In this study, the effects of adsorbed fulvic acid, a naturally occurring organic acid, on the kinetics of hematite aggregation and on the resulting structure of hematite aggregates are investigated. A model based on the colloid stability theory, which also accounts for the structure of the aggregates formed, is used to describe the aggregation kinetics of these adsorbed particles. The study of aggregate structure shows that the fractal dimensions of hematite aggregates that are partially coated with fulvic acid molecules are higher than those obtained with no adsorbed fulvic acid. The scattering exponents obtained from static light-scattering experiments of these aggregates range from 2.83 ± 0.08 to 3.42 ± 0.1. The scattering exponents of greater than 3 indicate that the scattering is the result of an object that contains pores that are bounded by surfaces with a fractal structure and can be related only to surface fractal dimension. The high fractal dimensions are due to restructuring within the aggregates, which only occurred at low coverage by the organic acid. At high fulvic acid concentrations, restructuring is not observed and flocculation bridging occurs, resulting in more open structures, with scattering exponents ranging from 2.07 to 2.72. © 1992.
