Kinetic modeling and experimental design of the sodium arsenojarosite decomposition in alkaline media: Implications

The alkaline decomposition of a synthetic sodium arsenojarosite sample with formula [Na0.87(H3O)0.13]Fe2.50[(SO4)1.95(AsO4)0.05][(OH)4.45(H2O)1.55] was studied in NaOH and Ca(OH)2 media. The experimental data on the progressive conversion period are consistent with the spherical particle model with decreasing core and chemical control. Partially decomposed particles observed by SEM confirm the presence of a non-reacting arsenojarosite core, a reaction front and an amorphous ash halo consisting of Fe(OH)3 with adsorbed AsO4, which coincide with the mentioned model. An experimental design was also developed to determine the effects of the variables and their interactions that are directly involved in the decomposition reaction. It was found that T, [OH−], and [OH−]∙T interactions are the factors with the greatest influence on the reaction. With the data obtained from the chemical kinetics and experimental design, we proposed a series of equations that satisfactorily describe the decomposition process in function of conversion and time.