Kinetics of humic acid adsorption at solid-water interfaces

The adsorption kinetics of purified Aldrich humic acid (PAHA) onto hydrophi lic (Fe2O3 and Al2O3) and hydrophobic (polystyrene and silanized SiO2) surf aces are studied by reflectometry. The initial rate of adsorption depends o n the rate of transport and the rate of attachment. Attachment on hydrophil ic surfaces is relatively fast at low pH where surface and HA attract each other electrostatically. Moreover, carboxylic and phenolic groups are expos ed 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 tr ansport-limited. At high. pH, where surface and HA repel each other electro statically, attachment is slow, and the adsorption rate is attachment-limit ed. At hydrophobic surfaces attachment of HA takes place through hydrophobi c attraction. Hydrophobic groups are hidden in the inner part of HA molecul es, and structural rearrangements are required before attachment. can occur . The slow attachment leads to an attachment-limited rate. Increasing the p H increases the number of charged groups at the outside of the molecules, a nd the rate of attachment becomes even slower. Adsorption rate variations w ith electrolyte concentration also reveal the dynamics and flexibility of t he HA molecules and the sensitivity of attachment for electrostatic effects .