Aminofunctionalized silica monolith for Pb2+ removal: synthesis and adsorption experiments

105 (2018) 287–297 February Desalination and Water Treatment www.deswater.com doi:10.5004/dwt.2018.22132 Aminofunctionalized silica monolith for Pb2+ removal: synthesis and adsorption experiments Hakimeh Sharififarda,c, Paolo Apreab,*, Domenico Caputob, Francesco Pepec,* a Department of Chemical Engineering, Amirkabir University of Technology, No.424, Hafez Ave., Tehran, Iran, email: hakimeh.sharifi@gmail.com b ACLabs – Applied Chemistry Laboratories, Department of Chemical, Materials and Industrial Production Engineering, Università Federico II, P.le Tecchio 80, 80125, Napoli, Italy, email: paolo.aprea@unina.it (P. Aprea), domenico.caputo@unina.it (D. Caputo) c Department of Engineering, Università del Sannio, Piazza Roma 21, 82100 Benevento, Italy, email: francesco.pepe@unisannio.it Received 15 February 2017; Accepted 31 January 2018 ABST R AC T Lead(II) removal from aqueous solutions by adsorption on APTMS-functionalized silica monolith (APTMS-Monosil) was investigated. Functionalized silica monolith was selected as adsorbent due to its ease of synthesis and versatility. Adsorption experiments were performed in a batch system, and the effects of various operating parameters, such as solution pH, initial concentration and solid to liquid ratio were evaluated. According to the Response Surface Methodology results, the optimum operating conditions for Pb2+ removal by APTMS-Monosil were pH = 6.24, initial Pb2+ concentration of 89.5 mg L–1 and solid to liquid ratio of 1.84 g·L–1. The kinetic data suggested that chemical adsorption, rather than mass transfer, was the controlling step for lead capture. Equilibrium isotherms were analyzed using different models, and data were well fitted to the Langmuir isotherm. The thermodynamic parameters of the adsorption process (∆H° = 30.9 kJ·mol–1, ∆S° = 0.187 kJ·mol–1 K–1, and ∆G° = –24.826 kJ·mol–1 at T = 298 K), calculated from three isotherms at T = 30– 60°C, revealed a spontaneous, endothermic process, with a strong chemical nature. The maximum adsorption capacity of APTMS-Monosil for lead was 450 mg·g–1, which is a high value if compared with other materials presented in the literature. Keywords: Silica monolith; Amine functionalization; Lead adsorption; Response surface method 1. Introduction Heavy metal contamination of water resources is one of the greatest environmental concerns because of the toxic effects that these substances have for human beings and other animals and plants in the environment, and because of their tendency to bioaccumulate. One of the most ubiquitous heavy metals is lead, that is released to environment through a number of industrials activities such as refineries, printing, production of pigments etc. [1–5]. The presence of lead in water, even at very low concentrations, is extremely harmful to the aquatic environment and human health. Lead can damage, among others, the nervous system, kidney, and reproductive system [6]. The World Health Organization (WHO) and US Environmental Protection Agency (US EPA) have set a maximum guideline concentration of 0.01 and 0.015 mg·L–1 for Pb in drinking water, respectively [7]. Several technologies, such as chemical precipitation, electrocoagulation, ion exchange, membrane processes and adsorption have been tested for heavy metal removal from industrial wastewater [8–17]. Among them, adsorption seems to be the most suitable method in case of low concentration of contaminant, due to its relatively low cost and high efficiency [18,19]. With the increase in global awareness for environmental pollution, there is a growing *Corresponding author. 1944-3994 / 1944-3986 © 2018 Desalination Publications. All rights reserved. 288 H. Sharififard et al. / Desalination and Water Treatment 105 (2018) 287–297 demand for novel adsorbents characterized by high performances and efficiency for removal of heavy metals from aqueous systems, and quite recently several adsorbents have been tested, such as metal oxides/hydroxides [20,21], zero valent iron composites [22,23], and surfactant modified materials [24,25]. Among the materials of interest for adsorption applications are ordered mesoporous silicas (OMS). OMS constitute a large class of mainly amorphous materials, whose greatest advantages are their relatively large, yet uniform pore size, large surface area, and easily controllable surface chemistry [26,27]. However, since OMS have no strong affinity towards heavy metals, it is necessary to modify them by insertion of appropriate functional groups, such as amino groups, in order to increase their adsorption performances [13,28–31]. OMS are typically synthesized as powders, and their textural properties are subject to degradation during aggregation/shaping processes. An interesting alternative to powder functionalization followed by aggregation is the use of substrates directly obtained as aggregate materials, such as macro/mesoporous silica monoliths that can be subjected to a post synthesis functionalization process [32]. Silica monoliths, which can be produced by different synthesis routes [32–34], feature high chemical and mechanical stability, and their surface can be readily functionalized via grafting of organic and inorganic moieties [35]. According to published literature, there are few research works regarding the application of silica monoliths for heavy metal removal. For example, Sugrue and coworkers [36] modified a commercially available chromatographic silica column with iminodiacetic acid, reporting some encouraging results about its ion-exchange properties. Awual and coworkers [37,38], on the contrary, performed a direct synthesis of silica monoliths followed by a direct immobilization of functional acids, in order to perform an efficient removal of copper and cobalt from aqueous solutions. The goal of the present investigation is the evaluation of a silica monolith, named Monosil [35], functionalized with 3-aminopropyl-trimethoxysilane for the removal of Pb2+ ions from aqueous solutions. The effect of several operating parameters, such as initial pH, lead concentration and solid to liquid ratio on removal efficiency is investigated in batch conditions using Central Composite Method (CCD) with response surface methodology (RSM), and kinetic and equilibrium data are thoroughly analyzed in order to better clarify the lead adsorption mechanisms. 2. Materials and methods 2.1. Synthesis and characterization of APTMS-Monosil Monosil silica monoliths were synthesized following the approach adopted in [35]. 23.15 g of deionized water and 2.3 g of HNO3 (68%) were mixed for 15 min at 0°C. Afterwards, 2.4 g of polyethylene oxide (PEO, 20 kDa) was added, and the mixture was stirred for 1 h. After that, 18.9 g of tetra-ethyl-ortho-silicate (TEOS) was added, and the new mixture was further stirred for 1 h. The solution was then poured into plastic tubes of 10 cm length and 8 mm internal diameter, and was kept at 40°C for 24 h to allow complete solidification. The monoliths, of cylindrical shape, were extracted from the tubes, washed in water, and then treated in an ammonia solution (0.1 M) at 40°C for 24 h. Finally, they were dried at 40°C overnight and calcined at 550°C for 8 h. In order to perform adsorption experiments, the monoliths were crushed and sieved to select 2–3 mm particles. The selected particles were functionalized with 3-aminopropyl-trimethoxysilane (APTMS) using a wet impregnation method. Initially, they were dried at 100°C for 24 h; then, grafting of aminic groups was carried out by refluxing 0.1 g of Monosil in a 50 mL dry toluene solution containing 0.25 mL of 97% pure APTMS at 110°C for 24 h. The APTMS functionalized particles were then collected and washed with toluene, and then dried at room temperature for 24 h. All reagents for the synthesis procedure were purchased from Sigma Aldrich Co., while deionized water (s≤0.055 mS/cm) was prepared in house. The surface area (SBET) and pore size distribution of both raw Monosil and APTMS–Monosil were determined from adsorption–desorption isotherms of N2 at 77 K. A Quantachrome NOVA 1000 surface area analyzer was used to determine the isotherms. Furthermore, pHZPC (point of zero charge) of APTMS–Monosil was measured using the following technique: 0.3 g of adsorbent was added to 50 mL of a 0.1 N NaCl solution, the pH of which was adjusted using aqueous HCl or NaOH solutions. The pHZPC of the adsorbent was assumed to coincide with the initial solution pH if this remained constant after addition of the adsorbent and thorough mixing [15]. Fourier Transform Infrared (FT-IR) spectra of the samples were recorded between 4000 and 400 cm−1 using the KBr method, with a resolution of 4 cm−1 with a Nicolet FT-IR spectrophotometer (NEXUS 670). 2.2. Effect of adsorption parameters The effects of three operating parameters, namely solution pH, solid to liquid ratio and initial concentration, were investigated. Batch Pb2+ adsorption experiments were performed by adding a measured weight of APTMS-Monosil in 50 mL of Pb2+ solutions with an assigned initial Pb2+ concentration and pH. Reagent grade Pb(NO3)2 from Sigma-Aldrich Co. and deionized water were used to prepare the solutions. The initial pH of the solutions was adjusted with HCl and NaOH solutions. The mixtures, kept in 100 mL Erlenmeyer flasks, were shaken at 30°C using an orbital shaker (FINEPCR-SH30) at 150 rpm for 3 h. At the end of the experiment, the adsorbent was separated from liquid phase by filtration on filter paper (Whatman No. 40). The residual Pb2+ concentration in solution was determined using Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES, Perkin Elmer Optima 2100 DV), and the comparison between initial and final Pb2+ concentrations allowed to evaluate the removal percentage. With the purpose of assessing the role of the three operating parameters mentioned above, CCD was used. Since the number of independent parameters is three, 20 experiments consisting of 8 factorial points, 6 axial points and 6 replicates at the center points were carried out [39–43]. The range of variables investigated is given in Table 1. 289 H. Sharififard et al. / Desalination and Water Treatment 105 (2018) 287–297 Table 2 Porosity parameters of Monosil and APTMS-Monosil Table 1 Experimental ranges and levels of independent variables Independent parameter X1 X2 X3 pH Solid to liquid ratio Initial Pb2+ concentration Actual form of coded levels Parameter –1 0 +1 2 0.5 10 4.35 1.75 55 6.7 3 100 BET Sp. Surf. area (m g ) Mesopore volume (mL g–1), BJH method Micropore volume (mL g–1), HK method Average pore radius (Å) 2 –1 Monosil APTMS-Monosil 543 0.844 163 0.327 0.032 0.042 32.3 43.3 2.3. Kinetic studies The kinetics of adsorption were studied by placing 0.05 g of adsorbent in flasks containing 50 mL of 100 mg·L–1 Pb2+ solution at pH 6.0. The flasks were agitated in the orbital shaker at 150 rpm for a time ranging between 2–360 min at 30°C. After that, the suspension was filtered, and the solution concentration was analyzed. The metal uptake was calculated as: q= V (C0 − C f ) m (1) where q is the amount of solute adsorbed per unit weight of adsorbent (mg·g–1), V and m are solution volume (L) and dry weight of adsorbent (g), respectively, and Coand Cf are the initial and final metal ion concentration (mg·L–1), respectively. 2.4. Equilibrium studies The adsorption isotherms for Pb2+ on APTMS-Monosil were determined by placing 0.025 g of adsorbent in a series of flasks containing 50 mL of metal ion solutions at different initial concentrations (10–220 mg L–1). Initial pH was adjusted at 6.0 in all experiments. The flasks were agitated in an orbital shaker at 150 rpm for 24 h at various temperatures (30°, 45°C and 60°C). After this period, the suspension was filtered and the residual Pb2+ concentration was determined. The amount of lead adsorbed, q, was calculated using Eq. (1). Fig. 1. BJH differential pore size distribution of: (a): Monosil, (b): APTMS-Monosil. 3. Results and discussion was determined by HK (Horvath−Kawazoe) and BJH (Barrett, Joyner, and Halenda) methods, respectively [45,46]. BJH differential pore size distribution (Fig. 1) shows that both Monosil and APTMS-Monosil have a mesoporous structure. It can be seen that functionalization of Monosil with APTMS causes a reduction in surface area and pore volume: this can be interpreted by assuming that the molecules of APTMS diffuse into the mesoporous structure of Monosil, limiting the accessibility of N2 molecules to smaller pores. As a result, the average pore radius of the APTMS-Monosil is higher compared to that of Monosil. The pHZPC of APTMS-Monosil is 4.9. The FT–IR spectra of Monosil and APTMS-Monosil are shown in Fig. 2. In both spectra, the two peaks located at around 3450 cm−1 and 1630 cm−1 can be attributed to the stretching and bending modes of adsorbed water molecules, while the peak at around 950 cm−1 is due to asymmetric vibrations of (Si–OH), thus revealing the presence of hydroxyl groups. The asymmetric, stretching and bending vibrations of (Si–O–Si) are responsible for the peaks at 1090 cm−1, 800 cm–1 and 460 cm–1, respectively [47]. After modification of Monosil with APTMS, new peaks are detected between 1610 cm–1 and 1715 cm–1. These new peaks represent the vibration of different amine groups such as NH2 [26]. 3.1. Characterization of the adsorbents 3.2. Effect of separation parameters 2.5. Regeneration and reuse of APTMS-Monosil The regeneration tests of APTMS-Monosil were carried out as follows: 0.05 g of adsorbent, previously saturated with Pb2+ ions by contacting it with a 100 mg L–1 Pb2+ solution at pH 6.0 for 48 h, was treated with 5 ml of 0.1 M EDTA solution under vigorous stirring at 50°C for 1 h, followed by washing with deionized water for several times and drying at 80°C for 2 h. The regenerated adsorbent was then used for the next adsorption test. Five consecutive adsorption– regeneration cycles were performed to check the reusability of the APTMS-Monosil for capturing Pb2+ [44]. The porosity parameters of Monosil and APTMS-Monosil are presented in Table 2. The surface area was calculated by applying the 3-point BET (Brunauer−Emmett−Teller) method. The volume (mL·g–1) of micropore and mesopore 3.2.1. Regression model equation development The complete design matrix and the response values obtained from the experimental works are reported in 290 H. Sharififard et al. / Desalination and Water Treatment 105 (2018) 287–297 Table 3, together with a comparison with the prediction of Eq. (2). Eq. (2) represents the empirical quadratic model that was used to fit the experimental data, with the coefficients of this model obtained by multiple regression analysis technique using Design-Expert® 7.0 software: Removalpercentage = −95.56 + 53.59X1 + 52.32X 2 − 0.45X 3 −5.69X1X 2 + 7.44 ⋅ 10 −3 X1X 3 + 7.77 ⋅ 10 −3 X 2 X 3 (2) −3.74 X12 − 4.26 X 22 + 3.47 ⋅ 10 −3 X 32 As indicated in Table 1, in this equation X1, X2 and X3 are solution pH, solid to liquid ratio (g L–1) and initial concentration of Pb2+ (mg L–1), respectively. The analysis of variance (ANOVA) was applied in order to critically evaluate the role played by the different variables considered. The ANOVA results are shown in Table 4. According to the rule, the best regression model is deter- mined by highest Fisher’s F values and lowest p-values [43,48]. Data in Table 4 indicate that the quadratic model was significant at the 90% confidence level. As it can be seen from inspection of Table 4, the F value for the model is 77.1, and correspondingly the p–value is lower than 0.0001 and SS is quite high (SS = 1.81·104). This implies that the model is significant, and that it can appropriately explain the relationship between response and variables individuated as independent. This conclusion is also confirmed by the fact that both R2 and R2adj approached unity. Furthermore, p-values that are lower than 0.05 allow to individuate the model terms which are significant, while values greater than 0.1 allow to individuate model terms are not significant, and can be omitted [49]. According to the results reported in the ANOVA table, the linear, quadratic and combined effects of pH (X1) and solid to liquid ratio (X2) are the most significant variables, while Pb2+concentration plays a less relevant role, at least in the concentration range explored here. The fact that Pb2+ concentration is practically irrelevant probably depends on the fact that the adsorbent is far from saturation. Eliminating the less significant terms from Eq. (2) and refining the model, Eq. (2) may be simplified to: Removalpercentage = −95.56 + 53.59X1 + 52.32X 2 − 5.69X1X 2 −3.74 X12 − 4.26 X 22 Fig. 2. FT–IR spectra of Monosil and APTMS-Monosil. (3) A further confirmation of the validity of the analysis now presented comes from the diagnostic plot of Fig. 3, in which the values predicted by Eq. (3) are reported vs. experimental data, with a very satisfactory comparison. The different roles played by the three independent variables under consideration are graphically described in Table 3 Experimental conditions and results for lead removal Run pH Solid to liquid ratio (g L–1) Initial concentration (mg L–1) Actual removal (%) Predicted removal (Eq. (2), %) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1.49 4.35 4.35 2.00 7.21 6.70 4.35 4.35 6.70 6.70 4.35 2.00 4.35 2.00 4.35 6.70 2.00 4.35 4.35 4.35 1.75 1.75 1.75 3.00 1.75 0.50 3.27 1.75 0.50 3.00 1.75 0.50 1.75 0.50 0.23 3.00 3.00 1.75 1.75 1.75 55.00 55.00 55.00 10.00 55.00 10.00 55.00 55.00 100.00 10.00 55.00 10.00 0.28 100.00 55.00 100.00 100.00 55.00 109.72 55.00 30.1 89.9 90.4 76.1 83 100 96 92 100 100 92.6 11 100 6.1 58.5 100 74.7 91.6 95.2 93.6 27.42 90.26 90.26 77.23 92.57 98.30 97.31 90.26 96.72 96.44 90.26 12.10 101.88 7.65 60.53 96.88 74.66 90.26 99.43 90.26 291 H. Sharififard et al. / Desalination and Water Treatment 105 (2018) 287–297 Table 4 ANOVA table for lead removal using APTMS-Monosil Source Sum of squares (SS) df Mean square F–Value p–value Model X1 X2 X3 X1 X 2 X1 X 3 X2 X3 X12 X 22 18145.067 8206.843 3459.752 0.114 2947.968 7.430 6.319 1109.030 9 1 1 1 1 1 1 1 2016.118 8206.843 3459.752 0.114 2947.968 7.430 6.319 1109.031 77.143 314.021 132.381 0.004 112.799 0.284 0.242 42.435 < 0.0001 < 0.0001 < 0.0001 0.9486 <0.0001 0.6055 0.6335 <0.0001 517.404 5.989 261.346 0.00 1 1 10 517.404 5.989 26.134 19.797 0.229 0.0012 0.6424 X32 Residual Pure Error R2 R 2adj Fig. 3. Predicted versus experimental values plot for Pb2+ removal using APTMS-Monosil (predicted values: Eq. (3)). the perturbation plot of Fig. 4. As expected from the analysis of the ANOVA table, Fig. 4 indicates that the effect of pH (X1) on removal efficiency is quite strong, followed by the effect of solid to liquid ratio (X2), with a weak dependence on Pb2+ initial concentration (X3). 3.2.2. Response surface plots Three-dimensional response surface plots were generated to further investigate the effects of the three process parameters considered, i.e. solution pH, solid to liquid ratio and initial concentration on Pb2+ removal. 3.2.3. Effect of pH Figs. 5a and 5b show the effect of solution pH on the removal efficiency. According to these figures, the removal 0.986 0.973 Fig. 4. Perturbation plot for Pb2+ removal at central point of design parameters. X1: pH; X2: solid to liquid ratio; X3: Pb2+ initial concentration. See Table 1 for the actual values of each parameter. efficiency of Pb2+ increases with pH ranging from 2 to 6.5 at any fixed initial concentration and solid to liquid ratio, even though this effect is smaller when solid to liquid ratio increases. Presumably, the importance of solution pH in the removal process derives from its effect on the surface functional groups and the speciation of lead in solution. Indeed, the fact that pHZPC for APTMS-Monosil is 4.9 means that the surface charge of the adsorbent is positive when pH < 4.9 and negative when pH > 4.9. Since the dominant forms of lead at initial pH < 6–6.5 is Pb2+ cations [50,51], at pH < pHZPC the surface of APTMS-Monosil is positively charged, and mainly an electrostatic repulsion occurs between Pb2+ cations and positive adsorption sites of APTMS-Monosil, while at pH > pHZPC, the adsorbent surface is negatively charged and it can more easily adsorb Pb2+. 292 H. Sharififard et al. / Desalination and Water Treatment 105 (2018) 287–297 3.2.4. Effect of initial concentration The effect of initial concentration is shown in Figs. 5b and 5c. The results indicate that this variable does not have a significant effect on removal efficiency regardless of pH and solid to liquid ratio, confirming the indications deriving from ANOVA table (p value = 0.9486). 3.2.5. Effect of solid to liquid ratio According to Figs. 5a and 5c, the removal efficiency increases with increasing solid to liquid ratio. This tendency was expected because, as the ratio increases, so does the number of adsorbent active sites, and thus more Pb2+ ions can be removed. Based on the optimum conditions, complete Pb2+ removal was predicted by the model under operating conditions of pH = 6.24, solid to liquid ratio 1.84 g L–1 and initial concentration of 89.5 mg L–1 for APTMS-Monosil. This result was validated experimentally (99.5% Pb2+ removal). 3.3. Adsorption kinetics The adsorption kinetics were investigated in order to analyze the mechanism of adsorption and the potential rate controlling phenomena, such as mass transfer and chemical reaction. The adsorption kinetics of Pb2+ onto APTMS-Monosil were investigated by three common models, namely pseudo-first-order model, pseudo-second-order model and intraparticle diffusion model. The pseudo-first-order model assumes that the limiting step of the adsorption process is the interaction between adsorbate molecules (here Pb2+ ions) and adsorbent active sites, and that this interaction can be kinetically described by a first order kinetic equation, with no role played by liquid–solid and intraparticle diffusion. If this model is appropriate, then the adsorption kinetics can be described by the following equation: ( qt = qe 1 − e k1t ) (4) where t (min) is time, qe and qt (mg g–1) are the amount of Pb2+ adsorbed at equilibrium and at time t, respectively, and k1 (min–1) is the rate constant for the adsorption (pseudo) reaction. The experimental data were also analyzed by the pseudo–second–order model, which once again assumes that the overall process is limited by the adsorbent–adsorbate interaction, but then assumes that this can be described by a second order kinetic equation. In this case the adsorption kinetics can be described by the following equation: t 1 t = + qt k 2 qe2 qe (5) in which k2 (g mg–1min–1) is the rate constant for the adsorption (pseudo) reaction. Eventually, the possibility that adsorption process is controlled by intraparticle diffusion was considered. If this is the case, then the adsorption kinetics can be described by the Morris–Weber model [52], according to which it is: qt = kidt1/2 + θ Fig. 5. Response surfaces plots for Pb2+ removal using APTMS-Monosil. (a): Effects of X1 and X 2; (b): effects of X1 and X3; (c): effects of X 2 and X3 (X1: pH; X 2: solid to liquid ratio; X3: Pb2+ initial concentration). (6) in which kid (mg·g–1min1/2) is the intraparticle diffusion rate constant, and θ (mg g–1) is a constant, the value of which depends on the role played by external (fluid–solid) mass transfer. In order to compare the three models, the kinetic data relative to Pb2+ adsorption with solid to liquid ratio of 1 g·L–1, initial Pb2+ concentration of 100 mg L–1 and initial pH of 6.0 have been reported in Fig. 6. In particular, Fig. 6a refers to the pseudo-first-order model, and is a plot of ln(qt) vs. t; H. Sharififard et al. / Desalination and Water Treatment 105 (2018) 287–297 293 confirmed by the fact that the R2 value for the pseudo-second-order model is significantly closer to 1 than those relative to the two other models considered. This suggests that adsorption of Pb2+ on APTMS functionalised Monosil is controlled by a pseudo–chemical reaction, which can be described by a second order kinetic equation. In particular, the fact that the Weber–Morris model fails to describe the available kinetic data (see Fig. 6c) appears to indicate that intraparticle diffusion plays a secondary role in the overall adsorption process, at least in the experimental conditions considered. 3.4. Equilibrium and thermodynamic of adsorption The equilibrium data relative to Pb2+ adsorption on APTMS-Monosil at 30°, 45° and 60°C are reported in Fig. 7. The most striking feature of Fig. 7 is that adsorption capacity increases with temperature, rather than decreasing, as is the case of most adsorption phenomena. In particular, the highest Pb2+ adsorption capacity, about 450 mg·g–1, was observed at the highest temperature explored, i.e. T = 60°C, with a behavior similar to the one exhibited by a number of amine–functionalized silicas toward gas–phase adsorption, in particular of CO2 [27]. In order to have a clearer understanding of the adsorption process, the available data were fitted using the Langmuir, Freundlich and Dubinin-Raduskevich isotherms. The Langmuir isotherm is based on the assumptions that all the adsorption sites are equivalent for what concerns adsorbent–adsorbate interactions, that each site only interacts with a single adsorbate molecule/ion and that adsorbate–adsorbate interactions are negligible. The Langmuir isotherm equation is described by the following equation: qe = qmax K LCe 1 + K LCe (7) where Ce (mg g–1) is the liquid phase concentration of adsorbate at equilibrium and KL (L·mg–1) and qmax (mg·g–1) are the model parameters: qmax represents the maximum amount of adsorbate which can be theoretically adsorbed per unit mass of adsorbent when the monolayer coverage is complete, while KL is a measure of adsorbent–adsorbate affinity. The Freundlich isotherm is an essentially empirical equation, which assumes that the adsorption process takes place on a heterogeneous surface, and is described by the following equation: Fig. 6. (a) Pseudo-first-order model; (b) Pseudo-second-order model; (c) Intraparticle diffusion model for Pb2+ removal. Fig. 6b refers to the pseudo-second-order model, and is a plot of 1/qt vs. t; eventually, Fig. 6c refers to the intraparticle diffusion model, and is a plot of qt vs. t1/2. Parameters and R2 values for the three models are reported in Table 5. Inspection of Fig. 6 clearly indicates that experimental results are much better interpreted by the pseudo-second-order model than by the other models taken into account. This is also qe = K f (Ce ) 1 n (8) where Kf and n are the model parameters: Kf (mg g–1 L1/n mg–1/n) is an indicator of the adsorption capacity of adsorbent and n is a measure of favorability of adsorption. The Dubinin–Radushkevich (D-R) isotherm is a model originally proposed to describe gas–phase adsorption of subcritical vapors on porous sorbents, and can in general be used to describe adsorption on a heterogeneous surface characterized by a Gaussian distribution of adsorbent– adsorbate interaction energy [53]. The D–R equation is described by Eq. (9): 294 H. Sharififard et al. / Desalination and Water Treatment 105 (2018) 287–297 Table 5 Constants and regression coefficient for kinetic models Pseudo-first-order k1 (min–1) qe (mg g–1) 0.034 46.54 R2 0.92 Pseudo-second-order k2 (g mg–1·min–1) 0.0009 qe (mg g–1) 104.16 R2 0.998 Intra-particle diffusion kid (mg g–1·min–1/2) 5.32 θ 31.63 R2 0.778 Table 6 Langmuir, Freundlich and D-R isotherm constant parameters. Isotherm Parameters 30ºC 45ºC 60ºC Langmuir qmax (mg g ) 337.3 413.6 448.1 KL (L mg–1) R2 Kf (mg g–1 L1/n mg1/n) n (–) R2 E (kJ mol–1) qmax (mol g–1) R2 0.232 0.964 85.81 3.396 0.874 0.543 313.1 0.828 0.248 0.981 101.17 3.286 0.926 0.459 364.0 0.918 0.383 0.973 130.33 3.636 0.840 0.743 423.3 0.964 Freundlich D-R Fig. 7. Adsorption isotherms of Pb2+ on APTMS-Monosil T = 30°, 45° and 60°C (points: experimental data; continuous curves: Langmuir isotherms with parameters taken from Table 6).   1 ε 2 qe = qmax exp  −      2  E      1  ε = RT ln  1 + C   e  –1 (9) where R and T are the gas constant (kJ mol–1 K–1) and absolute temperature (K), respectively, and qmax (mg·g) and E (kJ mol–1) are the model parameters. As indicated above, qmax represents the maximum possible amount of adsorbate; E, on the other hand, is the mean free energy of adsorption per mole of adsorbate when it is transferred to the surface of solid from infinity in the solution. In order to compare the three isotherms considered, the optimal values of the parameters were estimated starting from the linearized forms of Eqs. (7–9). The results of the regressions, together with the regression coefficients R2, are presented in Table 6. The comparison among the different regressions indicates that the Langmuir isotherm consistently gives the best fit to experimental data, and for this reason the curves relative to the Langmuir isotherms using the parameters taken from Table 6 have been reported in Fig. 7. The results of Table 6 and the very good agreement between Langmuir isotherms and experimental data shown in Fig. 7 suggest that Pb2+ adsorption on APTMS-Monosil takes place as a monolayer or, in other words, that all the adsorbing sites are equivalent, and capable of capturing a single Pb2+ ion each. In particular, the amino groups available on the silica surface can form complexes with Pb2+ ions [13], and it can be assumed that each couple of amino groups adsorbs a Pb2+ ion according to the scheme of Fig. 8. Fig. 8. Schematic of adsorption mechanism of Pb2+ by APTMS-Monosil nanocomposite. Furthermore, it is interesting to observe that both the maximum adsorption capacity qmax and the affinity constant KL significantly increase with temperature: qmax has an almost fivefold increase between 30° and 60°C: this could be due to a more active chelating effect of amino groups with increasing temperature. For what concerns KL, its dependence on temperature was used to determine the thermodynamic parameters of the adsorption process according to the following equations:  ∆G o = − RT ln K L  − ∆H o ∆So  + ln K L = RT R  o o o  T ∆S = ∆H − ∆G  (10) where ∆Go, ∆Ho and DSo are the standard changes in Gibbs free energy, enthalpy and entropy, respectively. ∆Ho and ∆So H. Sharififard et al. / Desalination and Water Treatment 105 (2018) 287–297 295 can be obtained as the slope and intercept of a plot of ln (KL) vs. 1/T (Fig. 9). Calculated thermodynamic parameters are reported in Table 7. The positive value of ∆H° confirms that the Pb2+ adsorption on APTMS-Monosil has an endothermic nature. Furthermore, its relatively high value is in good agreement with the adsorption mechanism devised above, which has a strong chemical (rather than purely physical) nature. Eventually, it must be pointed out that the high value of DSo counterbalances the value of ∆Ho, leading to a negative ∆Go (spontaneous process). The maximum adsorption capacity of APTMS-Monosil for Pb2+ (about 450 mg·g–1 at T = 60°C.) was also compared with the adsorption capacity of other adsorbents proposed for Pb2+ removal [50,51,54–67]. The results of this comparison, reported in Fig. 10 and in Table 8, indicate that APTMS-Monosil has very interesting performances. Fig. 9. Relationship between ln (KL) and 1/T for Pb2+ adsorption on APTMS-Monosil. 3.5. Regenerability tests Table 7 Thermodynamic parameters at T = 298 K for Pb2+ adsorption on APTMS-Monosil ∆H°, kJ·mol–1 30.9 DS° , kJ·mol–1 K–1 ∆G°, kJ·mol–1 0.187 –24.826 In order to verify the capacity of APTS–Monosil to withstand a number of repeated adsorption–desorption cycles, a series of experiments in which the same sample was used to remove Pb2+ from a solution having a Pb2+ concentration of 100 mg/l was carried out. As mentioned above, between two removal experiments the sample was regenerated by treatment with 0.1 M EDTA. The results for adsorption-desorption cycles of Pb2+ are presented in Fig. 11. As the results show, the adsorption percentage of lead decreased from Table 8 Comparison of Pb2+ adsorption capacity (mg g–1) of different adsorbents [45,46,49–62] Adsorbent Pb2+ adsorption capacity (mg g–1) Reference Goethite nanoparticles HAp/Fe3O4 microspheres Modified mesoporous carbon APTMS-Monosil Chitosan nanoparticles Hydrated maganes oxide (HMO) Polymer-based hybrid Zeolite (Clinoptilolite)pretreated Iron-activated carbon (IAC) nanocomposite MHC/OMCNTs Co0.6Fe2.4O4 micro-particles Activated carbon/Fe3O4 nanocomposite Activated carbon Pine cone activated carbon Fe3O4 nanospheres Fe3O4/SiO2 nanocomposite Nanometer titanium dioxide/silica gel 820.5 540 500 49 62 50 450 400 325 (This work) 51 52 181.4 122 53 58 121.9 61 116.3 80.32 71.42 54 59 60 51.81 27.53 18.47 17.65 3.16 46 45 55 56 57 Fig. 10. Comparison of Pb2+ adsorption capacity (mg g–1) of different adsorbents [45,46,49–62]. Fig.11. Removal efficiency of Pb2+ on APTMS-Monosil as a function of regeneration cycles of the adsorbent. 296 H. Sharififard et al. / Desalination and Water Treatment 105 (2018) 287–297 100% to 86.4%. The limited extent of decrease in adsorption indicates a very high regenerability of the proposed material. [9] [10] 4. Conclusions Amino-functionalized silica monolith (APTMS-Monosil) was synthesized and tested in several batch experiments as a highly porous adsorbent for Pb2+ removal from aqueous solution. The RSM results indicated that solution pH and solid to liquid ratio are the most significant removal parameters, and that the optimum operating conditions are pH of 5.9, solid to liquid ratio of 1.74 g L–1 and initial Pb2+ concentration of 84 mg L–1. In these conditions, practically 100% of Pb2+ present in solution can be removed by APTMS-Monosil. The kinetics of Pb2+ removal is controlled by the adsorption pseudo–reaction rather than intraparticle diffusion, and this reaction can be described by a second order kinetic equation. The Langmiur isotherm satisfactorily describes equilibrium adsorption of Pb2+ on APTMS-Monosil. Results show that this process has a markedly chemical nature, and that the maximum adsorption capacity of APTMS-Monosil is about 450 mg·g–1 at 60°C. The comparison between the adsorption capacity exhibited by APTMSMonosil and the capacities reported in the literature for other adsorbing materials is very favorable. Furthermore, APTMS-Monosil exhibits a good capacity of withstanding several adsorption-regeneration cycles. Based on this analysis, APTM-functionalized Monosil adsorbent could be an excellent candidate for practical applications in the removal of Pb2+ from wastewater, and continuous removal experiments in columns systems should be carried out in order to better evaluate it. 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