Algal cell inactivation and damage via cold plasma-activated bubbles: Mechanistic insights and process benefits

Cold plasma-based oxidation, wherein several reactive species are produced by combining a gas with electrical discharges, can treat algal-impacted waters. A novel approach to discharge cold plasma via bubbles, known as cold plasma-activated bubbles (CPABs), has gained attention due to increased interfacial area and residence time of bubbles, thereby effectively transporting reactive species. In this study, the efficacy of air-CPABs and oxygen-CPABs in treating Chlorella vulgaris-laden MilliQ and phosphate buffered saline (PBS) in the immediate and long-term (168 h) was evaluated via flow cytometry. Air-CPABs and O₂-CPABs were equally effective in the immediate and long-term for reducing cell numbers (65–100 % MilliQ; 0–45 % PBS) and increasing cell damage and inactivation (100 % in MilliQ and PBS in 3 h and 168 h, respectively). However, O₂-CPABs are preferable due to negligible nitrite and nitrate concentrations in the treated water compared to air-CPABs where nitrite and nitrate concentrations were 36–122 mg·L⁻¹ and 42–298 mg·L⁻¹, respectively, thereby, exceeding regulatory guideline limits. After air-CPAB treatment, the ratio of “intact but inactive” cells to “damaged but active” cells in PBS increased from 0.8 to 1 to 1.2–33.6, suggesting that healthy cells were inactivated prior to damage. This ratio declined from 3.6 to 16 to ∼ 1 in O₂-CPABs, caused by increasing numbers of “damaged but active” cells. For both, [rad]O₂⁻ and [rad]OH radicals had the greatest impact on algal cell removal in MilliQ (0–28 %); O₃ and [rad]O₂⁻ caused the most damage and inactivation (36–41 %) when using O₂-CPABs in PBS. CPAB energy yields (3.95–8.82 × 10¹⁰ cells·kWh⁻¹) were comparable to- or ∼ 3–7 × greater than those achieved via non-bubble plasma discharges at relatively lower discharge power (2.2–4.3 kW), indicating that CPABs efficiently utilised the discharged power for algal treatment and promoted sustainability.