(584g) MOF-Incorporated NF Membrane for Selective Removal of Selenium from Produced Water

Selenium is one of the components of the produced water (the waste stream in the oil extraction process with high salinity ranging from 500 ppm to 300 ppt), which limits the reuse of the treated stream. Selenium shows the tendency of bioaccumulation in the food chain and is harmful to aquatic and wild animals when ingested at high concentrations. A concentration of selenium greater than 0.05 ppm may cause tremors and heart attacks in human beings; hence, removing it from drinking water is crucial. Nanofiltration (NF) membranes have been used for the treatment of produced water. Although NF membrane showed acceptable rejection of heavy metals (~99% of Arsenic), the rejected heavy metals stay in the retention stream and therefore another separation process is needed to remove them for safe recycling. The adsorptive membranes with selectivity towards selenium will adsorb the selenium during filtration, hence eliminating the need for another separation process. Metal-organic frameworks (MOFs) are gaining significant attention recently as adsorbents due to their high porosity, adsorptivity, and selectivity. In this study, we report the synthesis of MOF-adsorptive membranes for the selective removal of selenium from high salinity (35000 ppm) waste streams such as produced water. The zirconium-based MOF (MOF-808) was synthesized via solvothermal method and iron-based MOF (Fe-MOF) via a non-thermal and solvent-free process using respective metal salts and trimesic acid. Based on the acceptable removal performance of MOF-808 (about 75 mg/g for selenite) and Fe-MOF (about 60 mg/g for selenite), they were selected for grafting on the TFC membrane for the generation of MOF-adsorptive membranes. The mentioned MOFs were grafted on the polyamide layer through chemical grafting. The blank NF membrane showed 5.64 LMH/bar permeability with ~86% rejection of selenite. Also, the blank membrane showed ~4.2 LMH/bar and ~0.1 LMH/bar of permeability for 2000 ppm and 35000 ppm saline (Na₂SO₄) feed respectively. The blank membrane showed 92% and 55% salt rejection for 2000 ppm and 35000 ppm salt solution respectively. The Fe-MOF membrane showed 6.63 LMH/bar permeability with enhanced selenium removal (about 88%). The effect of MOFs on the physicochemical properties of MOF-NF membrane and the selective performance along with the salt rejection of the fabricated MOF-NF membrane at different operation conditions (temperature (25-45°C), pH (2-12) and varying ionic strength (SO₃^2- and SO₄^2-)) are studied.