Rejection of trace organic compounds by membrane processes: mechanisms, challenges, and opportunities

Abstract

This work critically reviews the application of various membrane separation processes (MSPs) in treating water polluted with trace organic compounds (TOrCs) paying attention to nanofiltration (NF), reverse osmosis (RO), membrane bioreactor (MBR), forward osmosis (FO), and membrane distillation (MD). Furthermore, the focus is on loopholes that exist when investigating mechanisms through which membranes reject/retain TOrCs, with the emphasis on the characteristics of the model TOrCs which would facilitate the identification of all the potential mechanisms of rejection. An explanation is also given as to why it is important to investigate rejection using real water samples, especially when aiming for industrial application of membranes with novel materials. MSPs such as NF and RO are prone to fouling which often leads to lower permeate flux and solute rejection, presumably due to cake-enhanced concentration polarisation (CECP) effects. This review demonstrates why CECP effects are not always the reason behind the observed decline in the rejection of TOrCs by fouled membranes. To mitigate for fouling, researchers have often modified the membrane surfaces by incorporating nanoparticles. This review also attempts to explain why nano-engineered membranes have not seen a breakthrough at industrial scale. Finally, insight is provided into the possibility of harnessing solar and wind energy to drive energy intensive MSPs. Focus is also paid into how low-grade energy could be stored and applied to recover diluted draw solutions in FO mode.

Role of Membrane–Solute Affinity Interactions in Carbamazepine Rejection and Resistance to Organic Fouling by Nano-Engineered UF/PES Membranes

“…Carbamazepine is a transphilic organic solute with a log K ow value of 2.45 [ 57 ]. Previous studies have reported that the rejection of organic compounds is also controlled by membrane–solute hydrophobic interactions, where hydrophobic membranes show lower solute rejection due to the adsorption of the compounds onto the membrane surface and facilitate diffusion into the permeate side, leading to lower rejection [ 9 , 13 , 58 ]. There was a clear increase in carbamazepine rejection with the increase in membrane hydrophilicity (due to the addition of nanoparticles).…”

“…However, the trend was not very clear because the performance of the fabricated membranes depends mainly on the pore size and pore size distribution, porosity and morphological structure as well as the charge of the membrane and solute [ 20 ]. There is a tradeoff between water permeability and solute rejection properties, which implies that water permeability and rejection cannot be maximized at the same time [ 9 , 31 ]. At a working polymer/nanoparticle mass ratio, both permeability and rejection were improved without compromising either of the two.…”

“…Ag-GO and ZnO-GO membranes had the highest electron donor components (Table 4). The rejection of organic compounds is controlled by both the membrane and solute properties, where the major rejection mechanisms are size exclusion, electrostatic interactions and non-electrostatic interactions, which include hydrophobic interactions and the formation of hydrogen bonds [9]. Carbamazepine is a neutral compound; therefore, no charge interactions with the membrane were expected.…”

“…Membrane treatment using ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes has become a promising technique for the removal of organic contaminants in water, where the retention of organic compounds is controlled by membrane-solute interactions. This makes both membrane properties (e.g., Membranes 2023, 13, 744 2 of 21 molecular weight cut-off (MWCO), hydrophobicity and surface zeta potential) and solute properties (e.g., solute size and shape, charge, polarity and hydrophobicity) fundamental parameters in determining solute rejection [9]. Thus, electrostatic and non-electrostatic affinity interactions (e.g., hydrogen bonding and van der Waals interactions) are major membrane-solute interactions [10][11][12][13].…”

Removal of Algae and Algal Toxins from a Drinking Water Source Using a Two-Stage Polymeric Ultrafiltration Membrane Process

“…The removals of MC-LR and MC-RR both show a trend of first a decrease and then an increase to a stable state. The rejection of organic pollutants during the UF process is possibly due to two reasons, including physicochemical interactions between the membrane surface and algal toxins, e.g., intermolecular forces, hydrogen bonds, and hydrophobic interactions, as well as the membrane mechanical screening [31,32]. The decrease after the initial high rejection is due to the gradual saturation of the adsorption sites on the membrane surface.…”