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Utilization of coal tailings (CTs) to enable ammonia (NH 3) 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 NH 3. The CTs treated at 300 and 250 °C for 5 h achieved an NH 3 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 NH 3 uptake and concentration of acidic surface functional groups was found (R2 = 0.99). Spectroscopic analysis showed that NH 3 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 °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 NH 3 emissions. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • The higher the charge of the oxyanion, the easier the ligand exchange mechanism. • The mechanism of LDHs removing harmful anions favors the surface adsorption from a rate-determining step perspective. • 003 surface is more attractive to monovalent anions. • Divalent and trivalent anions are more inclined to interact with the metal atoms on the 110 surface. Environmental pollution caused by harmful anions is becoming an increasingly prominent problem and should be addressed urgently. Layered double hydroxides (LDHs) have been used as excellent adsorbents and stabilizers. Here, three removal mechanisms (surface adsorption, anion exchange and ligand exchange) of Mg 2 Al–NO 3 LDH for 13 harmful anions (F−, Cl−, Br−, I−, ClO 3 −, BrO 3 −, IO 3 −, CrO 4 2−, SeO 4 2−, MoO 4 2−, AsO 4 3−, VO 4 3−, PO 4 3−) are investigated by density functional theory (DFT). The adsorption energy of anions on 0 0 3 and 1 1 0 surface, the Gibbs free energy change of anion exchange and ligand exchange reactions are calculated thermodynamically. It is found that both divalent and trivalent harmful anions can be removed by the above three mechanisms. All anions except I− can undergo surface adsorption, with divalent and trivalent anions preferring to interact with 1 1 0 surface and monovalent more easily adsorbed on 0 0 3 surface. Kinetically, the energy barriers of the rate–determining steps for three mechanisms are further calculated. The results indicate that all harmful anions removed by LDHs favor the surface adsorption mechanism. Anion exchange mechanism cannot occur for anions with Mulliken electronegativity and ionic potential below 1.7 eV and 5 nm−1, respectively. Generally, the higher the charge of the oxyanion, the easier the ligand exchange mechanism. [ABSTRACT FROM AUTHOR]