Comparison of adsorption behavior of multiple inorganic ions on kaolinite and silica in the presence of humic acid using the multitracer technique

Abstract

The influence of humate formation on the adsorption of various ions on kaolinite and amorphous silica was studied using the multitracer technique, i.e., the simultaneous application of a variety of radioactive tracers. The technique simultaneously provided the solid-aqueous distributions of 32 elements, namely, Be, Na, Sc, V, Cr, Mn, Fe, Co, Zn, Ga, As, Se, Rb, Sr, Y, Zr, Tc, Ru, Rh, Ag, Te, Ba, Ce, Pm, Eu, Gd, Tm, Yb, Lu, Hf, Re, and Pt, in the absence and presence of humic acid. Speciation calculations under our experimental conditions were also conducted for 19 elements among them, using our previous results on the stability constants of their humate complexes. Comparison of the pH dependences of each element and humic acid dissolved in the aqueous phase enabled us to evaluate the interaction between them. With the aid of the speciation calculation, we discussed the predominant species that would control the environmental behavior of each ion. Among the elements studied, it was suggested that humate formation greatly affects the behavior of rare earth elements, which would indicate that the environmental behavior of these ions is influenced by humic substances. For alkaline earth metals (Be, Sr, Ba), divalent transition metals (Mn, Co, Zn), trivalent transition metals (Cr, Fe), and Ag, the influence of the formation of humate complexes on their distribution behavior was observed. It was estimated that the organic-inorganic complex made up of inorganic particles (e.g., kaolinite and silica in this study) coated with humic substances, plays an important role in the behavior of various ions in the environment. For the other ions of V V, Ga, Rb, Zr, Ru, Rh, Te, Hf, and Pt, humate formation was not important, but other factors such as hydrolysis and adsorption on solid surfaces were predominant factors. Based on the present results and an equilibrium model expressed by stability constants of hydroxides and carbonates or oxalates, the environmental behaviors of various cations are compared. For the oxoanions found in this study, such as As V, Se IV, Tc VII, and Re VII, humate formation also was not important. The inhibiting effect, on the adsorption of ions onto the solid surface, of the humic acid coating on the surface was observed for the distributions of V V, As V, Se IV, Zr, Ru, Rh, Hf, and Pt. These results show that the adsorption of humic substances alters the nature of inorganic particles in adsorbing metal ions.