Cold plasma activation and regeneration of walnut shells for enhanced pollutant adsorption in water

Walnut shells (WS) were activated for methylene blue (MB) adsorption using a plane-to-plane dielectric barrier discharge (DBD) reactor powered by high-voltage (HV) nanosecond pulses. A plasma bubble reactor, also driven by HV nanopulses, was used for their regeneration following adsorption saturation. The activated WS were characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Brunauer-Emmett-Teller analysis to evaluate structural and surface modifications. Activation was carried out using three different gases to assess their effect on surface material properties and adsorption performance, with the process exhibiting high energy efficiency (900 g-_WS/kWh). Adsorption experiments investigated parameters such as adsorbent dosage, adsorption time and initial MB concentration. Compared to the raw WS (43.7 %), air- and oxygen-activated WS achieved the highest MB removal efficiencies (98.1 % and 95.3 %, respectively), whereas argon activation resulted in lower efficiency (78.8 %), indicating a more limited surface modification effect. The maximum MB adsorption capacity increased significantly from 89.2 mg/g-_WS for raw WS to 175.2 mg/g-_WS after plasma activation. Isotherm modeling showed that MB adsorption followed the Freundlich model, while kinetic analysis favored the pseudo-second-order model. Beyond initial activation, the plasma bubble reactor was successfully applied to regenerate saturated WS over multiple adsorption cycles. The regeneration process was energy-efficient (100 g-_WS/kWh) and restored or even improved adsorption performance. Overall, this work demonstrates that cold plasma is a sustainable, energy-efficient, and effective method for both the activation and regeneration of biosorbents, offering significant potential for water purification applications.