Leveraging iron sulphide destruction via ORP-controlled microareation to improve phosphate recovery

Vivianite (Fe₃(PO₄)₂·8H₂O) is an attractive product for phosphorus recovery and can form in digested sludge, sediments, and manure when sufficient Fe(II) is available. In wastewater treatment, however, iron sulphides typically outcompete vivianite under anaerobic conditions. To reduce the iron dosage required for vivianite formation, this study investigated whether controlled microaeration at oxidation–reduction potential (ORP) levels ranging from -220 to 0 mV can oxidize iron sulphides, release Fe(II), and thereby promote vivianite formation. Microaeration produced two contrasting effects. Mössbauer spectroscopy indicated a loss of ~20% of the Fe(II) vivianite fraction across all ORP conditions, while a pyrite-like iron sulphide phase remained largely unchanged. In contrast, in the soluble phase, sulphur species increased (~40% of total sulphur), and phosphorus decreased (~70% of soluble phosphorus), suggesting iron–phosphate binding or adsorption under microaerobic conditions. These observations support a time-dependent reaction sequence. During early aeration, more labile FeSₓ phases are likely to have oxidized to elemental sulphur, providing free iron sites for phosphorus binding/adsorption. After prolonged aeration (24 h), both sulphur and iron species shifted toward oxidized products, including sulphate and Fe(III) minerals. Targeting only the initial microaeration phase may offer a pathway to selectively convert iron sulphides into more desirable recovery products—elemental sulphur and vivianite. Additionally, phosphorus removal, at ORP as low as -220 mV highlights microaeration as a potential low-chemical alternative to peroxide-based treatments.