CHAPTER 2
Materials and methods
2.1 Materials
All the chemicals non expressly cited are of analytical grade. All the reagents were employed without further purification. Water, purified by pumping demineralized water through a Milli-Q apparatus, was used to prepare the solutions and also as the reaction medium.
2.1.1 Metals
Cadmium was used in the form of hydrated metal ion perchlorate, purchased from Fluka. A stock solution of Cd(ClO4)2 was obtained by dissolving the salt in water; an aliquot of the resulting solution was titrated as HClO4 after ion exchange (Malatesta et al. 1999).
Palladium was used in the form of tetrachloropalladate. Aqueous solutions of K2PdCl4 were purchased from CHIMET S.p.a. Stock solutions were prepared by diluting weighed amounts of the solution in water.
Platinum was used in the form of tetrachloroplatinate purchased from CHIMET S.p.a. Stock solutions were prepared by dissolving the salt in acid solution of water.
Gallium was used in the form of gallium perchlorate. Ga(ClO4)3 was prepared by dissolution of a known weight of the pure metal in a known excess of perchloric acid. The dissolution process is very slow due to hydrogen overvoltage; hence, the metal was put in contact with a platinum wire and the solution was slightly warmed. This procedure led to a dramatic increase in the speed of dissolution. The concentrations of gallium(III) stock solutions were checked by titrating with EDTA (Orrichi, 1962) and they were found to coincide with those calculated from the weight of the dissolved metal.
2.1.2 Ligands
The ligands pyridine-2-azo-p-dimethylaniline (PADA) and 8-hydroxyquinoline (HQ), whose structures are shown in Figure 2.1, were obtained from Sigma-Aldrich. Stock solutions were prepared by dissolving weighed amounts of the solid in water. In the case of PADA 1% of ethyl alcohol was added while preparing the stock solution to increase ligand solubility.
N
N N N CH3
CH3
N OH
Fig 2.1 Molecular structures of (a) pyridine-2-azo-p-dimethylaniline (PADA) and (b)
8-hydroxyquinoline (HQ).
2.1.3 Surfactants
Sodium dodecylsulphate (SDS) was from Sigma-Aldrich and dodecyltrimethylammonium chloride (DTAC) was from Fluka. In both cases stock solutions were prepared by dissolving weighed amounts of the solid in water.
2.1.4 Ultrafiltration membranes
Ultrafiltration membranes made of regenerated cellulose (YM 3, Millipore) of diameter 44.5 mm and with a molecular weight cut off (MWCO) of 3000 daltons were used. UF membrane having higher MWCO could also be used taking into account the aggregation number of SDS (Yoshikiyo et al., 1987) and DTAC (Bales and Zana, 2002), this will result in a increase in the permeate flux. The change of
(a)
MWCO has, however, only minor effect on the metal rejection. The membranes were pre-treated and stored according to the method recommended by Millipore.
2.2 Methods
2.2.1 Spectrophotometry
A Perkin-Elmer 17 UV-Vis spectrophotometer was used to record absorption spectra and to perform spectrophotometric titrations. All measurements were made at 25 ± 0.1°C. Titrations were performed by adding with a microsyringe (Mitutoyo) increasing volumes of metal solution to 2.0 ml of the ligand solution directly in the spectrophotometric cell. The acidity and ionic strength were kept constant at the desired value during each titration. Experimental data, recorded at selected wavelengths, were analysed by means of non-linear least-square fitting procedures (Jandel – AISN).The equilibrium constants were obtained by means of the iterative procedure described in Appendix I.
2.2.2 Kinetics
The kinetic experiments were all performed using the stopped-flow technique, by monitoring, after fast mixing of the reactants, absorbance changes vs. time. This technique enables reactions taking place between milliseconds and many minutes time range to be studied. The apparatus, assembled in our laboratory, was equipped with a Biologic SFM-300 mixing unit connected to a spectrophotometric line by two optical fibres. The UV radiation from a Hamamatsu L248102 “quiet lamp” was passed through a Baush and Lomb 338875 high-intensity monochromator and split into two beams. The reference beam was sent directly to a 1P28 photomultiplier. The output from the two photomultipliers was balanced before each shot. The acquisition system keeps a record of a number of data points ranging from 10 to 8000 with a sampling interval in the 50 µs to 10 s time scale. Figure 2.3 shows the internal scheme of a generic stopped flow apparatus.
Figure 2.3 Internal scheme of a stopped flow apparatus. A)Light input from monochromator and
mirror box B)Light output to photomultiplier tube. 1)plunger 2)syringes 3)reservoir syringes 4)mixing chamber 5)cell 6)thermostating block 7)drain tube 8)drain valve block 9)stop syringe 10)adjustable stop nuts 11)mechanical stop 12)leaf-type trigger switch.
2.2.3 Ultrafiltration
The MEUF and LM-MEUF experiments were carried out in batch stirred cells (Amicon, model 8050) with a capacity of 50 ml and an effective membrane area of 13.4 cm2. A schematic representation of the cell is shown in Figure 2.4. The cell was initially loaded with 20 ml of solution under an applied nitrogen pressure of 3 bar. The permeate solution was collected until 2 ml of the retentate solution remained. In the stripping experiments the retentate was then mixed with 10 ml of stripping solution and then a second ultrafiltration step was performed which allows separation of the metal-containing phase (the permeate in this case) from the surfactant-containing phase (the retentate); this procedure enabled us to collect 10 ml of permeate solution. Shortly after the conclusion of the procedure the ultrafiltration membranes were flushed with deionized water and if necessary they were regenerated according to the method recommended by Millipore (Amicon). The amounts of metal ions extracted or recovered by ultrafiltration were assessed through atomic absorption spectroscopy. In any case only the permeate was analyzed, while the metal ion in the retentate was obtained by difference between the metal ion in the permeate and in the initial solution. The reproducibility of
concentration measurements in the extraction and stripping experiments was within ± 2 % and 4% respectively.
Figure 2.4 Schematic view of an ultrafiltration cell.
2.2.4 Atomic Absorption Spectroscopy
A Perkin Elmer HGA-800 atomic absorption spectrophotometer was used to measure the metal ions concentration in the permeate. Samples were atomised in an air/acetylene flame; in the case of palladium and platinum a graphite furnace was employed too. The light source was a hollow-cathode lamp of the element that was being measured.
2.2.5 pH, conductivity, surface tension, calorimetry measurements
A Metrohm 713 pH-meter equipped with a combined glass microelectrode was used in order to measure the pH of the solutions. The electrode was calibrated automatically employing buffers in the 4-9 pH range and solutions of known HCl concentration below pH 4.
Conductivity, surface tension and calorimetry experiments were used in order to measure the critical micelle concentration of DTAC and/or SDS. Conductivity and surface tension measurements were performed using respectively an Amel 160
conductivity meter and a KSV Sigma 703 manual digital tensiometer whereas a 2277 Thermal Activity Monitor calorimeter (TAM) was employed to perform the calorimetric measurement.
