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

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Switzerland : 1996), 2023-02, Vol.454, p.140304, Article 140304
• Main Authors: Rao, N.R.H., Chu, X., Hadinoto, K., Angelina, Zhou, R., Zhang, T., Soltani, B., Bailey, C.G, Trujillo, F.J., Leslie, G.L., Prescott, S.W., Cullen, P.J., Henderson, R.K.
• Format: Article
• Language: English
• Subjects: Cyanobacteria; Reactive oxygen and nitrogen species; Residual oxidation; Scavengers; Water treatment
• ISSN: 1385-8947; 1873-3212
• DOI: 10.1016/j.cej.2022.140304

[Display omitted] •O2- and air-CPABs had similar cell inactivation and damage in immediate and long-term.•O2-CPABs discharged fewer nitrites and nitrates into the effluent than air-CPABs.•O2− and OH drove algal cell damage for both CPABs when discharged in MilliQ.•O2−, H2O2, O3, OH, NO equally influenced O2-CPAB treatment when discharged in PBS.•CPABs were highly efficient in utilising the discharged power for treating algae. 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, O2− and OH radicals had the greatest impact on algal cell removal in MilliQ (0–28 %); O3 and 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.