Effect of pretreatment on Cd(II) and Pb(II) biosorption by Nannochloropsis oculata microalgae biomass

Microalgal biomass has proven to be highly effective in the removal of heavy metals such as Pb²⁺ and Cd²⁺. 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 Pb²⁺ and Cd²⁺. 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 CaCl₂ and 0.2 M NaOH for 24 h with slow stirring. FTIR studies indicated that carboxyl functional groups were mainly responsible for binding Pb²⁺ and Cd²⁺ during biosorption. The maximum biosorption capacity of Pb²⁺ was 227.86 mg g^-1 and 57.17 mg g^-1 for Cd²⁺ in the untreated biomass. The treatment with CaCl₂ (Bs1) was more effective in the removal of Pb²⁺ than NaOH treatment (Bs2), otherwise NaOH treatment (Bs2) was better in the absorption of Cd²⁺ than CaCl₂ treatment (Bs1). The experimental results and biosorption isotherms suggest that the microalgae showed a greater affinity for the removal of Pb²⁺ than for Cd²⁺ in all of the experiments. Both biomass treatments had a negative effect on Pb²⁺ and Cd²⁺ biosorption. Furthermore, the equilibrium data align well with the Sips isotherm model in the Langmuir derivation, suggesting a complex monolayer biosorption process.