CO$_2$-responsive photothermal sponge for efficient viscous oils removal and bidirectional oil/water emulsions separation

CO$_2$-responsive materials that use CO$_2$as a benign external trigger to regulate surface wettability have emerged as promising candidates for oily wastewater treatment. However, the efficient treatment of high-viscosity oils and bidirectional stable oil-in-water emulsions using CO$_2$-responsive oil/water separation materials remains a significant challenge. Herein, a multifunctional CO$_2$-responsive photothermal sponge (CCF@PDA-PMS) by integrating a CO$_2$-switchable polymer and low-cost biomass-derived carbonized cotton fibers (CCF) into a polydopamine (PDA) modified melamine sponge (MS) was successfully fabricated. This rational design combined reversible wettability switching with efficient solar-driven self-heating in a single three-dimensional porous framework, enabling adaptive treatment of complex oily wastewater. As expected, the resulting sponge exhibited rapid CO$_2$-triggered wettability switching from superhydrophobic (water contact angle of 151.7°) to superhydrophilic (0°). Meanwhile, the sponge demonstrated high adsorption capacities toward a wide range of oils, absorbing 20 to 45 times its own weight. Furthermore, under 1.0 sun irradiation, the CCF@PDA-PMS surface reached a temperature of 98.0 °C within 25 s owing to efficient photothermal conversion, which effectively reduced the viscosity of crude oil, enabling the CCF@PDA-PMS to rapidly absorb crude oil with a capacity of up to approximately 30gg$^{−1}$. ... mehrMore importantly, the integrated CO$_2$-responsive wettability allowed gravity-driven bidirectional separation of both water-in-oil (W/O) and oil-in-water (O/W) emulsions with separation efficiencies exceeding 98.2% while maintaining stable permeation fluxes. Additionally, the prepared sponge exhibited outstanding stability and recyclability. Overall, these multifunctional characteristics highlight the potential application prospects of the CO$_2$-responsive sponge in controllable oil-water separation and the remediation of viscous oil spills.