Microstructured calcium alginate (Ca-Alg) hydrogel exhibiting superhydrophilicity and underwater superoleophobicity is prepared for high speed and highly efficient oil/water separation. The fabricated mesh works in highly acidic or basic, salty, and high temperature environments because of the stability of Ca-Alg. Moreover, nonwoven fabric used as a template for Ca-Alg is capable of separation of an oil-in-water emulsion.