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Scanning Electron Microscopy of a Humic Acid and of a Fulvic Acid and its Metal and Clay Complexes

Chen, Y., Schnitzer, M.
Soil Science Society of America journal 1976 v.40 no.5 pp. 682-686
aluminum, copper, fulvic acids, humic acids, iron, pH, scanning electron microscopes, scanning electron microscopy, solubility
A Scanning Electron Microscope was used to explore the effect of varying the pH on the shape, size, and degree of aggregation of humic and fulvic acid particles. The structure of the complexes formed by fulvic acid when reacted with Cu²⁺, Al³⁺, Fe²⁺, and Fe³⁺ was also studied, as was the reaction product of Na-montmorillonite and fulvic acid. At low pH (2–3), fulvic acid occurred mainly as elongated fibers and bundles of fibers, forming a relatively open structure. With increase in pH (4–7), the fibers tended to mesh into a finely-woven network to yield a sponge-like structure. Above pH 7, a distinct change in structural arrangement and improved orientation was observed. At pH 8, the fulvic acid formed sheets which tended to thicken at pH 9. At pH 10, fine, homogeneous grains were visible. The effect of pH on the humic acid structure was similar to that observed on fulvic acid except that because of low solubility in water, the pH range had to be narrowed to between 6 and 10 and the pH at which the major transitions occurred was higher. Micrographs of fulvic acid-metal and fulvic acid-clay complexes show interesting possibilities for the use of this approach in investigations on metal- and clay-humic complexes.