Investigating iron–sulphur–phosphate transformation dynamics during redox-controlled microaeration of digested sludge towards improved resource recovery

Abstract

Iron is an attractive binding partner for phosphorus removal from digested sludge, with recovery as the Fe(II) phosphate mineral vivianite (Fe₃(PO₄)₂·8H₂O) representing a promising resource recovery pathway. However, sulphides compete strongly with phosphate for Fe(II), limiting vivianite formation and often requiring higher iron dosages. Because both iron and sulphur undergo redox transformations, controlled microaeration may alter the balance between iron sulphides, iron phosphates, and oxidized iron and sulphur species. This study investigated the effect of oxidation–reduction potential (ORP)-controlled microaeration (−220 to 0 mV) on Fe–S–P transformations in digested sludge. Microaeration showed contrasting effects. Mössbauer spectroscopy indicated an approximately 20% decrease in the Fe(II) associated with the vivianite fraction under all ORP conditions, while a pyrite-like iron sulphide phase remained largely unchanged. In contrast, the soluble phase showed increasing sulphur concentrations (ultimately recovered as sulphate) and a ∼70% decrease in soluble phosphorus, suggesting enhanced iron–phosphate binding or adsorption during microaeration. Although the applied aeration conditions ultimately did not lead to vivianite formation, the proposed mechanism identifies an early microaeration window that favours phosphorus removal, providing a basis for further optimization of microaeration strategies for phosphorus recovery from anaerobic sludges.