Isolating elemental phosphorus from sewage sludge ash by electrochemistry

Natural phosphorus resources are depleting. Phosphorus recovery from waste has become a strategic issue to address the potential threat of phosphorus shortage. Technologies to recycle phosphorus as fertilizers remain in limited applications due to cost and quality reasons. Here, we demonstrate sewage sludge ash as an appropriate resource to prepare elemental phosphorus. By leaching in molten CaCl2, PO43- in the ash can be set free and electrochemically split into elemental phosphorus and O2−. We identify two technical advantages of using sewage sludge ash. Metal cations released from the ash boost the leaching rate of PO43−, while SiO2 in the ash prevents the accumulation of O2- in the melt. Electrochemical isolation can keep most impurities in phosphorus within 1.00 ppm, far better than the industrialgrade commercial product. The estimated cost is less than 2 USD (kg-P)−1. This facile technology may contribute to closing the phosphorus loop for sustainable utilization.