Continuous circuit coprecipitation of arsenic(V) with ferric iron by lime neutralization: The effect of circuit staging, co-ions and equilibration pH on long-term arsenic retention

Highlight
• The circuit configuration influences the stability of As within coprecipitates.
• Multistage continuous coprecipitation provides solids with improved arsenic stability.
• Concentration of As after aging depends on co-ions present during coprecipitation.
• Presence of Ni2 + and Al3 + has beneficial effects on arsenic retention.

This article presents the results of an investigation on the impact of the continuous neutralization circuit and solution parameters on the stability of Fe(III)–As(V) coprecipitates produced from acidic sulfate solutions by lime neutralization. The work was undertaken to better understand the link between the process of arsenic removal via continuous circuit coprecipitation as practiced in industrial effluent treatment and the stability of arsenic in the resulting coprecipitates. In particular the work sought to establish the effects of circuit staging (1–3 stages, with and without recycling) and the presence of Ni2 + and Al3 + co-ions on coprecipitate stability. The latter was monitored during long-term laboratory aging (300–400 days and in some cases for ~ 1500 days) at various temperatures (4, 22, 40 and 70 °C) under fixed (pH 8) or drift pH oxic environment. Of the different circuit configurations the two-stage neutralization/coprecipitation circuit operating at pH = 4 (stage 1) and pH = 8 (stage 2) yielded the most stable coprecipitate in terms of arsenic retention. Recycling had no apparent effect on stability. It is proposed that stage-wise neutralization promotes the formation of ferric arsenate in addition to arsenate-bearing ferrihydrite resulting in enhanced arsenic retention. Furthermore arsenic retention was found to improve under pH drift aging conditions, where coprecipitates rapidly attained equilibrium values (< 0.2 mg/L As and pH ~ 7.2 at 22 °C). Coprecipitates aged under drift pH condition exhibited resistance to arsenic release upon pH elevation. Finally, arsenic release was found to be further suppressed in the case of coprecipitates produced in the presence of Ni2 + and Al3 +.