Detection and characterization of colloids in acid rock drainage solution from an ore mine - Application of X-ray Absorption Spectroscopy (XAS)

In the aqueous environment of abandoned ore mines, iron sulfide ore crystals em-bedded in clay play a key role in the sulfide oxidation process and water mineraliza-tion process occurring there. Highly mineralized, red-coloured solutions are formed by weathering of ore particles which causes acid rock drainage (ARD) that can be extremely acidic. The access of ARD solutions to the bulk waters of a mine is a major factor influencing the composition of mining effluents. Very little is known about the speciation of heavy and toxic metals and about the mineralogy of the colloidal parti-cles of such ARD solutions. In particular the colloid-chemistry of such waters is little understood. However, colloids can significantly influence the properties of ARD solu-tions.
Acid rock drainage (ARD) solution from an abandoned ore mine (pH 2.7, sulfate con-centration 411 mMol/L, Fe concentration 93.5 mMol/L) was investigated by photon correlation spectroscopy, centrifugation, filtration, ultrafiltration, scanning electron microscopy, ICP-MS, AAS, ion chromatography, TOC analysis, and X-ray absorption (XAS/EXAFS) spectroscopy. A colloid concentration of m 1 g/L was found. The pre-vailing particle size was < 5 nm. Fe, As and Pb were the metal constituents of the colloidal particles. According to EXAFS spectroscopy, the most probable mineralogi-cal composition of these particles is a mixture of hydronium jarosite (HFe3(SO4)2(OH)6) and schwertmannite (ideally Fe8O8(OH)6SO4). A small amount of a relatively coarse precipitate was formed in the ARD solution during the months af-ter sampling. The colloid particles are obviously an intermediate in the precipitate formation process. Our results suggest that the arsenate is bound to the colloid parti-cles by the formation of a bidentate binuclear inner-sphere surface complex. How-ever, the transformation of the colloidal material to the more aggregated long-term precipitate results in the incorporation of the arsenate into the interior of the iron hy-droxy sulfate crystal structures. Pb seems to occur as anglesite (PbSO4).