(92a) Utilizing Metal-Organic Frameworks for Selective Removal of Selenium from Produced Water

Selenium tends to accumulate in the food chain and has been known to decrease the lifespan of aquatic and wild animals. When the level of selenium in the drinking water exceeds 0.05 ppm, it can pose a risk to human health. Therefore, it is essential to remove it from the drinking water. Additionally, selenium is a component of produced water, which is the waste stream generated during the oil extraction process and has high salinity levels ranging from 5000 ppm to 300 ppt. This makes it difficult to reuse the treated stream. Metal-organic frameworks (MOFs) have attracted considerable interest as adsorbents due to their remarkable porosity, tunability, and selectivity. In this study, zirconium-based MOF (MOF-808) and iron-MOF (MIL-100(Fe)) were synthesized via different methods (such as solvothermal and acid-free-nonthermal processes) using metal salts and ligands. Isotherm adsorption tests were conducted for a 500 ppm MOF with 60 ppm selenium at 25°C for 24 h. MOF-808 (adsorption of 62% selenite ions and 39% selenate ions) and MIL-100(Fe) (45% selenite ions) exhibited better adsorption of selenite than selenate in water. The superior performance of MOF-808 is attributed to the coordination bonding between the charged metal in the MOF nodes and selenite and selenate molecules. To determine the selectivity of the MOFs toward selenium and selenate, isotherm adsorption tests were conducted in the presence of different salts (Na₂SO₄ (2000 ppm) and NaHSO₃ (2000 ppm)) for MOF-808 and Fe-MOF. The data show that MOF-808 could remove 53.55% of selenite and 2.15% of selenate in the presence of Na₂SO₄ (2000 ppm), whereas it could remove 5.2% of selenite and 1.26% of selenate in the presence of NaHSO₃ (2000 ppm) in water. The adsorption kinetics followed a pseudo-second-order model for the adsorption of Se from water and a high salinity oil-water emulsion. The adsorption isotherm was best fitted to the Langmuir isotherm for the adsorption of both selenite and selenate ions using MOF-808, with a maximum adsorption capacity of 78 mg/g for selenite. Coordination bonding between the metal nodes in the MOFs and selenite/selenate was the dominant adsorption mechanism compared with hydrogen bonding and p-anion bonding. MOF-808 showed an acceptable adsorption capacity for both selenite and selenate ions even after four adsorption/desorption cycles.