3.1 Effects of MICP on Compressive Strength
Surface strength measurement of tailings in each pilot cell was performed 28 days after treatment to assess the mechanical improvements of the material resulting from MICP. A comparison of average material strength between test cells is shown below in Figure 2.
The average surface strength was found to be the lowest in the Control, which was measured as 45.46 kPa. The Spray treatment yielded the highest average surface strength of 73.55 kPa, with some regions of its surface measuring as high as 122 kPa, however variations in strength measurements were still observed in this cell. The Till treatment exhibited an average surface strength of 52.61 kPa, an approximate increase of 15% from Control. Similarly, the average surface strength of the Trench treatment was 52.10 kPa. Based on the one-way ANOVA completed, Spray was found to be the only MICP-treated cell in which a significant increase in material surface strength was observed compared to the Control.
As a result of the variations in strength measurements and the large surface area of each pilot cell, a graded map of reported measurements was also created to illustrate localized improvements in material strength (Figure 3). The Control exhibited the widest variation in strength measurements, with a standard deviation of 24.1 kPa. Till exhibited the most homogeneity across surface with a standard deviation of 14.7 kPa. In comparison, the spray-on cell had a standard deviation of 19.13 kPa. The strength measurements in Trench showed a larger variation than the Control, with a standard deviation of 23.21 kPa. In the region of the cell in which the initial trench was cut, where higher concentrations of UBS and CS solutions were assumed to be present, the average surface strength was 49.03 18.53 kPa.
Improvements in material strength was considered a key metric in evaluating the efficacy of different MICP application methods (Ng et al., 2012). Results indicated Spray to be the only application method to yield a significant increase in material strength. This was consistent with previous studies that have successfully used a non-intrusive surface application to distribute treatment solutions within a soil sample (Cheng & Cord-Ruwisch, 2012; Hoang et al. 2019). This method showed the least disturbance to the material, allowing the treatment solution to permeate into the pore space of the sediment, thus leading to the nucleation of CaCO3 crystals between soil and rock particles. Additionally, the saturated state of the material upon initial application may have allowed the substrates from treatment solutions to be mobilized by the tailings pore water via diffusion. In contrast, both Till and Trench application methods were more disruptive to the tailings material, which likely impacted the extent of cementation. In the case of the Till treatment, the use of tilling equipment that effectively mixed the tailings with the treatment solutions provided the most homogenous result, however significant improvements in strength were not observed. It is believed that the treatment application caused soil disturbances that have the potential to impact the stress profile of the soil being evaluated (Mujah, Shahin, & Cheng, 2017).
From visual observation, the surface texture of each pilot cell varied considerably (Figure 4). The surface of tailings sampled from Spray remained intact upon excavation, indicating that a cemented crust formed over the test duration. Till and Trench exhibited a film-like surface that was lighter in colour and appeared more fine-grained than the material underneath. One possible reason could be the addition of treatment solution and mechanical agitation causing the segregation of granular and fine-grained tailings, which has been observed in soil-freshwater mixtures (Ganesalingam, Sivakugan, & Ameratunga, 2013). This can be supported by particle size data for Trench, as the mean grain size of the tailings surface samples was found to be 85-115 m, while the mean grain size 0.1m below the surface was 307.1 m (Figure 5).
It should also be noted that the film-like appearance of Till and Trench may also be the result of extracellular polymeric substances (EPS) excreted by organisms in the tailings environment (Redmile-Gordon & Chen, 2017). Changes in environmental factors such as pH, temperature, and salinity, as well as the presence of stressors such as heavy metals, can trigger bacterial colonies to produce EPS (Redmile-Gordon & Chen, 2017). Both clay minerals and EPS exhibit effective moisture retention (De Kretser, Scales, & Boger, 1997; Guo et al., 2018), and their establishment at the material surface may have inhibited pore water evaporation, as tailings samples from Till and Trench had elevated moisture levels compared to material sampled from Control and Spray (Table 4).
Table 4: Moisture content of tailings samples collected from Control, Spray, Till, and Trench cells at surface-level (0m) and 0.3m below the surface. All samples were collected 28 days after treatment at 18˚C.
Moisture (%)
|
Control
|
Spray
|
Till
|
Trench
|
0m
|
9.9
|
11.4
|
88.5
|
80.9
|
0.3m
|
23.9
|
18.7
|
83.0
|
83.1
|
A previous study reported that under 100% saturation, the site of calcite crystal formation occurs in pore spaces or sediment surfaces, whereas as 20% saturation, crystal formation occurs at sediment junctions, which are optimal for improving the mechanical properties of soil (Cheng et al., 2013). Therefore, it is possible that the high-moisture environment maintained in Till and Trench over the 28-day treatment period did not facilitate effective crust formation that would improve material strength. It is noted that the relationship between EPS production and carbonate precipitation has been studied, but few studies have looked at the combined influence of these two factors on the mechanical properties of soil (Ercole et al., 2007; Or, Phutane, & Dechesne, 2007).
3.2 Effects of MICP on Soil Geochemistry
Surface samples collected from each pilot cell after 28 days were analyzed to measure calcite (CaCO3) content and soil inorganic carbon content, or the measure of carbonate compounds within soil (Figure 5). A strong positive correlation was observed between measured CaCO3 content and inorganic carbon, indicating that inorganic carbon can also be used as a strong indicator for CaCO3 precipitation. Results from the XRD analysis indicated that a CaCO3 content of 5.4% (77 kg/m3 tailings) and 6.2% (88 kg/m3) in Spray and Till after their respective treatments. These were observed increases relative to the Control, which was found to have a CaCO3 content of 2.3% (33 kg/m3). A CaCO3 content of 3.2% (45 kg/m3) was recorded for the Trench, which was lower than that observed for the other treated cells.
Of the three treatment applications, Till was found to have the greatest increase in CaCO3 and inorganic carbon. While detrimental to the structural properties of the material, the mixing of the tailings material with UBS and CS may have increased levels of dissolved oxygen in the tailings, potentially creating more favourable conditions for S. pasteurii to initiate CaCO3 formation (Garcia-Ochoa et al., 2010). Conversely this may have also established favourable conditions for native species to proliferate, enabling competition for nutrients (Reeksting et al., 2020). Based on both measured calcite and inorganic carbon, Trench was found to have relatively small improvements in precipitated calcite, suggesting the conditions of this treatment application method did not facilitate sufficient calcite precipitation compared to the Spray and Till methods. It is possible that the segregation of fine particles at the surface of the Trench cell could have impacted MICP, as small pore spaces may restrict bacterial movement and the transport of nutrients (Rebata-Landa & Santamarina, 2006; Wang & Nackenhorst, 2020).
Acid-base accounting completed on samples from each pilot cell provided insight into the geochemical stability of treated tailings (Table 5). Results indicated that a 28-day treatment may not be sufficient to improve the net neutralizing potential of the material. However, analysis of sulfur species showed that all treatments had lower sulfate levels relative to the Control, suggesting the treatment contributed to minimizing the weathering of pyrite (Blowes & Jambor, 1990). It should be noted that longer-term kinetic tests would be necessary to understand the effect of this treatment on geochemical weathering.
Table 5: Measured acidification potential (AP), neutralizing potential (NP), and percent sulfate of total sulfur for Control, Spray, Till, Trench.
|
Control
|
Spray
|
Till
|
Trench
|
AP (kg CaCO3/tonne)
|
104.31
|
110.34
|
116.34
|
107.1
|
NP (kg CaCO3/tonne)
|
17.23
|
23.73
|
29.99
|
18.2
|
SO4/Total Sulfur (%)
|
9.54
|
6.09
|
4.05
|
5.83
|
3.3 Viability of Bacterial Colonies
Bacterial plates counts completed on surface tailings of Control, Spray, Till, and Trench cells 14 and 28 days after initial treatment provided insight into changes to the viability of bacterial populations over the period of testing. Colony forming units (CFU) measured from Control samples were all found to be below the range considered statistically significant (30-300 CFU) therefore these results were not reported (Sanders, 2012). A summary of bacterial plate counts is shown below in Figure 6. It was found that the decrease in viable bacteria in the Spray, Till, and Trench cells from 14 to 28 days was statistically significant, however there were no statistically significant differences in CFUs between Spray, Till, and Trench treatments.
Higher concentrations of CFUs were measured in Spray, Till, and Trench relative to the Control both 14 days and 28 days after treatment. Spray exhibited the highest number of CFUs, with 7.11 ´ 105 (± 2.25 ´ 105) CFU/g found in tailings sampled 14 days after treatment, and 1.05 ´ 105 (± 8.63 ´ 104) CFU/g from samples taken 28 days after treatment. Till exhibited the highest observed decrease in CFUs between sampling periods of the three treated cells with 2.10 105 (± 3.68 ´ 104) CFU/g after 14 days, and 8.4 103 (± 2.26 103) CFU/g after 28 days. Trench showed relatively stable concentrations of viable cells. Tailings samples taken from the trenched region were found to have a colony count of 3.0 104 (± 2.83 ´ 103) CFU/g after 14 days, and 4.54 104 (± 3.11 ´ 103) CFU/g after 28 days. Some of the decreases in cell counts could be attributed to the depletion of substrates utilized by bacterial cultures over the 28 day testing period (Mugwar & Harbottle, 2016). Due to the suspected uneven distribution of treatment solution using the Trench method, additional samples were collected from a different location of the shallow trench (region B) after 28 days. The tailings taken from trench region were found to have a concentration of 1.31 ´ 105 (± 5.32 ´ 104) CFU/g at 28 days, and while this was higher than the concentration initially measured within the shallow trench (region A), the difference was not found to be statistically significant.
The plate counts are indicative of the total amount viable colonies present in each test cell, however, they do not indicate the composition of native microbial populations versus S. pasteurii cultures. To account for the possible biostimulation of other native species, a bench-scale testing was completed on VJTB tailings samples, in which treatment solutions were applied both with and without S. pasteurii. It was found that after a 28-day period, samples treated with S. pasteurii had an average concentration of 3.62 105 CFU/g, compared to nutrient-enriched samples, which had a concentration of 2.05 105 CFU/g. Control samples did not exhibit significant colony growth to enable CFU determination. This would suggest that S. pasteurii can remain viable in the tailings environment, however, native species may also utilize treatment solutions as substrate, which could result in competition for nutrients. It is not known whether native species in these tailings possess enzyme pathways to facilitate CaCO3 precipitation, therefore a more in-depth microbiological analysis of bacterial species within the tailings and their interactions would be suggested in future work to gain a better understanding of its impact on MICP.
3.4 Effects of MICP on Water Quality Parameters
Porewater pH was used to monitor the progression of MICP in each cell, as well as potential ARD formation. The recorded pore water pH for each test cell is shown in Figure 7. The initial porewater pH of Spray, Till, and Trench was lower than the Control which was likely due to the need for additional site water cap (pH = 6.3) to saturate the tailings in these pilot cells.
The analysis of porewater from WS-B in the Control indicated no decline in pH approximately 0.5 m below surface level during testing, providing a positive outlook on the short-term geochemical stability of this material under neutral conditions. The recorded pH of the porewater collected from WS-B in MICP-treated cells steadily increased over the 28-day period, with porewater samples stabilizing approximately 21 days after treatment, and reaching a final pH of 7.0-7.5. A similar observation was made for samples collected from WS-A. Due to reduction in moisture from evaporation, some WS-A samplers could not collect a sufficient volume of pore water for analysis. Results from shake flasks extractions completed on tailings from a 0.3m depth 28 days after treatment, were found to be approximately 7.6 in Spray, Till, and Trench treatments, suggesting further improvements in water quality occured over the remainder of the test period.
The measurement of ammonia (NH3) is also commonly used to indicate the extent to which MICP has occurred. NH3 concentrations measured from pore water samples from each cell are shown in Figure 8. Marginal increases in NH3 were also observed in WS-B over the 28-day period, indicating the permeation of treatment solution to depths of 0.46 m below the surface.
NH3 measured in the Control remained low throughout the test duration, averaging 0.163 0.14 mg/L. In treated test cells, NH3 concentrations were found to be slightly above levels measured in the Control after initial application. After 14 days, an observable increase in the concentration of NH3 occurred in each of the three treated cells. The increase was more visible in samples taken from WS-A, as it was situated closer to the tailings surface in each cell, indicating urea hydrolysis was more prevalent in the upper layer of tailings. Measurements taken from WS-A over the remainder of the test period showed NH3 concentrations stabilizing or decreasing slightly. Tailings leachate analyzed by shake flask extraction showed similar results to the porewater samples. NH3 measured from the tailings leachate of the Control samples remained low, as shown in Table 4. The concentration of NH3 in the leachate of Spray, Till, and Trench was found to be higher than the Control. Notably, the NH3 measured from tailings at a 0.3m depth in Till was lower than the concentrations measured in Spray, Till, and Trench. This was an unexpected result, as previous studies had reported mixing treatment solutions with soil material provided a homogenous distribution with increasing sample depth (Mujah et al., 2017).
Table 6: Ammonia concentrations (mg/L) measured in tailings sample leachate using shake flask extraction method.
|
NH3 Concentration (mg/L)
|
Depth (m)
|
Control
|
Spray
|
Till
|
Trench
|
0
|
< 0.05
|
59
|
63
|
138
|
0.3
|
0.14
|
21
|
3.4
|
31
|
Results from the water quality analysis show this treatment has the potential to reduce the potential for acid generation by sulfidic tailings. The near-neutral pH maintained may have slowed the incidence of Fe (II) oxidation reactions that contribute to ARD formation. This is supported by lower fractions of sulfate measured in MICP-treated cells relative to the Control. In all cases, the MICP-treated cells exhibited increases in pore water pH over the course of the test period. This increase is likely attributed to the resultant carbonate, ammonium, and hydroxyl ions as products of urea hydrolysis (Choi et al., 2020; Okwadha & Li, 2010).
In treated cells, the elevated NH3 concentrations observed in the porewater indicated the occurrence of ureolysis and confirm the ureolytic activity of S. pasteurii in the tailings environment. It was observed that NH3 concentrations peaked after 14 days, then they began to decrease. There was no statistically significant correlation between viable bacteria and NH3 concentrations, indicating that observed decreases NH3 within porewater were independent of the decrease in microbial population. This change could be attributed to the volatilization of converted ammonia over the test duration (Kirk & Nye, 1991). As the pH of porewater increases, the conversion of NH4+ (pKa = 9.25) to NH3 is more favourable, which increases volatilization potential (Gui et al., 2018). This can also be attributed to a decrease in the availability of urea for ureolysis, as this would ultimately decrease NH3 formation (Bhaduri et al., 2016; Lauchnor et al., 2015).
3.5 Future Work
While the results show promise for MICP as potential for tailings attenuation, additional research and analysis is required prior to further scale-up. Assessing the behaviour of MICP-treated tailings over a longer monitoring period will provide a better understanding of its structural and geochemical behaviour and provide insight into the long-term viability of S. pasteurii in a tailings environment. Further studies should also assess the robustness of this treatment under more representative climate conditions, i.e., in a field pilot, to asses the effects of seasonal temperatures, humidity, and rainfall patterns on treatment longevity and potential maintenance requirements. Additionally, further a more in-depth analysis of the interactions between S. pasteurii and native microbial populations is warranted for better understanding of its impact on MICP.