Precipitação de metais provenientes de drenagem ácida de minas por sulfeto gerado por via biológica

Abstract

Sulfide minerals exposed to atmospheric conditions may cause impacts. The tailings from mining activity, reduced metals in contact with oxygen and water result in the release of protons, sulfate ions and metals in aqueous environment, a process accelerated by microorganisms. The designation for this effluent is Acid Mine Drainage (AMD). Bioremediation has been investigated for the treatment of this effluent. Sulfate-reducing bacteria enable simultaneous pH elevation, sulfate removal and consequently metals removal by precipitation as metal sulfides. Previous published researches reveal satisfying results, although few have investigated simultaneously the generation of alkalinity, sulfate reduction, oxidation of ethanol and removal of metals in conditions that gradually approach the real conditions of DAM. This research investigated the process of precipitation of the metals iron, zinc and copper by sulfide generated from a biological batch reactor for sulfate reduction of acid mine drainage, having the ethanol as electron donor. The study was divided into two stages, the first of which had variation in pH and the second, in the concentration of metals. It was observed that regardless of the initial pH, alkalinity generation has occurred and the final pH observed in all cycles remained between 6 and 7. The decline of the initial pH was directly related to the best sulfate removal rates and the achieved removals 39, 44 and 52%, for the initial pH of 6, 5 and 4, respectively. The sulfate removal was also improved as increased the concentration of metals. While there was only iron, sulfate removal was 57% on average; with iron and zinc, it was 61%; and with iron, zinc and copper removal reached 82% on average. In all conditions the COD removal was high, with at least 91% average and reaching 100%. The removals of metals reached 92% on average for iron, 99% for zinc and 100% for copper. The kinetic parameter (k) varied according to different values ??of the sulfate removal. In the initial phase, in which there was adaption of the inoculum, it did not represent well the behavior of the reactor, reflecting instability in this phase of the operation (0.14 ± 0.15) h-1. As the initial pH decays, the kinetic adjustment of order 1 represents well the behavior of the data and the parameter k increases considerably; plus, when all the metals had been added, the highest value was obtained (0.44 ± 0.04) h-1. As for the iron, the kinetic parameters obtained indicated that the value of k was reduced when the precipitation of iron competed with the presence of zinc and copper. The evaluation of sulfate in this configuration of biological reactor allowed us to establish favorable conditions of operation and showed that sulfetogenic systems respond better at low pHs and in the presence of certain metals, as the conditions of the synthetic DMA approach the actual ones. This condition is essential when one aims to use sulfetogenic systems for bioremediation of acid mine drainage.