3.1. Induced carbonate production (CO3−2)
In this study, the free carbonate ions (CO32−) produced by microbial-induced carbonate (MICP) and CO2-induced carbonate (CICP) were compared in Figure 3. As it can be seen from the figure, a value of 42.1 and 34.8 g/lit were obtained for CICP and MICP methods, respectively.
The effect of Zeolite treatment on ammonium concentration for MICP to generate an ammonium-free carbonate solution is shown in Figure 4. The ammonium content of the solution was initially around 1.39g/lit, as seen in the figure. After 15 percent Zeolite treatment, the ammonium concentration dropped to an average of 3.4 105 g/lit. The concentration meets the requirement specified by some drinking-water standards (e.g. lower than 5 ×10−4 g/lit, Australian Drinking Water Guidelines and Guidelines for Drinking-water Quality Management for New Zealand, 2016).
3.4 Carbonate mineral characterization
Figure 6 illustrates the crystal morphology of CaCO3 which was produced by MICP and CICP methods. As it can be seen, the CaCO3 crystals have been grown spherical with diameters of 2-10 µm in MICP (Fig. 6a), while crystals were in rhombohedral shape and smooth with diameters of 2-6 µm in CICP (Fig. 6b). Figure 7 shows EDS and XRD analysis of CaCO3 deposition. The EDS layered image presents the distribution of C, O, and Ca elements in the test area. The result of XRD justified vaterite minerals (100%) (Fig. 7a) and calcite (100%) (Fig. 7b) in MICP and CICP, respectively. In CICP, deposition and morphology of CaCO3 are dependent on additives such as CO2 and Ca2+ which are precipitated in three anhydrous polymorphic modifications (calcite, aragonite, and vaterite) and mostly is calcite (Kralj et al. 2004). MICP is largely dependent on metabolic processes by ureolytic bacteria that influence crystal growth (Rahman and Oomori 2009; Marvasi et al. 2010; Wei et al. 2015; Bielak et al. 2021). It has also been reported that the amino acid (Glu (red) and Asp (blue)) in the urease enzyme favored the formation of vaterite (Sondi and Salopek-Sondi 2005).
Figure8 shows the crystal morphology of FeCO3 which was produced by MICP and CICP methods. There was a similar crystal shape (spherical) for both of them. The particle size of FeCO3 in MICP was with diameters of 0.5-3 µm while the particle size was bigger (2-6 µm) in CICP. Figure 9 shows EDS and XRD analysis of FeCO3 samples. The EDS layered image presents the distribution of C, O, Mg, and Fe elements in the test area. The result of XRD justified siderite minerals (100%) for MICP and CICP. Less information is available concerning the mineralization or morphogenesis of iron carbonates (siderite). This is probably because natural siderite is often associated with other coexisting elements (e.g., Mn, Mg, and Ca), and contains a small amount of hematite (Fe2O3) due to partial oxidation in natural air (French 1971; Isambert et al. 2003). The biological aspects of iron biomineralization have been investigated by studying iron-bearing biominerals, owing to their significance in identifying microbe-sediment-water interactions, as well as mineral and biogenic origins (Frankel and Bazylinski 2003; Dong et al. 2009).
Figure 10 shows the crystal morphology of calcium magnesium carbonate which is produced by a mixture of magnesium and calcium ions with free carbonate ions (CO32−). As it can be seen from the figure, there was a botryoidal crystal shape of calcium magnesium carbonate in the MICP method (Fig. 10a) while a flower-like shape was observed in the CICP method (Fig. 10b). There was the main difference between the shape and size of crystals in both methods. The calcium magnesium carbonate was precipitated with diameters of 5-16 µm and 2-5 µm in MICP and CICP, respectively. Minerals deposited in a larger size in MICP than in CICP, as can be seen plainly. The EDS layered image presents the distribution of C, O, Mg, and Ca elements in the test area. XRD analysis of samples showed Ca-Mg (CO3) deposition while was as dolomite (40%) and calcite (60%) in MICP (Fig. 11a) and high-Mg calcite (61%) and magnesium carbonate hydrate (Nesquehonite) (39%) in CICP (Fig. 11b), respectively.
Previous laboratory research revealed that aerobic bacteria may have a role in the creation of high-magnesium calcite or dolomite, in the microenvironment around the cell (Sánchez-Román et al. 2009; Sánchez-Román et al. 2011; Al Disi et al. 2017). The increase of Mg2+ concentration (and the Mg2+:Ca2+ ratio) can also cause precipitation of dolomite and magnesite (Balci et al. 2016; Al Disi et al. 2019). Bacteria can adsorb Mg2+on their membranes during induce dolomite formation, while calcite precipitation is induced by preferential adsorption of Ca2+. González-Muñoz et al. (2010) found that Mg probably plays a key role in the development of the morphologies of the precipitates since these morphologies had never been observed in the absence of Mg. CaCO3 mineral that has precipitated from seawater changes from calcite to aragonite was experimentally determined as a function of temperature and additives. Results indicate precipitation of high-Mg calcite and dolomite is largely dependent on Mg: Ca ratio over a relatively small temperature range in an aqueous solution (Morse et al. 1997). In conclusion, the morphology, type, and size of calcium magnesium carbonate minerals are influenced by the chemical and biochemical environment in CICP and MICP.
Figure 12 illustrates the crystal morphology of MgCO3 which was produced by MICP and CICP methods. As it can be seen, the MgCO3 crystals have been grown in the radial needle (Fig. 12 (a, c)) and radial blades shape (Fig. 12 (b, d)) by MICP and CICP, respectively. There was the main difference between the sizes of crystals in both methods. The thickness of radial needle crystals was diameters of 1-10 µm in MICP, while it was precipitated in a greater thickness of about 5-20 µm as radial blades in CICP. It was clearly shown that the potential of CICP to precipitate MgCO3 is more than MICP. Figure 13 shows EDS and XRD analysis of MgCO3samples. The EDS layered image presents the distribution of C, O, and Mg elements in the test area. The result of XRD explained the mineralization of magnesium carbonate hydrate (Nesquehonite) for both methods (Fig. 13). The magnesium carbonate (as hydrate) can display four different morphologies during the reaction process such as needle, sheet, rose, and nest-like with different carbonation temperatures (Zhang et al. 2007; Chen et al. 2020). Gautier et al. (2014) have studied the growth process of hydromagnesite by calculating its growth kinetics in an aqueous solution from 25 to 75°C. A previous study showed a similar morphology and type of MgCO3 (H2O)3 (Nesquehonite) that precipitated MICP (Zhao and Zhao 2017) and CICP (Glasser et al. 2016).
The mean size distribution of minerals was measured by SEM photos. Figure 14 shows the average size of carbonate minerals produced by MICP and CICP methods. As can be seen from the results, the average size of crystals was larger in CICP. Although there was only a small difference in calcium magnesium carbonate precipitation in MICP compared to CICP. The largest crystals in CICP, however, were for magnesium carbonate hydrate (Nesquehonite). In this work, CICP demonstrated a great capacity for precipitating carbonate minerals with larger sizes. The minerals that develop during biomineralization are frequently characterized by low crystallinity, structural well-ordering, and a limited size range. (Frankel and Bazylinski 2003). Under the same environmental conditions, biologically induced mineralization is equal to inorganic mineralization, and the minerals are expected to contain crystal-chemical characteristics that are indistinguishable from minerals formed by inorganic chemical reactions. However, cell walls of bacterial surfaces including biofilms, dormant spores, and slim sheaths can act as important sites for the adsorption of ions for growth and nucleation minerals (Bäuerlein 2003).
A summary of the characterization of carbonate minerals produced by MICP and CICP techniques is shown in Table 3. In both procedures, all carbonate minerals were precipitated in various sizes, shapes, and types.
Table 3
Characterization of produced carbonate minerals by MICP and CICP
Name
|
Method
|
Type of Mineral
|
Size (µm)
|
Crystal Shape
|
Calcium carbonate
|
MICP
|
Vaterite
|
2-10
|
Spherical
|
CICP
|
Calcite
|
2-6
|
Rhombohedral
|
Ferrous carbonate
|
MICP
|
Siderite
|
0.5-3
|
Spherical
|
CICP
|
Siderite
|
2-6
|
Spherical
|
Magnesium calcium carbonate
|
MICP
|
Dolomite
|
5-16
|
Botryoidal
|
CICP
|
High-Mg calcite
|
2-5
|
Flower-like
|
Magnesium carbonate
|
MICP
|
Nesquehonite
|
1-10
|
Radial needle
|
CICP
|
Nesquehonite
|
5-20
|
Radial blades
|
Less information is available concerning the usage of other carbonate minerals for soil improvement. The solubility of carbonate minerals decreases by more than two orders of magnitude on transformation from Ca- to Mg-rich (Farsang et al. 2021). Mg carbonates such as magnesite and dolomite are semi-soluble and hydrophilic (Wonyen et al. 2018). Long et al. (2014) showed that the increase of Mg ions in the Mg-containing calcite improves the mechanical properties of biogenic minerals. As well as, ferrous carbonate has low solubility in an aqueous solution (Sun et al. 2009). It seems that carbonate minerals with substitution elements such as Mg and Fe affect on strengthens behavior of crystals in the soil improvement. Depending on the polymorph of CaCO3 crystals (i.e. calcite, vaterite, or aragonite), the strength of biocemented soil can be affected (Al-Thawadi 2013; Dhami et al. 2013). It appears that calcite crystals with rhombohedral shape and high-Mg calcite with flower-like shape in CICP have the capability of interlocking due to their surface roughness.
3.5 Unconfined compressive strength
Figure 15 shows the results of the UCS tests carried out on carbonate minerals-treated soil. As can be seen from the figure, the treated specimens with condensed Ca-Mg(CO3) and FeCO3 minerals, produced by the MICP method gained about 58.5 kPa and 63.0 kPa UCS strength respectively (Fig. 15). Moreover, MgCO3 minerals, produced by the MICP method had the lowest strength of about 25 kPa. The treated specimens with CaCO3 content gained higher compressive strength compared to the specimens with MgCO3 content in this study. The treated specimens with the CICP method gained higher compressive strength compared to the specimens treated with the MICP method. The minerals produced by the CICP method have a 15 to 28 percent higher compressive strength than those produced by the MICP method, according to the data.
The results of UCS showed that siderite and calcium magnesium carbonate (i.e. High-Mg calcite and dolomite) had more efficiency in soil strength as it seems to be because of the stability of Fe and Mg carbonate crystals. The treated soils by siderite in CICP had slightly more strength more than MICP which may be due to the larger particle size of siderite crystals. While the higher strength of high-Mg calcite in CICP is related to surface roughness and more interlocking of particles. The strength of calcite-treated soil in CICP was slightly more than vaterite-treated soil in MICP as well as may be related to the surface roughness of particles. The UCS result of magnesium carbonate hydrate (Nesquehonite) treated soil was lower than other carbonate minerals in this study. However, Nesquehonite (because of blade structure) had higher efficiency in clay soil in the previous study (Romiani et al. 2021). Moreover, the effect of Nesquehonite by CICP with more thickness was more than needle minerals in MICP in the UCS test.