Highly efficient treatment of oily wastewater using magnetic carbon nanotubes/layered double hydroxides composites

Water contaminated by emulsified oily wastewater seriously threatens the security of water resources globally. Herein, a three-dimensional composite, containing carboxylated carbon nanotubes (CNTs), magnetic particles (Fe3O4), and layered double hydroxides (LDHs), was prepared and utilized to separate emulsified oil from crude oil, paraffin oil, and hexadecane oil-in-water (O/W) emulsions. The morphology and physicochemical properties of the synthesized MCNTs/LDHs composites were studied by TEM, SEM, FT-IR, XRD, XPS, VSM, and zeta-potential analyzer. The effect of mass ratio of LDHs:MCNTs, temperature, and ionic strength on the removal of emulsified oil was also investigated. Due to the synergy of each component, the MCNTs/LDHs composites exhibited superior oil/water separation performance. It was found that the oil/water separation efficiencies of higher than 99% could be achieved at an optimal condition. Moreover, maximum oil/water separation efficiency for crude oil O/W emulsions was attained at a composite with a mass ratio of LDHs:MCNTs = 1:1, and maximum oil/water separation efficiency for paraffin oil and hexadecane O/W emulsions were attained at a composite with a mass ratio of LDHs:MCNTs = 1:5. The adsorption of emulsified oil onto MCNTs/LDHs composites constituted an endothermic reaction. The emulsified oil adsorption capacities were found to increase as ionic strength increased. According to the results of dynamic interfacial tension and characterization of optical microscope, the oil/water separation process involved interfacial adsorption, demulsification of stable emulsions, and magnetic separation. In addition, the MCNTs/LDHs composites still possessed prominent oil/water separation performance after five times of reuse, suggesting their wide application prospects in the treatment of oily wastewater.