Energy-efficient ozone-enzymatic valorization of cardboard waste for functional bio-adsorbents

The increasing generation of dye-contaminated wastewater highlights the urgent need for cost-effective and sustainable treatment methods. This study aimed to evaluate whether corrugated cardboard waste can be upcycled into a functional low-energy bio-adsorbent through short ozone pretreatment, enzymatic hydrolysis, and mild acetic acid washing, while also generating fermentable sugars as a co-product. Structural, spectroscopic, and electrokinetic analyses showed that ozone pretreatment selectively removed lignin and mineral fillers, lowered crystallinity, and created larger amorphous, hydroxyl-rich regions, while subsequent hydrolysis exposed additional reactive surface sites. Although the ozonation-enzymatic treatment resulting adsorbent had one-tenth the BET-N 2 surface area compared to cardboard-based biochar, it removed considerably more methylene blue and Rhodamine B than either untreated cardboard or cardboard-based char. In contrast, adsorption of anionic dyes was significantly better for pyrolyzed cardboard-based char. Adsorption kinetics were generally described by a pseudo-second-order model (R 2 > 0.99), likely dominated by hydroxyl interactions. Notably, streaming ζ-potential showed a stronger relationship with adsorption capacity than colloidal ζ-potential. A cradle-to-gate life cycle assessment indicated that production of this adsorbent required 0.43 kWh/g, representing a 72% reduction in energy consumption compared to 350 °C pyrolysis (or 0.06 kWh/g when excluding drying), while yielding ∼1.8 g sugars per g of adsorbent, offering an additional valorization pathway to offset greenhouse gas emissions. These findings demonstrate that targeted chemical and enzymatic processing, rather than maximizing surface area, is key to optimizing cationic dye adsorption, positioning upcycled cardboard as a viable circular-economy solution for textile wastewater treatment.