DIVISION S-9—SOIL MINERALOGY Rheological Properties of Aqueous Suspensions of Palygorskite

physical behavior of wetted soils is of paramount importance. The rheology of aqueous suspensions of six palygorskites with When the electrolyte concentration in the percolating different particle morphologies has been investigated as a function solution in soil is below the flocculation value of the of clay concentration, adsorbed ion, pH, and electrolyte concentration, clay, clay dispersion occurs. Under such conditions wausing a rotary viscometer. Rheological parameters (plastic viscosity, ter flow in the soil changes from flow of solution to flow Bingham yield value, and apparent viscosity) increase with the length/ width ratio of individual palygorskite fibers. The plastic viscosity inof clay suspension (Pupisky and Shainberg, 1979). The creases linearly with clay concentration within the range investigated presence of clay particles in the percolating solution (up to 5% w/v). The rheological parameters are higher for clays may increase significantly the viscosity of the flowing saturated with divalent ions than those for clays saturated with monosuspension and therefore may affect the hydraulic convalent ions. For ions of the same valency the rheological parameters ductivity of the soils, which is important for the managetend to increase with decreasing radius of the hydrated ion. The flow ment of irrigation. The suspension viscosity increases of suspensions is pseudoplastic regardless of the nature of the adsorbed even further when particle–particle interactions occur. ion. Measurements of electrophoretic mobility in very dilute suspenThe rheological parameters of clay suspension can sions suggest that the point of zero charge of the palygorskite surface be used to evaluate particle–particle interactions. The is at pH 4 to 4.5. Scanning electron microscopy indicates significant model of edge-to-face particle association was first prodifferences in fiber arrangement between low and high pH values. posed by Hofmann and Hausdorf (1945). The concept Face-to-face particle association occurs at low pH values, giving rise to close-packed domains of fibers, while at high pH values the fibers of face-to-face, edge-to-face, and edge-to-edge type asadopt a random orientation. The flow of suspensions is pseudoplastic sociations in suspension of platy clay minerals (such as at pH # 7 and changes to Newtonian at pH $ 9. At pH # 7 the montmorillonite and kaolinite) was developed further rheological parameters remain relatively constant even at high electroby van Olphen (1977). These simple models of particle– lyte concentrations, but at pH $ 9 they are influenced significantly particle interactions have been helpful for understandby electrolyte addition. Electrolyte-free palygorskite suspensions at ing the stability and rheological behavior of clay– pH 7 are antithixotropic. When the pH falls below 7 or when electrowater systems. lyte is added, the suspensions coagulate and become thixotropic. Some Several studies have been devoted to the rheological rheological properties of palygorskite are similar to those of platy properties of montmorillonite and kaolinite suspensions clay minerals such as kaolinite and montmorillonite, while other rheo(Heath and Tadros, 1983; Brandenburg and Lagaly, logical properties deviate considerably. The models developed to ex1988; Lagaly, 1989; Permien and Lagaly, 1994, 1995; plain the rheological behavior of platy clay minerals do not always account for the behavior of palygorskite, because of differences in Keren, 1988, 1989, 1991), which are found to be influparticle morphology and surface structure. enced by the size and shape of particles, clay concentration, nature of the exchangeable ion, pH, and electrolyte concentration. Palygorskite has a wide range of industrial applicaS soils in arid and semiarid areas contain palygortions because of its special sorptive, colloidal-rheologiskite in their clay fraction. Palygorskite is a so called cal, and catalytic properties, one of which is its usage “special clay”, characterized by a microfibrous morpholas drilling muds. Palygorskite muds have the advantage ogy and a relatively low surface charge. The effect of over other clays (such as smectite) in being less sensitive the common clay minerals, such as smectite, kaolinite, to salts; that is, the desired rheological properties remain and illite, on the properties of soils has been studied virtually constant even at high electrolyte concentraand is known in great detail (e.g., Dixon and Weed, tions (Galan, 1996). 1989). However, with regards to palygorskite, no inforThe rheological properties of a palygorskite clay from mation of this nature is available. This constitutes a Ukraine (Ovcharenko, 1963), Florida (Kheng, 1989), serious impediment for the proper management of these and Central Spain (Galan et al., 1994) have been investisoils. Nearly all palygorskite-containing soils are found gated more from the perspective of their application in dry areas, where irrigation is indispensable for profitthan in terms of particle–particle interactions. Moreable agriculture. For proper irrigation practices, the over, the samples used contained impurities of phyllosilicates (smectite for the clay from Florida, smectite and The Seagram Center for Soil and Water Sciences, The Faculty of illite for the clay from Ukraine and Central Spain) and Agricultural, Food and Environmental Quality Sciences, The Hebrew other minerals (quartz, calcite). These impurities can University of Jerusalem, Rehovot 76100, Israel. Received 10 Dec. 1998. *Corresponding author ([email protected]). Abbreviations: EM, electrophoretic mobility; MB, Methylene Blue; PZC, point of zero charge. Published in Soil Sci. Soc. Am. J. 64:427–436 (2000).