Composites Communications 12 (2019) 101–105
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Soy protein acid hydrolysate/silica hybrid material as novel adsorbent for methylene blue
Ahmed Salama Cellulose and Paper Department, National Research Centre, 33 El-Bohouth st., Dokki, P.O. 12622, Giza, Egypt
A R T I C LE I N FO
A B S T R A C T
Key words: Soy protein hydrolysate – Silica – Hybrid – water puriﬁcation
The current study purposes to use a sustainable, low cost and abundant plant protein, soy protein hydrolysate (SPH), as a functional polymer to support silica gel matrix. Tetraethoxysilane (TEOS) was used as silica precursor and 3-glycidoxypropyltrimethoxysilane (GPTMS) was used as crosslinking agent between SPH and silica gel. SPH/silica hybrid was prepared through facile chemical crosslinking method. FTIR showed that GPTMS act as covalent crosslinking agent between SPH and silica network in the crosslinked hybrid. SEM analysis exhibited a homogenous morphology for the prepared SPH/silica hybrid, suggesting a well integrity between SPH and silica components. The novel hybrid was also characterized through XRD and TEM. The crosslinked SPH/silica hybrid exhibited high absorption capacity for methylene blue (MB) removal from aqueous solutions. The adsorption results ﬁtted well with pseudo second order and Langmuir isotherm model and the maximum removal eﬃciency was 357 mg/g. These results demonstrated that SPH/silica hybrid is eﬀective adsorbent for removal of MB, which would provide a new sustainable adsorbent for water puriﬁcation.
1. Introduction Higher consumption of synthetic dyes for many industrial applications has led to generating hazardous wastes. Synthetic dyes which commonly used in printing and dyeing industries are often highly toxic, carcinogenic and mutagenic [1–3]. They can cause sever damages to human beings and aﬀect the aquatic ecosystem even at low concentrations. New strategies are required to remove the toxic dyes from waste water [4–7]. Last decades, many technics have been applied to remove organic dyes from contaminated water. Among these strategies, adsorption has been considered as eﬃcient technique for adsorption of coloring substances [8,9]. The high cost of the current adsorbents encourages the researchers to develop economical alternatives. Many attempts have been carried out for preparing cheap and eﬀective adsorbents containing natural polymers [10,11]. Hybrid materials have recently emerged as eﬃcient adsorbent for waste water and photodegradation [12–17]. Class II hybrid materials are characteristic by the presence of a real covalent bond between the organic and the inorganic components at the molecular level, and thus they are homogenous. Hybrid materials composed of natural polymers and silica have potential to combine the properties of an elastic organic polymer and inorganic silica. Recently, 3-glycidoxypropyl trimethoxysilane (GPTMS) has been used as crosslinking agent through nucleophilic ring opening reaction by carboxylic, hydroxyl or amino groups of
natural polymers to incorporate functionalized polymers in the silca gel . For example, D. Wang et. al. prepared chitosan/silica hybrid scaﬀold through the sol-gel process using GPTMS and TEOS as coupling agent and silica precursor, respectively. They proved that the inorganic/organic ratio can largely inﬂuence the hybrid performance . Alginate/silica hybrids was synthesized through nucleophilic ring opening reaction of GPTMS by carboxylic groups. The reaction of alginate with covalent coupling reagent was followed using diﬀerent analytical methods. Bioactivity and mechanical properties of the resultant alginate/silica hybrid monoliths were investigated . Soy protein hydrolysate (SPH) based materials have received considerable attention to use in food texture enhancers and as a pharmaceutical ingredients . Moreover, soy protein has recently applied as an adsorbent because of its chemical stability, high reactivity and high adsorption capacity. These properties are due to the presence of chemical reactive groups, carboxyl and amino groups . These functional groups have the capability to interact by physical and chemical forces with a wide variety of molecules. Soy protein based materials have been reported as low-cost materials for removal of heavy metals and organic dyes, and a valuable tool for protecting the environment. Soy protein contains 20 diﬀerent amino acids which provide versatile structure with several polar functional groups. However, the solubility of soy protein in acidic or basic mediums hampers its application as adsorbent. Few articles studied the probability of soy proteins as
E-mail address: [email protected]
https://doi.org/10.1016/j.coco.2019.01.002 Received 16 October 2018; Received in revised form 28 December 2018; Accepted 8 January 2019 Available online 09 January 2019 2452-2139/ © 2019 Elsevier Ltd. All rights reserved.
Composites Communications 12 (2019) 101–105
2.3.3. XRD XRD was performed using X-ray diﬀractometer (PANalytical, Netherlands) with a monochromatic CuKα radiation source (λ = 0.154 nm) in step-scan mode with a 2θ angle ranging from 5 to 70°.
organic dyes as adsorbent. In our previous study, the functionality of cellulose was improved by grafting soy protein isolate. Cellulose graft soy protein isolate was reported as a bioactive material for calcium phosphate mineralization and the produced hybrid was investigated as new sustainable adsorbent for dyes removal . In another study, soy protein isolate was immobilized on konjac glucomannan to remove MB from aqueous solutions. The prepared porous material showed adsorption capacity of 272 mg/g . Poly(methacrylate)/silica nanohybrid materials were prepared and investigated for MB adsorption and the maximum adsorption capacity reached to ~ 91.3 mg/g . In the current study, new sustainable covalently crosslinked SPH/ silica hybrid material was prepared and applied for the removal of MB, a cationic dye, from its aqueous solutions. GPTMS was used as a silane coupling agent and TEOS was used as silica precursor to form homogenous hybrid material. The current covalently crosslinked SPH/silica hybrid is a novel material and not reported previously. The removal of MB from its aqueous solutions was calculated as a function of time, pH and concentration. The kinetic and thermodynamic studies were performed to better evaluate the adsorption performance.
2.4. Batch adsorption studies The adsorption of MB was studied in 50 ml glass bottles with 50 mg of covalently crosslinked SPH/silica. The adsorbent was soaked in the aqueous buﬀer before adding MB solution. The adsorption experiments were conducted at diﬀerent pHs, diﬀerent times and diﬀerent dye concentrations. After the adsorption process, MB was quantiﬁed colorimeterically by measuring at 664 nm using spectrophotometer (JASCO V-650). The MB concentration was measured at adsorption equilibrium, qe (mg/g), and calculated according to the following equation:
(Co − Ce ) V W
where Co and Ce are the initial and equilibrium MB concentrations (mg/ L). For desorption study, MB-loaded crosslinked SPH/silica was agitated with 10 mL of 0.01 M HCl. The adsorbent was dried and adsorption–desorption cycle was repeated four times. Calculated results are representative of three experiments, and standard deviations are less than 6.0%.
2. Experimental section 2.1. Materials Soy protein acid hydrolysate, Tetraethyl orthosilicate (TEOS, 99.9%) and (3-Glycidyloxypropyl) trimethoxysilane were purchased from Sigma Aldrich. All other reagents were of analytical grade and used as received without further purifying.
3. Results and discussion 3.1. Synthesis of SPH /silica hybrid
2.2. Preparation of SPH/silica hybrid
Generally, TEOS can form silica network by sol gel technique, so the current article aims to use it as matrix to hold SPH for preparing new kind of hybrid materials. GPTMS was used as crosslinking agent to crosslink SPH with silica gel network. GPTMS can react with amino groups of SPH by a ring-opening reaction and react with TEOS to construct the functionalized silica gel network. In the acidic solution, GPTMS reacted with SPH through the acid-catalyzed amino-oxirane addition reaction. Moreover, the methoxysilane groups hydrolyze to form silanol groups which condense with TEOS to construct the silica network. The possibility of H-bonding between the carboxyl groups of SPH and the not condensed hydroxyl groups of silica gel may have an important role for increasing the homogeneity between the organic and inorganic part of the formed hybrid . (Scheme 1). After successful synthesis of SPH/silica hybrid, various techniques were used to prove its structure. Fig. 1(A) shows the FTIR spectra of SPH and SPH/silica hybrid samples. A broad band appeared at 3300–3500 cm-1 and corresponds to the stretching vibrations of –OH and –NH in SPH. The FTIR spectra of SPH displayed band located at 1643 cm-1 associated with the amide I band. Moreover, the weaker band at 1517 cm-1 represents the N-H binding of the primary amine (amide II). The amide I band shows no signiﬁcant change after the hybrid formation. However, the amide II band was completely
1 gm of SPH was dissolved in 20 mL distilled water followed by the addition of 2 mL coupling agent (GPTMS). The reaction continued for 12 hours at 40 °C in acidic medium. Then, 5 mL silica precursor (TEOS) was added at 60 °C for 2 hours to ensure complete hydrolysis of TEOS molecules. The resulting homogenous solution was heated in a water for 48 h until the formation of homogenous gel. The synthesized covalently crosslinked SPH/silica hybrid was washed with distilled water and subsequently dried in a vacuum oven.
2.3. Characterization 2.3.1. FTIR Fourier transform infrared spectroscopy (FT-IR) was done on a FTIR (Mattson 5000 FTIR spectrometer) using KBr discs in the range of 4000500 cm-1.
2.3.2. SEM Scanning electron microscopy was done on Model Quanta 250 FEG (Field Emission Gun) attached with EDX Unit (Energy Dispersive X-ray Analyses), with accelerating voltage 30 K.
Scheme 1. Synthesis of crosslinked SPH/silica hybrid. 102
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A. Salama 50
Fig. 1. FTIR spectra of SPH (A) and SPH/silica hybrid (B).
of the silica crosslinker (GPTMS) which responsible for covalent bond formation between silica gel and SPH. This morphology suggests a good adhesion between the two phases and homogenous distributions of SPH in silica gel network. The porous and homogenous structure of SPH/ silica hybrid provides suitable environment to adsorb diﬀerent types of water pollutants. TEM image of SPH/silica hybrid shows collected spherical nanoparticles without no distinct size or shape as showed in Fig. 3C. 3.2. Application of the SPH/silica hybrid for MB adsorption
Fig. 4. : Eﬀect of the pH on the adsorption capacities of SPH/silica hybrid for MB adsorption; dye concentration: 100 mg/L; sample dose: 0.05 g/50 mL and time: 60 min.
Covalently crosslinked SPH/silica hybrid was applied for the removal of MB from aqueous solutions. Removal of MB was reported to depend on diﬀerent parameters such as solution pH, adsorption time and the initial concentration of the dye. These various parameters were investigated in order to evaluate the potential of SPH/silica hybrid as new and sustainable adsorbent for cationic dyes.
2-Theta, degree Fig. 2. XRD pattern of SPH (A) and SPH/silica hybrid (B).
disappeared after the reaction with GPTMS which implied that the primary amine group of SPH reacted with GPTMS to form a secondary amines (CSPH-NH-CGPTMS). (Fig. 2). X-ray diﬀraction was directed to explore the changes of the structure of SPH/silica prepared by sol-gel technique. SPH shows relative intense peaks at 6.1, 11.6, 19.5, 29.1, 31.1, 43.7 and 44.6. These peaks originate from the β-sheet structures of the soy protein secondary structure. However, SPH/silica exhibited a relatively weak characteristic peak at 2θ = 22.8°. This peak may be assigned to the amorphous glassy structure of the hybrid materials . The SEM micrographs of SPH/silica hybrid is shown in Fig. 3. The SPH/silica hybrid shows homogenous rough surface with macroporous structure. Moreover, the morphology of the SPH/silica hybrid at higher magniﬁcation (Fig. 3b) looks to be homogenous and no clear evidence of phase-separated structure. This homogeneity is due to the presence
3.2.1. Eﬀect of pH on MB adsorption It has been reported that, the solution pH aﬀect the charge of the adsorbent and the ionization degree of the dye. Fig. 4 shows the adsorption of MB by covalently crosslinked SPH/silica hybrid at diﬀerent pH values. Higher adsorption capacity of MB on SPH/silica hybrid was favored by ascending pH of the solution, i. e. at pH 7–8. At high pH the ionic interactions between cationic MB molecules and the anionic groups in the hybrid are responsible for the high adsorption eﬃciency. Moreover, lower pH would oﬀer higher concentration of hydronium ions, which strongly compete with MB leading decreasing adsorption sites of the adsorbent. The competition between hydronium ions and MB will decrease with an increasing pH, and more adsorption sites will be available. As a result, the next adsorption experiments were carried out at pH 7, which taken as optimum pH value. Increasing the
Fig. 3. : SEM pattern of SPH/silica hybrid at diﬀerent magniﬁcations and TEM image of SPH/silica hybrid. 103
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Fig. 5. : Eﬀect of the adsorption time on adsorption capacity of SPH/silica and the ﬁtted Pseudo-second-order kinetic model. Adsorption experiments: MB concentration:100 mg/L; sample dose: 0.05 g/50 mL and pH: 7.0.
Fig. 6. : Eﬀect of the initial MB concentration on adsorption capacity of covalently crosslinked SPH/silica hybrid and the ﬁtted Langmuir isotherm model. Adsorption experiments: sample dose: 0.05 g/50 mL; pH: 7.0 and equilibrium time 80 minutes.
adsorption capacity of MB with increasing pH values has been reported in other diﬀerent studies .
Langmuir and Freundlich, were tested with the experimentally obtained equilibrium data. The theoretical Langmuir isotherm model assumes that adsorption occurs at speciﬁc homogeneous sites within the adsorbent. This equation can be written in linearized form as follows: 
3.2.2. Eﬀect of contact time The adsorption capacity-time proﬁle of SPH/silica hybrid was measured and displayed in Fig. 5. The adsorption of MB on SPH/silica hybrid is rapid for the ﬁrst 30 minutes after which it slows down and ﬂattened oﬀ as contact time increased (~30 to ~60 min.). This may be attributed to the formation of monolayer of dye molecules at outer surface of the hybrid. The kinetic studies of MB adsorption onto crosslinked SPH/silica were analyzed by two kinetic models, i.e. the pseudo-ﬁrst-order and pseudo-second-order models. The two models (Eqs. 2 and 3) are stated as follows:
log(qe − qt ) = log(qe ) −
K1 t 2.303
Ce Ks C = + e qe qmax qmax
The plot of the experimental Ce/qe against Ce for the experimental data presented high correlation coeﬃcient (R2 > 0.996). The linearized Langmuir equation show that the Langmuir model can describe the adsorption of MB on covalently crosslinked SPH/silica hybrid (see Table 2). From the slope and the intercept of the straight line, the values of qmax and Ks are estimated to be 357 mg/g and 41.25 mg/l, respectively. The Langmuir isotherm model assumes the monolayer adsorption of the MB on the surface of covalently crosslinked SPH/silica hybrid and all adsorption sites have equal energies and enthalpies. The Freundlich equation is given by:
t t 1 = + qt qe K2 qe2
log qe =
The parameters for the ﬁrst- and second-order kinetic models are listed in Table 1. The ﬁts of the experimental results show that the pseudo second order possesses higher R2 value. These results suggest that the pseudo second-order kinetic adsorption process is more suitable for describing MB adsorption and the adsorption is a chemical process.
1 log Ce + log P n
Freundlich parameters shows that the linear coeﬃcient was 0.889. Moreover, the values of the Freundlich model constants P and n are 19.5 and 2.06 respectively (Table 2). Regeneration of adsorbents for reuse is a crucial parameter in industrial applications. The adsorption study of MB onto crosslinked SPH/silica hybrid was dependent on pH value. In acid solution, desorption would be produced with the electrostatic interaction disappearing between the dye and the crosslinked SPH/silica hybrid. The adsorption–desorption study of crosslinked SPH/silica hybrid decreased slightly from 94 in the ﬁrst cycle to 89 after the fourth cycle.
3.2.3. Eﬀect of MB concentration and adsorption equilibrium study Fig. 6 shows the eﬀect of MB initial concentration on the adsorption capacity of covalently crosslinked SPH/silica hybrid. It is clear that the dye removal increases from 22 to 310 mg/g with increasing MB concentration from 25 to 500 ppm then tend to levels oﬀ with higher concentrations. This trend may be explained by the fact that the initial MB concentration provides a suﬃcient force to overcome the resistance of mass transfer from the aqueous phase to the solid phase . For describing the mechanism of the interaction between MB and the covalently crosslinked SPH/silica hybrid, two isotherm equations,
4. Conclusion SPH/silica hybrid was synthesized via sol-gel process as a new sustainable material for waste water puriﬁcation. FTIR results showed Table 2 Parameters for MB adsorption by crosslinked SPH/silica hybrid according to diﬀerent equilibrium models.
Table 1 Kinetic parameters for MB adsorption by crosslinked SPH/silica hybrid. Pseudo ﬁrst order-model
Pseudo second- order model
Langmuir isotherm constants qe, g) 81
qe, cal (mg/g)
qe, cal (mg/g)
K2 (g mg-1 min-1)
1.46 * 10-3
Freundlich isotherm constants
Composites Communications 12 (2019) 101–105
that SPH and silica network were covalently crosslinked via the amino group of SPH and the epoxide ring of the coupling agent. This was conﬁrmed by the disappearance of the amide II band of SPH after the reaction with GPTMS. The equilibrium adsorption data were best represented by the Langmuir model with a maximum adsorption capacity of 357 mg g-1 at pH 7. MB adsorption obeyed pseudo-second-order kinetics with R2 value of 0.997. The utilization of covalently crosslinked SPH/silica hybrid as sustainable and biocompatible adsorbent seems promising for removal of organic pollutants from aqueous solutions.
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