Effect of pH on Cu, Ni and Zn removal by biogenic sulfide precipitation in an inversed fluidized bed bioreactor

Abstract

Mining and metallurgical operations have resulted in the disposal of large quantities of non-treated metal rich effluents into the environment. From a techno-economic and environmental view-point, metal recovery from such waste streams can solve this persisting problem. This study compares the performance of two sulfate reducing inversed fluidized bed bioreactors (IFBs) to recover heavy metals (HMs) as metal sulfide from wastewater at neutral and acidic pH. The IFBs were operated at pH7.0 and 5.0 to study the effect of the pH on the metal (Cu, Ni and Zn) removal efficiency from a synthetic acid mine drainage containing the metals. The electron donor used in the study was ethanol, supplied at an organic loading rate of 1gCOD/Lday and a COD/sulfate ratio of 1.0. The average sulfide production was 220.9mg/L and 152.6mg/L, respectively, at pH7.0 and pH5.0 at a hydraulic retention time of 24h. The Cu and Zn removal efficiencies were >90% at an initial concentration of 25mg/L at both operational pH values; however, high concentrations of Ni (25mg/L) inhibited the sulfate reducing activities. When the influent concentration of each metal was decreased to 10mg/L, the sulfate reducing efficiency in the IFB was restored. Cu and Zn removal in the IFBs were satisfactorily predicted by the MINTEQ model, while large errors in the Ni removal efficiency were noticed, irrespective of the operational conditions, probably because the MINTEQ model does not take into account NiS polymorphism and binding with the organic compounds present in the biogenic effluent.