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• The novel magnetic activated biochar nanocomposites were first time prepared using wakame. • Loading nickel on the surface of wakame biochar is conducive to its recycling. • The adsorption process is a spontaneous endothermic process. • At 293 K, the maximum adsorption capacity of MB is 479.49 mg/g. In this work, magnetic wakame biochar nanocomposites for the first time had been synthesized to investigate their adsorption to methylene blue dye. As-prepared magnetic biochar samples were obtained by the impregnation method to load nickel on wakame biochar via one-step carbonization with activation agent KOH at 800 °C. The prepared samples were characterized by BET, XRD, FTIR, Raman, SEM, TEM and so on. The results exhibited that the maximum adsorption capacity of BW(Ni)0.5 to methylene blue could reach 479.49 mg/g at 20 °C. The adsorption behavior was more suitable for Langmuir isotherm equation and the kinetic data were most consistent with the pseudo second-order model. And also, the adsorption reaction was a spontaneous and endothermic process. After five cycles, it was found that BW(Ni)0.5 had a high adsorption capacity for methylene blue (117.58 mg/g). This study demonstrated that wakame biochar could have great potential in dye wastewater treatment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Acrylate-modified hydrochar with excellent adsorption properties was prepared easily. • The removal amounts of Pb(II) and methylene blue exceed 291.96 and 1014.96 mg/g. • Acrylate plays a key role in the adsorption process. • The acrylate content was more influential on adsorption than specific surface area. • The structure of hydrochar significantly affects its functional modification. Hydrochars are promising sorbents for wastewater treatment. Herein, two acrylate-modified hydrochars (AMHC 1 and AMHC 2) were obtained by grafting acrylic acid on the surface of two hydrochars (MHC 1 and MHC 2 hydrothermally carbonized in water and acidic medium respectively) with free radical polymerization. Characterizations show that MHC 2 is more prone to free radical polymerization than MHC 1 does, and has higher carboxylate content after modification. The adsorption amounts of AMHC 2 over methylene blue (MB) and Pb(II) are much higher than those of AMHC 1. Pseudo-second-order kinetic and Langmuir isotherm equations well fit the Pb(II) and MB sorption data of AMHC 2. The Pb(II) adsorptive mechanism is mainly inner-surface complexation accompanied by ion exchange and cation-π interaction. MB adsorption involves ion exchange, electrostatic interaction, H-bonding and π-π interaction. Hence, the one-step modification method of free radical polymerization under alkaline condition has great potential for preparing carboxylate-modified hydrochars to adsorb cationic pollutants. [ABSTRACT FROM AUTHOR]