Low Solute Rejection and Selectivity in NF

NF membrane has been widely applied in water-related applications. However, low solute-removal rate for certain applications is still not effectively solved. For instance, monovalent nitrate removal in water treatment is considered as nonideal, as the nitrate rejection rate using commercial NF membrane is still considered low (Bohdziewicz et al. 1999 Van der Bruggen et al. 2001 Garcia et al. 2006). Removal of nitrate ions from the water stream is important as synergistic toxicity effects potentially occur in combination with some other compounds such as pesticides (L Haridon et al. 1993). Low rejection of NF membrane toward other elements or compounds based on boron was also reported by Geffen et al. (2006) and Dydo et al. (2005). 

In addition to the solute rejection capability, solute selectivity in NF processes plays an important role in determining the NF efficiency. The selectivity of solute 1 over solute 2, termed as separation factor by certain researchers, was calculated using Equation (4.1) (Ahmadiannamini et al. 2010 Umpuch et al. 2010)  

Cp i represents permeate concentration for component 1 Q,2 represents feed concentration for component 2 Cf i represents feed concentration for component 1 Cp 2 represents permeate concentration for component 2 represents the rejection of component 1 / 2 represents the rejection of component 2 

This selectivity equation is popularly used for both charged and noncharged solutes (Umpuch et al. 2010). 

During the salt fractionation or selective separation (Oumar et al. 2001 Hilal et al. 2007), the removal of a certain solute over another is one of the most important applications of NF membranes. Unlike RO processes where almost none of the solute is permeable through the membrane, NF processes can partially allow the permeation of monovalent ions, but restrict the permeation of multivalent ions through 

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