Preparation of aluminosilicates with a high cation exchange capacity from agro-industrial waste

D.S. Paz, E.L. Foletto^†, D. A. Bertuol, M.C.M. Castoldi, S.L. Jahn, R. Hoffmann and M.A. Mazutti

Department of Chemical Engineering , Federal Universtiy of Santa Maria, 97150-900, Santa Maria,RS, Brazil ^† E-mail: efoletto@gmail.com

Abstract— Rice husk ashes are a potential source of pollution, thus there is continued interest in its recycling. An environmentally friendly solution is its conversion into products such as aluminosilicates that are designed for use as fertilizer. Aluminosilicates that present a high cationic exchange capacity becomes an excellent material to be used in soil for accumulating and liberating nutrients slowly to the plants. The aim of this study is to investigate the effects of the alkali concentration in the cationic exchange capacity of an aluminosilicate obtained from rice husk ashes. The obtained products were characterized by X-ray diffraction, scanning electron microscopy, atomic absorption spectroscopy, and cationic exchange capacity. The particles that were produced are composed of crystals smaller than 5 μm. The experimental conditions employed in this work produced a semi-crystalline aluminosilicate with a high cationic exchange capacity. The alkali concentration used had an influence on this property.

Keywords— Aluminosilicate; Cation Exchange; Rice Husk Ash; Preparation.

I. INTRODUCTION

In the Federal State of Rio Grande do Sul (Brazil), rice production is approx. 5,137 million t per year. Since the husks represent 20 wt.% of this value, the annual production of this residue in the state is approximately 1,027,400 t. These husks are burned for energy production, in turn generating great amounts of ashes (≈200,000 t per year). As the rice husk ashes (RHA) present more than 90 wt.% of silicon, its use as a silicon source in aluminosilicates synthesis constitutes an alternative for the reduction of costs and environmental damage. The disposal of ash in the soil is limited because of its chemical composition (Prasetyoko et al., 2006).

The ashes generally have small amounts of nutrients, an undesirable value of pH, salinity and traces of toxic elements such as As, B, Ba, Cd, Cr, Pb, Hg, Mo, and Se (Pandey et al., 2009). The RHA can be used as adsorbent in the process of gold extraction, in the production of silicon carbides (SiC), as a load in polymers, as an additive for cement, in the manufacture of concrete, and as support for the preparation of nickel-based catalysts (Foletto et al., 2005). An alternative technology is to use the ashes as a source of silicon for aluminosilicates synthesis with a high cation exchange capacity. These aluminosilicates can be used to wrap and slowly release nutrients or micronutrients for plants, especially NH₄⁺, K⁺, Mg²⁺ e Ca²⁺, thus contributing to an increase in the efficiency of the use of fertilizers by reducing the amount required to be applied in plantations. The use of such materials in the soil has as advantages: insoluble in water, which makes it adhere to the particles of soil and remain in the root zone and, therefore, is not leached; the use of potassium aluminosilicate as fertilizer increases the content of sugar and amino acids in plants, generating greater resistance to insects and diseases; enhances the ion exchange capacity of the soil resulting in less need for the use of fertilizers; promotes better plant growth (Kikuchi, 1999).

Several works have been published on the synthesis of aluminosilicates with high cation exchange capacity obtained from different sources of silicon (Berkgaut and Singer, 1996; Inada et al., 2005; Yaping et al., 2008), however, reports on the preparation of aluminosilicates with a high cation exchange capacity obtained from rice husk ashes are scarce in the literature.

In this sense, this work aimed to synthesize an aluminosilicate with a high cation exchange capacity using rice husk ash as a source of silicon.

II. MATERIALS AND METHODS

A. Synthesis

As a silicon source, rice husk ashes generated from an industrial burner of a local industry (INDUBER, Santa Maria, RS, Brazil) was used. As an aluminum source, potassium aluminates were used. They were prepared by the dissolution of 6 g of a aluminum wire (1mm diam., 99.99%, Aldrich) in 288 mL of a potassium hydroxide solution (11 wt.%), using a heating system with reflux. The time of reflux was 30 minutes. For the aluminosilicate formation in alkaline medium experiments were carried out varying the concentration of potassium hydroxide (KOH) in the reaction medium. Based on previous works (Querol et al., 2002; Juan et al., 2007; Ríos et al., 2009) experiments were carried out with different KOH concentrations (0.5 to 4 N), a temperature range from 80 to 200^oC, and a duration of 3 to 120 h. The composition of the reaction medium was the following: Al₂O₃:2.1SiO₂: XK₂O: 40H₂O, where X = 0.71, 1.08, 1.44, 1.8, and 2.16, resulting in concentrations of 1.0, 1.5, 2.0, 2.5, and 3.0 N, respectively. The reactions were realized in an open system (at atmospheric pressure) at 100° C, differing from the conventional procedures, where the solids are usually synthesized in a Teflon lined stainless steel autoclave by a hydrothermal process (Murayama et al., 2002; Moutsatsou et al., 2006). The preparation of the reaction mixture was carried out in accordance with the following procedure: a certain quantity of ash was added to a glass bottle (1L), coupled to a heating system, a thermometer and a reflux condenser were used to maintain the volume of the solution constant. Finally, potassium aluminate was added and the mixture was heated until it reached 100°C for 5 h. The formed solid was filtered and washed with deionized water to remove the excess alkali. The obtained solid was dried at 110°C for 24 h. The samples are designated as 1N, 1.5N, 2N, 2.5N and 3.0 N, in reference to the KOH concentration used for the reaction process.

B. Characterization of the samples

The RHA and the synthesized aluminosilicates samples were characterized by X-ray diffraction (Shimadzu diffractometer, model XD-7A, with radiation Cu-Ka) and by scanning electron microscopy (SEM; Model 2000FX, JEOL Co.). The chemical composition was determined by atomic absorption spectrometry (Analytik JENA, Vario 6, Germany). The Brunauer-Emmett-Teller (BET) surface area measurements were carried out by N₂ adsorption-desorption at 77 K using a ASAP 2020 instrument. In the determination of the cation exchange capacity, a sample of 0.5 g was placed in contact with excess (by the use of ammonium acetate), washed with ethanol, and then calcinated for the ammonia release. The released free ammonia was collected in water and this solution was then titrated with H₂SO₄, expressing the values in meq.g^-1 of the sample.

III. RESULTS AND DISCUSSION

The chemical composition of the RHA used in the synthesis was (wt.%): 94.4 (SiO₂), 1.21 (MgO), 1.06 (K₂O), 0.83 (CaO), 0.77 (Na₂O), 0.61 (Al₂O₃), 0.59 (MnO), 0.03 (Fe₂O). It can be observed that the ashes generated in the burning of rice husks have a high SiO₂ content and small amounts of other substances that are common trace elements in RHA (Foletto et al., 2006). The diffractogram (Fig. 1) indicates that the RHA is formed by silica in crystalline form, resulting from the predominant presence of cristobalite (2θ = 21.9) (Prasetyoko et al., 2006). The presence of silica crystalline, amorphous or in both forms, depends on the burning temperature or the method used for ash production (Rozainee et al., 2008). When the burning temperature of RHA is high, the silica contained in the ash is predominantly crystalline (Foletto et al., 2009). A small increase in the diffraction band in the range 2θ = 15-30^o indicates the presence of amorphous material.

Fig. 1. X-ray diffraction of RHA (Q: quartz), C: cristobalite).

In Fig. 2, it can be observed that the ash particles present a non-uniform shape. These particles are formed by small crystallites clusters. The formation of these clusters is caused due to the non-uniform distribution of temperature and mass flow during the combustion of husk (Foletto et al., 2009). The average of the particles size is smaller than 5 mm. Table 1 shows the chemical analysis results of the samples synthesized with different KOH concentrations. It was observed, for concentrations between 1 and 2N that the Al₂O₃ content remained almost constant, decreasing its content for higher KOH concentrations, resulting in an increase of the Si/Al ratio. All the samples presented high levels of potassium on its structure; which demonstrate that this material is a promising source of cations in the formulation of fertilizer (Kikuchi, 1999).

Fig. 2. SEM image of the rice husk ash.

Table 1. Chemical analysis of the samples. (wt. %).

Figure 3 presents the X-ray diffractograms from the samples prepared with different KOH concentrations. In all the samples, the presence of a certain amount of amorphous solids were observed (Fig. 3), as indicated by the rise in the diffraction band in 2θ between 25-35°. It was observed for all KOH concentrations that the solid has a substantial amount of non-reacted cristobalite. The presence of non-reacted cristobalite in the samples after 5 h of reaction at 100°C demonstrated the low reactivity of this phase in an alkaline solution. We carried out the aluminosilicate synthesis in a short reaction time (5 h) and moderate temperature (100°C) because long reaction times and high temperatures are not appropriate for a commercially viable industrial production. Depending on the reaction medium conditions, the cristobalite can require long reaction times (24 h) to dissolve (Prasetyoko et al., 2006). The X-ray diffractograms showed that the hidroxisoladite formation (Belviso et al., 2010) is favored when a high concentration [3N] of KOH is used in a reaction medium (Juan et al., 2007).

Fig. 3. X-ray diffractograms of the samples synthesized with KOH 1N, 2N and 3N. (Q: quartz, C: cristobalite; *: hydroxisodalite).

Figure 4 presents the images of samples synthesized using different KOH concentrations. The particles are formed by crystals clusters with irregular shapes. It can be observed that the morphology of the samples synthesized at higher concentrations (3N) tend to be of a spherical shape with a smaller particle size (approx. 3 μm). The particles size tends to decrease with the increase of the alkali concentration, indicating a re-crystallization in the system (Foletto et al., 2009). On the other hand, the increase of the alkali concentration leads to an increase of the surface area. The surface area of the samples prepared at 1N, 2 and 3 N KOH were 5.21, 9.40 and 16.43 m².g^-1, respectively. These features are of great importance because it allows a greater release of nutrients to the soil.

Fig. 4. SEM images of the samples: 1N, 2N and 3N.

Figure 5 presents the cation exchange capacity (CEC) results, for the samples synthesized with KOH. It can be observed that CEC increased linearly with the increase in the alkali concentration used in the synthesis. The maximum value of CEC obtained was approx. 9.1 meq.g^-1 for the sample obtained at the higher alkali concentration, 3N.

Fig. 5. Influence of alkali concentration on the CEC of the synthesized samples.

Table 2 presents the CEC values of the aluminosilicates investigated in this study and also by other researchers. The CEC obtained in this work has a superior value when compared with other aluminosilicates (see Table 2), clearly indicating that the sample can be used as an auxiliary product in agriculture.

Table 2. Cation exchange capacity of the different aluminosilicates.

IV. CONCLUSIONS

In this work, a semicrystalline solid with a high cation exchange capacity was obtained by a relatively simple reaction treatment in a KOH medium. This solid consisted of particles of a small size (approx. 1-5 μm). The semicrystalline solid obtained presents high levels of potassium in its chemical composition (see Table 1, 36 to 40 wt.% K₂O). The cation exchange capacity and surface area are strongly dependent on the alkali concentration used in the synthesis of the solid. Both exchange capacity and surface area are very improved with the increase of the alkali concentration. The described method could represent a promising application to obtain industrial products from agro-industrial waste in an economically advantageous way.

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Received: December 16, 2011.
Accepted: June 11, 2012.
Recommended by Subject Editor María Luján Ferreira.