Effects of zinc, copper and sodium ions on ferric arsenate precipitation in a novel atmospheric scorodite process

A novel process has been developed for the fixation of arsenic using the formation of scorodite. Stable scorodite particles were produced by introducing an oxidizing gas into a reaction mixture containing ferrous sulfate and arsenic(V) at high concentration to convert the ferrous ion to ferric ion. This approach to scorodite synthesis enabled the reaction to proceed in the presence of sulfuric acid under atmospheric pressure. The influence of zinc, copper, and sodium ions in the solution was investigated; as well as the effect of pure O₂ gas and air as the oxidizing agent and the effect of arsenic concentration and pH of the reaction mixture. Scorodite grains were precipitated even though the reaction system contained co-existing ions at high concentration. It was found that zinc did not affect the synthesis and that the presence of copper yielded a good scorodite product. However, the presence of sodium inhibited the formation of scorodite. An initial pH under 2 was significant for producing well-crystallized scorodite and avoiding conditions favorable for jarosite formation.