Tei Kim¹,Brinley Dai²,Jerry Gao³,Tony Guo Feng Tung⁴,Ashritha Kalakuntla⁵,Haaris Alam⁶,Elise Ngo⁶,Isana Alicea⁷,Aaron Sloutski⁸,Yiwei Fang⁸,Marcia Simon⁸,Jay Gao⁸,Miriam Rafailovich⁸,Steve Larson⁹,Fengxiang Han⁹

Stanford Online High School¹,Experimental High School Attached to Beijing Normal University²,Beijing No. 80 High School³,Shanghai High School International Division⁴,Ed W. Clark High School⁵,Portola High School⁶,Patchogue Medford High School⁷,Stony Brook University, The State University of New York⁸,US Army Corps of Engineers Research and Development Center⁹

Abstract

Preventing soil erosion is important in areas vulnerable to flooding, such as watersheds in the Mississippi River Basin. A potential solution is extracellular polymeric substances (EPS), which can increase and maintain the structural stability of soil by increasing resistance to environmental stresses. EPS can improve subterranean root growth, which can stabilize topsoil and help in erosion control. Furthermore, much research in the last decade has focused on explaining EPS-clay/soil interaction in order to improve its applicability. This project aims to investigate the chemical properties of Rhizobium tropici bacteria-derived EPS (RT-EPS) and its interactions with clay to model the on-site interactions with soil. EPS produced from the bacteria was used to generate an Ethanol Precipitate Material, RT-EPM, whose chemistry was researched in depth. First, the chemical composition of RT-EPM was analyzed with thermogravimetric analysis (TGA) and high-performance liquid chromatography (HPLC). TGA results indicated that most of the EPM was organic matter, and HPLC results demonstrated a significant presence of polysaccharides, with glucose being the main component (73%) and a significant presence of galactose (25%). Moreover, the chromatography also showed a dispersity of 1.013, indicating uniformity of the polysaccharides obtained from the RT bacteria. The interactions between RT-EPM and clay was explored through rheology. To examine the reformability of EPM-clay interactions in aqueous medium, an amplitude sweep, an angular frequency test, and a shear rate test were conducted. Comparing the viscosity values across the three freeze-dry and hydration cycles of the EPM, a negligible change in the mechanical properties was identified, indicating the reversible nature of the dynamic cross-linking.To examine the anchoring properties of the EPM to the soil, the rheological properties of the EPM mixed with various concentrations of clay were tested. An overall increasing soil shear strength as the clay concentration increased was found, demonstrating the anchoring capabilities of the EPM to clay. Rheology displayed the mixture’s desirable mechanical properties, which simulate RT-EPM’s on-site reinforcement of soil via shear strength enhancement. In conclusion, beside a significant polysaccharide presence, a small fraction of proteins was also found by running respective BSA-assays. It was also determined that the EPM, due to its adhesive properties, once mixed with clay forms interactions that are reversible, significant, and are environmentally-influenced. Future directions include in-depth research on the protein fraction RT-EPM contains, with a focus on their influence on the interactions with clay. In order to study the EPM-production influence on the interactions with clay, different dialysis cutoff standards will be applied to treat the EPM. Then, HPLC and rheology tests will be run separately for each cutoff (2K and 14K dialysis). C-NMR and H-NMR will also be applied to further shed light on the chemical composition of the EPM and interactions with clay. Zeta potential of the RT-EPM and clay solution will be used to measure the electrochemical potential and the stability of the particle interactions. The authors would like to thank the US Army Corps of Engineers (ERDC) for their support (W912HZ-20-2-0054) in this research.