Variscite dissolution rates in aqueous solution: does variscite control the availability of phosphate in acidic natural waters?

The dissolution rates of natural well-crystallized variscite (AlPO4 center dot 2H(2)O) were measured from the evolution of aqueous Al and P concentrations in closed and mixed-flow through reactors at 25 degrees C and from 1.5 to 9 pH. Measured dissolution rates decrease with increasing pH from 5.05 x 10(-16) mol/cm(2)/s at pH = 1.51 to 4.92 x 10(-17) mol/cm(2)/s at pH = 5.89 and then increase with increasing pH to 1.64 x 10(-17) mol/cm(2)/s at pH = 8.99. Estimates of the time required to equilibrate a mildly acidic, initially Al- and P-free solution with variscite based on measured dissolution rates and solubility products suggests it takes no more than several weeks to equilibrate this mineral with soil pore fluids. This result suggests that variscite can buffer aqueous phosphate concentrations in a significant number of near surface environments.