Bidirectional Permeation of Electrolytes in Osmotically Driven Membrane Processes

摘要

Osmotically driven membrane processes (ODMP) are emerging water treatment and energy conversion technologies. In this work, we investigated the simultaneous forward and reverse (i.e., bidirectional) solute fluxes that occur in ODMP. Numerous experiments were conducted using ternary systems (i.e, systems containing three distinct ions) and quaternary systems (i-e, systems containing four distinct ions) in conjunction with a membrane in a forward osmosis orientation. Ten different combinations of strong electrolyte salts constitute the ternary systems; common anion systems studied included KCI-NaCl KBr-NaBr, KNO_3-NaNO_3, KCl-CaCl_2 and KCl-SrCl_2; and common cation systems explored were KCl-KH_2PO_4 NaCl-NaClO_4 NaCl-Na_2SO_4 NaCl-NaNO_3 and CaCl_2-Ca(NO_3)_2. For each combination, two experiments were conducted with each salt being used once in th* draw solution and once in the feed solution. Quaternary systems studied were NaCl-KNO_3, Nad-MgSO_4 MgSO_4-KNO_3, and NaCl-K_2SO_4. Experimental fluxes of the individual ions were quantified and compared to a set of equations developed to predict bidirectional electrolyte permeation for ODMP in a forward osmosis orientation. Results demonstrate that ion fluxes from the draw solution to the feed solution are well predicted; however, ion fluxes from the feed solution to the draw solution show slight deviations from the model that can be rationalized in terms of the electrostatic interactions between charged ions. The model poorly predicts the flux of nitrate containing solutions; however, several unique mass transfer mechanisms are observed with implications for ODMP process design.