Direct observation of macromolecular structures of humic acid by AFM and SEM

Humic substances (HSs) can be easily adsorbed on natural solid surface and thereby affect the physicochemical behavior of metal ions in the natural environment. The influence of HSs is mainly attributed to the macromolecular structures and shapes of solution soluble and surface adsorbed HSs. Herein, to our knowledge, we firstly use both AFM and SEM to image and to compare the macromolecular structures of humic acid on the basal-plane surface of mica and silicon. Under our experimental conditions, spherical colloids are formed at low pH and high salt concentrations, while network or linear structures are formed at high pH and low salt concentrations. The shapes of HA are heterogeneous under different natural environmental conditions. The knowledge of macromolecular structures and shapes of HA in the natural environment is significant for the evaluation of metal ions’ behavior in HA–metal ion–mineral ternary systems.