Fractionation and leaching of Cd, Cu, Fe, Pb, and Zn from smelter residues of an old environmental liability in Argentina.

An integrated chemical and mineralogical characterization approach was applied to smelter wastes collected from 50-year-old dump sites in Argentina. Characterization included pseudo-total element concentrations, acid generation/neutralization potential, sequential extractions, pH-dependent leaching kinetics, and mineralogical analysis of all residues. These analyses provided detailed information on the reactivity of the minerals in the waste material and associated metal release. Cadmium and Zn were the elements of greatest environmental concern due to their high mobility. On average, the release of Zn and Cd in pH-dependent leaching essays reached 17.6% (up to 5.24 mg g-1) and 52.7% (up to 0.02 mg g-1) of the pseudo-total content, respectively. Moreover, Cd and Zn were also the metals that showed the higher proportions of labile fractions associated to the adsorbed and exchangeable fraction (60-92% for Cd and 19-38% for Zn). Since Cd and Zn concentrations in the residue are not high enough to form their own minerals, a large proportion of these elements would be weakly adsorbed on Fe oxyhydroxides. In contrast, the low release of Cu, Pb and Fe would be associated with these elements being incorporated into the crystalline structure of insoluble or very poorly soluble minerals. Lead is incorporated into plumbojarosite and anglesite. Copper was mainly in association with Fe oxyhydroxides and may also have been incorporated into the plumbojarosite structure. The latter could act as a sink especially for Pb under the acidic conditions of the smelter residue. Despite the elevated concentrations of Pb observed in the residue, it showed a very low mobility (≈0.1%), indicating that it is mostly stabilized. Nevertheless, the smelter residue is a continuous source of metals requiring remediation.

Chemical and ecotoxicological indicators for evaluating the treatment of coal mining wastes.

This paper consists of the evaluation in regards to the ecotoxicological effectiveness of a treatment applied to a coal mining waste. The treatment consisted of separating the particles based on gravimetric concentration in spirals, generating three fractions: heavy, intermediate, and light, with high, moderate, and low pyrite content, respectively. The intermediate fraction represents the larger disposal volume of the waste on soils. To evaluate the effectiveness of the treatment, metal determination and bioassays Eisenia andrei, Folsomia candida, Lactuca sativa, Daphnia similis, and Raphidocelis subcapitata were applied to the intermediary fraction. To evaluate the toxicity to aquatic organisms, elutriates were generated from the unprocessed waste and the intermediate fraction. The intermediate fraction showed a decrease of metal concentrations compared to the untreated waste. Metal concentrations in the intermediate fraction were below the Brazilian thresholds for soil quality. Avoidance bioassay with E. andrei and germination tests of L. sativa showed no significant effects. The bioassay with F. candida indicated a significant reduction in reproduction at the highest doses used (24% and 50%). Bioassays with D. similis and R. subcapitata revealed a reduction in toxicity of the intermediate fraction compared to the untreated waste. However, the toxicity levels of the intermediate fraction to aquatic organisms still require attention, especially in regards to pH that played a crucial role in the toxicity. Finally, the results suggest that the treatment performed on the coal waste was efficient, even though significant toxicity have still been detected in the treated waste and additional steps are still required for adequate final disposal.

The dynamics of arsenic and copper in solid and aqueous phases in reactive confluences receiving acid drainage: The role of turbidity and particle size.

The fate of suspended solids in aqueous systems enriched with copper (Cu) and arsenic (As) is still poorly understood, especially in mildly acidic streams with natural turbidity. This study integrated field, laboratory, and modeling to determine how turbidity, particle size distribution, and the partition of Cu and As interact in two model river confluences in an Andean watershed (upper Elqui, North-Central Chile). The mildly acidic Toro River (40.4 mgL-1; CuTOTAL>8 mgL-1) was diluted and neutralized at two consecutive confluences, resulting in dissolved As and Cu lower than 0.04 and 0.1 mgL-1, respectively. On-site laser scattering measurements showed that the size of suspended sediments was dominated by ultrafine (d<6 µm) and fine (6200 µm) were not observed, contrasting with other reactive Andean confluences that work as natural coagulation-flocculation reactors. Laboratory mixing experiments with filtered endmembers followed closely the trends observed in the field measurements. SEM observations and thermodynamic calculations, suggested that As-rich amorphous Fe minerals dominated the fine suspended solid inflow (d<15 µm) from the Toro River, while XRD did not reveal significant amounts of crystalline forms of Fe, As, or Cu minerals. Despite fresh precipitates that further associated dissolved As and Cu, the particles from the Toro River grew only slightly after the confluences, thus limiting particle settling potential and a significant metal-(loid)s removal. Consequently, the seasonal variation in the size and chemical nature of suspended solids in acid drainage inflows control the distinct physical and chemical fates of As and Cu after neutralization, as well as hydrodynamic or hydraulic conditions likely also constrain sediment deposition. The combined monitoring of chemical parameters and particle size distributions is a simple and cost-effective method to obtain information about the behavior of metal(loid)s and sediments.

Partitioning of copper at the confluences of Andean rivers.

The fate of copper (Cu) in rivers impacted by acid drainage remains poorly studied in waters with comparatively low Al and Fe concentrations. This work addresses the role of confluences in controlling the physical and chemical fate of Cu in a system with total molar ratio Cu/Al > 0.2 and Cu/Fe > 0.15. Two consecutive confluences were studied in the upper Mapocho watershed, a densely populated basin with intensive mining located in the Chilean Andes. The inflow had acidic conditions with seasonal variations and Cu up to 9 mg L-1. Lability measurements with diffusive gradient in thin films showed that Cu entered as a dissolved labile form. However, downstream from the confluences a higher pH shifted Cu toward nonlabile compounds and solid phases enriched with Cu. Measurements of x-ray absorption spectroscopy of freshly formed particles showed that composition was dominated by sorbed Cu and Cu(OH)2(s) precipitates, with a higher proportion of sorbed Cu downstream from confluences when pH < 5. Particle size distributions (PSD) measured in field showed that downstream from the confluences the total volume and average diameter of the suspended particles grew progressively, with estimated mean settling velocities increasing from 0.3 to 4.2 cm s-1. As a result, 7-30% of the influent Cu was removed from the river flow. These results highlight that shifts in chemical partition and PSDs in river confluences and the hydrodynamic environments at the river reach level control the mobility of Cu in systems with high Cu/Al and Cu/Fe.