Microbial Fe(II) oxidation and heavy metal co-precipitation in the Rio Tinto region, Spain
Mineralogy, Petrology and Geochemistry
Final Report Abstract
The Rio Tinto river region in the province of Huelva, Spain, is of great interest as it is one of the most polluted fluvial-estuarine systems in the world. Open-pit mines located at the headwaters of the river are subject to high levels of weathering, controlled by acidophilic microorganisms. The biooxidation of sulfide ores is accompanied by the release of S (2-16 g L^-1), Fe (0.2-20 g L^-1) and many other metals such as Сu (3.7 mg L^-1), K (9.2 mg L^-1), Mg (445 mg L^-1), Na (12.6 mg L^-1), Ca (70 mg L^-1), Co (3.7 mg L^-1), Cr (0.6 mg L^-1), Zn (116.2 mg L^-1) and Pb (0.1 mg L^-1). The fate of these dissolved, and in many cases toxic, elements over a 90-km stretch of the river from the headwaters to the estuary is poorly understood. Nevertheless, it is known that the extremely contaminated water of the Rio Tinto can transport up to 0.8% and 3.7% of global gross flux of dissolved Cu and Zn into the World Ocean. The present study revealed the impact of the Fe(II)-oxidizing microbial community on the fate of heavy metals (HMs) in the Rio Tinto river. We demonstrated that Acidithiobacillus spp. Leptospirillum spp. were the main microorganisms responsible for Fe(III) formation in the acidic water column of the Rio Tinto. The pH increase caused by microbial oxidation of Fe(II) and oversaturation of Fe(III) led to precipitation of schwertmannite and ferrihydrite as well co-precipitation of Al, Pb, As, Zn, Cu, Cr, Co, Cd. Mineral particles remained suspended due to their small size, low density and river velocity. In vitro experiments proved that microbial oxidation of Fe(II) led to i) formation of cell-mineral aggregates similar to those that have been identified in situ and ii) co-precipitation of HMs (e.g. As, Pb, Cr etc.). The nucleation of schwermannite, ferrihydrite and goethite occurred in proximity to the cell surfaces, supposedly at the hot-spots of Fe(II) oxidation (e.g. cytochrome c) or at the EPS. Therefore, Fe(II)-oxidizers can play a key role in immobilization of As, Cr, Pb, Al, Co and Cd at the water column of the river. Sorption of HMs is probably a less important process due to the electrostatic repulsion between the HM cations and the positive charge on the particle/cell surfaces, and/or due to the protonation of adsorption sites at low pH. Thus, microbial oxidation of Fe(II) in the northern region of the river lead to advection of SPM-associated HMs to the river estuary. Furthermore, we demonstrated for the first time that at the upper estuary, daily pH and chloride fluctuations contributed to a dominance of Acidihalobacter spp., as well as Marinobacter spp. and Mariprofundus spp. supposedly acting as the main drivers of Fe(II) oxidation. Fe(III)-reducers were identified as Acidiphilium spp. and Acidibacter spp. and Acidobacterium spp. Additionally, our data suggests that Fe cycle in the estuarine sediment can be operated not only exclusively by acidophilic microorganisms but also typically neutrophilic Fe(II)-oxidizing Marinobacter spp., Mariprofundus spp. and unclassified Gallionellacea as well neutrophilic Fe(III)-reducing Thermoanaerobaculum spp., Geobacter spp., Geothrix spp., unclassified Deferribacteraceae and SRB. Thus the upper estuary of the Rio Tinto can be considered as unique environments where very different groups of microorganisms can meet and interact for example acidophilic and neutrophilic Fe(II)-oxidizers (Acidihalobacter spp. and Marinobacter spp.) or marine and freshwater microaerophilic Fe(II)-oxidizers (Mariprofundus spp. and unclassified Gallionellacea). In summary, our study revealed that although a part of the SPM continuously settled down along the riverbed and led to burial of HMs in the sediment, another part of the SPM remained in the water column and was transported via the river flow to the Atlantic Ocean. For example, we demonstrated that up to 100% of the As and Cr load could enter the coastal sediment in the SPM-associated state.
Publications
- (2020) Role of biogenic Fe(III) minerals as a sink and carrier of heavy metals in the Rio Tinto river, Spain. Science of the Total Environment 718, 137294
Abramov, S., Tejada, J., Grimm, L., Schädler, F., Bulaev, A., Tomaszewski, E., Byrne, J., Straub, D., Thorwarth, H., Amils, R., Kleindienst, S., Kappler, A.
(See online at https://doi.org/10.1016/j.scitotenv.2020.137294)
