Stable Polyelectrolyte Multilayer-Based Hollow Fiber Nanofiltration Membranes for Produced Water Treatment

Produced water (PW) constitutes a massive environmental issue due to its huge global production as well as its complexity and toxicity. Membrane technology could, however, convert this complex waste stream into an important source of water for reuse, but new and more efficient membranes are required. In particular, in the last few years, polyelectrolyte multilayers (PEMs) established themselves as a very powerful method to prepare hollow fiber-based nanofiltration (NF) membranes, and this membrane type and geometry would be ideal for PW treatment. Unfortunately, the presence of surfactants in PW can affect the stability of polyelectrolyte multilayers. In this work, we investigate the stability of polyelectrolyte multilayers toward different types of surfactant, initially on model surfaces. We find that chemically stable multilayers such as poly(diallyldimethylammonium chloride) (PDADMAC)/poly(sodium 4-styrenesulfonate) (PSS), based only on electrostatic interactions, are substantially desorbed by charged surfactants. For poly(allylamine hydrochloride) (PAH)/PSS multilayers, however, we demonstrate that chemical cross-linking by glutaraldehyde leads to surfactant stable layers. These stable PEM coatings can also be applied on hollow fiber support membranes to create hollow fiber NF membranes dedicated for PW treatment. Increased cross-linking time leads to more stable and more selective separation performance. These newly developed membranes were subsequently studied for the treatment of synthetic PW, consisting of freshly prepared oil-in-water emulsions stabilized by hexade-cyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) in the presence of a mixture of ions. For both types of produced water, the membranes show excellent oil removal (∼100%) and organics removal (TOC reduced up to ∼97%) as well as good divalent ion retentions (∼75% for Ca2+ and up to ∼80% for SO42–). Moreover, we observe a high flux recovery for both emulsions (100% for CTAB and 80% for SDS) and especially for the CTAB emulsion a very low degree of fouling. These stable PEM-based hollow fiber membranes thus allow simultaneous deoiling and removal of small organic molecules, particles, and divalent ions in a single step process while also demonstrating excellent membrane cleanability.