Sugarcane bagasse-derived biochar modified by alkali for enriching surface functional groups to effectively treat ammonium-contaminated water.

In this study, sugarcane bagasse (SB), which was preliminarily treated with H₃PO₄, was utilized to produce biochar (SB-BC). The SB-BC was subsequently modified with KOH to enrich oxygen-containing functional groups (OCFGs) for the enhanced adsorption of NH₄⁺ from wastewater. Batch tests revealed that KOH-modified SB-BC (SB-MBC) increased the maximum Langmuir adsorption capacity of NH₄⁺ by approximately twofold, from 27.1 mg/g for SB-BC to 53.1 mg/g for SB-MBC. The optimal operational conditions for NH₄⁺ adsorption onto SB-MBC were pH of 7.0 and a biochar dose of 3.0 g/L for the removal of 50 mg/L NH₄⁺ at room temperature (25 ± 2 °C) over 180 min of contact. The enhanced adsorption capacity of NH₄⁺ onto SB-MBC was due to the important contribution of the OCFGs enriched on the surface of biochar, which was increased by about fourfold, after being modified by KOH. The NH₄⁺ adsorption dynamics were better fitted by the Elovich and the NH₄⁺ adsorption isotherms were better described by Langmuir and Sips models, showing that the adsorption process was dominated by monolayer chemisorption. The properties of the adsorption materials before and after adsorption of NH₄⁺ confirmed that cation exchange, electrostatic attraction and surface complexation were the main mechanisms controlling the adsorption process. The desorption and reusability tests of NH₄⁺-saturated SB-MBC revealed that NH₄⁺ adsorption slightly decreased after three successive sorption‒desorption cycles. The findings suggested that SB-MBC is a promising and feasible adsorbent for the effective treatment of NH₄⁺-contaminated water sources. Future work should conduct tests for treatment of NH₄⁺-rich real wastewater and utilize NH₄⁺-saturated SB-MBC as slow releasing fertilizer for plants growth.