Adsorption of natural humic acid and analog compounds onto goethite

Natural organic matter is responsible for carrying a great deal of elements in natural waters due to its complexing capacity and its sorption ability onto the surface of iron oxy-hydroxides. This work is aimed at analyzing the process of adsorption onto goethite of a natural humic acid and five organic acids with functional group analogs to those of humic acids. The results show that adsorption is favored at acid pH values and high ionic strength. For humic, phthalic, 2,4 and 2,5 dihydroxybenzoic (DHB) acids, there is electrostatic repulsion at neutral-alkaline pH values and low DOC concentration, also a cooperative process occurs between the solute molecules, which results in an increase in the affinity of the surface towards the adsorbate. In the case of the benzoic and salicylic acids neither steric impediments nor lateral interactions are observed, as a consequence their adsorption follows the pattern of a partition coefficient. For humic, phathalic and 2,4 DHB acids, the presence of high concentrations of electrolytes favors the formation of the adsorbate-surface bond, where the solvated Na is attracted onto the goethite surface forming an outer sphere complex, and carboxylic groups of the organic acids form a cationic bridge bond with Na+. Several layers are formed, where adsorbed molecules can offer hydrogen bridges or dipolar momentum capable of interacting with the solvent molecules thus facilitating the formation of successive layers.