Stability And Mechanism Of Different Gas Nanobubbles In Deionized Water And Salt Aqueous Solutions

Nanobubbles have many peculiar properties,such as small volume,high internal pressure,large specific surface area,high interface zeta potential,free radical initiation,strong oxidation,mass transfer enhancement and no secondary pollution.It is widely concerned in the fields of environmental treatment,cleaning,agriculture,aquatic products,water oxygenation,medical treatment,food processing,daily life and so on.However,these applications are in a relatively complex environment,and the properties of nanobubbles in different environments and the time-dependent change rule are not clear,which are bound to their further application in related fields.In this paper,the stability of air,N₂,O₂,Ar+8%H₂（Mixture of 92%Ar and 8%H₂）and CO₂ nanobubbles in deionized water and electrolyte salt solutions of different valence states were studied.The properties of nanobubbles in deionized water and electrolyte solutions with different valence states were studied by measuring the average diameter,zeta potential of nanobubbles,p H and Eh values of bubble suspensions.To further explain the stability of nanobubbles in different aqueous solutions,the properties of nanobubbles in deionized water and electrolyte solutions with different valence states were calculated and discussed by using the extended DLVO theory.The main conclusions are as follows:（1）The surfaces of air,N₂,O₂ and Ar+8%H₂ nanobubbles in deionized water are negatively charged,and the absolute values of zeta potential are greater than 15 m V.Air,N₂,O₂ and Ar+8%H₂ nanobubbles can be stable in deionized water for more than 60 days,showing long-term stability:the average bubble size gradually increases in the initial 30 days,and slowly decreases in30-60 days;the solubility of CO₂ nanobubbles in water is high,the generated H⁺are adsorbed on the surface of the bubbles,so that the surface of the bubbles is positively charged,and the zeta potential is less than+10 m V.CO₂ gas bubbles are unstable in deionized water,and can only exist for 5 days.Ostwald ripening effect intensifies and the bubble particle size increases,so that they float up and disappear in water after 5 days.（2）The cations in salt solutions neutralize the negative zeta potentials of air,N₂,O₂ and Ar+8%H₂ gas nanobubbles,resulting in the compression of the electric double layer.The Debye thickness of air,N₂,O₂ and Ar+8%H₂ gas nanobubbles decreases from 300 nm in deionized water to a few nanometers in electrolyte solutions,and the stabilization time decreases significantly which reduce to 1-2 weeks;The positive zeta potentials of CO₂ bubbles increase with the presence of cations in the salt solutions.The existence time of CO₂ bubbles is prolonged and the stable time increases from 5 days in deionized water to 2weeks in salt solutions.（3）On the one hand,the extended DLVO theory shows that the total potential energy of air,N₂,O₂ and Ar+8%H₂ gas nanobubbles in deionized water is greater than 20 k T on the first day,which is greater than 15 k T,indicating that gas bubbles can exist stably in deionized water;On the other hand,there are no energy barriers for air,N₂,O₂,Ar+8%H₂ and CO₂ gas bubbles in 1 mmol/L salt solutions of three valence states,indicating that different gas bubbles cannot exist stably in electrolyte solutions.