Sustainable Water Treatment using SpiruSpheres: A Biosorbent Based on Spirulina platensis and Sodium Alginate

Water pollution with hexavalent chromium [Cr(VI)] poses a serious threat to both environmental and public health due to its extreme toxicity, mobility, and persistence. The present work aimed to develop an efficient, sustainable, and cost-effective biosorption technique for Cr(VI) removal using immobilized Spirulina platensis (SpiruSpheres), a filamentous cyanobacterium rich in metal-binding functional groups. SpiruSpheres were tested using a gravity-driven separation funnel, simulating continuous flow conditions, a novel approach not widely explored in previous studies on Spirulina-alginate biosorption. Comprehensive characterization using FTIR, SEM-EDX, BET, and TGA confirmed the presence of functional groups and a structure conducive to adsorption. Biosorption performance was statistically optimized using response surface methodology, with a focus on pH, contact time, and initial Cr(VI) concentration. Maximum removal efficiency of 84.05% was achieved under acidic conditions (pH 3.61) after 150 min, with an initial concentration of 20 mg/L. The kinetic modeling suggested that the process may involve chemisorption as a potential step in the removal of Cr(VI), although further thermodynamic validation is required to confirm this mechanism. Isotherm analysis showed that the Freundlich model best fit the data, suggesting multilayer adsorption on a heterogeneous surface. The reusability of SpiruSpheres was demonstrated over four cycles, maintaining structural integrity and high performance, which adds significant practical value to this biosorbent for large-scale applications. In conclusion, this work presents a promising, reusable, and eco-friendly solution for Cr(VI) removal, especially in regions lacking advanced treatment infrastructure, and contributes to the advancement of green water treatment solutions.