Precipitation behaviour of As(V) during neutralization of acidic Fe(II)−As(V) solutions in batch and continuous modes

• Ferrous iron proved to be very effective in removing arsenate down to less than 1 mg/L.
• Ferrous arsenate is the major phase in the Fe(II)/As(V) coprecipitate in range of pH4~5.5.

Ferrous iron (Fe(II)) is usually present in a considerable amount in acidic hydrometallurgical effluents. However, little is known about its role on arsenate (As(V)) precipitation during ambient temperature neutralization. This work investigated the precipitation behaviour of As(V) during hydrolysis of ferrous sulfate solutions in both batch and continuous modes. In the batch mode experiments, the pH of acidic ferrous sulfate solution with and without the presence of As(V) was raised from 1.6 up to 11.5 by the addition of NaOH or CaO. In the absence of As(V), Fe(II) ions started to precipitate out of solution at ~ pH 7 and were completely removed at ~ pH 8.5 as ferrous hydroxide, whereas in the presence of As(V), rapid precipitation of Fe(II) occurred at > ~ pH 4 accompanied by simultaneous removal of As(V) with the latter almost completely precipitated at > ~ pH 5.5. This indicates strong association of Fe(II) with As(V) in the precipitates. Elemental analysis after digestion of the Fe(II)–As(V) precipitates yielded Fe(II)/As(V) molar ratios similar to that of symplesite (Fe3(AsO4)2·8H2O). During the steady state regime of the 2-stage continuous coprecipitation/treatment of an acidic Fe(II)–As(V) solution (Fe(II)/As(V) = 4), with the first and second reactor pH kept constant at 6 and 8 respectively, the majority of As(V) was found to precipitate in the first reactor together with ferrous iron at an Fe(II)/As(V) molar ratio close to that of symplesite. These results imply that ferrous iron is an important contributor to arsenic (V) removal/precipitation and should be taken into account in the treatment of arsenic-bearing hydrometallurgical effluents and disposal of produced coprecipitated solids.