Abstract
Industrial adoption of nanofiltration (NF) for treatment of low-pH wastewater is hindered by the limited membrane lifetime at strongly acidic conditions. In this study, the electroplating wastewater (EPWW) filtration performance of a novel pH-stable NF membrane is compared against a commercial NF membrane and a reverse osmosis (RO) membrane. The presented membrane is relatively hydrophobic and has its isoelectric point (IEP) at pH 4.1, with a high and positive zeta potential of +10 mV at pH 3. A novel method was developed to determine the molecular weight cut-off (MWCO) at a pH of 2, with a finding that the membrane maintains the same MWCO (~500 Da) as under neutral pH operating conditions, whereas the commercial membrane significantly increases it. In crossflow filtration experiments with simulated EPWW, rejections above 75% are observed for all heavy metals (compared to only 30% of the commercial membrane), while keeping the same pH in the feed and permeate. Despite the relatively lower permeance of the prepared membrane (~1 L/(m2·h·bar) versus ~4 L/(m2·h·bar) of the commercial membrane), its high heavy metals rejection coupled with a very low acid rejection makes it suitable for acid recovery applications.
Highlights
Membranes have proven to be a key technology to achieve process intensification in unit operations concerning separation and purification
It has a polyamide skin layer supported on a polysulfone substrate and can operate continuously at pH 2–11, with it being possible to expose it during short periods of time to more extreme pH conditions for cleaning purposes
The two micrographs on the left (A and B) show the cross-section of Membrane A with two different magnifications. They reveal an asymmetric structure composed of a dense thin top layer on its surface turning into finger-like macrovoids in the membrane bulk
Summary
Membranes have proven to be a key technology to achieve process intensification in unit operations concerning separation and purification. One of the main reasons behind the success of polymeric membranes in so diverse applications is their optimization to deal with harsh process conditions, such as high temperature, organic solvents, extreme pH and an oxidative environment. Such development can be seen in the history of polymeric membranes: the first commercial cellulose acetate (CA) membranes in the 1970s were stable only within a pH range of 4–8, whereas current polyamide composite membranes can be operated continuously at pH values between 2 and 12 [2]
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