In situstudies into the formation kinetics of potassium jarosite

Abstract

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−1was estimated for the first stage of growth, in good agreement with previous work.