Al3+-Modified Melamine Sponge for Downhole Oil–Water Separation

Abstract Increasing water production during the later stages of oilfield development generates large volumes of produced water with low oil content, creating challenges for efficient treatment. Downhole oil–water separation can reduce the treatment load of surface facilities but requires materials with high oil selectivity and stable performance. Herein, a hydrophobically modified melamine sponge was prepared through a simple AlCl3 immersion method. SEM-EDS, FTIR, and XPS confirmed the introduction of Al3+ onto the sponge surface and supported its interaction with electron-donating sites in the melamine framework, while the original porous structure was preserved. The modified sponge exhibited water contact angles above 130° and adsorption capacities of approximately 100 g/g for crude oil and 89 g/g for paraffin oil. TGA indicated a modest improvement in thermal stability. Oil–water separation was further evaluated through adsorption, in situ extraction, and filtration. The adsorption mode achieved nearly 100% separation efficiency, while in situ extraction reached 97.5% efficiency at a rate of 6.25 mL/s. Filtration further demonstrated the potential for polishing low-oil-content wastewater. These results highlight Al3+-modified melamine sponge as a simple and versatile material for staged oil–water separation in oilfield applications.