Gleichgewicht und Kinetik der Sorption von Schwermetallionen an reinem und polymermodifiziertem Goethit

The thesis deals with the reaction of the heavy-metal ions Pb$^{2+}$, Cd$^{2+}$ and Ni$^{2+}$ with pure goethite as well as with polymethacrylic acid (PMA) modified goethite. Subject of investigations are the charging behaviour of the oxide goethite, the equilibrium and the chemical kinetics. The results obtained are interpreted by means of two surfeve complexation models. Potentiometric titrations of goethite at different ionic strengths have shown that in the pH range between 3.0 and 6.0 the surface is positively charged. The charging behaviour is described quantitatively well by the triple layer model (TLM). All pH-dependent adsorption isotherms of Pb$^{2+}$, Cd$^{2+}$ and Ni$^{2+}$ on goethite at constant ionic strength show the same S-shape course. Due to the increased screening of charge with increasing ionic strength the adsorbed amount of heavy-metal ions on goethite also rises. Lead ions - in general - show a higher affinity to goethite than cadmium and nickel ions. With respect to different adsorption mechanisms considered in the triple layer model the experimental data are interpreted best if the adsorption of the hydroxo-species NOH$^{+}$ in the 1-layer is assumed. This result is also confirmed by the evaluation process developed by Kallay which combines the double layer model (DLM) with electrophoretic mesurements. Further information about the adsorption mechanism is obtained by determining rate constants from kinetic measurements. For this purpose the pressure jump relaxation technique (p-jump) is used where the reaction is monitored by electrical conductivity. Two diffusion processes are observed: (1) the fast transport of MOH$^{+}$ species t the goethite surface followed by (2) the slower diffusion of MOH$^{+}$ in the porous goethite. The rate constants show no pH-dependence and only a slight dependenve on temperature. The adsorption isotherm of polymethacrylic acid (PMA) on goethite is of Langmuir type and the polymer is adsorbed irreversibely on the oxide surface forming loops and tails. Goethite modified with PMA shows a significant decrease of the surface charge caused by the deprotonation of the carboxylic residues at the surface. With respect to the binding of the heavy-metal ions this leads to an increased adsorbed amount compared to pure goethite.