Preferential Transport of Divalent Ions through PEDOT:PSS Membranes and Coatings*

There is a need for polymeric membranes and processes with selectivity towards specific salts or counterions. This would lower the cost for recovery of valuable ions such as lithium or copper and enable more cost-effective approaches to the removal of scalants and toxic ionic contaminants such as nitrates and phosphates. Here, we demonstrate that the commercial conductive and sulfonated polymer PEDOT:PSS can be used to significantly enhance the selectivity for removal of divalent counterions in a membrane capacitive deionization (MCDI) process. PEDOT:PSS represents a useful model material because the sulfonate functionalities provide preferential permeability to divalent ions and the crosslink density and water content of swollen membranes can be systematically tuned. We characterize the physical and transport properties of PEDOT:PSS and incorporate it as an ion-selective layer in an MCDI system. We show that preferential transport of Ca²⁺ over Na⁺ can be attributed to a higher membrane diffusivity of Ca²⁺ in the membrane.  The use of the PEDOT:PSS ion exchange membrane increases ion selectivities from 2:1 to 10.5:1 under optimized conditions. This work demonstrates a new and readily available polymeric material for selective extraction of divalent ions.