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 SiO{sub}2 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 (<10 nm) where repulsive forces arising from steric interference of the NOM molecules were also present. At high ionic strength (e.g., seawater) or low pH, the electrostatic forces were largely absent, allowing steric repulsion forces to dominate. In addition, adhesive bridging between surfaces by adsorbed NOM was observed, creating a strong energy barrier to spontaneous disaggregation of colloid aggregates. Our results demonstrate that adsorbed NOM dominates the surface forces and thus stability with respect to aggregation of natural water colloids.
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