PREDICTING HUMIC ACID ADSORPTION USING ATOMIC FORCE MICROSCOPY (AFM) FORCE SPECTRA ANALYSIS

Humic substances play an integral role in environmental systems. Their function as a major constituent of total dissolved organic matter as well as their buffering capacity proves important to a large variety of geochemical reactions, but of even greater interest concerns their interactions with other compounds and elements in the environment.

Atomic force microscopy (AFM) lends itself well to the study of interfacial surface reactions found in environmental systems. The ability to manipulate the surface of an AFM probe by coating it with various materials as well as the same flexibility in the choice of a substratum allows for the study of the interactions between unlimited combinations of AFM tips and surfaces. This research exploits the use of AFM as a means to study the adhesion force between humic acid-coated AFM tips and common mineral surfaces.

The ability to use AFM adhesion force data to predict humic acid adsorption would constitute a great success in geochemical scaling. To date there has been a general disagreement between AFM adhesion data and macroscale bacterial adhesion results, so we looked at the discrepancy between these two data sets and tried to better “scale-up” AFM data to the bench scale, with a specific focus on the reaction between humic material and mineral surfaces..

One problem in attempting to perform this cross-scale research is that the scientific community has not established a consistent method of analyzing AFM force data. Some researchers have looked at histogram analyses as a crude means to look at the frequency of adhesion force measurements between two surfaces, but the amount of information obtainable from these approaches is limited to broad categorizations. Others have reported specific adhesion forces between a tip and surface without accounting for issues such as tip reproducibility and adhesion force variability. Usually the analysis of AFM data reported in the literature is either too general or lacks a quantitative nature. In this paper we propose a new and comprehensive way to interpret AFM data using particle size spectra analyses used over the years by researchers in other fields. The preliminary results from adapting these methods to AFM “force spectra” analyses look promising at establishing a correlation between nanoscale AFM adhesion data and macroscale humic acid adsorption experiments.