The present investigation aims at identifying the molecular properties of the dye which control
membrane fouling during nanofiltration. Three negatively charged molecules, namely Acid red 87, Direct
blue 53 and Acid black 1 and three positively charged molecules, namely Azure A, Basic blue 9 and Basic
green 4 are investigated. Dye molecules of 50 mg/L dissolved in 2000 mg/L of NaCl were subjected to
nanofiltration using NF 270 membrane (Dow Filmtec with an isoelectric point of 3.3) at pH-3, pH-7
and pH-10. The flux decline, salt rejection and dye rejection were measured using Sterlitech cross flow
cell (CF042) with an active membrane surface area of 14.6 104 m2. Flux decline due to membrane
fouling was also calculated by measuring pure water flux after washing the fouled membrane with water
at the same pH for 30 min. These results indicate electrostatic interactions between the charged dye
molecules and the membrane charge, which depend on the pH. Strong sulfonic acid containing dye
molecules (Direct blue 53 and Acid black 1) do not get adsorbed on the membrane surface. High flux rate
and dye rejection were noticed in all the three pH media investigated for these dyes. Weak carboxylic
acid (Acid red 87) shows strong flux decline and membrane fouling in acidic pH. Positively charged
dye molecules with relatively low molecular weights, exhibit strong fouling effect in neutral as well as
alkaline pH. The effects of molecular-membrane electrostatic interactions and acid–base interactions
on membrane fouling are highlighted in this work along with the molecular size effect.
The present investigation aims at identifying the molecular properties of the dye which controlmembrane fouling during nanofiltration. Three negatively charged molecules, namely Acid red 87, Directblue 53 and Acid black 1 and three positively charged molecules, namely Azure A, Basic blue 9 and Basicgreen 4 are investigated. Dye molecules of 50 mg/L dissolved in 2000 mg/L of NaCl were subjected tonanofiltration using NF 270 membrane (Dow Filmtec with an isoelectric point of 3.3) at pH-3, pH-7and pH-10. The flux decline, salt rejection and dye rejection were measured using Sterlitech cross flowcell (CF042) with an active membrane surface area of 14.6 104 m2. Flux decline due to membranefouling was also calculated by measuring pure water flux after washing the fouled membrane with waterat the same pH for 30 min. These results indicate electrostatic interactions between the charged dyemolecules and the membrane charge, which depend on the pH. Strong sulfonic acid containing dyemolecules (Direct blue 53 and Acid black 1) do not get adsorbed on the membrane surface. High flux rateand dye rejection were noticed in all the three pH media investigated for these dyes. Weak carboxylicacid (Acid red 87) shows strong flux decline and membrane fouling in acidic pH. Positively chargeddye molecules with relatively low molecular weights, exhibit strong fouling effect in neutral as well asalkaline pH. The effects of molecular-membrane electrostatic interactions and acid–base interactionson membrane fouling are highlighted in this work along with the molecular size effect.
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