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• H 2 O 2 modification of hydrochar increased surface functional groups on hydrochar. • Modified hydrochar adsorbed NH 4 + better than pristine hydrochar by two times. • NH 4 +adsorption was mainly controlled by cation exchange and electrostatic attraction. • Modified hydrochar can be used as a promising alternative adsorbent for NH 4 + removal. This work investigated adsorption behaviors of NH 4 + onto H 2 O 2 modified paper waste sludge-derived hydrochar (HPSH). Effect of various operational parameters, including modification ratio, solution pH, adsorbent dosage, initial NH 4 + concentration and contact time on NH 4 + adsorption was studied. Besides, the NH 4 + adsorption isotherms, kinetics and thermodynamics at different operational temperatures were systematically investigated. The textural, morphology, crystalline structure and surface chemistry characteristics of hydrochars were analyzed. Specially, NH 4 + adsorption mechanisms controlled by enriched oxygen-containing surface functional groups by treatment of H 2 O 2 onto hydrochars were deeply discussed. The results showed that the optimal treatment ratio between H 2 O 2 and PPSH was 40. Characterization data indicated H 2 O 2 modification increased the oxygen-containing functional groups on the hydrochar surface. The maximum adsorption capacity of HPSH-40 was 7.941 mg/g which was higher that of PPSH about double. NH 4 + adsorption onto hydrochars was well described by the pseudo-second-order model and Langmuir isotherm. The thermodynamic studies showed NH 4 + adsorption onto hydrochars was endothermic in nature. The mechanisms of NH 4 + adsorption onto hydrochars included π-cation interaction, cation exchange, surface complexation and electrostatic attraction which were attributed to the vital role of enriched oxygen-containing surface functional groups on hydrochar's surface. [Display omitted] [ABSTRACT FROM AUTHOR]