Within the membrane community there has been an increasing focus on the filtration technique forward osmosis (FO), where filtration is driven by a concentration gradient as opposed to the traditional pressure gradient known from reverse osmosis (RO). FO membranes have gained interest in several disciplines and are finding applications in different markets such as seawater and brackish water desalination, wastewater treatment, treatment of high salinity waters, fertigation, textile industry, dairy, food, and beverage, power generation, and pharma industry.
One of the key performance indicators for FO membranes is the rejection of organic molecules. In drinking water or wastewater treatment, high rejection is important to ensure that micropollutants such as pesticides, pharmaceuticals, and endocrine disruptors do not end up in the final product. In concentration or dilution applications, such as downstream processing of pharmaceuticals and fertigation, it is also important to control the transport of other molecules than water across the membrane to avoid loss of valuable components.
In Niada Bajraktari’s et.al study, the transport of two peptides of molecular sizes 375 Da and 692 Da with flexible structures across a biomimetic forward osmosis membrane with a dense selective layer was investigated. The membrane exhibited high but incomplete rejection rates where 1% of the peptides were still able to cross the membrane barrier in spite of their relatively large size. The peptide with the smallest molecular mass was found to have the lowest permeability, which might be explained by the radius of gyration. A higher radius of gyration enables the peptide to assume several conformations when in contact with the membrane matrix, which ultimately could increase the probability of the peptide being transported through the membrane. The transport mechanism was found to be diffusion based. The overall finding of this study was that even relatively large molecules can cross an otherwise dense membrane layer. This is significant primarily for choosing the optimal operating conditions and run time of the system in downstream processing and needs to be considered for each and every forward osmosis application. Ultimately, it has a significance in the robustness of the membrane fabrication process where the incorporation of biomolecules in the selective layer is one of the key steps.