INFLUENCE OF HUMIC SUBSTANCES ON CO2+ SORPTION BY A SUBSURFACE MINERAL SEPARATE AND ITS MINERALOGIC COMPONENTS

The sorption of Co2+ (10(-6) mol/L) was measured on subsurface mineral materials in the absence and presence of a sorbed leonardite humic ac id (LHA) to (1) evaluate the sorptive role of mineral-bound humic subs tances, and (2) establish approaches to model metal ion binding in com posite materials. The subsurface materials were a <2.0 mu m size fract ion of an ultisol saprolite (CP) and this same material treated with d ithionite-citrate-bicarbonate (DCB) to remove Fe-oxides (DCP). Compara ble experiments (with and without LHA) were also performed with minera l sorbents representing dominant phases in the CP separate (gibbsite, Al-goethite, and kaolinite) to evaluate their potential contributions to Co sorption. The mineral-bound LHA ranged in concentration between 0.1-0.4 mg-C/m(2), representing approximately 0.7% of the subsurface i solate by mass. The sorption-desorption of LHA on the mineral surfaces , and its affinity for Co as a aqueous phase complexant were also dete rmined. Batch measurements were employed (sorbents at 20-90 m(2)/L; LH A-DOC at approximate to 11 mg-C/L) over a range in pH and ionic streng th (I) at I = 0.01 and 0.1 in NaClO4. The LHA strongly sorbed to the s ubsurface mineral isolates (CP and DCP), and to all the specimen sorbe nts except kaolinite. Maximum sorption of LHA occurred at lower pH (ap proximate to 4.5). In solid-free suspensions, the affinity of LHA for Co increased with pH and decreasing I(K-d ranging 20-450 L/g). Mineral -bound LHA increased Co sorption on all the sorbents by factors of 10- 60%, with the greatest augmentation noted at pH values (4.5-6.5) where (1) maximum LHA sorption occurred, and (2) Co sorption to the mineral phase was weak and dominated by ion exchange. The LHA appeared simply to augment, rather than to change the intrinsic adsorption behavior o f the mineral sorbents. Accordingly, predictions of the K-d for Co on the LHA-coated subsurface materials (DCP, CP) based on a linear additi vity model agreed well with the experimental data, suggesting that the complex humic-mineral association acted as a noninteractive sorbent m ixture at low aqueous Co concentrations.