Biosorption of three acid dyes by the brown macroalga Stoechospermum marginatum: isotherm, kinetic and thermodynamic studies

The brown macroalga Stoechospermum marginatum was tested for its ability to remove the acid dyes Acid Blue 25 (AB25), Acid Orange 7 (AO7) and Acid Black 1 (AB1) from aqueous solution. Biosorption was investigated under different conditions of pH, biomass and initial dye concentrations, temperature, contact time and biosorbent particle size. The results at equilibrium were successfully described by the Freundlich model, and the estimated sorption capacities were 22.2, 6.73 and 6.57 mg/g for AB25, AO7 and AB1, respectively. Among the pseudo-first-order, pseudo-second-order and intraparticle diffusion models, which were applied to kinetically describe the biosorption, the second one showed the best fit of the experimental data (R2 = 0.99). The thermodynamic parameters of the process (ΔG°, ΔH° and ΔS°) were estimated from tests carried out at different temperatures (283, 298 and 313 K). For all dyes, the standard changes of the Gibbs free energy were negative (-5.22 < ΔG° < -3.68 kJ/mol for AB25; -2.49 < ΔG° < -1.72 kJ/mol for AO7; -1.94 < ΔG° < -0.94 kJ/mol for AB1) and those of the enthalpy positive (ΔG° = 10.8, 5.52 and 8.46 kJ/mol for AB25, AO7 and AB1, respectively), hence pointing out that biosorption was a spontaneous and endothermic process. A decrease in particle size of S. marginatum biomass increased AB25 and AB1 removal efficiencies per unit weight of sorbent. FT-IR analysis demonstrated the involvement of different functional groups, mainly hydroxyl and amine groups. This study highlights the potential of the selected macroalga as an effective biosorbent for the removal of dyes from wastewaters.