From waste to water purification: Self-floating orange peel cellulose microspheres via in-situ CO2 foaming for methylene blue adsorption.

The treatment of organic dye wastewater calls for sustainable and high-performance technologies. Herein, we present an innovative strategy to upcycle waste orange peels into self-floating porous cellulose (OPC) hydrogel microspheres via a novel in situ CO2 foaming technique for efficient methylene blue removal. A synergistic acid-alkali-enzyme method was employed to extract OPC, which was subsequently engineered into a three-dimensional porous architecture with an average pore size of 46.7 nm, as evidenced by SEM and BET analyses. Under alkaline conditions, the adsorption process followed pseudo-second-order kinetics and the Freundlich isotherm model, yielding a high theoretical capacity of 98.0 mg/g. Remarkably, the microspheres maintained 91.2% of their initial adsorption efficiency after eight consecutive cycles, and continuous-flow tests showed a stable MB removal rate of 91.4% over five cycles. Mechanistic investigations confirmed that adsorption was governed by pore filling, electrostatic attraction, and hydrogen bonding, accompanied by a synergistic regeneration mechanism involving electrostatic reversal, competitive desorption, and structural preservation. This work provides a readily recyclable adsorbent along with fundamental insights for sustainable wastewater treatment.