Reaction mechanism for the formation of ammonium jarosite: thermodynamic studies and experimental evidence

Experimental and thermodynamic studies on the iron–sulphate–water system were carried out in order to appreciate the possible reaction mechanisms for the formation of ammonium jarosite. The mole ratio of ferric and sulphate species in the precipitates obtained during the kinetic studies was analysed. Thermogravimetric analyses (TGAs) and differential thermal analyses (DTAs) were conducted to examine the weight losses from ammonium jarosite during its thermal decomposition. Based on the results of both the thermodynamic studies and the experimental test work, a possible reaction mechanism for the formation of ammonium jarosite is proposed. This proposed reaction mechanism and the possible multiple pathways by which jarosite precipitation could occur are discussed. When ammonia solution is used as the ammonium ion source, the formation of ammonium jarosite is believed to start with the species FeSO₄⁺ and Fe(OH)₃. Solid basic ferric sulphates, [Fe(OH)₂]₂SO₄ and Fe(OH)SO₄, may be precursors for the formation of ammonium jarosite. Fe(OH)₃ can undergo reactions forming hydroxyl ferric Fe(OH)₂⁺ ions and these are able to form basic ferric sulphates. There appears to be a gradual solid state conversion of the basic ferric sulphates/hydroxides into jarosite over time.