Magnetic amphiprotic catalysts ( MACs) were synthesized by coating magnetic nanoparticles (MNPs) stepwise with polydopamine (Pdop), a polyethyleneimine (PEI) brush, octadecylamine (ODA), and Au. The amphiprotic characteristics of the MACs were tuned by varying the portions of the hydrophilic and hydrophobic moieties. The MACs normally dispersed in the hydrophobic layer but were forced into water to catalytically decompose aqueous pollutants upon the application of a magnetic field. When the magnetic field was turned off, the MACs naturally returned to the hydrophobic layer and catalytically decomposed non-aqueous pollutants. Next, the purified oil was readily separated from the oil/water mixture by using the magnetic field because the MACs absorbed oil. Oil-in-water and water-in-oil wastewater emulsions containing aqueous and non-aqueous pollutants, respectively, were also purified and separated into pure oil and water. A series of wastewater purification steps, including oil/water separation and the decomposition of aqueous and non-aqueous pollutants in both phases, was performed by in situ and continuous processes using the MACs. The MACs also performed catalytic organic reactions in organic solvents.