Robust, all-state superhydrophobic-superoleophilic coating for ultrafast selective dye adsorption and oily wastewater purification

Superwettable surfaces have high application potential in liquid separation and dye adsorption, but considerable challenges remain in separately and quickly removing different contaminants in industrial wastewaters such as effluents from textile dying and wool scouring processes. Herein, a robust, all-state superhydrophobic-superoleophilic (SHP-SOI) composite coating was fabricated, using cobalt 2-methylimidazole nanoparticles (ZIF-67 NPs), polydimethylsiloxane (PDMS) and octadecylamine (ODA), and applied to various substrates via a facile two-step dip-coating method. The ZIF-67/PDMS-ODA coated fabrics turned superhydrophobic with a water contact angle (WCA) of over 150°, and oils could easily spread into the fabric with an oil contact angle (OCA) of 0° either in air or water. Owing to the synergistic effect of the coating components, the coating showed ultrafast and repeatable adsorption to anionic dyes (e.g. methyl orange, MO) with the maximum equilibrium adsorption capacity of 205.7 mg/g (MO concentration 100 mg/L, pH = 3). More importantly, 97.9 % of MO was adsorbed in only 5 min and the adsorption reached equilibrium in 30 min. In addition, owing to the coating's strong affinity toward different oils, the treated fabrics could efficiently separate/adsorb oils from oil contaminated dye solutions. This all-state SHP-SOI coating was robust and could withstand various severe treatments without losing its original wettability and dye/oil adsorption/separation capability; it also exhibited heat induced self-healable property.