Kinetics and modelling of the decomposition of a solid solution of potassium–ammonium arsenojarosite in NaOH and Ca(OH) 2 media

The kinetics of the reaction of a solid solution of potassium–ammonium arsenojarosite in NaOH and Ca(OH) 2 was studied. The dissolution curves showed an induction period, during which the concatenation of the ions of the medium with the surface of the particle created active sites until a reaction front was established. The ions of the medium and of the arsenojarosite diffused through the reaction front. This was followed by the progressive conversion period, where the concentrations of K + , SO 4 2− , NH 4 + and AsO 4 3− ions increased until the reaction remained constant. SEM results show an unreacted arsenojarosite core, a reaction front and a layer of ash made of amorphous Fe(OH) 3 with adsorbed AsO 4 . The experimental data of the progressive conversion period are well described by the shrinking core kinetic model, where the chemical reaction controls the process. The values of the activation energy obtained for the progressive conversion period were 67.9 and 58.7 kJ mol −1 for NaOH and Ca(OH) 2 , which indicates that the mechanism that controls either system is the chemical reaction. With the data obtained from the chemical kinetics, the equations are proposed which describe the decomposition process in the studied media. The kinetic modelling was also carried out, thus confirming that the established expressions favorably describe the decomposition process in NaOH and Ca(OH) 2 media.