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DIVISION S-2—SOIL CHEMISTRY

Rheology of Sodium-montmorillonite suspensions

Effects of humic substances and pH

^a Field Service, Extension Service, Ministry of Agriculture, P.O. Box 28, Bet-Dagan 50250, Israel
^b Department of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel

* Corresponding author (yonachen{at}agri.huji.ac.il)

Organic matter (OM) is considered to act as a soil structure stabilizer. However, under certain conditions, either in suspension or in soils, addition of low concentrations of humic substances (HS) can result in particle dispersion. In this research, the rheology of montmorillonite suspensions was studied as a function of exchangeable cation, HS concentration (0–4000 mg L^-1) and pH (4–10). The Na-montmorillonite suspensions exhibited non-Newtonian rheology at all pH values. The differential viscosity of the clay suspension decreased with increasing pH. The pseudoplastic non-Newtonian flow resulted from the associations between clay platelets. Addition of HS to clay suspensions changed the flow behavior from non-Newtonian to Newtonian as the HS concentration increased. At a shear stress of 0.1 Pa, the differential viscosity of the Na-montmorillonite was 33.7 mPa s. At a HS concentration of 100 mg L^-1, the suspension showed a decrease in differential viscosity to 11.6 mPa s, and a further increase in HA concentration to 400 mg L^-1 reduced the differential viscosity to 3.92 mPa s. Addition of 100 mg L^-1 HS at the lower pH values (4, 6, and 8) caused a decrease in the attraction forces between the clay particles resulting in reduced differential viscosity at low shear stress (0.1 Pa). This phenomenon is in accordance with the edge-charge reversal mechanisms (from positive to negative) reported previously. An additional mechanism influences the differential viscosity of the suspensions. This mechanism is possibly associated with the formation of mixed micelles of HS with the clay platelets.

Abbreviations: DOM, dissolved organic matter • HA, humic acid • HS, humic substances • FA, fulvic acid • FV, Flocculation value • OM, organic matter • PZC, Point of zero charge