Binary Competitive Biosorption of Cd(II), Co(II), and Cu(II) onto Microalgae: pH-Dependent Behavior and Evaluation of Multicomponent Adsorption Models

The competitive biosorption of Cd(II), Co(II), and Cu(II) onto Chlorella vulgaris, Scenedesmus sp., and Spirulina platensis was evaluated at pH 4.0 (acidic) and 7.0 (near-neutral). Single-solute Langmuir and Freundlich parameters for the three metals, previously obtained for the same biomasses, were used as fixed inputs, and new binary competitive experiments were conducted for Cd/Cu, Co/Cu, and Cd/Co systems at a 1:1 initial concentration ratio. Single-solute capacity and affinity depended strongly on metal type, microalgal species, and pH, but the binary results showed that single-solute capacity did not predict competitive resistance. Cu(II) dominated in Cu-containing systems and strongly suppressed Cd(II) and Co(II) uptake, whereas Cd(II) was consistently less suppressed than Co(II) in Cd/Co systems, so competitive behavior could not be reduced to a single universal selectivity sequence. The Extended Langmuir (EL) model served as a useful ideal baseline but frequently overestimated adsorption of the weaker competitors. The Modified Competitive Langmuir (MCL) and Sheindorf–Rebuhn–Sheintuch (SRS) models described the non-ideal, asymmetric competition substantially better than EL. Because no independent ternary experiments were performed, the binary-derived parameters should be treated as preliminary inputs for future ternary modeling. Overall, single-solute isotherms alone can overestimate metal removal in mixed-metal waters, and binary competition data are necessary for realistic evaluation of microalgal biosorbents.