Characterisation of active and porous sublayers of composite membranes from hydrodynamic and electrical measurements

Abstract

Composite membranes can be considered as a series association of two homogeneous elements coupled in series, which present different structural and transport properties [1]: a thin and dense active layer, and a thick but porous support to give mechanical stability to the whole system. Although it is commonly assumed the active (or skin) layer controls membrane selectivity and fluxes, the porous sublayer can also slightly affect them [2]. For determining transport behavior of composite membranes, it is necessary to know the characteristic coefficients for each layer but information on the active/ porous layer interface is also of interest. In this work, hydrodynamic and impedance spectroscopy measurements were carried out with a composite reverse osmosis polyamide/ polysulfone membrane with various electrolytes at different concentrations, to compare membrane parameters depending on the electrolyte type. Two opposite membrane configurations were used for hydrodynamic filtration experiments, the active layer or the porous sublayer facing the high-pressure side, which allows the estimation of average active layer rejection coefficient (sa) and salt permeability (wa), active/porous layers intermediate concentration (Ci) and salt permeability in the porous sublayer (wp). Impedance spectroscopy (IS) measurements were carried out for a wide range of frequency with the membrane separating an electrolyte solution of a given concentration and it is a successful tool to determine the electrical properties (electrical resistance and capacitance) of heterogeneous systems formed by a series array of layers with different electrical/structural properties by using equivalent circuits as models [3–4].