Magnetically responsive multi-wall carbon nanotubes as recyclable demulsifier for oil removal from crude oil-in-water emulsion with different pH levels

A magnetically responsive multi-walled carbon nanotubes (M-MWCNTs) appropriate for acidic to alkaline conditions of crude O/W emulsion was designed and prepared via regulating the content of surface functional groups (such as -NH2, -OH and -COOH). The M-MWCNTs was confirmed as an efficient demulsifier and the corresponding demulsification efficiency achieved the highest when its dosage was increased to 400 mg/L. Significantly, the M-MWCNTs has superior demulsification performance under wide pH levels. What's more, the M-MWCNTs still has good demulsification efficiency after 5 cycles, suggesting it has high stability. The M-MWCNTs has strong interfacial activity, which can reach the oil-water interface and interact with asphaltenes. The synergistic π-π and/or n-π interaction of aromatics/functional groups of MWCNTs and asphaltenes as well as the electrostatic attraction between the M-MWCNTs and the asphaltenes promoted the oil separation from acidic crude O/W emulsion. In the neutral condition, the interfacial adsorption of π-π and/or n-π interactions was the main demulsification mechanism. Moreover, the electrostatic repulsion between the M-MWCNTs and crude O/W emulsion decreased due to the increase of amino groups under the alkaline condition. The protective film stabilizing the crude O/W emulsion was more facile broken, thusly promoting oil droplets separation from crude O/W emulsion.