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[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]

[Display omitted] • Two biochars were prepared from APS/KPS-pretreated bamboo. • Cation types in persulfate impact physicochemical properties of biochar. • The KPS-pretreated bamboo-derived biochar has a higher adsorptive capacity for MB. • The discrepancy in adsorption capacity of two biochar for MB was analyzed. • Electrostatic action is the main adsorption mechanism for MB onto biochars. The adsorption behaviors of biochar are largely impacted by biomass feedstock. In this study, two biochars were prepared from torrefaction of ammonium persulfate- and potassium persulfate-pretreated bamboo and then activated by cold alkali, which are named as ASBC and KSBC, respectively. The two biochars were characterized by different instruments, and their adsorption properties over cationic methylene blue (MB) were compared. The type of persulfates little affected the specific surface areas, but significantly impacted O (29.54 % vs. 35.113 %) and N (12.13 % vs. 3.74 %) contents, functional groups, and zeta potentials of biochars. MB adsorption onto ASBC/KSBC is a single-layer chemical endothermic process and ASBC/KSBC exhibit high adsorption capacity over MB (475/881 mg·g−1) at 303 K. Obviously, the sorption capacity of MB onto KSBC much surpasses that of MB onto ASBC. These results indicate biomass pre-treatment is a cheap and convenient method to prepare biochars with unique physicochemical and adsorptive properties. [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]