Coprecipitation of arsenate with iron(III) in aqueous sulfate media: effect of time, lime as base and co-ions on arsenic retention.

The removal and immobilization of arsenic from industrial mineral-processing effluents typically involves lime neutralization and coprecipitation of arsenate with ferric iron. Despite the wide practice and environmental importance of this technique, no laboratory study has focused on the roles of lime as base and third ions like Ca2+, Ni2+ and SO(2)4(-) on the kinetics of arsenic retention by the coprecipitates. In this work, coprecipitation was performed at 22 degrees C by fast (10 min) neutralization of industrially relevant concentrated arsenate-iron(III) (Fe/As=2, 4) acidic sulfate solutions to different pHs (4, 6, 8) in batch reactors, and the concentration of arsenic was monitored up to 1 year. The tests showed that maximum removal of arsenic was achieved upon neutralization to the target pH. Arsenic was found to be released back into solution from the precipitates upon continuing mild agitation at constant pH. Near-equilibrium was attained at different times depending on the applied pH: 10 days at pH 4, 6 months at pH 6 and 9 months at pH 8. An aging treatment at pH 4 significantly enhanced arsenic retention (arsenic release was reduced by at least 50%) after the system was finally stabilized at pH 8. The retention of arsenic at pH 8 was multifold improved (by a factor x 25) when lime was used instead of NaOH. Similarly, the retention of arsenic was enhanced by the presence of calcium and nickel ions in the starting solution. Finally, evidence of Ca(II)-Fe(III)-As(V) association was found, but not sulfate incorporation at pH 8.