Study on Halide Ions Selectivity of Industrial Grade Anion Exchange Resin Auchlite A-378

The thermodynamic approach was applied to predict the trend selectivity of industrial grade anion exchange resin Auchlite A-378 in chloride form towards iodide and bromide ions in the solution. The study was conducted by performing the Clˉ/Iˉ and Clˉ/Brˉ uni-univalent ion exchange reactions under gradually increasing temperature conditions. The thermodynamic equilibrium constants K values were used to calculate the enthalpies of the two uni-univalent ion exchange reactions. It was observed that the K values for Clˉ/Iˉ ion exchange reaction were higher than Clˉ/Brˉ exchange reaction under identical experimental conditions. Also the enthalpy values for the two uni-univalent ion exchange reactions were calculated as -39.51 and -18.38 kJ/mol respectively. The high K and low enthalpy values obtained for Clˉ/Iˉ ion exchange reaction is responsible for higher selectivity of the resin towards iodide ions as compared to that towards bromide ions, when both the ions are present in the same solution.


INTRODUCTION
Ion exchange is the most appropriate and the most efficient method for the treatment of variety liquid industrial waste streams. With respect to economy and efficacy, ion exchange stands between the other two major liquid waste treatment processes of chemical precipitation and evaporation [1]. Chemical precipitation is often less expensive but is not always effective in treatment of industrial liquid waste. Evaporation method on the other hand may yield higher decontamination factors; but it is more costly than ion exchange. The development of new ion exchangers is narrowing the gap in decontamination factors between evaporation and ion exchange. Ion exchangers containing sulpho-and phospho-acidic groups and those containing tetra-ammonium basic groups are strong acidic and strong basic exchangers, respectively, whereas those containing phenolic and primary amino groups are weak acidic and weak basic exchangers, respectively. Exchangers with carboxy groups and tertiary amino groups take a medium position between strong and weak acidic and basic exchangers, respectively. Efforts to develop new organic ion exchangers for their specific industrial applications are continuing [2,3] and various aspects of ion exchange technologies have been continuously studied to improve the efficiency and economy of their application in  [4][5][6][7][8][9]. However, for proper selection of ion exchange resin, it is essential to have adequate knowledge regarding their physical and chemical properties, which forms the complementary part of resin characterization study . Generally the selected ion exchange materials must be compatible with the chemical nature of the liquid waste such as pH, type of ionic species present as well as the operating parameters, in particular temperature [37][38][39][40].
Hence in the present investigation thermodynamics of uni-univalent ion exchange reaction was studied to understand the selectivity behaviour of industrial grade anion exchange resin Auchlite A-378 towards the iodide and bromide ions in the solution.

EXPERIMENTAL
The ion exchange resin Auchlite A-378 as supplied by the manufacturer (Auchtel Products Ltd., Mumbai, India) was the weak base industrial grade anion exchangers in hydroxide form. The resin was having polystyrene matrix having tertiary ammonium functional group. The particle size of the resin was 0.3-1.2 mm, having moisture content of 48%, operating pH range of 0-7 and maximum operating temperature of 60.0 °C. For present investigation, the resin grains of 30-40 mesh size were used. The soluble impurities of the resin were removed by repeated soxhlet extraction using water and occasionally with distilled methanol to remove non-polymerized organic impurities. In order to bring about complete conversion of resins in chloride form, the resins were conditioned with 10 % potassium chloride solution in a conditioning column. The resins were washed with distilled deionised water until the washings were free from chloride ions. The resins in chloride form were air dried over P 2 O 5 and used for further studies. The exchange capacity of the resin was experimentally determined by the standard method [41] using sodium nitrate and titrating against standard silver nitrate solution. Ion exchange resins (0.500 g) in chloride form were equilibrated with iodide ion solution of different concentrations at a constant temperature of 35.0 °C for 3 h. From the results of kinetics study reported earlier ; it was observed that this duration was adequate to attain the ion exchange equilibrium. After 3 h, the iodide ion solutions in equilibrium with ion exchange resins in chloride form were analyzed for their chloride and iodide ion concentrations by potentiometric titration with standard 0.1 M silver nitrate solution. From the results the equilibrium constant K for the reaction was determined at 35.0 °C. The equilibrium constants K for the above Clˉ/Brˉ system was determined for different temperatures in the range of 30.0 °C to 45.0 °C.

International Letters of Chemistry, Physics and Astronomy Vol. 30
In the present study, a semi-micro burette having an accuracy of 0.02 mL was used in the titrations against silver nitrate solution. The titration readings were accurate to ±0.02 mL. Considering the magnitude of the titre values, the average equilibrium constants reported in the experiment are accurate to ±3 %.

RESULTS AND DISCUSSION
The equilibrium constant for reactions 1 and 2 can be written as here, A is the ion exchange capacity of the resin and Xˉ represents Iˉ and Brˉ ions. For different concentrations of Xˉ ions in solution at a given temperature, K values were calculated and an average K value for this set of experiments was found (Tables 1, 2). Similar K values were calculated for the Clˉ/Iˉ and Clˉ/Brˉ system at different temperatures (Table 3).    The log K values were plotted against 1/T (in Kelvin), which gives a straight line graph (Figure 1) from the slope of this graph enthalpy values of the ion exchange reaction 1 and 2 were calculated (Table 3).
Bonner and Pruett [42] studied the temperature effect on uni-univalent exchanges involving some divalent ions. In all divalent exchanges, the equilibrium constant decreases with rise in temperature resulting in exothermic reactions. Similar results were obtained in the present investigation for Auchlite A-378 resins, during Clˉ/Iˉ and Clˉ/Brˉ uni-univalent ion exchange reactions having the enthalpy values of -39.51and -18.38 kJ/mol respectively ( Table 3). The equilibrium constants K for Clˉ/Iˉ ion exchange reaction were found to be higher than that for Clˉ/Brˉ exchange under identical experimental conditions (Table 3). Similarly, the enthalpy value for Clˉ/Iˉ ion exchange reaction was lower than that calculated for Clˉ/Brˉ ion exchange reaction. The relatively high K and low enthalpy values obtained for Clˉ/Iˉ ion exchange reaction indicate higher selectivity of the resin towards iodide ions as compared to that towards bromide ions, when both the ions are present in the same solution [43][44].

CONCLUSION
Ion exchange technology is widely being used for separation of particular ionic species in presence of other. The selection of suitable ion exchange material is still more critical when the process involves separation of two or more chemically same ionic species in the solution. Under such critical conditions the present experimental technique will be useful in deciding about the selection of suitable ion exchange material. The technique used here can be extended further to standardise the process parameters in order to bring about the efficient separation of the desired ionic species from the solution.