Unravelling phosphorus mobilisation from sludge incineration ash: Organic-iron-phosphate complexation and aqueous speciation

Abstract

Developing sustainable technologies to recover phosphorus from iron-rich waste, such as incinerated sewage sludge ash, represents a critical engineering challenge. Conventional acid-leaching processes suffer from high chemical consumption, equipment corrosion, and poor selectivity, necessitating energy-intensive and costly operations. Here, a novel and highly efficient recovery process is presented that overcomes these limitations by operating under mild conditions (pH 5, ambient temperature). The process achieved over 90% phosphorus extraction efficiency from iron phosphate (Fe-P), a result that defies traditional reliance on strong acidity. To uncover the mechanism enabling this breakthrough, a multiscale investigation combining kinetic analysis, spectroscopy, and computational chemistry was conducted. The results revealed a previously unrecognized phosphate-modulated dissolution pathway whereby phosphate anions actively induce the transformation of poorly soluble polynuclear iron-citrate aggregates into a highly stable and soluble mononuclear complex (CA-Fe³⁺-P, denoted as CF3P), confirmed by its spontaneous formation energy (ΔG = −117.47 kcal/mol). By forming this stable complex, iron and phosphate are sequestered together in a soluble aqueous form, which directly prevents reprecipitation of insoluble iron phosphate. These findings establish a new design principle for sustainable resource recovery, demonstrating that process efficiency can be achieved through targeted manipulation of molecular speciation under mild conditions, rather than relying on aggressive, high-input chemical approaches.