Single electrolyte. The skin layer of NF membranes is
generally made from hydrophilic amphoteric polymers, for example,
polyamide, which possesses dissociable carboxylic and amine groups,
exhibiting positive or negative surface charge dependent on pH [20].
For the strong electrolyte, the effect of pH on solute rejection can be
simply explained by membrane charge variation and electrostatic
repulsion/attraction between ions and membrane. Generally, the salt
rejection increases with membrane charge and is lowest at the
isoelectric point