# Enhancing the Strength of Mine Residue Soil by Bioremediation Combined with Biopolymers
## 1 Introduction
XG forms a highly viscous gel when hydrated, coating soil particles and creating bonds between soil particles, namely clay particles, via electrostatic (ionic and hydrogen) interactions [4; 9; 12], promoting an increase in the shear strength and stiffness of the bio-stabilized soil [8; 9; 11; 13], increasing the optimum water content and the plasticity characteristics (mainly the liquid limit) [13; 15], reducing permeability [16], and enhancing erosion resistance [4; 17]. The high molecular weight and long-chain structure of XG contribute to its effectiveness in soil stabilization. Regarding the mine residue soils, a very limited number of studies have been published on the subject. Studies from [PERSON] et al. [17] and [PERSON] et al. [18] indicated that the addition of XG effectively reduced mine tailings/residue erosion. Nevertheless, it is not clear what effect the addition of XG to a mine residue soil has in terms of unconfined compressive strength and stiffness. Carboxymethyl cellulose (CMC) is a biopolymer derived from cellulose, a major component of plant cell walls, and on wooden banks of trees [3]. CMC is widely used in various industries [19] due to its high water-retaining capacity, adhesion, ability to change the viscosity of the system [7], and film-forming properties. In soil stabilization, CMC acts as a binder and a water-retaining agent, filling the voids between soil particles and forming a cohesive matrix. These features result in reduced soil permeability and water infiltration rate [3; 7; 20], increased optimum water content and decreased maximum dry unit weight [10], and increased cohesion between particles, and therefore the strength and stiffness of the bio-stabilized soil [3; 19; 20; 21], which promotes vegetation growth and increases soil erosion resistance [20], contributing to the enhancement of soil properties [20]. Regarding the effect of CMC in contaminated soils, a field assessment of CMC-bridged chlorapatite microparticles was performed by [PERSON] and co-authors [22], who demonstratedthe effectiveness, long-term stability, and mechanism of CMC in reducing the mobility of lead in contaminated soil, with the best results achieved at a CMC content of 0.5%. The effect of CMC on mine residue soils has not yet been studied, and its effect in terms of unconfined compressive strength and stiffness is unknown. Bioremediation techniques offer another avenue for soil stabilization by harnessing the power of microbial activity to enhance soil properties and/or degrade contaminants. Biostimulation involves stimulating the native microbial population in soils by providing suitable growth conditions, such as nutrients or oxygen. This stimulation enhances microbial activity, leading to the degradation or transformation of contaminants or improvements in soil properties. [PERSON] et al. [16] demonstrated a reduction in soil permeability using bacteria-produced biopolymers through biostimulation. On the other hand, bioaugmentation involves the introduction of specific microorganisms or microbial communities into the soil to remediate contaminants or improve soil properties. These microorganisms can degrade pollutants or transform them into less harmful substances, or induce the calcium carbonate precipitation that binds soil particles [4]. [PERSON] et al. [23] explored the combined application of CMC-stabilized nanosized Fe\({}^{0}\) and mixed anaerobic microorganisms for the enhanced stabilization of chromium (VI) in soil. The authors observed that CMC enhanced the dispersion of nanosized Fe\({}^{0}\) and at the same time promoted the activity of microorganisms. Despite the research to date, there are still gaps in the knowledge regarding the application of these bio-techniques (biopolymers and bioremediation) to mine residue soils. In addition, the combined application of these two bio-techniques is a subject that has been less discussed but can bring technical benefits and therefore deserves to be studied.