INSTRUMENTAL TECHNIQUES

Transcription

1 INSTRUMENTAL TECHNIQUES The fly ash, mainly contain inorganic material along with several minerals which are quite complex in structure and composition. Sophisticated spectral techniques are required for its perfect characterization, because of its fine grain size structure and trace amounts of its constituents. In order to examine chemical composition of BFA and synthesized zeolitic materials (CZBFA and FZBFA), X- ray Fluorescence technique was used. Structural units, nature and mineralogical characterization of the sorbents were carried out by X-ray Diffraction patterns and Fourier Transform Infrared Spectroscopy (FTIR). The surface morphology was characterized by Scanning Electron Microscopy (SEM). INSTRUMENTAL METHODS AND ANLYSIS X-RAY FLUORESCENCE (XRF) The XRF technique is widely used to measure the elemental composition of materials. The method is fast and non-destructive to the sample. In X-ray fluorescence, when a primary X-ray excitation source from an X- ray tube or a radioactive source strikes the sample, a part of the X-ray can either be absorbed by the atom or scattered through the material surface. The part of the X- ray absorbed by the atom transferred all of its energy to an inner most electrons, is called as the photoelectric effect. During this process the primary X-ray having sufficient energy, eject the electrons from the inner most shells to outer most shells and create vacancies in the inner shells. These vacancies represent an unstable condition for the atom. As the atom returns to its ground level condition, the electrons from the outer shells are transferred to the inner shells. During returning to ground state, atoms give off a characteristic X-rays whose energy is the difference between the two binding energies of the corresponding shells. The emitted X-rays produced from this process are called X-ray Fluorescence or XRF. The process of detecting and analyzing the emitted X-rays is called X-ray Fluorescence Analysis. A typical X-ray spectrum from an irradiated sample will ~ 58 ~

2 display multiple peaks of different intensities. The energy of peaks leads to the identification of elements present in the sample (qualitative analysis), while the peak intensity provides the relevant or absolute elemental concentration (quantitative analysis). The chemical constituents present in BFA, CZBFA and FZBFA were determined by X-ray fluorescence technique using X-ray XDL - B, Fischer scope, Japan at Faucet Division, Kohler India Corporation Pvt. Ltd., Ankleshwar. FOURIER TRANSFORM INFRARED SPECTRA (FTIR) Infrared spectroscopy is widely used spectral technique in academic and industrial laboratories, as an instrumental method for structural and functional group analysis of compounds. Since IR absorption is related to the vibrationalrotational motions of covalent bonds in a molecule, the spectra can provide much detailed information about the structure of the molecular compounds. It is possible by examination of a large number of spectra of known materials to correlate specific vibrational absorption maxima with the specific functional group. These characteristic peaks permit the chemist to obtain useful structural information by correlating them with the generalized characteristic. The FTIR spectra of BFA, CZBFA, FZBFA and sorbents loaded with phenols were recorded using KBr pellet method on Thermo-Nicolet is-10 FTIR spectrometer at Chemistry Department, Veer Narmad South Gujarat University, Surat. For this, highly purified and desiccated spectroscopic grade 1.0 g KBr and mg of well dried sorbent sample was mixed thoroughly and ground in a mortar. The finely ground mixture was then transferred to the mould and the pellets prepared in this manner were used to scan FTIR spectra of the sorbent samples by standard procedure. ~ 59 ~

3 POWDER X-RAY DIFFRACTION SPECTROSCOPY (PXRD) X-ray diffraction is versatile and non-destructive technique for characterizing the crystalline samples including nano material. It reveals detailed information about phase identification and crystal structure of natural and engineered materials. X-ray diffraction techniques are based on the elastic scattering of X-rays from structures that have long range order. The powder diffraction (PXRD) is a technique used to characterize the crystallographic structure, crystalline size (grain size) and preferred orientation in polycrystalline or powder solid samples. Powder diffraction technique is commonly used to identify unknown substances and mineralogical characteristics of sorbents. Powder X-ray diffraction (XRD) was carried out with Panalyticals X.Pert Pro instrument at Panjab University, Panjab, using a monochromatized X-ray beam from nickel-filtered Cu K α (λ = A o ) radiation and Ni filter at room temperature and operated at 45.0 kv and 40.0 ma. The powder diffractograms of the samples were recorded over a range of 2θ values from 5 to 70. The scanning rate was set at /min. Analysis of sorbents by X-ray diffraction provides d-spacing values of their corresponding minerals and crystalline phase, by comparing the diffraction data against a database maintained by the International Centre for Diffraction Data. Thus, identification of crystalline characteristics and mineralogy of solid material can be done on the basis of the data provided by Joint Committee on Powder Diffraction Standards (JCPDS) [241]. SCANNING ELECTRON MICROSCOPY (SEM) In the fields of material science and surface chemistry detailed knowledge of the physical nature of the surfaces of solids is of great importance. Characterization of surface morphology is often of vital importance in understanding the physical and chemical behaviour of the material. In Scanning Electron Microscopic (SEM) analysis, accelerated electrons carry significant amounts of kinetic energy and this energy is dissipated as a variety ~ 60 ~

4 of signals produced by electron-sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons (that produce SEM images). Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography of samples. Natural, synthetic silicates and aluminosilicates possess polymeric structure. Silicates are the salts of silicic acid, having the general formula xe n O m.ysio 2.zH 2 O, here, E representing Na, Ca, Al, Mg and other similar atoms. Silicates may have lamellar or three dimentional crystalline structure. Some of the natural aluminosilicates of lamellar structure are kaolinite and montmorillonite zeolite. Lamellas are primary structural elements, whose different arrangement causes a great diversity in the structure forms of crystal polymers. Thus, structure formation on the basis of lamellar crystals develops when concentration of solution is high. In such cases crystalline structural characteristic of the zeolites are formed as polyhedrons. {hedrites and axiallites or ovals (ovoids)}. The SEM micrograph of BFA, CZBFA, FZBFA and phenol loaded sorbents were observed by Scanning Electron Microscope (SEM). The SEM images were acquired by using HITACHI S-3400N (operating at 15 kv) at Sardar Vallabhbhai National Institute of Technology, Surat. The detailed examination of SEM at different magnifications provides the information about the modifications in the fold structures of lamellar polymeric structure. ~ 61 ~

5 RESULTS AND INTERPRETATIONS X-RAY FLUORESCENCE (XRF) The chemical and mineralogical compositions of the BFA, CZBFA and FZBFA obtained from XRF technique are summarized in Table 3.1 in the form of stable oxides. The amounts of SiO 2 and Al 2 O 3 within zeolites (CZBFA and FZBFA) were lower than corresponding oxides in the original BFA. This may be due to the dissolution of glass phase (aluminum silicate) into the alkaline solution during alkaline hydrothermal treatment. An increase in Na 2 O contents of CZBFA and FZBFA is caused by captured of sodium ions to neutralize the minus charge on aluminate in zeolite structure when zeolite crystal is formed [242]. The content of calcium magnesium oxides and other components after synthesis were reduced. This indicates that these impurities were dissolved into alkaline solution during alkaline hydrothermal synthesis. During alkaline hydrothermal treatment and fusion given to BFA, the aluminate and silicate ions get dissolved from BFA in alkaline hydrothermal solution, thus forming precursor aluminosilicate gel covering the outer surface of BFA. The precursor gel is motivated by sodium ions in alkaline media to crystallize in the form of zeolitic material during crystallization time (CZBFA and FZBFA). Alkali fusion enhances the dissolution of fly ash converting most of the ash into sodium salts (silicate and aluminate). The fused ash can then be dissolved in water and reacted by hydrothermal synthesis to produce zeolites. The mechanism for zeolite synthesis from fly ash in a fusion and hydrothermal synthesis process has three stages (Figure 3.1) as proposed by Murayam et al. [242]: 1) The dissolution of aluminium and silicon from fly ash, 2) The deposition of aluminosilicate gel on ash surface, 3) The crystallisation of zeolite from aluminosilicate gel ~ 62 ~

6 Figure 3.1 Illustration of reaction mechanism for the batch hydrothermal Conversion of fly ash to zeolites Table 3.1 Chemical composition of BFA, CZBFA and FZBFA analyzed by XRF Constituents Weight percentage determined BFA ZBFA FZBFA SiO Al 2 O Fe 2 O CaO Mgo Na 2 O K 2 O ~ 63 ~

7 FTIR SPECTROSCOPY FTIR spectra of BFA, CZBFA, and FZBFA are shown in Figure 3.2(a), Figure 3.3(a) and Figure 3.4(a) respectively. Phenol loaded BFA, CZBFA, and FZBFA are shown in Figure 3.2(b), Figure 3.3(b) and Figure 3.4(b) respectively. FTIR spectra of sorbent materials indicate weak and broad bands in the region of 4000cm cm -1. FTIR spectra of sorbents show a broad band between 3700cm -1 and 3100cm -1 indicating the presence of - OH group on the sorbent surface. This stretching is due to the silanol groups (Si-OH) of sorbents. The band observed between 1250cm cm -1 can be ascribed to TO 4 (where T = Si, Al) asymmetric stretching vibrations of internal tetrahedral. The bands between 720cm cm -1 and 420cm cm -1 can be due to symmetric stretching and TO 4 bending mode of internal tetrahedral respectively. FTIR spectrum of BFA (Figure 3.2(a)) shows asymmetric stretching of TO 4 at cm -1 and symmetric stretching of TO 4 at cm -1. The band at cm -1 and cm -1 in BFA are attributed to bending of tetrahedral. The shift of the band maximum of cm -1 in the unzeolitized BFA to cm -1 in CZBFA confirms the tetrahedral coordination of aluminum in the zeolite framework [243, 244]. In the zeolite spectra this band is the main asymmetric stretching vibration of the tetrahedral which is the result of asymmetric stretching vibration of external linkages of primary structural units. The bands at cm -1 and 668cm -1 correspond to symmetric stretching vibration of Si-O-Si and Si-Al-O respectively. The bands at about 467cm -1 and 454cm 1 can be assigned to O-T-O bending. The band at about cm 1 attributed to deformation of - OH vibration of absorbed water and bending vibration of H 2 O. Newly observed bands confirm the formation of zeolite from BFA. In case of FZBFA, the shifts of bands from cm -1 to and cm -1 to cm -1 confirm the formation of zeolite phases which can be assigned to asymmetric and symmetric stretching of internal tetrahedral (TO 4 ) of amorphous aluminosilicates formed by the reaction of dissolved Si +4 and Al +3. Decrease in frequency of asymmetric stretching vibration of tetrahedral indicates that the amount of aluminium is increased in tetrahedral sites of the aluminosilicate ~ 64 ~

8 framework of the zeolite. The bands at 765cm -1 and 685cm -1 correspond to symmetric stretching vibration of internal tetrahedral (TO 4 ). The band at 609.4cm -1 can be ascribed to double ring mode of external linkage of zeolite framework. The band at about 461cm -1 can be assigned to TO 4 bending in internal tetrahedral. The band at about 420cm -1 is observed due to pore opening of external linkage of zeolite framework. The band at about 1651cm 1 and cm -1 are attributed to deformation of - OH vibration of absorbed water and bending vibration of H 2 O [244]. The broad band at about 3440±20cm -1 in CZBFA and FZBFA indicates the presence of water in the mineral of synthesized zeolite. It is known that one of the most important properties of zeolites is the occurrence of zeolite water in their structures, namely neutral water molecules in voids and channels. The band observed at about 2360±30cm -1 is due to the atmospheric carbon dioxide. In all phenol loaded FTIR spectrum of sorbents, the asymmetric and symmetric vibration bands of internal tetrahedral are shifted little bit due to sorption of phenols. In phenol loaded spectra of CZBFA and FZBFA, the deformation and bending vibration are suppressed due to the pore filling of zeolites by sorbate molecules. Phenol content is less as compare to OH groups of zeolitic network (Si-OH) in the taken sample, So, it does not increase the intensity of OH band (3400cm -1 ). In phenols loaded spectrums some new peaks are observed in region of 1600cm cm -1 due to benzene ring of phenols. Although some inference can be drawn about the surface functional groups from IR spectra, the weak and broad bands do not provide authentic information about the nature of the surface oxides. ~ 65 ~

9 Figure 3.2(a) FTIR spectra of BFA Figure 3.2(b) FTIR spectra of Phenol loaded BFA ~ 66 ~

10 Figure 3.3(a) FTIR spectra of CZBFA Figure 3.3(b) FTIR spectra of phenol loaded CZBFA ~ 67 ~

11 Figure 3.4(a) FTIR spectra of FZBFA Figure 3.4(b) FTIR spectra of phenol loaded FZBFA ~ 68 ~

12 POWDER X-RAY DIFFRACTION The powder X-ray diffraction pattern of BFA, CZBFA and FZBFA are shown in Figure 3.5, Figure 3.6 and Figure 3.7 respectively. BFA is primarily composed of partially crystalline phases along with the amorphous glass phase material. Amorphous material of BFA mainly composed of aluminosilicates. A wide huge hump at lower diffraction angle in BFA diffraction pattern indicates the presence of glass phase in BFA [245]. The diffractogram of BFA shows α-quartz peaks (JCPDS 5-490), several small peaks of mullite (JCPDS ), a small peak of Stilbite and other amorphous materials [246]. After treatments the broad hump of amorphous phase at lower diffraction angle weakens and several new diffraction peaks were detected in PXRD patterns of CZBFA and FZBFA. Further, dissolution of α-quartz phase also occurs under given experimental condition. This indicates that the dissolution of amorphous phase and α-quartz parts result in the formation of zeolites. Some mullite peaks were observed in CZBFA due to it s the low reactivity under given condition of alkaline treatment. CZBFA shows comparatively low flat and low intensity hump after treatment and also gives several sharp diffraction peaks, which were not seen in BFA. The most intense new peaks were observed at 2θ=15.97 and 2θ=26.13 which can be attributed to formation of zeolites. In CZBFA, positive identification was made for zeolite P (JCPDS ), Analcime (JCPDS ) and Chabazite (JCPDS ). The observed d-spacing values for Phillipsite (P) = 6.95, 4.22, 3.74, 3.66, 3.32, 2.89, 2.68, 1.97, 1.89, 1.80, 1.71, 1.68 and Analcime (A) = 5.54, 4.82, 3.41, 2.27, 2.22, 1.74 in CZBFA indicate their presence as major content [247]. PXRD pattern of FZBFA shows that the hump at lower diffraction angle is also suppressed as compared to BFA and CZBFA. Several peaks which have been observed in CZBFA have disappeared and intensity of few peaks is increased after fusion treatment. The crystalline phases identified in the FZBFA were zeolite P (JCPDS ), Analcime (JCPDS ), Zeolite A (JCPDS 14-90), Zeolite X (JCPDS ), ZSM 12, calcined (JCPDS ). Zeolite P and Analcime ~ 69 ~

13 were found to be dominant in zeolite formation during fusion treatment. The diffractograms show that the original crystalline phases quartz and mullite of BFA are mostly absent in the zeolitic materials after fusion treatment. From the disappearance of quartz and mullite peaks, it can be deduced that both quartz and mullite in the BFA have reacted with NaOH during fusion to form sodium aluminosilicates. The results from the FZBFA are probably related to the fact that more aluminosilicates have been produced and dissolved in the solution due to the formation of sodium silicates and amorphous aluminosilicates during fusion and crystallization respectively. Figure 3.5 Powder X-ray diffraction pattern of BFA ~ 70 ~

14 Figure 3.6 Powder X-ray diffraction pattern of CZBFA Figure 3.7 Powder X-ray diffraction pattern of FZBFA ~ 71 ~

15 SCANNING ELECTRON MICROSCOPY Scanning electron microscopic (SEM) analysis has found great use in understanding the surface morphology of material. SEM photographs of BFA, phenol loaded BFA (BFA-Phenol), ortho-chlorophenol loaded BFA (BFA-OCP), para-nitrophenol loaded BFA (BFA-PNP), CZBFA, phenol loaded CZBFA (CZBFA-Phenol), ortho-chlorophenol loaded CZBFA (CZBFA-OCP), paranitrophenol loaded CZBFA (CZBFA-PNP), FZBFA, phenol loaded FZBFA (FZBFA-Phenol), ortho-chlorophenol loaded FZBFA (FZBFA-OCP) and paranitrophenol loaded FZBFA (FZBFA-PNP) were acquired at 500X magnification to observe their surface texture (Figure 3.8 to Figure 3.10). Several observations of the samples were made before the SEM images were taken, to ensure that the photographed regions were representative of the overall samples. The obtained photomicrographs reveal the surface texture and porosity of sorbents. The SEM image of BFA shows fibrous structure along with large irregular silicate masses and lamellar structure with large but shallow pore size and strands in each fold (Figure 3.8(a)). It also indicates the irregular shapes and surfaces of BFA. The virgin BFA mostly contain non-crystalline glass phase, with a loose structure and possess smooth surface particle because the surface is covered by an aluminosilicate glass phase. After hydrothermal treatment, the CZBFA particles seem to be more fluffy and porous (Figure 3.9(a)). This may be due to the change in the surface of the particles which show crystalline forms with compact structures and honeycomb aperture with holes. The surface shows hollow particles with interior voids, creating more folded strands with deeper pores. The SEM photograph of CZBFA indicates the sorbent to be porous. SEM micrograph of FZBFA exhibits a typical size of trapezohedral analcime crystals which is approximately 10µm and the morphology is similar to those reported by Lin et al [247]. Overall, the transformation of fly ash to different phases of zeolite can be demonstrated by SEM, albeit some crystals with the exception of analcime are not well distinct. ~ 72 ~

16 The SEM micrographs exhibit the occurrence of spherical particles in CZBFA and FZBFA (Figure 3.9(a) and 3.10(a)) due to the treatment in alkaline media. It reduces the glass phase characteristic as shown in X-ray diffraction patterns [245]. It is also apparent form the SEM that after modification process, spherical particles have covered with small crystallites. Many small crystallites can be observed in FZBFA. This is consistent with nucleation occurring rapidly on the particle surface. Jansen [248] has suggested that the presence of impurities in the reactants may affect the crystal form and chemical properties of the zeolite formed. Such impurities can increase nucleation at the expense of crystal growth and lead to the formation of many small crystallites. Because BFA is not pure quartz and mullite but also comprises of other materials, it is not surprising that nucleation processes at the base concentration produced better defined crystallites. Moreover the particle size of CZBFA and FZBFA has been decreased and surface area has been increased as compared to original BFA especially in FZBFA. This may be due to the change in the structure of fly ash in which more spaces occurred within the structure. The increase in surface area enhances the sorption. The SEM micrograph of phenol loaded BFA (Figure 3.8(b-d)) shows that the layered strands get diminished showing that the sorption of phenols creates more compact nature. The spherical beads fill up the surface showing very intense phenol sorption on the surface throughout in CZBFA and FZBFA (Figure 3.9(b-d) and 3.10(b-d)). The SEM micrographs of phenol loaded CZBFA and FZBFA exhibit agglomeration of many small spherical particles forming large nonspherical particles due to sorption of phenol. In case of FZBFA, it can be expected that the agglomeration of particles form clusters, it could be due to its softer texture having pores [249]. From the micrograph of phenol loaded sorbents, a horizontal layer type of sorption is observed on the smooth surface. Accumulation of contaminants on the edges of the rough surfaces as well as in the macropores is also observed. ~ 73 ~

17 (a) (b) (c) (d) Figure 3.8 SEM micrograph of (a) BFA, (b) BFA-Phenol, (c) BFA-OCP and (d) BFA-PNP at 500X magnification ~ 74 ~

18 (a) (b) (c) (d) Figure 3.9 SEM micrograph of (a) CZBFA, (b) CZBFA-Phenol, (c) CZBFA-OCP and (d) CZBFA-PNP at 500X magnification ~ 75 ~

19 (a) (b) (c) (d) Figure 3.10 SEM micrograph of (a) FZBFA, (b) FZBFA-Phenol, (c) FZBFA-OCP and (d) FZBFA-PNP at 500X magnification ~ 76 ~