# A Review on the Importance of Microbial Biopolymers Such as Xanthan Gum to Improve Soil Properties
## 2 Biological Processes in Geotechnical Engineering
### Soil-Biopolymer Interaction  
Recent studies indicate that the effect of the use of biopolymers in soil treatment is due to the association and the balance of two main factors, the creation of additional links between the soil particles and the filling of the soil's voids (at least partially) with hydrogels. The creation of links in the soil skeleton explains the better effectiveness of this methodology in strengthening clayey soils in relation to sandy soils [1]. In fact, biopolymers have high specific surfaces with electrical charges that enable them to interact directly with the clay's minerals, and also provide firm matrices of high strength biopolymer-soil due to the electrical charges [6]. This type of link is responsible for the increase in the unconfined compressive strength and cohesion resistance observed in clay-biopolymer mixtures in relation to the results obtained with pure sands [1; 12]. Although the effect of biostabilisation with biopolymers is higher in the presence of clays than in sands, the experimental results show that the impact of this methodology in sandy soils is also positive. The SEM (Scanning Electron Microscopy) image of the structure of a sand stabilized with xanthan gum (Figure 1) clearly shows that the stabilization of sandy soils with biopolymers creates a network of links between the soil particles. \begin{table}
\begin{tabular}{c c} \hline
**Biopolymer** & **Effect** \\ \hline Casein and Sodium Caseinate salt & Stabilize sand dunes [1]. \\ \hline Curdlan & Clogs the soil [2]. \\ \hline Chitosan gum & Decreases hydraulic conductivity [11]; Remediates wastewater and contaminated soil [1; 5]. \\ \hline Xanthan gum & Decreases permeability; \\  & Retains water due to strong hydrogen bonding [1; 12; 13]. \\ \hline \(\beta\)-Glucan & Increases the resistance to soil erosion and promotes the growth of vegetation [1; 14]. \\ \hline Gellan Gum & Improves mechanical properties [1; 4; 14; 15]. \\ \hline Polyacrylamide (PAM) & Decreases soil erosion due to PAM hydrogels [1; 15]. \\ \hline Agar Gum & Rapid gelation and increases water infiltration [16]. \\ \hline  & Increases mechanical properties [1; 4; 17; 18; 19]; \\ Guar Gum & Reduces the collapsible potential of plastic soil [13]; \\  & Decreases the hydraulic conductivity of silty sand [20]; \\  & Increases the liquid limit of kaolinite clay [21]. \\ \hline \end{tabular}
\end{table} 
Table 1: Effect of common biopolymers on soil properties. Previous studies have demonstrated that this methodology is suitable for use in the majority of natural soils, which are composed of a mixture of coarse and fine particles (silt and clay), where there is a combination of the strength of the biopolymer-clayey-soil matrices (improvement in cohesion), and the aggregation of coarse particles (improvement in the friction angle) can be observed [11]. Taking these results into consideration, a previous in situ soil characterization is recommended to better understand its composition and to determine the most appropriate method and biopolymer to maximize the strengthening effect [12]. A second effect of the use of biopolymers in soil treatment is the formation of viscous hydrogels capable of filling the soils' pores spaces, especially in sand, which causes the bioclogging effect, which has, as a main consequence, the reducing of the hydraulic conductivity by about 3-4 orders of magnitude [13]. Therefore, biopolymers have the potential to be applied in several hydraulic purposes such as grouting, slurry walls and infiltration barriers (temporary) [14]. The amount of water is another essential factor to be considered in soil-biopolymer interaction. A study by [PERSON] et al. [1] showed that when soils have a higher water content, the system formed by the soil and the biopolymer, whether it is a dehydrated gel or a matrix, has the ability to adsorb and transfer water to hydrogels.