Experimental determination of struvite dissolution and precipitation rates as a function of pH

Struvite (MgNH4PO4·6H2O) was dissolved and precipitated in closed-system reactors at 25 °C and 7.76 < pH < 10.65. Reactive fluid Mg and P concentrations attained a stationary-state after ∼10 h in all experiments. The temporal variation of these concentrations was evaluated to retrieve struvite solubilities, and dissolution and precipitation rates. Resulting struvite solubilities are consistent with an equilibrium constant for the reaction: MgNH 4 PO 4 · 6 H 2 O ⇔ Mg 2 + + NH 4 + + PO 4 3 - + 6 H 2 O of 10−13.24±0.5, which is similar to values previously reported in the literature. Measured struvite dissolution and precipitation rates (r) are found to be consistent with r = r + 1 - exp A 2 RT where r+ symbolizes the forward dissolution rate, A refers to the chemical affinity of the dissolution reaction, R designates the gas constant, and T represents absolute temperature. Values of r+ are found to be independent of pH and equal to 10−12.85±0.15 mol/cm2/s in initially NH4-free reactive fluids. The presence of 0.06 mol/kg NH4 in the initial reactive fluid led to non-stoichiometric element release interpreted to stem from the precipitation of a NH4-rich phosphate phase. Use of measured rates in geochemical modeling calculations indicate that both undersaturated and supersaturated aqueous fluids will rapidly equilibrate with struvite, supporting its potential use as a renewable fertilizer.