Cycling of As, P, Pb and Sb during weathering of mine tailings: implications for fluvial environments

Abstract

The weathering and oxidation of mine tailings has the potential to contaminate water and soil with toxic elements. To understand the mechanisms, extent and products of the long-term weathering of complex Bolivian tailings from the Cerro Rico de Potosí, and their effects on As, Pb, P and Sb cycling, three-year long laboratory column experiments were carried out to model 20 years of dry- and wet-season conditions in the Pilcomayo basin. Chemical analysis of the leachate and column solids, optical mineralogy, X-ray diffraction, scanning electron microscopy, electron probe microanalysis, microscale X-ray absorption near edge structure spectroscopy, Bureau Commun de Référence sequential extraction and water-soluble chemical extractions, and speciation modelling have shown that the weathering of As-bearing pyrite and arsenopyrite, resulted in a loss of 13–29% of the original mass of As. By contrast, Pb and Sb showed much lower mass losses (0.1–1.1% and 0.6–1.9%, respectively) due to the formation of insoluble Pb- and Sb(V)-rich phases, which were stable at the low pH (~2) conditions that prevailed by the end of the experiment. The experiment also demonstrated a link between the cycling of As, Sb, and the oxidation of Fe(II)-bearing sphalerite, which acted as a nucleation point for an Fe-As-Sb-O phase. Phosphorus was relatively immobile in the tailings columns (up to 0.3% mass loss) but was more mobile in the soil-bearing columns (up to 10% mass loss), due to the formation of soluble P-bearing minerals or mobilization by organic matter. These results demonstrate the influence of mine tailings on the mobility of P from soils and on the potential contamination of ecosystems with As, and strongly suggest that these materials should be isolated from fluvial environments.

Arsenopyrite weathering under conditions of simulated calcareous soil

“…S3b). This finding suggests that arsenopyrite weathering reactions can be influenced by forming an Fe arsenate compound associated to accessible preexisting arsenic in the system, as is evident in the continuous accumulation of total arsenic in mining environments (e.g., Craw et al 2002Craw et al , 2003Murciego et al 2011;Kossoff et al 2012), including calcareous systems (e.g., Aguilar et al 2004;Romero et al 2011;Molinari et al 2014). …”

Arsenic-Microbe-Mineral Interactions in Mining-Affected Environments

“…The stability and potential mobility of As in mine wastes is controlled by their uptake in minerals [1,2]. Many of the reactions that result in the dissolution of primary As-bearing minerals, and formation of secondary products, are mediated by prokaryotes (i.e., Bacteria and/or 6 Fe oxyhydroxysulfates Figure 1.…”

“…And (C) Electron microprobe X-ray map of secondary Fe-As-Sb-O phase in solids from Bolivian tailings weathering column experiments. Reprinted with permission from [2], Mineralogical Society of Great Britain & Ireland.…”

“…In these cases, the As is taken up by sorption onto the mineral surface [24], or incorporated into the mineral structure by co-precipitation. The most common of these occurs in mine wastes include Fe oxides and hydroxides (Table 1) [2,5,14,25], and minerals of the jarosite family [4,5,19,25,26]. The dominant form of As in these secondary phases is As(V), but As(III) has been reported in roaster oxide phases [12] (Table 1), in secondary oxide and oxyhydroxide phases (claudetite, manganarsite, Table 1) in an underground gold mine [11] and in products of experiments involving bacteria and acid mine drainage water bacteria-acid mine drainage water experiments [27].…”

“…These are produced by the natural weathering of primary sulfides, or by metal extraction processes (e.g., roasting, [12,13]). Secondary As-bearing minerals containing structural As include Fe arsenates such as scorodite [2] and kankite [14], Fe sulfoarsenates such as tooeleite [14,15], Ca-Fe arsenates such as yukonite ( Figure 1b) and amorphous forms [14,16,17], and Ca-Mg arsenates such as pharmacolite [18]. Arsenic can also be incorporated in trace amounts in a variety of minerals such as Fe, Mn or Al oxides (Figure 1c), hydroxides and sulfates, other hydrous and anhydrous sulfates, phosphates and hydrous silicates [4,5,13,14,[19][20][21][22][23].…”