Minerals are more complex than previously thought because of the discovery that their chemical properties vary as a function of particle size when smaller, in at least one dimension, than a few nanometers, to perhaps as much as several tens of nanometers. These variations are most likely due, at least in part, to differences in surface and near-surface atomic structure, as well as crystal shape and surface topography as a function of size in this smallest of size regimes. It has now been established that these variations may make a difference in important geochemical and biogeochemical reactions and kinetics. This recognition is broadening and enriching our view of how minerals influence the hydrosphere, pedosphere, biosphere, and atmosphere.

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Humic substances are high molecular weight, heterogeneous organic materials which dominate the natural organic matter pool in many aquatic environments and often form coatings on mineral surfaces. Humic substances sorbed to mineral surfaces may bind and hence immobilize trace metals, radionuclides, and nonionic organic pollutants, and they may also alter clay-mineral flocculation kinetics. Determining the physical shapes and forms of sorbed humic substances is essential for development of realistic pollutant-binding models. To this end, we are using direct, in-situ (in 0.01 M CaCl 2 , e100 mg C L -1 solution, pH ∼5), nanometer-scale atomic force microscopy (AFM) to image the physical shapes and forms of humic substances sorbed to the basal-plane surface of mica. Under our experimental conditions, the sorbed molecules form ring-shaped aggregates with diameters on the scale of several tens of nanometers; smaller nanometerscale rings present along the circumference could potentially represent hydrophobic domains. The highly porous threedimensional copolymeric nature of the sorbed humic substances has important implications for reactivity at the soil particle-solution interface. Ongoing research focuses on determining the effects of changing solution conditions and sorbent properties on the shapes and forms of sorbed humic substances.

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The molecular weight of humic substances influences their proton and metal binding, organic pollutant partitioning, adsorption onto minerals and activated carbon, and behavior during water treatment. We propose a lognormal model for the molecular weight distribution in aquatic fulvic acids to provide a conceptual framework for studying these size effects. The normal curve mean and standard deviation are readily calculated from measured M n and M w and vary from 2.7 to 3 for the means and from 0.28 to 0.37 for the standard deviations for typical aquatic fulvic acids. The model is consistent with several types of molecular weight data, including the shapes of highpressure size-exclusion chromatography (HP-SEC) peaks. Applications of the model to electrostatic interactions, pollutant solubilization, and adsorption are explored in illustrative calculations.

In aerobic, circumneutral environments, the essential element Fe occurs primarily in scarcely soluble mineral forms. We examined the independent and combined effects of a siderophore, a reductant (ascorbate), and a low-molecular-weight carboxylic acid (