Results & Discussions
Statistical Analysis
Following the creation of exploratory graphics, further graphical and statistical analysis was completed. To start, a bar graph was created for both nickel concentration and alkalinity in order to make it easier to identify potential trends. These graphs include a 95% confidence interval for each rock type/amendment rate.
Figure 1. Bar chart for nickel concentration (left), and alkalinity (right), with error bars representing a 95% CI.
Next, a one-way ANOVA was run to determine if the nickel concentration or alkalinity observed in any of the amended leachates were statistically different (α = 0.05) from the values observed in the control leachates.
Figure 2. ANOVA results comparing each rock type (at varying amendment rates) to control values for nickel concentration (left), and alkalinity (right).
With statistically significant differences identified in both nickel concentration and alkalinity from the control, post hoc testing was then performed. More specifically, a Tukey Honestly Significant Difference (HSD) test was performed, for all rock types that demonstrated a significant difference in either analysis, in order to avoid type 1 errors. Following the Tukey HSD test, new bar graphs were created with Tukey labels in order to demonstrate at which amendment rates significant differences from the control values occurred.
Figure 3. Final bar graphs for rock types whose leachates showed a significant difference in nickel concentration from the control leachates, with labels representing significantly different groups.
Figure 4. Final bar graphs for rock types whose leachates showed a significant difference in alkalinity from the control leachates, with labels representing significantly different groups.
Conclusions
Nickel Contamination: Based on the above graphical and statistical analyses, it is possible to identify some trends in nickel contamination for certain rock amendment types. Interestingly, both basalt and metabasalt showed a statistically significant decrease (α = 0.05) in nickel concentration in leachates at high amendment rates (50 kg/m2 for basalt, 10 and 50 kg/m2 for metabasalt), and while this was unexpected, it can likely be explained due to sorption occurring between the nickel ions in solution and the mineral grain surfaces. On the other hand, serpentinite demonstrated a significantly significant increase (α = 0.05) in nickel concentration in leachates at the highest amendment rate (50 kg/m2). This was expected, as the serpentinite was the rock type with the highest concentration of nickel within it. It should be noted however, that while the nickel was elevated for the high amendment rate serpentinite leachates, reaching a maximum value of 11.3 ug/L, it was still statistically significantly lower than the North American water standard of 100 ug/L (p = 3.06 e-04).
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Overall, while some increase in nickel contamination was seen for the serpentinite amendment, it occurred at a low enough level that it should not lead to negative ecological impacts, even at high spreading rates.
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Alkalinity: Based on the above graphical and statistical analyses, it is possible to identify consistent trends in alkalinity for all rock amendment types. Namely, each rock type demonstrates a statistically significant increase (α = 0.05) in alkalinity for at least two spreading rates above the control alkalinity values. While this statistically significant difference was seen starting at a spreading rate of 10 kg/m2 for basalt, kimberlite, metabasalt, and serpentinite, it was observed starting at a spreading rate of 5 kg/m2 for wollastonite.
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Overall, these increasing trends in alkalinity observed for each rock type with increasing amendment rate demonstrates that carbon dioxide was being removed from the atmosphere and stored within the soil leachates. Further analysis of this data can be used to quantify just how much CO2 was removed in each pot.
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Recommendations & Discussion
While this trial demonstrates the potential to capture carbon dioxide without serious nickel contamination concern for both freshly mined and mine waste rock amendments, further research should be pursued on the subject. This includes:
1.Larger (field) scale trials to confirm or refute the carbon capture and contamination results
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2. Longer timeframe trials to analyze the carbon capture and contamination potential over the whole weathering timeframe of the minerals
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3. Life cycle assessment and techno economic analysis for enhanced rock weathering to determine the net CO2 and economic feasibility