The global phosphorus challenge involves two interconnected issues: securing a sustainable phosphorus supply and reducing environmental pollution caused by phosphorus losses. Phosphorus is essential for all living organisms and is a critical component of mineral fertilizers. However, phosphate rock reserves are concentrated in a limited number of regions, including Morocco, Russia, United States, and China, creating supply vulnerabilities for import-dependent regions such as the European Union. At the same time, the largely linear use of phosphorus leads to accumulation in landfills, agricultural soils, and aquatic systems, where it drives eutrophication and ecosystem degradation. Recovering phosphorus from secondary streams such as manure, wastewater treatment sludge, and lake sediments offers a promising route to address both challenges.

This thesis focuses on phosphorus recovery through the formation of vivianite (Fe₃(PO₄)₂·8H₂O), a ferrous phosphate mineral with high thermodynamic stability and paramagnetic properties, enabling magnetic separation. Vivianite formation was investigated across manure, wastewater treatment sludge, and lake sediments to identify key barriers and opportunities for recovery.

The role of organic matter in vivianite formation was first examined. A phosphorus-to-dissolved-organic-carbon scale was introduced to evaluate how different organic compounds influence iron–phosphorus interactions. Organic ligands including bipyridine, citrate, humate, alginate, acetate, and dissolved organic matter from pig manure were tested for their effects on vivianite formation and dissolution. Citrate strongly inhibited vivianite formation and dissolved a substantial fraction of existing vivianite, while humate had a moderate inhibitory effect. In contrast, manure-derived dissolved organic matter had minimal influence on vivianite stability and primarily affected crystal morphology. These results showed that organic matter alone cannot explain reduced vivianite formation in pig manure.

The thesis then identified carbonate competition as a major limiting factor. In aged and digested manure, siderite (FeCO₃) formed preferentially over vivianite due to high dissolved inorganic carbon concentrations, causing carbonate to outcompete phosphate for ferrous iron. Reducing carbonate availability increased vivianite formation, both in fresh manure and in digested manure treated by gas stripping. Similar carbonate inhibition was observed in thermally hydrolyzed digested sludge, demonstrating that this limitation extends beyond manure.

Vivianite formation in dredged lake sediments was also investigated. Despite elevated phosphate concentrations typical of eutrophic systems, vivianite precipitation occurred only after additional phosphate dosing. This indicates that phosphate availability in sediments is limited and that competing anions such as carbonate and sulphide strongly influence iron binding. Analytical challenges further complicated vivianite quantification, highlighting the need for combined spectroscopic, microscopic, and extraction-based approaches. Overall, vivianite recovery from lake sediments appears limited.

Finally, the thesis examined the role of iron sulphides in limiting vivianite formation. Controlled oxidation of iron sulphides in digested sludge through microaeration was studied as a potential strategy to promote phosphorus recovery. Although sulphide destruction could not be conclusively demonstrated, reduced dissolved phosphate concentrations indicated enhanced iron–phosphate interactions. These findings suggest a practical pathway to improve phosphorus removal without increasing iron dosing.

Overall, this thesis shows that the dominant barriers to vivianite formation are carbonate competition in high-alkalinity systems such as manure and certain sludges, limited phosphate availability in sediments, and iron sulphide formation across all matrices. The carbonate barrier can be overcome, making vivianite recovery from pig manure technically feasible. More broadly, successful implementation of vivianite-based phosphorus recovery requires stronger integration across agricultural, environmental, and sanitation sectors. Addressing the global phosphorus challenge therefore demands not only technical innovation but also sustained transdisciplinary collaboration. ... Read more

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. ... Read more

Phosphorus runoff from agricultural land is a major driver of eutrophication, with manure serving as a significant source of phosphorus input. In regions such as the Netherlands, high livestock densities and limited land availability pose challenges for manure management, particularly in pig farming. Recovering phosphorus from manure and redistributing it to phosphorus-deficient areas offers a sustainable solution. This study explores phosphate recovery via vivianite (Fe₃(PO₄)₂·8H₂O) precipitation—a method previously demonstrated in municipal wastewater treatment plant sludge—and evaluates its applicability to pig manure. Vivianite formation was investigated in fresh, 4-month-aged, and digested pig manure, as well as in Thermal Hydrolysis Plant (THP) derived digested sewage sludge. A key finding is that high dissolved inorganic carbon (DIC) concentrations inhibit vivianite formation by promoting siderite (FeCO₃) precipitation. In digested manure, a DIC threshold of approximately 3 g/L HCO₃⁻ was identified, below which vivianite formation is favored. THP sludge, characterized by elevated DIC, exhibited similar inhibitory effects. More generally, vivianite was shown to form without significant competition with siderite if the DIC concentration is <2.5 times the iron concentration. Experimental results were compared with thermodynamic predictions using Visual MINTEQ and experiments in ultrapure water, revealing discrepancies which may be attributed to the ionic composition in environmental matrices. Strategies such as combining ammonia and DIC stripping or targeting fresh manure were shown to enhance vivianite formation. These findings can be used to propose the integration of vivianite-based phosphorus recovery into broader resource recovery frameworks, including biomethane production, ammonium recovery, and carbon capture. ... Read more