Efficient metal ions biosorption on red and green algae biomass: Isotherm, kinetic and thermodynamic study

In this study two types of marine algae: red algae (Callithamnion corymbosum – CC-RAB) and green algae (Ulva lactuca – UL-GAB), were used for the retention of Cu²⁺, Zn²⁺ and Co²⁺ ions from aqueous media, by biosorption. Both types of marine algae are abundant on the Romanian coast of the Black Sea and, since they have no uses, they represent a serious problem for the beach area. Therefore, their use as biosorbents for the recovery of some metal ions of strategic industrial importance (such as Cu²⁺, Zn²⁺ and Co²⁺ ions) may represent a way to valorise this biomass resource. In order to evaluate the biosorptive performances of the red algae biomass (CC-RAB) and green algae biomass (UL-GAB), batch experimental studies were carried out at different initial solution pH, biosorbent dose, initial metal ions concentration contact time and temperature. The optimal conditions (pH = 5.0; 2.0 g biosorbent L^-1, 3 h, 25 ±1 °C) were then used to obtain kinetic curves and biosorption isotherms, which were modelled. The pseudo-second order kinetic model best fits the kinetic data, while the biosorption isotherms are described by the Langmuir model, for all studied metal ions on both biosorbents. The maximum biosorption capacity depends on the nature of algae biosorbent, and follows the order: Cu²⁺ (81.25 mg g^-1) > Zn²⁺ (73.69 mg g^-1) > Co²⁺ (27.89 mg g^-1) in the case of CC-RAB, and Zn²⁺ (69.29 mg g^-1) > Cu²⁺ (43.47 mg g^-1) > Co²⁺ (26.15 mg g^-1) in the case of UL-GAB. The thermodynamic parameters (∆G⁰, ∆H⁰ and ∆S⁰) were also evaluated, and the obtained values indicate that all biosorption processes are spontaneous and endothermic. In addition, desorption of metal ions is quantitative in acid media, but the biosorption capacities decrease significantly after the first cycle of use. All these aspects have important environmental implications, and may provide benchmarks in the design of a strategy for the valorisation of this biomass resource.