Acid drainage and metal bioleaching by redox potencial changes in heavy polluted fluvial sediments

N. PORZIONATO; M. MELLOTA; R. CANDAL; G. CURUTCHET

Advanced Material Research

Trans Tech Publications

Año: 2013 p. 496 - 499

1022-6680

Acid Mine Drainage (AMD) is a well documented process and it´s mechanisms are well known. Fluvial sediments contaminated are in some ways similar to a low grade sulphidic ore. The high organic load leads to an anaerobic environment and under this conditions, activity of sulphate reducer bacteria produce high sulphide concentration and alcalinization. The heavy metals precipitate as metal sulphides or hydroxides and are adsorbed in the sediment matrix. Changes in the redox conditions of the stream lead to enhance activity of sulphur and iron oxidizing bacteria with consequent acidification and metal release, which is also observed in AMD processes. In this article we characterize fluvial heavy polluted sediments from Reconquista river basin (a heavy polluted river near Buenos Aires, Argentina) and study the mechanism of acidification and bioleaching that occurred by redox potential changes. Cores of sediments were taken and conserved with unaltered redox condition until its analysis in the laboratory. The cores were cut at several depths and the samples were analyzed as follow. Sequential metal extraction was performed for acid-base balance following Forstner method. Kinetic assays of re-suspension and bioleaching were done in agitated flasks. Native consortia of bacteria with iron reducing and oxidizing bacteria, sulphate and manganese reducing bacteria and sulphur oxidizing bacteria including acidophilic ones were collected from all the samples and characterized by their metabolic activity. The results indicate a high degree of contamination in the sediments. The concentration of Zn, Cu and Cr reaches values higher than the intervention value for sediment quality guidelines in several countries. The acid base balance and kinetic experiments shown high risk of acidification and release of metals in an eventual change of redox potential of the basin (as in a dredging). By the other hand, the same biooxidation process under controlled variables could be used as a tool for decontamination of the sediments and recovery of valuable metals by bioleaching.