Influence of ferric iron source on ferrate's performance and residual contamination during the treatment of gold mine effluents

Ferrate [Fe(VI)] is increasingly used for the treatment of several contaminants in drinking water and municipal/industrial waste water. Recent findings also show that Fe(VI) is a promising alternative for the treatment of gold mine effluents contaminated by cyanides (CN−), thiocyanates (SCN−), and ammonia nitrogen (NH3-N). However, the ferric [Fe(III)] salt used in Fe(VI) synthesis could affect this alternative's efficiency and costs, as well as residual contaminants in the treated effluent. The objective of this study was to evaluate the influence of three Fe(III) salts on the performance of Fe(VI) in the treatment of gold mine effluents and the residual contaminants produced in the process. To do so, the performance of wet Fe(VI) synthesized with ferric nitrate [Fe(NO3)3], ferric chloride (FeCl3), or ferric sulfate [Fe2(SO4)3] was evaluated for the treatment of a real gold mine effluent, with final adjustment of pH at ∼7. The results showed that for the effluents treated with Fe(VI) synthesized in the presence of Fe(NO3)3, the final NO3− concentrations were 4 times higher relative to those in the treated effluents with Fe(VI) synthesized from either FeCl3 or Fe2(SO4)3. The use of Fe2(SO4)3 in the wet Fe(VI) preparation did not entail a significant increase of the final SO42− concentrations, which were very similar to initial ones (1220 vs. 1012 mg/L). At the same time, Fe(VI) synthesized from Fe2(SO4)3 was the only one to remove the CN− and NH3-N almost totally (∼99%). Finally, the production yield of Fe(VI) with FeCl3 was lower than Fe2(SO4)3 or Fe(NO3)3 (3400 vs. 5500 and 5635 mg/L, respectively). This low yield production of Fe(VI) from FeCl3 entails more costly production costs. Thus, Fe2(SO4)3 is potentially the most appropriate Fe(III) salt source for Fe(VI) synthesis for use in the treatment of contaminated mine effluents.