Nanobubbles of Oxygen, Air, and Ozone Gas for the Degradation of Reactive and Cationic Dyes from Wastewater

The presence of dyes in industrial effluents causes significant environmental harm. Traditional wastewater treatment technologies are insufficient to remove dyes rapidly. This study examined the degradation efficiency of dyes (Methylene Blue (MB) and Remazol Brilliant Blue R (RBBR)) by using nanobubbles (NBs) of ozone (O₃), oxygen (O₂), and air. For their generation, hydrodynamic cavitation was selected. The impacts of the flow rate, pH, reaction kinetics, and initial pollutant concentration were investigated. As expected, the flow rate affected NB size and concentration, impacting pollutant removal efficiency. ζ potential showed that O₃@NBs achieved the highest absolute value of 27.8 mV at pH 7.5, exhibiting the best stability and performance. Experimental results show that the implantation of O₃@NBs rapidly removes 100% of MB and RBBR within 15 min, independent of pollutant concentration or pH. O₂ and air NBs had lower removal efficiencies, indicating the higher oxidative potential of O₃@NBs. In addition, the soluble O₃@NBs managed to degrade 40 and 65% of the total organic content for MB and RBBR, respectively. Kinetics analysis showed that all NBs follow a first-order kinetic model. The stability of produced NBs was explored over the span of 1 year, revealing O₂@NBs as the most stable. Exploring the application in real textile wastewater showed that O₃@NBs can effectively be employed to obtain clear water, since it removed >70% of both the dye and total dissolved solids present in the solution. Also, scavenger studies revealed that hydroxyl radicals are highly responsible for the degradation of both MB and RBBR. Overall, this work provides a mechanistic understanding of the reactivity of O₃@NBs, O₂@NBs, and Air@NBs and sheds light on the importance of nanobubble features and reaction parameters in optimizing advanced oxidation processes for wastewater treatment applications.