Eco-friendliness and low cost are critical when investigating new oil–water separation agents with high separation efficiencies for the treatment of emulsified oily wastewater in industrial applications, including crude oil exploitation. Treatment methods specifically suited to wastewater containing ultra-small oil droplets are lacking and urgently required. This study investigated the one-pot synthesis of humic acid and polydimethyldiallylammonium chloride coated Fe₃O₄ nanoparticles. A low dosage of the nanoparticles (375 μg mL⁻¹) exhibited excellent separation efficiency (nearly 100%) and reusability when applied to hexadecane–water emulsions containing ultra-small droplets (200–300 nm). Electrostatic interactions and the strong interfacial activity of the nanoparticles played essential roles in achieving oil–water separation. This study provided an efficient extraction material synthesized by a facile and cheap method for separating ultra-small oil droplets from emulsions.