Desorption of humic acids from an iron oxide surface

The adsorption and desorption of different humic acids on iron oxide-coated silicon plates was studied in stagnation point flow using a reflectometric technique. The adsorption increased by decreasing the pH. Desorption studies were conducted by first adsorbing the humics at low pH and then desorbing them by either increasing the pH or rinsing with supporting electrolyte (0.1 M KNO3) at a constant pH. Two different desorption processes can be distinguished depending on the method used for desorbing the humics: desorption upon change in pH without changing the HA concentration and desorption upon dilution without changing the pH. When desorption is induced by changing the pH, the desorption is fast and reversible, and the rate of desorption increases by increasing the pH. At the new pH the surface becomes oversaturated with HA molecules because of increased molecule-molecule repulsion (electrostatic) and decreased surface-molecule attraction (electrostatic and specific). Desorption upon dilution at a constant pH is so slow that the equilibrium condition cannot be reached in practice. Since there is no change in pH, the interactions in the adsorbed layer remain unchanged, and the only driving force for desorption is given by the decrease of the HA concentration in the bulk solution. Such a. change is much less effective than a direct change of the interactions in the adsorbed layer (by changing the pH), and it is insufficient to promote a fast desorption.