# Reducing Soil Permeability Using Bacteria-Produced Biopolymer
## 4 Conclusions  
This work compares the ability of the xanthan-like gum obtained directly from a strain of _S. maltophilia_ Faro439 species (LabXLG), with commercial xanthan gum (XG) and Portland cement to decrease the permeability coefficient of a sandy soil. In parallel, the effect of the curing time (3, 7, 14, and 28 days), the hydraulic gradient (6, 8, 2, 9, 4, and 10.6), and the XG content (0.16 and 0.33%) and cement (0.33 and 1.0%) are also analysed. From the experimental testing programme, the following conclusions may be drawn:  
1. The treatment of the soil with both types of XG induces a significant decrease in the coefficient of permeability compared to the untreated soil. 2. The comparison of the two types of XG shows that the LabXLG is less effective than the commercial XG, which is related to its lower level of purity. The lyophilized biomass formed by _S. maltophilia_ Faro439 also contains lyophilized cells as well as the biopolymer. 3. The use of a 0.33% content of commercial XG is more effective to reduce the coefficient of permeability of the treated soil than the use of 1% cement, namely for a curing time of 7 and 14 days. These results indicate that XG can replace the use of cement in the short term, for instance in temporary works. 4. The increase in the hydraulic gradient induces a slight decrease in the permeability coefficient. This is consistent with the descendent water flow used in the permeability tests since the increase in the hydraulic gradient tends to increase the vertical effective stress with the consequent decreases in the void ratio and the soil's permeability. 5. The treatment with both types of XG demonstrates a slight decrease in the permeability coefficient during the first 14 days of curing, followed by an increase for longer times. The initial decrease in the soil's permeability is associated with the hydration of the hydrogels, while the increase in the soil's permeability over time probably reflects the existence of some biodegradation of the XG and/or the eventual dehydration of the hydrogels associated with the consequent shrinkage. 6. The microstructure of the treated soil depends on the type of XG, which is linked with each one's effectiveness to reduce the soil's permeability. Thus, the LabXLG creates a network of fibres that links the soil particles and decreases the voids in the soil, while the commercial XG induces the partial filling of the pore spaces with a homogeneous paste, probably due to the hydration of the hydrogels. Finally, it should be mentioned that taking the production process and its effectiveness in reducing the permeability coefficient into consideration, the experimental results of this work show that the xanthan gum produced using the _S. maltophilia_ Faro439 strain emerges as an interesting alternative for use in the stabilisation of sandy soils. Although _Stenotrophomonas maltophilia_ is included in the BSL-2 facultative pathogen group, there are no regulations available for the release of this type of microorganism into the environment, and the number of cells detected in the assays with soil is very low. Therefore, future studies are still needed to ensure that this material is fully safe. Conceptualization, A.C.P., P.V.M. and A.P.C. ; writing--original draft preparation, A.M., A.C.P. and P.V.M. ; writing--review and editing, A.M., P.V.M., P.J.V.O. and A.P.C. ; funding acquisition, P.V.M. and P.J.V.O. All authors have read and agreed to the published version of the manuscript. This work was supported by the projects ERAMIN2 REIVIVING funded by Fundacao para a Ciencia e Tecnologia (FCT), project PTDC/CTA-AMB/31820/2017 and FCT UIDB/00285/2020, BIORECOVER H2020 grant agreement 821096, project POCI-01-0145-FEDER-028382 and the R&D Unit Institute for Sustainability and Innovation in Structural Engineering (ISISE), under reference UIDB/04029/2020. Not applicable. Data Availability Statement: No new data were created or analysed in this study. Data sharing is not applicable to this article.