<p>This study evaluated the potential of spent microalgal biomass, after lipid extraction, and its biochar derivatives for cadmium removal from wastewater. Adsorption was species-dependent. <i>Scenedesmus</i>-derived biochar exhibited better performance, with a maximum Cd²⁺ adsorption capacity of 79 mg/g, attributable to its higher surface area (37.1 m²/g) and surface charge (surface zeta potential of − 38.5 mV at pH 6). On the other hand, <i>Spirulina</i>-derived biochar showed 22 mg/g of Cd²⁺ adsorption, hindered by its high ash content, pore blockage, low surface area (5.2 m²/g), and low surface charge (− 10.5 mV at pH 6). Cd²⁺ adsorption reached equilibrium within 30 min and was described by pseudo-second-order (<i>R</i>² &gt; 0.98) kinetics. Cd²⁺ adsorption on spent biomass was best described by the Langmuir model. Cd²⁺ adsorption on biochar conformed more closely to the Freundlich model. Results from single-metal and multi-metal systems, supported by thorough characterisation, confirmed that surface adsorption, primarily governed by electrostatic attraction, was the dominant mechanism. Results from this study underscore the feasibility of valorising spent microalgal biomass into effective adsorbents for wastewater treatment.</p>