Determination of Selectivity Coefficients for Hardness for ResinTech CN Grades

Determination of Selectivity Coefficients for Hardness for ResinTech CN Grades

SUMMARY 

Selectivity coefficients for ResinTech CNS and CN8 Strong Acid Cation (SAC) resins were determined. Each resin was submerged in calcium chloride and magnesium chloride solutions with agitation via magnetic stir bar at ambient temperatures. An aliquot of each solution was collected after stirring for four hours and was tested via ICPMS to calculate the selectivity coefficients for hardness over sodium ion for each resin. 

Table 1: Calculated selectivity coefficients from ICP-MS analysis of CN8 and CNS resins. 

Details 

The selectivity coefficients of Na⁺ vs Mg²⁺ and Na⁺ vs. Ca²⁺ were calculated for CN8 and CNS resins. This was done by preparing a solution of CaCl² (400 ppm as Ca) and a solution of MgCl² (240 ppm as Mg). The initial concentration of ions in each solution was measured via ICP-MS. Then, 7 grams of each strong acid cation was added to 1L of each solution and stirred for 4 hours at room temperature via a magnetic stir bar. After 4 hours, an aliquot of each solution was taken and filtered for ICP-MS analysis. Each selectivity study was performed in triplicate to get an average selectivity coefficient for both cations. The selectivity coefficient for Ca²⁺ and Mg²⁺ of each non-solvent cation was calculated and shown in
Table 1. 

According to the values depicted in Table 2, the selectivity of Ca²⁺ and Mg²⁺ for CG8 is 4.06 and 2.59. The CN8 resin Resintech currently sells had an average selectivity of 5.17 and 2.46. Despite the significant difference between values, these are reasonable when accounting for the presence of hydrogen ions when in gel form. The nonsolvent resin appears to present a higher affinity for Ca²⁺ than solvent-based resins and a slightly lower affinity for Mg²⁺. This difference in selectivity can be potentially attributed to the chlorinated hydrocarbons used as a swelling agent in resins like CG8. 

Table 2: Reference values from ResinTech, Inc. technical files.

Additionally, the selectivity for CNS lined up well with what was expected. CNS is a styrene-divinylbenzene crosslinked copolymer with approximately 6.5% crosslinking. The literature values for the selectivity of CG8 and CG4 values can be found in Table 2. If we were to average those values to estimate the selectivity for a solvent-based resin with 6% DVB we could assume a selectivity of about 3.6 for Ca²⁺ and 2.41 for Mg²⁺. The selectivity of CNS which is about 6.5% DVB resulted in 4.82 for Ca²⁺ and 2.57 for Mg²⁺. While the level of crosslinking is slightly higher than what we can assume for a CG6 the additional discrepancy can be explained by the chlorinated hydrocarbons used as a swelling agent for solvent-based resins. The same trend was observed for the difference between a CN8 and a CG8 resin. The added hydrogens present from the chlorinated hydrocarbon used to swell the resin bead appears to hinder the selectivity of the ion exchange resin for Ca²⁺ significantly but have no significant effect on the selectivity for Mg²⁺. In both cases the nonsolvent cations exhibited a higher selectivity for Ca²⁺ and a comparable selectivity for Mg²⁺. In the future more data will be collected for the selectivity of both CNS and CN8 for additional ions such as Li⁺. 

Raw Data from Calculations of Selectivity Coefficients of Nonsolvent Cations

Acknowledgements 

The author would like to thank both Tom Smith and Troy Mulligan for their help and contributions to this body of work. 

Submitted by: R. Ross