Molecular size distribution and shape of humic substance and ferrihydrite coprecipitated complexes

To compare the interaction between humic substances (HS) and ferrihydrite during coprecipitation, we prepared five ferrihydrite-HS complexes through coprecipitation using a Leonardite. The reaction products were studied by transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis. DLS data of unfiltered samples showed a bimodal size distribution, with a prevalent presence (62%) of very large particles (average size of 255 nm with a standard deviation of 272 nm) and 38% of smaller particles with 85 nm of size (standard deviation of 14 nm). The TEM images showed irregular subangular macroparticles with colloidal structure and with rough or irregular surfaces rounded by very small particles. Indeed, most of the rounded and angular particles could be small ferrihydrite aggregates that are also observed as discrete particle association. The smooth surfaces of the round and angular particles suggested that they were poorly crystalline Fe phases. The TEM results coincided with DLS analysis that the complexes were a woven network of micelles structure.