Different influences of organic ligands on vivianite formation and dissolution

Vivianite (Fe₃(PO₄)₂·8 H₂O) has emerged as a promising mineral for phosphorus (P) recovery from digested sludge, and it may also contribute to phosphate management in lake sediments and manure, given the similar anaerobic conditions across these environments. However, organic ligands in these matrices have been proposed to complex with iron (Fe), thereby reducing the efficiency of vivianite formation. This study aims to elucidate the impact of organic ligands on vivianite formation, particularly focusing on their binding strength with iron and the subsequent effects on vivianite formation in pig manure. Organic ligands not only form complexes with iron but also influence the crystal growth process. We investigated how different organic ligands affect the formation and dissolution of vivianite, assuming that ligands with higher iron-binding strength would enhance phosphate solubilization. Our findings revealed that citrate nearly completely inhibited vivianite formation (up to 100 %) and caused a 50 % dissolution of existing vivianite, while humate hindered vivianite formation by 40 % but only led to a 10 % dissolution. Interestingly, pig-derived dissolved organic matter had minimal effects on the precipitation of iron and phosphorus but significantly altered the morphology of the resulting products, which varied depending on the age of the manure filtrate. While the iron binding strength of organic ligands does influence vivianite formation, it does not solely account for the reduced vivianite formation observed in complex matrices like manure. Therefore, a more nuanced assessment of the role of organic matter in vivianite formation is warranted.