Synthesis and Characterization of Mesoporous Aluminosilicates for Copper Removal from Aqueous Medium

In this study the characterization of an aluminosilicate synthesized from commercial Al ₂(SO ₄) ₃ and colloidal SiO ₂ is presented, as well as its capacity for the removal of copper from aqueous solution. Characterization of the synthesized material was performed using X-ray diffraction, BET nitrogen adsorption-desorption, mass titration and the Boehm method. In order to obtain stable agglomeration and enhance its surface area (165 - 243 m ²/g) and solid adsorbing capabilities, the molar ratio SiO ₂:Al ₂O ₃ (1:3, 1:1 and 3:1) was studied, the solubility of the preparation material, synthesis-procedure time and solution pH function were also examined. The maximum capacity to remove copper ions from an aqueous solution by synthesized aluminosilicate was 16 mg/g at pH 4 and 25℃. The Langmuir model fitted better to the copper adsorption experimental data.

• Mesoporous Aluminosilicates; Surface Properties; Copper Adsorption

1. Introduction

The disposal of waste materials for a number of Industries is an issue which has attracted special attention due to the importance that this concern has from the standpoint of: both reutilization of available resources and environmental protection [1,2]. Public awareness has raised concerns about water pollution generated by industries, which has led to legislation on waste control, becoming more demanding, introducing ever more restrictive rules on the permitted content of toxic compounds or hazardous industrial discharges [3-5]. The dilemma posed by the public health protection against the progress, is the requirement calling for the incorporation of economically viable technologies to be implemented quickly as possible [6-8].

Mining solid wastes and acid liquid mine drainage have contributed significantly to pollution with dissolved copper going to industry effluent streams in the Bajío Mexican region (Guanajuato, Mexico) which is renowned as a mining zone and consequently has been facing this problem for a long period of time [9,10]. Other industrial sources are: fertilizer chemicals, refining petroleum products, paints and pigments production, steelworks, foundries, electroplating etc [11-13]. It is known; copper is an essential element for life, but acute doses cause disorders in the human metabolism. The average concentration of copper in rivers and lakes is 0.01 ppm while in groundwater is around 0.005 ppm [14]. Nevertheless, in some cases levels up to 2.7 ppm have been measured in groundwater. The USEPA1 standard for drinking water is 1.3 ppm [15]. Copper poisoning causes stomach and intestinal distress, kidney damage, hemolytic anemia, neurological abnormalities and corneal opacity [16]. For that reason, the industry traditionally have used chemical and electrochemical methods to precipitate and reduce metals [17-25]; however, some of those are expensive and/or lead to other problems such as large quantities of waste solids [26]. Research and development of processes for metals removal of wastewater from industries, through the use of adsorbents, aims to address specific issues raised in relation to water pollution and its treatment costs.

The adsorption process of metals in aqueous solutions is one of the most efficient and economical techniques for contaminant removal, particularly if it uses a low cost adsorbent. Mesoporous materials2 are attractive for adsorptive processes due their potential for isomorphous substitutions and ease of access to the pores relative to micropores. It has been reported that this type of porosity facilitates more efficient mass transfer inside the pores and at same time makes more effective the process of heavy metals removal [27]. Furthermore, they can be reused with the implementation of an efficient desorption and regeneration process [28].

In this work is presented an alternative to commercial aluminosilicates and natural clays at low cost is presented. These materials always have been attractive in adsorption field, but they have been currently used as powders having some notorious drawbacks when utilized in packed columns, due to the high pressure drop that they cause. Furthermore, aluminosilicates pellets have shown to be more efficient inside applications in continuous processes. Therefore, this work was focussed on the synthesis of aluminosilicates with low-cost precursors in alkaline media varying the SiO₂/Al₂O₃ molar ratio. Also, several characterization methods to identify their crystallographic and surface features were carried out. In addition, the agglomerated pellets have been used to remove effectively copper ions (Cu²⁺) from aqueous solution and; finally, to add sustainable to this process, three different copper desorption treatments were performed.

2. Methodology

2.1. Synthesis of Mesoporous Aluminosilicates

The synthesis of aluminosilicates was based on the methodology previously reported by González Figueroa [29]. The of Al₂(SO₄)₃, is dissolved in distilled water with a ratio of 12 ml for 1 g of aluminium sulphate (Prosi PSV Chemicals, technical), the solution is maintained in a glass vessel with magnetic agitation, while increasing the temperature by heating up to 80˚C under vacuum. The final solution pH value is approximately 2. After that step, a three neck round bottle flask is utilized to simultaneously: 1) add Al₂(SO₄)₃ dropwise, 2) add colloidal SiO₂ (30 wt% suspension in H₂O, Aldrich) dropwise, and finally, 3) inject the hydrolyzing agent, ammonia gas (anhydrous, Refriamonia). The reactant amount depends on the required molar ratio (1-3). The reaction is carried out until all reagents are depleted, the pH is measured again, and it should show a value of 10 - 12. The precipitated solid was filtered and was washed with a solution of ammonia-water (10%) followed by 1 L of hot water (90˚C), additionally cold water is needed to ensure that no sulphates compounds remain in the solid, (a test is performed with barium chloride, BaCl₂). The wet cake is dried at room temperature overnight and subsequently goes to the oven for a period of time of 24 h at 110˚C. Finally, the collected dry material is ground and meshed in a Tyler sieve (0.6 - 0.7 mm). In parallel, some dried material was mixed vigorously with a dilute nitric acid solution to form a suspension, which was dripped into a glass column containing two phases: an upper liquid zone 10 cm high of an organic phase which is composed of hexane (99.8%, J.T. Baker) and a 60 cm lower liquid zone of a ammonium hydroxide solution (30%, Karal) with the purpose of shaping spherical agglomerates (pellets).

2.2. Characterization of Aluminosilicates

2.2.1. Structural and Textural Properties

X-ray diffraction (XRD) patterns were obtained for each of materials synthesized at different molar ratios to determine the material structure. The XRD patterns were obtained with a Diffractometer INEL model Equinox System Cu/λ = 1.54 Ǻ, with 30 kV and 20 mA. The textural characterization (surface area, pore volume and pore diameter) was performed using the technique of S_BET. To the latter case, a physisorption analyzer Micromeritics model ASAP 2010 was used.

2.2.2. Surface Acidity Determination

The active sites of the aluminosilicate were determined by Böehm titration method [30,31]. The total acid and basic sites were neutralized with 0.1 M NaOH and 0.1 M HCl, respectively. The determination of the active sites was performed by adding 1 g of material to each of the polypropylene tubes, which contained 50 ml of neutralizing solution. The tubes were partially submerged in a bath at constant temperature (25˚C) for 5 days until equilibrium was reached, during this period, the tubes were agitated in laboratory vertical agitation equipment. Then a sample of 10 ml from each tube was taken and was titrated with 0.1 M solutions of HCl or NaOH, as appropriate.

2.2.3. Point of Zero Charge Determination

Sample of 1 g of aluminosilicate was poured into 50 ml polypropylene tubes with 10 ml of 0.1 M NaCl. The container was covered and left for 48 h in magnetic agitation. After this time, the solution’s pH is measured and the final pH was plotted against the mass of activated alumina.

2.3. Aluminium Solubility Determination

Aluminum (Al³⁺) in solution was measured after contact with deionized water at different pH values. Addition of 4 ml of water to a sample of 0.02 g of the material was performed, at different pH values (1-6), the setup was then left under magnetic agitation for 1, 2, 3, 4, and 24 h respectively, after that period of time, the sample was centrifuged and the Al³⁺ concentration was measured in the supernatant volume with the Inducted Coupled Plasma Optical Emission Spectrometry (ICP-OES) model PerkinElmer Optima 4300 DV equipment.

2.4. Adsorption

2.4.1. Kinetics

Samples of 0.02 grams of the synthesized material (SiO₂: 3Al₂O₃) were in contact with 4 ml of 100 ppm copper solution in continuous agitation for 5, 10, 15, 20, 30 and 60 min, respectively; afterwards samples were centrifuged and copper (Cu²⁺) concentration was measured in the supernatant with the ICP-OES equipment.

2.4.2. Isotherms

The adsorption isotherms were obtained in batch studies that were carried out subjecting a sample (0.02 g) to mechanical shaking for a period of 30 min at 25˚C and 35˚C respectively. Volume utilized was 4 ml of Cu²⁺ solution, the different concentrations used were 1, 20, 40, 60 and 100 ppm, at pH 4 (pH for better copper adsorption). For each tube the material was separated using a centrifugation step (6000 rpm) for over 5 min. Copper concentrations in the supernatant were determined with the ICPOES equipment. All assays were carried out three times and the mean values are plotted

3. Results and Discussion

3.1. Aluminosilicates Structural and Textural Properties

Figure 1 shows the XRD patterns of the powders obtained by the hydrolysis-precipitation reaction. Characteristic peaks of SiO₂ at 23˚ and γ-Al₂O₃ peak at around 43˚ and 64˚. The large peak in Figure 1 at ~3˚ demonstrates the mesoporous nature of the structure. Also, it

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