Rates and Mechanism of Vivianite Dissolution under Anoxic Conditions

Abstract

The iron phosphate mineral vivianite Fe(II)₃(PO₄)₂·8H₂O has emerged as a potential renewable P source. Although the importance of vivianite as a potential P sink in the global P cycle had previously been recognized, a mechanistic understanding of vivianite dissolution at the molecular level, critical to its potential application, is still elusive. The potential of vivianite as a P sink or source in natural or engineered systems is directly dependent on its dissolution kinetics under environmentally relevant conditions. To understand the thermodynamic and kinetic controls on bioavailability, the oxidation and dissolution processes of vivianite must be disentangled. In this study, we conducted controlled batch and flow-through experiments to quantitatively determine the dissolution rates and mechanisms of vivianite under anoxic conditions as a function of pH and temperature. Our results demonstrate that vivianite solubility and dissolution rates strongly decreased with increasing solution pH. Dissolution was nonstoichiometric at alkaline pH (>7). The rapid initial dissolution rate of vivianite is related to the solution saturation state, indicating a thermodynamic rather than a kinetic control. A defect-driven dissolution mechanism is proposed. Dissolution kinetics over pH 5-9 could be described with a rate law with a single rate constant and a reaction order of 0.61 with respect to {H⁺}: $R_{\exp }=36.0·e^{-1.41·\mathrm{pH}}·{[1-e^{(0.2·\Delta G/\mathrm{RT})}]}^{4.7}$ The activation energy of vivianite dissolution proved low (E_a = 20.3 kJ mol^-1), suggesting hydrogen bridge dissociation as the rate-determining step.

Keywords: P recovery; P sink; recycled fertilizer; stoichiometric dissolution; sustainable P cycling.