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Colloids and Surfaces A: Physicochemical and Engineering Aspects, 15 January 2013.
Motoyoshi Kobayashi, Miki Nitanai, Naoya Satta, Yasuhisa Adachi
Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan
Faculty of Agriculture, Iwate University, 3-18-8 Ueda, Morioka, Iwate 020-8550, Japan
Abstract
Imogolite, an aluminum silicate clay mineral obtained from volcanic ash soil, has several unique properties: a nano-tubular structure, having positive charge below pH 9, and coagulation beyond pH 6–7. Thus, imogolites can play an important role when volcanic ash soil-based coagulants are used for coagulation.
To understand the mechanism of imogolite-induced coagulation and subsequent sedimentation, we studied the coagulation and charging of three types of latex spheres in the presence of purified imogolites, obtained from natural soil in Kitakami, Iwate, Japan. Coagulation and dispersion were analyzed by observing latex–imogolite mixed suspensions with the unaided eye and by measuring the supernatant absorbance. The charging property was studied by electrophoretic mobility. These experiments were carried out as a function of the ratio of the concentration of imogolites (CI) to that of latex particles (CL) at three different pH values (pH 4, 6.5, and 10).
The experimental results clarified that the mechanisms of imogolite-induced coagulation depend on the charging and coagulation properties of the imogolites. That is, when the latex and the imogolites are oppositely charged and the imogolites are dispersed, coagulation occurs only around the isoelectric point. Here, the main coagulation mechanism is charge neutralization. If the latex and the imogolites are oppositely charged and the imogolites are coagulated, coagulation occurs not only around the isoelectric point but also at higher CI/CL ratios. This result indicates that the particles coagulate due to both charge neutralization and sweep coagulation, in which the latex particles are enmeshed by the flocs of the imogolites. When the latex and the imogolites are similarly charged and the imogolites are coagulated, the particles aggregate at higherCI/CL ratios through sweep coagulation only. We also found that the efficiency of sweep coagulation depends on the electrokinetic potential of the imogolites and that the charge neutralization is determined by the imogolite dosage per total particles surface area