Speciation of Hydroxy‐Aluminum Solutions by Wet Chemical and Aluminum‐27 NMR Methods

Abstract

Partially neutralized Al solutions with OH/Al molar (ñ) ratios ranging from 0.25 to 2.5 were studied employing 27Al nuclear magnetic resonance spectroscopy and the ferron‐timed colorimetric assay. The monomeric Al concentrations (Ala) estimated by the ferron assay were consistently higher than the actual concentration determined by NMR spectroscopy. The differences between the methods indicate the presence of a small polymer which reacts rapidly with ferron and is erroneously assigned to the monomeric Al fraction. The polymeric Al component (Alb) estimated by the ferron method could be partitioned into a more rapidly reacting fraction (≤1 h) and a slowly reacting fraction (>1 h). The amount of Al associated with the rapidly reacting polymeric fraction corresponded to the amount of [AlO4 Al12(OH)24(H2O)12]7+ polymer directly determined by NMR spectroscopy. The slower reacting component of the Alb fraction is believed to be a polymer of the hexameric ring morphology. Direct NMR measurements provided evidence for specific adsorption of Al13 to a cation exchange resin and differential precipitation kinetics of this polymer with added sulfate. These data indicate that characterizing hydroxy Al solutions without direct measurements can lead to erroneous interpretations concerning the distribution between monomeric and polymeric Al and the nature and types of polymeric Al present.

The electrochemical production of highly effective polyaluminum chloride

“…The method used to characterize PAC and other alum solutions was a timed spectrophotometric procedure involving reaction of Al with ferron (Bertsch et al, 1986). Three fractions designated: Ala (monomers), Alb (polymers) and Alc (precipitated Al) were determined by the procedure.…”

“…It is known that, the process of Al 13 formation requires the presence of Al(OH) 4 À as a precursor (Bertsch et al, 1986;Bertsch, 1987 andBottero et al, 1987). During the electrolysis, OH À homogeneously evolves on the surface of the cathode sheets that are more fully arranged in the whole space of ECR, leading to the formation of Al(OH) 4 À at the metal±solution interface.…”

Reaction of OH-Al Polymers with Smectites and Vermiculites

“…Brosset et al (1954), followed by Hsu and others, first proposed that the OH-AI polymers have a hexagonal-ring structure resembling fragments of AI(OH)3 crystals (Hsu, 1989). Many recent studies (Akitt et al, 1972;Akitt and Farthing, 1978;Bottero et al, 1980;Bertsch et al, 1986aBertsch et al, , 1986b) favored the Alia Keggin structure for the OH-A1 polymers in solution, originally proposed by Johansson (1963). The presence of the Alia species in the interlayer ofsmectites has also been suggested (Vaughan and Lussier, 1980;Pinnavaia et al, 1984;Plee et al, 1985Plee et al, , 1987Schutz et al, 1987;Fripiat, 1988).…”

Mechanisms of Gibbsite Crystallization from Partially Neutralized Aluminum Chloride Solutions

“…The AI-13 polymer consists of a four-coordinated A104 tetrahedron surrounded by four trioctahedral OH-A1 units at the comers of a truncated tetrahedron (Johansson, 1960(Johansson, , 1963. 27A1 nuclear magnetic resonance (NMR) studies demonstrated the existence of four-coordinated AI in OH-A1 solutions (Akitt et aL, 1972;Farthing, 1978, 1981;Teagarden et aL, 1981;Bertsch et aL, 1986aBertsch et aL, , 1986b. Such a structure, however, should not be stable in aqueous solutions because of the following reasons (Denney and Hsu, 1986): (1) A1-O is unstable in aqueous solution and tends to hydrate to A1-OH or A1-H20.…”