Factors affecting the flow behavior of soil allophane suspensions under low shear rates

Abstract

The flow characteristics at low rates of shear (<60 sec −1) of aqueous suspensions of an allophane clay, separated from soil, have been investigated using a programmable rotational viscometer. The untreated clay gives an initial rise in shear stress followed by a rapid decline before reaching steady-state conditions. The acceleration and deceleration pathways enclose an appreciable hysteresis loop. These observations have been interpreted in terms of a network structure in which globular particles, held together by primary and secondary bonds, become elongated on shearing. The primary (electrostatic) bonds remain largely intact during shear while the secondary (van der Waals) bonds are ruptured but may partially reform. Treatment with a solution of sodium dithionite-citrate-bicarbonate (DCB), which removes both hydrous iron oxides and allophane-like constituents from the sample, gives rise to a system of discrete particles with relatively weak secondary bonding while a suspension of air-dried (untreated) allophane consists of very weakly interacting aggregates. Successive shearing disrupts the gel structure which, however, can be partially restored by resting. Being amphoteric, allophane particles tend to repel each other on the acid and alkaline side of the point of zero charge (PZC) while maximum flocculation and particle-particle interactions occur at the PZC (pH 6) of the sample. The deflocculation process at pH > 7, however, may be largely offset by the alkali-induced hydrolysis and polymerization of the associated allophane-like constituents. Scanning electron microscopy of the unsheared suspension and of the material after shearing, DCB treatment, and air drying provides support for the proposed structural model of allophane.