Rare Earth Ion-Induced Wettability Transition of Melamine Sponge for Durable Oil/Water Separation

The development of 3D hydrophobic porous adsorbents for oil/water separation that are inexpensive, easy to fabricate, highly efficient, and physically and chemically durable continues to be a major research focus and a critical issue to be addressed. In this study, a facile and universal wettability transition strategy of amphiphilic melamine sponge (MS) was developed via simple dip-coating MS in rare earth ion solutions, taking Ce ion as a representative example. The formation of abundant coordination bonds between Ce ions and nitrogen atoms on a MS skeleton effectively decreased the sponge's surface energy and tailored its microtopography and roughness, which collectively endowed the Ce ion-modified sponge (Ce-MS-10) with excellent hydrophobicity, as evidenced by a water contact angle of 143.1°. Owing to its ultrahigh porosity (>99%) and excellent oil/water selectivity, the Ce-MS-10 sponge exhibited a maximum oil adsorption capacity of up to 171.7 g/g. Meanwhile, as a result of its well-preserved mechanical elasticity from the MS sponge, Ce-MS-10 facilitated efficient oil/water separation through simple physical manipulation and can be reused repeatedly, as evidenced by its stable adsorption performance and excellent recyclability. The high stability of the coordination bonds formed between Ce ions and N atoms in the MS framework also enabled the Ce-MS-10 sponge to retain robust hydrophobic stability under various harsh physical (friction, adhesion, and oscillation) and chemical (strong acid or base, salty condition, and chemical solvents) surroundings, further validating its potential for practical application in real-world water remediation. In addition, the broad applicability of this facile Ce ion-based hydrophobization strategy for MS was further demonstrated with other commonly used rare earth element ions (La³⁺, Pr³⁺, Nd³⁺, and Y³⁺). The present study facilitates an ultrafacile strategy to fabricate MS-derived hydrophobic oil adsorption sponge to realize efficient and durable oil/water separation.