Improved adsorption capacity of ammonium from aqueous solution by modified biogas residue biochar

Abstract Residue biochar can be utilized as an adsorbent for ammonium nitrogen (NH 4 + -N) to prevent non-point source pollution. However, the limited adsorption capacity has restricted its extensive application. In this study, biochar was modified with hydrogen peroxide ( H 2 O 2 ), potassium permanganate (KMnO 4 ), and sodium hydroxide (NaOH) to enhance its adsorption performance. A comparative analysis of the biochar surface characteristics was used to investigate the adsorption systems. The results indicated that the adsorption capacities of the modified biochar (MB) were significantly enhanced compared with the raw biochar (RB). At the highest NH 4 + -N concentration of 150 mg L −1 , the adsorption capacities of RB-H 2 O 2 , RB-NaOH, and RB-KMnO 4 increased to 3.0, 3.2, and 4.0 times that of RB, respectively. As predicted by the Langmuir isotherm model, the maximum adsorption capacities of these three MB were 13.93, 41.00, and 68.15 mg g −1 , respectively. Ammonium adsorption on the MB surfaces was affected by surface adsorption, liquid membrane diffusion, and intra-particle diffusion. The specific surface area and pore volume of RB-KMnO 4 were significantly enhanced, with an increase in active sites on the pore surfaces, thereby strengthening its adsorption capacity for NH 4 + -N. In contrast, the adsorption of NH 4 ⁺-N by RB-H 2 O 2 and RB-NaOH primarily relied on the substantial increase in –C–O functional groups, with additional contributions from other oxygen-containing functional (e.g. –OH, –COOH, and Fe–O). In conclusion, RB-KMnO 4 exhibited the highest adsorption efficiency, with pore-based adsorption playing a dominant role over functional group-based adsorption. These findings highlight the critical role of pore structure optimization in enhancing the biochar adsorption capacity for NH 4 + -N. Graphical Abstract