Abstract

The present work is concerned with synthesis of aluminophosphate zeolites with tridymit framework by employing microwave technique and alanine as structure directing agent. Aluminophosphate zeolite was characterised using XRD, SEM- EDAX, BET and FTIR. Adsorption efficiency of the synthesised aluminophosphate zeolite was evaluated using water soluble dye methylene blue as a model pollutant. Influence of pH, Adsorbent dosage, Dye concentration and contact time were studied. Experimental data fitted well with pseudo bimolecular kinetic model and freundlich’s adsorption isotherm (R²=1) with yielding a maximum adsorption capacity of 37.016ppm mg^-1.

1. Introduction

Unusual physicochemical properties of zeolites and zeolite like frameworks such as Silicates, aluminosilicates, aluminophosphates and metal substituted aluminophosphate zeolites play a significant role in augmenting their industrial applications mainly as adsorbents, catalysts and in membrane separation ^{1, 2, 3}. Aluminophosphates specifically are important members of this family as they have silicon free composition but are also similar to zeolites in morphology and function.

Microporous nature of aluminophosphates retards the rate of diffusion on to molecular sieves there by reducing the efficiency of the molecules as adsorbents, catalysts or membrane separation. Synthesis of mesoporous aluminophosphate with ordered structure will enhance diffusion of reactants on to zeolite frameworks ⁴. Intensive research is being carried out in this field of designing and synthesis of aluminophosphates with desired structural and physicochemical properties for specific applications by altering various synthetic parameters such as Aluminium phosphate ratio, by using different structure directing agents such as surfactants, quaternary ammonium salts, amines, organosilanes etc., changing synthesis conditions like Ionothermal synthesis ⁵, hydrothermal synthesis ⁶, Solvothermal synthesis ⁷, vapour phase synthesis ⁸, solvent free synthesis ⁹ etc. Combinatorial methods are being employed for the synthesis of aluminophosphate zeolites ¹⁰ conventional techniques involve High temperature and pressure along with long reaction times.

Water soluble dyes or colouring agents such as Methylene blue, Malachite green, trypan blue, Evan’s Blue, sulphur dyes etc., are used extensively in many industries such as paper, pulp, leather, textiles and hence are major threat to environmental safety, particularly to our water bodies. With growing awareness about environment and its wellbeing massive research work is being done worldwide in developing materials and techniques to aid in removing these pollutants from water bodies. Although various techniques both physicochemical and biological techniques such as flocculation ¹¹, precipitation, coagulation, ozonation, photo degradation, membrane filtration are available, it has been found tough to remove organic dyes and pigments using conventional methods owing to their complex structures and stability towards action of light or oxidizing agents and resistance to aerobic digestion. Adsorption is one of the favourable techniques for removing water soluble dyes ¹² and Aluminophosphate zeolites are promising materials in removal of these water soluble dyes by adsorption.

Microwave assisted synthesis of aluminophosphates offers the advantage of quick synthesis of aluminophosphates under normal temperature and pressure ^{5, 13, 14, 15}. Current study gives a brief account of synthesis of Aluminophosphate with tridymit framework employing microwave assisted synthetic technique and amino acids as structure directing agents. Structure and morphology of aluminophosphates were studied using XRD, IR, SEM-EDAX and BET. Adsorption capacity of the synthesised aluminophosphates in removing water soluble dyes was evaluated using Methylene Blue as model adsorbent.

2. Materials and Methods

Aluminium Hydroxide (LR grade) used as source of Aluminium, Ortho phosphoric acid (LR grade) used as source of Phosphorus and (±) alanine(LR grade) was used as structure directing agent for the synthesis of aluminophosphates. Methylene Blue (LR grade) was used to evaluate adsorption capacity of AlPOs.

3.9g (1mmol) of Aluminium hydroxide was dispersed in 10mL of distilled water and stirred for 15 min. 5.6mL (2mmol) of orthophosphoric acid was added dropwise to the dispersion with constant stirring. Reaction mixture was stirred for 1h at room temperature. 2.22g (0.05mmol) of Alanine was dissolved in 10mL of distilled water and added to the reaction mixture with constant stirring. The gel was aged for 1h at room temperature and subjected to microwave irradiation (160W, 20 min) after noting the Initial and Final pH. White precipitate of Aluminophosphates formed was washed thoroughly with distilled water under ultra-sonication. The product was then filtered and dried in hot air oven at 200°C for 2h.

Phase purity of the synthesised Aluminophosphates was examined using powder x-ray diffraction pattern (XRD smart lab x-ray diffratometer (CuK α , λ=1.5406 å). FTIR spectra of the particles give an understanding in to the nature of bonding. Details pertaining to surface morphology, elemental composition and crystal size were obtained using SEM with EDAX (VEGA 3 TESCAN). Surface area was determined using Quatachrome instruments-Nova station 0, version 3. Adsorption studies with respect to methylene blue were studied using colorimeter.

Batch sorption experiments were conducted to evaluate the effect of various factors such as pH, adsorbent dosage, initial dye concentration, contact time ^{16, 17, 18}.

100ml of 5ppm methylene blue solution was adjusted to different initial pH values (pH 3, 5, 7, 9 and 11). The solutions were then stirred with 5mg of adsorbent. 2ml of the solution was withdrawn at regular intervals, centrifuged and absorbance of the centrifugate was measured. Percentage of adsorption was calculated using eq. 1.

(1)

100ml of 5ppm methylene blue solution was stirred with different amount of adsorbent (5, 10, 15, 20, 25 mg). 2ml of dye solution was withdrawn at regular interval of time and absorbance was measured after removing catalyst by centrifugation. Percentage of adsorption was calculated using eq. 1.

100ml of dye solution of varying concentrations (1, 2, 3, 4, 5ppm) was stirred with 10mg of adsorbent. Dye solution was withdrawn at regular interval of time, centrifuged and absorbance of centrifugate was measured. Percentage of adsorption was calculated using eq.1.

The results obtained were analysed using kinetic models and adsorption isotherms to understand the nature of adsorption of methylene blue by aluminophosphate zeolites.

3. Results and Discussions

X-Ray diffraction pattern of Ala templated alumminophosphates presented in Figure 1a. All the peaks in the diffraction pattern of the obtained compound can be indexed as belonging to ALPO₄-tridymit (orthorhombic) phase (a= b= 7.082 and c= 6.993) corresponding to JCPDS no-11-500. Average particle size D calculated using Debye- Scherrer equation (2) ¹⁹,

(2)

Willamson–Hall (W-H) method was used to calculate the lattice strain from the following modified Scherrer equation (3) ¹⁹,

(3)

W-H plot of βcosθ against 4sinθ is shown in Figure 1b. Dislocation density (δ) is estimated using the below equation (4), the calculated physical parameters are given in Table 1.

(4)

SEM images of synthesized Aluminophosphate zeolites are given in Figure 2 and from the image it is evident that the particles have a spherical morphology and EDAX (Figure 3) confirms the purity of the synthesized Aluminophosphate zeolites.

Surface area and Pore size analysis: Nitrogen adsorption, desorption isotherms of Aluminophosphates (Figure 4a) shows no typical hysteresis indicating the presence of blind cylindrical, cone shaped or wedge shaped pores (Figure 4a) ²⁰.

A linear plot of [1/W ((P_o/P)-1)] vs P/P_o presented in Figure 4b. The surface area of the aluminophosphates was found to be 2.743m²/g. From nitrogen desorption isotherm using BJH analysis, pore volume and pore radius were determined and found to be 0.004cm³/g and 19.2Ǻ respectively (Figure 4c). Hence synthesized aluminophosphates can be considered to be mesoporous in nature ²¹.

Infrared spectroscopy: FTIR spectrum of Aluminophosphate zeolites shown in Figure 5. Peaks around 468-545cm^-1 corresponds to bending vibrations of PO₄, Sharp peak at 615 cm^-1 can be attributed to interaction between alumina and Al³⁺. Symmetric stretching of PO₄ tetrahedra is confirmed by sharp peaks in the region 700-740cm^-1. Characteristic peak of PO₄ tetrahedra is observed at 1115 cm^-1. Broad band in the region 3000-3100cm^-1 corresponds to the presence of surface hydroxyl groups ²¹.

4. Adsorption studies of Methylene Blue on to Aluminophosphates

Methylene blue was used as a model dye to study the effectiveness of aluminophosphate zeolites as adsorbents. From Figure 6 it is evident that adsorption is maximum at pH 7. Adsorption attains equilibrium with in initial 20 minutes irrespective of adsorbent dosage and initial dye concentrations (Figure 7 and Figure 8). Aluminophosphate zeolites were found to be more effective as adsorbents at lower dosages but efficiency decreases with increasing dosage. This is due to blocking of pores on zeolite surface due to aggregation of adsorbents with increased concentration. A compilation of previous results of methylene blue adsorption from literature is given in Table 2.

Decrease in adsorption efficiency can be attributed to low surface area of ala templated aluminophosphate zeolites and also to the blind pores in the adsorbent.

Kinetic parameter data presented in Table 3 confirms that the process of adsorption follows pseudo bimolecular kinetics. In this model, rate of adsorption depends on the amount of dye adsorbed on to the surface of adsorbent and to the amount adsorbed at equilibrium. A plot of t/q_t vs time in minutes gives a straight line graph with pearson correlation coefficient R² value equal to 0.9697 (Figure 9). From the plot, rate of adsorption was found to be 7.21 x 10² ppm mg^-1 min^-1. It also implies that the adsorption process is a multilayer process which is augmented by the results of study of adsorption isotherm models. A plot of Ce/qe vs Ce (Langmuir’s isotherm) and –logC_e vs –log q_e (Freundlich’s Isotherm) were presented in Figure 10 (a) and (b) respectively. From the two plots, it can be derived that plot of –logC_e vs –log q_e gives a straight line graph with slope 1/n and intercept –log K_f. Adsorption process fits well with freundlich’s adsorption model of multilayer adsorption as suggested by pearson correlation factor which was calculated to be 1 and dimensional quantity n which corresponds to intensity of adsorption was determined to be 1 indicating that the process of adsorption is moderate ²⁶. Adsorption of methylene blue on to Aluminophosphate zeolite follows freundlich adsorption isotherm indicating a multilayer adsorption process (Figure 10) ^{16, 17}. Experimental parameters corresponding to Kinetic study and adsorption isotherm study of adsorption of methylene blue on ala templated aluminophosphate are presented in Table 3 ²⁴.

5. Conclusion

Aluminophosphates with orthorhombic phase was synthesized using alanine as structure directing agent under ambient conditions. Scanning electron microscope images confirm aggregates of roughly spherical plate like structures with dimensions in the range of 230nm. Surface area of Aluminophosphates was found to be 2.743m²/g and pore radius 19.2Ǻ. Adsorption of methylene blue by amino acid templated aluminophosphates follows pseudo second order kinetics with rate 7.21 x10² ppm mg^-1min^-1 and follows freundlich adsorption isotherm indicating a multilayer process.

Acknowledgement

Authors are grateful to STIC, Cochin for providing instrumentation facilities for characterization.

References