Structure, dissolution, and plant uptake of ferrous/zinc phosphates

Development of slow release fertilizers by tuning dissolution kinetics can reduce the environmental impact of (micro) nutrients added to crops. Mixed metal compounds may have different dissolution kinetics and plant uptake than single metal compounds. In this study, mixed Fe(II)/Zn(II) phosphates (0-100 at% Zn) were prepared by aqueous precipitation and their structural characteristics and dissolution kinetics in a sand column were measured as model for divalent metal and phosphate release in soil. Three minerals were identified, namely vivianite (Fe₃(PO₄)₂·8H₂O) at 0-20 at% Zn, phosphophyllite (Zn₂Fe(PO₄)₂·4H₂O) at 20-79 at% Zn, and hopeite (Zn₃(PO₄)₂·4H₂O) at 79-100 at% Zn. The Fe-rich materials had high SSA of 42-64 m² g^-1, which decreased to ≤4 m² g^-1 for ≥79 at% Zn. The Fe K-edge and Zn K-edge XANES spectroscopy measurements show that the samples had comparable local structure and contained 13-72% of Fe as Fe(III) due to partial oxidation. In the sand column, Zn(II) and Fe(II) phosphates dissolved near-congruently at steady state (>7 h), whereas mixed Fe(II)/Zn(II) phosphates showed preferential release of Zn over P and Fe, likely due to reprecipitation of Fe. Pot experiments demonstrate that Zn from Fe(II)/Zn(II) phosphates is absorbed by bird's eye chili plants (C. annuum), in agreement with the preferential dissolution of Zn(II). These results may provide insight into the dissolution of other divalent metals, which not only aids in the growth of plants and resulting foodstuff but ultimately leads to reductions in environmental contamination.