Synergistic iron ion and sulfate removal via chitosan-engineered yeast biosorbents

The sustainable removal of iron ions (Fe^2+/3+) and sulfate (SO₄^2-) from contaminated water remains a major challenge, particularly in mining-affected regions such as Lusatia (Germany), where both contaminants frequently co-occur at elevated levels. Here, a dual-modification strategy based on chitosan (Cs)-modified yeast cells (Cs@YC) and chitin-glucan complexes (CGC), is presented to enhance biosorption performance. Dried Cs@YC showed improved adsorption capacities compared to unmodified YC, while the combined Cs-functionalized CGC (Cs@CGC) exhibited the highest uptake capacities, reaching 2.09 mmol g^-1 for Fe^2+/3+ and 0.79 mmol g^-1 for SO₄^2- at pH 6. Microscopic and spectroscopic analyses (SEM-EDX, TEM, AFM, XPS) showed that Cs surface functionalization combined with alkaline-induced CGC restructuring increases the density and accessibility of amino and hydroxyl groups and promotes controlled iron (oxyhydr)oxide nucleation and crystallization. Equilibrium modelling using Langmuir, Freundlich, and Sips isotherms demonstrated that Cs@YC provides high-affinity metal-binding sites with positively charged surface domains that facilitate electrostatic association and adsorption of SO₄^2-. To the best of our knowledge, this is the first study to demonstrate the synergistic interaction between Cs modification and CGC-based structural tuning in yeast-derived biosorbents for dual Fe^2+/3+/SO₄^2- remediation. The results demonstrate that Cs@YC offers promising adsorption performance under controlled laboratory conditions, warranting further evaluation in real mining-affected waters containing competing ions and organic matter.