Geochemistry of acidic Rio Tinto headwaters and role of bacteria in solid phase metal partitioning

Artikel i vetenskaplig tidskrift, 2004

Mining operations in the Rio Tinto area, southwest Spain, have created a historical pollution problem in the river and estuary. The fate of metals along the Rio Tinto river and estuary has been previously characterized, but little is known about transport and behaviour of metals and other elements in the headwaters of the river. The headwaters are situated approximately 100 km (north east) from Huelva at the Mediterranean Sea, with three investigated sites in this area distributed over 100 m. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) of aqueous and solid samples revealed Ca, Fe, Mg, Na, S, Si, and Al concentrations above 0.5 mM in the stream surface waters at all sites. Trace elements included K, Cu, Li, Mn, and Zn. Concentrations of particulate elements generally accounted for less than 10% of the total load. In deeper pools along the stream, brine water was found with notably higher levels of Fe, Mg, S, Si, and Al compared to the stream water. The brine had a pH of 1.9, while the surface stream waters had pH values of about 2.7. Groundwater from the seep on the bank of the stream was found to be chemically distinct from both surface stream water and the brine. Fe, S and Al were in the solid samples at 100 mmol/kg or more. K, Na, P, Si, As, Ba, Cu, Li, and Mn were characterized by intermediate concentrations, 1.0–100 mmol/kg. A progressive downstream increase in particulate elemental concentrations was observed for P, Ba, Mn, Pb, Sr, V and Zn. Scanning electron microscopy (SEM) showed a variety of bacterial-like forms among the suspended particulates. In some instances, the bacteria appeared to be closely associated with or mineralized by inorganic precipitates. Transmission electron microscopy (TEM) revealed extensively mineralized bacterial cells in the solid sediment samples. The chemical composition of the investigated stream is attributed to inputs from groundwater and brines that likely evolve in response to evaporation during prolonged periods of moisture deficit in the summer. Planktonic bacteria were prominent components of the suspended particulate fraction in the stream; however, pH sorption edges for dissolved metals on bacteria are generally above pH 4.0, and therefore, it is unlikely that bacterial cells are an important component for transport of dissolved metals in the stream. Bottom sediments contain abundant highly mineralized bacteria with coatings of poorly crystalline iron oxyhydroxides and hydroxysulfate precipitates. Such bacterial–mineral composites likely contribute to increased solid phase partitioning of dissolved metals in the Tinto river system.