Kinetics of Humic Acid Adsorption at Solid-Water Interfaces

The adsorption kinetics of purified Aldrich humic acid (PAHA) onto hydrophilic (Fe2O3 and Al2O3) and hydrophobic (polystyrene and silanized SiO2) surfaces are studied by reflectometry. The initial rate of adsorption depends on the rate of transport and the rate of attachment. Attachment on hydrophilic surfaces is relatively fast at low pH where surface and HA attract each other electrostatically. Moreover, carboxylic and phenolic groups are exposed to the outside of the HA molecules, and these groups form complexes with surface hydroxyl groups. Due to the high attachment rate the process is transport-limited. At high pH, where surface and HA repel each other electrostatically, attachment is slow, and the adsorption rate is attachment-limited. At hydrophobic surfaces attachment of HA takes place through hydrophobic attraction. Hydrophobic groups are hidden in the inner part of HA molecules, and structural rearrangements are required before attachment can occur. The slow attachment leads to an attachment-limited rate. Increasing the pH increases the number of charged groups at the outside of the molecules, and the rate of attachment becomes even slower. Adsorption rate variations with electrolyte concentration also reveal the dynamics and flexibility of the HA molecules and the sensitivity of attachment for electrostatic effects.