Oral bioaccessibility of metal(loid)s in dust materials from mining areas of northern Namibia.

Ore mining and processing in semi-arid areas is responsible for the generation of metal(loid)-containing dust, which is easily transported by wind to the surrounding environment. To assess the human exposure to dust-derived metal(loid)s (As, Cd, Cu, Pb, Sb, Zn), as well as the potential risks related to incidental dust ingestion, we studied mine tailing dust (n = 8), slag dust (n = 5) and smelter dust (n = 4) from old mining and smelting sites in northern Namibia (Kombat, Berg Aukas, Tsumeb). In vitro bioaccessibility testing using extraction in simulated gastric fluid (SGF) was combined with determination of grain-size distributions, chemical and mineralogical characterizations and leaching tests conducted on original dust samples and separated PM10 fractions. The bulk and bioaccessible concentrations of the metal(loid)s were ranked as follows: mine tailing dusts < slag dusts ≪ smelter dusts. Extremely high As and Pb bioaccessibilities in the smelter dusts were caused by the presence of highly soluble phases such as arsenolite (As2O3) and various metal-arsenates unstable under the acidic conditions of SGF. The exposure estimates calculated for an adult person of 70 kg at a dust ingestion rate of 50 mg/day indicated that As, Pb (and also Cd to a lesser extent) grossly exceeded tolerable daily intake limits for these contaminants in the case of slag and smelter dusts. The high risk for smelter dusts has been acknowledged, and the safety measures currently adopted by the smelter operator in Tsumeb are necessary to reduce the staff's exposure to contaminated dust. The exposure risk for the local population is only important at the unfenced disposal sites at Berg Aukas, where the PM10 exhibited high levels of bioaccessible Pb.

Transformation of arsenic-rich copper smelter flue dust in contrasting soils: A 2-year field experiment.

Dust emissions from copper smelters processing arsenic-bearing ores represent a risk to soil environments due to the high levels of As and other inorganic contaminants. Using an in situ experiment in four different forest and grassland soils (pH 3.2-8.0) we studied the transformation of As-rich (>50 wt% As) copper smelter dust over 24 months. Double polyamide bags with 1 g of flue dust were buried at different depths in soil pits and in 6-month intervals; then those bags, surrounding soil columns, and soil pore waters were collected and analysed. Dust dissolution was relatively fast during the first 6 months (5-34%), and mass losses attained 52% after 24 months. The key driving forces affecting dust dissolution were not only pH, but also the water percolation/retention in individual soils. Primary arsenolite (As2O3) dissolution was responsible for high As release from the dust (to 72%) and substantial increase of As in the soil (to a 56 × increase; to 1500 mg kg-1). Despite high arsenolite solubility, this phase persisted in the dust after 2 years of exposure. Mineralogical investigation indicated that mimetite [Pb5(AsO4)3(Cl,OH)], unidentified complex Ca-Pb-Fe-Zn arsenates, and Fe oxyhydroxides partly controlled the mobility of As and other metal(loid)s. Compared to As, other less abundant contaminants (Bi, Cu, Pb, Sb, Zn) were released into the soil to a lesser extent (8-40% of total). The relatively high mobility of As in the soil can be seen from decreases of bulk As concentrations after spring snowmelt, high water-extractable fractions with up to ∼50% of As(III) in extracts, and high As concentrations in soil pore waters. Results indicate that efficient controls of emissions from copper smelters and flue dust disposal sites are needed to prevent extensive contamination of nearby soils by persistent As.

Characterization and pH-dependent environmental stability of arsenic trioxide-containing copper smelter flue dust.

Increasing amounts of impurities (especially As) in Cu ores have aggravated the problem of flue dust generation in recent years. As an example from a smelter processing As-rich Cu ores, we characterized a flue dust particularly rich in As (>50 wt%) to understand its mineralogy and pH-dependent leaching behavior, with special emphasis on binding, release and solubility controls of inorganic contaminants (As, Bi, Cd, Cu, Pb, Sb, Zn). Whereas arsenolite (As2O3) was the major host for As and Sb, other contaminants were bound in sulfides, arsenates, alloys and slag-like particles. The EU regulatory leaching test (EN 12457-2) indicated that leached As, Cd, Sb and Zn significantly exceeded the limit values for landfills accepting hazardous waste. The pH-dependent leaching test (CEN/TS 14997) revealed that As, Sb and Pb exhibited the greatest leaching at pH 11-12, whereas Cd, Cu and Zn were leached most under acidic condition (pH 3) and Bi leaching was pH-independent. Mineralogical investigation of leached residue coupled with geochemical modeling confirmed that newly formed Ca, Pb and Ca-Pb arsenates (mimetite, Pb5(AsO4)3Cl) partly control the release of As and other contaminants under circumneutral and alkaline conditions and will be of key importance for the fate of smelter-derived contamination in soils or when stabilization technology is employed.

Copper isotopic record in soils and tree rings near a copper smelter, Copperbelt, Zambia.

The copper (Cu) content and isotopic composition were studied in soils and in pine tree rings at locations close to and far from the Cu smelter, located at Kitwe, Zambia. The soil in the remote area contained 25-75mgkg-1 Cu, whereas the soil close to the smelter contained 207-44,000mgkg-1 Cu. The δ65Cu at the remote area and close to the smelter varied in the range -0.40 to -0.11‰, and -0.44 to 0.01‰ respectively. The δ65Cu of the surface soil at both profiles (-0.44 to -0.40‰) is similar to the isotopic composition of the concentrates processed in the smelter (-0.75 to -0.45‰), i.e. both locations are affected by Cu ore dust. The increase in the δ65Cu in the direction towards the centre of the profile is caused by the oxidative dissolution of Cu(I) from ore minerals, during which heavier Cu is released. In deeper parts of the profile, there is a slight decrease in δ65Cu because of easier mobilisation of the lighter isotope. The tree rings at the two locations differ in the total contents and isotopic composition. At the less contaminated site, the Cu contents equal 0.4 to 1.1mgkg-1 while, at the polluted site, the Cu contents vary in the range 3 to 47mgkg-1. Whereas, at the less contaminated location, the tree rings are substantially enriched in lighter Cu (δ65Cu=-0.76 to -2.2‰), at locations close to the smelter the tree rings have an isotopic composition (-0.31 to -0.88‰) similar to that of the contaminated soil or processed ore. The isotopic compositions of the tree rings close to the smelter are affected particularly by interception of dust containing Cu ore. The δ13C in tree rings demonstrate the interconnection of acidification and Cu mobility.

The pH-dependent leaching behavior of slags from various stages of a copper smelting process: Environmental implications.

The leaching behaviors of primary copper (Cu) slags originating from Ausmelt, reverbatory, and converter furnaces operating under a single technological process were compared to a residual slag tailing obtained by slag re-processing via flotation and metal recovery. The EN 12457-2 leaching test, used for assessment of the hazardous properties, was followed by the CEN/TS 14997 pH-static leaching test (pH range 3-12). Both leaching experiments were coupled with a mineralogical investigation of the primary and secondary phases as well as geochemical modeling. Metals (Cd, Cu, Pb, Zn) exhibit the highest leaching at low pH. Under acidic conditions (pH 3-6), Ausmelt slag and slag tailing exhibited higher metal leaching compared to other slag types. Very low leaching of metals (far below EU limits for non-hazardous waste) was observed at natural pH (7.9-9.0) for all the studied slag samples. In contrast, relatively high leaching of As was observed over the entire pH range, especially for Ausmelt slag (exceeding the EU limit for hazardous waste by 1.7×). However, geochemical modeling and scanning electron microscopy indicated that formation of stable Ca-Cu-Pb arsenates and the binding of As to newly formed Fe (oxyhydr)oxides play an important role in efficient As immobilization at the slag-water interface. In contrast, no controls were predicted for Sb, whose leaching was almost pH-independent. Nevertheless Sb leached concentrations at natural pH were below EU limit for hazardous waste. Re-processing of primary Cu slags for metal recovery, and subsequent co-disposal of the resulting slag tailing with dolomite-rich mine tailing and local laterite is suitable for stabilizing the remaining contaminants (except Sb) and limiting their leaching into the environment.