Received 28 September 2015; accepted 3 January 2016; published 6 January 2016
Coordination compounds play a vital role in our lives and in various fields. Also, the complexes have the ability to chelate metal ions via several sites such as nitrogen, oxygen, and/or sulfur atoms  . Recently, since the increasing use of coordination compounds in analytical, bio-, medicinal chemistry and pigments, many investigators are embarked to these topics, especially the important roles of the complexes derived from hydrazone- oximes. There has been considerable interest in the development of novel compounds with anticonvulsant, antidepressant, analgesic, anti-inflammatory, antiplatelet, antimalarial, antimicrobial, anti-mycobacterial, and anti-tumor, and vasodilator, antiviral and anti-schistosomiasis activities. Hydrazones possess azometine moiety, which constitutes an important class of compounds for new drug development. Therefore, many researchers direct to synthesize these classes of compounds as target structures and evaluate their biological activities. These observations have been guided for the development of new hydrazones that possess varied biological activities  . Mercury is a highly toxic element that is found both naturally and as an introduced contaminant in the environment. The risk is determined by the likelihood of exposure, the form of mercury present (some forms are more toxic than others) and the geochemical and ecological factors that influence how mercury moves and changes form in the environment. Numerous techniques for the separation and/or pre-concentration of trace metals from different analytes have been reported such as volatilization, liquid-liquid extraction, selective dissolution, precipitation, electrochemical deposition and dissolution, ion exchange, liquid chromatography, flotation, freezing and zone melting and cloud point extraction (CPE)  . Of these techniques flotation has the particular merit of providing efficient, quick, simple preconcentration of trace elements both as anionic or cationic species from: a) media of low and high salinity  and b) large solution volumes  ; it therefore has a considerable potential in the determination of very small amounts of metal ions in solution. The flotation technique can be classified into precipitate flotation and ion flotation. In ion flotation technique, the desired trace ions in an aqueous solution are converted into hydrophobic species by adding ligands and/or surfactants floated with the aid of numerous bubbles and concentrated in a scum or copious foam layer on the solution surface  .
The lack of any studies reported in literature concerning the synthesis and characterization of [Hg(L1)Cl] Cl・5H2O gives us the push to investigate the Hg2+ complex. Also, the aim of the present study is to throw more light on the synthesis and characterization of Hg2+ complex. Moreover, our goal is extended to introduce 2,3- butanedionemonoxime Girard’s T hydrazone as a new reagent for the flotation and CVAAS determination of total Hg2+ traces in water samples. Finally, the different experimental factors affecting the flotation process have been investigated in details.
2.1. Materials and Reagents
All the chemicals used were of analytical grade and used without further purification. A saturated solution of Hg2+ (1000 mg L−1) was used after appropriate dilution with double deionized distilled water. Other chemicals and reagents were from BDH quality. Oleic acid (HOL) stock solution, 6.36 × 10−2 mol・dm−3 was prepared by dispersing 20 cm3 of HOL, (food grade with sp. gr. 0.895, provided by JT Baker Chemical Co.), in 1 dm3 kerosene. L1 was prepared as described earlier  . A Perkin-Elmer model 2380 AAS was used, inconnection with a mercury hydride system (MHS-10). Nitrogen or argon was used as a purge gas and NaBH4 as reluctant. Elemental analyses (C, H, M) were performed with a Perkin-Elmer 2400 series II analyzer at the Microanalytical Center at Cairo University, Egypt. Chloride was determined gravimetrically the as AgCl  . The IR spectrum of [Hg(L1)Cl]Cl・5H2O was recorded as KBr discs on Mattson 5000 FTIR spectrophotometer (400 - 4000 cm−1). 1H-NMR spectra were recorded on Jeol-90Q Fourier Transform (200 MHz) in d6-DMSO at Cairo University, Egypt. Mass spectra were recorded on MS 70 eV EIGC, MS QP-1000 EX Shimadzu (Japan) mass spectrometer at Cairo University. Thermal analyses measurements (TG, DTG) were recorded with a Shimadzu Thermo gravimetric Analyzer TGA-50 using α-Al2O3 as a reference material at Mansoura University.
2.2. Flotation Cells
Two types of flotation cells were used throughout this work have been described earlier  . Flotation cell (a) is a cylindrically graduated glass tube of 16 mm inner diameter and 290 mm length with a stopcock at the bottom. Such cell is used to study the different factors affecting the efficiency of flotation. Flotation cell (b) is a cylindrical tube of 6 cm inner diameter and 45 cm length with a stopcock at the bottom and a quick fit stopper at the top; this cell is used to separate mercury from 1 dm3 of different water samples. The pH of each sample was adjusted in the range 2 - 10 using Hanna Instruments 8519 digital pH meter with glass and saturated calomelelectrodes calibrated on the operational state using standard buffer solutions.
The structure of L1 and its Hg2+ complex, geometry optimization and conformational analysis has been performed using of MM+ force ﬁeld as implemented in Hyperchem 8.0  . The low lying obtained from MM+ was then optimized at PM3 using the Polak-Ribiere algorithm in RHF-SCF set to terminate at an RMS gradient of 0.01 Kcal・mol−1.
2.4. Analytical Procedures
Two mL of aqueous EtOH solution of 1 × 10−4 mol・L−1 and L1 were introduced into a flotation cell containing 1 × 10−6 mol−1 of Hg2+ solution then the pH was adjusted to 5.0 using HCl and/or NaOH and the solution was mixed thoroughly. The mixture was then diluted to 10 mL with redistilled water. To the above solution 3 mL of oleic acid with a definite concentration (2 × 10−4 mol・L−1) were added. The cell was then turned upside down twenty times by hand and kept upright for 5 min to ensure complete flotation of the Hg2+ complex species. The scum layer was eluted with 5 mL of L1 mol・L−1 HCl (1:1) solution to complete trapping  . The concentration of Hg2+ was determined using CVAAS measurements at 253.7 nm with a Perkin-Elmer 2380 atomic absorption spectrometer. The separation efficiency (%F) was calculated from the relation:
where, Cs and Ci denote the scum and the initial concentrations of Hg2+, respectively.
2.5. Analysis of Water Samples
Water samples were obtained as follows: distilled water, tap water, river Nile and underground water from Mansoura City. All samples were filtered through G4 sintered glass. For total organic mercury in water, the samples were digested in a closed system using the sequence of 10 mL of 5% KMnO4, 10 mL of 8N HNO3, 10 mL of 18 N H2SO4 and 20 ml of 4% K2S2O8. The samples were heated at <90˚C for 30 min, allowed to cool and then 4 mL of 10% NH2OH・HCl was added to reduce excess oxidant immediately before the flotation procedure was carried out. To large flotation cells, five water samples (1 L each) containing a defined amount of Hg2+ chloride and 5 mL of 10−3 M L1 were added and the pH was adjusted to 5 - 6. The reaction mixture was shaken to ensure complete complex formation. Then, 8 mL 10−3 M HOL was added to each flotation cell and the cells are shaken upside down for five min. The scum layer was separated and eluted with 1 mol L−1 HCl. The final volume was 10 mL.
2.6. Synthesis of L1
L1(C9H19N4O2Cl) was synthesized as described earlier  and can be represented by keto/enol forms as shown Figure 1. The product is white in color and soluble in H2O and most polar organic solvents and the value of molar conductance in DMSO (28.3 ohm−1・cm2・mol−1) suggesting the electrolytic nature of L1. The structure of L1 is confirmed using elemental analyses (Calcd: C = 40.2, H = 7.9, Cl = 13.2; Found: 40.4, 7.3, 12.9) and spectral measurements.The melting point of L1 is 172˚C, which matches the results reported value  .
2.7. Synthesis and Characterization of [Hg(L1)Cl]Cl・5H2O
The Hg2+ complex, [Hg(L1)Cl]Cl・5H2O, was synthesized by refluxing solutions of equivalent amounts of L1 and
Figure 1. The keto and enolforms of L1.
HgCl2 in absolute EtOH for 0.5 h. The product was filtered off, washed several times with hot EtOH and Et2O and finally dried in a vacuum desiccator over anhydrous CaCl2. The yield of the Hg2+ complex is 96%. The structure of the complex is confirmed by its melting point (212˚C) and elemental analyses (Calcd: C = 17.7, H = 4.8, Hg = 32.7, Cl = 17.4; Found: 16.9, 4.8, 32.4 and 18.1) and represented in Figure 2.
3. Results and Discussion
3.1. Infrared Spectra
The IR spectrum of the free L1 (Figure 1(a)) in KBr shows a strong band at 1697 cm−1 assignable to the ν(C=O) vibration  in addition to medium and weak bands at 1650, 1614, 1405 and 1020 cm−1 assigned to the azomethine of hydrazone ν(C=N1), azomethine of oxime ν(C=N2), δ(OH) and ν(N-N) vibrations, respectively  . Also, the two bands observed at 3124 and 3214 cm−1 are assigned to the free ν(NH) and hydrogen bonded, respectively. The bands observed at 3395 and 3454 cm−1 are attributed to the (OH) free and bonded hydrogen, respectively. The broad weak bands in the 1800 - 1200 cm−1 and 2200 - 2400 cm−1 regions are taken as an evidence for the existence of intra-molecular hydrogen bonding of the type (OH… N) (Figure S1)  .
The IR spectrum of [Hg(L1)Cl]Cl・5H2O (Figure 3) suggests that L1 coordinates as a neutral bidentate via the two azomethine groups as shown in Figure 2. The mode of chelation is supported by the IR spectrum where; i) the negative shift of both the bands of azomethine (C=N1) and (C=N2) groups and ii) the bands of the (C=O) and (OH) groups remainexisted after coordination indicating that these groups are not participated in the coordin- ation.
Figure 2. Structure of Hg2+ complex.
Figure 3. IR spectrum of [Hg(L1)Cl]Cl・5H2O.
3.2. 1H-NMR Spectra
The 1H-NMR spectrum of L1 in d6-DMSO (Figure 4) shows two signals at 11.74 ppm and 11.25 ppm, down- field with respect to TMS, which disappear upon adding D2O. These signals are attributed to the protons of (OH) of the oxime group and (CONH) group, respectively. The signals in the 1.98 - 2.19 ppm range are assigned to the three methyl groups (CH3)3. Also, the signals observed at 2.50, 4.56 and 4.76 ppm are attributed to the protons of
Figure 4. 1H-NMR spectrum of L1 in d6-DMSO and D2O.
(CH3) and (CH2) of the oxime group and (CH3) of the hydrazone group, respectively. All these foundations are taken as evidence that L1 is mainly existed in the keto form either in the free case or in the hydrogen bonded.
The 1H-NMR spectrum of [Hg(L1)Cl]Cl・5H2O in d6-DMSO (Figure S2) shows two signals at 11.73 ppm and 11.24 ppm, downfield with respect to TMS, which disappear upon adding D2O (Figure S3). These signals are attributed to the protons of (OH) of the oxime and the NH of the (CONH) group, respectively. The signals in the 1.97 - 2.12 ppm range correspond to the three methyl groups (CH3)3. Also, the observed signals at 2.49 - 2.51 ppm, 4.3 ppm and 4.72 ppm are attributed to the protons of (CH2) and (CH3) of the oxime group and (CH3) of the hydrazone group, respectively. All these observations confirm that the complex exists in the keto form.
3.3. Mass Spectra
The mass spectrum of L1 (Figure S4) shows the molecular ion peak at m/z = 250. This suggests that the proposed structure for L1 is correct and has the chemical formula; C9H19N4O2Cl and the M. wt. = 250.726. Also, the results of elemental analyses and 1H-NMR are taken as strong evidences for the proposed structure (Figure 2). The mass fragments of L1 are shown in Scheme S1. The mass spectrum of [Hg(L1)Cl]Cl・5H2O (Figure S5) shows the molecolare ion peak at m/z = 613 while the theoretical value is 612.29.
3.4. Molecular Modeling
The molecular modeling along with atom member of L1 and its Hg2+ complex, [Hg(L1)Cl]Cl・5H2O, are shown in Figure 5 and Figure 6. The data are calculated using quantum mechanics for the complexes. Semi-empirical molecular Mechanics Optimization method is used.
1) Some bond lengths don’t change as in N(8)-O(20) of L1, N(3)-O(13) of complex, N(7)-N(13) of L1 and
Figure 5. Molecular modeling of L1.
Figure 6. Modeling structure of [Hg(L1)Cl]Cl・5H2O.
Scheme S1. The fragmentation pattern of L1.
N(4)-N(15) of complex which have the same values 1.316 Å and 1.352 Å, respectively.
2) On the other hand, some bonds are elongated as C(1)-N(7) and C(6)-N(8) of L1 (1.244 Å) but C(1)-N(4) and C(2)-N(3) of complex (1.377 Å).
3) Other bonds are shortened as C(1)-C(2) of L1 (1.540 Å) changed into 1.337 Å in the complex.
4) The same notifications can be discussed in bond angles. These differences take place on coordination and formation of the five-membered ring, which ensures the minimum energetic state of the complex.
3.5. Thermo Gravimetric Analysis
Thermal studies of the Hg2+ complex is studied in the range 30˚C - 800˚C to insight about its thermal stability, the nature of the solvent molecules and the general scheme for their thermal decomposition. The data showed that the water of crystallization is volatilized within the temperature range 75˚C - 125˚C. The TGA decomposition steps with the temperature range and weight loss for the Hg2+ complex.
3.6. Kinetic Studies
Figure 7. Modeling structure of [Hg(L1)Cl]Cl・5H2O.
Table 1. Some of energetic properties of L1 calculated by DMOL3 using DFT-method.
1) All decomposition stages showed a best fit for n = 1, while the other values have no better correlation.
2) The activation energy (Ea) decreases for the subsequent degradation steps revealing a less energy needed for the thermal decomposition of the remaining parts.
3) The negative value of the entropy of activation (ΔS*) of the decomposition steps of the metal complex indicates that the activated fragments have more ordered structure than the undecomposed complex and/or the decomposition reactions are slow  .
4) The negative sign of the enthalpy of activation ΔH* of the decomposition stages reveals that the decomposition stages are easier.
The positive sign of free energy of activation (ΔG*) indicates that the free energy of the final residue is higher than that of the initial compound and hence all the decomposition steps are nonspontaneous processes. Moreover, the values of ΔG* increase significantly for the subsequent decomposition stages of a given compound. This conclusion, as a result of the increasing of TΔS* reflects that the rate of removal of the subsequent species is lower than that of the precedent one  -  .
3.7. Analytical Studies
3.7.1. Influence of pH
The pH of a solution is a very important factor for metal chelate formation and for the flotation process. There- fore, the effect of pH on the flotation of Hg-L1 chelate was studied in the pH values ranging from 2.0 to 9.0. The results are shown in Figure 8. In the absence of L1 (Figure 8(a)) the flotation efficiency of Hg2+ is very low over the pH range tested. The maximum flotation efficiency (~92%) was recorded over pH values ranging from 4.5 to 6.0. According to Figure 8 (A andb) the effective role of L1 is clear; it forms a complex with Hg2+ ions rendering them more hydrophobic and easily separated from the solution bulk using the HOL surfactant. At higher pH valuesthe decrease in the flotation efficiency is attributed to the formation of a white emulsion and due to the formation of excessive foams of sodium oleate. This will hinder the reaction to complete.
3.7.2. Influence of Oleic Acid Concentration [HOL]
The surfactant concentration (oleic acid) is very important parameter; up to a certain concentration of HOL the floatability increase. Figure 9 shows that the floatability remains at higher up to value (~99%) over the concen- tration range (2 - 6 × 10−4 mol・L−1) of oleic acid and decreases gradually as the concentration increases. The decrease in the flotation efficiency at higher HOL concentrations is due to the collection of the surfactant molecules together forming micelles  . These micelles compete with colligend molecule, [Hg(L1)Cl]Cl・5H2O and since they stay in the solution, they reduce the effectiveness of separation. In addition, the concentration of surfactant changes the bubble size with the size getting smaller as the surfactant increases.
Figure 8. Influence of pH on the flotation efficiency of Hg (II): (a) in absence of L1 and (b) in the presence of 1.0 × 10−4 mol・L−1 L1 and 2 × 10−4 mol・L−1/HOL.
Figure 9. Influence of oleic acid (HOL) concentration on the flotation efficiency of Hg (II).
3.7.3. Influence of Ligand Concentration (L1)
On fixing the various optimum conditions, the variety of L1 concentration was examined. The data obtained show that the floatability of the Hg2+ ion increases clearly reaching its maximum percentage (99%) at M:L ratio of (1:1). Moreover, excess amount of collector has no effect on the flotation process. Therefore, a concentration of 1 × 10−4 mol・L−1 L1 was used.
3.7.4. Influence of Temperature
The maximum flotation efficiency is obtained in the range (25˚C - 50˚C). The proposed flotation procedure is performed at room temperature (25˚C).
3.7.5. Influence of Volume
A series of experiments was achieved to float different concentrations of Hg2+ solution from different aqueous volumes using suitable large flotation cells under the recommended conditions. The results obtained revealed that, up to 30 μg of Hg2+ could be quantitatively separated from one liter into 10 mL of HOL with a preconcentration factor of 100.
3.7.6. Influence of Ionic Strength
Table 2 illustrates the effect of varying the ionic strength of different salts on the floatation efficiency of the studied metal ion using the optimum conditions. The salts used in adjusting the ionic strength generally similar natural water samples. It is quite clear that the ionic strength of the medium has not markedly affected the flotation process.
3.7.7. Influence of Foreign Ions
In order to study the tendency of L1 to form complexes with number metal ions, the effect of foreign metal ions on the flotation of Hg2+ ion using the optimum conditions is examined. These foreign ions are selected on the basis that they are normally present in fresh and saline waters. The tolerable amounts of each ion giving a maximum error ±5% in the flotation efficiency are summarized in Table 3. The experimental data showed that most of the investigated foreign cations (Na+, K+, Ca2+ and Mg2+) and anions (acetate, oxalate, Cl−, I−, , and) did not interfere in the recovery of Hg2+ ion using the optimum conditions whereas other foreign ions have little interfering effects (~2%). All of these interferences were completely removed by increasing the concentration of L1.
3.7.8. Mechanism of Flotation
The nature of the interaction between oleic acid surfactant and the formed complex must be studied to approach the actual mechanism of flotation. The proposed mechanism may proceed through: i) a physical interaction; ii)
Table 2. Influence of ionic strength on the flotation (% F) of Hg2+.
Table 3. Influence of foreign ions on the floatability of Hg2+ under the optimum conditions.
Table 4. Removal of Hg2+ from water samples (L1 = 10 - 3 M, pH = 5, 25˚C, n = 3).
by forming a hydrogen bond between the hydrophilic part of HOL and the active sites in the ligand complex or iii) by an interaction between oleic acid and the complex, formed in solution through a coordinate bond forming a self-floatable (Hg2+-L1-HOL) species. In all cases, the hydrophobic part of the surfactant attaches to air bubbles and floats separating the analyte containing species.
3.7.9. Analytical Application
The determination of the Hg2+ ion in natural water samples was carried out by CVAAS after the flotation preconcentration was carried out. The samples were collected from different places. The analytical results are summarized in Table 4.
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