Heavy metal adsorption from aqueous solution onto modified biochar derived from date seed biomass was investigated. Modification methods included pre-treatment of the biomass prior to pyrolysis using NaOH and HCl (labelled as DSB-PB and DSB-PA respectively) and post–treatment of the biochar pyrolyzed at 550 °C for 3 h with NaOH and HCl (labelled as DSB-BW and DSB-AW respectively). Modified biochars were compared to unmodified one in terms of their surface chemistry, morphology, metal uptake, and adsorption kinetic behaviour. Scanning electron microscopy images show that acid and base treatment enhanced the biochar porous structure. Fourier-transform infrared spectroscopy revealed that acid treatment had enriched the functional groups such as CC, CO, CO, and phenolic groups on the biochar surface which further facilitated metal ions adsorption. Alkali treatment had less significant impact on the biochar surface functional groups. Modification improved the adsorption capacity for all metals. Most notably, DSB-PA biochar showed the highest adsorption capacities of 0.911, 0.705, and 0.692 mmol g−1 for Pb2+, Cu2+, and Ni2+ respectively. Compared to the unmodified biochar, the adsorption capacities of DSB-PA for Pb2+, Cu2+, and Ni2+ increased by 27%, 66% and 98%, respectively. Sips model adequately described the adsorption isotherms indicating heterogeneous adsorption systems. Adsorption rates of metal ions onto the DSB-PA were fast with almost 94% of the total amount of metals adsorbed within the first hour compared to 3–6 h for unmodified biochar. Pre-treatment of biochar with HCl is an economical method to produce an effective low-cost adsorbent with low environmental impact.