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 sulfa te or aqueous nitrate media on iminodiacetic (IDA) resin Lewatit TP-207 was studied. A novel approach for evaluation of the resin phase composition fr om ct versus T dependencies has been proposed and the validity of such an a pproach is demonstrated. The temperature dependence of the equilibrium sepa ration factor, alpha, in the Cu2+-Ni2+ exchange varies from nitrate to sulp hate medium and it depends on the formation of different IDA complexes with the minor component (Ni2+) in the resin phase. In the nitrate medium, form ation of (LDA)IDA 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 s ulfate medium is characterized by the formation of nickel complexes of both stoichiometries within the whole temperature range studied. The dependence of alpha on T in Zn2+-Co2+ exchange system has been shown to be weaker tha n 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 inte rval of working temperatures (Delta T) and its position on the temperature scale. The efficiency of thermostripping rises with an absolute Delta T val ue and also increases following the shift of temperature interval to the lo wer temperature range. (C) 2000 Elsevier Science B.V. All rights reserved.