Non-linear isotherm models, cadmium kinetics, and biosorption thermodynamics of dried biomass of native Aphanothece sp. in a batch system

Abstract

Dried biosorbent was prepared from Aphanothece sp. cyanobacteria harvested from a photobioreactor system fed with atmospheric carbon dioxide. Cadmium-ion biosorption of the prepared biosorbent from aqueous solution was characterized by non-linear (Langmuir, Freundlich and Dubinin-Radushkevich) isotherms, non-linear kinetics (pseudo first-order and pseudo second-order) and thermodynamic analysis. The optimum conditions were pH 8.0, 30°C, 0.1 g/L biomass, and 60 min contact time. The biosorption efficiencies exceeded 90%. The low-range data (initial Cd concentration C0 = 1.09-6.23 mg/L) and highrange data (C0 = 5.41-83.07 mg/L) were best fitted to the Langmuir model, with maximum uptake capacities of 12.01 and 187.5 mg/g (R2 = 0.995 and 0.996). In the Dubinin-Radushkevich isotherm model, the mean biosorption energy was 12.91 kJ/mol, suggesting that ion exchange was the working mechanism. The biosorption apparently followed pseudo second-order kinetics (R2 = 0.994- 0.999; k2 = 2.04 E-03 to 3.86 E-02 g/mg min). The biosorption process was energetically feasible (ΔG0 = -13.47--8.88 kJ/mol), exothermic (ΔH0 = -74.82 kJ/mol) and tended to become more ordered (ΔS0 = -0.204 kJ/mol K) towards the end of the process. The biosorbent was reusable through three adsorption/desorption cycles in 1 M HCl.