Prof. Bernhard Dold UNIL-CAM (Main Applicant), Prof. Christof Holliger, Dr. Pierre Rossi, Nouhou Diaby
Southern Peru Copper Corporation
September 2005 – August 2008
Ezio Buselli and Rodolfo Vicetti, Environmental Services, Southern Peru Copper Corporation, Ilo, Moquegua, Peru.
Mining activities produce enormous amounts of waste material known as tailings which are composed of fine to medium size particles. The tailings often contain sulfides which oxidation lead to acid and metal contamination of water; hence the need for remediation. In this work a tailings bioremediation was studied by an interdisciplinary approach including geochemistry, mineralogy and microbiology. The aim of the work was to study the effect of the implementation of wetland above oxidizing tailings on the hydrogeology and the biogeochemical cycle, and to assess the system evolution over time.
To reach these goals, processes occurring in a marine shore tailings deposit were investigated. The studied tailings deposit is located at the Bahìa de Ite, Pacific Ocean, southern Peru, where between 1940 and 1996 the tailings were discharged from the two porphyry copper mines Cuajone and Toquepala. After the end of deposition, a remediation approach was initiated in 1997 with a wetland implementation above the oxidizing tailings. Around 90% of the tailings deposits (total 16 km2) were thus treated, except the central delta area and some areas close to the shoreline.
The isotopic study showed that the tailings were saturated with fresh water in spite of the marine setting, due to the hydraulic pressure resulting from the wetland implementation. The hydrogeology controls the seawater limitation in the tailings water body. A submarine groundwater discharge (SGD) was flowing at the tailings-former seabed interface and was a source of SO42-, Cl–, Na+, Fe2+, and Mn2+ input into the tailings, due to local geological setting.
The aquatic geochemistry analysis, the X-Ray diffraction (XRD), and the sequential extractions showed that iron and sulfur oxidation were the main processes in the non-remediated tailings which had a top low-pH oxidation zone with strong accumulation of efflorescent salts at the surface due to capillary upwards transport of heavy metals (Fe, Cu, Zn, Mn, Cd, Co and Ni) from solution in the arid climate. These analyses showed also that the implementation of the wetland resulted in very low concentrations of heavy metal in solution (mainly under the detection limit) due to the near neutral pH and more reducing conditions. The heavy metal ions that decreased in solution precipitated in the mineral phase as hydroxides and sulfides or were bound in the organic matter.
The bacterial community composition analysis by Terminal Restriction Fragment Length Polymorphism (T-RFLP) and cloning and sequencing of 16S rRNA genes combined with a detailed statistical analysis revealed a high correlation between the bacterial distribution and the geochemistry. Acidophilic autotrophic oxidizing bacteria were dominating the oxidizing tailings whereas neutrophilic and heterotrophic reducing bacteria were driving the processes in the remediated tailings below the wetland. At the subsurface of the remediated tailings, an iron cycling was highlighted with oxidation and reduction processes thanks to micro-aerophilic niches provided by the plant rhizosphere in this overall reducing environment.
The in situ bioremediation experiment showed that the main parameter to take into account for the effectiveness was the water table and chemistry which controls the system. The constructed remediation cells were more efficient and rapid in metal removal when saturation conditions were available. The effect of the remediation was slower when the tailings saturation could not be achieved. The metal removal mechanisms observed were Fe(III)hydroxide precipitation/co-precipitation and sulfide precipitation. The change in geochemistry was highly correlated to the bacterial community structure which shifted according to the evolution of the geochemical conditions. This study showed that the bioremediation by wetland implementation can be an effective and rapid treatment for copper mine tailings deposits. However, the water saturation of the tailings has to be managed on a long-term basis in order to guarantee stability of the no-impact situation reached.