Abstract
In this research, we investigated the influence of feedwater ionic strength on diffusion of divalent ions through a hollow-fiber nanofiltration membrane. The results indicated that solute flux of magnesium was increased as a result of elevating the ionic strength in the feedwater. Specifically, the feedwater ionic strength was observed to have a nonlinear impact on the diffusion of magnesium during the nanofiltration process, which was under-predicted by the homogeneous solution diffusion (HSD) model. This result suggested that elevating the feedwater ionic strength had reduced the strength of the electrostatic double layer at the membrane surface. We then developed a modification of the HSD model (referred to as the HSD-IS model) which incorporated an empirical term related to the effect of feedwater ionic strength (IS) on diffusion of magnesium. The root mean squared error of the HSD-IS model was improved by 77% as compared to the HSD model, which did not incorporate a term related to feedwater ionic strength. This improvement suggested that feedwater ionic strength should be considered when modeling hardness removal during nanofiltration.
Highlights
Nanofiltration (NF) is a pressure-driven membrane process often used in water treatment for removal of divalent metal ions
Feedwater ionic strength was observed to have a nonlinear impact on the diffusion of magnesium during a NF process
A modification of the homogeneous solution diffusion (HSD) model was developed and proposed, which incorporated an empirical term related to the effect of feedwater ionic strength on diffusion of magnesium
Summary
Nanofiltration (NF) is a pressure-driven membrane process often used in water treatment for removal of divalent metal ions. This process is often referred to as hardness removal or membrane softening. Solute mass transfer through an NF membrane is widely considered to be a diffusion-controlled process and is commonly modeled by the homogeneous solution diffusion (HSD) model which is considered the first model developed for high recovery reverse osmosis (RO). The HSD model was developed using fundamental mass balance equations considering a single membrane element, while assuming the mass transfer of water and solutes are diffusion-based and occur due to pressure and concentration gradients, respectively.
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