Enhanced adsorption of ammonia and total nitrogen by activated carbon fibres dual-modified with HNO3 and NaOH

Abstract Nitrogen serves as an important driver of eutrophication in water bodies, with its excessive inputs posing considerable threats to aquatic ecosystems. In order to remove nitrogen pollutants efficiently, activated carbon fibres (ACFs) were dual-modified with HNO3 and NaOH to improve their adsorption performance in aqueous solution. Batch experiments on surface characteristics and adsorption effects on ammonia nitrogen (NH3-N) and total nitrogen (TN) were conducted to assess the adsorption performance and related mechanisms of modified ACFs (MACFs). The surface roughness of MACFs was increased, and –OH, C=O and C=C double bonds and Na+ were loaded on their surfaces. Among the nine MACFs, ACF50-0.4—modified with 50% HNO3 and 0.4% NaOH—achieved the highest adsorption capacity and removal rate of nitrogen pollutants. Its adsorption effect could be maximized and a high removal rate could be achieved at the dose of 1.0 g l−1, initial NH3-N and TN concentrations of 15 and 25 mg l−1, respectively, pH of 7 and temperature of 25°C. Moreover, the nitrogen adsorption mode was multilayer heterogeneous surface adsorption, which was spontaneous, endothermic and dominated by chemisorption. The main adsorption mechanisms of MACFs included surface adsorption, electrostatic attraction, ligand and ion exchange.