Simultaneous phosphorus recovery as vivianite crystallization and hydrogen generation from acidified oil wastewater by Fe-C micro-electrolysis

Phosphorus recovery from industrial wastewater is an important route for alleviating phosphorus resource depletion and reducing eutrophication risk. Acidified oil wastewater generated during vegetable oil refining is characterized by strong acidity, high organic load, and high phosphorus content, making conventional treatment both costly and chemically intensive. Herein, an Fe-C micro-electrolysis process was developed to achieve phosphorus recovery from acidified oil wastewater through in situ vivianite crystallization, while simultaneously generating hydrogen as a value-added by-product. Commercial Fe-C filler was directly introduced into the strongly acidic wastewater, and the electrochemical reactions promoted the release of Fe²⁺, the consumption of protons, and the gradual increase in pH value, thereby creating favorable conditions for vivianite formation without the addition of exogenous iron salts or alkaline reagents. The effects of Fe-C filler particle size on phosphorus recovery efficiency, H₂ generation, vivianite crystallization rate, as well as the removal efficiencies of chemical oxygen demand (COD) and NH₄ ⁺-N were systematically investigated. Under the optimal condition using 3–6 mm Fe-C filler, the pH increased to 5.6 after 96 h of reaction, the phosphorus recovery efficiency reached 84.7%, and the vivianite crystallization rate reached 82.3%. Concurrently, per liter of wastewater treated, the process yielded 291 mL of H₂ and approximately 100 g of vivianite, while simultaneously removing 48.2% of COD and 23.2% of NH₄ ⁺-N. Compared with traditional chemical crystallization, this process realizes synergistic phosphorus recovery, hydrogen generation and pollutant degradation in a single system, providing a promising strategy for sustainable treatment and resource recovery from acidic phosphorus-rich industrial wastewater.