Arsenic mineralogy and mobility in the arsenic-rich historical mine waste dump.

A more than 250 year-old mine dump was studied to document the products of long-term arsenopyrite oxidation under natural conditions in a coarse-grained mine waste dump and to evaluate the environmental hazards associated with this material. Using complementary mineralogical and chemical approaches (SEM/EDS/WDS, XRD, micro-Raman spectroscopy, pore water analysis, chemical extraction techniques and thermodynamic PHREEQC-2 modeling), we documented the mineralogical/geochemical characteristics of the dumped arsenopyrite-rich material and environmental stability of the newly formed secondary minerals. A distinct mineralogical zonation was found (listed based on the distance from the decomposed arsenopyrite): scorodite (locally associated with native sulfur pseudomorphs) plus amorphous ferric arsenate (AFA/pitticite), kankite, As-bearing ferric (hydr)oxides and jarosite. Ferric arsenates and ferric (hydr)oxides were found to dissolve and again precipitate from downward migrating As-rich solutions cementing rock fragments. Acidic pore water (pH3.8) has elevated concentrations of As with an average value of about 2.9 mg L(-1). Aqueous As is highly correlated with pH (R2=0.97, p<0.001) indicating that incongruent dissolution of ferric arsenates controls dissolved As well as the pH of the percolating waste solution. Arsenic released from the dissolution of ferric arsenates into the pore water is, however, trapped by latter and lower-down precipitating jarosite and especially ferric (hydr)oxides. The efficiency of As sequestration by ferric (hydr)oxides in the waste dump and underlying soil has been found to be very effective, suggesting limited environmental impact of the mine waste dump on the surrounding soil ecosystems.

Natural attenuation of arsenic in soils near a highly contaminated historical mine waste dump.

Arsenic-contaminated soils near historical As-rich mine waste in Jáchymov (Czech Rep.), resulting from the smelting and seepage of the mine waste pore water, were studied to examine As partitioning between solid phases and pore waters. Mineralogical and geochemical analyses showed that As is exclusively associated with unidentified amorphous Fe oxyhydroxides, poorly crystalline goethite and hematite as adsorbed and coprecipitated species (with up to 3.2 wt.% As). Adsorption of As by Fe oxyhydroxides is likely to be a major control on the migration of As in the soil pore water containing only up to 15 µg L(-1) As(V). The slight variations in the dissolved As(V) concentrations do not follow the total contents of As in the soil or adsorbed As, but appeared to be a function of pH-dependent sorption onto Fe oxyhydroxides. The geochemical modelling using PHREEQC-2 supported the efficiency of As(V) adsorption by Fe oxyhydroxides in the soil affected by As-rich waste solution seepage. It also suggested that active Fe oxyhydroxides has a strong attenuation capacity in soil that could effectively trap the aqueous As(V) from the unremitting waste seepage for the next approx. 11600 years.

Secondary arsenic minerals in the environment: a review.

Information on arsenic (As) speciation in solid materials is critical for many environmental studies concerned with As stability and/or mobility in natural As-impacted soils and mining or industrial sites contaminated by As. The investigation of these systems has provided evidence for a number of secondary As minerals that have often played a significant role in As mobility in the solid phase-water system. This paper presents a list of environmentally important secondary As minerals in contaminated soil and waste systems, summarizes the information about their origin, occurrence, environmental stability and thermodynamics, and proposes several important avenues for further investigation.

Raman microspectroscopy as a valuable additional method to X-ray diffraction and electron microscope/microprobe analysis in the study of iron arsenates in environmental samples.

In this paper, we demonstrate that combined application of X-ray diffraction (XRD), electron microscope/microprobe analysis (EMPA), and Raman microspectroscopy is an available and powerful approach for identification and characterization of iron arsenate minerals in complex environmental samples. Arsenic-rich material from the medieval mining dump close to the Giftkies mine in the Jáchymov ore district (Czech Republic) has been studied. Scorodite, kankite, amorphous iron arsenate (pitticite), and, to a lesser extent, native sulfur were determined in the studied samples as products of low-temperature arsenopyrite weathering. Scorodite and kankite form mixed nodules and crusts, which are locally coated by hardened gel-like amorphous pitticite. Pitticite also borders fractures in the mineralized rock fragments in the dump. Native sulfur, in microscopic crystals and grainy aggregates, originates directly in places with dissolved arsenopyrite and forms pseudomorphs. The Raman spectra presented in the paper can serve as comparative data for phase identification in other contaminated areas. New Raman data for the hydroxyl stretching region of scorodite (important bands: 3514, 3427, and 3600 cm(-1)) and the whole Raman spectrum for pitticite (important bands: 472, 831, 884, 2935, 3091, 3213, 3400, and 3533 cm(-1)) are a valuable output of this paper.

Mineralogical and geochemical controls of arsenic speciation and mobility under different redox conditions in soil, sediment and water at the Mokrsko-West gold deposit, Czech Republic.

Naturally contaminated soil, sediment and water at the Mokrsko-West gold deposit, Central Bohemia, have been studied in order to determine the processes that lead to release of As into water and to control its speciation under various redox conditions. In soils, As is bonded mainly to secondary arseniosiderite, pharmacosiderite and Fe oxyhydroxides and, rarely, to scorodite; in sediments, As is bonded mainly to Fe oxyhydroxides and rarely to arsenate minerals. The highest concentrations of dissolved As were found in groundwater (up to 1141 microg L(-1)), which mostly represented a redox transition zone where neither sulphide minerals nor Fe oxyhydroxide are stable. The main processes releasing dissolved As in this zone are attributed to the reductive dissolution of Fe oxyhydroxides and arsenate minerals, resulting in a substantial decrease in their amounts below the groundwater level. Some shallow subsurface environments with high organic matter contents were characterized by reducing conditions that indicated a relatively high amount of S(-2,0) in the solid phase and a lower dissolved As concentration (70-80 microg L(-1)) in the pore water. These findings are attributed to the formation of Fe(II) sulphides with the sorbed As. Under oxidizing conditions, surface waters were undersaturated with respect to arsenate minerals and this promoted the dissolution of secondary arsenates and increased the As concentrations in the water to characteristic values from 300 to 450 microg L(-1) in the stream and fishpond waters. The levels of dissolved As(III) often predominate over As(V) levels, both in groundwaters and in surface waters. The As(III)/As(V) ratio is closely related to the DOC concentration and this could support the assumption of a key role of microbial processes in transformations of aqueous As species as well as in the mobility of As.

Oxidation of the arsenic-rich concentrate at the Prebuz abandoned mine (Erzgebirge Mts., CZ): mineralogical evolution.

Ore concentrate with up to 65 wt.% of arsenic (by-product of cassiterite extraction) exposed to climatic conditions was studied from the mineralogical point of view. Detailed sampling, X-ray diffraction analyses, energy-dispersive microanalysis (EDAX) and especially scanning electron microscopy (SEM) were applied to study the arsenopyrite-löllingite-concentrate weathering. The studied concentrate contains very small proportion (<5 vol.%) of gangue minerals such as quartz and feldspars; the oxidation of arsenopyrite and löllingite (and accessory pyrite) is thus practically not complicated by interference with additional minerals and elements. Arsenolite, scorodite, kaatialaite and native sulphur were found to be the main secondary phases originating by dissolution of arsenopyrite and löllingite. New secondary phases precipitate on the surface of the ore-concentrate body but also form cement among the grains of finely milled material. The following succession of secondary minerals was determined: arsenolite, scorodite+native sulphur and kaatialaite. Significant arsenic migration into the proximal environment was revealed: 2580 and 13,622 mgkg(-1) were the highest arsenic concentrations in two sections excavated at distances of 0.5 and 1.5 m from the concentrate body.