Engineering long-life, high-performance coalescence media based on hierarchical carbon@ZnO porous spheres for oily wastewater treatment

Coalescence process offers an environment-friendly physical approach for oily wastewater treatment, harnessing micro- and nano-structured materials for efficient separation performance. Yet, the development of durable, scalable coalescing media remains a challenge. In this study, we present a hydrothermal–pyrolysis strategy to synthesize hierarchically porous, carbonized ZnO (C@ZnO) microspheres, optimized for efficient oil/water separation. The hierarchically porous ZnO microspheres serve as a scaffold, while the ultrathin, chemically inert carbon shell imparts favorable wettability and corrosion resistance. These C@ZnO microspheres exhibit a high surface area, engineered porosity, and ideal hydrophobic/oleophilic wettability - all of which contribute to enhanced stability and separation efficiency, even under challenging conditions involving moderate acidic/alkaline pH and high salinity. The oil/water separation efficiency remains above 98% after exposure to a wide pH range (4−10) and can reach up to 99% in highly saline environments. The material maintains 98% oil removal efficiency over multiple reuse cycles. This work presents a practical and scalable solution for continuous oil–water coalescence separation in industrial applications.