Sedimentation behavior of dispersed particles of clay and silt in acidic and alkaline suspensions of inorganic materials from various volcanic ash soils

Abstract

Various volcanic ash soils pre-treated with hydrogen peroxide were dispersed by physical and ultrasonic treatments. Amounts of clay and silt particles suspended in acidic (pH 4) solution and those in alkaline (pH 10) solution were determined by a pipet method, respectively. The mass ratios of clays in acidic I alkaline suspensions were classified into three categories: much less than unity (G1), approximately equal to unity (G2), and much higher than unity (G3). Both G1 and G2 clays were stably suspended in alkaline solution, indicating that clay particles consisting of abundant layer silicate minerals of the 2 :1 an 2 :1:1 types and their inter grades were dispersed. In acidic solution, G2 clay was also stably suspended, whereas G1 clay settled rapidly, suggesting the existence of an interaction between the negatively charged layer silicates and the positively charged colloids, e.gFe (hydr)oxides. The G3 clay remained suspended in acidic solution, but settled rapidly in alkaline solution, The dispersion of G3 clay under acidic conditions can be explained by mutual repulsion due to the presence of alto phane (Si/Al=about 1 : 2), imogolite, and dithionitet citrate-extractable components (DCEC), e.gFe (hydr)oxides, and allophane-like constituents, The rapid sedimentation of the G3 clay in alkaline solution was attributed to (1) mutual flocculation with imogolite, and probably (2) coating (and bridging) of allophane with the colloids of DCEC. The sedimentation behavior of the silt particles was generally similar to that of the clay particles. It was concluded that rapid sedimentation of the silt particles of (1) G3 in alkaline suspension, and (2) G1 in acidic suspension was mostly due to the adhesion of clay particles onto the surface of larger silt particles.