Abstract
Cobalt, a toxic heavy metal commonly found in water and wastewater, presents serious environmental and public health concerns, while simultaneously representing a critical resource for recycling. This work investigated the performance of a novel and environmentally friendly continuous liquid phase plasma discharge (CLPD) process for the removal and recovery of cobalt from aqueous media. The CLPD reactor is designed with two conductive channels and maintains stable electric discharge at the orifice of two dielectric plates positioned between opposing electrodes. The effects of key operating parameters, including liquid flow rate (25-100 mL/min) and applied power (200-300 W), were evaluated. Optimal cobalt removal efficiency (93%) was achieved within 25 minutes at a 50 ml/min liquid flow rate and an applied power of 300 W, without the use of external discharge gases. Subsequently, different discharge gases—air, argon, helium, and hydrogen—were introduced to enhance cobalt removal and energy efficiency. After 30 minutes of treatment at 200 W and a gas flow of 0.2 L/min, cobalt removal efficiencies followed the trend: hydrogen (92%) > helium (90%) > argon (89%) > air (74%). Among these, helium achieved the highest energy efficiency (0.393 g/kWh) within the first 20 minutes, with argon showing comparable performance. Both helium and argon also exhibited the most rapid cobalt removal kinetics. Notably, the introduction of discharge gases enhanced energy efficiency by 34.5% relative to gas-free CLPD operation. Reaction kinetics studies with scavenger tests revealed that oxidative reactive species primarily hydroxyl radical (•OH) and superoxide radicals (O₂•⁻) generated in the discharge zone were primarily responsible for cobalt removal through the formation of cobalt oxide particle.
The influence of co-existing heavy metals (zinc, copper and lead), which are commonly found alongside cobalt in wastewater, was also investigated. The presence of these metals adversely affected cobalt removal efficiency, with the most pronounced reduction (91%) observed in multi-metal systems. At equimolar (1:1) concentrations relative to cobalt, copper exhibited the greatest inhibitory effect, reducing cobalt removal efficiency by 85%, followed by zinc (53%) and lead (52%) after 30 minutes of treatment, compared to cobalt-only systems.
Beyond cobalt removal, the CLPD process also enabled the synthesis of cobalt oxides particles. In mixed-metal solutions, the recovered particles consisted of cobalt-metal oxide composites with average particle size ranging from 2.5 to 3 µm, indicating potential for applications in various technological fields. Overall, these findings demonstrate that the CLPD is a robust, energy-efficient, and highly effective method for both cobalt removal and recovery, contributing to sustainable wastewater treatment and resource reclamation.