Ion exchange on resins with temperature-responsive selectivity: III. Influence of complex formation stoichiometry on temperature dependence of resin selectivity

The influence of temperature (293<T<353 K) on Cu2+–Ni2+; Zn2+–Co2+, and Cu2+–Co2+ ion-exchange equilibrium at pH=1.55–3.0 from either aqueous sulfate or aqueous nitrate media on iminodiacetic (IDA) resin Lewatit TP-207 was studied. A novel approach for evaluation of the resin phase composition from α versus T dependencies has been proposed and the validity of such an approach is demonstrated. The temperature dependence of the equilibrium separation factor, α, in the Cu2+–Ni2+ exchange varies from nitrate to sulphate medium and it depends on the formation of different IDA complexes with the minor component (Ni2+) in the resin phase. In the nitrate medium, formation of (IDA)2–Ni complex is prevailing at 293<T<308 K, while at T>308 K 1:1 IDA:Ni2+ complex is dominating. The Cu2+–Ni2+ exchange equilibrium from sulfate medium is characterized by the formation of nickel complexes of both stoichiometries within the whole temperature range studied. The dependence of α on T in Zn2+–Co2+ exchange system has been shown to be weaker than that in the Cu2+–Ni2+ system. This result is in a good agreement with the predictions made in the first communication of this series. The results of thermostripping experiments carried out for Cu2+–Co2+ exchange have shown that the efficiency of the thermostripping process depends on both the interval of working temperatures (ΔT) and its position on the temperature scale. The efficiency of thermostripping rises with an absolute ΔT value and also increases following the shift of temperature interval to the lower temperature range.