Detection and Characterization of ultrafine Fe-As-Pb colloids in acid rock drainage solution from an ore mine

The processes of sulfide oxidation, water acidification and water mineralization in abandoned ore mines is closely associated with the existence of gangue fissures in the host rock that contain clay minerals and finely-divided sulfide ores. These fis-sures release highly mineralized, red-colored acid rock drainage (ARD) solutions which can easily be collected from pools in front of the fissures. ARD solution from an abandoned Zn-Pb-Ag mine at Freiberg, Germany, (pH 2.7, sulfate concentration 411 mmol/l, Fe concentration 93,5 mmol/l) was investigated by photon correlation spec-troscopy, centrifugation, filtration, ultrafiltration, scanning electron microscopy, ICP-MS, AAS, ion chromatography, TOC analysis, and X-ray absorption fine structure (EXAFS) spectroscopy. The question was if this ARD solution contains colloidal par-ticles of the lower nanometer range or if it is particle-free after the common filtration through a 450-nm filter. We found that the solution contains a small amount (about 20 mg/l) of submicron particles of about 100 nm in size. However, the major colloidal component was shown to be a population of ultrafine particles of less than 5 nm. The concentration of these particles is about 1 g/l. They consist of Fe, As and Pb com-pounds. According to EXAFS spectroscopy, the most probable mineralogical compo-sition of these particles is a mixture of hydronium jarosite (HFe3(SO4)2(OH)6) and schwertmannite (ideally Fe8O8(OH)6SO4). We also observed the formation of a rela-tively coarse precipitate of a similar mineralogy in the colloidal solution over a time span of months. The ultrafine colloids are obviously an intermediate in the formation process of the long-term precipitate. The arsenic is probably bound onto the ultrafine colloidal particles as a bidentate binuclear arsenate surface complex (inner-sphere complex). However, the transformation of the colloids to the more aggregated long-term precipitate leads to the incorporation of the arsenic into the interior of the iron hydroxy sulfate structures. The mechanism of this incorporation could be the substi-tution of arsenate for sulfate in the jarosite structure and/or the formation of very small scorodite clusters as occlusions within the oxyhydroxysulfate structures. The lead might occur as anglesite (PbSO4).