Ultrasonics Sonochemistry 16 (2009) 686–691
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Trivalent manganese as an environmentally friendly oxidizing reagent for microwave- and ultrasound-assisted chemical oxygen demand determination Claudia E. Domini a, Lorena Vidal b, Antonio Canals b,* a b
Departamento de Química, Universidad Nacional del Sur, Av. Além 1253, 8000 Bahía Blanca, Argentina Departamento de Química Analítica, Nutrición y Bromatología, Universidad de Alicante, P.O. Box 99, 03080 Alicante, Spain
a r t i c l e
i n f o
Article history: Received 25 November 2008 Received in revised form 20 January 2009 Accepted 22 January 2009 Available online 4 February 2009 Keywords: Chemical oxygen demand determination Mn(III) Ultrasound energy Microwave radiation Optimization Experimental design
a b s t r a c t In the present work manganese(III) has been used as oxidant and microwave radiation and ultrasound energy have been assessed to speed up and to improve the efficiency of digestion step for the determination of chemical oxygen demand (COD). Microwave (MW) and ultrasound-assisted COD determination methods have been optimized by means of experimental design and the optimum conditions are: 40 psi pressure, 855 W power and 1 min irradiation time; and 90% of maximum nominal power (180 W), 0.9 s (s1) cycles and 1 min irradiation time for microwaves and ultrasound, respectively. Chloride ion interference is removed as hydrochloric acid gas from acidified sample solutions at 150 °C in a closed reaction tube and captured by bismuth-based adsorbent suspended above the heated solution. Under optimum conditions, the evaluated assisted digestion methods have been successfully applied, with the exception of pyridine, to several pure organic compounds and two reference materials. COD recoveries obtained with MW and ultrasound-assisted digestion for five real wastewater samples were ranged between 86–97% and 68–91%, respectively, of the values obtained with the classical method (open reflux) used as reference, with relative standard deviation lower than 4% in most cases. Thus, the Mn(III) microwave-assisted digestion method seems to be an interesting and promising alternative to conventional COD digestion methods since it is faster and more environmentally friendly than the ones used for the same purpose. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Oxygen demand is an environmentally important parameter which reflects the degree of organic pollution in water. Since the degradation of organic compounds requires oxygen, their concentrations can be estimated by the amount of required oxygen [1,2]. Chemical oxygen demand (COD), biological oxygen demand (BOD) and total organic carbon (TOC) are three main indexes used to assess this organic pollution in aqueous systems. In particular COD, which is defined as the amount of oxygen equivalents consumed in the oxidation of organic compounds by strong oxidants (such as dichromate, permanganate, etc.) [3], represents the total pollutants load of most wastewater discharges [4]. As a pollution monitoring parameter, COD has the advantage of speed and simplicity compared with BOD, and requires less equipment than to TOC determination. Thus, COD is preferred for estimating organic pollution in water [2,5,6]. In the conventional COD evaluation methods, a known excess of oxidant is added to a sample and the mixture is boiled. Whereas the sample is digested, COD material in the sample is oxidized by the oxidant. After the oxidation has proceeded for a finite period * Corresponding author. Tel./fax: +34 965 909790. E-mail address:
[email protected] (A. Canals). 1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2009.01.008
of time, the initial concentration of organic species can be calculated by determining the amount of the remaining oxidizing agent commonly by titration or UV–vis spectrophotometry [7]. The conventional methods, however, require the time-consuming process (about 2 h) of closed refluxing samples to achieve more complete oxidation. Moreover, these methods consume some expensive (Ag2SO4) and toxic (hexavalent chromium and mercury) chemicals [8–10]. For these reasons, many efforts have been done to develop new COD tests that shorten the digestion time and eliminate metals such as Cr(VI), Hg, and Ag [11]. For the latter purpose, manganese(III) has been recently suggested as a new environmentally friendly oxidizing reagent. The reaction occurring in this COD method is best represented by the following equation, where the Mn(III) oxidant is reacted with potassium hydrogen phthalate (KHP):
2KC8 H5 O4 þ 30Mn2 ðSO4 Þ3 þ 24H2 O $ 16CO2 þ 60MnSO4 þ 28H2 SO4 þ 2KHSO4
ð1Þ
However, this new oxidant still needs 1 h to digest organic matter at 150 °C [11]. The acceleration of chemical reactions is a feature shared by microwave (MW) and ultrasound radiation. The advantages of
C.E. Domini et al. / Ultrasonics Sonochemistry 16 (2009) 686–691
microwaves in the analytical laboratories are well known [12]. Microwaves have also been used for COD determination with good recoveries and an important reduction in the time of digestion step [12–18]. Other advantages of microwave digestion over the conventional hot-plate digestion methods include reduced contamination, lower reagent and sample usage and easy to handle. The microwave oven heats the reaction mixture to a high temperature very rapidly and the closed vessel helps to prevent losses due to volatilization of compounds. However, one of its main drawbacks is the high initial cost of the equipment and safety limitations (i.e., high pressure and temperature). On the other hand, power ultrasound is being extensively used in a great variety of applications such as solution degasification, cleaning, aerosol generation, metal extraction and organic synthesis [19–26]. Applying of continuous systems for this step has not been frequently used, even though this approach speeds sample treatment up to considerably [27–30]. Other advantages of continuous ultrasound-assisted leaching are modest consumption of sample and reagents, and the need for few or none of the chemicals required for dissolution in manual methods [31]. However, surprisingly ultrasound-assisted sample preparation is still not commonly used for analytical purposes. Recently, Domini et al. [18] Canals and Hernández [32] and Canals et al. [33] have used ultrasound energy for the determination of chemical oxygen demand (COD). Sample preparation depends on a variety of experimental factors controlled by the operator. This set of conditions must be optimized and several optimization approaches could be used. Among them, multivariate statistics (i.e., factorial design) shows the next main advantages: (i) if the effects of the factors are additive, then the factorial design needs fewer measurements than the classical ‘‘one-at-a-time” approach in order to give the same precision (i.e., in general, for k factors, a classical approach involves k times as many measurements as a factorial one with the same precision); (ii) the factorial experiment detects and estimates any interaction between factors that could affect the result [34]. Both MW and ultrasound-assisted digestions have been used to speed up the sample digestion for COD determination using hexavalent chromium as oxidant. One recent trend is that analytical methods decrease their output of hazardous materials. This includes laboratory wastes that contain toxic materials (hexavalent chromium, mercury, and silver metals). Therefore, the goals of this work were: (i) to use manganese(III) as oxidizing reagent for COD determination assisted by microwave radiation (closed) and ultrasound energy and; (ii) to apply a factorial design technique in order to accelerate the optimization process of these digestion steps in order to suggest a fast, safe, easy to handle and environmentally friendly COD method.
2. Experimental 2.1. Apparatus A microwave system (MSP 1000, CEM corp., Matthews, N. Carolina, USA) with 950 W effective power output and a pressure control system was used for the microwave digestion. Advanced composite vessels (CEM corp., Matthews, N. Carolina, USA) with maximum pressure of 200 psi were used. The sonication system used has been previously described [18]. A 200 W, 24 kHz ultrasonic processor (Dr Hielscher, Teltow, Germany) was used as the sonic generator. An all-glass cylindrical sonotrode (12 mm o.d.; 125 mm long, reference SG12, Dr Hielscher, Teltow, Germany) was directly introduced 4.5 cm into the reaction mixture. On this length the higher recoveries were obtained.
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A spectrophotometer UV–vis (Unicam Instruments, Helios Gamma & Delta, Great Britain) and a quartz cell (HELLMA), 10 mm length pass, were used to measure the excess of oxidants (chromium(VI) and manganese(III)). The method used in this study for the reduction of chloride interference in the determination of COD using bismuth-based adsorbents instead of Hg(II) has been previously described [11,35]. 2.2. Reagents and samples All reagents were of analytical grade. Distilled and deionized water was used throughout (18 MX cm resistivity). Classical COD assay solutions were prepared according to the closed reflux reference method [7]. Briefly, the digestion solution was prepared by mixing 10.216 g K2Cr2O7, 33.3 g HgSO4, and 167 mL H2SO4 (concentrated), and then diluting up to volume of 1000 mL with deionized water. Previously, silver sulphate was added to the sulfuric acid in the proportion 5.5 g Ag2SO4 per kg of H2SO4. The manganese(III) digestion solution was prepared by mixing two solutions. One solution was prepared by dissolving approximately 1.0 g of KMnO4 in deionized water and diluting to 0.5 L. The solution was boiled during 2 h and was settled an overnight. Latter, the solution was filtered. Second solution was prepared dissolving approximately 5.0 g of MnSO4 in the minimum amount of deionized water and diluting to 0.5 L with concentrated sulfuric acid. Finally, both solutions were mixed and on this way a digestion solution of 1.6 g/L of Mn(III) on 18 N H2SO4 (saturated with MnSO4) was prepared. Solution concentration was determined by titration against standardized 0.25 M FeSO4(NH4)2SO4 6H2O (FAS) solution that was prepared by dissolving 98 g of iron(II) ammonium sulphate on 20 mL of concentrated sulfuric acid, and after cooling diluting to 1000 mL. This solution was standardized daily against standard potassium dichromate solution [7]. Manganese(III) has a broad absorption band in the region between approximately of 420 and 620 nm, with a single maximum at 510 nm [11]. Calibration graphs were prepared using dilutions of potassium hydrogen phthalate (KHP) standard solution. The working range of the test is 50–1500 mg/L COD. The calibration slope is negative because manganese(III) color decreases as COD concentration increases, being the slope 0.0005 and 0.0006 absorbance/ (mg/L) and the correlation coefficient 0.9977 and 0.9967 for microwave and ultrasound, respectively. For optimization, calibration graphs and interference studies a standard solution of potassium hydrogen phthalate (KHP), corresponding to 2000 mg/L COD, was prepared by dissolving 1.702 g of dried (120 °C, overnight) KHP in water and diluting up to 1 L with water. Appropriate dilutions gave solutions of different COD values. To compare digestion methods, solutions of seven organic compounds (D-glucose, lactic acid, salicylic acid, methyl isobutyl ketone, acetic acid, pyridine and picric acid) other than KHP were prepared, encompassing a wide range of oxidability. The theoretical COD values of each of these solutions would be about 100 mg O2/L, assuming that the oxidation was completed. Standard solution of chloride was prepared for interference studies corresponding to 20000 mg/L, and solutions of different concentration were prepared by appropriate dilutions. Bismuthate-based adsorbent (as S1 on Ref. [35]) and basket were prepared as described by Vaidya et al. [35]. Blank determinations were carried out on distilled and deionized water. Two reference materials (RMs) (a 200 mg O2/L reference material, Reagecon, Ireland, UK, and GBW08624B, National Research Center for Certified Reference Materials, PR of China) were used to evaluate the bias on the proposed methods. Real wastewater samples were supplied by a local private water-analysis laboratory
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(Labaqua, Alicante, Spain). All reference materials and real samples were stored at 4 °C until subsequent analysis.
3. Results and discussion 3.1. Optimization of the microwave-assisted digestion method
2.3. Procedures 2.3.1. Classical COD determination method [7] An aliquot of sample was refluxed during approximately 2 h in a strong acid solution with a known excess of potassium dichromate (K2Cr2O7) in the presence of Ag2SO4 and HgSO4. Chromium(VI) was reduced to chromium(III) during oxidation. The oxidant excess was determined by a UV–vis spectrophotometer set at 620 nm. 2.3.2. Microwave-assisted digestion method The sample (0.5 mL) was placed directly in the vessels, previously washed with 40% (w/v) nitric acid, and 5.0 mL of Mn(III) digestion solution was added. The vessels were then sealed (closed microwave digestion) and introduced in the microwave oven. Three parameters (pressure, power and irradiation time) were programmed according to the experimental design optimization. After the program completion, the vessels were removed from the microwave oven and were left to cool for 30 min. In all experiments six vessels were used. For optimization purposes, four sample replicates and two blanks were done. Blanks were located on opposite positions in the carousel in order to introduce the possible lack of homogeneity in the microwave oven. Otherwise stated, five replicates and one blank were simultaneously irradiated since no statistical difference was found between blanks on the optimization study. 2.3.3. Ultrasound-assisted digestion method The mixture to sonicate was made by mixing, in a glass tube, 0.5 mL of sample with 5.0 mL of Mn(III) digestion solution. Afterwards, the glass probe was directly introduced 4.5 cm into the reaction mixture and the mixture was sonicated. Three parameters (sonication time, cycles and intensity) were programmed according to the experimental design optimization. After the sonication program completion, the sonotrode was removed and the reaction mixture was left to cool during 15 min. Since the precision was quite good, during the optimization study one sample and one blank were only done. However, otherwise stated, for statistical significance, five replicates and one blank were always done. When both assisted digestion methods were applied to real samples firstly they were treated in order to remove chloride interference (see Section 3.3). Finally, on both Mn(III) assisted digestion methods, Mn(III) excess was determined by a UV–vis spectrophotometer set at 510 nm. The statistical design and analysis of the response variables was supported by statistical graphics software (STATGRAPHICS Plus, version 5.1 STSC, Rockville, MD, USA). In all the design matrices low and high levels are denoted by () and (+), respectively, and the percentage of COD recovery was used as response factor.
Manganese(III) closed microwave-assisted digestion method was optimized by a three-factor design with two levels for each factor. In this case, the design requires eight experiments plus one in the middle which were performed in duplicate and were randomly run. The extra experiment was included to have an estimation of the response in the centre of the design. The parameters optimized were digestion pressure (A), power (B) and irradiation time (C). Table 1 shows the description of the experiments and the relation between codified and real experimental values chosen in this work. Maximum recovery was found for experiment 5 (Table 1). Hence, the values of pressure, power and irradiation time were fixed at 40 psi, 855 W and 1 min, respectively. The statistical analysis of the results was performed considering all possible interactions between the different variables. As can be observed in Fig. 1, only variable A (pressure) and AC (interaction) can be considered significant, showing a negative and positive effect, respectively. The other factors and interactions were found to be statistically negligible (95% probability). 3.2. Optimization of the ultrasound-assisted digestion method Manganese(III) ultrasound-assisted digestion method was optimized by a three-factor design with two levels for each factor. In this case, the design also requires eight experiments plus one in the middle, which were performed in duplicate and were randomly run. The extra experiment was included for the same reason as before with microwaves (i.e., to have an estimation of the response in the centre of the design). The selected parameters were cycles (A), intensity (B), and sonication time (C). The instrument can be used in pulsed mode to enable rhythmic processing of media. With a pulse setting of ‘‘1” the reaction mixture is sonicated without interruption whereas with a pulse setting, for example, of ‘‘0.5” the reaction mixture is sonicated for 0.5 s and then sonication stops for 0.5 s. Hence, in pulse mode the ratio of sound-emission time
Fig. 1. Pareto chart of the standardized effects in the 23 factorial design for Mn(III) closed COD microwave-assisted digestion method optimization.
Table 1 Factor levels and design matrix in the 23 factorial design for Mn(III) closed COD microwave-assisted digestion method optimization. Factor
Pressure (psi) Power (W) Irradiation time (min)
Key
A B C
Levels
Run
Low ()
High (+)
Middle (0)
1
2
3
4
5
6
7
8
9
40 655 1
90 855 5
65 760 3
+ +
+ +
+
+
+ + +
+ +
+
0 0 0
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C.E. Domini et al. / Ultrasonics Sonochemistry 16 (2009) 686–691 Table 2 Factor levels and design matrix in the 23 factorial design for Mn(III) COD ultrasound-assisted digestion method optimization. Factor
Key
Levels Low ()
High (+)
Middle (0)
1
2
3
4
5
6
7
8
9
Cyclesa (s (s1)) Intensity (%) Sonication time (min)
A B C
0.2 20 1
0.9 90 5
0.6 55 3
+
+
+ +
+
+ +
+ +
+ + +
0 0 0
a
Run
See text for explanation.
Fig. 2. Pareto chart of the standardized effects in the 23 factorial design for Mn(III) COD ultrasound-assisted digestion method optimization.
to cyclic pause time can be adjusted continuously from 0% to 100% per second. Table 2 lists the design matrix for experiments and the values given to each factor. With this assisted sample digestion method, the maximum recovery was found for experiment 4, hence, 0.9-second pulses each second, 90% of maximum nominal power (180 W) and 1 min were used for cycles, intensity and sonication time, respectively. Fig. 2 shows that only one of the three main factors, cycles (A), is significant showing a positive effect (95% probability). What seems like contradiction between Pareto chart (Fig. 2) and optimum conditions (experiment number 4 on Table 2) for variable B (intensity) is due to the interactions between variables. However, taking into account interactions AB and BC the best B value is + (i.e., 90%). When there are interactions between variables prevents from interpreting the main effects directly. A factor can not be said to have a single effect when it interacts with another. The analysis should be carried out directly on the interactions, using the first order interaction diagrams (not shown) [36].
Fig. 3. Chloride ion interference study (n = 3). Theoretical COD value: 100 mg/L.
ker filled with water and heated by a conventional heating plate at 150 °C for 2 h. The basket was then removed from the reaction tube and the Mn(III) oxidant was then added to the sample and digestion was done under the optimized conditions. In this work, interference from chloride was investigated between 300 and 4830 mg/L and interference concentration was considered significant when produced a deviation of 10% in COD values. As can be seen in Fig. 3, interferences are significant from 300 mg/L of chloride ion when microwave radiation is applied, whereas ultrasound energy shows negligible interferences until 600 mg/L of chloride ion. Under the latter chloride concentration the measured COD value was 109 ± 6 (standard deviation). When bismuth-based adsorbent was used the interference of chloride ion was not significant until 1000 mg/L with both assisted digestion methods (Fig. 3). 3.4. Application of methods
3.3. Interferences Chloride ions are the most important interference in COD determination because they are amenable to oxidation by dichromate and can precipitate the silver used as catalyst. This problem is usually solved by adding HgSO4 to the sample and the concentration of HgSO4 required is related to the concentration of chloride in the sample. Hence, if this interference could be reduced or even eliminated the method would be more environmentally sound. Automated methods of analysis are a good choice to reduce or even eliminate this interference. When digestion is performed by flow injection, chloride is tolerated up to concentrations of 30,000 mg/ L and 10,000 mg/L when Ce(IV) is used as oxidizing agent or microwaves as heating method, respectively [37,38]. For chloride ion removal, the sample solutions were prepared by adding 1.0 mL of aqueous sample and 1.0 mL of concentrated sulfuric acid to a reaction tube. A pre-wetted basket with the adsorbent [35] was then inserted into the reaction tube, which was subsequently capped and gently shaken to mix the sample with sulfuric acid. Next, the reaction tube was introduced in a ba-
3.4.1. Application to pure organic compounds As a first test of the general viability of the optimized sample pre-treatment methods, the COD values of various pure organic compounds were determined by the Mn(III) digestion methods assisted by microwaves and ultrasounds. The results are given in Table 3. For comparison purposes the COD values determined by the classical closed reflux method are also included in the table. The recovery values are ranged between 96–100%, 61–100% and 51– 94% for classical, closed microwaves and ultrasound-assisted digestion methods, respectively. Pyridine was poorly oxidized with all digestion methods as in other previous COD studies [18], but for the acetic acid the assisted digestion methods supplied higher recovery values (75% and 62% for microwaves and ultrasounds, respectively) than a previously published work using Mn(III) as oxidant (24%) [11]. The corresponding recovery values for some compounds (i.e., salicylic acid, methyl isobutyl ketone, acetic acid and picric acid) are from 51% to 76% for both assisted methods. These limitations of the assisted digestion methods as regards the type of organic compound have also been described previously
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Table 3 COD values and relative standard deviations for some pure organic compounds using the Mn(III) assisted digestion methods and the classical digestion method.a Compound
D-Glucose Lactic acid Salicylic acid Methyl isobutyl ketone Acetic acid Pyridine Picric acid a
Theoretical COD value (mg/L)
Cr(VI) classical method
Mn(III) closed microwaves method
Mn(III) ultrasound method
COD (mg/ L)
RSD (%)
Recovery (%)
COD (mg/ L)
RSD (%)
Recovery (%)
COD (mg/ L)
RSD (%)
Recovery (%)
102.3 100.8 101.8 97.7
102.6 98.0 98.0 97.0
1.1 1.6 0.9 2.2
100 97 96 99
102.0 90.4 72.0 60.0
5.1 5.6 6.0 6.4
100 90 71 61
96.5 93.2 68.4 49.6
5.1 1.5 4.5 1.7
94 92 67 51
102.8 99.05 100.2
102.6 12.1 100.3
2.4 2.2 1.4
100 12 100
77.2 20.0 75.4
3.0 8.4 5.1
75 20 75
63.7 19.5 76.5
1.1 5.0 4.6
62 20 76
Number of replicates = 5.
Table 4 Mean COD recoveries and RSD values obtained with the digestion methods applied to RMs.a Reference material
Reference COD value (mg/L)
Digestion method
Obtained COD value (mg/L)
RSD (%)
Recovery (%)
Reagecom
199–201
GBW08624B
1277 ± 1.2
Cr(VI) classical method Mn(III) closed microwave Mn(III) ultrasound Cr(VI) classical method Mn(III) closed microwave Mn(III) ultrasound
205 198 198 1260 1173 1174
2 2 1 3 4 0.9
103 99 99 99 92 92
a
Number of replicates = 5.
Table 5 Mean COD recoveries and RSD values obtained with the digestion methods applied to real samples.a Sample number
1 2 3 4 5 a
Cr(VI) classical method
Mn(III) closed microwaves method
COD (mg/L)
RSD (%)
COD (mg/L)
RSD (%)
Recovery (%)
Mn(III) ultrasound method COD (mg/L)
RSD (%)
Recovery (%)
5075 17325 1231 788 866
0.08 0.02 0.5 1.0 1.2
4851 16764 1083 678 814
0.2 0.1 0.9 3.6 1.1
96 97 88 86 94
4123 11746 918 711 615
1.4 3.2 2.2 3.3 1.9
81 68 75 91 71
Number of replicates = 5.
for an ultrasound-assisted [33] and the official COD methods [39,40]. Most of relative standard deviation (RSD) values are ranged between 1.0% and 6.5%, being the highest the ones obtained with Mn(III) digestion assisted by microwaves (3.0–6.5%). 3.4.2. Reference materials The COD values of two COD reference materials (RM) were determined by the two Mn(III) assisted digestion methods. The results are given in Table 4. For comparison purposes the COD values determined by the classical digestion method are also included in the results. In almost all cases, recovery values ranged from 92% to 103% and RSD values were always lower than 4% (n = 5). 3.4.3. Application to real samples Table 5 shows the COD and RSD values obtained with the two optimized assisted digestion methods and the classical digestion one when applied to five real wastewater samples from an industrial plant. COD values obtained with the classical open reflux method were used as reference for recovery calculations. Bismuth-based adsorbent was applied to remove interferences to all real samples. Recovery values were ranged between 86–97% and 68–91% for the closed microwave-assisted digestion and ultrasound-assisted digestion methods, respectively. This range of recoveries is similar of previous work with the same oxidant, however, in this work a significant reduction on the digestion time is observed (1 min vs. 60 min) [11]. The low recovery values obtained
with the ultrasound-assisted method could be due to a less energetic conditions obtained with this type of energy and type of organic compounds that constitute the samples [33,39,40]. It should be borne in mind that the optimization studies were done with KHP solutions (i.e., an easily oxidized compound). In almost all cases, RSD values were lower than 4% (n = 5).
4. Conclusions Manganese(III) has been proved as an efficient oxidizing reagent for COD assisted determination methods in wastewater samples. Microwaves and ultrasound have been evaluated to assist Mn(III) as oxidant, however satisfactory results have only been obtained assisted by microwave radiation. Factorial design has been used to speed up the optimization of two assisted digestion methods and the optimum values obtained for the different experimental variables studied in each case were: 40 psi pressure, 855 W power and 1 min irradiation time (microwave-assisted digestion) and 90% of maximum nominal power (180 W), 0.9-second pulses per second and 1 min irradiation time (ultrasound-assisted digestion). Both irradiations times are similar than previously published works about COD assisted methods using more toxic oxidants [18], however, when compared with a previously published work using the same Mn(III) oxidant [11] the digestion time has been significantly reduced from 1 h to 1 min.
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The bismuth adsorbent minimizes the chloride ion interference in COD determinations avoiding the use of silver and highly toxic mercury salts. Then, the new manganese(III) microwave-assisted COD method with chloride pre-treatment offers a functional, affordable, rapid and environmentally sound alternative to the current chromium– mercury–silver COD determination systems.
[18]
[19]
[20]
Acknowledgements [21]
The authors gratefully acknowledge the Spanish Ministry of Science and Technology (Project numbers DPI2002-04305-C02-01, CTQ2005-09079-C03-01/BQU and PTR1995-0882-OP-02-01) for the financial support of this work. C.E.D. also thanks to AGEVAL and the University of Alicante (‘‘Vicerrectorado de Coordinación y Comunicación”) for her PhD fellowship. This work is part of the Doctoral Thesis of Claudia E. Domini.
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