Atomic force microscopy and surface characteristics of iron oxides formed in citrate solutions

Surface geometry of minerals greatly influences the physical, chemical, and biological processes occurring on the surface. However, a quantitative or even a qualitative description of the surface geometry of minerals has proven to be extremely difficult. In our study, the fine-scale morphology (1 by 1 micrometer scale) and surface geometry described by mean surface roughness and surface fractal dimension of Fe oxides formed at various concentrations of citrate, which is common in terrestrial and aquatic environments, were investigated by atomic force microscopy (AFM). Specific surface area and Point of Zero Salt Effect (PZSE) of the Fe oxides, as well as P adsorption, were also studied. Citrate present during the formation of Fe oxides significantly altered the fine-scale morphology, surface geometry, and other surface characteristics of the products. The mean surface roughness and surface fractal dimension determined by AEM measured the degree of the disorder of surface structure of Fe oxides. The modification of the surface characteristics of the Fe oxides by coprecipitated citrate through fundamental structural changes and the blocking of P-adsorption sites by citrate affected the P adsorption. Due to the hindrance of the crystallization process, P adsorption per unit weight of the Fe oxides formed at 10(-3) M citrate was very significantly enhanced. The fine-scale morphology, surface geometry, and related surface characteristics of Fe oxides formed under the influence of organic acids merit close attention as we advance our understanding of their surface chemistry pertaining to dynamics and transformations of nutrients and pollutants in terrestrial and aquatic environments.