Utilization of vivianite as a slow-release phosphorus fertilizer: efficiency and mechanisms

Background Vivianite is an intensively studied phosphorus (P) recovery product from wastewater treatment plants. However, its downstream application has not been well-addressed thus restricting P cycling. This study investigated the P fertilizer efficiency of vivianite in pot experiments using ryegrass ( Lolium multiflorum L.) and tomato ( Solanum lycopersicum L.) with two soilless growing media (perlite and quartz). The variation of vivianite during fertilization was examined to elucidate the mechanisms of P release from vivianite, using a recently developed sequential P extraction protocol coupled with light microscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). Results The mineral fertilizer equivalence (MFE) of vivianite was determined by comparing the P uptake by plants under vivianite treatment to that under mineral P fertilizer treatment, i.e., Ca(H 2 PO 4 ) 2 in this study. It shows that the vivianite-P uptake of ryegrass was positively correlated with time in a 3-month period and its uptake in quartz was faster than that in perlite, leading to a higher MFE of vivianite in quartz than in perlite (15.8–20.3% vs 6.6–7.4%). This should be attributed to the higher P adsorption capacity and lower hydraulic conductivity of perlite, which restricted the vivianite-P dissolution and mobility. The vivianite variation revealed that vivianite-P was mainly released upon vivianite oxidation to ferric (hydr)oxides in perlite, probably given the high aeration condition in this type of growing medium. In quartz, however, the release of vivianite-P was most likely due to vivianite dissolution driven by the plant uptake of P, during which a higher plant uptake of Fe was observed than that in perlite. Conclusions The findings reveal that vivianite can serve as a slow-release P fertilizer, and its fertilizer efficiency is highly related to the properties of the growing media. Quartz is a preferred growing medium over perlite for the efficient fertilization of this sparingly soluble P source. The findings deepen the understanding of P fertilization of vivianite and lay foundation to its utilization as a slow-release P fertilizer for a circular economy of P. Graphical