Mechanistic Study of Adsorption of Acid Orange-7 over AluminumOxide Nanoparticles

e adsorption behavior of acid orange-7 (AO-7) on aluminum oxide nanoparticles (ANP) generated by sol-gel method has been investigated to understand the physicochemical process involved and to explore the potential use of nano particles in textile effluent treatment and management. e results revealed that ANP can remove AO-7 dye up to 97.6mg/g at 303K. e adsorption process is found to be pH dependent and the optimum pH obtained is 2.0. e equilibrium was established in 1 h. Langmuir, Freundlich, and Temkin Isotherm models were applied on the system. Scanning electron microscopic analysis reveals eye-catching nanoporous morphology of the material. e results of FTIR spectroscopy reveal that the process is electrostatic complexation mechanism driven. XRD studies revealed nanocrystalline structure of ANP. BET surface area measurement suggests high pore volume and surface area of adsorbent. e kinetic measurements suggest pseudo-second-order kinetic processes. e thermodynamic measurements suggest that all processes are endothermic accompanied with negative ΔG and positive ΔS, ΔH.


Introduction
It is well recognized that the presence of dyes in aquatic environment can result in several problems.Most dyes are soluble in water.Textile waste water is one of the major environmental concerns.Many industries use dyes and pigments to color their �nal products.Consequently the waste water effluents are highly colored and the disposal of this waste in water bodies causes damage to the environment.ese dyes affect photosynthetic activities of aquatic �ora due to reduced penetration of sun light.Most of the dyes are xenobiotic and nonbiodegradable in nature.e conventional waste water treatment technologies depend upon removal of biological oxygen demand but reduction of BOD is ineffective against color removal.e combination of technologies like adsorption and dye degradation can provide the solution to the problem [1], although these processes are still in laboratory stage of development.Chemical oxidation of dyes is very successful for azo dyes as it can initiate the cleavage of azo bond.e problem of secondary pollution due to formation of oxidized amines and chlorine (in case of NaOCl) is suspected [2].In recent times UV/O 3 has been used as treatment technology.is is applicable in gaseous state and almost all types of dyes can be removed successfully without formation of any sludge.e process is pH dependent and associated with high cost and also suffers from limitation of UV-light penetration [3,4].Sonoelectrolysis is also evaluated for the azo dye removal.e electrooxidation of dye (50 ppm) in saline solution involving in situ generation of hypochlorite ion was enhanced using ultrasound (20 kHz, 22 w) when carried out in a semisealed cell which reduced the ultrasonic degassing [5].Adsorption is a conventional technology for dye removal with very high efficiency and simple process.Granulated activated carbon is used for the removal of azo dye, acid orange-7 from aqueous solution [6].e carbon is regenerated using microwave radiations.e efficiency of carbon was very high due to surface modi�cation.Activated carbon is very efficient adsorbent and very efficient for cationic and anionic dyes.e adsorbent use of carbon in wastewater treatment is impractical due to competitive adsorption of other organic molecules along with dye molecules; the carbon as adsorbent can be thus used in end of treatment steps exclusively for color removal.e nonconventional adsorbents are in application due to their easy availability [7].Agricultural and industrial waste products with and without modi�cation are efficient in removing color [8][9][10][11][12].e sludge disposal problem is a setback hence recyclable and more efficient adsorbents are required [13].Adsorption technology is nondestructive technology involving phase change from aqueous phase to solid surface immobilization.Some techniques involving adsorption with catalytic degradation of dye seem to be more lucrative.Chen et al. [14] removed aromatic compounds on single-walled carbon nanotubes.e application of nanoparticles as adsorbents has come up as an interesting area of research because of their small particle size and high surface area.e active sites are also more and capable of interacting with pollutant species [15,16].e application of ANP has been evaluated for the removal of hexavalent chromium from aqueous medium [17].A previous study indicates that adsorption capacity remains unchanged aer regeneration of nanosized adsorbent [18].ANPs are prepared by using solgel method.e material of high purity can be prepared by this method [19].In the present study ANPs prepared by solgel method are used for adsorptive removal of azo dye AO-7 from aqueous medium.[20].is dye shows  max at 542 nm.1000 ppm (1000 mg L −1 ) solutions were prepared by dissolving appropriate amount of dye in water and stored in dark colored bottle and diluted by adding suitable amount of distilled water to the stock solution as per requirement.

Preparation of Nanoparticles.
Nano-Al 2 O 3 particles (ANP) were synthesized by sol-gel method [17].For this purpose saturated solution of aluminium sulphate was precipitated with 6 N ammonia solution till a gel is obtained.e gel is then calcined in a muffle furnace at 1073 K for 1 h.e powder ANP was stored in a dessicator till further use.

Adsorption
Experiments.e series of experiments were conducted by placing 50 mL of 100 ppm dye solution in an Erlenmeyer �ask and adding the required amount of adsorbent to that in an incubator shaker.e pH of different solutions was adjusted with 0.1 N HCl and 0.1 N NaOH.Aer attainment of equilibrium the aqueous phase was analyzed for residual dye concentration using UV visible spectrophotometer.From the absorbance data   (mg g −1 ) was determined using where  0 is initial dye concentration,   is �nal dye concentration, and  is volume of dye in liters and  is mass of adsorbent in g.Duplicate experiments were performed to get concordant results.e results showed variation in the range of ±5%.e kinetics of dye removal was studied using required dye concentration.e samples were withdrawn at regular intervals and residual concentration was analyzed aer centrifugation using ultracentrifugation at 1000 rpm.e isotherms were studied by using 50 mL of dye solution within optimum range of concentration at 303 K, 313 K, and 323 K. Aer the attainment of equilibrium the residual dye concentration was analyzed spectrophotometrically. e dye concentration before and aer adsorption was determined by using Shimadzu (2101 PC) spectrophotometer.A standard plot is drawn for known concentrations and the concentration of dyes was determined by converting the optical density to corresponding concentration.e dyes were analyzed at their respective  max .e pH zpc (pH zero point charge) was determined by the method reported earlier [3].e pH of dye solutions and pH zpc were determined by using pH meter by Toshvin (TMP-85).An autoarranging conductivity meter TCM+15, provided with temperature compensator made by Toshnival, India, was used for determination of surfactant characteristics and nature of the �ltrate of adsorbent and its derivative washings.e weighing was carried out on a digital weighing balance of accuracy up to 0.1 mg by citizen Co. BET surface area measurement is carried out by using micrometrics surface area analyzer.is also gave monolayer volume of N 2 and pore volume of the adsorbents.e FTIR of adsorbents and surface derivatives was carried out with Perkin Elmer spectrophotometer in the range of 400-4000 cm −1 using perkin elmer spectrophotometer.e adsorbent is mixed with anhydrous KBr to make a pellet and 400 scans are carried out to give the average FTIR scan.e adsorbent samples were also characterized by powder X-ray diffractometry using an X'PERT PRO PANalytical with Cu-K  radiation.SEM is carried out by using ZEOL scanning electron micrograph.TEM provided topographical, morphological, compositional, and crystalline information of nanoadsorbents.e images allow us to view samples on a molecular level, making it possible to analyze structure and texture.e data analysis was carried out by using correlation 0.253 Total surface basicity (mmol g −1 ) 6.879 analysis employing least squares method and sum of error square was calculated by SPSS-17 statistical soware.

Results and Discussion
3.1.Characterization of the Adsorbent.e characteristics of ANP are given in Table 1.e SEM and TEM of ANP are shown in Figure 2 indicating morphology of ANPs.e X-ray diffraction analysis of the ANP was carried out and shown in Figure 3. e d spacing was measured between the rows of atoms in crystal structure of ANP.e small peak formation indicated the nanocrystalline structure of the ANP.FTIR spectra (Figure 4) revealed that O-H vibrations 3459 cm −1 due to the water in the lattice [21] and O-H bending vibrations appear at 1644 cm −1 .Weak bands are observed at 1386 cm −1 due to Al-O bond vibrations.e bands at 1071 cm −1 and 846 cm −1 appear due Al-O bonds [22].BET surface area analysis shows that surface area of ANP is 82.91 m 2 g −1 and the pore volume is 0.389 cm 3 g −1 ; the density of ANP was determined and found to be 0.976 g cm −3 .

Effect of Adsorbent Dose.
To investigate the effect of adsorbent dose on adsorption of dye on ANP, the experiments were conducted with adsorbent dose between 0.1 g-12.5 g in 100 mL at 303 K and it was found that with an increase in the dose, the adsorption increases.is may be credited to the reason that at lower adsorbent dosage the number of dye molecules is comparatively higher as compared to availability of adsorption sites.e results are shown in Figure 5.It was observed that when adsorbent dose was doubled the percentage dye removal increased by a the range of 2-9 while initial concentration (100 mg L −1 ), adsorbent dose (0.1 g/100 mL), and temperature (303 K) were kept constant.e effect of initial pH on the dye removal is shown in Figure 6.e adsorption capacity decreases when pH increases.e maximum adsorption of AO-7 occurs at pH 2. is may be ascribed to the reason that in the aqueous medium the functional groups such as hydroxyl groups develop on the surface which are protonated in acidic medium causing more electrostatic interactions between protonated ANP and anionic dye.e pH zpc measurement suggests that pH zpc for ANP is 8.0; thus beyond pH zpc negative charge develops on the surface so anionic dyes are best adsorbed below their pH zpc .As the pH of the system increases the number of positively charged sites decreases; hence the adsorption of anionic dye also decreases.A positively charged site on the adsorbent favors the adsorption of anionic dyes due to electrostatic attractions.

Effect of Contact Time. e effect of contact time
was investigated in the batch mode at dye concentration 100 mg L −1 .e results have been shown in Figure 6 which suggests that the adsorption capacity of dyes increases with increasing contact time.e rate of dye removal is high initially due to high-concentration gradient and more availability of adsorption sites.e rapid transport of dye molecules from aqueous solution to the bulk makes the adsorption fast.e surface of ANP is charged and the net charge at the surface at a particular pH governs the adsorptive removal of dye from the solution.
3.5.Effect of Temperature.e effect of temperature on adsorption of AO-7 on was studied by carrying the temperature-controlled equilibrium experiments.e equilibrium adsorption capacity increases as a function of temperature as revealed by adsorption isotherms.e results advocate that the adsorption capacity of ANP increases with the increase in temperature suggesting that all the adsorption processes are endothermic in nature.e increase in adsorption at higher temperature can be attributed to the fact that at higher temperature kinetic energy of dye and water molecules increases in aqueous medium and the dye binds preferably to the active sites having higher surface area.e positive effect of temperature also indicates chemisorption [23] as a mode of binding of dye with ANP.Similar results have been found for the removal of basic dyes by other workers [23] suggesting that in this work the behavior of ANP towards dyes remains the same and the binding energy of adsorbate and adsorbent varies as the slope of different curves is different.
3.6.Adsorption Isotherms.e Freundlich, Langmuir, and Temkin isotherm models have been successfully applied to all of the above systems at various temperatures 303 K, 313 K, and 323 K (Figure 7) and thermodynamic parameters are calculated accordingly.For the equilibrium concentration of adsorbate (  ) and amount of dye adsorbed at equilibrium (  ), the following linear forms of Langmuir (2), Freundlich (3), and Temkin (4) isotherms were studied.
Langmuir adsorption model is a well-known twoparameter model of adsorption.It has produced a good agreement with the experimental data suggesting the monolayer adsorption [24] of AO-7 on ANP.
In the Langmuir adsorption the primary binding forces are physical and it is assumed that adsorption capacity of all binding sites is equal and binding on one site does not affect another. 0 and  are Langmuir constants in the following equation: 0 (mg g −1 ) and  (L mg −1 ) indicate maximum dye uptake and Langmuir equilibrium constant, respectively.ese isotherms characterize the equilibrium parameters of homogenous surfaces, monolayer adsorption, and allocation of adsorption sites.is isotherm �tted well on data (Table 2) suggesting that the adsorption of AO-7 on the active site does not affect the neighboring site.e values of  0 decrease with the increase of temperature due to increase in kinetic energy of dye molecules in aqueous medium.e essential characteristics of Langmuir isotherm can be calculated by using a dimensionless adsorption constant   which can be de�ned as follows: where  0 is initial dye concentration.In this experiment the value of   was found in the range of 0.00086-0.0075which lies between 0 and 1 suggesting favorable adsorption.e value of   is very small suggesting that the size of adsorbent sites is uniform.
Freundlich isotherm is the earliest known two-parameter isotherm model based on the fact that exponentially decaying adsorption site energy distribution can be applied to nonideal sorption.  and  are Freundlich constants: e Freundlich parameters calculated for the adsorption of AO-7 on ANP suggest that values of  range between 0.93 and 2.02, since the values of 1/ for the process lie in the range 0 to 1.07.is indicates favorable adsorption [25].e value of   indicates adsorption capacity in L g −1 and   increases with the increase of temperature suggesting stronger binding forces operating at higher temperature between ANP and AO-7.On the basis of statistical analysis it is clear that Freundlich isotherm �ts superiory on the process.Temkin isotherm assumes that the fall in heat of adsorption of AO-7 on ANP would decrease linearly for the ANP and AO-7 interactions.e isotherm can be represented by the following linear form: where   is Temkin constant related to the heat of sorption J mol −1 and   is Temkin isotherm constant (L g −1 ).e value of   lies very close to 1.000 at all temperatures and   decreases with the increase of temperature suggesting endothermic nature of the adsorption.Langmuir and Freundlich isotherms �tted the data better than Temkin as this is clear from the statistical analysis of the data.e changes in the reaction on ANP expected during the process require the brief idea of the thermodynamic parameters which were also calculated from the above data using ( 6)-( 8): where  is Langmuir equilibrium constant.e values Δ ∘ and Δ ∘ are determined from intercept and slope of (7).e values of thermodynamic parameters are given in Table 3. e Gibbs free energy, Δ ∘ , was found to be negative at all temperatures, indicating spontaneous process at all the temperatures while enthalpy, Δ ∘ , was positive suggesting endothermic and irreversible nature of the process.e positive value of entropy, Δ ∘ , suggests favorable randomness factor though its value is small.is suggests that in spite of small particle size of adsorbent the reaction is energetically favorable.e thermodynamic parameters were calculated and are given in Table 3.

Kinetic Studies
e rate of removal of AO-7 has been studied as a function of time on ANP as shown in Figure 6.e equilibrium was attained in 1 h.Adsorption rate constant study was carried out with the famous Lagergren rate equation: e pseudo-�rst-order equation (Figure 8) was evaluated.e  1 was found to be 1.0319 × 10 −2 min −1 .e suitable agreement was found between Pseudo �rst order data in statistical analysis, the correlation coefficient is high and standard error is low.e second-order equation was also tested on the whole range of time.e plot is shown in Figure 9 indicating the pseudo second order equation e pseudo second order kinetic model �tted better than Lagergren model.e correlation constant was higher than �rst-order model.e rate constant  2 was found to be 9424 × 10 −4 mg⋅g −1 min −1 .Elovich kinetic model (10) was also applied on the data to evaluate the possibility of chemisorption: e plot between   versus ln  was a straight line.e regression coefficient was found to be 0.9855; a very small standard error was obtained.Several kinetic parameters are calculated for different models and represented in Table 4.

Desorption Studies
e desorption studies were also carried out.AO-7 was desorbed by using 1% w/v NaOH; it was found that the ANP can be used as adsorbent for more than 11 cycles without where   is concentration of dye aer desorption and   is concentration of dye aer sorption.e results of sorption and desorption cycles are represented in Figure 10.e amount of dye desorbed was higher than the desorption results obtained for other adsorbents reported in the literature [26][27][28][29].

Conclusion
is study shows that AO-7 can be successfully removed from the aqueous solution by adsorption on ANP and this can be an effective and valuable mean for controlling water pollution due to dyes.e following conclusions can be drawn from this study.e batch adsorption experiments show that the adsorption of the AO-7 over ANP and its derivatives is dependent on pH, amount of adsorbent, concentration, contact time, and temperature, and 100% dye removal could be accomplished.e thermodynamic parameters obtained in both cases con�rm the feasibility of the process at each concentration.e Freundlich, Langmuir, and Temkin isotherms �tted the data well.e results of kinetic experiments show that the adsorption proceeds via pseudo second order kinetics over all adsorbents.e desorption studied indicates that ANP can be used as speedy and efficient secondary treatment option.

F 4 :F 5 :F 6 :
FTIR of ANP.factor of 1.5.Almost optimum adsorption was observed at 0.1 g per 100 mL.3.3.Effect of pH.e dye adsorption is affected by solutionpH and in the present study the effect of pH is studied in Effect of adsorbent dose on percentage dye removal, 100 ppm dye concentration, 303 K, pH = 5 of AO-7.Effect of initial pH on percentage dye removal, 100 ppm dye concentration, and 303 K.

F 7 :
Adsorption isotherm plot of   versus   of AO-7 on ANP at 303 K, 313 K and 323 K.