COMPLEXATION OF HUMIC SUBSTANCES WITH OXIDES OF IRON AND ALUMINUM

Complexation of HA (and FA) by goethite, hematite, gibbsite, and boehm ite was studied in pre-dried systems, Hematite showed the highest fixa tion of HA at various oxide: HA ratios and at all pH greater than or e qual to 7.0. A gradual reduction in HA/FA fixation from 2.0 to 10.0 wa s 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 ha ve a remarkably dissimilar influence on HA complexation by the four mi nerals. 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 goet hites revealed a mixed trend, It is inferred that two major modes of H A bonding are operative in hematite and goethite, viz,, cation bridges forming oxide-M-HA links and direct bonding of HA to coordination cen ters at the oxide surface; forces of such bonding are strongest in hem atite, In boehmite, cation bridging is the major interactive mode, whe reas in gibbsite, HA fixation occurs primarily to coordination centers at the surface.