Biosorption of malachite green dye over Spirulina platensis mass: process modeling, factors optimization, kinetic, and isotherm studies

Spirulina platensis is one of the types of blue-green algae that was used as a biosorbent in this study. The aim of this study was to investigate the efficiency of S. platensis in removing MG from aqueous solutions and also to evaluate the biosorbent capacity using different kinetic models and isotherms. To obtain the optimum condition for MG biosorption using BBD, input factors included the initial level of MG 20–200 mg.L⁻¹), dose of S. platensis (0.1–1.5 g.L⁻¹), pH (4–9), and contaact time (5–80 min). The statistical method of BBD was considered to evaluate the removal rate of MG dye from aqueous solutions. The prediction of MG removal efficiencies and the evaluation of variable interactions were performed using a polynomial equation. The maximum removal efficiency of MG was obtained as 94.12% under MG level of 100.54 mg. L⁻¹, pH of 7.57, contact time of 52.43 min, and S. platensis dose of 0.98 g. L⁻¹. The removal MG efficiency enhanced with the increase in pH, reaction time, and S. platensis dose, and reduced with the decrease in MG level. The quadratic model suggested that the pH had a high impact on MG removal. The isotherms and kinetics data could be properly illustrated by the Freundlich model and the pseudo-second-order equation. Thermodynamic factors, including ΔG⁰, ΔH⁰, and ΔS⁰ showed the adsorption of MG onto S. platensis was spontaneous and exothermic. The acquired findings also showed that the physisorption mechanism mainly govern the MG sorption process. As a result, S. platensis showed excellent adsorptive properties and hence could be offered as a viable option for eliminating MG from aqueous solutions.