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Title: ROLE OF CATION DEMIXING AND QUASICRYSTAL FORMATION AND BREAKUP ON THE STABILITY OF SMECTITIC COLLOIDS

Author
item PILS, JUTTA - IOWA STATE UNIVERSITY
item Laird, David
item EVANGELOU, V - DECEASED

Submitted to: Journal of Applied Clay Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/23/2006
Publication Date: 11/27/2006
Citation: Pils, J., Laird, D.A., Evangelou, V.P. 2006. Role of cation demixing and quasicrystal formation and breakup on the stability of smectitic colloids. Journal of Applied Clay Science. 35:201-211.

Interpretive Summary: Soil structure influences the ability of soils to supply air and water to growing plants. Soil structure also influences how easily soils are compacted by heavy loads and how easily soils are eroded by rain drop impact or running water. The mechanisms and processes controlling the stability of soil structure are only partially understood. We have discovered that the distribution of cations on clay surfaces influences both shrinking and swelling of clays and the breakup and formation of clay aggregates. Monovalent cations (sodium, potassium, and ammonium) tend to accumulate in certain zones within clay aggregates creating planes of weakness. Clay aggregates tend to split apart along these planes. This paper is of primary interest to chemists who seek to understand the stability of colloidal systems, soil scientists who seek to understand the formation and stabilization of soil structure, and engineers who seek to develop technology for stabilizing soils.

Technical Abstract: The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has been extensively used to explain colloid stability. This study investigated the effect of demixing of monovalent and divalent cations and crystalline swelling on the breakup and formation of smectite quasicrystals (QCs) and how these processes affect flocculation and dispersion of natural soil clay-humic complexes. The results indicated that in a Ca-dominated system the formation of large QCs enhanced flocculation and that increasing the concentration of Na+, K+, or NH4+ resulted in the breakup of large Ca-QCs, which enhanced dispersion. In low ionic strength systems, dispersion was caused by expanded double layers (DLVO) and the formation of small QCs. X-ray diffraction analyses showed that as large Ca-QCs break up, monovalent cations resided primarily on the external surfaces and Ca2+ was preferentially retained in the interlayers. In high ionic strength systems increasing concentrations of monovalent cations also decreased the size of QCs but the effect was partially counteracted by compression of double layers between QCs. X-ray diffraction analyses indicated that monovalent cations were sorbed on both the external surfaces and in the interlayers in high ionic strength systems.