A new National Science Foundation-backed project from South Dakota State University will explore if biofilms made from dental plaque can help improve the stability of problematic soils.

In South Dakota, "problematic soils" — soils whose physical properties have the potential to expand, collapse or erode — negatively affect the fertility of the land, agricultural production and crop health. Under certain climatic conditions, these soils can also cause infrastructure, like roads and bridges, to crumble. With an ever-changing climate, soil erosion is expected to increase, worsening the problem.

A new project from South Dakota State University is looking to address this issue by using a rather unusual substance: dental plaque.

Aritra Banerjee is an assistant civil engineering professor at SDSU. Since earning his doctorate from the University of Texas at Arlington in 2017, he has been studying problematic soils and believes dental plaque could offer a sustainable solution.

"The inspiration for the proposed project stems from the observation that dental plaque needs specialized tools to be dislodged from teeth after hardening," Banerjee said. "If the process can be replicated to bind soil particles together, it will result in stronger soils that are resistant to erosion and may mitigate scouring."

Through a $299,797 National Science Foundation-backed project, Banerjee and his research team will explore the feasibility of using biofilms, such as dental plaque and sulfate-reducing bacteria, to stabilize problematic soils and prevent soil erosion.

Research process

First, the research team will determine what strain of bacteria will grow best in the soil. Each strain of bacteria found in the dental plaque will be cultured and tested. The strain that grows and binds best in the soil will be utilized.

"We are using strains of bacteria that are commonly found in dental plaque as the biofilms," Banerjee said. "The bacteria present in the biofilms will be utilized to regenerate the plaque in the soil so that it binds together."

Traditionally, weak soils have been stabilized with cement or lime. However, these techniques are not environmentally friendly and, for certain soils, may not even be suitable. For example, when lime or concrete is added to sulfate-rich soils, a mineral called ettringite slowly forms in the presence of water. Eventually, ettringite will lead to the concrete or pavement expanding and cracking — a phenomena otherwise known as "sulfate-induced heave." This causes civil infrastructure, like bridges and roads, to crumble well before their expected lifespan.

"Addressing the issues of sulfate-induced heave and the effect of changing climatic conditions will result in fewer maintenance activities during the life of the pavement," Banerjee said.

One of the ways to mitigate this issue is to remove sulfate from the soil prior to stabilization. The research team will look to use sulfate-reducing bacteria to eliminate sulfate before the application of lime.

"This will prevent sulfate-induced heave in bio-treated soils and mitigate erosion," Banerjee added.

Removing sulfate before the application of lime is needed for better crop health. South Dakota, Texas, Colorado, Wyoming and the southwestern United States are home to the expansive, problematic soils outlined above.

"The successful integration of sulfate-reducing bacteria in sulfate-rich soils will also allow the use of lime in such soils without facing issues of sulfate-induced heave, which causes cracks in soils and results in erosion during flooding and windstorms," Banerjee said. "Since the state of South Dakota has a significant presence of sulfate-rich expansive soils and is a major agricultural producer, such innovative measures of bio-stabilization have immense potential for adaption in agriculture and civil infrastructure development, rehabilitation and repair."

Banerjee points out that the potential, proposed solutions could enhance the health of crops and plants in drought-prone areas as biofilms have high water retention capabilities.

The project will be in collaboration with faculty members from Arizona State University and the Center for Bio-mediated and Bio-inspired Geotechnics, ASU's NSF-funded research center.

The work is expected to last approximately two years.