Magnetized hydrochar for the adsorption of cadmium and a method of synthesizing magnetized hydrochar from microalgal biomass are provided. The magnetized hydrochar may be synthesized by subjecting a microalgal biomass to a hydrothermal carbonization (HTC) reaction to produce hydrochar, chemically activating the hydrochar with H2O2, and magnetizing the activated hydrochar through coprecipitation. The microalgal biomass may be selected from Chlorella vulgaris FR751187 or Scenedesmus obliquus GU732418. The resulting mHC may be used to adsorb cadmium ions from an aqueous environment, including but not limited to waste effluent.