Atomic Force Microscopy Studies of Metal Ion Sorption


Costas Tsouris Separations and Materials Research Group, Oak Ridge National Laboratory

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Sorption plays an important role both in natural waters and in water treatment processes, and is one of the techniques used to remove metal ions from aqueous solutions. However, the uptake of metal ions from aqueous solutions is limited by interactions between ions and surfaces in contact with the solution. Moreover, the fate and transport of metal ions are controlled by their reactions with surfaces at the solid–liquid interface. For example, inorganic colloids such as clays, metal oxides, and carbonates have been reported as effective sorbents of metal ions through ion exchange and surface complexation reactions. It has also been noted that surface properties of sorbents, such as surface charge and electrical double layer, change continuously during sorption of metal ions. As a result, physical transport, chemical reactivity, and/or bioavailability of the pollutants is altered during the sorption process.

Earlier studies have shown that sorption of ions from aqueous solutions by colloidal particles affects colloidal stability, mobility, and reactivity. This sorption process also has significant influences on interparticle forces and the zeta potential of the colloidal sorbent. In most cases, reactions of metal ions with functional groups on the surface of particles are time-dependent. A quantitative understanding of the changes in surface properties caused by chemical interactions between the particles and the ions present in the surrounding environment is critical to the prediction and control of the behavior of the colloidal system. This knowledge can be obtained by using atomic force microscopy (AFM), which provides direct and real-time measurements of colloidal forces during sorption of ions. However, the extent of metal ion sorption by colloidal particles is strongly influenced by several factors such as solution pH, ionic strength, size and type of the substrate, concentration of metal ions and particles, reaction time, and temperature. Atomic force microscopy is employed here to elucidate the influence of solution ionic strength and concentration of metal ions on sorption and colloidal behavior. The AFM cantilever with an attached colloidal particle can be regarded as a sensor in water/wastewater systems.