Chitosan-based Fe₃O₄/TiO₂-microalgae hybrid microrobots for efficient adsorption and magnetic removal of nanoplastics

The increasing prevalence of nanoplastics in aquatic environments presents serious ecological and health challenges, highlighting the need for sustainable remediation materials based on biological macromolecules. In this study, chitosan-based hybrid magnetic microrobots composed of Fe₃O₄/TiO₂-coated microalgae were developed for the efficient adsorption and recovery of nanoplastics from water. Chitosan, a renewable biopolymer rich in amine (-NH₂) and hydroxyl (-OH) groups, plays a central role in facilitating electrostatic interactions and hydrogen bonding with negatively charged polystyrene nanoplastics. The hybrid system integrates magnetic Fe₃O₄ for external manipulation and recovery, TiO₂ providing hydroxyl-rich surfaces, and microalgae contributing biocompatibility and heterogeneous surface chemistry. Strong electrostatic attraction between nanoplastics (ζ = -29.1 mV) and the positively charged composite (ζ = +15.1 mV) resulted in removal efficiencies of 98-99% at concentrations of 2-5 mg/mL. Adsorption behavior followed a Freundlich isotherm model, indicating multilayer binding, while pseudo-second-order kinetics suggested chemisorption-dominated uptake. The material retained over 70% of its removal efficiency after five reuse cycles, enabled by magnetic recovery without energy-intensive separation. These findings demonstrate the potential of chitosan-based hybrid bio-composites as sustainable and reusable macromolecular adsorbents for nanoplastic remediation in aquatic systems. It aligns with the Sustainable Development Goals: SDG 6 (Clean Water and Sanitation) and 13 (Climate Action), represents a scalable and sustainable solution for nanoplastic pollution.