In situ studies into the formation kinetics of potassium jarosite

This paper reports the results of time-resolved synchrotron powder diffraction and small-angle scattering experiments designed to investigate the kinetics of formation of potassium jarosite by co-precipitation at 353, 368 and 393 K. Only jarosite was produced in these syntheses, and the particles that formed nucleated on the walls of the capillary reaction vessels with a disc-like shape. Relative Rietveld scale factors indicating jarosite abundance have been used as the basis for kinetic modelling of the nucleation and growth stages using a modified form of the Avrami kinetic equation. The results show that jarosite forms by a single nucleation event followed by two distinct stages of growth, each characterized by a different Avrami exponent. The value of this exponent is initially between 1 and 2, and then reduces to around 1. This suggests that jarosite growth after nucleation is controlled by effects at the solution-boundary interface, with the first stage best described by two-dimensional growth which transitions to one-dimensional growth later in the reaction. An activation energy of 89 kJ mol^-1 was estimated for the first stage of growth, in good agreement with previous work.