Synergistic recovery of aluminum phosphorus from incinerated sewage sludge ash via acid and alkaline treatment

The "phosphorus (P) crisis" has increased interest in improving P recovery methods and their efficiency. Incineration of sewage sludge ash is a recognized method for efficient P recovery, but challenges persist because of low recovery efficiency using thermochemical or biological methods and the issue of heavy-metal contaminants. Traditional wet-chemical processes, which yield calcium phosphate as the main product, are complex and costly. In contrast, aluminum (Al) phosphate (AlP) not only serves as a slow-release phosphate fertilizer but also has diverse high-value industrial applications, such as its use in battery coatings. To address these challenges, we developed a novel recovery route based on AlP extraction from Al-rich incinerated sewage sludge ash via acid leaching and alkaline precipitation. Our findings indicate that nitric acid (HNO3) achieves the highest Al leaching efficiency. The leaching kinetics of PO43- and metals follow second-order kinetics. Furthermore, pH adjustment with NaOH showed that the P precipitation efficiency from oxalic acid leachate was lower than that from hydrochloric acid, sulfuric acid (H2SO4), and HNO3 (88.0%-99.8%) because of buffering effects. These results underscore the potential of Al-rich ash for effective P recovery and considerably broaden the range of applications for P products.