Relative Permeation of Ions in Reverse Osmosis Processes using Ion-exchange selectivity Data: Theoretical Approach and Experimental Validation
The general objective of this research was to study the widely accepted solution-diffusion model to predict the differential rejection of various electrolytes through RO membranes. The central theme rests on the premise that ions diffuse through the RO membranes as electrically neutral solutes and their effective diffusivities (i.e. Salt permeability coefficients) are strongly correlated to the hydrated ionic radii of individual ions. It is well recognized that the affinity of an ion towards a polymeric ion exchanger is strongly correlated to its hydrated ionic radius. Thus, more easily obtained ion exchange selectivity data may serve as the basis in determining the difference in permeation rates of different electrolytes. According to information in the open literature to date, ion exchange selectivity data have not been used as yet for modeling solute permeation through RO membrane.
Reverse Osmosis (RO)/ Nanofiltration (NF) are pressure driven membrane processes, which are widely used to separate dissolved solids from water. RO and NF are non-selective processes i.e semi-permeable membranes is not capable of removing targeted solutes selectively. It has however been observed that the membrane rejects different electrolytes to a different extent. Also rejection of a particular electrolyte is affected by accompanying electrolytes in the feed water, all other conditions remaining identical. In the area of water and wastewater treatment and water reuse, the degree of rejection (or permeation) of environmentally regulated species are important. Progress has been made in developing models, which integrates Nernst planck equation with Donnan effect to predict the relative permeation of ions of dissimilar valence i.e. hetero-valent ions. Nernst planck equation with or without Donnan effect is however unable to differentiate the permeation between ions of Relative Permeation of Ions in Reverse Osmosis Processes using Ion-exchange selectivity Data: Theoretical Approach and Experimental Validation identical valence i.e homo-valent ions. In this work a novel approach has been developed to determine the relative permeation rates of different electrolytes through semi-permeable membranes; of particular importance is the ability of this new approach to predict differential permeation among ions of the same valence.
The central theme of the model resides on the premise that ions diffuse through membranes as electrically neutral solutes and their permeation rates are functions of their inter-diffusion coefficients. Stokes-Einstein equation relates diffusivity of ions with their individual hydrated ionic radii (rH) and relative rH is related to their ion exchange selectivities towards polymeric ion exchangers. Therefore, more easily obtained data from ion exchange selectivity are used to estimate the relative rH of ions which, in turn, can predict their relative permeation rates through the membrane. In a multi-component system, ions with a higher charge are rejected more than the ions with lower charge. For ions with identical charges, the one with smaller rH (or higher ion exchange selectivity) will permeate faster with higher rH (or lower selectivity).
Mukherjee, P. and SenGupta, A.K. "Ion exchange selectivity as a surrogate indicator of relative permeability of ions in reverse osmosis processes", Environ. Sci. Technol., (2003), 37, 1432-1440.
Mukherjee, P. and SenGupta, A. K. “Some observations about electrolyte permeation mechanism through reverse osmosis and nanofiltration membranes.” Jour. Membr. Sci., (2006), 278, 301-307.