Keratin/casein sponges: fabrication, characterization and use as novel green bio-polymers for nitrate ion sequestration

A novel keratin/casein bio-composite sponge capable of removing nitrate ions from aqueous solutions is presented. Compared to many conventional bio-adsorbents such as chitosan derivatives, eggshell composites, and clay-based materials, the keratin/casein sponge demonstrates enhanced removal efficiency, better mechanical integrity, and a more sustainable preparation pathway. By applying a green alkaline hydrolysis method, the keratin extraction process from chicken feather was optimized, leading to 14% recovery. Different proportions of keratin and casein were used for the production of sponges by the freeze drying method. Materials characterization was carried out by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, thermal gravimetric analysis and dynamic light scattering techniques. Adsorption experiments demonstrated an impressive sodium nitrate adsorption capacity of 51 mg g⁻¹ at pH = 3. It is shown that integral sponges with high adsorptive properties are produced in a limited range of the keratin/casein weight ratio. While casein imparts cohesion to the composite, it is also protonated under acidic conditions, exhibiting a synergetic effect for nitrate anion adsorption. It is shown that adsorption follows a Langmuir isotherm, and accordingly, it is mathematically demonstrated that the adsorption kinetics should obey a pseudo-second order model. Such a prediction was confirmed by the dynamic experiments. Given its renewable composition, cost-effectiveness, and regeneration potential, the new bio-composite shows strong promise for practical deployment in water treatment and other environmentally relevant applications.