Evaluation of metal partitioning and mobility in a sulfidic mine tailing pile under oxic and anoxic conditions.

Mining-influenced water emanating from mine tailings and potentially contaminating surface water and groundwater is one of the most important environmental issues linked to the mining industry. In this study, two subsets of Callahan Mine tailings (mainly comprised of silicates, sulfides, and carbonates) were collected using sealed containers, which allowed keeping the samples under anoxic conditions during transportation and storage. Among the potential contaminants, in spite of high concentrations of Cu, Mn, Pb, and Zn present in the solid mine tailings, only small amounts of Mn and Zn were found in the overlying pore water. The samples were subjected to leaching tests at different reduction-oxidation (redox) conditions to compare metal and S mobilization under oxic and anoxic conditions. It was observed that Cd, Cu, Mn, Pb, S, and Zn were mobilized at higher rates under oxic conditions, while Fe was mobilized at a higher rate under anoxic conditions in comparable constant pH experiments. These results suggest that metal mobilization is significantly impacted by redox conditions. When anoxic metal mobilization assessment is required, it is recommended to always maintain anoxic conditions because oxygen exposure may affect metal mobilization. A sequential extraction performed under oxic conditions revealed that most of the metals in the samples were associated with the sulfidic fraction and that the labile fraction was associated with Mn and moderate amounts of Pb and Zn.

Metal removal efficiency and ecotoxicological assessment of field-scale passive treatment biochemical reactors.

Anaerobic biochemical reactors (BCRs) are useful for removing metals from mining-impacted water at remote sites. Removal processes include sorption and precipitation of metal sulfides, carbonates, and hydroxides. A question of interest is whether BCRs remove aquatic toxicity. Influent and effluent samples from the Luttrell Repository and Peerless Jenny King, both in Montana, USA; Park City, Utah, USA; and Standard Mine, Colorado, USA, were examined and compared for removal of metals and aquatic toxicity. Effluent samples from Standard Mine included those having solely BCR treatment and those having BCR treatment followed by aeration in a polishing cell. Metal removal for all sites was >90%. All influent samples were acutely toxic to Ceriodaphnia dubia and Pimephales promelas; toxicity was removed following treatment, except in the Luttrell Repository and Standard Mine BCR samples. Laboratory aeration of undiluted samples eliminated (Standard Mine BCR) or significantly reduced (Luttrell Repository, 65% survival) acute toxicity, most likely through removal of hydrogen sulfide. A toxicity identification evaluation suggested that metals also might be contributing to toxicity in the Luttrell Repository effluent samples; metals other than Mn were either not detected or very low (Fe and Pb) in the Standard Mine BCR samples. Field-aerated samples were not acutely toxic, and only the Luttrell Repository and Standard Mine samples showed short-term subchronic toxicity. Overall, results indicated BCR treatment had high metal removal efficiency and that inclusion of in-field aeration was beneficial in removal of acute and short-term subchronic toxicity.

Microbial community analysis of two field-scale sulfate-reducing bioreactors treating mine drainage.

The microbial communities of two field-scale pilot sulfate-reducing bioreactors treating acid mine drainage (AMD), Luttrell and Peerless Jenny King (PJK), were compared using biomolecular tools and multivariate statistical analyses. The two bioreactors were well suited for this study because their geographic locations and substrate compositions were similar while the characteristics of influent AMD, configuration and degree of exposure to oxygen were distinct. The two bioreactor communities were found to be functionally similar, including cellulose degraders, fermenters and sulfate-reducing bacteria (SRB). Significant differences were found between the two bioreactors in phylogenetic comparisons of cloned 16S rRNA genes and adenosine 5'-phosphosulfate reductase (apsA) genes. The apsA gene clones from the Luttrell bioreactor were dominated by uncultured SRB most closely related to Desulfovibrio spp., while those of the PJK bioreactor were dominated by Thiobacillus spp. The fraction of the SRB genus Desulfovibrio was also higher at Luttrell than at PJK as determined by quantitative real-time polymerase chain reaction analysis. Oxygen exposure at PJK is hypothesized to be the primary cause of these differences. This study is the first rigorous phylogenetic investigation of field-scale bioreactors treating AMD and the first reported application of multivariate statistical analysis of remediation system microbial communities applying UniFrac software.

Statistical validation of sulfate quantification methods used for analysis of acid mine drainage.

Turbidimetric method (TM), ion chromatography (IC) and inductively coupled plasma atomic emission spectrometry (ICP-AES) with and without acid digestion have been compared and validated for the determination of sulfate in mining wastewater. Analytical methods were chosen to compare the performance of a portable field turbidimetric instrument and to validate the underlying assumption utilized in conversion of total sulfur to sulfate during ICP-AES analysis. Accuracy and precision of analytical techniques were compared to one another using control and field samples collected from a mine site using the Bonferroni multiple comparison test. Effects of sample dilution, filter pore size and acidification on sulfate quantification were also studied. The results showed that IC and ICP-AES with and without acid digestion provided excellent recoveries in the case of control samples (within 90-110%). These analytical methods also showed lower relative standard deviation for both control and field samples. On the other hand, performance of the turbidimetric method was severely affected by sample dilution and acidification, and also revealed poor sulfate recoveries for control samples ranging from 0 to 83.5%. Analysis of variance (ANOVA) was used to evaluate the response (sulfate concentration) obtained from factorial design. Analytical method had significant effect (P<0.0001) on the sulfate quantification. The interaction between determination method and sample dilution was more significant than other two-way interactions.