Effects of Feed Water Inorganic Chemistry On Polyamide NF/RO Membrane Structure, Interfacial Properties and Removal of Trace Organics

Recently, we have been exploring the effects of feed water composition (ionic strength, pH and divalent cation content) on water permeability, trace organic solute rejection (Ethylene glycol, NDMA, 1,4-dioxane, phenol, methanol), interfacial properties of three commercial polyamide thin film composite membranes. Here, we use experimentally determined water flux, solute rejection and interfacial energy in a convection-diffusion-affinity NF/RO membrane transport model to deduce changes in polyamide thin film structure (thickness or state of swelling and free volume or porosity). All membranes tested became quantifiably more hydrophilic and swollen (increased free volume and thickness as indicated by the model parameters of apparent pore radius, r_p, and structure factor, ε/Δx) with increasing feed solution ionic strength, pH, Ca²⁺ and Mg²⁺ content. Water permeability decreased with ionic strength, Ca²⁺ and Mg²⁺ content, but increased with pH. For RO membranes, organic solute permeability increased with pH, but decreased with ionic strength and Ca²⁺ and Mg²⁺ content, while for NF membranes organic permeability increased with ionic strength, pH and Ca²⁺ and Mg²⁺ content. This study offers a practical way to describe and quantify the response of membrane interfacial and structural properties to changes in water chemistry and in turn how that related to membrane performance, which may help inform new membrane designs specifically targeting removal of trace organic contaminants from water.