Can vivianite be produced when iron (oxyhydr)oxides react with sulfide?

Abstract

Phosphorus is a vital nutrient element, but excessive amounts in water can lead to eutrophication. Vivianite (Fe3(PO4)2 · 8H2O), an iron phosphate mineral, can be an important sink for P in aquatic environments. To effectively remove P, the formation of vivianite has been considered a promising pathway. Iron (oxyhydr)oxides, exhibit a strong adsorption capacity for phosphate (P) and have been added to lakes to remove excessive phosphorus. However, when iron (oxyhydr)oxides are buried in sediments and enter anaerobic layers where sulfide (S(-II)) is present, iron (oxyhydr)oxides can undergo reduction, releasing P and exacerbating water quality issues. It has long been believed that vivianite cannot form in the presence of S(-II) due to the binding of Fe(II) in the form of FeS during the S(-II) oxidation by iron (oxyhydr)oxides. Thermodynamic calculations, however, indicate possibly formation of vivianite during the S(-II) oxidation by iron (oxyhydr)oxides in the presence of high concentration of P. Although it is still unclear whether vivianite formation might impeded during this process due to reaction kinetics. In this study, we sought to investigate the formation of vivianite in the presence of S(-II) using a flow-through reactor (FTR). The experiments were conducted at a pH of 7.3, using lepidocrocite or ferrihydrite as iron (oxyhydr)oxides with a range of P concentrations from 0.1 mM to 4 mM. When employing ferrihydrite as the iron source, a S(-II) concentration of 0.5 mM, P concentration of 4 mM, and a flow rate of 0.1 ml/min, resembling vivianite crystals were observed in the solid phase through TEM analysis, while S(-II) remained detectable. Following a no-flow period of 11 days, vivianite formation was confirmed through XRD analysis. Consistent with our thermodynamic calculations, vivianite was not detected at lower P concentrations (0.1-2 mM P). Additionally, in batch experiments using the same P concentration and total amount of S(-II) as the FTR experiment, vivianite was not detected, despite varying the no-flow period from 7 to 60 days. Based on the experimental findings we concluded that the built up of sufficient high Fe2+ concentrations is pivotal for vivianite formation. At different reaction stages, the accumulation of Fe2+ may originate from different processes. In FTR experiments without a no-flow period, Fe2+ could be generated during the reduction of ferrihydrite before the formation of FeS or regional FeS dissolution. On the other hand, during the no-flow period after the FTR experiments, Fe2+ accumulation could be attributed to ferrihydrite acting as a sink for S(-II) to promote FeS dissolution. This study presents a new possible pathway for the vivianite formation in the presence of S(-II) in the natural environment. However, the formation of vivianite only occurs under highly restrictive conditions, where S(-II) is effectively consumed, Fe(II) is released into the solution, and the concentration of P is high, resulting in the supersaturation of vivianite in the solution and subsequent precipitation.