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

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). 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(H2PO4)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. 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.