Surface modification of coal tailings by thermal air oxidation for ammonia capture

Utilization of coal tailings (CTs) to enable ammonia (NH3) capture is of interest from pollution control and waste management perspectives. In this work, CTs were surface modified by air oxidation at different temperatures and varying duration to increase the uptake of NH3. The CTs treated at 300 and 250 degrees C for 5 h achieved an NH3 uptake of 52.5 and 45.3 mg g-1, respectively, which was significantly higher than that of the untreated CT (2.1 mg g-1). A linear relationship between NH3 uptake and concentration of acidic surface functional groups was found (R2 = 0.99). Spectroscopic analysis showed that NH3 can be retained on the oxidized CT through chemisorption involving carboxylic groups, leading to the formation of amides. Kinetic studies in the temperature range of 200-300 degrees C revealed an activation energy of 50.2 kJ mol-1 for the formation of acidic surface functional groups on CTs. These comparably mild conditions for the oxidative surface modification make CTs versatile and readily available materials for reducing agricultural NH3 emissions.