A Low‐Cost Multifunctional <scp>PDMS</scp> Sponge Based on Boron Compounds and Waste‐Derived Candle Soot for Flame‐Retardancy, Oil–Water Separation, and Self‐Cleaning

ABSTRACT Traditional sorption materials suffer from several significant drawbacks, including low sorption capacity, poor effectiveness in removing heavy oils, limited efficiency in oil–water separation, and inadequate flame‐retardant properties during oil spill incidents. In this study, an environmentally benign and multifunctional composite sponge with enhanced flame‐retardant properties was developed by modifying a polydimethylsiloxane (PDMS) sponge fabricated via a low‐cost and simple sugar molding method. Boric acid (BA) and borax (BX), both halogen‐free and low‐toxicity boron‐based compounds, were incorporated at different ratios to improve the thermal stability of the sponge, with a 1:1 BA/BX composition showing the most favorable performance. Subsequently, a dispersion containing candle soot (CS), a waste‐derived carbonaceous material, was impregnated into the sponge to increase surface roughness and hydrophobicity. The resulting PDMS@BA 1 /BX 1 @CS sponge demonstrated improved self‐cleaning behavior against common contaminants, with a static contact angle of approximately ~139°, along with high oil–water separation efficiency. The modified sponges showed sorption capacities ranging from 6.5 to 11.3 g/g for various oils and organic solvents. Moreover, the PDMS@BA1/BX1@CS sponge extinguished the ignited flame within 12 s after adsorbing the flammable solvent. These results reveal that the developed sponge has great potential for applications such as oil–water separation, rapid fire extinguishing, and self‐cleaning.