(272g) Ion Partitioning Between Brines and Ion Exchange Polymers

Ion partitioning between brines and ion exchange polymers

 

Michele Galizia^‡, Donald R. Paul, Benny D. Freeman

J. McKetta Department of Chemical Engineering, 200 E. Dean Keeton Street, 78712 Austin,

and Center for Energy and Environmental Resources, 10100 Burnet Road, 78758 Austin,

The University of Texas at Austin, TX (USA)

^‡ Permanent address: Department of Chemical, Biological and Materials Engineering, 100 E. Boyd St., 73019 Norman, University of Oklahoma, OK (USA)

For long time, the validity of the Donnan model [1] to describe ion partitioning between electrolyte solutions and ion exchangers was questioned [2]. As recently reported in the literature [3], quantitative failure of Donnan model in predicting ion partitioning between diluted electrolyte solutions and charged polymers arises from neglecting non-idealities in the polymer phase. When a charged polymer is equilibrated with diluted salt solutions, ion activity coefficients in the membrane do not match those in the contiguous, external solution, so corrections for membrane non ideal behavior are needed. However, when salt concentration in the external solution exceeds 1 mol/L for NaCl and 0.1 mol/L for CaCl₂, ion activity coefficients in the membrane and in solution rapidly collapse on one another. In these conditions, the environment in the membrane becomes thermodynamically similar to that in the external solution. Indeed, since most of fixed charges are screened by sorbed counter-ions, the membrane environment is essentially made from water and ions, while polymer chains just slow down ion diffusion due to tortuosity/area effects.

In this study, a combined experimental and theoretical approach was used to test this hypothesis. Specifically, a fundamental, quantitative study of NaCl and CaCl₂ partitioning between a cation exchange membrane based on a cross-linked poly(p-styrene sulfonate-co-divinylbenzene) and concentrated brines is reported. In these conditions, the experimentally determined ion activity coefficients in membrane match those in the contiguous, external solution, and the Donnan model provides an accurate prediction of co-ion and counter-ion concentration in the polymer.

Experimental findings were interpreted using the recently developed Donnan-Manning model [3], which permits to track the change in polymer structure as a function of the external salt concentration, and build structure-property correlations for ion sorption in commercial membranes for water desalination and power generation applications.

[1] F.G. Donnan, Chem. Rev. 1924 1 73

[2] E. Glueckauf et al., Nature 1961 191 904

[3] J. Kamcev et al., PCCP 2016 18 6021