Chemical and structural studies of aluminium and zirconium aqueous chemistry

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Shafran, K.L., 2003. Chemical and structural studies of aluminium and zirconium aqueous chemistry. PhD, Nottingham Trent University.

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Abstract

Systematic studies of chemical and structural aspects of aqueous chemistry of aluminium and zirconium over a wide range of pH conditions with special emphasis on the formation of large polynuclear hydrolytic species have been performed. Medium-concentrated (0.05-0.25 mol/L) solutions of aluminium and zirconium chloride were used and acid-base titration was chosen as a method for pH variation. The effect of anions (chloride, nitrate, acetate, sulphate and carbonate), temperature (room temperature and 90°C), ionic strength, strength and type of alkali (KOH, KHCO3, NH4OH) and the presence of selected complexing agents including glycine (for complexation with zirconium only) on hydrolysis-condensation of A1 and Zr-ions were investigated.

At high temperature (90 °C) Keggin polycation Al13O4(OH)24(H2O)127+ (A113) dominated in the pH range 4≤pH≤6, but was gradually replaced by Al13O8(OH)56(H2O)3218+ (A130) polycation after 12 hours of hydrolysis. The kinetic rate of the A113 polycation formation was at least 2 orders of magnitude lower than that of the Al13-mer. The A130-mer appears to be formed directly from the A113-mer and monomers. Sulphate-ions totally change the speciation patterns of Al-ions under acidic conditions, prohibiting the formation of Keggin polycations (A113 and A130).

Spray-freeze-drying of solution samples with 4<pH<6 (1.0<h<2.8) gave largely aluminium polycations (A113 and A130) in the solid state. The solid samples were mainly amorphous (pseudoboehmite) with minor quantities of gibbsite.

Hydrolysis of zirconium-ions at 90°C led to a mixture of monoclinic zirconia and zirconyl salts with variable hydration at very low pH (pH<0.6). At higher pH values amorphous zirconia dominated the equilibrium. At alkaline pH tetragonal zirconia was the major solid phase formed. Particle size of the hydrolytic zirconium polymers varied with pH, being the lowest at pH~0.6 (d~0.2-0.4 nm) and rapidly increasing with pH increase. At alkaline conditions, partial dissolution of hydrous zirconia led to smaller particle sizes, particularly in the presence of carbonate-ions from alkali which formed strong complexes with zirconium.

The effect of sulphate and acetate ions on the speciation of zirconium-ions was pH-dependent. At equimolar ratios of SO42-:Zr4+, crystalline zirconium sulphate was formed at pH<0.5. At 0.5<pH<3.5 basic zirconium sulphates began to prevail as solid products. At pH>3.5 amorphous zirconia dominated. Acetate-ions behaved similarly to sulphate-ions, albeit amorphous zirconia began to form at lower pH. Unidentate, bridging and chelating complexes of sulphate and acetate-ions with zirconium polymers were identified. The reaction of glycine with zirconium was quantitative in acidic conditions and complexes formed were stronger than those with acetate. The presence of glycine hindered condensation of Zr polymers below pH?2.0. Five complexes between zirconium and glycine were identified.

Item Type:

Thesis

Creators:

Shafran, K.L.

Date:

2003

ISBN:

9781369316346

Identifiers: