Abstract

Basic aluminum sulfate can be prepared by homogeneous precipitation starting from admixtures of ammonium aluminum sulfate and ammonium bisulfite solutions. Powder agglomeration problems usually arise in wet precipitation processes mainly at high concentration of precursors. Some organic additives have been used in order to modify the physical and chemical properties of the final product during precipitation processes. In this work, the effects of cationic, anionic and neutral organic additives on the physical and chemical characteristics of basic aluminum sulfate (BAS) obtained by homogeneous precipitation were investigated. Dodecylethyl-dimethyl-ammonium bromide (DEA) and chitosan were used as cationic additives, whereas dodecyl lithium sulfate (DLS) was selected as anionic additive. Furthermore, poly(vinyl alcohol) (PVA) and poly(ethylene glycol) (PEG) were used as neutral additives. The products were characterized by Fourier transformed infrared spectroscopy (FTIR), powder x-ray diffractometry (XRD), thermal analysis (TG and DTA) and scanning electron microscopy (SEM). The chemical composition and crystallinity of the final product were not modified by the presence of the studied additives in this reaction media. The basic aluminum sulfates were amorphous, with chemical composition 2Al2O3·SO3·9 H2O. However, the morphology and agglomeration state of the final product were modified by the nature of the used additives. Anionic additive, DLS, produced irregular agglomerated particles. On the other hand, neutral additives, PVA and PEG, provided particles with low agglomeration state. The highest effect on dispersion was observed when cationic additives were used. Fine well-dispersed particles of basic aluminum sulfate were obtained by addition of chitosan during the precipitation process.
Keywords

Organic additives, agglomeration, basic aluminum sulfate
Introduction

Aluminas have been in use for many years as catalyst, adsorbents, desiccants, abrasives, plastic fillers as well as fire retardants for many chemical process industries. Aluminas for water adsorption, which are traditionally known as desiccants, were first introduced in 1932 [1]. Since long time ago, synthetic aluminas have been used in the chromatographic purification of biological compounds. Recently aluminas have found widespread usage in applications as diverse as municipal wastes treatment as well as in polymer and pharmaceutical industries [2].

Alumina chemicals have been applied to treat industrial and municipal waters [3]. On the other hand, aluminum sulfate is useful as a coagulant for removal of various metals, particles and undesirable organic compounds from industrial waste water. Alumina trihydroxide (gibbsite) and monohydrate (pseudoboehmites) also have been accepted as flocculants in drinking-water. Transition forms of aluminas have been used as adsorbents for removal of undesirable contaminants in both municipal and industrial waters.

As a result of more stringent conservation requirements large alumina adsorption processes have been developed for water treatment. Successful new environmental applications of alumina adsorbents include the removal of phosphate, mercaptan, arsenic, fluoride and colloidal silica compounds in ground and drinking water [4].

The most important source of aluminum hydroxides is the Bauxite refining plant. More than 94% world alumina production is accounted for by the Bayer process for Bauxite refining. The alumina product from the Bayer process is usually 99.5% pure with soda (Na2O) as the mayor impurity [5]. Therefore, when a pure alumina product is required, it must be obtained by another chemical process that uses pure aluminum salts as a raw material.

The interest to find alternative routes for the preparation of ceramic powders that avoid the inconveniences of the traditional techniques has been growing. For this reason, the development of new synthetic routes is required that allow to obtain ceramic powders with specific properties, such as small particle size, spherical in shape, narrow particle size distribution, absence of agglomerates and high chemical purity.

One of the most interesting recent developments in the preparation of aluminum hydroxide and alumina has been the introduction of BAS as monosized alumina precursors with uniform and controllable morphology. Matijevic [6] originally produced particles that contained an appreciable amount of sulfate but this contaminant could be exchanged for hydroxide species, in much the same manner as the one described by Gordon et al. [7], converting the BAS to hydrous aluminum oxide.

Other researchers have also used basic aluminum salts as high purity alumina precursors. Cornilsen and Reed studied amorphous basic aluminum succinate and BAS as potential precursors [8]. Whereas Sacks et al. [9] and Blendell et al. [10] used spherical BAS as the specific starting material for conversion to alumina.

The BAS studied by the above mentioned researchers, were obtained by homogeneous precipitation of BAS in aqueous medium. In this case, agglomeration problems of the BAS particles generally arise, especially when the precipitation process is performed at high concentrations of aluminum salt. Therefore, under this condition it is difficult to obtain monosized and non-agglomerated particles.

The use of low aluminum concentration solutions in the precipitation process allows the preparation of non-agglomerated BAS particles. However, at these low alumina concentrations the amount of product obtained is too low, from a practical point of view.

In one alternative process to prevent agglomeration, organic compounds are added to the precipitation medium. Generally, the preparation of mono-dispersed alumina hydroxide by sol-gel process was achieved, using high alumina concentration and hydroxyl-propylcellulose as a steric agent to prevent agglomeration.

In this work, the effects of cationic, anionic and neutral organic additives on the physical and chemical characteristics of BAS obtained by homogeneous precipitation were investigated. DEA and chitosan were used as cationic additives, whereas DLS was selected as anionic additive. Furthermore, PVA and PEG were used as neutral additives. The products were characterized by XRD, TA (DTA and TG), FTIR and SEM.
Experimental

The organic additives used in this study were reagent-grade products obtained from Sigma-Aldrich. The BAS was obtained by the following procedure. The aluminum bisulfite solution was prepared by dissolving previously precipitated basic aluminum sulfate with sulfur dioxide in aqueous media. The organic polymers described above were added to the aluminum bisulfite solution. This solution is added drop by drop to previously heated water with vigorous agitation. After the total quantity of the solution was added, the solution was stirred for 15 minutes in order to complete the precipitation. The obtained BAS precipitates were separated by filtration, washed with hot water and dried at 75°C for 24 hours. The solids were characterized by X-ray diffractometry (XRD) (Model D-500, Siemens, Germany) using Ni-filtered CuKa radiation. Infrared analysis was performed using KBr pellets and the samples were run on a Fourier transform infrared (FTIR) spectrometer (Model 1600 series FTIR, Perkin Elmer, Norwalk, Connecticut, USA). Differential thermal analysis (DTA) and thermal gravimetric analysis (TGA) of the samples were obtained by heating 20 mg of sample up to 1300°C, at a rate of 10°C/min, in air (Model SDT 2960, TA instruments, New Castle, Delaware). The morphology of the particles was determined by scanning electron microscopy (SEM) (Model jsm-35C Jeol, Tokyo, Japan).
Results and Discussion
Effects of Organic Additives on Chemical Composition of BAS

Chemical composition of BAS obtained by homogeneous precipitation in presence of organic additives was determined by thermogravimetric analysis. Typical TG curve of BAS is shown in Figure 1. Table 1 summarized the results of thermal analysis.

In TG curve of Figure 1, the mass loss occurred below 800°C corresponds to the loss of water. On the other hand, the loss weight between 800 – 1280°C can be attributed to the sulfur trioxide. So, considering the weight losses of the samples, the amount of water, sulfate and aluminum oxide can be calculated. The chemical formula of BAS with different additives can be estimated by this method and the values varied in the range of (1.9-2.3)Al2O3 SO3 (8.4-11)H2O. On the other hand, the chemical formula of BAS prepared without organic additives was 2Al2O3 SO3 9H2O. Thus, we can conclude that the organic polymers used here have not effect on the chemical composition of the precipitated BAS.

AZoJomo - The AZO Journal of Materials Online - Typical TG and DTA curves for BAS with additive

Figure 1. Typical TG and DTA curves for BAS with additive.
Effects of Organic Additives on Crystallinity and Chemical Structure of BAS

The XRD spectra of all BAS samples obtained with different additives show only amorphous phase (data not shown).

The FTIR spectra corresponding to a typical BAS precipitated in presence of additives correspond to a hydrated basic aluminum sulfate. The FTIR spectrum in Figure 2 indicates that the compound was a hydrate, because of the strong and broad absorption band in the region from 3000 to 3700 cm-1 and the absorption band which peaks at 1655 cm-1 [11, 12]. Furthermore, the strong and broad band centered at 1135 cm-1 and the small shoulder at 998 cm-1 could been assigned to sulfate absorptions (n3) and (n1) respectively [11-13]. The strong and broad absorption band centered at 613 cm-1 probably resulted from the combined absorptions of sulfate (n4), the Al-O stretching vibrations and the Al-OH wagging vibrational mode of molecular water [11, 12]. Therefore this compound corresponded to a hydrated basic aluminum sulfate, very similar to those obtained and studied by Matijevic [14] and Saks [15].

AZoJomo - The AZO Journal of Materials Online - Typical IR spectrum of BAS obtained with additives

Figure 2. Typical IR spectrum of BAS obtained with additives.

It should be notice that the additives were not detected in any BAS samples by FTIR spectroscopy. This is probably due to the small amount of additive adsorbed on the BAS surface. Non-ionic macromolecules such as polyethylene oxide and polyvinyl alcohol are linear, flexible molecules with no charge, which adsorb non-specifically on the surface of oxides. Because the interaction with the surface takes place through hydrogen bonds between polar functional groups of the polymer chain and the hydroxylated and protonated groups on the surface, the adsorption density decreases with increasing pH, irrespective of the nature of the oxide. Furthermore, the affinity of the polymer for the surface will be weaker when the surface sites are more prone to salvation [16]. In this case, low adsorption of non-ionic polymers on BAS surface is expected owing to the high solvation energy of aluminum ion (4700 kJmol-1, 25°C) [17].

Table 1: Results of thermal analysis (TG and DTA) of typical BAS with additive.

Temperature

(°C)


Weight Loss

(%)


Thermal Effects

(°C)


Decomposition

Stage

30-400


30.21


Endo (121 °C)


- H2O

400-800


4.68


----


- H2O (de OH)

800-1000


17.59


Endo (956 °C)


- SO3

1000-1280


- 0.9


Exo (1259 °C)


- SO3

On the other hand, polyelectrolytes and ionic surfactants absorb more strongly than neutral molecules on charge surfaces, because the adsorption energy contains an electrostatic contribution from sites of opposite charge on the polymer and on the oxide. The surfactant is attached to the surface of the particle through electrostatic interaction between the polymer charged head and opposite charged sites on the solid surface. In this case, the adsorption density depends on the sign and number of surface charges [16]. Because BAS is positively charged (+9.0 mV, Z-potential) at precipitation pH, it is expected that the negatively charged additive DLS will adsorb more strongly on the positively charged BAS than the positively charged additives DEA and chitosan.

Although DLS additive is not detected on the BAS sample by FTIR at used additive concentration, when the added amount of DLS is higher than 4%, the precipitated BAS becomes hydrophobic, i.e. the dodecylsulfate negative ion is adsorbed on the surface of BAS.
Effects of Organic Additives on Agglomeration Grade of BAS

The chemical nature of the organic polymers affects the morphology and agglomeration grade of BAS. As shown in Figure 3, the precipitation of BAS with DLS produces agglomerates consisting of fine particles (0.2 mm). Furthermore, irregular plate like particles (>1 mm) were formed.

AZoJomo - The AZO Journal of Materials Online - Scanning electron micrograph of BAS obtained with DLS

Figure 3. Scanning electron micrograph of BAS obtained with DLS.

As described above, this anionic polymer strongly interact with the positively charged BAS. So, the molecules of DLS are attached to the surface of the solid through electrostatic interaction between the negative head of polymers and charged sites on the solid of BAS. At the same time, the hydrophobic tales of polymer escape contact with the water molecules by adsorbing passively at the surface of the solution. This is the reason why plate like particles were formed in the air-water interface. This type of additive is not adequate to obtain regular shape particles.

Cationic additives, such as DEA and chitosan, produced the BAS with fine spherical particles (< 0.5 mm) and lower grade of agglomeration as shown Figure 4.

AZOJomo - The AZO Journal of Materials Online - Scanning electron micrographs of BAS obtained with additives DEA

(a)

AZOJomo - The AZO Journal of Materials Online - Scanning electron micrographs of BAS obtained with additives chitosan

(b)

Figure 4. Scanning electron micrographs of BAS obtained with additives (a) DEA and (b) chitosan.

For comparison, the scanning micrograph of BAS obtained without additives is shown in the Figure 5.

Generally, the precipitated BAS particles without additives are large agglomerates (>10 mm). In the early stage of homogeneous precipitation, small particles of BAS are produced. Such small particles are attracted between them with strong force, consequently this produces large agglomerates. The presence of a cationic polymer, especially polyelectrolyte chitosan, prevents formation of hard agglomerates or aggregates of BAS through electrosteric mechanism. Owing to the fact that chitosan polymer has positive charge at the precipitation pH of BAS, it is adequate to produce fine non-agglomerated particles of BAS.

AZoJomo - The AZO Journal of Materials Online - Scanning electron micrograph of BAS obtained without additives

Figure 5. Scanning electron micrograph of BAS obtained without additives.

On the other hand, neutral polymers, such as PVA and PEG, affect the agglomeration state of the BAS in a lower grade than the ionic additives. This can be attributed to the weak interaction force between the neutral polymers PVA and PEG with the positively charged surface of BAS. In this case, the interaction of neutral additives with the surface takes place through hydrogen bonds between the functional groups of the polymer and the hydroxylated and protonated groups of the BAS surface [16]. According to the chemical compositions of PVA and PEG, the first additive is expected to interact more strongly with the BAS surface than the second one. PVA contains hydroxyl group, which can form hydrogen bonds with the hydroxilated groups of the BAS surface, whereas PEG contains only ether group which is less polar than hydroxyl. In Figure 6, the scanning electron micrographs of BAS obtained with neutral additives can be seen. SEM micrograph of BAS obtained with PVA shows that this additive exhibits higher effect on the agglomeration grade of the BAS particles, owing probably to a steric effect.

AZOJomo - The AZO Journal of Materials Online - Scanning electron micrographs of BAS obtained with additives PVA

(a)

AZoJomo - The AZO Journal of Materials Online - Scanning electron micrographs of BAS obtained with additives PEG.

(b)

Figure 6. Scanning electron micrographs of BAS obtained with additives (a) PVA and (b) PEG.
Conclusions

Chitosan is the best dispersant for BAS obtained by homogeneous precipitation. All additives used here could modify the agglomeration grade without modifying the composition and crystalline grade of BAS. The chemical compositions of BAS slightly vary in the range of (1.9-2.3)Al2O3 SO3 (8.4-11)H2O. The crystalline nature of all BAS was amorphous. The adsorption of all additives used here on the surface of BAS solid was not observed by IR spectroscopy.