The bacterium Rhizobium tropici attaches to the roots of leguminous plants and produces nodules which convert atmospheric nitrogen to ammonia. It uses carbohydrates to produce an extracellular polymeric substance (EPS), a biopolymer, which can be implemented as an additive to increase the viscosity of substances, such as aqueous solutions. It is of great interest to take advantage of these naturally developed biopolymers and use them beyond the simple symbiotic advantages between legumes and rhizobial microbes. There are many applications of these biopolymers in the civil, agricultural, and military sectors. Our project aims to improve upon the physical properties of soil in order to contribute to those sectors, such as creating more sustainable training grounds, detecting soil disturbances, and suppressing dust formation for the military. To effectively alter the physical properties of the Rhizobium tropici EPS and ultimately increase the substance’s overall viscosity, we sought to modify the biopolymer’s surface charge (i.e. the zeta potential). We hypothesized that adjusting the surface charge of a biopolymer would alter the biopolymer’s viscosity. More specifically, our hypothesis is that as the zeta potential of a biopolymer decreases, the viscosity of a biopolymer increases. It is further hypothesized that when negatively charged clay neutralizes the excess positive charge of the biopolymer samples, it increases the content of neutral particles, leading to increased Van der Waals interactions and increased viscosity. This hypothesis was confirmed by measuring the viscosity of Xanthan Gum and Locust Bean Gum biopolymer mixtures of various molar and mass ratios at different pH levels, as well as adding either 10% Edgar Plastic Kaolin clay or 10% halloysite clay. These varying characteristics allowed us to compare their zeta potentials with their measured viscosity. Further characteristics of the EPS were also revealed in this study through the following tests: thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). From these initial observations, we believe that altering the zeta potential of the Rhizobium tropici EPS can serve as a potential method for modifying the biopolymer’s viscosity and enhance its application for the military and beyond.

Modifying the rheology of rhizobium tropici EPS through surface charge to enhance soil applications

ACS Spring 2022

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