Microalgal biomass has proven to be highly effective in the removal of heavy metals such as Pb 2+ and Cd 2+ . Moreover, there are treatments available to enhance the heavy metal removal capabilities. This study investigates the impact of alkaline pretreatment on Nannochloropsis oculata biomass on the biosorption of Pb 2+ and Cd 2+ . Nannochloropsis oculata was cultured in fed-batch reactors at 25 °C, with 166 μmol photons m -2 s -1 light intensity, in f/2 Guillard medium for 21 days. The biomass was treated with 0.2 M CaCl 2 and 0.2 M NaOH for 24 h with slow stirring. FTIR studies indicated that carboxyl functional groups were mainly responsible for binding Pb 2+ and Cd 2+ during biosorption. The maximum biosorption capacity of Pb 2+ was 227.86 mg g -1 and 57.17 mg g -1 for Cd 2+ in the untreated biomass. The treatment with CaCl 2 (Bs1) was more effective in the removal of Pb 2+ than NaOH treatment (Bs2), otherwise NaOH treatment (Bs2) was better in the absorption of Cd 2+ than CaCl 2 treatment (Bs1). The experimental results and biosorption isotherms suggest that the microalgae showed a greater affinity for the removal of Pb 2+ than for Cd 2+ in all of the experiments. Both biomass treatments had a negative effect on Pb 2+ and Cd 2+ biosorption. Furthermore, the equilibrium data align well with the Sips isotherm model in the Langmuir derivation, suggesting a complex monolayer biosorption process. Graphical Abstract