Epoxide Functional γ-Al 2 O 3 / Fe 3 O 4 / SiO 2 Ceramic Nanocomposite Particles as Adsorbent for Reactive Azo Dye : Understanding Surface Property

In this investigation magnetic γ-Al2O3 ceramic nanocomposite particles bearing epoxide functionality are prepared following a multistep process. The ultimate nanocomposite particles are named as γ-Al2O3/Fe3O4/SiO2/poly(glycidyl methacrylate (PGMA). The surface property is evaluated by carrying out the adsorption study of Remazol navy (RN), a model reactive azo dye, on both γ-Al2O3/Fe3O4/SiO2 and γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles. The adsorption is carried out at the point of zero charge (PZC) to neutralize the effect of particle surface charge. The adsorption rate is very fast, reached equilibrium (qe) value within five min. Due to mesoporous structure of silica layer γ-Al2O3/Fe3O4/SiO2 nanocomposite particles possessed relatively higher specific surface area and magnitude of adsorption is dependent on the total specific surface area. The introduction of epoxide functionality favored high adsorption capacity in mass per unit surface area. The adsorption process strictly followed Langmuir model. Thermodynamic equilibrium parameters implied that irrespective of surface functionality the adsorption process is spontaneous and exothermic. Pseudo-second-order rate kinetic model is more appropriate to explain the adsorption kinetics.


Introduction
Alumina (Al2O3) nanocrystals constitutes important class of nanoscale ceramic materials, possess desirable surface properties such as high surface area, thermal, mechanical and chemical stability, Lewis acid property and porosity [1][2][3][4][5].These properties make them useful in hightemperature catalyst or catalytic support, tissue scaffolds, coating formulation, composite reinforcing materials, sorbent and membrane [6][7][8][9][10][11][12][13][14][15][16].Recently researchers are continuously thriving for new class of composite materials to widen the application potential as well as to overcome the limitations such as tendency to aggregation, poor functionality and poor compatibility with the aqueous environment generally observed for metal oxides.However, only few research articles are available on the preparation of inorganic-organic hybrid composite materials from Al2O3 nanoparticles.Khabibullin et al. grafted poly(methyl methacrylate) brushes on α-Al2O3 nanoparticles via surface initiated atom transfer radical polymerization [7].Popat et al. designed poly(ethylene glycol) (PEG) surface modified Al2O3 composite particles for targeted drug delivery system [8].The porous surface of Al2O3 was first hydroxylated and finally reacted with silane coupled PEG.Jackson et al. modified 10 µm sized Al2O3 particles with epoxy monolayer via self-assembly and curing of epoxy fluids [9].In another similar work ultrathin polypyrrole film was developed on Al2O3 particles using hexanoic acid as a template [10].In a recently published article Anaya et al. modified the surface of γ-Al2O3 particles with stearic, palmitic, erucic and oleic acids and finally prepared high performance biocompatible polysulfone/ γ-Al2O3 nanocomposite simply via self-assembly through cooling process [11].
In a recently published article Bristy et al. optimized the preparation conditions of epoxide polymer layered magnetic γ-Al2O3 nanocomposite particles named as γ-Al2O3/Fe3O4/SiO2/poly(glycidyl methacrylate (PGMA) [17].The preparation scheme of 4 nanocomposite particles is shown in Figure 1.γ-Al2O3 core particles were prepared by sol-gel technique and then doped with Fe3O4 nanoparticles.To improve the compatibility before next step seeded polymerization with GMA magnetic γ-Al2O3 particles were modified with mesoporous SiO2 layer.Size distribution, morphology, surface composition and magnetic property of nanocomposite particles were analyzed.In this investigation the surface property of γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles was evaluated by carrying out adsorption of Remazol navy (RN), a model reactive azo dye.RN is a widely used textile dye in the subcontinent.The leakage of dye containing wastewater into the environment is known to possess serious health hazards.The discharged dye molecules in water remains for long time because they are naturally non-degradable and most of them are strongly poisonous and proven to be carcinogenic [18][19][20][21][22][23][24].The removal of dye from water bodies is therefore indispensible.
Adsorption is an important technique for removing dye because it is easy to operate and also possible to reuse both dye and adsorbent.Here the adsorption behavior of RN on γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles was compared with that on γ-Al2O3/Fe3O4/SiO2 nanocomposite particles as reference material.The inclusion of epoxide functionality on γ-Al2O3 particles is expected to improve several properties like hydrophilicity, colloidal stability and reactivity with sensitive compounds like reactive dyes, diamines and fluorescent compounds [25][26][27].Magnetic nature would favor easy separation of adsorbent from the medium by applying magnetic field.It can be mentioned that only few number of adsorption studies are available with remazol group dyes (such as remazol brilliant blue, remazol red, remazol black b, remazol brilliant violet) on natural activated carbon and the adsorption process was found to be relatively slow, taken several hours for maximum adsorption [28][29][30].

Chemicals and Instruments
Monomer grade GMA from Fluka, Chemika, Switzerland was purified to remove inhibitors by passing through activated basic alumina column.was used for the measurement of magnetic property.

Preparation of Reference Fe3O4 Nanoparticles
Reference Fe3O4 nanoparticles were prepared by co-precipitation of Fe 2+ and Fe 3+ from their aqueous 25% NH4OH solutions (molar ratio 1:1.87) in a three necked round flask.The coprecipitation was carried out in a nitrogen atmosphere for 2 h at 85°C.The prepared Fe3O4 emulsion was treated with HNO3 (2M) for 15 min, washed with water to neutral pH and finally stabilized by slowly adding citric acid (40 g).Before characterization Fe3O4 nanoparticles were washed magnetically by repeated sedimentation to remove free citric acid.γ-Al2O3 particles were doped with Fe3O4 nanoparticles to produce γ-Al2O3/Fe3O4 nanocomposite particles.For this, in-situ co-precipitation of Fe 2+ (0.3753 g) and Fe 3+ (0.438 g) from their alkali solution (25% NH4OH) was carried out in presence of cationic CTAB (0.0125 g) stabilized γ-Al2O3 (0.5 g) particles.The yield of reference Fe3O4 nanoparticles was considered to fix the weight ratio of alumina/magnetite at 1/2.Before repeated washing the prepared black colored γ-Al2O3/Fe3O4 nanocomposite dispersion was treated with 2 M HNO3 (1.3 g) for 15 min.

Preparation of γ-
γ-Al2O3/Fe3O4 nanocomposite particles were stabilized before next step modification by adding 0.4 M citric acid and washed again repeatedly by magnetic separation and redispersion in distilled deionized water.
The surface modification of γ-Al2O3/Fe3O4 nanocomposite particles by mesoporous silica (SiO2) layer was carried out following a slightly changed process as available in literature [31,32].Deionized water (24 g), ethanol (4 g) and mesoporous template, CTAB (0.196 g) taken in a three necked round flask were mixed thoroughly at 60 °C.After the complete solubilization of CTAB, γ-Al2O3/Fe3O4 nanocomposite particles (0.5 g) were added to the mixture.Then pH of the mixture was adjusted at 9-11 using 25% NH3 solution (0.26 g), a favorable condition for the formation of SiO2 layer.Finally TEOS (0.5 g) was added dropwise and the reaction was continued for 2 h at 60 °C.The formation of γ-Al2O3/Fe3O4/SiO2 nanocomposite particles was confirmed as the black solution gradually turned fade.The nanocomposite dispersion was subjected to repeated washing (initially by ethanol and finally by deionized water) with five cycles of magnetic separation.

Characterization
For TEM observation a drop of diluted sample (0.01% solid) was placed on the carboncoated copper grid, dried at room temperature and then observed at an accelerating voltage of 100 kV.BET method was used to measure the specific surface areas (SBET) of the powdered nanocomposite particles at 77 K with NOVA3000e apparatus.Prior to the measurement the sample was dried in oven at 70 °C.The surface elemental composition of nanocomposite particles dried onto a carbon tape was evaluated by XPS.This was equipped with a monochromatic Al Ka radiation (1486.6eV)at 104 W and 20 kV and an X-ray current of 20 (micro)A.The pressure in the measurement chamber was ca.8.0 10 -7 Pa.The step size was 0.25eV for the both survey and high resolution spectra (pass energy 280eV).

Point of Zero Charge (PZC) of Nanocomposite Particles
The PZC of each of the γ-Al2O3/Fe3O4/SiO2 and γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles was determined by using salt addition method.20 mL of 0.1 M NaNO3 solution and 0.05 g of nanocomposite particles were mixed in a 100 mL beaker.The pH value was adjusted to 5, 6, 7, 8, and 9 respectively using either of the diluted NaOH or HNO3 solution.
The mixture was then magnetically stirred at 25 o C for 24 h.The change in pH value, ΔpH, (difference between initial and final pH) was plotted against initial pH value.The pH at which ΔpH is zero was taken as the PZC of nanocomposite particles.9 2.6.Adsorption of RN on Nanocomposite Particles 30 mL of RN (100 mg L -1 ) aqueous solution was mixed with 0.01 g of γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles and the pH value was immediately adjusted to the PZC (pH 7.45).The nanocomposite-dye mixture was magnetically stirred at 303 K for different time intervals to optimize the equilibrium adsorption time.After each specific time interval γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles were magnetically separated and finally centrifuged at 12000 rpm.Two-step separation was carried out to avoid the presence of dirt particles.The magnitude of dye adsorption was then estimated by measuring the absorbance of the supernatant using a UV-visible spectrophotometer at the λmax of 620 nm.Initial dye concentration and calibration curve were used for this purpose.
For comparative study adsorption on γ-Al2O3/Fe3O4/SiO2 nanocomposite particles was also carried out under the same conditions at pH 7.20 corresponding to the PZC.
Dye uptake at equilibrium, q (mg g -1 ), was determined by where Co and  (mg L -1 ) are the initial and equilibrium concentrations of the dye solutions,  (L) is volume of the solution, and  (g) is the mass of nanocomposite particles taken as adsorbent.
Dye adsorption efficiency (%) was calculated by using following expression: The effect of adsorbent dose was studied by mixing variable amounts of each γ-Al2O3/Fe3O4/SiO2/PGMA and γ-Al2O3/Fe3O4/SiO2 nanocomposite particles with 30 mL of 100 mg L -1 RN aqueous solution, pH was adjusted to the PZC and equilibrated for 5 min (optimized from the previous experiment) at 303 K to achieve the maximum adsorption.The equilibrium adsorption experiments were conducted as continuous experiment under identical conditions with variable initial RN concentrations (20, 40, and 45 mg g -1 ) and temperatures (283, 303, and 323 K) using fixed amount (0.01 g) of Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles.Whereas for γ-Al2O3/Fe3O4/SiO2 nanocomposite particles RN initial concentrations were varied between 40 to 60 mg g -1 .Langmuir, Freundlich and Temkin adsorption isotherms were used to describe the adsorption behavior.
Langmuir isotherm [33] a theoretical model valid for monolayer adsorption is expressed by the following nonlinear equation: where, qe (mg g -1 ) is the amount of dye adsorbed per unit mass of the nanocomposite particles at equilibrium, Ce (mg L -1 ) is the equilibrium concentration of dye left out in the supernatant, qmax is the theoretical monolayer adsorption capacity and b (L mg -1 ) is the Langmuir constant depicting the energy and affinity of adsorption.
The linear equation of Freundlich adsorption isotherm model [34] on heterogeneous surface can be expressed as: where Kf (mg g -1 ) is a constant relating to the adsorption capacity and n (g L -1 ) is an empirical parameter measuring the adsorption intensity.
The linear form of Temkin adsorption isotherm model [35] suitable for explaining the chemisorption adsorption mechanism is given below: where B (= RT/b) is Temkin constant representing the heat of adsorption, R is universal gas constant (8.314J/mol•K), T is the absolute temperature (K), AT (L mg -1 ) is the equilibrium 11 binding constant relating to maximum binding energy.The constants AT and B were determined by plotting qe vs ln Ce.
To understand the nature of interaction thermodynamic parameters such as changes in free energy, enthalpy and entropy (ΔG, ΔH and ΔS) were also explored using the following equations: Where, KC is the thermodynamic equilibrium constant.

Reuse of γ-Al2O3/Fe3O4/SiO2/PGMA Nanocomposite Particles
A desorption experiment was performed to investigate the reusability of the nanocomposite particles as adsorbent.The adsorption/recycle experiment on γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles was started with 0.035 g nanocomposite particles and 50 mg L -1 RN aqueous solution (30 mL).After each successive adsorption at 303 K the dye loaded adsorbent was treated with 30 mL 1 M NaOH solution for 24 h at 50 °C.Then nanocomposite particles were magnetically recovered, washed repeatedly (5 times) with distilled deionized water before studying the adsorption again.Similar adsorption/recycle measurement was also carried out with γ-Al2O3/Fe3O4/SiO2 nanocomposite particles using 30 mL of 100 mg L -1 dye solution.Relative to γ-Al2O3/Fe3O4/SiO2 nanocomposite particles the average thickness of hairy structure increased by around 4 nm after seeded polymerization.Flake shaped γ-Al2O3/Fe3O4 nanocomposite particles with light as well as dark background overlapped with hairy structure is also visible in either case.These results suggested that the surface of γ-Al2O3 particles has ultimately been modified according to the reaction protocol (Figure 1).The specific surface area (SBET) of γ-Al2O3/Fe3O4/SiO2 nanocomposite particles (149.63 m 2 g -1 ) is comparatively high 13 compared to γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles (53.16 m 2 g -1 ).This is possibly attributed to the reduction of surface porosity and increase in average size following seeded polymerization of GMA.quickly dispersed once the magnetic field is removed.It is also obvious that γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles can be easily recycled after recovery from aqueous solution (inset of Figure 4).From the economic viewpoint magnetic separable property of nanocomposite particles would be important in water treatment application.

Adsorption Study of RN
One way of measuring the surface activity of nanocomposite particles is to study the adsorption behavior.RN a kind of reactive azo dye is known to possess different types of reactive groups along with azo-group and is capable of forming covalent bond with textile fibers such as cotton [37].The use of azo dyes is posing serious threat as dye precursors or their biotransformation products are creating various toxicities like carcinogenic and mutagenic effects [38], teratogenicity in frog embryos [39], enzymic degradation metabolites toxicity [40], and phytotoxicity [41].The adsorption study was basically carried out on γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles.To see any improvement in adsorption following epoxide functionalization a comparative adsorption study on γ-Al2O3/Fe3O4/SiO2 nanocomposite particles was also carried out.
In order to avoid the ionic interaction, the adsorption of RN on nanocomposite particles was studied at the respective PZC which were found to be 7.20 and 7.45 for γ-Al2O3/Fe3O4/SiO2 and γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles respectively (Figure S1).PZC represents a state when the electric charge density of nanocomposite particles is zero.It is expected that ionization of sulfonic groups in RN at pH below the PZC would favor the adsorption on positively charged nanocomposite particles.
Figure 5 displays the effect of contact time on the adsorption of RN.It is observed that the adsorption of the dye is very rapid and reached equilibrium within 5 min on both γ-Al2O3/Fe3O4/SiO2 and γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles.Initially the rapid adsorption is associated with the availability of larger number of vacant active sites on adsorbent surface.It is reasonable to assume that after 5 min only few adsorption sites are available to accommodate additional dye molecules.In the following experiments the contact time was therefore adjusted to 5 min to attain maximum adsorption.The difference in adsorption magnitude of RN on two types of nanocomposite particles is typically attributed to the difference in surface properties and more importantly the total specific surface area.The total specific surface area of γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles (53.16 m 2 g -1 ) was found to be much less than that of γ-Al2O3/Fe3O4/SiO2 nanocomposite particles (149.63 m 2 g -1 ).So, adsorption capacity in mass per unit area (mg m -2 ) would be more acceptable for comparing the adsorption performance.From an economical point of view it is important to know the minimum amount of nanocomposite particles (adsorbent dose) required for maximum adsorption.Figure 6 suggests that the adsorption efficiency increases with the increase in amount of nanocomposite particles.
This behavior is ascribed to the increase in surface area with the increase in adsorbent dose.
Initially the adsorption efficiency of RN on γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles increases rapidly up to 0.03 g of nanocomposite particles and then slowed down to reach ~100% adsorption efficiency.Concurrently, with increasing adsorbent dose, the amount of adsorption decreases, thus causing a decrease in qe value (Figure S2). Figure 6 indicates that the optimum adsorbent amount is 0.06 g to achieve ~100% adsorption efficiency but 0.01 g adsorbent is the optimum amount for obtaining the maximum adsorption density.The reason may be that at lower adsorbent dose the dye molecules are more easily accessible.Therefore with increase in adsorbent dose there is less commensurate increase in adsorption leaving many adsorption sites unoccupied during adsorption [42,43].Some authors also accounted for the interaction of nanocomposite particles at higher solid content causing partial overlapping or aggregation resulting in a decrease in effective adsorbent surface area [44].Compared to this on γ-Al2O3/Fe3O4/SiO2 nanocomposite particles the adsorption efficiency of RN reached ~100% rapidly at relatively low adsorbent content (0.03 g).The larger specific surface area of γ-Al2O3/Fe3O4/SiO2 nanocomposite particles might have contributed to this adsorption behavior.
The optimum amount of γ-Al2O3/Fe3O4/SiO2 nanocomposite particles required for obtaining maximum adsorption density (Figure S2) remained as same (0.01 g) as epoxide functional nanocomposite particles.can be due to the increased solubility of dye molecules and dissociation of physical bonding (Van der Waals interaction) following increased entropy [45,46].Comparatively the adsorption amount on γ-Al2O3/Fe3O4/SiO2 nanocomposite particles (Figure 7B) dropped rapidly with increasing temperature.It is reasonable to assume that adsorption of RN on γ-Al2O3/Fe3O4/SiO2 nanocomposite particles is mainly controlled by Van der Waals interaction whereas the adsorption on γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles is controlled by both physical (Van der Waals interaction) and chemical bonding (hydrogen bonding and perhaps covalent linkage).displayed in Table 1.The data indicate that the Langmiur isotherm yielded the best fit, as supported by the highest correlation coefficient (R 2 ).This implies that homogeneous monolayer adsorption is preferably followed.The values of RL are between 0 and 1, confirming the adsorption process as favorable.The theoretical maximum adsorption capacity qmax is maximum (69.44 mg g -1 ) at 283K and decreases with increasing temperature.The value of Kf (Freundlich constant) also confirms that the adsorption of RN on nanocomposite particles is more favorable at lower temperature.Similarly the values of Temkin constant (B), which are related to the heat of adsorption of RN, decrease with increase in temperature and irrespective of temperature the value is lower than 8.0 Kj mol -1 .This indicates that the interaction between dye molecules and nanocomposite particle surface mostly followed physisorption [47].Relatively low AT values ascribe the low electrostatic interaction between RN dye and γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles at different temperatures [48].Physisorption is usually supported at lower temperature because at higher temperature the solubility and entropy of dye molecules are enhanced.The equilibrium adsorption isotherm models on γ-Al2O3/Fe3O4/SiO2 nanocomposite particles are also analyzed (Figures S3-S5).The obtained equilibrium data and related empirical constants presented in Table 1 indicate that Langmuir isotherm is the best fit model.Hence the same monolayer surface coverage is preferably followed.The theoretical maximum adsorption capacity (qmax) of γ-Al2O3/Fe3O4/SiO2 nanocomposite particles decreases from 129.87 to 121.95 mg g -1 with the increase of temperature from 283 K to 323 K. Therefore it can be said that adsorption of RN is favorable on both nanocomposite particles at lower temperature.The theoretical maximum adsorption capacity of γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles (1.30 mg m -2 ) is relatively high compared to that of γ-Al2O3/Fe3O4/SiO2 nanocomposite particles (0.87 mg m -2 ).This result suggests that reactive epoxide functionality on γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles acts as a driving force for the increase of adsorption capacity.The structure of RN is unknown due to commercial purpose but one can normally expect the presence of different types of functional groups such as S=O, -CONH2/-CONH-, aromatic phenol and even may be primary or secondary amine.So there is enough chance to form hydrogen and/covalent bonds among ester-epoxide groups on the particle surface and reactive groups of RN dye molecules.Whereas the formation of such hydrogen and/covalent linkages with γ-Al2O3/Fe3O4/SiO2 nanocomposite particles is scarcely possible.Thermodynamic equilibrium constant, Kc, calculated from intercept of the plots of ln (qe/Ce) against qe (Figure S6), decreases with increasing temperature irrespective of nanocomposite particles (Table 2).Thermodynamic parameters ΔH and ΔS were calculated from the slope and intercept of the linear plot of ln KC against 1/T (Figure S7) using Van't Hoff equation (eq.6) and subsequently ΔG values were obtained from eq. 7. Irrespective of temperature ΔG values are negative and lie in the range -20 < ΔG < 0 kJ/mol.This indicates that the adsorption process is physical and thermodynamically favorable [47].The more negative value of ΔG at lower temperature again supports that adsorption is preferable at lower temperature.The negative value of ΔH suggests the adsorption process as exothermic.
Comparatively the more negative value of ΔS for adsorption on γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles indicates that adsorbed dye molecules are less disordered at the interface [49,50].This perhaps indicates the formation of physical as well as chemical bonding (preferably hydrogen bonding) between RN and epoxide functionality on γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles.t/qt = 1/K2 q 2 e + (1/qe) t ---------------- (8) where K2 is the equilibrium rate constant of the P-S-O adsorption (g mg -1 min -1 ), qe is the maximum adsorption capacity (mg g -1 ) for the P-S-O adsorption, and qt is the adsorption capacity (mg g -1 ) at any adsorption time t (min).For γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles the obtained value of R 2 (0.9996) from the plot of t/qt against t (Figure 11) shows that this kinetic model is applicable to describe the adsorption kinetics and the experimental qmax (88.08 mg g -1 ) is consistent with the calculated qe (87.72 mg g -1 ) value.Similarly for γ-Al2O3/Fe3O4/SiO2 nanocomposite particles P-S-O kinetic model (Figure S8) is applicable as the value of R 2 is close to unity (0.9966) and the experimental qmax (141.33 mg g -1 ) is close to the theoretical value of qe (142.86 mg g -1 ).Regeneration and reuse of adsorbent materials are crucial from the view point of industrial application, process economy and preventing pollution from used adsorbent.The adsorptiondesorption-reuse cycles were carried out three times and the results are displayed in Figure 12.
Treatment of dye loaded γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles with 1M NaOH solution did not decrease the adsorption magnitude of RN in the third cycle as the recovered particles retained almost 99% adsorption efficiency.Hence it can be said that γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles can be used as adsorbent for the removal of dye from contaminated water coming out from dyeing industry.It is worthwhile to mention that γ-Al2O3/Fe3O4/SiO2 nanocomposite particles also had the same adsorption efficiency in the third cycle (data not shown).

Conclusion
Flake shaped and porous γ-Al2O3 particles prepared by sol-gel technique were doped with magnetic iron oxide nanoparticles.The magnetic nanocomposite particles were then modified with mesoporous SiO2 layer and finally with PGMA layer via seeded polymerization.The introduction of epoxide PGMA layer slightly reduced the magnetic property but still they were strongly paramagnetic and moved under the external magnetic field.The surface activity of γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles was studied by measuring the adsorption behavior of RN a reactive azo dye and the results were compared with those of γ-Al2O3/Fe3O4/SiO2 as reference materials.The adsorption rate was very fast and reached equilibrium in 5 min.The amount of adsorption was dependent on the initial concentration of RN and adsorption was favorable at lower temperature (283 K).Irrespective of the nature of nanocomposite particles the correlation coefficients (R 2 ) of Langmuir, Freundlich and Temkin confirmed that adsorption process at any temperature could be best explained by Langmuir isotherm.The introduction of epoxide functionality increased the maximum theoretical adsorption capacity per unit surface area from 0.87 mg m -2 in γ-Al2O3/Fe3O4/SiO2 nanocomposite particles to 1.30 mg m -2 in γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles.The increased adsorption capacity of γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles was speculated to be due to the formation of hydrogen bonding in addition to Van der Waals interaction with dye molecules.The possibility of the formation covalent linkage is also there provided RN dye molecules contained reactive amine groups as most of the reactive azo dyes.The negative value of ΔG and ΔH (-10.60 kJ/mol) suggested that the adsorption process was spontaneous and exothermic in nature.A comparison between experimental and theoretical adsorption capacities suggested that kinetic data can be described by the pseudo-second-order equation.
Al2O3/Fe3O4/SiO2/PGMA Nanocomposite Particles γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles were prepared following a multistep process.In the first step γ-Al2O3 particles were prepared from Al(NO3)3,9H2O (35 g) and urea (72 g) maintaining the molar ratio of Al 3+ /urea at 1/13.The aluminium-urea saturated solution was heated in a three necked round bottomed flask immersed in thermostat oil bath maintained at 90 °C for 12 h.With the progress of the reaction the pH gradually increased from ~ 2 to ~6.The Al2O3 sol produced was heated for another 3 h to a transparent gel.The Al2O3 gel was finally dried at 300 °C for 3 h in presence of air to produce amorphous γ-Al2O3 powder.
TEM images of γ-Al2O3 particles and corresponding nanocomposite particles are illustrated in Figure 2. The particles were all washed by repeated replacement of continuous phase with Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 11 July 2018 doi:10.20944/preprints201807.0205.v1fresh distilled water before characterization.γ-Al2O3 particles possessed some interesting hexagonal flake shaped morphology.The size is ranged between 100 to 400 nm.The formation of spherical, cubic as well as tetragonal shaped particles is also possible as the image shows only a portion of the sample.The difference in contrast between dark and light parts in the magnified inset image is attributed to the porous γ-Al2O3 particles.After magnetization the morphology of γ-Al2O3/Fe3O4 nanocomposite particles (Figure 2b) changed a bit.The deposition of Fe3O4 nanoparticles increased the contrast of γ-Al2O3 particles.Fe3O4 nanoparticles are arranged into needle like fashion and grown from the surface of γ-Al2O3 particles.As the γ-Al2O3/Fe3O4 nanocomposite particles were magnetically washed prior to the TEM observation some free needle like Fe3O4 nanoparticles might also be present.TEM image of γ-Al2O3/Fe3O4 nanocomposite particles supported the assumption made regarding the formation of rectangular shaped γ-Al2O3 particles along with hexagonal flake shaped particles.In order to improve the compatibility with organic PGMA the surface of γ-Al2O3/Fe3O4 nanocomposite particles was modified with mesoporous SiO2 layer following treatment with TEOS [36].The penetration of GMA monomer into the mesoporous SiO2 channel may also favor seeded polymerization, hindering secondary nucleation.Both γ-Al2O3/Fe3O4/SiO2 and γ-Al2O3/Fe3O4/SiO2/PGMA nanocomposite particles almost retained the hairy needle like morphology of γ-Al2O3/Fe3O4.

Figure 5 .
Figure 5. Contact time dependent change in adsorption amount of RN on γ-

Figure 7 .
Figure 7. Initial RN concentration and temperature dependent adsorption behavior on A) γ-

Figure 12 .
Figure 12.Relationship between reuse cycle and the percent adsorption efficiency of RN dye

Table 1 .
Empirical constants for the adsorption of RN on nanocomposite particles.
rate expression.Pseudo-first-order model was not used as the value of R 2 obtained from the plot of ln (qe -qt) versus t was less than 0.50.The pseudo-second-order (P-S-O) model was therefore used to investigate the kinetics of the adsorption of RN dye on nanocomposite particles.The equation of the P-S-O kinetic model is as follows: