Coal fly ash (CFA) as an industrial solid waste has an extremely adverse influence on the environment. The high value utilization of CFA was a hot research topic with sustainable development for years. In this work, we found that CFA exhibits underoil superhydrophobicity and underwater superoleophobicity due to the special microstructures and chemical composition. And for the first time, CFA was used as a filter material for the separation of various oil/water mixtures and emulsions under the driven of gravity without any further chemical modification. Both light oil/water and heavy oil/water immiscible mixtures can be completely separated through CFA with almost 99.9% efficiency. More importantly, due to the “size-sieving” effect, CFA was capable of purifying various emulsions with high separation efficiency. After being pre-wetted with water, CFA displayed excellent separation capacity more than 92% and good flux as fast as 1050 L m⁻² h⁻¹ for water-in-oil emulsions. After being pre-wetted with oil, CFA exhibited a higher separation efficiency of 99.5% and a flux of over 935 L m⁻² h⁻¹ for water-in-oil emulsions. Therefore, this work provided an innovative, green, and low-cost approach for various oil/water mixtures and emulsions separation, which can also be widely used in wastewater treatment.

粉煤灰（CFA）作为工业固体废物对环境具有极其不利的影响。CFA的高价值利用是多年来可持续发展的热门研究课题。在这项工作中，我们发现由于特殊的微观结构和化学成分，CFA表现出底油超疏水性和水下超疏油性。首次将CFA用作过滤材料，以在重力的驱动下分离各种油/水混合物和乳液，而无需进行任何进一步的化学修饰。轻油/水和重油/水不混溶的混合物都可以通过CFA完全分离，效率几乎达到99.9％。更重要的是，由于具有“筛分”效果，CFA能够以高分离效率纯化各种乳液。用水预湿后，^-2  h ^-1（油包水型乳剂）。在用油预润湿后，CFA对油包水型乳剂的分离效率高达99.5％，通量超过935 L m ^-2  h ^-1。因此，这项工作为各种油/水混合物和乳浊液的分离提供了一种创新，绿色，低成本的方法，该方法也可广泛用于废水处理。