Biomass Concentration Determines Langmuir and Freundlich Isotherm Parameters in Biosorption by Chlorella vulgaris

Adsorption isotherm parameters are widely used to characterize biosorbent performance and are often interpreted as intrinsic properties of the adsorbent. However, the extent to which these parameters depend on biosorbent concentration remains insufficiently understood. Here, the adsorption of Ni²⁺ and Cd²⁺ by Chlorella vulgaris was systematically investigated over a broad range of biomass concentrations by determining complete adsorption isotherms independently at each biomass level. Increasing biomass concentration consistently decreased the specific adsorption capacity and shifted the adsorption isotherms toward lower apparent saturation capacities for both metal ions. Langmuir and Freundlich models were fitted using both linearized and nonlinear regression. Despite differences in the absolute parameter values obtained by the fitting approaches, all models revealed the same biomass-dependent trend. In particular, the Langmuir maximum adsorption capacity (q_max) and Freundlich adsorption coefficient (K_F) decreased systematically with increasing biomass concentration. These results demonstrate that biomass concentration influences the experimentally observed adsorption behaviour and the isotherm parameters derived from it. Consequently, q_max and K_F should not be interpreted as universal material constants without considering the biomass concentration at which they were determined. Biomass concentration should therefore be reported and considered when comparing biosorption isotherms across independent studies.