Complexation of Humic Substances with Oxides of Iron and Aluminum

Abstract

Complexation of HA (and FA) by goethite, hematite, gibbsite, and boehmite was studied in pre-dried systems. Hematite showed the highest fixation of HA at various oxide: HA ratios and at all ph ≥ 7.0. A gradual reduction in HA/FA fixation from 2.0 to 10.0 was observed for all minerals except gibbsite, which showed a very sharp decrease at pH > 7.0 and a maximum at pH 5.0. Exchangeable cations have a remarkably dissimilar influence on HA complexation by the four minerals. Thus, various cationic forms of gibbsite showed a drastic loss of HA fixation capacity compared with the original (pronated) surface, whereas in boehmite, the reverse behavior was observed. Most of the metal ion-substituted hematites showed excellent HA fixation, but goethites revealed a mixed trend. It is inferred that two major modes of HA bonding are operative in hematite and goethite, viz., cation bridges forming oxide-M-HA links and direct bonding of HA to coordination centers at the oxide surface; forces of such bonding are strongest in hematite. In boehmite, cation bridging is the major interactive mode, whereas in gibbsite, HA fixation occurs primarily to coordination centers at the surface.