Can Tailoring the Polyamide Active Layer of the Desalination Membranes Ease the Recycling of End-of-Life Membranes for Seawater Pretreatment Applications.

Millions of End-of-Life (EoL) reverse osmosis (RO) membranes are discarded each year by desalination facilities worldwide. This poses a significant environmental challenge, as large amounts of solid waste are often managed through landfilling or incineration. Recycling EoL RO membranes for reuse as nanofiltration (NF) or ultrafiltration (UF) membranes is an attractive pathway for the effective management of solid waste. However, the commonly adopted route to degrade the polyamide active layer of the EoL membranes for recycling and reuse requires high-intensity exposure of the membranes to the NaOCl solution. There is a need to develop a route to make membranes more responsive to the low dosage of NaOCl for recycling and reuse of the membranes. The current study explored that tailoring of the membrane active layer could generate a more responsive polyamide membrane to a mild concentration of NaOCl. This was achieved by incorporating monobenzoyl chlorinated aromatic reactants 4-nitrobenzoyl chloride (4-NC) and 3,5-dinitrobenzoyl chloride (3,5-DNC) into the active layer of the membranes. A minor concentration of the 4-NC and 3,5-DNC was added to the trimesoyl chloride-containing organic phase during interfacial polymerization with m-phenylenediamine. Owing to the presence of only one reactive site in 4-NC and 3,5-DNC, the reaction with m-phenylenediamine can hinder crosslinking in certain regions of the m-phenylenediamine and trimesoyl chloride polyamide active layer. This limited number of uncross-linked regions not only increases the membrane permeability with reasonable salt rejection but also makes the membranes more responsive to chlorine. This was realized when the permeate flux was found to be 108, 114, and 200 L m-2 h-1 for the M1 (control), M2 (4-NC), and M3 (3,5-DNC) membranes, respectively, at a feed pressure of 30 bar using 100 ppm of NaOCl for 24 h. On the other hand, the rejection of NaCl was decreased from 97 to 9.6, 93 to 7.1, and 91 to 3.6% with an increase in contact time from 2 h to 24 h for M1, M2, and M3 membranes, respectively. This exposure intensity was significantly less than that commonly used in the literature. Furthermore, the recycled modified membrane, especially M3, showed reversible fouling when the membranes were tested using bovine serum albumin as a model foulant in seawater scenarios.