Hydrated aluminium sulfate precipitation by enzyme-catalysed urea decomposition

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Abstract

Hydrated basic aluminium hydroxide precipitates were obtained from aluminium sulphate solution at room temperature and at 92°C by using urea. Enzyme urease was used to decompose urea at room temperature. It was observed that precipitates obtained at room temperature were much finer than those obtained at 92°C but they had similar chemistry. On calcination at 1200°C, each precipitate obtained at room temperature converted to a discrete α-Al2O3 crystal whereas that obtained at 92°C converted to a rather densely packed agglomerate of α-Al2O3 crystals. As a result of this, α-Al2O3 powder obtained from 25°C precipitates showed much better sintering behaviour.

Introduction

Powder characteristics such as particle size and shape and agglomeration play a major role in preparation of ceramics, especially for high performance applications. Powders having spherical morphology, fine particle size, narrow particle size distribution and no agglomeration are needed to obtain homogeneous microstructure and thus high mechanical properties.

Homogeneous precipitation by urea has been used to obtain uniform particles of aluminium hydroxide with controlled morphology.1, 2, 3, 4 In this method, upon heating a solution containing aluminium ions and dissolved urea to over 70°C, urea decomposes to CO2 and NH3. NH3 reacts with water producing hydroxyl ions which causes a uniform rise in pH of the solution until the solubility limit of aluminium sulfate hydrate is exceeded. Uniform rise in pH prevents the occurrence of very high local supersaturation (as in the case with the additions of a base) allowing nucleation homogeneously throughout the solution. Then, the nucleates grow uniformly with no further nucleation forming uniform size precipitates.

Decomposition of urea in a reasonable time requires temperatures over 85°C. However, the decomposition reaction can take place at room temperature with a trace amount of enzyme urease.5 Very recently, Simpson et al.6 published the results of the first study on precipitation of basic aluminium sulfate at room temperature by urease catalysed decomposition of urea. They successfully obtained precipitates with sizes in the range of 20–600 nm.6 The precipitates were amorphous. Their investigation on the effect of urease amount on the decomposition kinetic of urea in a solution containing 0.16 M aluminium acetate, 0.19 M sodium sulfate and 0.12–0.56 M urea revealed that at least 4.8 units urease per mL of the solution was required to complete the reaction in the given time frame of 25 h.6 Other studies concerning precipitation of hydrated basic aluminium sulfate by urea in the literature 1, 2, 3, 4 deals with precipitation by the thermal decomposition route of urea above 80°C. In this study, precipitation of aluminium sulfate hydrate by urea decomposition at 92°C (thermal decomposition) and at room temperature (urease catalysed decomposition) was investigated. Precipitate morphologies and characteristics were compared.

Section snippets

Experimental

Chemicals used in this work were analytical reagent grade from Merck (Darmstadt, Germany). 16.6 gr Al2(SO4)3.16 H2O was dissolved in 1 l of distilled water in a beaker. Then, 30 g urea was added and dissolved. pH of this solution was 4. This is referred to here as the standard solution. The beaker covered by an aluminium foil was put in a still water bath and temperature of the solution was raised to 92°C and kept there for 3 h for precipitation.

For precipitation experiments at room

Results

Precipitation at 92°C was completed within 3 h and pH of the solution after the completion was 7. Precipitation at room temperature took longer. No study was undertaken to find the minimum time necessary for complete precipitation but it was completed after 24 h and pH of the solution after the completion was 8. The precipitates produced at both temperatures were easily filterable. Precipitation was not observed with urease concentration of 3 and 15 mg.

There were various experimental

Precipitation

In homogeneous precipitation by the decomposition of urea with the effect of temperature (over 80°C), precipitates nucleate and grow throughout the solution with no secondary crystallisation.7 Therefore, the number of nuclei formed initially determines the particle size. Increasing urea concentration at constant aluminium ion concentration was observed to decrease the particle size of the precipitates from aluminium sulphate solutions.4 This was explained to be due to more urea decomposition

Conclusions

Precipitation by enzyme urease catalysed urea decomposition at room temperature produces very fine precipitates of aluminium sulfate hydrate compared to those produced by thermal decomposition of urea at 92°C. For successful precipitation at room temperature, at the urease amount studied here, it was important that urease remained undissolved and undisturbed in the solution. Higher solution concentrations caused urease to dissolve and thus prevented the occurrence of precipitation.

As-produced

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