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Comparative study of modified/non-modified aluminum and silica aerogels for anionic dye adsorption performance

Developing effective and low-cost adsorbents is of great significance for controlling water contamination. To eliminate anionic water contaminants, four modified/non-modified aluminum (AlHMD, AlOSSD) and silica (SiHMD, SiOSSD) aerogels have been successfully employed. The four as-prepared aerogels were applied as adsorbents for removal of an anionic dye (acid orange 7, AO) from aqueous solution. Compared to silica aerogels, aluminum aerogels showed efficient adsorption performance for anionic water contaminants. The AO maximum adsorption capacity of Al2O3 aerogel is twice as high as that of SiO2 aerogel. The maximum adsorption capacity of aerogels was in the following order: AlHMD > AlOSSD > SiHMD > SiOSSD. Adsorption kinetics and isotherms of AO dye on the four as-prepared aerogels have also been studied. The kinetic data fitted well with the pseudo first-order kinetics model and the adsorption isotherm could be described by the Langmuir model. Adsorption rate of AO dye was mainly governed by film diffusion and intra-particle diffusion. Based on the adsorption mechanism, this work provides an idea for the design of superior adsorbents for anionic water contaminants.


Introduction
Water contamination is a very signicant problem for human society due to tens of thousands of dyes being ubiquitous in sewage waters from multiple industries such as textiles, rubbers, cosmetics, food, printing, paper, carpets, leather, etc. Various methods for treatment have been developed, such as oxidation-ozonation, 1 photocatalysis, 2 coagulation-occulation, 3 membrane separation, 4 biological methods 5 and adsorption. [6][7][8] Among these methods, the physical adsorption method is generally considered attractive owing to its effectiveness, ease of operation, convenience, simplicity of design, economic and environmental considerations, with the selection of adsorbent being the key to dye decolorization. 9 Aerogels as a new type of nanometric materials, with low density, high porosity, and large specic surface area, are becoming the ideal choice of adsorbent. 10,11 To date, silica aerogels have been used for removal of dyes from aqueous solution as can be found in previous literature. Silica aerogels as sorbent materials were used to remove cationic dye rhodamine B from aqueous solutions by Li et al. 12 Hydrophobic/hydrophilic silica aerogel was successfully prepared to remove cationic dyes methylene blue and rhodamine B from aqueous solutions by Han et al. 13 Hydrophilic/hydrophobic transparent silica aerogel was synthesized to remove cationic dye rhodamine B from aqueous solutions by Zong et al. 14 Moreover, some other materials used for adsorption have also been reported. A nitrilotriacetic acid anhydride-modied lignocellulosic material was prepared to remove cadmium and lead from aqueous solutions by Huang et al. 15 The biosorption potential of dried activated sludge as a biosorbent for zinc(II) removal from aqueous solution was investigated by Yang et al. 16 A dried biomass waste from biotrickling lters has been prepared for removing lead [Pb(II)] ions from aqueous solutions by Cheng et al. 17 Summarizing the above literature, it was found that the composites as adsorbents were mostly studied for cationic dyes while the adsorption of anionic dyes has been treated much less. Water pollutants include not only cationic pollutants, but also anionic pollutants, so it is of great signicance to study the adsorption of anionic pollutants.
The surface properties including surface areas and surface functional groups of aerogels can affect their adsorption capacity, and a larger specic surface area is benecial to enhance adsorption capacity of adsorbents. 18,19 In this work, we combined the advantages of aluminum and silicon aerogels with a large surface area, and successfully prepared hexamethyldisilazane (HMDS)-modied Al 2 O 3 aerogel (Al HMD ) and SiO 2 aerogel (Si HMD ) by an organic solvent exchange method. Adsorption capacities of the aerogels with different properties for the anionic dye acid orange 7 (AO) have been compared. The adsorption kinetic and isothermal parameters were evaluated. Experimental results suggested that the modied Al 2 O 3 aerogel was more efficient, and Al 2 O 3 aerogels were more suitable for the removal anionic dye AO from aqueous solutions than SiO 2 aerogels.

Materials
Aluminum isopropoxide (AIP) was chemically pure and obtained from Sinopharm Co. Ethanol (EtOH), HMDS (C 6 H 19 NSi 2 ), n-hexane (C 6 H 14 ), ethyl silicate (TEOS), acetonitrile (C 2 H 3 N), nitric acid (HNO 3 ), ammonium hydroxide (NH 3 $H 2 O) and AO (C 16 H 11 N 2 NaO 4 S$5H 2 O) were of analytical grade and obtained from Sinopharm Co. All chemicals were used without any further purication. The molecular structure of the AO is shown in Fig. 1. Deionized water was used throughout this work.

Aerogel synthesis
Synthesis of Al 2 O 3 aerogel. Al 2 O 3 wet gel was formed by the method of reux in a water bath. AIP (2.5 g) was added to a round-bottom ask, aer which the solution was heated to 80 C with vigorous stirring. The volume ratio of deionized water to ethanol was 1 : 3. The solution was heated to reux for 3 hours. Then the reaction mixture was heated at 80 C for another 1 hour without stirring. A certain concentration of HNO 3 was added to the solution, aer which the solution was moved to a beaker and cooled to room temperature. The sol was stirred for 1 hour and aged for several days. The Al 2 O 3 wet gel was thus prepared. The drying method adopted was one of two ways. The rst way was hydrophobic modication drying (HMDS). Briey, the gel was aged in EtOH and soaked in C 6 H 14 , C 6 H 19 NSi 2 /C 6 H 14 (at a volume ratio of 1 : 2) and C 6 H 14 in turn. Then the aerogel was formed aer drying at 60 C for 12 hours in an oven. The obtained Al 2 O 3 aerogel was named Al HMD . The other way was organic solvent sublimation drying (OSSD). The wet gel was aged in EtOH and soaked in 50%, 80%, 100% exchanging solvent C 2 H 3 N/EtOH (v/v) for every 12 h, and then dried at 50 C under 80-100 kPa in a vacuum drying oven. The obtained Al 2 O 3 aerogel was designated Al OSSD . The chemical principle for the preparation of the Al 2 O 3 aerogels is depicted in Fig. 2.
Synthesis of SiO 2 aerogel. SiO 2 wet gel was obtained by the method of acid and base two-step catalysis. 13 The sol was prepared with the following reagent volume proportion: TEOS5.5 : EtOH5.75 : HNO 3 1.8 (0.008 M). Then the sol was formed aer stirring for 12 hours. An amount of 0.15 mL (0.5 M) of NH 3 $H 2 O was added to the sol and stirred for several minutes, affording the SiO 2 wet gel aer standing. The wet gel was dried using the two drying methods mentioned above. The obtained SiO 2 aerogels were denoted Si HMD and Si OSSD , respectively.

Characterization
The small angle X-ray diffraction (SAXD) patterns of samples were obtained with a D/Max-gA X-ray diffractometer (Rigaku, Tokyo, Japan) at 40 kV and 100 mA with monochromatic Cu (divergence slit (DS) ¼ 0.1 , scattering slit (SS) ¼ 0.1 , receiving slit (RS) ¼ 0.15 mm, scan 0.6-10 , rate of scanning 0.5 min À1 ). The microstructures of the aerogels were observed with  scanning electron microscopy (SEM, Philips XL-30SEManalyzer) and transmission electron microscopy (TEM, Philips Tecnnai-12, operating at 120 kV). Contact angles were measured by using a contact angle system (Harke-SPCA, Harke, China). The Brunauer-Emmett-Teller (BET) specic surface area was obtained from N 2 adsorption/desorption analyses at 77 K (NOVA2000; Quantachrome, Boynton Beach, FL). Pore size distribution and pore volume were calculated by the Barrett-Joyner-Halenda method. Mutual effects between the modier and the wet gel were analyzed by Fourier transform infrared (FT-IR) spectra, which were obtained with the KBr wafer technique. The spectra were recorded from 500 to 4000 cm À1 . The concentration of dyes was determined using a UV-visible spectrophotometer (UV-2450; Shimadzu, Kyoto, Japan).

Adsorption experiment of AO
An accurately weighed quantity of the dye AO was dissolved in deionized water to prepare stock solution (500 mg L À1 ). To prevent decolorization by direct sunlight, the stock solutions were stored in a dark bottle and kept in a dark place before use. Experimental solutions of the desired concentration were obtained by successive dilutions the stock solution in denite proportions.
All adsorption experiments were conducted by batch experiments mixing 0.02 g aerogel powder with 20 mL AO aqueous solution of known concentration in a series of 50 mL conical asks. The mixture in a ask with the required concentration and appropriate pH was shaken at a speed of 180 rpm in a constant temperature oscillator at room temperature. 20 The dye solution pH was adjusted to different values between 1 and 13 with hydrochloric acid (0.1 M) or sodium hydroxide solution (0.1 M). Aer a xed time interval, the suspension was centrifuged. The residual concentrations of the solutions were measured by a UV-visible spectrophotometer.
The equilibrium adsorption capacity Q e (mg g À1 ) and percentage removal of aerogel powder for AO aqueous solution were calculated through the following equations: where C 0 (mg L À1 ) and C e (mg L À1 ) are the initial and equilibrium adsorbate concentrations of AO in the solution, respectively; V (L) and m (g) are the volume of solution and the mass of adsorbent, respectively. The data of equilibrium obtained were analyzed using Langmuir and Freundlich isotherm models, which can be described as: where Q m (mg g À1 ) and b (L mg À1 ) are the maximum adsorption capacity and Langmuir constant related to the energy of adsorption, respectively. K f (mg g À1 ) is the Freundlich constant related to the adsorption capacity and n is an empirical constant.
The linearized pseudo-rst-order Lagergren equation is expressed as: where k 1 (min À1 ) and t (min) are the adsorption equilibrium rate constant of pseudo-rst-order kinetics and contact time, respectively.

Results and discussion
Physicochemical properties of the as-prepared aerogels The SAXD patterns of the as-prepared aerogels are shown in Fig. 3(a). It can be seen that the samples of Al HMD , Al OSSD and Si HMD showed diffraction peaks in the position of 1 to 2 , and all the diffraction peak positions were at approximately 1 . This indicates that the three aerogels possess similar mesoporous structure and aperture. The diffraction peak of Si OSSD is indistinct in comparison with the others. This indicates that the regularity of mesopores in Si OSSD was poorer than in the others. 21 Thus it can be seen that HMDS modication made the internal structure change in the silica aerogel. For further research on the inuence of the drying method on aerogel structure, the FT-IR spectra were studied. Fig. 3(b) shows the FT-IR spectra of the as-prepared aerogels. As can be seen from the gure, aer using the HMDS modication drying, the hydroxyl peak of Al HMD and Si HMD became weaker than that of Al OSSD and Si OSSD at 3480 cm À1 . This is because the -OH group on the aerogel surface was replaced by the organic group aer modication drying. The medium width adsorption band near 1634 cm À1 in the spectrum of Al OSSD can be attributed to the bending vibration of H 2 O. 22 The absorption peaks of Al HMD and Al OSSD near 1063 cm À1 can be ascribed to Al-O-Al vibrations, and absorption bands near 753 cm À1 can be attributed to bending of the O-Al-O bonds. The peaks at 2987 cm À1 , 1360 cm À1 and 926 cm À1 correspond to the asymmetric, symmetric and in-plane vibrations of -CH 3 group. This indicates whether the modication of Al 2 O 3 aerogel has no inuence on the existence of methyl. As seen from Fig. 3(b), the Si HMD shows asymmetric methyl vibration and swing vibration at 2958 cm À1 and 901 cm À1 , respectively, compared with Si OSSD . It can be inferred that the silica aerogel contains methyl on the surface aer modication drying. 23 The absorption peaks of Si HMD and Si OSSD near 1061 cm À1 can be ascribed to Si-O-Si vibrations, and absorption bands near 789 cm À1 can be attributed to bending of the O-Si-O bonds. Fig. 4 presents SEM micrographs of the four as-prepared aerogels. Fig. 4(a) displays the SEM image of Al HMD , the structure of which presents porous agglomerates composed of Al 2 O 3 particles. 24 Compared with the compact stacked bulk morphology of Al OSSD , Al HMD has a more uffy and porous sheet structure. As can be seen from the SEM image (Fig. 4(c)) of Si HMD , it exhibits 20-50 nm colloidal particles constituting a three-dimensional branching network structure, 25 but Si OSSD tends to be agglomerated into bulk in Fig. 4(d). In short, Al OSSD and Si OSSD aerogels dried through OSSD consist of agglomeration of several micrometers or hundreds of micrometers. The agglomeration dimension of Al OSSD is smaller than that of Si OSSD . On the basis of this, it is speculated that the size of agglomeration is the reason why the adsorption capacity of Al 2 O 3 aerogel better than that of SiO 2 aerogel. 26 The TEM images of the as-prepared aerogels are shown in Fig. 5. As shown in Fig. 5, the compositions of the aluminumbased aerogels are dense and pleated, and the compositions of the silica-based aerogels are more loose and porous. In order to determine the hydrophobic conditions of aerogels, the contact angle of sample and water was measured, as shown in the insets of Fig. 5. The contact angle of Al 2 O 3 aerogel and water is much less than 90 , presenting ease of wetting by water and good hydrophilic energy (insets in Fig. 5(a and b)). The inset of Fig. 5(c) shows the water droplet on the surface of the HMDS-modied SiO 2 aerogel is basically spherical, the water contact angle being more than 90 and not easily wetted by water, exhibiting good hydrophobic properties. This shows that the modied aerogel surface organic groups have excellent hydrophobic properties. 27 The water contact angle of SiO 2 aerogel dried by OSSD is much less than 90 , showing good hydrophilic properties (inset in Fig. 5(d)). The water contact angle of aerogels dried by OSSD approaches 0 , indicating super-hydrophilic properties (insets in Fig. 5(b and d)).
The aerogel powder textural properties were further analyzed by BET adsorption-desorption measurements. The N 2  adsorption-desorption isotherms and pore size distribution curves of the as-prepared aerogels obtained at 77 K are shown in Fig. 6. The adsorption and desorption isotherms obtained for all samples are of type IV which is the performance characteristic of mesoporous materials according to the classication by IUPAC, as shown in Fig. 6(a and b). [28][29][30] The hysteresis loop in the isotherms is caused by the capillary condensation of N 2 gas occurring in the mesopores. The shape of the pores determines the hysteresis loop shape. 31 The Al HMD sample shows H1 type of hysteresis loop, while the other samples present the shape of the hysteresis loop between type H 2 and H 3.

32,33
In addition, the four as-prepared aerogel pore size distributions are depicted in Fig. 6(c-f). The pore size distributions are unimodal with the peak of Al HMD , Al OSSD , Si HMD and Si OSSD pore radius at 20, 10, 3 and 5 nm, respectively. The monomodal pore size distribution indicates the mesoporous nature of the  synthesized aerogels. The BET surface area, pore volume, and average pore radius of the four as-prepared aerogel samples are summarized in Table 1. For alumina aerogels, the aerogel modied/non-modied by HMDS is not signicantly different in terms of surface area. But the pore radius of Al 2 O 3 aerogel aer HMDS modication (17.865 nm) is about three times that prepared by OSSD (5.622 nm). This shows that the modication has little effect on the specic surface area of the aluminum aerogel but it does affect the pore size. On the contrary, the surface area of silica aerogel is signicantly increased aer the surface modication of HMDS (671.762 m 2 g À1 ) compared to the OSSD method (151.240 m 2 g À1 ) and the pore size distribution is almost the same. This indicates that the modication has a great inuence on the specic surface area of the silica aerogel, but hardly affects the size of the pores.
The as-prepared aerogels are porous materials which are composed of colloidal agglomerations of different dimensions. The FT-IR analysis and water contact angle illustrate that modication reduces hydroxyl groups on the surface of aerogels and results in the hydrophobicity of Si HMD . For the four asprepared aerogels, the aluminum-based aerogels have moderate specic surface area and larger pore radius aer modication. For the silica-based aerogels, the non-modied aerogel has a lower specic surface area while the modied aerogel has a larger specic surface area; but both of the aerogels have almost the same pore size. The data for specic surface area and porosity can benet the contact between the adsorption sites of aerogels and AO molecules. To sum up, the as-prepared aerogels can be used as adsorbents because they are good porous materials and easily modied by functional groups.

Factors affecting dye adsorption
In order to investigate the adsorption properties of the asprepared aerogels for anionic dyes, the solution pH, contact time, and initial dye concentration will be taken into consideration. The inuence of these various parameters on the removal of AO dye is studied as follows.
Effect of solution pH and zeta potential. The pH of the dye solution plays an important role in AO dye treatment process, particularly for the adsorption capacity. 34 The efficiency of adsorption depends on the solution pH because variation in pH brings about a variation in the surface properties of the adsorbent and the degree of ionization of the adsorbate molecules present in the solution. 35,36 Gupta et al. 37 studied the effect of solution pH on the adsorption of AO by waste materials (bottom ash and de-oiled soya) and they discovered that the maximum adsorption was at pH 2. Fig. 7 shows the adsorption capacity of all of the prepared aerogels for AO at pH varying in the range of 1-13. The maximum removal rates of AO for the four prepared aerogels are at pH ¼ 2, pH ¼ 4, pH ¼ 3 and pH ¼ 3, respectively. From Fig. 7 and considering the optimization of subsequent experimental conditions, all subsequent experiments were implemented at pH 3. As shown in Fig. 7, when the pH is less than 7, the all aerogels adsorb AO and the removal rate increases rst then decreases with the increase of pH. When the initial pH is more than 7, there is almost no adsorption capacity for anionic dye AO of the aerogels. This is because of the presence of excessive OH À , which competes with the dye anions for the adsorption sites at alkaline pH. 38 In Fig. 7(a), the maximum AO removal rate of Al HMD is 42.3%, which is about 5% higher than the maximum adsorption rate of Al OSSD of 37.6%. The AO removal rates of Si HMD and Si OSSD only show a slight change in Fig. 7(b). The reason why the removal efficiency of silica aerogels before and aer the modication is not signicantly improved may be that the pore radius of the silica aerogel before and aer the modication does not change signicantly. The alumina aerogels have a larger pore radius than the silica  aerogels, and the maximum removal rate of Al 2 O 3 aerogel for AO is twice that of the SiO 2 aerogel at acidic pH. Therefore, this further conrmed that the pore radius of the aerogel is one of the important parameters affecting the AO removal rate. The surface properties of adsorbents are also important factors in the dye adsorption process. Zeta potential is the potential difference between the dispersion medium and the stationary layer of uid attached to a dispersed particle. 39 Thus, zeta potential is used to determine the potential of aerogel surface to conrm the results of adsorption. The surfaces of the four as-prepared aerogels (Al HMD , Al OSSD , Si HMD , and Si OSSD ) have charges of +0.83 mV, À0.34 mV, À4.34 mV, and À4.43 mV, respectively. This indicates that the surfaces of the aerogels are negatively charged, except Al HMD . The positively charged surfaces can enhance the adsorption of the negatively charged AO anions by the adsorbents through electrostatic forces of attraction. 40 One is able to infer the adsorption capacity of the four as-prepared aerogels is in the order Al HMD > Al OSSD > Si HMD > Si OSSD according to the size of the zeta potential. Meanwhile, it has been reported that for a certain range of electrolyte concentrations, for colloidal particles of the same size and surface charge density, the surface of the particles is different in hydrophilicity and the electrophoretic mobility is different. [41][42][43] Since the surface charge is surrounded by the hydrophilic layer, and the hydrophilic surface increases the bonding of the ions and increases the surface conductance and causes the shear plane to expand, the hydrophilic surface of aluminum aerogels causes a decrease in the electrophoretic mobility and absolute values of the zeta potential. Moreover, Leonard et al. 44 have reported that using materials that have different surface potentials causes an increased attraction of ions during the removal process. Suchithra et al. 45 have reported that the surface charge affects the adsorption process, negative surface charge attracting ions carrying net positive surface charge. So, it can be concluded that the surface charge of the materials is also an important factor of adsorption.
Effect of contact time. The contact time between adsorbent and adsorbate is also an important physical parameter in industrial wastewater treatment. 46 From Fig. 8 it is clear that rapid adsorption of AO dye took place within 30 min for the four prepared aerogels, and then adsorption became slow and almost reached equilibrium within 120 min. At the initial contact time the adsorption sites are exposed and these make the rate of adsorption to be fast, and, consequently, the diffusion process from the bulk solution to the adsorption sites. 47 It is also seen from Fig. 8 that the removal of dye by adsorption on aerogels is very fast at the initial period of contact but slowed down with time. This is because each adsorbent has limited adsorption sites and, aer a certain time, these are exhausted and the adsorption process attained an equilibrium state. 48 More importantly, the Al 2 O 3 aerogel exhibited twice the adsorption rate compared to the SiO 2 aerogel, which can be ascribed to the surface potential and active groups. As can be seen from Fig. 8, the adsorption capacity of the four as-prepared aerogels is in the order Al HMD > Al OSSD > Si HMD > Si OSSD .
Effect of initial AO dye concentration. The initial dye concentration has a pronounced effect on its removal from aqueous solutions. 49 The adsorption capacity for AO dye of the four as-prepared aerogels increases with the initial dye concentration as shown in Fig. 9. It is found that when the initial dye concentration increases from 10 to 200 mg L À1 , the adsorption capacity of AO onto Al 2 O 3 aerogel is 3.7 to 52 mg g À1 and the adsorption capacity of AO onto SiO 2 aerogel is 2.36 to 20.2 mg g À1 . Basically, from Fig. 9, the adsorption percentage decreases and the adsorption capacity increases with increasing initial dye concentration. It is likely that the resistance to mass transfer of dye between the aqueous solution and the solid phase is overcome by the driving force provided by the initial dye concentration. Perhaps the higher initial concentration solutions have stronger driving forces in adsorption and fewer available adsorption sites of constant adsorbent compared to the lower initial concentration solutions, so AO removal depends upon the initial concentration. 50 Besides, with the increase in initial concentration, the interaction between dye and adsorbent is enhanced. Therefore, the increase of the adsorption of dye will follow the increase in initial dye concentration.

Adsorption kinetics
The study of adsorption kinetics is a prerequisite for the quantitative description of the course of a reaction with time, giving valuable insights into the chemical reaction and mechanism of the adsorption process. The time versus log(Q e À Q m ) plots (Lagergren plots) were found to be linear for the four asprepared aerogels, as shown in Fig. 10. The calculated kinetic parameters from these plots such as rate constants and correlation coefficients R 2 for the different aerogels are given in Table  2. It is clear that high correlation coefficients (R 2 > 0.99) were observed for all ts, the AO adsorption process being of a pseudo-rst-order nature in each case. The slope values evaluated from these straight line plots for Al HMD , Al OSSD , Si HMD , and Si OSSD are found to be À5.613 Â 10 À2 , À5.203 Â 10 À2 , À2.405 Â 10 À2 , and À2.800 Â 10 À2 , respectively. Results show that the overall AO adsorption process rate should be controlled by the chemical process in conformity with the pseudo-rst- order kinetics mechanism. Adsorption kinetics reveals the rate of solute uptake and evidently this rate controls the residence time of the AO dye at the solution interface.
Mass transfer of an adsorbate to an adsorbent particle is governed by a number of relationships obtained mainly from diffusion and mass balance equations. There may occur three consecutive steps in the process of organic matter adsorbed to porous adsorbents from aqueous solution, and these steps are as follows: (i) lm diffusion, where the adsorbate is instantaneously transferred to the adsorbent exterior through available    Fig. 11 Kinetic plots of intra-particle diffusion model for the adsorption of AO dye on the four as-prepared aerogels.
active sites on the surface lm; (ii) particle diffusion, where the adsorbate molecules migrate from the surface to the interior of the adsorbent particle; and (iii) equilibrium, where the adsorption of the adsorbate occurs on the inner surface of the adsorbent. 36 The intra-particle diffusion model was used to identify the diffusion mechanism during the AO adsorption process. 51 It is described using the following equation: where k i (mg g À1 min À1/2 ) is the intra-particle diffusion constant and C is a constant which describes the boundary layer effects. Fig. 11 depicts that the intra-particle diffusion model for AO adsorption tted well (R 2 > 0.98) the data if the whole adsorption process was divided into two linear phases involving a rst stage and a second stage. The two phases in the intra-particle diffusion plot represent surface adsorption to the adsorbent and the intraparticle diffusion of AO to adsorption sites. 52 The intra-particle diffusion rate constant k i (mg g À1 min À1/2 ) is evaluated from the slope of the second linear portion of Q m versus t 1/2 , the values being presented in Table 3. Initially, within the rst stage, it is postulated that AO molecules from bulk solution are transported to the external aerogel surface through lm diffusion and its rate is very high. A larger slope can be observed from the rst sharp section suggesting that the rate of dye removal is higher rstly attributed to the active adsorption sites. Aer saturation of the surface, the AO molecules enter into the aerogel by intra-particle diffusion until equilibrium is reached. Hence, the AO adsorption process might be described by lm diffusion followed by intraparticle diffusion process. 53

Adsorption isotherms
The adsorption isotherms show the adsorbed molecules are distributed at the interface between the solid phase and the liquid phase when the adsorption process reaches an equilibrium state. To analyze the adsorption equilibrium data by tting them to different isotherm models is of fundamental importance to nd the suitable model that can be applicable for design purposes. 54 There are several isotherms such as the Langmuir and Freundlich models, which are most widely  Paper used to describe the adsorption isotherms. The Freundlich and Langmuir adsorption isotherm models were each successfully applied to the adsorption capacity variation vs. concentration for the four as-prepared aerogels, shown in Fig. 12, and the parameters and correlation coefficients calculated for the Langmuir and Freundlich linear models are given in Table 4. Normally the best-tting model can be chosen based on the linear regression correlation coefficient (R 2 ) values. Table 4 shows that the least-squares correlation coefficient (R 2 ) values of the Langmuir model are higher than those of the Freundlich model for all samples, which demonstrate that the isotherms of all of samples are well tted by the Langmuir equation.
The kinetic data are well tted with the Lagergren rst-order kinetics model according to the adsorption kinetics and the isotherm adsorption data are well tted with the Langmuir model. The results of kinetic experiments and Langmuir isotherm conrm that, for all adsorbents, the adsorption proceeds via lm diffusion and intra-particle diffusion.
In consideration that dyes have high molecular weight and complex structures, the as-prepared aerogels are used for adsorption of small molecule ions to compare the results with the adsorption of dyes.

Adsorption mechanism
There are no chemical bonds produced between the aerogel and AO dye, and thus the adsorption process is physical adsorption. The adsorption capacity is compared with the theoretical model, and the adsorption capacity of the aluminum aerogel is superior to that of the silica aerogel, with the modied aerogel being better than the non-modied aerogel. This indicates that the modication could promote adsorption. The data of specic surface area and porosity can benet the contact between the adsorption sites of aerogels and AO molecules. The aluminum aerogel has stronger adsorption capacity than the silica aerogel due to the positively charged surfaces of the aluminum aerogel being able to provide adsorption sites for electrostatic interaction with negatively charged AO dye molecules. The reason why Si HMD has the largest specic surface area but lower adsorption capacity is the electrostatic repulsion between the negatively charged active sites on the adsorbent and the anionic AO dye molecules.

Conclusion
In summary, a comparative study of modied/non-modied aluminum and silica aerogels used to remove the dye AO is presented. HMDS-modied Al HMD aerogel and Si HMD aerogel show smaller colloidal agglomerates than Al OSSD and Si OSSD . The surface area of Al HMD aerogel has a slight change (from 364.601 to 330.096 m 2 g À1 ) aer being modied by HMDS; the surface area of Si HMD aerogel increased from 151.240 to 671.762 m 2 g À1 aer being modied by HMDS. In this work, it is found that the surface charge property of the adsorbent is the key factor. The adsorption capacity of aerogels is in the order: Al HMD > Al OSSD > Si HMD > Si OSSD . The kinetic data are well tted with the Lagergren rst-order kinetics model according to the adsorption kinetics and the isotherm adsorption data are well tted with the Langmuir model. The results of kinetic experiments and Langmuir isotherm conrm that, for all adsorbents, the adsorption proceeds via lm diffusion and intra-particle diffusion. In short, the factors affecting dye adsorption are not only specic surface area and porosity, but also surface functional groups, electrostatic effect and surface charge. It is benecial for designing a high-efficiency adsorbent to remove anionic aqueous contaminants.

Conflicts of interest
There are no conicts to declare.