EFFICIENT RECOVERY OF NICKEL (II) FROM A MIXTURE OF NICKEL (II) AND CHROMUIM (VI) IONS USING DUOLITE A7 RESIN

Separation of Ni(II) and Cr(VI) from a solution containing a mixture of both ions was conducted using adsorption technique with Duolite A7 resin as the adsorbent. The effects of ionic strength (1.0 - 5.0 M), resin dosage (1.0 - 5.0 g), temperature (30 - 50°C) and equilibration time (5 - 150 mins) on the separation of both ions were investigated. Up to 91% adsorption of Ni(II) by the resin was observed at pH 4.5. It was observed that Ni(II) ion under each of the conditions showed significantly higher binding affinity to the resin than Cr(VI). Adsorption of Ni(II) was rapid and was completed in about 10 min equilibration time. There was a continuous decrease in the adsorption of both ions as the temperature of the mixture increased. The FTIR spectra of the unloaded and loaded resinshowed that Duolite A7 is highly selective for the optimum recovery of Ni(II) from a solution containing a mixture of Ni(II) and Cr(VI) ions under the considered experimental conditions.


INTRODUCTION
Recovery of metals from waste alloys is generally regarded as a preferred method of lowering the concentration of these metals in the environment compared to incineration or landfilling which continue to pose serious threats to human health and the environment [1].The recovery system has been phrased as a method of converting waste to wealth.Among the potential contaminants, nickel(II) is one of the most widespread pollutants in the environment that has prominent industrial and commercial applications [2].
Wastewater containing nickel are produced in chemical, metallurgical, electroplating, battery, and paint industries [2,3].Recovery of nickel from waste-water or alloy will therefore be of great economic importance.
Among most methods of metal recovery and management, sorption technique has been widely used for the separation of metals from aqeous solution.This is because the method is easy to design and has a high efficiency to cost ratio [4].
Moreover, for removal and recovery of valuable metals, ion exchange has been recognized as a promising alternative technique to traditional methods of precipitation coupled with filtration [5].The chelating resins were commonly employed as ion-exchange materials once their ligands can selectively bind to certain metal ions through ionic and coordinating interactions.
Previous studies have shown that resins generally could be used for selective removal and recovery of chromium and copper [6].This is preferred to conventional methods such as chemical precipitation, coagulation/flocculation, complexation, electrochemical deposition and biological treatment, which have considerable drawbacks that include imperfect metal removal, high reagent and energy requirement, and generation of large amount of toxic sludge that must be disposed in additional steps [7].
In this study, the ion exchange resin used is Duolite A7 [8].Duolite A7 is a phenolic weak base anion resin that is highly resistant to physical attrition.It has a high acid capacity, excellent physical stability, low basicity, and it is resistant to foul smell [9].Due to its high porous structure, this hydrophilic resin is useful both for reversible adsorption of large organic molecules and for acidity removal.Also, since Duolite A7 is a weak base anion exchange resin, it is regenerated rapidly and efficiently with little more than stoichiometric amount of alkali [8].

Materials
Nickel(II) chloride and potassium dichromate(VI) are both analytical grade reagents and were purchased from Sigma Aldrich.Duolite A7 free base and sodium chloride were purchased from Rohm Hass, USA.

Equipment and methods
pH values of the solutions were measured using a pH meter (Hanna, USA).The concentration of the metal ions was determined using a Perkin Elmer 400 Atomic Absorption Spectrometer (AAS), USA.A thermally controlled magnetic stirrer (AM3250B) was used for metal ion-Duolite A7 resin equilibration.FTIR analyses were done using a SHIMADZU FTIR-8400S spectrometer.

Fourier Transmittance Infra Red (FTIR) Analysis
The solution of nickel (II) chloride and potassium dichromate (VI) was allowed to react with the resin in aqueous phase for 10 mins.The metal ion-loaded resin was removed by filtration and dried.The filtrate was analyzed using AAS to determine the concentration of the left-over ions.
The dry and the pristine resins were analyzed by FTIR spectroscopy to determine the change in the functional groups on the Duolite resin surface as a result of adsorption.

Recovery of metals from the loaded resin
The metal-ion-loaded resins were soaked in 5 M aqueous ammonia solution for 24 hours.The solution was filtered and the concentration of the metal ions recovered was determined by AAS.

Ionic strength
Ionic strength enhances phase separation of metals [10].The effect of ionic strength on the separation of nickel and chromium is presented in

Resin dosage
Effect of resin dosage was studied at room temperature (30°C) by varying the weight of the resin from 1.0 g to 5.0 g as shown in Figure 3.In

Surface functional groups of the adsorbent
The unloaded duolite A7 resin and the loaded resin were analyzed by FTIR in order to study the interaction between the metal ions and the resin.
The result is summarized in Table 1.The results

Recovery of metal ions from the loaded resin
To verify the applicability of Duolite A7 resin, the elution of the metal ions from the resin was done to recover the adsorbed ions.The resin loaded with metal ions was soaked overnight in 5 M aqueous ammonia, after which it was agitated for 10 mins at room temperature.The resin was separated from the solution by filtration to be able to determine the concentration of metal ions released into the solution using AAS.The recovery of Ni(II) and Cr(VI) ions were determined to be 85.4% and 82.1% respectively.
A similar result was reported for the modified Duolite XAD-761 by Shah and coworkers [5].

A
solution of Ni(II) and Cr(VI) was prepared by weighing 10 mg of NiCl2 and 10 mg of K2Cr2O7 into in 1 litre of deionized water to simulate a Ni(II) and Cr(VI)-containing industrial wastewater.For each of the experiments, 50 ml of the simulated industrial wastewater was subjected to Ni(II) adsorption using Duolite A7 resin after determining the initial concentration of Ni(II) and Cr(VI) by AAS.The optimum conditions for the removal of Ni(II) were determined by varying the pH, resin dosage, temperature, ionic strength, and equilibration time of the systems.The ion-loaded resins were filtered off after 10 mins in the solution and the filtrate was collected in a clean 250 ml beaker.The filtrate was thereafter analyzed by AAS to determine the final concentration of the metal ions.

Figure 1 .
Figure 1.The effects of ionic strength on the degree of adsorption of Ni(II) and Cr(VI) ions to Duolite A7 resin was investigated in the presence of 1.0, 2.0, 3.0, and 5.0 mol/L NaCl at the room temperature (30°C), pH 1, 10 mins equilibration time and 1.0 g resin dosage.As shown in Figure 1, increase in ionic strength significantly increased the adsorption of Ni(II) to above 80%.A very sharp increase was observed at 1.0 mol/L NaCl.The increase became slow at 2.0 mol/L NaCl showing that the optimum ionic strength for Ni(II) adsorption should be in the range 1.0 -2.0 mol/L.Cr(VI) adsorption at increasing ionic strength was poor.Less than 30% of the initial concentration of Cr(VI) ions was adsorbed even at 4.0 mol/L ionic strength (Figure 1).The results showed that at the specified conditions, Ni(II) can be efficiently separated from a mixture of containing Ni(II) and Cr(VI) ions.

Figure 1 :
Figure 1: The plot showing the effect of ionic strength on the percentage of Ni(II) and Cr(VI) adsorbed onto Duolite A7 resin.

Figure 2 :
Figure 2: Effect of pH on Ni(II) and Cr(VI).

Figure 3 ,
Figure 3, the adsorption of nickel only increases at a dosage of 1.0 g after which subsequent increase in the dosage amount did not lead to a significant change in the percentage of metal ions

Figure 3 :
Figure 3: A plot showing the effect of resin dosage on the adsorption of Ni(II) and Cr(VI) ions to Duolite A7 resin.Variation of equilibration time Adsorption of Ni(II) and Cr(VI) ions at different equilibration time ranging from 5 to 150 mins was investigated at 30°C, 1.0 g resin dosage, pH 1.0 and ionic strength of 1.5.Figure 4 reveals that the percentage of Ni(II) removed significantly increased only from 5 -10 mins, after which it remained constant with increasing equilibration time.The plot also showed that the resin had reached saturation at the prevailing experimental condition for the removal of Ni(II) after 10 mins

Figure 4 :
Figure 4: A plot showing the effect of time of equilibration on the adsorption of Ni(II) and Cr(VI) ions to Duolite A7 resin.

Figure 5 :
Figure 5: A plot showing the effect of temperature on the adsorption of Ni(II) and Cr(VI) ions to Duolite A7 resin.
showed a shift at O-H stretch from 3441 -3431 cm -1 and C-H stretch from 2935 -2935 cm -1 did not change as expected.A shift in the C=O vibrational frequency at 1660 -1653 cm -1 and a new band attributed to metal-oxygen bond (M-O bond) at 569 cm -1 were observed ( Ni(II) and Cr(VI) were generated in the laboratory at concentration ratio of 1:1.The characterization of the adsorbent with FTIR confirmed the adsorption of the metal ions to Duolite A7 resin.The resin showed far higher binding affinity to Ni(II) ion than Cr(VI) ion under the operating conditions, and suggests it can be used to selectively remove Ni(II) from a solution containing a mixture of both ions.The uptake capacity of the resin was largely pH, ionic strength and temperature dependent.The equilibration time and resin dosage did not significantly affect the uptake of the resin for either of the two ions.