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• Solvothermal synthesis of pyridinium functionalized chitosan. • High efficiency of functionalized chitosan for methyl orange and Cr(VI) sorption. • A key role of pyridinium plays in adsorption. • Stable sorption and desorption properties for 5 successive cycles. A pyridinium functionalization chitosan (PCS) at high yield was facilely and solvothermally obtained from reactions of chitosan with N-2,4-dinitrophenyl-pyridinium chloride. The morphology and physical–chemical properties of PCS were tested with various techniques. Its sorption behaviors towards methyl orange (MO) and Cr(VI) were systematically investigated. Pseudo-second-order kinetic and Langmuir equations well fitted the sorption kinetics and isotherms, respectively. Thermodynamics analyses revealed the spontaneous and endothermic sorption of these two contaminants. PCS exhibited high sorption capacity of 1649.30 mg·g−1 MO and 200.46 mg·g−1 Cr(VI) at 308 K. The superior sorption performance of PCS over MO is ascribed to ion exchange, intermolecular hydrogen bond, and electrostatic and π-π interactions, while sorption of PCS over Cr(VI) is mainly driven by electrostatic forces, reduction and ion exchange. Moreover, the PCS exceeded 95 % of its original capacities during five cycles. This high sorption capacities and high reusability make PCS an excellent sorbent candidate towards anionic contaminants. [ABSTRACT FROM AUTHOR]

[Display omitted] • Functionalized hydrochar was prepared by free radical post-synthetic modification. • Hydrochar with − N+H 2 R groups exhibited high adsorption capacity towards Cr(VI). • •Nitrogen-containing groups in hydrochar play multiple roles in adsorption procedure. Chemical modification on hydrochars can significantly improve their ability of removing heavy metal ions from wastewater, but so far no research has focused on the chemical modification through free radical reaction. In this work, a cation functionalized hydrochar (CFHC) bearing − N+H 2 R was synthesized by grafting-polymerization of glycidyl methacrylate (GMA) onto bamboo hydrochar under initiation by benzoyl peroxide, followed by the amination with the introduced epoxy group and diethylenetriamine and a subsequent hydrochloric acid treatment. The resulted CFHC exhibited a superior removal capacity of 424.09 mg·g−1 for Cr(VI), and the highest sorption occurred at pH of 2. Combining a series of characterizations and tests, it was concluded that the sorption conformed to the pseudo-second-order and Freundlich equations, indicating a multilayer chemisorption process that mainly driven by electrostatic reaction, reduction, and surface complexation. This research proved that a free radical polymerization treatment could effectively transform hydrochars into super adsorbents for wastewater treatment. [ABSTRACT FROM AUTHOR]

• N-doped hydrochar was synthesized by one-step hydrothermal reaction. • Cr(VI) adsorption capacity of hydrochar was enhanced by N-doping. • NH 4 Cl acts as a catalyst and a nitrogen source in preparation of N-doped hydrochar. • N-groups can adsorb Cr(VI) by electrostatic interaction and reduce Cr(VI) to Cr(III). N-doped biochar can effectively eliminate toxic Cr(VI). Here, N-doped hydrochar (NHC) was successfully synthesized by one-pot hydrothermal carbonization (HTC) of NH 4 Cl and bamboo, and employed to adsorb Cr(VI). The specific surface area, pore volume, and carbon and nitrogen contents of NHC all increase compared with the undoped hydrochar (HC). NH 4 Cl acts as a cheap nitrogen source to enhance the nitrogen content of hydrochar and as an acid catalyst to accelerate hydrochar carbonization. Adsorption experiments show NHC has higher adsorption capacity than HC for Cr(VI). XPS and FTIR imply the dominant mechanisms of adsorbing Cr(VI) onto two hydrochars are electrostatic attraction, reduction and complexation, but the contributions of surface functional groups in two hydrochars for elimination of Cr(VI) differ. The doped nitrogen in NHC is pivotal in adsorbing and reducing Cr(VI). Hence, NHC prepared from bamboo and NH 4 Cl by one-step HTC is a cheap and efficient adsorbent to eliminate aqueous Cr(VI). [ABSTRACT FROM AUTHOR]