Extraction and Separation of Some Metal Ions by Diethylenetriamine Polysiloxane Immobilized Ligand System

An extraction chromatographic solid porous polysiloxane functionalized by chelating diethylenetriamine ligand of the general formula P-(CH2)3-NH(CH2)2NH(CH2)2NH2, (Where P represents [Si-O]n siloxane network) has been evaluated for the separation of Co(II), Ni(II) and Cu(II) from aqueous solutions. The chromatographic parameters of the separation method have been optimized. The ligand system retained Co(II), Cu(II) and Zn(II) effectively when used as a metal ion extractant by controlling the pH value. The ligand system also shows a good separation of a mixture of metal ions Co(II), Ni(II) and Cu(II) when used as chromatographic stationary phase. The optimum separation pH values were 4.5, 4 for Co(II) and Ni(II) respectively, while a solution of 0.1 M HNO3 was used to elute Cu(II). Metal ions were also preconcentrated at pH 5.5. The chemisorbed metal ions were regenerated from the solid extractant using 0.5 M HCl.


Reagents and Materials
Tetraethylorthosilicate, diethylenetriaminopropytrimethoxysilane, were purchased from MERCK and used as received.Diethyl ether and methanol (spectroscopic grade) were used as received.Solutions of metal ions of the appropriate concentrations were prepared by dissolving the metal chloride (analar grade) in deionized water.pH range of 3-6 was controlled by using acetic acid (0.1 M)/sodium acetate (0.1 M) buffer solutions.

General Techniques
Analysis for carbon, hydrogen and nitrogen was carried out, using an Elemental Analyzer EA 1110-CHNS CE Instrument.The concentrations of metal ions in their aqueous buffer solutions were measured using a Perkin-Elmer A Analyst-100 spectrometer.The infrared spectra for the materials were recorded on a Perkin-Elmer FTIR spectrometer using KBr disk in the range 4000 to 400 cm −1 .All pH measurements were carried out using HM-40 V pH Meter.

Preparation of Polysiloxane-Immobilized Diethylenetriamine Ligand System (P-DTA).
Polysiloxane-Immobilized diethylenetriamine (P-DTA) was prepared by adding diethylenetriaminopropytrimethoxysilane (13.27,50 mmol) to a stirred solution of tetraethylorthosilicate (20.83 g, 100 mmol) in 15 mL methanol and HCl (4.95 mL, 0.42 M).Gelation occurred within few seconds.The product was left to stand for 12 h then dried in vacuum oven at 90 o C. The material was crushed, sieved, washed successively with 50 mL portions of 0.025 M NaOH, water, methanol and diethyl ether and then dried in vacuum oven at 90 o C at 0.1 torr for 10 h.

Batch Experiments
A 100 mg of the functionalized polysiloxane-immobilized ligand system, P-DTA was shaken with 25 cm 3 of 0.02 M aqueous solution of the appropriate metal ions (Co 2+ , Ni 2+ and Cu 2+ ) using 100-cm 3 polyethylene bottles.Determination of the metal ion concentration was carried out by allowing the insoluble complex to settle down, withdrawing an appropriate volume of the supernatant using a micropipette and then diluting to the linear range of the calibration curve for each metal using AAS.The maximum metal ion uptake capacity was calculated as mg of M 2+ /g ligand.Each study was performed at least in a triplicate.

Column experiments-Preconcentration Experiment
A glass column (250 mm long, 10 mm diameter) was washed sequentially with 0.1 M nitric acid, water and acetone.It was then oven-dried and packed with a bed (5.0 g, 60-80 mesh) of the diethylenetriamine immobilized ligand system, P-DTA.The packed material was activated for each run by washing with 15 cm 3 of aqueous solution of 0.5 M hydrochloric acid, followed by deionized water and finally with acetate buffer solution at pH 5.5.Solutions (50 cm 3 , 100 ppm) of each metal ion at different pH values were eluted with a flow rate at 1.0-1.5 cm 3 min -1 by gravity.When needed, vacuum pump was used to reach the desired flow rate.In another experiment, the column containing the bed (5.0 g, 60-80 mesh) of P-DTA, was activated as mentioned previously and solutions of different concentrations (0.0005 -1.0 M) buffered at pH 5.5 were passed through the column with a flow rate at 1.0-1.5 cm 3 min -1 .The chemisorbed metal ion was eluted by passing 50 cm 3 of an aqueous 0.5 M hydrochloric acid.The metal ions were quantified by atomic absorption spectroscopy.

Column Separation Experiment
The column was packed with the functionalized diethylenetriaminopolysiloxane, P-DTA (5.0 g, 60-80 mesh).After each use the column was flushed with 0.50 M hydrochloric acid, followed by deionized water, to remove any uneluted metal contaminant.Before any sample injection, the column was preconditioned by passage of 25 cm 3 of the appropriate buffered aqueous solution to equilibrate the column as that of the working solution.Solution of a mixture of metal ions (Co 2+ , Ni 2+ and Cu 2+ , each of 100 ppm) was injected in the column., then buffered solutions of controlled pH were passed through the column at a flow rate of 1.0 -1.5 cm 3 min -1 by gravity.The eluates were collected in fractions with a volume range 5-10 cm 3 .Each fraction was diluted to 50 cm 3 and the amount of metal ion (mg) in each fraction was determined using atomic absorption spectroscopy.The retained metals on the ligand system were eluted with 10 cm 3 of 0.50 M hydrochloric acid.The solution was then diluted to 50 cm 3 and the metal concentrations in the solution were determined by using atomic absorption spectroscopy.

Synthesis of Polysiloxane-immobilized Triamine Ligand System (P-DTA)
The diethylenetriamine polysiloxane ligand system (P-DTA) was made by hydrolytic polycondensation between TEOS and diethylenetriaminopropyltrimethoxysilane (Scheme 1).The elemental analysis results of the prepared polysiloxane are given in Table 1.The functional group content of the ligand system is higher than previously reported results 15 .It is found that percentages of C and N are slightly lower than expected due to formation of small oligomers which leached during the washing process 10,[29][30][31] .Formation of these small oligomers is enhanced by the presence of self base catalyzed amino groups which lead to rapid gelation, so small amounts of non-cross linked oligomers are formed 10,[29][30][31] .The equal ratios of both expected and found results confirm that complete functionalized ligand leached.

Metal Uptake Capacity of P-DTA
The metal ion uptake capacity of Co 2+ , Ni 2+ and Cu 2 + , was determined by shaking the functionalized ligand system, P-DTA with buffered metal ions solutions for 48 h.The results in mg M 2+ / g ligand are given in Table 2.The elemental analysis of nitrogen of the immobilized ligand (P-DTA) as given in Table 1 was 11.89% i.e. 2.8 mmol N/g ligand.Comparing this value with the maximum metal ion uptake, it is possible to suggest that not all functional groups are accessible to binding with the metal ions assuming the ligand to metal complexation ratio is 1:1.It is clear that uptake of metal ions increases in the following order:

Effect of pH on the metal ions chemisorption
The influence of pH of the aqueous solution on the retention of cobalt, nickel and cupper was investigated in the pH range 3-6 by using acetate buffer solution.The results are depicted in Figure 2. The recovery values of the analyte ions were generally found to be very small at low pH values and increases by increasing pH.Maximum retention occurs at pH 5.5.In case of cupper about 100% was retained while 93% and 88 % were retained in case of nickel and cobalt respectively.

Preconcentration of metal ions
The ability of P-DTA to extract cations from aqueous solution was evaluated using a series of concentrations of metal ions.The results are given in Figure 3.The amount of metal ion recovered by the ligand system increases with increasing concentration up to the maximum value.Based on elemental analysis of N (11.89 %) of the ligand system (each ligand contains 3 N atoms), there was 2.8 mmol ligand per gram of immobilized system.Assuming 1:1 complex formation of M:L mole ratio the maximum loading (theoretically) of metal ion would be 179 mg Cu, 164 mg Ni and 165 mg Co per gram ligand system.The maximum chemisorption values were obtained at concentration of 0.5 M of the eluted metal ions and found to be 128 mg Cu, 104 mg Ni and 97 mg Co per gram ligand system.These values represent preconcentration efficiency of the column in percentages of 73.5 %, 63.4 % and 58.8 % for copper, nickel and cobalt respectively.At low metal ios concentration, ∼100 % extractions were achieved.This promises the column to be an excellent preconcentration system for these metal ions.

Separation of metal ions
Separation of a mixture of metal ions Cu(II), Ni(II) and Co(II) was performed by elution with buffer solutions at different pH values.Three bands were observed by pH control.A blue color band of Cu(II) was observed upstream followed by a green and a pink bands of Ni(II) and Co(II) respectively.These metal ions were eluted cleanly from the mixture by pH control.Figure 4 shows the separation of Cu(II), Ni(II) and Co(II) metal ions as a function of elution volume at variable pH values.Complete separation of Co(II), Ni(II) and Cu(II) from solution mixture was performed and improved by stepwise pH control of the eluent .The desorbed amount of metal was calculated from the total fractions of 500 cm 3 .Three well resolved peaks of cobalt, nickel and copper ions were obtained, at pH 4.5, 4 and 0.1 N HNO 3 respectively.This promises the ligand system to be efficient in a clean separation of these metal ions.

Conclusions
The immobilized diethylenetriamine ligand system was prepared by direct sol-gel hydrolytic polycondensation reaction of diethylenetriaminopropyltrimethoxysilane and tetraethylorthosilicate monomers at ambient temperature.This ligand system has been shown to be an effective solid-phase preconcentration agent for cobalt, nickel and copper at pH 5.5.
The ligand system exhibits high potential for separation of a mixture of Co 2+ , Ni 2+ and Cu 2+ metal ions from aqueous solution by pH control.

Figure 2 .
Figure 2. Chemisorption of metal ions by P-DTA versus pH values

Table 2 .
Maximum metal uptake by P-DTA