Chemical upcycling of phosphoric acid recovered from sewage waste

Global concerns surrounding the depletion and strategic importance of phosphorus resources have heightened the demand for the recovery and recycling efforts. However, the phosphorus compound obtained from waste exists primarily as chemically inert phosphoric acid or its salts, and the direct conversion into high-value chemicals has remained largely elusive. Our objective was to develop an innovative technology that bypasses the use of white phosphorus and enables the direct esterification of phosphoric acid, leading to the production of phosphate triesters widely utilized across versatile applications. Tetraalkyl orthosilicates emerged as highly effective reagents for the direct triple esterification of 85% phosphoric acid, as well as the esterification of organophosphinic and phosphonic acids. Through both experimental and theoretical investigations, we have elucidated a novel mechanism governing the reaction, wherein tetraalkyl orthosilicate not only serves as an alkyl transfer reagent but also facilitates the formation of multimolecular aggregates as a reaction template, assisting the proton shuttle process. By optimizing the recovery method of phosphoric acid from sewage sludge ash, we successfully achieved the direct esterification of the recovered phosphoric acid with tetraalkyl orthosilicate, thus pioneering a groundbreaking upcycling pathway from sewage waste to valuable phosphorus chemicals.