Geochemical and mineralogical control on the mobility of arsenic in a waste rock pile at Dlouhá Ves, Czech Republic

Abstract

A waste rock pile with initial high sulfide (10–20 wt.%) and low carbonate content (1–2 wt.%) located at Dlouhá Ves in the Czech Republic has been investigated in two profiles (excavation and outcrop) using powder X-ray diffraction, electron microprobe analysis, bulk composition analysis and Mössbauer spectroscopy. The mobility of arsenic and other contaminants was evaluated by leaching experiments. The primary source of the arsenic was arsenopyrite, which was significantly oxidized in both profiles. The principal As-bearing phase at the excavation profile was goethite, located at the top of the profile, and minerals of the jarosite group which were found down to its base. Melanterite, rich in copper and zinc, was found in a sulfide-rich, lower part of the profile together with anglesite. At the outcrop profile, minerals of the jarosite–beudantite group, scorodite and kaňkite prevail and no Fe(II)-minerals were found. The paste pH was lower at the excavation profile (minimum about 1.9) than at the outcrop profile (minimum of about 2.8). Processes in the pile are affected by the pyrite/arsenopyrite ratio, where high pyrite content decreases the As/S ratio and results in the formation of jarosite group minerals and low pH conditions. Where arsenopyrite predominates, sulphides are coated by scorodite and other Fe–As phases like schwertmannite, which limit their further oxidation. Arsenic concentrations released during the leaching experiments were generally low; maximum amounts (up to 0.56 ppm) were released from horizons with jarosite and arsenopyrite. In contrast, minimum amounts of arsenic were released from horizons with beudantite and scorodite. Melanterite found at the excavation profile was fully soluble, releasing copper and zinc into the solution. Differences between both profiles are caused especially by the different degrees of water flow through the pile material and the limited penetration of oxygen into the deep parts of the excavation profile. It seems that beudantite and scorodite represent a long-term option for immobilization of arsenic, but arsenic stored in jarosite can be mobilized relatively easily. Highly soluble Fe(II) minerals such as melanterite may represent an environmental problem. However, potential mineralogical transformations and stability of arsenic in beudantite group minerals have yet to be evaluated.