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• We prepare the "salt-based biochars" using the "one-step sintering" technique. • SiBC and MnBC (Si and Mn modified biochar) exhibit excellent adsorption properties. • Cu(II) adsorption mechanisms by SiBC and MnBC are reviewed. Novel salt-based biochar was prepared by loading silicon (Si) on cornstalk biomass with "one-step sintering" technique. Manganese (Mn) was also used to modify biochar with the same method as a control. Surface morphology, elemental composition, crystal structure and surface area of "salt-based biochars" were analyzed by SEM + EDS, XRD, FTIR and BET, and the effects of the dosage of absorbent and pH of solution on the adsorption process were explored. Si and Mn could be successfully attached on the biochar surface as oxide forms. SiBC exhibited a dense and agglomerated surface, while MnBC was a kind of porous and rough materials. The optimal adsorption capability would realize when putting 2 g/L of biochar composites at pH = 5–6. Adsorption isotherms, adsorption kinetics, combine with FTIR and XPS were carried out to help to elaborate the adsorption mechanisms. The maximum adsorption capacity of Cu (II) was 152.61 mg/g on SiBC and it could reach at 97% of removal rate within 10 min when the concentration was 100 mg/L, while MnBC had to take 500 min to achieve the same adsorption effect, and reached 187.76 mg/g of maximum adsorption capacity. Langmuir model and pseudo-second-order model were more suitable for both SiBC and MnBC, which meant the monolayer and chemical adsorption were dominated. Surface complexation and precipitation was attributed to SiBC. Specialistic adsorption, ion exchange and intra-particle diffusion was put it down to MnBC. [ABSTRACT FROM AUTHOR]