Wettability-switchable deep bed filtration for efficient oily wastewater treatment

Deep-bed filtration for oily wastewater purification has long been constrained by the engineering "Attachment-Detachment Paradox". Traditional passive hydraulic backwashing cannot overcome the severe interfacial energy barrier inherently associated with static, high-surface-energy media. To address this, we developed a temperature-swing deep-bed filtration process driven by dynamic interfacial energy regulation. This process utilizes a thermo-responsive filter bed operating in an oleophilic state during cold-water filtration (20°C) to actively capture oil droplets, and rapidly transitions to an oleophobic state during warm-water backwashing (40°C) to trigger spontaneous oil detachment. Results demonstrated a filtration efficiency of > 99% during the 20°C stage, and highly efficient spontaneous detachment (backwash efficiency >98%) simply by switching to the 40°C backwash. The underlying interfacial response mechanism was elucidated using extended DLVO (XDLVO) theory, atomic force microscopy (AFM), and molecular dynamics (MD) simulations. Temperature induced a conformational transition of the grafted polymer chains from a collapsed to an extended state. At low temperatures, the exposed hydrophobic backbone created a deep potential well and strong attachment (∼2-4 nN) to firmly anchor oil droplets. For the non-polar diesel oil phase, this strong attachment was mainly associated with van der Waals attraction, hydrophobic affinity, and enhanced short-range attractive interactions caused by the weakened hydration layer. As temperature rose above the critical threshold, the chains extended to reconstruct a dense, highly ordered interfacial hydration layer. This hydration-layer reconstruction weakened the short-range attractive interactions and switched the microscopic attachment force to a near-zero state (∼0-1 nN). The resulting hydration repulsion and steric hindrance fundamentally dismantled the high energy barrier for detachment. This research provides a low-energy, sustainable technology for oily wastewater treatment and establishes a general methodology for utilizing external stimuli to overcome interfacial attachment hysteresis.