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This study rigorously investigated the adsorption behaviors and mechanisms of strontium ions (Sr2+) by the pristine and magnetic biochars prepared via the pyrolysis of pristine and FeCl3-pretreated spent coffee grounds (SCG). The FeCl3-pretreatment could magnetize successfully the pristine SCG biochars (magnetization value of pristine SCG biochars = 0.19 emu/g; magnetization value of magnetic SCG biochars = 8.99 emu/g) and improved substantially their specific surface area (pristine SCG biochars = 5.3 m2/g; magnetic SCG biochars = 431.7 m2/g) and total pore volume (pristine SCG biochars = 0.002 cm3/g; magnetic SCG biochars = 0.186 cm3/g). However, the removal efficiency of Sr2+ with the pristine SCG biochars (33.4–34.1%) was not significantly different from that with the magnetic SCG biochars (33.1–33.9%). The calculated kinetic parameters indicate that the adsorption of Sr2+ by the pristine and magnetic SCG biochars was governed by the chemisorption (R2 of the pseudo-first-order kinetic equation ≤ 0.602; R2 of the pseudo-second-order kinetic equation = 1.000). The higher R2 values of the Langmuir isotherm (R2 = 0.999) model compared to the Freundlich isotherm model (R2 ≤ 0.950) are evident that the monolayer adsorption mainly determined the adsorptive removal of Sr2+ with the pristine and magnetic SCG biochars. Moreover, the adsorption of Sr2+ using the pristine and magnetic SCG biochars was the spontaneous and endothermic process. These observations indicate that the FeCl3-pretreatment might offer a viable way to effectively recover the residual SCG biochars after the Sr2+ adsorption through the magnetic separation (recovery rate = 90%).