Voyksner et al., 1989). These could be organic toxic species that do not absorb in the visible region. An efficacious remediation strategy should achieve a complete mineralization with the formation, as far as possible, of inorganic species N2, NOx, CO2, and SO3. Thus, it is necessary to be able to estimate the pathway of the degradation process and also to identify the intermediates to evaluate their possible toxicity or bioavailability. This is not an easy goal, because the destruction of a complex organic structure to give inorganic species is not a single process but proceeds by steps with the successive formation of many intermediate organic species that in turn degrade more or less easily as a function of their stability. The degradation studies of aromatic sulfonates reported in literature are few—alkylbenzene sulfonates (Fernández et al., 2004; González et al., 2004), aromatic sulfonates (Storm et al., 1999), dyes (Baiocchi et al., 2002; Gosetti et al., 2004) and drugs (Calza et al., 2001). These degradation processes are induced by catalysts or UV-irradiation and the degradation is studied by liquid (Fernández et al., 2004; González et al., 2004; Storm et al., 1999) and gas chromatography (Ding et al., 1999; Knepper, 2002; Reemtsma, 1996) with mass spectrometric detection.
A remediation strategy for waste destruction is based on electrochemical oxidation techniques that can be based on direct electrochemical oxidation (DEO) or on mediated electrochemical oxidation (MEO). The oxidants are electrochemically regenerated to minimize the possible formation of secondary wastes (Cañizares et al., 2002, 2003, 2004; Gennaro et al., 1997; Ravera et al., 2004; Saracco et al., 2001). In a previous paper (Ravera et al., 2004), a MEO electro-oxidative method on platinum electrodes was proposed for the degradation of 1,5-naphthalenedisulfonic acid (indicated in this paper as NDS), one of the most used naphthalene-based dye intermediates (Lynchand Scanlan, 1927). The destruction of NDS was performed by employing persulfate, which is a strong oxidizing agent that can be generated in the waste itself by oxidation of hydrogen sulfate. Hydrogen sulfate is highly concentrated in the waste (together with sulfate ions), because it comes from the synthesis of sulfonates. Under these conditions, as the degradation reaction proceeds, beside the dissipation of the sulfonate and the formation of sulfate ions, other possible degradation intermediates can occur. To estimate the degradation pathway with time, the HPLC technique interfaced with mass spectrometric detection was employed. Some intermediates could be identified, but it was not possible to determine the pathway up to the mineralization. The problems encountered were mostly due to high polarity of the analyte, as well as high concentration of sulfate and persulfate presented in treated waste water solution.
To improve our results and to reduce the interferences produced during the electrochemical oxidation, in this article new experiments are presented in which thermally activated (80°C) persulfate is used instead of electrochemically generated persulfate. Thermal activation also produces high reactive species such as sulfate radicals and hydroxyl radicals, but the reaction can be more easily controlled and likely leads to the formation of a lower number of interfering species that decrease the signal/noise ratio in the MS detection.
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MATERIALS AND METHODS
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Reagents
Ultrapure water from a Millipore Milli-Q system (Milford, MA, USA) was used for the preparation of all the solutions. The HPLC-grade methanol from Merck (Darmstadt, Germany) was filtered before use through a 0.45-µm membrane (Millex, Millipore). 1,5-Naphtalendisulfonic acid disodium salt (NDS), sodium persulfate, acetic acid, formic acid, and triethylamine (TEA)—all of analytical grade—were obtained from Merck (Darmstadt, Germany). Sulfuric acid 96% was purchased from Carlo Erba (Milan, Italy).
Instrumentation
The analyses were performed on a Finnigan Mat Spectra System equipped with a Degasser SCM1000, a gradient pump Spectra System P4000, an Autosampler Spectra System AS3000, interfaced by the module SN4000 to a diode array detector Spectra System UV6000LP and to ESI-MS ion trap detector Finnigan LCQ Duo (San Jose, CA, USA). A microprocessor pH meter (Hanna Instrument, Portugal), equipped with a combined glass-calomel electrode, was employed for pH measurements. The spectrophotometric analyses were performed on a spectrophotometer Jasco V-550.
Degradation Method
Two mixtures containing NDS and persulfate with molar ratios of 1:1 and 1:100 were prepared. The first solution was used for the degradation pathway evaluation, and the second mixture for the COD estimation. The degradation process was induced by adding 50.0 mL of a 0.26 mM solution of NDS to 50.0 mL of sodium persulfate at concentrations of 0.26 and 26.0 mM, respectively. These mixtures were heated to about 80°C.
Thermal activated process proceeds through the formation of SO4–· and HO· radicals according to the reactions:
Dissipation of the NDS during degradation reaction was measured for 20 h, collecting samples of solution each 20 min, both by HPLC-MS analysis and UV-Vis spectrometry with maximum absorbance at 287 nm.
Chemical Oxygen Demand Determination Method
In a distillation flask, 2.5 mL of 0.42 M potassium dichromate solution and 7.0 mL of sulfuric acid (96% w/w) were added to 2.5 mL of the degraded solution containing a NDS and persulfate mixture (each at 0.13 mM). This solution was kept at 20°C to avoid the possible loss of volatile species. The acid solution was then boiled for 2 h in reflux conditions, cooled, and three drops of ferroin-indicator were added. The dichromate excess was titrated with a 0.25 M solution of ammonium iron(II) sulfate hexahydrate until the solution color changed from blue-green to dark red.
To evaluate a possible interference in the COD determination by impurities contained in persulfate, a blank tritation was performed on the same persulfate concentration. The COD was then calculated (Method IRSA-CNR n. 5110, p. 285).
Liquid Chromatography–Mass Spectrometry Analysis
Method I
The separation of NDS was achieved on Lichrospher 100 RP-18 column (250 by 4 mm, 5 µm) (Merck, Darmstadt, Germany) with a Lichrospher RP-18 (15.0 by 4 mm, 5 µm) as guard precolumn. The mobile phase consisted of 5.00 mM triethylamine aqueous solution and methanol (90:10, v/v) adjusted to pH 4.0 by formic acid at flow rate of 0.30 mL min–1 (oven temperature was 25°C). A volume of 20.0 µL was injected into the HPLC-MS system.
Method II
The separation of the NDS and reaction products was performed on Polaris C18-Ether column (150 by 4.6 mm, 5 µm) (Varian, CA, USA); the mobile phase was a 2.00 mM triethylamine aqueous solution in methanol (90:10, v/v) acidified to pH 5.1 by acetic acid. Flow rate was 0.30 mL min–1, oven temperature was 25°C, and the injection volume was 20.0 µL. The HPLC system was connected to a Thermoquest LCQ DUO ion trap mass spectrometer from Finnigan equipped with an atmospheric pressure ionization (API) interface and an electrospray ion (ESI) source. High purity N was used as nebulizer (operating pressure at 80 of the arbitrary scale 0–100 of the instrument), and He (>99.999%) served as quenching agent. The ESI probe tip and capillary potential were set at 4.50 kV and –43.00 V, respectively. The heated capillary was set at 200°C and ion optics parameters were optimized to the following values: tube lens offset, –60.00 V; first octapole voltage, 8.75 V; inter octapole lens voltage, 14.00 V; second octapole voltage, 11.00 V. The mass to charge range (m/z) was 100 to 500 m/z and the mass spectrometer was operated in negative ion full-scan mode (3 micro scans, 50 ms inlet time) and in MSn mode. The eluent from the chromatographic column first enters the UV-VIS diode array detector, then the ESI interface and the ion trap mass analyzer. The conditions for the MS analysis are optimized for the peak at 287 m/z.
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RESULTS AND DISCUSSION
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The mass spectrometry analysis of NDS was performed for a solution that contained NDS and persulfate at the same concentration (0.13 mM). In these conditions, the proceeding of the degradation is much slower than that observed in excess of persulfate, so that the degradation pathway and the formation of intermediates can be more easily studied. In addition, lower concentrations of nonvolatile persulfate present advantages of a lower noise in mass analysis and greater sensitivity. Solutions containing a persulfate/NDS molar ratio equal to 100 were in turn prepared for the evaluation of COD during the degradation process.
Characterization of NDS with Mass Spectrometry
The first step in the mass spectrometric study consisted in the characterization of NDS, since the identification of the major products-ions, which form in the collisional sequential fragmentations of MSn analysis, could advantageously help in the identification of degradation intermediates.
The MS, MS2, and MS3 spectra of NDS were obtained by direct infusion of 10.0 mg L–1 methanol solution at flow rate of 20.0 µL min–1 in ESI ion source. Due to the presence in the molecule of two anionic sulfonated groups (Fig. 1)
, the characterization study was performed in negative ion-mode.
The molecular mass of NDS is 332 atomic mass units (amu); the molecule provides well recognizable signals corresponding to three negative pseudo-molecular ions, respectively, at 143, 287, and 309 m/z. The signal at 143 m/z is due to the bicharge molecule minus two sodium [M – 2Na]2–, whereas the signals at 287 and 309 m/z correspond respectively to the monocharge species [M – 2Na+H]– and [M – Na]–. A study performed as a function of ESI capillary temperature showed that the signals that corresponds to monocharge structures prevail at capillary temperatures greater of 180°C, whereas the signals that correspond to bicharge structures prevail at capillary temperatures between 120 and 150°C. Each signal of the pseudo-molecular ions so evidenced was then fragmented up to MS3 analysis and the characteristic product ions are summarized in Table 1.
Sample Analysis
Figure 2
shows the LC-MS chromatogram recorded for a solution containing NDS and persulfate, each at concentration 0.13 mM, under the chromatographic conditions of Method I (see Experimental section). The peak at 6.11 min corresponds to NDS. The mass signal was acquired only after about 1 min from injection, to avoid the interference on the mass signal of persulfate, that elutes at dead time.
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Fig. 2. Liquid chromatography–mass spectrometry (LC-MS) chromatogram of a mixture of 2.0 mg/L 1,5-naphthalendisulfonic acid disodium salt (NDS) standard solution and 2.0 mg/L persulfate standard solution. Cromatographic conditions: see method I in LC-MS analysis (experimental session). MS detection. Peak identification: 6.11 min NDS.
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Degradation Study by HPLC-DAD-MS
The NDS–persulfate mixture undergone to thermally activated degradation shows the same m/z signals of the degradation intermediates formed by mediated electrochemical oxidation (MEO) (Ravera et al., 2004). This indicates that the persulfate activation by electrochemical or thermic process is comparable; nevertheless, heat activation allows to estimate and explain in major details the chemical intermediates structures using sequentially MSn analysis.
The chromatographic mass analysis evidenced the presence of a peak to which corresponds a m/z of 303. The species, called intermediate I, can be identified as a monohydroxylated disulfonate, obtained by insertion of a ·OH radical in the naphtalene ring of NDS. The peaks of NDS and intermediate I are not resolved, but their identification is possible by extracting from the total ion current (TIC) the single ions, respectively, of m/z 287 (NDS) and m/z 303 (intermediate I). Anyway, the separation of two peaks was obtained on a Polaris-Ether C18 column (Method II). Using this stationary phase, a lower concentration of triethylamine is required in the mobile phase, which leads to a lower noise in mass analysis and increased sensitivity. A typical example of separation between the peaks of NDS and intermediate I, which forms after 20 min of degradation process, is presented in the chromatogram of Fig. 3
, where the peaks of NDS (Fig. 3a) and of the intermediate I (Fig. 3b) are present. Figure 3c and 3d show the mass spectra at 287 m/z corresponding to NDS and at 303 m/z for intermediate I.
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Fig. 3. Liquid chromatography–mass spectrometry (LC-MS) chromatogram of a solution containing 1,5-naphthalendisulfonic acid disodium salt (NDS) and persulfate (each 0.13 mM) after 20.0 min of the degradation process. Cromatographic conditions: see method II in LC-MS analysis (experimental session). Detector MS. Peak identification: 7.31 min NDS (Fig. 3a), 6.66 min intermediate I (Fig. 3b). Mass spectra of NDS [M – 2Na+H]– at 287 m/z (Fig. 3c) and of intermediate I at 303 m/z (Fig. 3d).
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The HPLC-MS analysis performed after 2 h and 20 min of the degradation process showed that some NDS is still present and the intermediate I is no more present at significant amount, whereas two new degradation intermediates have been formed, respectively, characterized by 253 m/z (intermediate II) and 257 m/z (intermediate III). Figures 4a, 4b, and 4c
show the chromatographic peaks of NDS, intermediate II, and intermediate III, respectively. It can be observed that NDS and intermediate II elute around 7 min and Fig. 4d shows the corresponding mass spectrum, where the signal at 287 m/z related to NDS and the signal at 253 m/z related to intermediate II are both present. Figure 4e shows the mass signal at 257 m/z that correspond to the intermediate III, eluted at 9.88 min as shown in Fig. 4c. To obtain a chemical structure for the intermediates II and III, a MSn sequential fragmentation study was performed.
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Fig. 4. Liquid chromatography–mass spectrometry (LC-MS) chromatogram of a solution containing a mixture of 1,5-naphthalendisulfonic acid disodium salt (NDS) and persulfate (each 0.13 mM) recorded after 2 h and 20 min of the degradation process. Cromatographic conditions: see method I in LC-MS analysis (experimental session). Detector MS. Peak identification: at 7.03 min coelute NDS and intermediate II (Fig. 4a,b) and at 9.88 min elutes intermediate III. Figure 4d reportes the mass spectrum of NDS (287 m/z) and intermediate II (253 m/z); Fig. 4e shows the mass spectrum of intermediate III (257 m/z).
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The mass spectrum of intermediate II shows a major intensity signal at 253 m/z and a signal at M + 2, probably due to a hydroxylated species. A sequential fragmentation on the 253 m/z signal leads to a loss of 28 amu that corresponds to a C=O group, and successively to a loss of 64 amu, likely due to the release of SO2 from a sulfonate group. This pathway is in agreement with the observation that electron donating substituents like OH facilitate desulfonation (Gilbert, 1965). The fragmentation study for the intermediate III (257 m/z) led first to a loss of 18 amu that corresponds to a water molecule and then to two sequential losses of 28 amu that correspond to two carboxyl groups.
The pathway degradation proposed is confirmed by UV-Vis spectra recorded as a function of degradation time ranging from t = 0 to times of 6 h and 20 min in the wavelength range between 225 and 400 nm. The absorbance at 287 nm, which corresponds to the maximum of absorbance of NDS, progressively decreases with time. On the other hand is progressively increasing the absorbance around 260 nm, which corresponds to a wavelength typical of monoaromatic species (Fig. 5)
. A linear relationship between ln(At/A0) and reaction time (where At and A0 are the absorbances of NDS at 287 nm and at time = t and at time = 0, respectively), indicates that the reaction rate is pseudo-first order with respect to NDS concentration, according to the data obtained by mediated electrochemical oxidation (Ravera et al., 2004). The value of rate constant k is 6.72 min–1 and t1/2 is 280 min.
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Fig. 5. Ultra-violet (UV) spectra of a solution of 1,5-naphthalendisulfonic acid disodium salt (NDS) and persulfate (each 0.13 mM mM) heated at 80°C as a function of time.
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The results obtained by liquid chromatography, MS and MSn spectrometry, and UV-Vis spectrophotometry lead to propose the destruction pathway of NDS presented in Fig. 6
. The mechanism is in agreement with those proposed by other authors for the H2O2 oxidation of p-toluensulfonic acid (Stöffler and Luft, 1999) and TiO2 catalyzed oxidation of differently substituted aromatic sulfonates (Baiocchi et al., 2002; Sangchakr et al., 1995).
It was previously reported (Sangchakr et al., 1995; Stöffler and Luft, 1999) that when an aromatic structure is hydroxylated and desulfonated, a fission of the aromatic ring takes place. It is very likely, therefore, that the reactions observed can lead to the destruction of the naphthalene structure and to the opening of one aromatic ring with the formation of oxygenated compounds like aldehydes, ketones, and carboxylic acids. In our conditions, mass spectrometric analysis could not support this hypothesis, due to the too large noise and low sensitivity.
To collect information about the extent of destruction of NDS reached by the degradation process, the solution undergone to degradation was analyzed for COD content. This evaluation could give a global information on the residual content of organic matter. The COD evaluation performed according to the IRSA-CNR method n. 5110 (p. 285) gave the following results. The COD value at t = 0 is equal to 240 mg/L where after 20 h of degradation process is only 23 mg L–1 with a decrease of about 90%. These data suggest that the mineralization process has achieved an extent of about 90%.
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CONCLUSIONS
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The HPLC-MS and HPLC-MSn techniques were employed to follow the degradation pathway in the persulfate assisted oxidative degradation of 1,5-naphthalenedisulfonic acid. The results obtained in conditions of NDS/persulfate ratio equal to 1, supported by the spectrophotometric data and COD values, suggest that the naphthalene structure is being broken during the oxidative process, making the pollutant more available to biodegradation. The presence of nonvolatile persulfate in the degraded solution precludes to perform a more accurate mass analysis, to better evaluate the extent of the mineralization undergone by the sulfonate. The information collected by COD determination allows to conclude that the mineralization process has taken place at percentage levels around 90%, when the persulfate/NDS molar ratio is around 100:1.
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ABSTRACT
INTRODUCTION
