Synthesis and nature of the reaction of a solid solution of ammonium-potassium arsenojarosite in NaOH medium

A solid solution of ammonium-potassium arsenojarosite was synthesized for this study. It was chemically and mineralogically characterized by Inductively Coupled Plasma Spectrometry (ICP), elemental analysis, Atomic Absorption Spectrophotometry (AAS), X-Ray Diffraction (XRD) and Scanning Electron Microscopy with Energy-Dispersive X-Ray Spectroscopy (SEM-EDS). The following composition was obtained: 26.67 % Fe, 36.41 % SO₄, 3.60 % K, 1 % NH₄, 6.71 % AsO₄ and 25.61 % OH^–+H₂O, which results in the following approximate formula: [(NH₄)_0.23K_0.77] Fe_2.47 [(SO₄)_1.69(AsO₄)_0.31] [(OH)_4.1(H₂O)_1.9]. The amount of arsenic was higher than the reported by different authors who have synthesized similar compounds. The XRD results confirm that the obtained sample corresponds to ammonium-potassium arsenojarosite. The granulometric analysis showed that the main particle sizes are 38 and 25 µm, corresponding to mesh sizes 400 and 500. This indicates that a fine size particle was obtained. The compound’s nature of thereaction in NaOH was studied as well, showing an induction period characterized by the formation of active sites until a reaction front is established. It is followed by the progressive conversion period, in which K⁺, SO₄^2- and NH₄⁺increase in the solution until reaching stabilization. The reaction was followed by determination of K with AAS. The obtained results are consistent with the kinetic model of constant-size spherical particles and unreacted core with chemical control. Partially decomposed particles were also observed by SEM, where an unreacted arsenojarosite core, a reaction front and an ash halo made of amorphous Fe(OH)₃ with adsorbed AsO₄ can be seen.