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+}:
The activation energy of vivianite dissolution
proved low (E
a = 20.3 kJ mol–1), suggesting hydrogen bridge dissociation as the rate-determining
step.