Alkaline decomposition of synthetic jarosite with arsenic

The widespread use of jarosite-type compounds to eliminate impurities in the hydrometallurgical industry is due to their capability to incorporate several elements into their structures. Some of these elements are of environmental importance (Pb²⁺, Cr⁶⁺, As⁵⁺, Cd²⁺, Hg²⁺). For the present paper, AsO₄^3- was incorporated into the lattice of synthetic jarosite in order to carry out a reactivity study. Alkaline decomposition is characterized by removal of sulfate and potassium ions from the lattice and formation of a gel consisting of iron hydroxides with absorbed arsenate. Decomposition curves show an induction period followed by a conversion period. The induction period is independent of particle size and exponentially decreases with temperature. The conversion period is characterized by formation of a hydroxide halo that surrounds an unreacted jarosite core. During the conversion period in NaOH media for [OH^-]?>?8?×?10^-3?mol?L^-1, the process showed a reaction order of 1.86, and an apparent activation energy of 60.3?kJ?mol^-1 was obtained. On the other hand, during the conversion period in Ca(OH)₂ media for [OH^-]?>?1.90?×?10^-2?mol?L^-1, the reaction order was 1.15, and an apparent activation energy of 74.4?kJ?mol^-1 was obtained. The results are consistent with the spherical particle model with decreasing core and chemical control.