Assessing microbially mediated vivianite as a novel phosphorus and iron fertilizer

Background Microorganisms can transform phosphorus (P)-enriched iron (Fe)-oxide sludge into products with higher P concentration or can directly promote the precipitation of P-rich compounds from water. However, there is no evidence of these products’ efciency as fertilizers. This study aimed to assess the efectiveness of microbi ally mediated vivianite (biovivianite) as P and Fe fertilizer for durum wheat and white lupin, respectively. Results To this end, two completely randomized block experiments were conducted with wheat (phosphorus (P) experiment) and white lupin (iron (Fe) experiment). The P and Fe sources used included biovivianite produced by microbial reduction of P-containing ferrihydrite at pH 6.5 (VivInsol6.5) and pH 7.0 (VivInsol7.0), biovivianite pro duced with soluble Fe(III) citrate (C6H5FeO7) in the presence of soluble phosphate at pH 7 (VivSol), and vivianite from a commercial company (ComViv). Potassium dihydrogen phosphate (KH2PO4) was used as a reference fertilizer in the P experiment, and Fe-EDDHA and Fe(II)-sulfate (FeSO4.7H2O) were used in the Fe experiment. Total P uptake by wheat plants from the product dominated by vivianite and phosphate-green rust (VivSol) was not signifcantly diferent from KH2PO4. The relative P use efciency, i.e., the equivalence in terms of P recovery of VivSol was 74% of KH2PO4, making VivSol the efective P source for durum wheat among the products tested (aside from KH2PO4). For Fe uptake, product dominated by vivianite and metavivianite (VivInsol7.0), was the most efective Fe source for white lupin followed by Fe-EDDHA, ComViv, and VivSol with VivInsol6.5 as the least efective but without signif cant diferences with Fe(II)-sulfate. The average crystallite sizes of the biovivianite were 59 nm, 63 nm, and 66 nm for VivSol, VivInsol7.0, and VivInsol6.5, respectively. Conclusions The mineral constituents of the biovivianite coupled with their nano-crystallite sizes explained its efectiveness as P and Fe fertilizers. The results reveal that biovivianite production is a novel way of producing efcient P and Fe fertilizers from P-enriched Fe sludge or P-rich water. Thus, it can be used for producing fertilizers with high P and Fe concentrations from water purifcation, providing new tools for a circular economy approach in the use of a non-renewable resource such as P.