Bacterial Adhesion to Metal Oxide Surfaces in the Presence of Natural Organic Matter

Open Access
Author:
Winslow III, Charles John
Graduate Program:
Environmental Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 29, 2009
Committee Members:
  • Bruce Ernest Logan, Thesis Advisor
  • William D Burgos, Thesis Advisor
  • James David Kubicki, Thesis Advisor
Keywords:
  • natural organic matter
  • metal oxides
  • atomic force microscopy
  • AFM
  • bacterial adhesion
Abstract:
An understanding of the interaction between natural organic matter and metal oxides is critical to predicting bacterial adhesion in natural systems. The objective of this research was to examine bacterial adhesion to various metal oxide surfaces in the presence of humic acids using nanoscale atomic force microscopy (AFM) experiments. AFM was used to measure the adhesion force between standard silicon nitride AFM probes (i.e. bacterial surrogates) and various surfaces before and after humic acid adsorption as a function of residence time. Five surfaces were analyzed in this study, including three metal oxide surfaces (Fe2O3, Al2O3, and TiO2), a glass microscope slide (SiO2), and freshly-cleaved muscovite mica. Adhesion to each surface was quantified in terms of both the overall interaction between the AFM probe and the surface (i.e. the average force or Favg) as well as the localized interaction between the AFM probe and the stickiest five percent of the surface sites (i.e. the sticky force or F5). Spectral force analysis (SFA) was introduced as a useful tool for analyzing the distribution of adhesion forces on a surface by simultaneously providing insight into the effect of humic acid adsorption, residence time, and the small number of highly sticky surface sites. Adhesion force to the uncoated surfaces increased in the order SiO2 < mica < TiO2 < Al2O3 < Fe2O3, irrespective of whether the average (Favg) or sticky (F5) forces were analyzed. Increasing the tip-surface residence time resulted in an overall increase in adhesion force but not a change in the relative stickiness of the five surfaces. At short residence times (i.e. zero seconds) humic acid adsorption resulted in an increase in adhesion force on all surfaces except mica and Fe2O3, irrespective of whether the average forces or sticky forces were analyzed. At long residence times (i.e. ten seconds) humic acid adsorption resulted in an increase in adhesion force on all surfaces except mica and Fe2O3 when considering the average forces. When considering the sticky forces at long residence times, humic acid adsorption resulted in an increase in adhesion force on all surfaces except Fe2O3. No consistent explanation could account for the observed results, although arguments based on electrostatics and surface roughness could explain the tip-surface interaction in certain instances. It was concluded that standard silicon nitride AFM probes are not ideal bacterial surrogates, so further work with colloid AFM and bacteria is needed to better understand bacterial adhesion to metal oxides in the presence of natural organic matter.