Biosorption of methylene blue onto Arthrospira platensis biomass: Kinetic, equilibrium and thermodynamic studies

In this study, Arthrospira platensis biomass was employed as a biosorbent for the removal of methylene blue (MB) dye from aqueous solutions. The kinetic data were better described by the pseudo-second order model and equilibrium was established within 60–120 min. The intra-particle diffusion was not the only rate-limiting step and film diffusion might contribute to MB biosorption process. The increase of temperature from 298 to 318 K caused a decrease of biosorption capacity. The Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherm models described well the experimental equilibrium data at all studied temperatures. The maximum monolayer adsorption capacity (qmax) was 312.5 mg MB/g at 298 K and pH 7.5. According to the results of the thermodynamic analysis and the release of exchangeable cations from the biomass surface, physical sorption and ion exchange were the dominant mechanisms of MB biosorption at lower temperature. Methanol esterification of the dried biomass showed the involvement of carboxyl functional groups in MB chemisorption. The thermodynamic parameters indicated that MB biosorption onto A. platensis was a spontaneous, favorable and exothermic process. The biosorption results showed that A. platensis could be employed as an efficient and eco-friendly biosorbent for the removal of cationic dyes.