Metal mobility in acid mine drainage-impacted waters: from rivers to oceans

Abstract

The complete pathway to the oceans for metals contained in acid mine drainages (AMD), from their release to river courses by weathering of sulfide-rich mining wastes, can be elucidated in the Iberian Pyrite Belt (FPI). The Odiel and Tinto rivers drain this vast expanse of massive sulfides, exploited since historical times, discharging a large amount of acidity and pollutants into a common estuary, the so-called Estuary of Huelva. The mixing of acidic river water and alkaline seawater in the estuary leads to a series of geochemical reactions that control the residence time of pollutants along the estuarine transition. This manuscript focuses its attention on that set of geochemical processes that ultimately determine the amount of pollutants that are subsequently transferred to the Atlantic Ocean. In the estuary, a progressive increase in pH is observed from the fluvial to the marine domain due to the water mixing. During the AMD neutralization, flocculation of particulate material occurs: Fe firstly precipitates as schwertmannite and later Al in the form of basaluminite. Other non-conservative elements that are removed from the water column by precipitation processes are Cu, rare earth elements (REE) and Y. The precipitation of schwertmannite produces the retention of As by adsorption at pH below 5.0. However, when schwertmannite particulate matter reach waters with higher pH values, As desorption occurs, which is released back to solution. Arsenic adsorption/desorption processes are associated with the zero-charge point for schwertmannite. Other contaminants such as Zn, Mn, Ni and Co behave conservatively, remaining in solution throughout their estuarine transit and, hence, significantly threatening the environmental conditions of the coastal areas of the Gulf of Cadiz.