The Role of Water Hydration States in a Bacteria Derived Biopolymer (EPS) /Nanoclay Hydrogel

With the onset of climate change, there is increasing concern about its effects on soil erosion. Microorganisms produce extracellular polymeric substances (EPS) that have shown potential in lessening the impact of environmental stressors. Specifically, a Rhizobium tropici (R. tropici) derived biopolymer has been reported to effectively strengthen the soil and mitigate soil erosion. Another study suggests that this biopolymer enhances the plant growth, proliferates leaf production and develops a compact root structure. To probe the mechanism behind EPS and its role in compacting soil and stimulating plant growth, we studied water retention and water-biopolymer, soil-biopolymer interactions.<br/>EPS produced from Rhizobium tropici bacteria were ethanol precipitated into EPM(ethanol precipitated materials). 5%, 10%, 20% EPM biopolymers rehydrated with different kaolinite (EPK) concentrations were prepared. For each concentration of biopolymer and water mixture, four samples were made, containing clay that are 0%, 5%, 10%, and 20% by mass. The hydrated water molecules in polymer can be divided into three classes: free water, intermediate water (weakly bonded), and non-freezing water (tightly bonded). Differential Scanning Calorimetry (DSC) was carried out on EPS/EPK hydrated samples by a cooling scan from 25°C to -50°C followed by a heating scan to 25°C at 10 °C/min rate. The proportion of the freezable water (free water), non-freezable water was calculated based on the enthalpy of the melting peaks. The calculated non-freezable water in rehydrated EPS increased from 5% to 21% with higher EPS concentrations. This indicates a possible formation of a more compact structure when EPS is at high concentration. In the EPS/EPK system, the presence of a secondary melting peak at -8°C which is not observed in EPK/water mixture indicates that kaolinite in EPS induces a weakly bounded water layer. On the other hand, the percentage of non-freezable decreases as more EPK is introduced in EPS. A possible explanation of this is that the surface of kaolinite contact water diminishes the surface where water is enclosed by entangled biopolymer EPS. A model of nacre-like structure of EPS-EPK-EPS is supported by cryo-SEM images, and postulated to be the mechanism of EPS stabilizing the soil.<br/>Next, the water retention behavior of EPS was investigated by two means. A low concentration of 1% rehydrated EPS water solution droplet ~ 5 μL was dropped on untreated silicon wafer. Subsequently, the droplet width and height versus time were measured at 3-minute intervals using the goniometer. Using water contact angle analysis, the rate of evaporation was determined for each sample at pH 5, 6, 7, 8, and 9 to simulate performance in real environmental soil conditions. Lastly, the water retention ability of biopolymer EPS was also assessed by taking one droplet~10 mg on a mass balance to record the rate of evaporation by mass over time.<br/>Calorimetry data indicated that higher concentrations of biopolymer and percent of non-freezable water were positively correlated. Also, the addition of kaolinite changes the surface chemistry of EPS biopolymer. Droplet analysis of all three solutions revealed that the biopolymer evaporated height-wise, while water evaporated equally in height and width. Biopolymer solutions from varying pH all evaporated height-wise, with greater height change in solutions further from the original 7 pH solution. The formation of a complex from the EPS/EPK may explain its soil strengthening ability. These suggest possible applications such as maintaining crop-soil moisture and aiding soil compaction during freeze-thaw cycles. Future goals are to solidify our understanding of the interaction between EPS and nanoclay with field tests, and properties that promote soil health, paving the way to an environmentally friendly future.<br/><br/>Work supported by the ERDC (W912HZ-20-2-0054) and the Morin Charitable Trust.