Fouling of nanofiltration membranes by dyes during brine recovery from textile dye bath wastewater

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 50mg/L dissolved in 2000mg/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 14.6×10^-4m². 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 30min. 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.