Mechanochemical Upcycling of Spent LIB Graphite into Photocatalytic Adsorbents for Wastewater Treatment

Spent graphite from retired lithium-ion batteries (LIBs) is a neglected resource with pressing environmental challenges—pyrometallurgical recycling for valuable metal recovery will emit over 3.2 million tons of CO₂ annually by 2030, while existing upcycling methods like the Hummers process consume massive harsh chemicals (30 kg H₂SO₄, 3 kg KMnO₄ per kg graphite) and suffer poor economic feasibility. Additionally, graphene-based adsorbents derived from spent graphite face critical desorption issues: traditional solvent regeneration (e.g., ethanol, acids) damages material structure, reducing adsorption capacity to 29.34% after five cycles. Thus, an environmentally friendly, low-cost, and recyclable upcycling strategy for spent graphite is urgently needed.

Now, a team at Shanghai Jiao Tong University has developed a scalable mechanochemical approach combined with electrostatic self-assembly to convert spent LIB graphite into a high-performance photocatalytic adsorbent, named OMG17@TiO₂. This composite achieves ultrahigh adsorption capacities of 673.67 mg/g for methylene blue (MB) and 966.58 mg/g for rhodamine B (RhB), significantly outperforming most reported graphene-based materials. Notably, it realizes 70% desorption efficiency via in-situ photocatalysis—51% higher than the simple mixture of OMG17 and TiO₂—and retains over 60% desorption efficiency after five consecutive adsorption-desorption cycles.

"Upcycling spent graphite not only mitigates the environmental burden of battery waste but also addresses the demand for low-cost, sustainable adsorbents in wastewater treatment," said the lead researcher. "Innovative mechanochemical conversion avoids harsh chemicals, while the integration of TiO₂ enables solvent-free regeneration. Our findings provide a closed-loop pathway for carbon resource recycling and pollutant remediation, bridging waste management and environmental protection."

Multi-dimensional Structure for Pollutant Adsorption

OMG17 is fabricated via high-energy ball milling of spent graphite, forming graphene-like porous structures with interconnected nanospheres and 1–2 nm surface cracks (induced by ball milling-induced internal stress). Its specific surface area reaches 665.96 m²/g—far higher than raw spent graphite’s 8.664 m²/g—providing abundant active sites for adsorption. Through electrostatic self-assembly, 20 nm TiO₂ nanoparticles are uniformly dispersed on the OMG17 matrix, with strong interfacial interactions confirmed by overlapping carbon and titanium elemental signals.

Ball milling also generates oxygen-containing functional groups (hydroxyl and carboxyl) on OMG17’s surface, enhancing affinity for organic pollutants via hydrogen bonding and electrostatic interactions. The material’s porous structure (2–14 nm pores) is highly matched with the molecular size of MB (3–5 nm), facilitating efficient pore-filling adsorption. Raman and XRD results verify the formation of few-layer graphene structures in OMG17, while TEM observations confirm retained high graphitization, ensuring structural stability.

Efficient Adsorption and Reversible Regeneration

The OMG17@TiO₂ composite follows the Langmuir monolayer adsorption model and pseudo-second-order kinetic law, indicating dominant chemisorption. For cationic dyes, sulfonating agent SDBS modification further improves adsorption capacity by introducing sulfonyl groups. Unlike traditional solvent regeneration (which causes irreversible pore blockage), in-situ photocatalytic regeneration triggers TiO₂ to generate reactive free radicals (·OH, O₂⁻) under UV irradiation, degrading adsorbed pollutants into small molecules.

This solvent-free process effectively preserves the material’s pore structure: BET analysis shows significant recovery of mesopore volume (2–13 nm) after regeneration. The composite maintains 70% adsorption capacity retention after the first regeneration and over 60% after five cycles, far outperforming ethanol-regenerated OMG17 (29.34% retention). Density functional theory (DFT) calculations confirm that hydroxyl and carboxyl groups on OMG17 reduce MB adsorption energy to -2.27 eV, enhancing adsorption stability.

Scalable Application for Wastewater Treatment

Based on OMG17@TiO₂’s synergistic adsorption-photocatalysis performance, the team designed a recyclable membrane filtration system for industrial dye wastewater treatment. Driven by atmospheric pressure, the system achieves 81.3% removal of 20 mg/L MB solution, retaining over 71.8% efficiency after five cycles—outperforming TiO₂-free membranes (47.2% retention) and blank membranes (11.1%). The membrane also exhibits ultrahigh water flux and stable operation under low or no pressure, suitable for large-scale wastewater treatment.

"The upcycling of spent graphite requires integrating resource recovery, cost-effectiveness, and environmental sustainability," the lead researcher noted. "Traditional methods fail to balance these factors—pyrometallurgy is polluting, chemical oxidation is costly, and solvent regeneration is unsustainable. Our mechanochemical-self-assembly strategy avoids these drawbacks, offering a scalable solution for battery waste management and pollutant control. This work paves the way for ‘waste-to-high-value-material’ closed-loop systems."