Analyzing the Rheology of a Rhizobium Tropici-Produced Entangled Exopolysaccharide and Its Effect on Soil Stability

A promising strategy to combat erosion has recently emerged from a Rhizobium tropici-produced exopolysaccharide, which effectively transforms the mechanical properties of soil, reducing erosion1. Research has also uncovered that applying Rhizobium tropici-produced extracellular polymeric substance (EPS) enhances root and vegetation growth, offering prospective improvements in soil stability¹. In addition, RT-EPS's prolific yielding, improvement of soil adhesion and nutrient and water retention, and biodegradability² provide a sustainable and inexpensive method for curtailing erosion. However, the properties and influence of RT-biopolymer entanglement in gelled solutions remain largely uncertain, limiting the efficacy of its field utilization. Thus, this study aims to understand RT-EPS’s rheological characteristics and how these properties can improve basin and littoral soil stability. It was hypothesized that RT-EPS forms entanglement hydrogels, with higher biopolymer concentrations resulting in increased viscosity.
Gels of deionized water and RT-EPS concentrations (c) ranging from 0.1 mg/mL to 15 mg/mL were utilized to explore RT-EPS’s rheological properties. RT-EPS gel shear viscosity (Pa●s) (η) was plotted against the shear rate (s^-1) from 0.001 s^-1 to 94 s^-1, and zero-shear viscosity values were determined. The flow curve of RT-EPS across all concentration regimes fits the classic Ostwald–de Waele relationship and revealed that biopolymer hydrogels exhibit shear thinning behavior, with the flow behavior index, (n), generally decreasing as RT-EPS concentration increases. Using the model η_sp = k(c)ⁿ, we determined that there is a superlinear relationship between solute concentration in RT-EPS hydrogels and gel viscosity, where η_sp ∼ c^0.851 in the dilute regime, η_sp ∼ c^2.88 in the semi-dilute regime, and η_sp ∼ c^3.48 in the entangled regime. The shear rate sweep also revealed an overlap concentration (c^*) of 1.875 ± 0.0125% wt% and an entanglement concentration (c_e) of 4.5 ± 0.5%. This showed that RT-EPS hydrogel is an entangled polymer solution after c > 4.5 ± 0.5%.
A shear stress (τ) versus shear rate plot for 0.001 s^-1 to 1000 s^-1 revealed that RT-EPS data fits the Herschel-Bulkley equation and also demonstrates shear thinning behavior. A positive relationship between yield stress (τ_y) and RT-EPS concentration was also found.
We also performed a frequency sweep over an oscillation cycle (0.001 Hz ≤ f ≤ 50 Hz) at 25°C. Storage modulus (G’) and loss modulus (G’’) versus frequency curves showed that RT-EPS generally demonstrates solid-like behavior at lower frequencies and elastic behavior at higher frequencies. We also solved for the frequency when tanδ (G''/G') is equal to one to obtain the gel points for each sample. Plotting gel point versus concentration showed that the RT-EPS hydrogel gel point decreases with increasing concentration.
200 mL samples containing 160 mL of sand and 40 mL of gelled RT-EPS samples concentrated with 1 mg/mL, 3 mg/mL, and 5 mg/mL EPM were used for soil stability analysis. Compared to a control mixture of 160 ml sand and 40 ml DI water, samples containing RT-EPS presented consistently higher G’ values with the 5 mg/mL samples being stiffer until ≈ 0.4% strain, where all experimental samples began to follow very similar stiffness trends. We found that RT-EPS could strengthen the soil as G’ increases with increasing RT-EPS concentration. Future work includes further soil stability testing.
We would like to thank the Louis Morin Charitable Trust and the US Army Corps of Engineers (EDRC) for supporting (W912HZ-20-2-0054) this research.

¹Luo, H., et. al (2022). Forestry Research and Engineering International Journal, 5(1), 17–20. https://doi.org/10.15406/freij.2022.05.00102
²Larson, S., et. al (2016). U.S. Army Engineer Research and Development Center. https://apps.dtic.mil/sti/pdfs/AD1011606.pdf.