Pre-pilot scale degradation of cationic and anionic dyes by ozone nanobubbles

Recently, nanobubble technology is increasingly applied in wastewater treatment. However, the production of ozone nanobubbles (O₃/NBs) and their applications for degradation of toxic and recalcitrant organic pollutants in water with large scale are quite limited and remain to be enlightened. Here, we report the production of O₃/NBs from a nanobubble generator combined with an ozone generator, and their application for removing Direct Blue 71 (DB-71) and Methylene Blue (MB) dyes in a 60 L tank. Different characteristics of the generated O₃/NBs were measured while different parameters impacting the degradation of these cationic and anionic dyes were systematically examined. Kinetics of the degradation processes and mineralization levels for each pollutant were also clarified. The generated O₃/NBs presented extremely high DO (45 ± 2.0 mg/L) and ORP (1160 ± 54 mV), which were stable for at least 20 min. O₃/NBs exhibited a unimodal particle distribution with an average diameter of about 140 nm, and a number concentration of 8.3 × 10⁷ particles/mL. Under optimal conditions, 100% of DB-71 (30 mg/L) were removed and mineraized within 16 min while 100% of MB (10 mg/L) were removed and over 58% of MB (30 mg/L) were mineralized to form non-toxic products. O₃/NBs based oxidation process is very effective for organic dye degradation with high loading, occurring much faster than other advanced oxidation processes. The current work provides an effective and promising approach for generating the O₃/NBs presenting high efficiency, easy operation, and high potential of scalable applications in wastewater treatment.