Impact of discharge gas on a novel continuous flow in-liquid plasma discharge process for cobalt remediation and recovery from wastewater

Cobalt contamination in wastewater poses significant health and environmental risks, necessitating efficient removal and recovery strategies. This study examines the role of various discharge gases-air, argon, hydrogen, and helium-in optimizing cobalt removal/recovery and energy efficiency using a novel continuous flow in-liquid plasma discharge (CFILPD) process. The effect of gas flow rate and applied power were assessed with air and argon to determine optimal conditions for maximizing cobalt removal. In air discharge, increased gas flow rate reduced cobalt removal due to the formation of acidic reactive species, whereas argon flow rate had no significant impact. Under the optimal operating condition at 200 W and 0.2 L/min gas flow, cobalt removal after 30-min treatment followed the trend: hydrogen (92 %) > helium (90 %) > argon (89 %) > air (74 %). Introduction of discharge gases enhanced energy efficiency of the CFILPD process by 34.5 % compared to gas-free operation. Helium gas yielded the highest energy efficiency (0.393 g/kWh) during the first 20 min, with similar result observed for argon, and both showed fastest kinetics in cobalt removal. The process yielded cobalt oxide particles as the sole byproduct, offering economic value for industrial applications. Morphological analysis revealed that particles recovered with argon exhibited smoother surfaces, while those obtained using air, helium, and hydrogen displayed porous structures. These findings underscore the potential of the CFILPD process as an effective and energy-efficient method for cobalt removal and recovery from wastewater, with argon emerging as the most cost-effective option.