Selective Removal of Selenium from Produced Water Using MOF-808 and MIL-100(Fe): Performance and Mechanism Assessment
Sweta Modak, Brenner Kar, Milad Rabbani Esfahani
Abstract
The presence of selenium (Se) in produced water limits the reuse of treated streams due to its bioaccumulation in the food chain, which can reduce the lifespan of aquatic and wildlife species. Metal–organic frameworks (MOFs) have gained significant attention as adsorbents owing to their high porosity, tunability, and selectivity. In this study, zirconium-based MOF (MOF-808) and iron-based MOF (MIL-100(Fe)) were synthesized using solvothermal and acid-free, nonthermal methods and evaluated for Se removal from a highly saline (35,000 ppm) oil–water emulsion in batch experiments. MOF-808 adsorbed 72% of selenite and 16% of selenate, and MIL-100(Fe) adsorbed 58% of selenite and negligible amount of selenate from a highly saline oil–water emulsion with 60 ppm Se and 500 ppm MOF concentration. To determine the selectivity of MOFs toward selenite and selenate, isotherm adsorption tests were conducted in the presence of different salts (Na 2 SO 4, NaHSO 3, and NaCl at 2000 ppm). The adsorption kinetics followed a pseudo-second-order model for the adsorption of selenite from both water and high-salinity oil–water emulsion. The maximum adsorption capacities for selenite and selenate using MOF-808 were 107.1 and 47.6 mg/g, respectively, and for selenite using MIL-100(Fe) was 184.2 mg/g from high-salinity oil–water emulsion. The adsorption isotherm of selenite changed from the Langmuir to Freundlich isotherm fitting in the presence of high salinity and oil compared with water, indicating multilayer adsorption. Both MOFs showed elevated efficiency for selenite adsorption in the presence of high salinity and oil compared to water, potentially due to the salting-out effect. The superior performance of MOFs in adsorbing selenite compared to selenate is attributed to the coordination bonding between the metal in the MOFs and selenite. Electrostatic interaction was the dominant mechanism for selenate adsorption using MOF-808. Efficient adsorption of selenite was achieved using MOF-808 from modeled-produced water, even in the presence of multiple salts, naphthenic acid, high salinity, and oil. MOF-808 showed an acceptable adsorption–desorption capacity for both selenite and selenate.