(701e) Relating Mechanical Properties of Biofilm-Mineral Composites to Bulk Consolidated Media Properties

Date:   April 2019

Relating mechanical
properties of biofilm-mineral composites to bulk consolidated media properties

Sobia
Anjum¹², Betsey Pitts¹²,
Adrienne Phillips¹², Robin Gerlach¹²

¹
Center for Biofilm Engineering,
Montana State University, Bozeman, MT, USA.

²
Chemical and Biological
Engineering, Montana State University, Bozeman, MT, USA.

Sponsors: Fulbright and the Department of Energy

Bacterial
biofilms are communities of bacterial cells that attach to surfaces and produce
extracellular polymeric substances (EPS). EPS act as scaffold that allows cells
to attach to surfaces but can also limit the growth of bacteria and their extracellular
reactions. The bacteria used in this study are Escherichia coli MJK2 and
Sporosarcina pasteurii ATCC11859, and the reaction of interest is
ureolysis. Ureolysis is the hydrolysis (breakdown) of urea resulting in an
increase in pH and induction of calcium carbonate precipitation in the presence
of dissolved calcium. The overall reaction can be summarized as follows:

(NH₂)₂CO
+ 2H₂O + Ca²⁺ à
2NH₄⁺+ CaCO₃(s)

This calcium carbonate
precipitation is also called biomineralization. Biomineralization carried out
by bacterial biofilms can lead to formation of conglomerates consisting of
biofilm-produced minerals and the surrounding biological materials. Such
micro-scale biomineral conglomerates can be used for the consolidation of
soils, improvement of wellbore cement integrity, to seal subsurface fractures
and as bioadhesives. The composition, spatiotemporal distribution, and
interaction of the biofilm and mineral components will affect the material and
mechanical properties of biofilm-mineral composites, which determine the
mechanical properties of e.g. consolidated surfaces. The study of structural
and mechanical features of biofilm-mineral composites has been limited, and we
are aiming to relate the microscale properties of biofilm composites to
macroscale properties such as bulk strength and stiffness of consolidated media.
In this study, spatiotemporal distribution and cohesive and adhesive strength
of biofilm-mineral composites produced by Sporosarcina pasteurii strain
ATCC11859 and Escherichia coli strain MJK2 biofilms are being assessed.
The structural features of and the mineral distribution in biofilm-mineral
composites, grown in Drip Flow Reactors (DFRs) are observed using Confocal
Laser Scanning Microscopy (CLSM) and Field Emission Electron Microscopy
(FE-SEM). The composition of the biofilm-mineral composites is analyzed as a
ratio of organic to inorganic content using Thermogravimetric Analysis (TGA).
The analyses show that the structure, composition and distribution of the
minerals varies with the bacterial strain and the availability of calcium
chloride and urea. Dynamic Mechanical Analysis (DMA) in compression and tension
modes are being optimized to measure elasticity and ductility of samples. While
a Double Cantilever Beam (DCB) method is being optimized to measure the
fracture strength of biofilm composites grown between two flat plates. We have
demonstrated that the mechanical and structural properties of biofilm-mineral
composites can be varied with operational parameters such as availability of
calcium and urea, and the bacterial strain used. We aim to develop
relationships between biofilm-biomineral composite structure, composition and
mechanical properties (such as elasticity, ductility and toughness) to aid in process
control and design of engineering applications in porous media and elsewhere.