The role of soil clays and clay-humic complexes in processes controlling colloidal stability and the sorption and degradation of tetracyclines
The Derjaguin-Landau-Verwey-Overbeek theory has been extensively used to explain dispersion and flocculation in colloidal systems, however little is known about the effect of crystalline swelling on colloidal behavior. The first topic addressed in the dissertation is the effect of monovalent and divalent cations on crystalline swelling and the breakup and formation of quasicrystals (QCs) and how these processes affect flocculation and dispersion of natural soil clay-humic complexes. The results indicate that high Ca 2+, levels enhance the formation of large QCs, which readily flocculate and settle out of suspension. Increasing the concentration of Na+, K+, or NH4+ results in the breakup of large Ca-QCs, which enhances dispersion.;The second part of the dissertation addresses the influence of soil smectites and humic substances and their interactions on sorption and desorption of tetracyclines. Tetracyclines are important in human medicine and are also used extensively in livestock production. Chlortetracycline and tetracycline are routinely used for growth promoting and therapeutic purposes in livestock production. To elucidate the environmental fate of these pharmaceuticals, sorption isotherms were obtained using dilute CaCl2 or KCl background solutions at different pHs for clays, humic substances, and clay-humic complexes. In all systems, the soil components sorbed >96% of added tetracyclines. The results indicate that tetracyclines are dominantly sorbed on soil clays and that HS compete with tetracyclines for sorption sites on the clay surfaces.;The third topic addressed in the dissertation is degradation and bioavailability of TC when it is adsorbed to whole soils, soil clay-HC, and soil clays. Specifically studied was the degradation of TC in both aqueous and colloidal systems, the antimicrobial activity of sorbed TC, whether sorbed TC is a long-term reservoir for slow TC release, and the ability of TC resistant bacteria to degrade sorbed TC. Studies indicated that sorbed TC inhibits soil bacteria, however, most likely due to slow TC release from soil components. Most of the adsorbed TC was retained by the soil and soil components in a form that was not released during the 149-day incubation. This stable pool of adsorbed TC appeared to be primarily associated with organic matter and humic substances.