Geochemical and mineralogical characterization of the arsenic-, iron-, and sulfur-rich mining waste dumps near Kank, Czech Republic
Abstrakt
The role of secondary minerals in controlling the migration of arsenic and selected metals (Cu, Pb, Zn) has been investigated in voluminous mining waste dumps at Kank (northern part of the Kutna Hora ore district, Czech Republic). These wastes initially contained a large amount of Fe sulfides and arsenopyrite and have been exposed to atmosphere for approximately 500 years.
The long-term weathering under acidic conditions caused dissolution of almost all sulfides and significant alteration of primary silicate minerals, producing deeply weathered As-, Fe-, and S-rich waste material (As: 13 g kg(-1); Fe: 74 g kg(-1); S: 44gkg-1) with large proportion of clayey matrix filling the space between rock fragments. Detailed mineralogical investigation (powder X-ray diffraction, electron microprobe, Raman spectroscopy) and sequential extraction revealed that arsenic in these wastes is principally stored in reactive but not highly soluble minerals such as poorly-crystalline Fe (oxyhydr) oxides, bukovskyite, and X-ray amorphous hydrated ferric arsenate (HFA).
Lesser fraction of As is contained in schwertmannite and less reactive scorodite, jarosite, and well-crystalline Fe (oxyhydr) oxides (especially goethite). Fe (oxyhydr) oxides also appear to be the main reservoirs of Cu and Zn; Pb is principally stored in jarosite.
Acidic pore waters (pH approximate to 2.8) collected after rainfall events contain much Al (113 mg L-1), As (2.5 mg L-1), Ca (500 mg L-1), Cu (24 mg L-1), Fe (58 mg L-1), Si (44 mg L-1), SO42-(2170 mg L-1) and Zn (16 mg L-1). Geochemical modeling revealed that most of these elements (Al, Ca, Cu, SO42-, Zn) are controlled by ephemeral Al-Ca-Cu-Zn sulfates (e. g. gypsum, aluminite), which form at the surface of the waste dumps as a result of evaporation of pore solutions during dry seasons.
Despite the fact that bukovskyite and other interesting Fe(III) (sulfo) arsenate minerals store a major part of As in the mining wastes, geochemical modeling supported the notion that mobility of As and Fe is controlled by the unspectacular poorly-crystalline As-bearing Fe (oxyhydr) oxides. The data show that the Fe(III) (oxyhydr) oxides, (sulfo) arsenates and hydroxysulfates in the 500-year-old mining waste dump retain arsenic efficiently but not completely.
Monitoring wells installed at the site before our research recorded up to 1440 mg L-1 As in shallow groundwater. They argue, however, that this aquifer is disconnected from the larger groundwater bodies at the site and hence does not represent arsenic accumulation of environmental concern.