Environ. Sci. Technol. 2003, 37, 3303-3308
Forces between Colloid Particles in Natural Waters LUKE M. MOSLEY AND KEITH A. HUNTER* Chemistry Department, University of Otago, P.O. Box 56, Dunedin, New Zealand WILLIAM A. DUCKER Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24060
The origin and nature of interparticle forces acting on colloid surfaces in natural waters has been examined using an atomic force microscope. Natural colloids were represented by a surface film of iron oxide precipitated onto spherical SiO2 particles, and the effects of adsorbed natural organic matter (NOM), solution pH, and ionic composition on the force-separation curves were investigated. NOM from both riverine and marine environments was strongly adsorbed to the iron oxide surface. Under conditions of low ionic strength, the interparticle forces were dominated by electrostatic repulsion arising from negative functional groups on the NOM, except at very small separations (12 h) 0.7-µm nominal pore size glass fiber (Whatman GF/F) filters. About 20 mL (ca. 200 times the AFM cell volume) of the solution containing the organic material was slowly injected into the AFM cell at a rate of ca. 1 mL every 2 min. The solution was left in the AFM cell for at least 16 h to allow for the adsorption of NOM and equilibration with the iron oxide surface. The same filtered seawater and river water was also used as the measuring solution for some of the subsequent AFM measurements. Following each experiment, the particle on the cantilever was examined, and its radius was measured using transmission electron microscopy (Cambridge Stereoscan). We also measured forces in the presence of a polymer of simple known chemistry, poly(methacrylic acid) (PMA). PMA is a linear polyelectrolyte that, like NOM, is dominated by carboxylic acid functional groups with similar charge densities on both (30). We are not suggesting that PMA is a suitable model for the more complex NOM. Rather, we used this simple polymer to see if any features of NOM interaction with the colloid particles and effects on interparticle forces are also exhibited by a much simpler polyelectrolyte. PMA with Mw ) 5510 and Mw/Mn ) 1.05 was obtained from Polymer Source Inc. (Montreal, Canada). A 100 mg/L solution was freshly prepared before each experiment by dissolving PMA in Milli-Q water. PMA was adsorbed onto the HFA-coated particle and plate surfaces by methods similar to that used for natural water NOM. All of the force curves we present below represent a typical case for each experiment, notionally the “average” of at least 30 measurements made with both different particles on the tip and in different regions of the plate surface for a given tip plus particle combination. These replicate measurements gave force separation curves that, while not always in exact quantitative agreement, were very similar in magnitude and general shape, except in some solution conditions noted below.
Results and Discussion AFM images of the silica following exposure to FeCl3 (Figure 1) showed that a relatively flat layer of iron oxide was formed and were consistent with adsorption of very small (
