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Figure 10. Mean particle size and ¢ potential of water collected from the kinetic column containing the waste rock from the Cordero Rojo Mine.

Figure 10 Mean particle size and ¢ potential of water collected from the kinetic column containing the waste rock from the Cordero Rojo Mine.

Related Figures (14)

Figure 1. Possible trend of solute release with weathering of fresh waste rock (modified from Langman et al. [18]).
Figure 1. Possible trend of solute release with weathering of fresh waste rock (modified from Langman et al. [18]).
Figure 2. Powder River Basin, Gillette Coal Field, and Cordero Rojo Mine, Wyoming, USA (modified from Martin and Langman [31]). potential new sources of As and Se in the waste rock through a comparison of kinetic (forward or predictive) models and kinetic column results from the weathering of the waste rock from the Cordero Rojo Mine. The output of the mineral weathering models is used to highlight the difficulty of predicting waste rock weathering from known contaminant sources present in the geologic state of the overburden.
Figure 2. Powder River Basin, Gillette Coal Field, and Cordero Rojo Mine, Wyoming, USA (modified from Martin and Langman [31]). potential new sources of As and Se in the waste rock through a comparison of kinetic (forward or predictive) models and kinetic column results from the weathering of the waste rock from the Cordero Rojo Mine. The output of the mineral weathering models is used to highlight the difficulty of predicting waste rock weathering from known contaminant sources present in the geologic state of the overburden.
Figure 3. Removal of overburden and waste generation during open-pit coal mining at the Cordero Rojo Mine, Powder River Basin, Wyoming, USA (Martin and Langman [31)).
Figure 3. Removal of overburden and waste generation during open-pit coal mining at the Cordero Rojo Mine, Powder River Basin, Wyoming, USA (Martin and Langman [31)).
Figure 4. Reconstructed landscape adjacent to the active Cordero Rojo Mine, Powder River Basin, Wyoming, USA. The coal pit has been filled with waste rock (backfill), graded, and covered with seeded topsoil along with reconstruction of water features. Figure 4. Reconstructed landscape adjacent to the active Cordero Rojo Mine, Powder River Basin,
Figure 4. Reconstructed landscape adjacent to the active Cordero Rojo Mine, Powder River Basin, Wyoming, USA. The coal pit has been filled with waste rock (backfill), graded, and covered with seeded topsoil along with reconstruction of water features. Figure 4. Reconstructed landscape adjacent to the active Cordero Rojo Mine, Powder River Basin,
Figure 5. Overburden and coal seam at the Cordero Rojo Mine, Powder River Basin, Wyoming, USA (Martin and Langman [31)).
Figure 5. Overburden and coal seam at the Cordero Rojo Mine, Powder River Basin, Wyoming, USA (Martin and Langman [31)).
Figure 6. View of the particle size variability in waste rock generated at the Cordero Rojo Mine in the Powder River Basin, Wyoming, USA. Waste rock can range from clays to boulders.
Figure 6. View of the particle size variability in waste rock generated at the Cordero Rojo Mine in the Powder River Basin, Wyoming, USA. Waste rock can range from clays to boulders.
Figure 7. The likely presence of arsenic [As] as arsenic-bearing pyrite and sorbed particles on ¢ pre-experiment coal particle from the Fort Union waste rock. The XRF analysis of the waste rock indicated 7 ppm of As in the Fort Union sample and 2 ppm in the Wasatch sample (Se was not part of the XRF analysis). Examination of the waste rock with the SEM-EDS (examples shown in Figures 7 and 8) indicated the presence of As in coal particles (Fort Union sample) and distributed concentrations of Se (0.5 wt. %) in areas with high clay content (Fort Union and Wasatch samples). Analysis of the floated coal particles from the Fort Union waste rock indicated that As was consistently present in trace amounts (0.1 to 0.6 wt. %) throughout the coal particles, and Se was detected (up to 7 wt. %) as clay particles sorbed to larger particle surfaces. Results suggest a coal source for As and sorbed or precipitated Se particles with a greater abundance of both elements in the Fort Union waste rock.
Figure 7. The likely presence of arsenic [As] as arsenic-bearing pyrite and sorbed particles on ¢ pre-experiment coal particle from the Fort Union waste rock. The XRF analysis of the waste rock indicated 7 ppm of As in the Fort Union sample and 2 ppm in the Wasatch sample (Se was not part of the XRF analysis). Examination of the waste rock with the SEM-EDS (examples shown in Figures 7 and 8) indicated the presence of As in coal particles (Fort Union sample) and distributed concentrations of Se (0.5 wt. %) in areas with high clay content (Fort Union and Wasatch samples). Analysis of the floated coal particles from the Fort Union waste rock indicated that As was consistently present in trace amounts (0.1 to 0.6 wt. %) throughout the coal particles, and Se was detected (up to 7 wt. %) as clay particles sorbed to larger particle surfaces. Results suggest a coal source for As and sorbed or precipitated Se particles with a greater abundance of both elements in the Fort Union waste rock.
Figure 8. The likely presence of a sorbed selenium particle (e.g., selenite [SeO3]) on pre-experiment aluminosilicate (e.g., plagioclase) particle in the Fort Union waste rock.
Figure 8. The likely presence of a sorbed selenium particle (e.g., selenite [SeO3]) on pre-experiment aluminosilicate (e.g., plagioclase) particle in the Fort Union waste rock.
Figure 9. Specific conductance and total (unfiltered) concentrations of arsenic and selenium in the in water collected from the kinetic column containing the waste rock from the Cordero Rojo Mine Non-detected selenium concentrations were set to 0.006 nmol/L (half the reporting limit).
Figure 9. Specific conductance and total (unfiltered) concentrations of arsenic and selenium in the in water collected from the kinetic column containing the waste rock from the Cordero Rojo Mine Non-detected selenium concentrations were set to 0.006 nmol/L (half the reporting limit).
Figure 11. Arsenic results for the basic arsenic model with oxidative dissolution of arsenic-bearing pyrite and total (unfiltered) and filtered arsenic concentrations from the column experiment. peak (or shoulder) followed by a larger peak before a substantial drop in As release near Day 20 (unfiltered) or Day 24 (filtered values). The unfiltered As trend is more variable compared to the filtered As trends, likely because of larger particles moving through the column. The two-peak release (shoulder and main peak) in the As trend likely occurred because of multiple source contributions, such as initial nanoparticle flushing (no differences in unfiltered or filtered As concentrations during the first week) and the subsequent degradation of the bulk sulfide mineral source with oxidative dissolution of As-bearing pyrite.
Figure 11. Arsenic results for the basic arsenic model with oxidative dissolution of arsenic-bearing pyrite and total (unfiltered) and filtered arsenic concentrations from the column experiment. peak (or shoulder) followed by a larger peak before a substantial drop in As release near Day 20 (unfiltered) or Day 24 (filtered values). The unfiltered As trend is more variable compared to the filtered As trends, likely because of larger particles moving through the column. The two-peak release (shoulder and main peak) in the As trend likely occurred because of multiple source contributions, such as initial nanoparticle flushing (no differences in unfiltered or filtered As concentrations during the first week) and the subsequent degradation of the bulk sulfide mineral source with oxidative dissolution of As-bearing pyrite.
The weathering regime to be tested with the enhanced As model is the meshins of nanomaterial contribution and bulk solid weathering to capture the two-peak trenc identified in the column results (Figure 11). An examination of the release of Fe during the column experiment indicates a similar trend of two peaks during the first 30 days o he column experiment (Figure 12). There appears to be a flushing of Fe-bearing particle: that may be carrying As. The range of particle sizes (e.g., nanoscale to microscale) wer arge given the filtered Fe concentration was substantial during the first peak (0.06 umol/L compared to the total Fe concentration (0.07 umol/L) and the mass of As (~0.03 pmol/L ir he total and dissolved concentrations). The initial peak of As and Fe likely is the result o he flushing of mostly nanoparticles with some larger particles from the waste rock befor the onset of pyrite oxidation and release of fully oxidized As. This flushing of As and Fi is a result of their association where As is commonly sorbed to Fe (oxyhydr)oxides [99 Nanoparticle Fe (oxyhydr)oxides are common and have a high surface energy that wil readily sorb other metal(loid)s such as As [14,100,101]. Figure 12. Unfiltered and 0.45-um filtered iron and arsenic concentrations recorded during the column experiment.
The weathering regime to be tested with the enhanced As model is the meshins of nanomaterial contribution and bulk solid weathering to capture the two-peak trenc identified in the column results (Figure 11). An examination of the release of Fe during the column experiment indicates a similar trend of two peaks during the first 30 days o he column experiment (Figure 12). There appears to be a flushing of Fe-bearing particle: that may be carrying As. The range of particle sizes (e.g., nanoscale to microscale) wer arge given the filtered Fe concentration was substantial during the first peak (0.06 umol/L compared to the total Fe concentration (0.07 umol/L) and the mass of As (~0.03 pmol/L ir he total and dissolved concentrations). The initial peak of As and Fe likely is the result o he flushing of mostly nanoparticles with some larger particles from the waste rock befor the onset of pyrite oxidation and release of fully oxidized As. This flushing of As and Fi is a result of their association where As is commonly sorbed to Fe (oxyhydr)oxides [99 Nanoparticle Fe (oxyhydr)oxides are common and have a high surface energy that wil readily sorb other metal(loid)s such as As [14,100,101]. Figure 12. Unfiltered and 0.45-um filtered iron and arsenic concentrations recorded during the column experiment.
Figure 13. Arsenic concentrations predicted from the enhanced arsenic model of nanoparticle contribution and oxidative dissolution of arsenic-bearing pyrite compared to the column experiment’s filtered (<450 um) arsenic concentrations.
Figure 13. Arsenic concentrations predicted from the enhanced arsenic model of nanoparticle contribution and oxidative dissolution of arsenic-bearing pyrite compared to the column experiment’s filtered (<450 um) arsenic concentrations.
Figure 14. Selenium results for basic selenium model with dissolution of a selenium-bearing gypsum and total (unfiltered) and filtered selenium concentrations recorded during the column experiment. Selenium concentrations recorded from the column experiment were the same for the total, 0.45-um filtered, and 0.2-um filtered concentrations. ‘igure 14. Selenium results for basic selenium model with dissolution of a selenium-bearing gypsum Analysis of Se in the unfiltered (total), 0.45-tum filtered, and 0.20-um filtered column results indicate similar Se concentrations for all sample types. The presence of Se salts in the waste rock [35,69] and the results of the column experiment suggest that Se was present as Se ions or flushed from the waste rock as <0.20-um nanomaterials. The column Se results indicated an initial spike in Se concentrations that quickly declined to very low concentrations by Day 10 and non-detectable levels by Day 17 (Figure 14). Such a quick Se release and decline are indicative of a fast mineral reaction (e.g., salt dissolution) and/or flushing of a nanomaterial source (e.g., desorption of selenite/selenate particles). Given the solubility of selenate, limited mobility of selenite, and oxygenated conditions recorded during the column experiment, Se likely was present as selenate derived from the oxidative dissolution of Se salts.
Figure 14. Selenium results for basic selenium model with dissolution of a selenium-bearing gypsum and total (unfiltered) and filtered selenium concentrations recorded during the column experiment. Selenium concentrations recorded from the column experiment were the same for the total, 0.45-um filtered, and 0.2-um filtered concentrations. ‘igure 14. Selenium results for basic selenium model with dissolution of a selenium-bearing gypsum Analysis of Se in the unfiltered (total), 0.45-tum filtered, and 0.20-um filtered column results indicate similar Se concentrations for all sample types. The presence of Se salts in the waste rock [35,69] and the results of the column experiment suggest that Se was present as Se ions or flushed from the waste rock as <0.20-um nanomaterials. The column Se results indicated an initial spike in Se concentrations that quickly declined to very low concentrations by Day 10 and non-detectable levels by Day 17 (Figure 14). Such a quick Se release and decline are indicative of a fast mineral reaction (e.g., salt dissolution) and/or flushing of a nanomaterial source (e.g., desorption of selenite/selenate particles). Given the solubility of selenate, limited mobility of selenite, and oxygenated conditions recorded during the column experiment, Se likely was present as selenate derived from the oxidative dissolution of Se salts.
The initial Se model identified the timing of the Se peak in the column results but no’ the post-peak decline of column concentrations (Figure 14). A model of salt dissolution and bulk solid Se-bearing pyrite would not replicate the column results but simply add to the peak concentration followed by a slower decline in concentrations, similar to the As-bearing pyrite oxidation and As release. An enhanced model of Se salt dissolution (part of peak concentration) and oxidation of Se-containing nanopyrite (surface-to-volume ratio of 4.2 but same log k of —8.19 for pyrite [86]) was able to replicate the column results (Figure 14). The enhanced model identified a peak concentration from salt dissolution and greater release o! Se with oxidation of a nanopyrite that produced a near replication of the post-peak decline visible in the column experiment concentrations (Figure 15). An increase of the surface to-volume ratio from the standard 0.3 value used for pyrite oxidation [86] represents 4 substantial increase in potential weathering applicable to nanomaterials generated during mining. Nanomaterials can be highly reactive because the area-to-volume ratio substantially increases as particle size decreases, which allows for a much greater available surface fo1 reactions [102]. The change in particle size during waste rock generation should depend on the mineral type, composition, and original particle shape [10]. The identification ot! nanopyrite in coal mine waste rock [63] supports the inclusion of such reactive sources fo! potential contaminants, such as Se. Figure 15. Selenium concentrations predicted from the enhanced selenium model of dissolution of selenium-bearing gypsum and selenium-bearing nanopyrite compared to the column experiment selenium concentrations (total and filtered selenium concentrations were identical during the column experiment).
The initial Se model identified the timing of the Se peak in the column results but no’ the post-peak decline of column concentrations (Figure 14). A model of salt dissolution and bulk solid Se-bearing pyrite would not replicate the column results but simply add to the peak concentration followed by a slower decline in concentrations, similar to the As-bearing pyrite oxidation and As release. An enhanced model of Se salt dissolution (part of peak concentration) and oxidation of Se-containing nanopyrite (surface-to-volume ratio of 4.2 but same log k of —8.19 for pyrite [86]) was able to replicate the column results (Figure 14). The enhanced model identified a peak concentration from salt dissolution and greater release o! Se with oxidation of a nanopyrite that produced a near replication of the post-peak decline visible in the column experiment concentrations (Figure 15). An increase of the surface to-volume ratio from the standard 0.3 value used for pyrite oxidation [86] represents 4 substantial increase in potential weathering applicable to nanomaterials generated during mining. Nanomaterials can be highly reactive because the area-to-volume ratio substantially increases as particle size decreases, which allows for a much greater available surface fo1 reactions [102]. The change in particle size during waste rock generation should depend on the mineral type, composition, and original particle shape [10]. The identification ot! nanopyrite in coal mine waste rock [63] supports the inclusion of such reactive sources fo! potential contaminants, such as Se. Figure 15. Selenium concentrations predicted from the enhanced selenium model of dissolution of selenium-bearing gypsum and selenium-bearing nanopyrite compared to the column experiment selenium concentrations (total and filtered selenium concentrations were identical during the column experiment).
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