Experimentally Derived Sticking Efficiencies of Microparticles using Atomic Force Microscopy: Toward a Better Understanding of Particle Transport

It is estimated that there are 5x1030 microorganisms on Earth and that approximately 50% live in unconsolidated sediment on the terrestrial subsurface. Subsurface disturbances caused by the constant search for natural resources and our dependence on groundwater make the abundance and diversity of these organisms a global concern. It is vital to many environmental fields, including bioremediation, water purification, and contaminant transport, that we understand how microorganisms and other colloidal particles attach to and detach from natural sediments and ultimately how they travel through porous media.

Sticking efficiency (alpha) is a major component of most particle transport theories. It is defined as the ratio of particles that adhere to a collector surface compared to the total number of particles that collide with that surface. In this study, the Interaction Force Boundary Layer (IFBL) model was used to determine the sticking efficiencies of inorganic colloidal particles and Enterococcus faecalis cells against a silica glass collector surface. Sticking efficiencies were derived from intersurface potential energies that were determined from integrated force-distance data measured by Atomic Force Microscopy (AFM). Force data were measured in buffered aqueous solutions of varying pH and ionic strength to determine the influence of solution chemistry on particle removal from solution. Zeta-potentials were measured to determine the impact of particle and collector surface charge on force measurements.

The results of this study indicate that alpha is strongly influenced by solution chemistry. The response of alpha to small changes in solution pH and ionic strength may be several orders of magnitude. Zeta-potential measurements imply that sticking efficiencies are strongly influenced by the electrical charges on both the particle and collector surfaces. Zeta-potentials of bacteria did not vary significantly with changing solution pH, but did respond to changing solution ionic strength.

Historically, alpha has been very difficult to predict. This study is the first to report sticking efficiencies measured using AFM and the first to successfully apply the IFBL model to colloidal particles. Æ nThe incorporation of empirical nanoscale interactions into the measurement of alpha promises to more successfully describe particle adhesion and, thus, particle transport.