The Electrophoretic Mobility of Imogolite and Allophane in the Presence of Inorganic Anions and Citrate

Abstract

Abstract--The purpose of this study was to investigate bonding mechanisms of representative inorganic anions and citrate with imogolite and allophane using electrophoresis. The electrophoretic mobility (EM) of synthetic imogolite and allophanes with A1/Si molar ratios of 2.02, 1.64, and 1.26 was determined in 0.001 and 0.01 M sodium solutions. The highest point of zero mobility (PZM) values for imogolite and the highest point of zero charge (PZC) values for allophane occurred in the presence of C104, NO3, Br, I, and C1. Below the PZM and PZC, C1 and t lowered the EM relative to the other anions but did not shift the PZM and PZC significantly. This indicates that Cl and I formed more outer-sphere complexes than the other ions. The EM of imogolite and allophane was negative at pH < 6 in 0.001 and 0.01 M NaF probably due to a phase change. We observed the formation of cryolite (Na3A1F6) with transmission electron microscopy (TEM) and X-ray diffraction (XRD) in the NaF systems at low pH. Conversely, phosphate at 0.001 and 0.01 M concentrations lowered both the PZM and the EM in imogolite and both the PZC and the EM in allophane compared with C104. Phosphate-treated allophane had the same PZC as a synthetic amorphous aluminum phosphate. The PZM values of imogolite and allophane with 2:1 A1/Si in 0.0001 M Na-citrate were 10.9 and 5.9, respectively. At pH 7.3, Na-citrate lowered allophane EM more than it lowered imogolite EM relative to C104.The EM in NaC104 and Na2SO4 was reversible by forward-and back-titration with NaOH and HC1Oa, indicated that C104 and SO4 were not specifically adsorbed. Chloride likely formed more outer-sphere complexes than C104. Imogolite EM and allophane EM in dilute NaF and NaHzPO4 solutions were not reversible, indicating either surface inner-sphere complexes or surface precipitates of aluminum fluoride and amorphous aluminum phosphate-like materials on these minerals. Sulfate gave a lower EM than the monovalent anions, implying a greater tendency to form outer-sphere complexes. Citrate appeared to form inner-sphere complexes on both imogolite and allophane, but formation was concentration-dependent. The tendency of anions to form surface complexes with imogolite and allophane is consistent with the tendency of anions to form soluble aluminum complexes.

Colloidal Stability of Imogolite Nanotube Dispersions: A Phase Diagram Study

“…The value of μ is fairly constant around 4.5 10 −8 m 2 V −1 s −1 for pH ≤ 6, indicating saturated protonation of the nanotube outer surface (−AlOH 2 + groups) 45,61 for such pH values. Then it decreases almost linearly with pH similarly to that observed previously for single-walled aluminosilicate 41,62,63 and aluminogermanate nanotubes. 42 Interestingly, the use of the isoelectrical point (IEP), i.e., the pH at which the electrophoretic mobility is zero, has been proposed as an alternative method to evaluate the formation of INTs.…”