*Effect of Biotreatment on Mine Tailings/Dust Control*

The various applications of the biotreatment of soils include the control of permeability, improvement of the bearing capacity, strength and stiffness development, and the control of dust due to erosion [172]. The air quality in the majority of cities worldwide is becoming a grave concern due to the increase in population and urbanization [173,174]. In this scenario, measures to curb the deterioration of air quality are developed to safeguard the environment. Dust contributes to the deterioration of air quality. According to Watson et al. [175], the major sources of air pollution in the major cities of the U.S. are vehicular emissions and dust from roads. With dust's severe impact on the air quality, methods to reduce dust emission are employed. These methods include using dust suppressants, spraying water, and providing wind shield walls against dust emission [176]. Chang et al. [177] reported that water spraying to control dust serves the purpose for a maximum duration of 4 h. Using water for dust suppression impacts water reserves, since water offers a temporary remedy and needs repeated application. Additionally, chemically activated suppressants for dust control are highly corrosive and hamper the environment. Sustainable and eco-friendly dust control techniques are on high demand [176]. Dust control using biotreatment can be achieved in the same way that sand solidification in the desert can be carried out; potential dust sources with dust particles can be dealt with through biocementation [178].

Sun et al. [176] conducted a study on dust near a quarry site in China by developing a simulation of rainfall erosion and by conducting field tests. Their test methods involved ascertaining surface strength, which is most vulnerable to wind erosion. Surface hardening was obtained by spraying biotreatment solutions on the surface; after spraying a thin and hard calcite on the surface, a crust of soil was formed. They confirmed that the cementation of dust particles using CaCO3 precipitates through the enzyme treatment reduced dust pollution and that implementing EICP for dust control could be efficient during sandstorm and rainfall. Meyer et al. [56] used *Sporosarcina pasteurii* to treat two soil types to control air pollution due to dust. The treated soils were made to pass through a wind tunnel, and the amount of reduction in soil mass after exposure in the wind tunnel was observed to express the amount of wind erosion. The results obtained from this work showed that microbially precipitated calcite was very much effective in controlling soil erosion, as proven by wind tunnel experiments. Naeimi and Chu [179] compared the effectiveness of dust control through the biotreatment method and conventional techniques. *Sporosarcina pasteurii* was used in their study to treat sand against dust emission. The comparison was done on the same sand treated with calcium lignosulfonate, water, and calcium chloride. Their results showed that the biotreatment of soil exposed to wind in the wind tunnel improved erosion resistance, and only 1.5% mass loss was observed. Other treatment methods showed greater loss in mass after wind tunnel testing; hence, the use of biomediated soils was the best treatment method used in their study. Table 3 provides different bio-stabilizers adopted for dust control in different non-plastic materials.


**Table 3.** Bio-stabilizers adopted for dust control in different non-plastic materials.

The erosion of deposits of mine tailings caused by wind erosion is one of the most serious environmental concerns [184,185]. Wind carrying mine tailings poses a threat to water bodies nearby and deteriorates air quality, which means serious risks to human and animal health [186]. Controlling dust by suppressants sprayed on the deposits is a common method. Dust suppressants agglomerate the fine particles and check the possibility of dust/finer mine tailings escaping with the wind movement; agglomerated dust particles form dense deposits on the ground, trapping dust sources beneath [187]. Chen et al. [188] conducted a study on the reduction of dust from mine tailings with a biopolymer coating on deposits. They found that the biopolymer coating was effective in mitigating mine tailing dust. Govarthanan et al. [189] used bacteria for the mineralization of lead contaminants found in mine tailing. They found that precipitates of calcium carbonate mineralized by bacteria were effective in lead bioremediation. Bacteria used in their study were effective in changing nitrates of lead to silicon oxides and sulfides of lead, thereby reducing the severity of lead on the atmosphere. Zamani et al. [190] studied the effect of MICP on mine tailing stability and found that microbial precipitates of CaCO3 were effective in improving the stability of slopes of mine tailing materials. It can be observed from the literature that the remediation of heavy metals and dust control from mine tailings are well addressed by the biocementation process.

#### **7. Limitations of Biocementation Techniques**

Miftah et al. [43] reviewed the effectiveness of the MICP and EICP techniques in soil improvement and expressed that these methods could be effective in many geotechnical applications. However, certain concerns limit the effectiveness of these techniques. In the MICP method, concerns such as the type of soil, environmental issues, and the uniform treatment of soil mass are factors that create problems for its application. In the EICP method, the cost of enzymes happens to be too high since 57–98% of the cost of enzyme solutions is incurred on the urease enzyme. The soil type also plays an important role in governing the effectiveness of the biotreatment. The MICP method is restricted to the subsoil, and other regions of the soil may not provide a feasible environment to the bacterial growth. MICP does not show good results when used on very fine soils because comparatively larger sizes of bacteria cannot be accommodated in the pores of fine soils. On the other hand, EICP does not pose any hindrance in its application due to its size. Miftah et al. [43] also discussed the environmental concerns related to the use of MICP. This technique leaves microbes in soils after treatment, which means that it may require the permission of the concerned authorities and regular inspection to ensure that the energy of microbes is not hazardous to the surroundings. Furthermore, the release of ammonia through MICP is dangerous to people and the ecology of the area where it is applied, especially to the air and water. Additionally, the increase in pH may develop potential corrosion, and further contamination of groundwater due to chloride may be possible after the precipitation of CaCO3. On the other hand, urease used in the EICP technique may not have a long-term impact on the environment because it becomes degraded after a certain time period. The use of microbes for soil treatment needs a specific environment in the soil mass for their cultivation, and the storage of bacterial strains is an expensive process. With these limitations in the use of alternate means for calcite precipitation, EICP seems to be better than MICP [191,192]. It has also been observed that high concentrations of calcium chloride and urea hinder the bacterial activity, reducing the amount of calcite precipitation. Conversely, using enzymes can very well be possible with high concentrations of calcium chloride and urea, which paves the way for a greater amount of calcite precipitation [109]. Therefore, the EICP technique is preferable over MICP. Yasuhara et al. [100] mentioned that maintaining bacteria for their cultivation requires technical expertise. Controlling bacterial activity also poses a challenge in the MICP method; the EICP technique is free of this constraint.

### **8. Conclusions**

Biostabilization of the soil is an emerging trend with relatively simple onsite applications; the application of the stabilizer of a particular type onsite is carried out by injecting the stabilizer in the work area. Biostabilization of soils through EICP and MICP have the potential to meet the ever-growing demands of setting new infrastructure and remediating contaminants in the soils. Furthermore, the challenge of reducing environmental pollution and developing sustainable techniques can be achieved by the implementation of these techniques. The development of precipitates in the voids of the soil is helpful in reducing hydraulic conductivity. The application of MICP/EICP has stretched to the extent that ponds can be created in regions with soils of high permeability by relying on CaCO3 precipitates. Salient observations made from the existing literature are summarized below.


**Author Contributions:** Conceptualization, A.A.B.M. and A.A.; Methodology, A.A.; Formal analysis, A.A.B.M.; Investigation, K.L.; Resources, A.A.B.M.; Data curation, K.L.; Writing—original draft preparation, M.A.L.; Writing—review and editing, M.A.L., A.A.B.M. and A.A.; Visualization, A.A.; Supervision, A.A.B.M.; Project administration, A.A.B.M.; Funding acquisition, A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is funded by College of Engineering Research Center and Deanship of Scientific Research at King Saud University in Riyadh, Saudi Arabia.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The content presented here was sourced from existing published literature, hence, this clause is not applicable.

**Acknowledgments:** The authors acknowledge the College of Engineering Research Center and Deanship of Scientific Research at King Saud University in Riyadh, Saudi Arabia for their financial support for the research work reported in this article.

**Conflicts of Interest:** The authors declare no conflict of interest.
