Behavior of Polyamide Brackish Water Reverse Osmosis Membranes in Acidic NaOCl Environment

Abstract

The chlorine sensitivity of polyamide reverse osmosis (RO) membranes has been a persistent challenge, as chemical alterations compromise their structural integrity, reducing their effectiveness in desalination and water treatment applications. This study examines the structural changes and performance impacts of chlorination of BWRO membranes under acidic conditions (pH 4) using NaOCl at concentrations of 500–10,000 ppm for 1, 3, and 6 h. Characterization techniques such as SEM, AFM, ATR-FTIR, XPS, contact angle, and GIXRD were employed. The results showed a significant decrease in pure water flux (PWF) after exposure to acidic hypochlorous acid (HOCl), which was subsequently recovered after salt passage. Interestingly, a slight increase in salt rejection (% SR) was observed up to 5000 ppm for 3 h of treatment, followed by a decline at higher exposures. This behavior of abrupt drop and recovery in flux with a marginal increase in salt rejection is attributed to conformational changes in the membrane. GIXRD analysis revealed reduced crystallinity and wettability at low HOCl concentrations, while higher levels caused membrane embrittlement. This study provides key insights into the structural and transport changes in RO membranes during acidic chlorination, highlighting the need for optimized cleaning protocols to enhance membrane lifespan.