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modified- TiO2 nanocomposites for removal of Cr (VI) from aqueous solutions
Progress in Organic Coatings 110 (2017) 24–34
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Surface modification of TiO2 nanoparticle by three dimensional silane coupling agent and preparation of polyamide/modified- TiO2 nanocomposites for removal of Cr (VI) from aqueous solutions
MARK
⁎
Mohammad Dinari , Atefeh Haghighi Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: s-Triazine based polyamides Surface modified TiO2 Thermal properties Cr(VI) ions removal
In the present investigation, the goal was the preparation, characterization and surface morphology of polyamide/modified-TiO2 nanocomposites (PA/m-TiO2 NCs). At first, a soluble aromatic PA was synthesized by direct polycondensation reaction of the synthesized diacid and 1,3-phenylenediamine in molten tetrabutylammonium bromide (TBAB) with good yield and moderate inherent viscosity. The structure and purity of these compounds were confirmed by different techniques. For the improvement of TiO2 nanoparticles dispersion and enhancing interactions between nanoparticles and polymeric matrix, the surface of TiO2 was successfully modified by 1,3,5-triazine based silane coupling agent. The functionalized nanoparticles were then inscribed in 0, 5, 10 and 15 wt.% into polymer matrix. The resulting hybrids were characterized by Fourier transforminfrared spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The TGA of the obtained PA/ m-TiO2 NCs proves the enhancement in the thermal stability with an increase in the percentage of titania nanoparticles. It was further investigated for Cr(VI) ion removal by optimizing the parameters including pH, contact time and concentration. Also, adsorption kinetics and adsorption isotherms of this polymer were investigated.
1. Introduction Increase in the use of heavy metals over the past few decades resulted in increasing awareness of the potentially hazardous effects of elevated levels of these materials in the environment and makes a great concern about the pollution by them [1]. Heavy metals pollution is a serious problem threatening human health and other organisms due to their high level toxicity, carcinogenicity and non-biodegradability, even though they are in low concentration [2–4]. Nowadays, a number of promising processes are used for elimination of heavy metal ions and dyes from wastewaters [5]. Polymer nanocomposites (NCs) adsorbents have been used for this purpose owing to their chemical functionality, dimension stability, adjustable surface area, easiness of handling and regeneration [6–8]. In the last years, there has been a strong emphasis on the development of polymeric NCs, where at least one of the dimensions of the filler material is of the order of a nanometer [9,10]. Polymer NCs incorporate the remarkable features of both nanoparticles (NPs) and polymers: the unique physical and chemical properties resulting from the large surface area to volume ratios, the high interfacial reactivity of nanofillers, and outstanding mechanical ⁎
Corresponding author. E-mail addresses: [email protected], [email protected] (M. Dinari).
http://dx.doi.org/10.1016/j.porgcoat.2017.04.044 Received 10 December 2016; Accepted 25 April 2017 0300-9440/ © 2017 Elsevier B.V. All rights reserved.
properties and compatibility owing to their polymer matrix, being also amenable to regeneration and reuse [11–14]. In the last decades, several NCs have been fabricated for the adsorptive removal of heavy metals from water and wastewater [15,16]. Overall, various effects contribute determining the whole efficacy of NCs action. Composite matrix can be aromatic polyamides (PA). Aromatic PAs are well known as high-performance polymers because of their combination of excellent thermal, mechanical, and chemical properties that make them useful in applications for advanced technologies [17–19]. They are used neat in fibers, films and may contain fillers in engineering plastics applications. Most frequently, much effort has been paid to the development of aromatic heterocyclic PAs for use as advanced composite matrix, structural adhesives or coatings in hightemperature applications [20]. However, the main trouble of aramids is associated with their insolubility owing to their rigid backbones and strong interchain interactions, leading to processing difficulties. Therefore, many efforts have been made to increase the solubility and processability of aramids through structural modification of their monomers [21–25]. One of the common approaches to increasing solubility without much compromising their thermal and mechanical
Progress in Organic Coatings 110 (2017) 24–34
M. Dinari, A. Haghighi
2914 (w), 1678 (m), 1576 (w), 1439 (w), 1271 (m), 1158 (w) and 576 (w).
stability is the use of monomers with bulky packing-disruptive moieties [26–28]. The incorporation of bulky groups increases the interchain spacing and reduces the packing efficiency thereby increasing the intrinsic microporosity [28]. Recently, blending of the polymers with inorganic nanomaterials has occupied an impressive platform of research because of their simplicity, stable performance and mild operating conditions [29]. The addition of NPs to the polymer matrix has been the most usually accepted technique for manufacturing polymer NCs and the hybrids prepared with uniform dispersion of NPs can offer good mechanical strength, antifouling or self cleaning, bactericidal and also to some extent, photocatalytic properties [30,31]. Among many nano-materials, TiO2 NPs gains its importance because of its photocatalytic activity, self cleaning property, availability, ease of preparation and economic feasibility [32,33]. Since TiO2 directs to the formation of agglomerates at higher concentration due to its high surface energy which offers a driving force for the formation of particle bonding, achieving a hybrid with excellent NPs dispersion is a challenge [34]. Generally, this problem would be resolved either by chemical or mechanical modification to TiO2 prior to NC preparation. The most common method used attaches a suitable organic group on the surface atoms using silane coupling agents [35]. This type of surface modification stabilizes the NPs against the agglomeration and also makes them compatible with the other phase [36]. The suitable surface modification of NPs, not only leads to enhanced dispersion and compatibility in polymer matrix, but also undergoes chemical or physical interactions with the polymer matrix [37]. Here, we introduce a new macromolecular construction, based on the nucleophilic aromatic substitution chemistry of cyanuric chloride for triazine-based aromatic PA. The thermally stable PA was produced by direct polymerization reaction of 4,4′-((6-morpholino-1,3,5-triazine2,4-diyl)bis(azanediyl))dibenzoic acid and 1,3-phenylendiamine in molten tetrabutylammonium bromide (TBAB)/triphenyl phosphite (TPP) system. Then, the surface of the TiO2 NPs was modified with 1,3,5-triazine core silane coupling agent. The functionalized TiO2 NPs were then inscribed in 0, 5 and 10 wt.% into PA matrix to formed different polymer NCs. Then, the application of this novel trazine-based PA and NC materials for adsorption of Cr(IV) from aqueous solution was examined. The contact time as a significant parameter on the removal of Cr(IV) ions and the adsorption mechanism was investigated.
2.3. Modification of TiO2 with cyanuric chloride and APTES (m-TiO2) For surface modification of TiO2 NPs, at first, a three dimensional silane coupling agent was synthesized as follow: 1.0 g (5.42 mmol) of cyanuric chloride was dissolved in 10 mL of dry toluene. Then, an excess amount of APTES (27.10 mmol) was added drop wise to the cyanuric chloride solution in toluene and it was refluxed for 6 h under nitrogen atmosphere. N,N-diisopropyleethylamine was added drop wise to the reaction mixture to neutralize the formed HCl. The product mixture separated in two layers: the higher layer was removed to emit solvent and un-reacted materials, and lower layer was distilled under vacuum to formed 1,3,5-triazine core silane coupling agent as a product for surface modification of TiO2 NPs as follow: 0.50 g of the dried TiO2 was dispersed in 10 mL of toluene and it was sonicated for 30 min. Appropriated amount of 1,3,5-triazine core silane coupling agent (with respect to 30% wt. of TiO2 NPs) was added to the mixture under ultrasonic irradiation. Then the reaction mixture was refluxed at 110 °C with stirring under nitrogen atmosphere for 24 h. The modified TiO2 (m-TiO2) NPs were collected by centrifugation of the reaction mixture at 5000 rpm and washed twice with toluene and ethanol to completely remove the residual silane coupling agent. It was dried in vacuum at 60 °C for 8 h. 2.4. Synthesis of PA/m-TiO2 NCs To obtain homogeneous mixed of PA/m-TiO2 composites with different amounts of m-TiO2 NPs (0, 5, 10 and 15 wt%), a two-step process was used. At first, two solutions were synthesized: 0.1 g of polymer was dissolved in 2 mL DMF and m-TiO2 was separately dispersed in 2 mL DMF solution with stirring for 24 h at 25 °C. Then, the solutions were mixed to reach the preferred weight percentages of m-TiO2 from 5 to 15 wt.%. The PA/m-TiO2 solutions were stirred for 24 h at 25 °C and then it was sonicated for 2 h in ice bath with frequency 2.259 × 104 Hz and power 100 W. To remove the solvent, obtained PA/m-TiO2 solutions were poured into glass petri dishes and after 3 day at ambient temperature, PA/m-TiO2 NCs were prepared. 2.5. Characterization techniques
2. Experimental Fourier transform-infrared (FT-IR) spectra of the samples were recorded at room temperature in the range of 4000–400 cm−1 at a resolution of 4 cm−1, on Jasco-680 (Japan) spectrophotometer. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance DRX 500 and 125 MHz, by using solutions in deuterated dimethylsulfoxide (DMSO-d6). The Proton resonances were designated as singlet (s) and multiplet (m).X-ray diffraction (XRD) patterns were recorded using CuKα radiation on a Bruker (Germany), D8Avance, diffractometer operating at current of 100 mA and a voltage of 45 kV. The diffractograms were measured for 2θ, in the range of 10–80°, using CuKα incident beam (λ = 1.51418 Å). Carbon, hydrogen and nitrogen content of the compounds were determined by pyrolysis method by Vario EL elemental analyzer. Thermal stability was measured with a thermogravimetry analysis (TGA) (TA instrument Co.) at a heating rate of 10 °C min−1 from room temperature to 800 °C under a continuous flow of nitrogen. Inherent viscosities were measured by a standard procedure using a Cannon-Fenske routine viscometer (Germany) at the concentration of 0.5 g/dL at 25 °C.
2.1. Materials All materials and solvents were obtained from Merck Chemical Co and Aldrich Chemical CO. Cyanuric chloride, tetrabutylammonium bromide (TBAB), triphenyl phosphite (TPP) and 1,3-phenylene diamine were used as conventional. TiO2 nanoparticles with average particle sized of 25–30 nm was purchased from nanosabz Co. (Tehran, Iran) and it was dried at 500 °C for about 5 h to ensure complete drying, before using. The coupling agent; 3-aminopropyltriethoxysilane (APTES) obtained from Merck Chemical Co. 2.2. Synthesis of aromatic polyamide with triazine moiety A mixture of 0.100 g (0.458 mmol) of synthesized diacid [38] and 0.590 g (1.832 mmol) of TBAB was ground until a powder was formed and then it was moved into a 25 mL round-bottom flask. 0.049 g (0.458 mmol) of 1,3-phenylene diamine was added to the mixture and it was heated until homogeneous solution was formed. Then, 0.436 g (1.832 mmol) of TPP was added. After that, the solution was stirred for 16 h at 130 °C and the viscous solution was precipitated in 30 mL of methanol. The light yellow solid was filtered off and dried to give 0.146 g (98%) of aromatic polymer. ▓▓▓▓FT-IR peaks (KBr, cm−1): 3423 (br), 3082 (w), 2985 (m),
2.6. Adsorption studies: Cr(VI) removal over PA and PA/m-TiO2 NCs The important parameters on the adsorption of Cr(VI) from aqueous solutions are pH, contact time and concentration. In this study, the adsorption behavior of Cr(VI) from aqueous solution by the aromatic 25
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the surface properties of TiO2 NPs. First, a 1,3,5-triazine core silane coupling agent was synthesized. Then, the surface of TiO2 NPs was modified with the above prepared coupling agent. The presence of siloxane end groups on the synthesized cross-linker helps in the effective modification TiO2 NPs and the highly polar triazine group improves the dispersibility of the modified nanoparticles in organic solvents [26,42]. After the modification of TiO2 NPs, the dispersity of NPs could be greatly improved in the polymer matrix and also this phenomenon facilitates its interactions with desired organic polymer. This silane functionalized TiO2 NPs were finally incorporated in different weight percentages such as 0, 5, 10 and 15% into the PA matrix with similar triazine segment (Scheme 2).
PA and PA/m-TiO2 NCs were investigated and their kinetic and isotherm was assessed. A stock solution of 1000 mg L−1 Cr(VI) was prepared by dissolving an weighed amount of analytical grade K2Cr2O7 (> 99.0% purity) in deionized water, the other solutions were prepared by dilution. To estimate the effect of pH, 6 tests was done at different pH levels (from 1 to 6) with 0.001 g of PA and PA/m-TiO2 NCs 5%, 10% and 15% by setting the initial Cr(VI) concentrations at 10 ppm in plastic container. The containers were placed in a shaker at a speed of 180 rpm and the contact time was 300 min. The adsorbents were filtered through a 0.45 μm filter membrane. The residual Cr(VI) concentrations were detected by flame atomic absorption spectrometer. pH = 4 is the optimum pH for the maximum removal of Cr(VI) by PA and PA/m-TiO2 NCs 5%, 10% and 15%. The solution pH was adjusted by 0.1 M NaOH and 0.1 M HCl. To determine the effect of contact time, 12 tests was done at different times (from 20 to 240 min) at optimum pH with 0.001 g of adsorbents and by setting the initial Cr(VI) concentrations at 10 ppm in plastic container. After the shaking of solutions, the adsorbents were filtered and the residual ions concentrations were detected. In order to determine the effect of concentration, 7 tests were done at different concentrations (from 5 to 700 ppm) at optimum pH and time with 0.001 g of adsorbents. The removal efficiency was calculated as follow:
%Removal =
3.3. Characterization techniques 3.3.1. FT-IR study Fig. 1 displays the FT-IR spectra of pure PA, m-TiO2 as well as NC with 0, 5, 10 and 15 wt.% of m-TiO2. For neat PA, a peak at 3420 cm−1 related to amidic NH groups in polymer backbone and strong peaks at 1680 cm−1 related to carbonyl of the amide groups. The absorption bands appeared around 3013–3100 and 2850–2930 cm−1 are related to the corresponding aromatic and aliphatic C–H stretching vibration, respectively (Fig. 1a). For m-TiO2 NPs, the broad peaks at 3300–3500 cm−1 correspond to stretching motions of the hydroxyl and NH groups. The bands at 2870–2928 cm−1 can be ascribed to the C–H symmetrical and asymmetrical stretching absorptions of the CH2 group of the APTES. The characteristic peak at 1610 cm−1 was correspond to C]N stretching of triazine ring. The stretching vibration of Si–O–C group was appeared at 1084 cm−1. The broad band observed in the region 450–800 cm−1 corresponds to Ti–O stretching peak (Fig. 1b). These results showed that the triazine-based silane coupling agent was grafted onto surface of TiO2 by chemical bond. To display the interactions between m-TiO2 NPs and PA chains, the FT-IR spectra of PA/m-TiO2 hybrid are shown in Fig. 1. Compared with pure polymer (Fig. 1a), a new broad absorption band around 450–800 cm−1 can be found in the FT-IR spectra of PA/m-TiO2 NCs, which increases with increasing TiO2 contents and can be ascribed to the vibration of Ti–O–Si groups (Fig. 1c–e). These results confirm the presence of m-TiO2 NPs in the PA matrix.
Ci − Ce Ci
That Ci and Ce are the initial and final (equilibrium) concentrations of the Cr(VI) ions in aqueous solution (mg L−1). 3. Results and discussion 3.1. Synthesis of organo-soluble PA Because of the rapid emergence of ionic liquids (ILs) as green solvents and replacement of a volatile and toxic organic solvent in the polymerization with a nonvolatile solvent, in this study, s-triazine containing PA was synthesized in green route by using molten TBAB as an IL media. This polymer was prepared by the direct polymerization reaction of s-triazine containing diacid monomer with 1,3-phenylenediamine as show Scheme 1. The inherent viscosity of the PA under optimized condensations was 0.35 dL/g and yield was 98%. The chemical structure and purity of this compound was also confirmed with elemental analysis, FT-IR and 1H NMR spectroscopy as reported in our recently published paper [38].
3.3.2. X-ray diffraction Fig. 2 displays the XRD patterns of s-triazine containing PA, m-TiO2 as well as NCs of PA with 5% and 10% of m-TiO2 NPs. For pure PA (Fig. 2d), it could be observed that except one crystalline peak, there is a lack of any diffraction peak in the range of 2θ angle, and this observation suggested the presence of a little proportion of crystalline phase compared to an amorphous one. For m-TiO2 (Fig. 2a), the presence of anatase and rutile phase was confirmed by the peaks appeared at 101, 110, 004, 200, 105, 211, 204, 220, 301, which approved the crystalline form of this compound [41]. The XRD patterns of NC show characteristic peaks of m-TiO2 and PA, suggesting that the surface modification and preparation process does not affect the morphology of NPs (Fig. 2b and c). The average size of the TiO2 is calculated to be 25–35 nm according to the Debye-Scherrer formula.
3.2. Surface modified TiO2 NPs and preparation of PA/m-TiO2 NPs The uniform distribution of modified nanoparticles in polymer matrices can lead to physical, morphological, and mechanical property enhancements in the resulting NCs [9,19]. Due to exist of OH groups on the surface of NPs, it is easy for TiO2 NPs to greatly agglomerate in polymer matrix. Silane coupling agents are one of the generally used materials for enhancing the dispersion stability of the NPs in the polymers [39]. The silane coupling agents lie onto metal oxide surface by the hydrolysis and condensation reactions of silane agents: R–Si–OR′ + H2O → R–Si–OH + R′OH
(1)
R–Si–OH → R–Si–O–Si–R + H2O
(2)
M–OH + R–Si–OH → M–O–Si–R + H2O
(3)
3.3.3. Electron microscope characterization of m-TiO2 and PA/m-TiO2 NCs In order to examine the microstructures and nanofiller distribution within the NCs, FE-SEM and TEM analysis were conducted. Fig. 3 shows the FE-SEM and TEM images of the m-TiO2 with two different magnifications. According to these images, m-TiO2 NPs have a nanoscale size with spherical form-like. The sizes of particle were in the range of 25–35 nm. Fig. 4 shows the FE-SEM micrographs of pure PA and PA/m-TiO2 NCs. It is natural to believe that a homogenous dispersion of NPs in material will bring best result. The FE-SEM images of NCs (Fig. 4c–f) indicate that, m-TiO2 were dispersed in the PA matrix but, as it can be
Where R and R′ are functional groups and methyl (or ethyl), respectively, and M stands for metal oxide [40]. When silane coupling agent dissolved in the water, ethoxy groups were hydrolyzed and Si–O–Si bonds formed. Then, by reaction with hydroxyl groups of metal NPs, the alkyl groups of coupling agent were placed on the surface of NPs [41]. In this study, APTES and cyanuric chloride were used to modify 26
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Scheme 1. Surface modification of TiO2 nanoparticles by 1,3,5-triazine containing silane coupling agent.
Scheme 2. Preparation of PA/m-TiO2 nanocomposites.
dispersion of modified NPs in the polymer matrix. Fig. 5 shows the TEM images of the PA/m-TiO2 NCs 5 wt% at different magnifications. In these images, no aggregation is visible between the m-TiO2 and relatively satisfactory dispersion of m-TiO2 can be observed. This
seen in PA/m-TiO2 NCs 10 wt% (Fig. 4e and f) the bright spots on the background images seems to be agglomerates of m-TiO2 NPs which increased with increasing the content of the filler in the polymer matrix. TEM analysis was used to investigate the shape, size and also 27
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PA/m-TiO2 NCs and pure PA were studied by TGA. Fig. 6 shows the thermal degradation behavior of the pure TiO2 and s-triazine modified TiO2 under nitrogen atmosphere. It is clear from Fig. 6 that the thermal stability of the surface modified NPs is lesser than the untreated TiO2 NPs. The unmodified TiO2 NPs had a less weight loss (about 2%) in the whole temperature ranging 0–800 °C, which can be attributed to the removal of water; and partial dehydroxylation of the TiO2 NPs. The TGA curve of m-TiO2 has a clear two-step decomposition route (Fig. 6b). The first step, mass loss happened in the temperature range 150–250 °C, which might be ascribed to the hydroxide group on the surface of the TiO2 NPs. The second decomposition happened in the temperature range 450–550 °C, which could be related to the thermal decomposition of organic groups such as 1,3,5-triazines and other organic chains present on the surface. TGA curves of the PA and PA/m-TiO2 NCs are shown in Fig. 7. The TGA curves reveal the weight loss of the material as it is heated. The gradual weight loss of the samples was due to the evaporation of trapped solvent and decomposition of the polymer. TGA data for thermal stability of the PA/m-TiO2 NCs was studied based on 5 and 10% weight loss (T5 and T10) and the residue at 800 °C (char yield). As observed from Fig. 7 and Table 1, with increasing amount of m-TiO2 in polymer matrix, thermal stability was increased due to the formation of highly cross-linked network structures. This can be due to thermally stable TiO2 lattice and presence of triazines containing silane crosslinker which restrict the movement of chains. In fact, m-TiO2 act as a thermal insulator and mass transport barrier to the volatile products generated during decomposition and hence increase the thermal stability of the polymer NCs. The char yield can be useful as a significant factor for appraising the limiting oxygen index (LOI) values. LOI is an amount of the percentage of oxygen that is requisite for burning of the materials. According to the Van Krevelen, LOI is increased with the increasing of char yield [43].
Fig. 1. FT-IR spectra of the aromatic PA (a), surface modified TiO2 (b), NCs with 5% (c), 10% (d) and 15% (e) of modified TiO2.
LOI = 17.5 + 0.4CR, Where CR = char yeild. The values LOI for PA/m-TiO2 NCs calculated in range of 43–46%. So, NCs could be classified as the self-quenching materials. 3.4. Adsorption studies: chromium removal over triazine containing PA and NCs Hexavalent chromium, which is primarily present in the form of chromate (CrO4−) and dichromate (Cr2O7−), possesses significantly higher levels of toxicity than the other valency states, because hexavalent chromium has high water solubility and, consequently, its high mobility poses a greater risk due to its carcinogenic properties [44–46]. In this study, Cr(VI) was selected for the adsorption experiments due to main problems that Cr(VI) can be caused in living organisms, such as heart problems, skin rashes, memory loss, high blood pressure, mental confusion and pancreas damages. The removal of Cr(VI) was performed using s-triazine containing polymer and PA/mTiO2 NCs. The amount of adsorbed heavy metal ions was calculated using the following equation [47,48]:
Q = (C 0 − C)V/m Where Q is the amount of metal ions adsorbed onto unit amount of sorbent (mg g−1); C0 and C are the concentrations (mg mL−1) of metal ions in the primary solution and in the aqueous phase after adsorption, respectively; V is the volume of the aqueous phase (mL); and m is the weight of the sorbent (g). The influence of three important parameters including contact time, pH and concentrations were evaluated in this study.
Fig. 2. XRD patterns of the surface modified TiO2 (a), NCs with 5% (b) and 10% (c) of modified TiO2 and pure aromatic PA (d).
indicates that modification cause easier dispersion during composite processing. From TEM images, it was estimated that the particle size of the resulting NCs are around 20–300 nm.
3.4.1. Effect of pH on the adsorption of Cr(VI) The pH value plays a very important role in the adsorption process. Fig. 8b shows the removal of Cr(VI) as a function of pH at different sorbent. To control optimum pH for the maximum elimination of Cr
3.3.4. Thermal properties One of the thermal analysis techniques is thermogravimetric analysis (TGA) that used to measure weight alteration and thermal decomposition of the sample. The thermal properties of the m-TiO2, 28
Progress in Organic Coatings 110 (2017) 24–34
M. Dinari, A. Haghighi
Fig. 3. FE-SEM (a, b) and TEM (c, d) images of surface modified TiO2 nanoparticles.
3.4.2. Effect of the contact time on the adsorption of Cr(VI) Fig. 8a indicates the effect of contact time on sorption of chromium ions by synthesized PA and PA/m-TiO2 NCs. It is observed that there is little change of sorption rate at 140, 140, 120 and 120 min for pure PA, PA/m-TiO2 NC 5%, PA/m-TiO2 NC 10% and PA/m-TiO2 NC 15%, respectively and leveled off gradually until the Cr(VI) adsorption showed no considerable increasing and removal has finally reached equilibrium. Therefore, the maximum of removal percentages at contact time 140, 140, 120 and 120 min for PA, PA/m-TiO2 NC 5%, PA/m-TiO2 NC 10% and PA/m-TiO2 NC 15%, respectively, is 71.81%, 76.36%, 80.90% and 85.90%.
(VI), the equilibrium adsorption of chromium was measured at different pH levels from 1.0 to 6.0 by setting the initial Cr(VI) concentrations at 10 ppm and the results are summarized in Table 2. The maximum removal percentages of Cr(VI) is obtained at pH = 4.0 for pure PA, PA/ m-TiO2 NC5%, PA/m-TiO2 NC10% and PA/m-TiO2 NC15% at about 76.25%, 80.83%, 83.75% and 86.25%, respectively. Decreasing in Cr (VI) removal at pH < 4, due to competition of Cr(VI) with H+. Also in very acidic pH, the high concentration of H+ ions caused protonation of nitrogen atoms on the surface of adsorbents and cause decrease of interaction with metal ions and surface of adsorbents. Decreasing in Cr (VI) removal at pH > 4, due to precipitation of chromium hydroxide and also due to conversion of Cr(VI)–Cr(III).
29
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Fig. 4. FE-SEM images of pure PA (a, b), NCs with 5% (c, d), 10% (e, f) and 15% (g, h) of modified TiO2.
30
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Fig. 5. TEM images of NCs with 5% of modified TiO2. Table 1 Thermal characterizations of the pure PA and different PA/m-TiO2 NCs. Entry
Material
T5a (°C)
T10a (°C)
Char yield (%)b
LOI (%)
1 2 3 4
Pure PA PA/m-TiO2 NC5% PA/m-TiO2NC10% PA/m-TiO2NC15%
397 453 496 540
437 476 512 580
59 64 69 72
41.7 43.1 45.1 46.3
a Temperature at which 5 and 10% weight loss was recorded by TGA at a heating rate of 20 °C/min in a nitrogen atmosphere. b Percentage weight of material left undecomposed after TGA analysis at maximum temperature 800 °C in a nitrogen atmosphere.
ln(q e − qt) = lnq e − k1t
Fig. 6. TGA thermograms of pristine TiO2 (a) and surface modified TiO2 (b).
That k1 is the rate constant of adsorption (min−1), qe and qt are the amounts of metal ions adsorbed per unit mass of the adsorbent (mg g−1) at equilibrium and time t, respectively, and were calculated according to:
q e = (C i − Ce)V/m qt = (C i − C t )V/m Where Ct (mg L−1) is the metal concentrations at time t. 2) The pseudo-second-order model which is expressed by the following equation:
t 1 1 = + t qt qe k adq 2e Fig. 7. TGA thermograms of neat PA (a), NC with 5% (b), 10% (c) and 15% (d) of modified TiO2.
That kad, is the rate constant of equation (g mg−1 min−1) and it can be calculated from the plots of t/qt versus t. and h = kad qe 2 (mg g −1 min −1) (Fig. 8c).
3.5. The adsorption kinetic studies
3) Elivich equation expression is given as:
To investigate the adsorption kinetics of metals, four kinetic models, pseudo-first-order, pseudo-second-order, Elovich, and intra-particle diffusion model, were used to the data [47,48].
qt =
1) The pseudo-first-order model that showed in below equation:
4) The intra-particle diffusion model can be calculated with below equation (Fig. 8d):
ln(hB) ln(t) + B B
Where β is the desorption constant (mg g−1 min−1).
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Fig. 8. The effect of contact time (a); the effect of pH (b); (Pseudo-second-order kinetics plots c) and the Elivich model (d) for the adsorption of Cr(VI) in the pure PA and NCs of PA with 5%, 10% and 15% of modified TiO2.
according to equation:
Table 2 Adsorption quality and distribution coefficient parameters for Cr(VI) solution (10 mg L−1, pH = 4), on the sorbents. Sample
PA NC 5% NC 10% NC 15%
Metal content solution (mg L−1)
Initial (Ci)
Final (Ce)
10 10 10 10
2.375 1.916 1.625 1.375
Removal efficiency (%)
Metal adsorbed on Sorbent mg g−1))
Kd(mL g−1) × 103
76.25 80.833 83.75 86.25
76.25 80.833 83.75 86.25
0.025 0.032 0.042 0.047
⎡ C − Ce ⎤ kd = ⎢ i ⎥ (V/m) ⎣ Ce ⎦ Where, V is the solution volume (mL). Higher kd values illustrated better sorbents for the elimination of metal. The correlation coefficient (R2) values of the four kinetic models and other related kinetic parameters were calculated and listed in Table 3. On the basis of the values of the correlation coefficient (R2), it was found that the pseudo-second-order kinetic model has the highest matching with the experimental data than the other models for interpretation of adsorption mechanism of Cr(VI) for pure PA, PA/mTiO2 NC5%, PA/m-TiO2 NC10% and PA/m-TiO2 NC15% (Fig. 8).
qt = k intra (t)1/2 + C
3.5.1. Effect of concentrations on the adsorption of Cr(VI) In order to investigate the behavior of adsorption isotherms, 7 tests was done at different initial Cr(VI) concentrations levels (from 5 to 700 ppm) with 0.001 g of adsorbent at the optimum of time and pH (Fig. 9a). To investigate the adsorption isotherm of adsorption, three models, Langmuir adsorption isotherm, Freundlich adsorption isotherm and Dubinin–Radushkevich (D-R) isotherms were used.
−1
Where kintra is the intra-particle diffusion rate constant (mg g min −1/2 ) which can be calculated from the slope between qt vs. t 1/2 and C is a constant. The kinetic model with a higher correlation coefficient R2 was selected as the best model. To estimate of sorption performance of sorbents the distribution coefficient (kd) of metal ions, was calculated Table 3 Comparison of the kinetic parameters for the PA and NCs in Cr (VI) adsorption. Sorbent
PA NC5% NC10% NC15%
qe(exp (mg g)
71.818 76.363 80.909 85.909
Pseudo first order
Elovich kinetic
Intra-particle diffusion
Pseudo second order
qe,cal (mg/g)
Kad (g/mg min) × 10−3
R1 2
β (mg/g min)
R2 2
Kintra (mg/g min)
C
R3 2
K1 × 10−3
R4 2
75.187 79.360 84.033 90.909
1.4 1.5 1.75 1.7
0.9995 0.9996 0.9995 0.9980
0.1324 0.1324 0.1317 0.1463
0.9411 0.9411 0.9231 0.9256
1.546 1.546 1.547 1.401
50.904 55.449 60.138 62.531
0.843 0.843 0.819 0.841
2.7 34.5 1.27 1.84
0.948 0.857 0.950 0.936
32
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Fig. 9. The effect of concentration (a); Langmuir adsorption isotherm (b); Freundlich adsorption isotherm (c); Dubinin–Radushkevich (D-R) isotherms (d) for the adsorption of Cr(VI) in the pure PA and NCs of PA with 5%, 10% and 15% of modified TiO2.
1) Langmuir adsorption isotherm: In this model, there is no interaction among adsorbed molecules and adsorption process happens on homogeneous surfaces. The Langmuir model showed in below equation [49]:
logq e = logkf +
That Kf (the adsorption capacity) and n (intensity of a given adsorbent) are the Freundlich isotherm constant (Fig. 9c). The values of the constants in both models are obtained from the slope and the position (Fig. 9b and c). Table 4 shows the results of the fit and of the constants of both models for four adsorbents including pure PA, PA/m-TiO2 5%, PA/m-TiO2 10% and PA/m-TiO2 15%. The values of n for pure PA, PA/m-TiO2 5%, PA/m-TiO2 10% and PA/mTiO2 15% were 9.26, 9.42, 9.37 and 9.97, respectively. The values between 1 and 10 for n in the adsorption process are favorable [50]. All the correlation coefficients and parameters obtained for the isotherm models from Table 4 reveal that the Langmuir isotherm is the best model to demonstrate the adsorption of Cr(VI) onto all adsorbents.
Ce/qe = 1/KLqmax + Ce/qmax Where Ce (mg/L), qe (mg/g) and qmax (mg/g) signify the equilibrium concentration, the adsorption capacity and the maximum adsorption capacity of the adsorbents in the aqueous solution, respectively. KL is a constant related to binding energy of the sorption system (L/mg) (Fig. 9b). 2) Freundlich adsorption isotherm: This model can be explained the multilayer adsorption of an adsorbate onto a heterogeneous surface of an adsorbent. The linear form of Freundlich isotherm model expression is given as:
3) Dubinin–Radushkevich (D-R) isotherms: For investigated the nature of adsorption, this model is used. The linear form of this model expressed by the following equation:
Table 4 Isotherm parameters for PA and PA/m-TiO2 in Cr(VI) adsorption.
Lnqe = lnqm − βε2
Models
Parameters
PA
NC 5%
NC 10%
NC 15%
Langmuir
qm/mg g−1 KL/L mg−1 R2
86.84 0.1241 0.9996
96.15 0.1139 0.9994
97.08 0.1307 0.9997
98.03 0.1992 0.9999
Freundlich
KF/mg g−1 N R2
48.82 9.26 0.9228
50.35 9.49 0.9180
51.27 9.37 0.9085
55.38 9.98 0.8147
89.09
89.10
91.03
95.87
4.7
4.6
4.7
4.8
1.041 0.9232
1.052 0.9356
1.041 0.9277
1.030 0.9910
DubbininRadushkevich
qm/mg g
−1
-BD × 10−7/ mol2 kj−2 ED/kJ mol−1 R2
1 logCe n
Where β is the activity coefficient related to mean sorption energy (mol2/kJ2), and ε is the Polanyi potential, that can be calculated from bellow equation:
ε = RTln(1 + 1/Ce) where R and T are the ideal gas constant (8.3145 J/mol K) and absolute temperature (K), respectively (Fig. 9d). Ea is the free energy change of adsorption (kJ/mol), which required transferring 1 mol of ions from solution to the adsorbent surface, that it can be calculated from bellow equation [51]:
Ea = 1/(−2β)1/2 33
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In this model, if Ea < 8 kJ/mol, the mechanism of adsorption is Physical adsorption, if 8 > Ea < 20 kJ/mol adsorption is dominated by chemical ion exchange and if Ea > 20 kJ/mol, mechanism of adsorption is chemical adsorption [52]. The E values 1.041, 1.052, 1.041 and 1.030 for PA, PA/m-TiO2 5%, PA/m-TiO2 10% and PA/mTiO2 15%, respectively. This suggests that metal adsorption onto this adsorbents is physical adsorption. 4. Conclusions In this study, new PA containing s-triazine ring was successfully synthesized via direct polycondensation of 4,4-(6-morpholine-1,3,5triazine-2,4-diyl)bis(azanediyl)dibenzoic acid and 1,3-phenylenediamine in the presence of molten TBAB and TPP in good yield and moderated inherent viscosity. Although, dispersion of nanoparticles is a key property to obtain organo-inorgano hybrid composite, so by effective surface modification of TiO2 nanoparticles using 1,3,5-triazine containing silane coupling agent, they were uniformly dispersed weight different percentages into PA matrix. Morphology study of resulting PA/m-TiO2 NCs showed well-dispersed m-TiO2 nanoparticles in the polymer matrix by FE-SEM and TEM analyses. TGA data indicated that thermal stability of the NCs have enhanced with increasing m-TiO2 nanoparticles content. Then, this polymer and NCs were used for Cr(VI) ions removal by optimizing the parameters including pH, contact time and concentration. The maximum uptakes of Cr(VI) at pH = 4.0 is about 76.25%, 80.83%, 83.75% and 86.25% for pure PA, PA/m-TiO2 NC5%, PA/m-TiO2 NC10% and PA/m-TiO2 NC15%, respectively. With due attention to, Ea obtained from Dubinin–Radushkevich isotherm, adsorption mechanism for these adsorbents is physical adsorption. Also, the four adsorption kinetics including pseudo-first-order, pseudo-second-order, Elovich, and intra-particle diffusion model, were discussed briefly to understand the adsorption procedure. Due to the proximity of R2 values of the investigated four kinetic models, pseudo second-order model could be consider as a suitable model for interpretation of adsorption mechanism of Cr(VI). Therefore, the polymer and it’s NCs could be considered as a good adsorbent for removal of Cr(VI) from aqueous solutions due to the presence of different functional group such as nitrogen atoms on the triazine ring and NH as a good platform for the absorption process. Notes The authors declare no competing financial interest. Acknowledgements We wish to express our gratitude to the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, for partial financial support. References [1] M.A. Hashim, S. Mukhopadhyay, J.N. Sahu, B. Sengupta, J. Environ. Manage. 92 (2011) 2355–2388. [2] Z. Lin, Y. Zhang, Y. Chen, H. Qian, Chem. Eng. J. 200 (2012) 104–112.
34
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Surface modification of TiO2 nanoparticle by three dimensional silane coupling agent and preparation of polyamide/modified- TiO2 nanocomposites for removal of Cr (VI) from aqueous solutions
MARK
⁎
Mohammad Dinari , Atefeh Haghighi Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: s-Triazine based polyamides Surface modified TiO2 Thermal properties Cr(VI) ions removal
In the present investigation, the goal was the preparation, characterization and surface morphology of polyamide/modified-TiO2 nanocomposites (PA/m-TiO2 NCs). At first, a soluble aromatic PA was synthesized by direct polycondensation reaction of the synthesized diacid and 1,3-phenylenediamine in molten tetrabutylammonium bromide (TBAB) with good yield and moderate inherent viscosity. The structure and purity of these compounds were confirmed by different techniques. For the improvement of TiO2 nanoparticles dispersion and enhancing interactions between nanoparticles and polymeric matrix, the surface of TiO2 was successfully modified by 1,3,5-triazine based silane coupling agent. The functionalized nanoparticles were then inscribed in 0, 5, 10 and 15 wt.% into polymer matrix. The resulting hybrids were characterized by Fourier transforminfrared spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The TGA of the obtained PA/ m-TiO2 NCs proves the enhancement in the thermal stability with an increase in the percentage of titania nanoparticles. It was further investigated for Cr(VI) ion removal by optimizing the parameters including pH, contact time and concentration. Also, adsorption kinetics and adsorption isotherms of this polymer were investigated.
1. Introduction Increase in the use of heavy metals over the past few decades resulted in increasing awareness of the potentially hazardous effects of elevated levels of these materials in the environment and makes a great concern about the pollution by them [1]. Heavy metals pollution is a serious problem threatening human health and other organisms due to their high level toxicity, carcinogenicity and non-biodegradability, even though they are in low concentration [2–4]. Nowadays, a number of promising processes are used for elimination of heavy metal ions and dyes from wastewaters [5]. Polymer nanocomposites (NCs) adsorbents have been used for this purpose owing to their chemical functionality, dimension stability, adjustable surface area, easiness of handling and regeneration [6–8]. In the last years, there has been a strong emphasis on the development of polymeric NCs, where at least one of the dimensions of the filler material is of the order of a nanometer [9,10]. Polymer NCs incorporate the remarkable features of both nanoparticles (NPs) and polymers: the unique physical and chemical properties resulting from the large surface area to volume ratios, the high interfacial reactivity of nanofillers, and outstanding mechanical ⁎
Corresponding author. E-mail addresses: [email protected], [email protected] (M. Dinari).
http://dx.doi.org/10.1016/j.porgcoat.2017.04.044 Received 10 December 2016; Accepted 25 April 2017 0300-9440/ © 2017 Elsevier B.V. All rights reserved.
properties and compatibility owing to their polymer matrix, being also amenable to regeneration and reuse [11–14]. In the last decades, several NCs have been fabricated for the adsorptive removal of heavy metals from water and wastewater [15,16]. Overall, various effects contribute determining the whole efficacy of NCs action. Composite matrix can be aromatic polyamides (PA). Aromatic PAs are well known as high-performance polymers because of their combination of excellent thermal, mechanical, and chemical properties that make them useful in applications for advanced technologies [17–19]. They are used neat in fibers, films and may contain fillers in engineering plastics applications. Most frequently, much effort has been paid to the development of aromatic heterocyclic PAs for use as advanced composite matrix, structural adhesives or coatings in hightemperature applications [20]. However, the main trouble of aramids is associated with their insolubility owing to their rigid backbones and strong interchain interactions, leading to processing difficulties. Therefore, many efforts have been made to increase the solubility and processability of aramids through structural modification of their monomers [21–25]. One of the common approaches to increasing solubility without much compromising their thermal and mechanical
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2914 (w), 1678 (m), 1576 (w), 1439 (w), 1271 (m), 1158 (w) and 576 (w).
stability is the use of monomers with bulky packing-disruptive moieties [26–28]. The incorporation of bulky groups increases the interchain spacing and reduces the packing efficiency thereby increasing the intrinsic microporosity [28]. Recently, blending of the polymers with inorganic nanomaterials has occupied an impressive platform of research because of their simplicity, stable performance and mild operating conditions [29]. The addition of NPs to the polymer matrix has been the most usually accepted technique for manufacturing polymer NCs and the hybrids prepared with uniform dispersion of NPs can offer good mechanical strength, antifouling or self cleaning, bactericidal and also to some extent, photocatalytic properties [30,31]. Among many nano-materials, TiO2 NPs gains its importance because of its photocatalytic activity, self cleaning property, availability, ease of preparation and economic feasibility [32,33]. Since TiO2 directs to the formation of agglomerates at higher concentration due to its high surface energy which offers a driving force for the formation of particle bonding, achieving a hybrid with excellent NPs dispersion is a challenge [34]. Generally, this problem would be resolved either by chemical or mechanical modification to TiO2 prior to NC preparation. The most common method used attaches a suitable organic group on the surface atoms using silane coupling agents [35]. This type of surface modification stabilizes the NPs against the agglomeration and also makes them compatible with the other phase [36]. The suitable surface modification of NPs, not only leads to enhanced dispersion and compatibility in polymer matrix, but also undergoes chemical or physical interactions with the polymer matrix [37]. Here, we introduce a new macromolecular construction, based on the nucleophilic aromatic substitution chemistry of cyanuric chloride for triazine-based aromatic PA. The thermally stable PA was produced by direct polymerization reaction of 4,4′-((6-morpholino-1,3,5-triazine2,4-diyl)bis(azanediyl))dibenzoic acid and 1,3-phenylendiamine in molten tetrabutylammonium bromide (TBAB)/triphenyl phosphite (TPP) system. Then, the surface of the TiO2 NPs was modified with 1,3,5-triazine core silane coupling agent. The functionalized TiO2 NPs were then inscribed in 0, 5 and 10 wt.% into PA matrix to formed different polymer NCs. Then, the application of this novel trazine-based PA and NC materials for adsorption of Cr(IV) from aqueous solution was examined. The contact time as a significant parameter on the removal of Cr(IV) ions and the adsorption mechanism was investigated.
2.3. Modification of TiO2 with cyanuric chloride and APTES (m-TiO2) For surface modification of TiO2 NPs, at first, a three dimensional silane coupling agent was synthesized as follow: 1.0 g (5.42 mmol) of cyanuric chloride was dissolved in 10 mL of dry toluene. Then, an excess amount of APTES (27.10 mmol) was added drop wise to the cyanuric chloride solution in toluene and it was refluxed for 6 h under nitrogen atmosphere. N,N-diisopropyleethylamine was added drop wise to the reaction mixture to neutralize the formed HCl. The product mixture separated in two layers: the higher layer was removed to emit solvent and un-reacted materials, and lower layer was distilled under vacuum to formed 1,3,5-triazine core silane coupling agent as a product for surface modification of TiO2 NPs as follow: 0.50 g of the dried TiO2 was dispersed in 10 mL of toluene and it was sonicated for 30 min. Appropriated amount of 1,3,5-triazine core silane coupling agent (with respect to 30% wt. of TiO2 NPs) was added to the mixture under ultrasonic irradiation. Then the reaction mixture was refluxed at 110 °C with stirring under nitrogen atmosphere for 24 h. The modified TiO2 (m-TiO2) NPs were collected by centrifugation of the reaction mixture at 5000 rpm and washed twice with toluene and ethanol to completely remove the residual silane coupling agent. It was dried in vacuum at 60 °C for 8 h. 2.4. Synthesis of PA/m-TiO2 NCs To obtain homogeneous mixed of PA/m-TiO2 composites with different amounts of m-TiO2 NPs (0, 5, 10 and 15 wt%), a two-step process was used. At first, two solutions were synthesized: 0.1 g of polymer was dissolved in 2 mL DMF and m-TiO2 was separately dispersed in 2 mL DMF solution with stirring for 24 h at 25 °C. Then, the solutions were mixed to reach the preferred weight percentages of m-TiO2 from 5 to 15 wt.%. The PA/m-TiO2 solutions were stirred for 24 h at 25 °C and then it was sonicated for 2 h in ice bath with frequency 2.259 × 104 Hz and power 100 W. To remove the solvent, obtained PA/m-TiO2 solutions were poured into glass petri dishes and after 3 day at ambient temperature, PA/m-TiO2 NCs were prepared. 2.5. Characterization techniques
2. Experimental Fourier transform-infrared (FT-IR) spectra of the samples were recorded at room temperature in the range of 4000–400 cm−1 at a resolution of 4 cm−1, on Jasco-680 (Japan) spectrophotometer. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance DRX 500 and 125 MHz, by using solutions in deuterated dimethylsulfoxide (DMSO-d6). The Proton resonances were designated as singlet (s) and multiplet (m).X-ray diffraction (XRD) patterns were recorded using CuKα radiation on a Bruker (Germany), D8Avance, diffractometer operating at current of 100 mA and a voltage of 45 kV. The diffractograms were measured for 2θ, in the range of 10–80°, using CuKα incident beam (λ = 1.51418 Å). Carbon, hydrogen and nitrogen content of the compounds were determined by pyrolysis method by Vario EL elemental analyzer. Thermal stability was measured with a thermogravimetry analysis (TGA) (TA instrument Co.) at a heating rate of 10 °C min−1 from room temperature to 800 °C under a continuous flow of nitrogen. Inherent viscosities were measured by a standard procedure using a Cannon-Fenske routine viscometer (Germany) at the concentration of 0.5 g/dL at 25 °C.
2.1. Materials All materials and solvents were obtained from Merck Chemical Co and Aldrich Chemical CO. Cyanuric chloride, tetrabutylammonium bromide (TBAB), triphenyl phosphite (TPP) and 1,3-phenylene diamine were used as conventional. TiO2 nanoparticles with average particle sized of 25–30 nm was purchased from nanosabz Co. (Tehran, Iran) and it was dried at 500 °C for about 5 h to ensure complete drying, before using. The coupling agent; 3-aminopropyltriethoxysilane (APTES) obtained from Merck Chemical Co. 2.2. Synthesis of aromatic polyamide with triazine moiety A mixture of 0.100 g (0.458 mmol) of synthesized diacid [38] and 0.590 g (1.832 mmol) of TBAB was ground until a powder was formed and then it was moved into a 25 mL round-bottom flask. 0.049 g (0.458 mmol) of 1,3-phenylene diamine was added to the mixture and it was heated until homogeneous solution was formed. Then, 0.436 g (1.832 mmol) of TPP was added. After that, the solution was stirred for 16 h at 130 °C and the viscous solution was precipitated in 30 mL of methanol. The light yellow solid was filtered off and dried to give 0.146 g (98%) of aromatic polymer. ▓▓▓▓FT-IR peaks (KBr, cm−1): 3423 (br), 3082 (w), 2985 (m),
2.6. Adsorption studies: Cr(VI) removal over PA and PA/m-TiO2 NCs The important parameters on the adsorption of Cr(VI) from aqueous solutions are pH, contact time and concentration. In this study, the adsorption behavior of Cr(VI) from aqueous solution by the aromatic 25
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the surface properties of TiO2 NPs. First, a 1,3,5-triazine core silane coupling agent was synthesized. Then, the surface of TiO2 NPs was modified with the above prepared coupling agent. The presence of siloxane end groups on the synthesized cross-linker helps in the effective modification TiO2 NPs and the highly polar triazine group improves the dispersibility of the modified nanoparticles in organic solvents [26,42]. After the modification of TiO2 NPs, the dispersity of NPs could be greatly improved in the polymer matrix and also this phenomenon facilitates its interactions with desired organic polymer. This silane functionalized TiO2 NPs were finally incorporated in different weight percentages such as 0, 5, 10 and 15% into the PA matrix with similar triazine segment (Scheme 2).
PA and PA/m-TiO2 NCs were investigated and their kinetic and isotherm was assessed. A stock solution of 1000 mg L−1 Cr(VI) was prepared by dissolving an weighed amount of analytical grade K2Cr2O7 (> 99.0% purity) in deionized water, the other solutions were prepared by dilution. To estimate the effect of pH, 6 tests was done at different pH levels (from 1 to 6) with 0.001 g of PA and PA/m-TiO2 NCs 5%, 10% and 15% by setting the initial Cr(VI) concentrations at 10 ppm in plastic container. The containers were placed in a shaker at a speed of 180 rpm and the contact time was 300 min. The adsorbents were filtered through a 0.45 μm filter membrane. The residual Cr(VI) concentrations were detected by flame atomic absorption spectrometer. pH = 4 is the optimum pH for the maximum removal of Cr(VI) by PA and PA/m-TiO2 NCs 5%, 10% and 15%. The solution pH was adjusted by 0.1 M NaOH and 0.1 M HCl. To determine the effect of contact time, 12 tests was done at different times (from 20 to 240 min) at optimum pH with 0.001 g of adsorbents and by setting the initial Cr(VI) concentrations at 10 ppm in plastic container. After the shaking of solutions, the adsorbents were filtered and the residual ions concentrations were detected. In order to determine the effect of concentration, 7 tests were done at different concentrations (from 5 to 700 ppm) at optimum pH and time with 0.001 g of adsorbents. The removal efficiency was calculated as follow:
%Removal =
3.3. Characterization techniques 3.3.1. FT-IR study Fig. 1 displays the FT-IR spectra of pure PA, m-TiO2 as well as NC with 0, 5, 10 and 15 wt.% of m-TiO2. For neat PA, a peak at 3420 cm−1 related to amidic NH groups in polymer backbone and strong peaks at 1680 cm−1 related to carbonyl of the amide groups. The absorption bands appeared around 3013–3100 and 2850–2930 cm−1 are related to the corresponding aromatic and aliphatic C–H stretching vibration, respectively (Fig. 1a). For m-TiO2 NPs, the broad peaks at 3300–3500 cm−1 correspond to stretching motions of the hydroxyl and NH groups. The bands at 2870–2928 cm−1 can be ascribed to the C–H symmetrical and asymmetrical stretching absorptions of the CH2 group of the APTES. The characteristic peak at 1610 cm−1 was correspond to C]N stretching of triazine ring. The stretching vibration of Si–O–C group was appeared at 1084 cm−1. The broad band observed in the region 450–800 cm−1 corresponds to Ti–O stretching peak (Fig. 1b). These results showed that the triazine-based silane coupling agent was grafted onto surface of TiO2 by chemical bond. To display the interactions between m-TiO2 NPs and PA chains, the FT-IR spectra of PA/m-TiO2 hybrid are shown in Fig. 1. Compared with pure polymer (Fig. 1a), a new broad absorption band around 450–800 cm−1 can be found in the FT-IR spectra of PA/m-TiO2 NCs, which increases with increasing TiO2 contents and can be ascribed to the vibration of Ti–O–Si groups (Fig. 1c–e). These results confirm the presence of m-TiO2 NPs in the PA matrix.
Ci − Ce Ci
That Ci and Ce are the initial and final (equilibrium) concentrations of the Cr(VI) ions in aqueous solution (mg L−1). 3. Results and discussion 3.1. Synthesis of organo-soluble PA Because of the rapid emergence of ionic liquids (ILs) as green solvents and replacement of a volatile and toxic organic solvent in the polymerization with a nonvolatile solvent, in this study, s-triazine containing PA was synthesized in green route by using molten TBAB as an IL media. This polymer was prepared by the direct polymerization reaction of s-triazine containing diacid monomer with 1,3-phenylenediamine as show Scheme 1. The inherent viscosity of the PA under optimized condensations was 0.35 dL/g and yield was 98%. The chemical structure and purity of this compound was also confirmed with elemental analysis, FT-IR and 1H NMR spectroscopy as reported in our recently published paper [38].
3.3.2. X-ray diffraction Fig. 2 displays the XRD patterns of s-triazine containing PA, m-TiO2 as well as NCs of PA with 5% and 10% of m-TiO2 NPs. For pure PA (Fig. 2d), it could be observed that except one crystalline peak, there is a lack of any diffraction peak in the range of 2θ angle, and this observation suggested the presence of a little proportion of crystalline phase compared to an amorphous one. For m-TiO2 (Fig. 2a), the presence of anatase and rutile phase was confirmed by the peaks appeared at 101, 110, 004, 200, 105, 211, 204, 220, 301, which approved the crystalline form of this compound [41]. The XRD patterns of NC show characteristic peaks of m-TiO2 and PA, suggesting that the surface modification and preparation process does not affect the morphology of NPs (Fig. 2b and c). The average size of the TiO2 is calculated to be 25–35 nm according to the Debye-Scherrer formula.
3.2. Surface modified TiO2 NPs and preparation of PA/m-TiO2 NPs The uniform distribution of modified nanoparticles in polymer matrices can lead to physical, morphological, and mechanical property enhancements in the resulting NCs [9,19]. Due to exist of OH groups on the surface of NPs, it is easy for TiO2 NPs to greatly agglomerate in polymer matrix. Silane coupling agents are one of the generally used materials for enhancing the dispersion stability of the NPs in the polymers [39]. The silane coupling agents lie onto metal oxide surface by the hydrolysis and condensation reactions of silane agents: R–Si–OR′ + H2O → R–Si–OH + R′OH
(1)
R–Si–OH → R–Si–O–Si–R + H2O
(2)
M–OH + R–Si–OH → M–O–Si–R + H2O
(3)
3.3.3. Electron microscope characterization of m-TiO2 and PA/m-TiO2 NCs In order to examine the microstructures and nanofiller distribution within the NCs, FE-SEM and TEM analysis were conducted. Fig. 3 shows the FE-SEM and TEM images of the m-TiO2 with two different magnifications. According to these images, m-TiO2 NPs have a nanoscale size with spherical form-like. The sizes of particle were in the range of 25–35 nm. Fig. 4 shows the FE-SEM micrographs of pure PA and PA/m-TiO2 NCs. It is natural to believe that a homogenous dispersion of NPs in material will bring best result. The FE-SEM images of NCs (Fig. 4c–f) indicate that, m-TiO2 were dispersed in the PA matrix but, as it can be
Where R and R′ are functional groups and methyl (or ethyl), respectively, and M stands for metal oxide [40]. When silane coupling agent dissolved in the water, ethoxy groups were hydrolyzed and Si–O–Si bonds formed. Then, by reaction with hydroxyl groups of metal NPs, the alkyl groups of coupling agent were placed on the surface of NPs [41]. In this study, APTES and cyanuric chloride were used to modify 26
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Scheme 1. Surface modification of TiO2 nanoparticles by 1,3,5-triazine containing silane coupling agent.
Scheme 2. Preparation of PA/m-TiO2 nanocomposites.
dispersion of modified NPs in the polymer matrix. Fig. 5 shows the TEM images of the PA/m-TiO2 NCs 5 wt% at different magnifications. In these images, no aggregation is visible between the m-TiO2 and relatively satisfactory dispersion of m-TiO2 can be observed. This
seen in PA/m-TiO2 NCs 10 wt% (Fig. 4e and f) the bright spots on the background images seems to be agglomerates of m-TiO2 NPs which increased with increasing the content of the filler in the polymer matrix. TEM analysis was used to investigate the shape, size and also 27
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PA/m-TiO2 NCs and pure PA were studied by TGA. Fig. 6 shows the thermal degradation behavior of the pure TiO2 and s-triazine modified TiO2 under nitrogen atmosphere. It is clear from Fig. 6 that the thermal stability of the surface modified NPs is lesser than the untreated TiO2 NPs. The unmodified TiO2 NPs had a less weight loss (about 2%) in the whole temperature ranging 0–800 °C, which can be attributed to the removal of water; and partial dehydroxylation of the TiO2 NPs. The TGA curve of m-TiO2 has a clear two-step decomposition route (Fig. 6b). The first step, mass loss happened in the temperature range 150–250 °C, which might be ascribed to the hydroxide group on the surface of the TiO2 NPs. The second decomposition happened in the temperature range 450–550 °C, which could be related to the thermal decomposition of organic groups such as 1,3,5-triazines and other organic chains present on the surface. TGA curves of the PA and PA/m-TiO2 NCs are shown in Fig. 7. The TGA curves reveal the weight loss of the material as it is heated. The gradual weight loss of the samples was due to the evaporation of trapped solvent and decomposition of the polymer. TGA data for thermal stability of the PA/m-TiO2 NCs was studied based on 5 and 10% weight loss (T5 and T10) and the residue at 800 °C (char yield). As observed from Fig. 7 and Table 1, with increasing amount of m-TiO2 in polymer matrix, thermal stability was increased due to the formation of highly cross-linked network structures. This can be due to thermally stable TiO2 lattice and presence of triazines containing silane crosslinker which restrict the movement of chains. In fact, m-TiO2 act as a thermal insulator and mass transport barrier to the volatile products generated during decomposition and hence increase the thermal stability of the polymer NCs. The char yield can be useful as a significant factor for appraising the limiting oxygen index (LOI) values. LOI is an amount of the percentage of oxygen that is requisite for burning of the materials. According to the Van Krevelen, LOI is increased with the increasing of char yield [43].
Fig. 1. FT-IR spectra of the aromatic PA (a), surface modified TiO2 (b), NCs with 5% (c), 10% (d) and 15% (e) of modified TiO2.
LOI = 17.5 + 0.4CR, Where CR = char yeild. The values LOI for PA/m-TiO2 NCs calculated in range of 43–46%. So, NCs could be classified as the self-quenching materials. 3.4. Adsorption studies: chromium removal over triazine containing PA and NCs Hexavalent chromium, which is primarily present in the form of chromate (CrO4−) and dichromate (Cr2O7−), possesses significantly higher levels of toxicity than the other valency states, because hexavalent chromium has high water solubility and, consequently, its high mobility poses a greater risk due to its carcinogenic properties [44–46]. In this study, Cr(VI) was selected for the adsorption experiments due to main problems that Cr(VI) can be caused in living organisms, such as heart problems, skin rashes, memory loss, high blood pressure, mental confusion and pancreas damages. The removal of Cr(VI) was performed using s-triazine containing polymer and PA/mTiO2 NCs. The amount of adsorbed heavy metal ions was calculated using the following equation [47,48]:
Q = (C 0 − C)V/m Where Q is the amount of metal ions adsorbed onto unit amount of sorbent (mg g−1); C0 and C are the concentrations (mg mL−1) of metal ions in the primary solution and in the aqueous phase after adsorption, respectively; V is the volume of the aqueous phase (mL); and m is the weight of the sorbent (g). The influence of three important parameters including contact time, pH and concentrations were evaluated in this study.
Fig. 2. XRD patterns of the surface modified TiO2 (a), NCs with 5% (b) and 10% (c) of modified TiO2 and pure aromatic PA (d).
indicates that modification cause easier dispersion during composite processing. From TEM images, it was estimated that the particle size of the resulting NCs are around 20–300 nm.
3.4.1. Effect of pH on the adsorption of Cr(VI) The pH value plays a very important role in the adsorption process. Fig. 8b shows the removal of Cr(VI) as a function of pH at different sorbent. To control optimum pH for the maximum elimination of Cr
3.3.4. Thermal properties One of the thermal analysis techniques is thermogravimetric analysis (TGA) that used to measure weight alteration and thermal decomposition of the sample. The thermal properties of the m-TiO2, 28
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Fig. 3. FE-SEM (a, b) and TEM (c, d) images of surface modified TiO2 nanoparticles.
3.4.2. Effect of the contact time on the adsorption of Cr(VI) Fig. 8a indicates the effect of contact time on sorption of chromium ions by synthesized PA and PA/m-TiO2 NCs. It is observed that there is little change of sorption rate at 140, 140, 120 and 120 min for pure PA, PA/m-TiO2 NC 5%, PA/m-TiO2 NC 10% and PA/m-TiO2 NC 15%, respectively and leveled off gradually until the Cr(VI) adsorption showed no considerable increasing and removal has finally reached equilibrium. Therefore, the maximum of removal percentages at contact time 140, 140, 120 and 120 min for PA, PA/m-TiO2 NC 5%, PA/m-TiO2 NC 10% and PA/m-TiO2 NC 15%, respectively, is 71.81%, 76.36%, 80.90% and 85.90%.
(VI), the equilibrium adsorption of chromium was measured at different pH levels from 1.0 to 6.0 by setting the initial Cr(VI) concentrations at 10 ppm and the results are summarized in Table 2. The maximum removal percentages of Cr(VI) is obtained at pH = 4.0 for pure PA, PA/ m-TiO2 NC5%, PA/m-TiO2 NC10% and PA/m-TiO2 NC15% at about 76.25%, 80.83%, 83.75% and 86.25%, respectively. Decreasing in Cr (VI) removal at pH < 4, due to competition of Cr(VI) with H+. Also in very acidic pH, the high concentration of H+ ions caused protonation of nitrogen atoms on the surface of adsorbents and cause decrease of interaction with metal ions and surface of adsorbents. Decreasing in Cr (VI) removal at pH > 4, due to precipitation of chromium hydroxide and also due to conversion of Cr(VI)–Cr(III).
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Fig. 4. FE-SEM images of pure PA (a, b), NCs with 5% (c, d), 10% (e, f) and 15% (g, h) of modified TiO2.
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Fig. 5. TEM images of NCs with 5% of modified TiO2. Table 1 Thermal characterizations of the pure PA and different PA/m-TiO2 NCs. Entry
Material
T5a (°C)
T10a (°C)
Char yield (%)b
LOI (%)
1 2 3 4
Pure PA PA/m-TiO2 NC5% PA/m-TiO2NC10% PA/m-TiO2NC15%
397 453 496 540
437 476 512 580
59 64 69 72
41.7 43.1 45.1 46.3
a Temperature at which 5 and 10% weight loss was recorded by TGA at a heating rate of 20 °C/min in a nitrogen atmosphere. b Percentage weight of material left undecomposed after TGA analysis at maximum temperature 800 °C in a nitrogen atmosphere.
ln(q e − qt) = lnq e − k1t
Fig. 6. TGA thermograms of pristine TiO2 (a) and surface modified TiO2 (b).
That k1 is the rate constant of adsorption (min−1), qe and qt are the amounts of metal ions adsorbed per unit mass of the adsorbent (mg g−1) at equilibrium and time t, respectively, and were calculated according to:
q e = (C i − Ce)V/m qt = (C i − C t )V/m Where Ct (mg L−1) is the metal concentrations at time t. 2) The pseudo-second-order model which is expressed by the following equation:
t 1 1 = + t qt qe k adq 2e Fig. 7. TGA thermograms of neat PA (a), NC with 5% (b), 10% (c) and 15% (d) of modified TiO2.
That kad, is the rate constant of equation (g mg−1 min−1) and it can be calculated from the plots of t/qt versus t. and h = kad qe 2 (mg g −1 min −1) (Fig. 8c).
3.5. The adsorption kinetic studies
3) Elivich equation expression is given as:
To investigate the adsorption kinetics of metals, four kinetic models, pseudo-first-order, pseudo-second-order, Elovich, and intra-particle diffusion model, were used to the data [47,48].
qt =
1) The pseudo-first-order model that showed in below equation:
4) The intra-particle diffusion model can be calculated with below equation (Fig. 8d):
ln(hB) ln(t) + B B
Where β is the desorption constant (mg g−1 min−1).
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Fig. 8. The effect of contact time (a); the effect of pH (b); (Pseudo-second-order kinetics plots c) and the Elivich model (d) for the adsorption of Cr(VI) in the pure PA and NCs of PA with 5%, 10% and 15% of modified TiO2.
according to equation:
Table 2 Adsorption quality and distribution coefficient parameters for Cr(VI) solution (10 mg L−1, pH = 4), on the sorbents. Sample
PA NC 5% NC 10% NC 15%
Metal content solution (mg L−1)
Initial (Ci)
Final (Ce)
10 10 10 10
2.375 1.916 1.625 1.375
Removal efficiency (%)
Metal adsorbed on Sorbent mg g−1))
Kd(mL g−1) × 103
76.25 80.833 83.75 86.25
76.25 80.833 83.75 86.25
0.025 0.032 0.042 0.047
⎡ C − Ce ⎤ kd = ⎢ i ⎥ (V/m) ⎣ Ce ⎦ Where, V is the solution volume (mL). Higher kd values illustrated better sorbents for the elimination of metal. The correlation coefficient (R2) values of the four kinetic models and other related kinetic parameters were calculated and listed in Table 3. On the basis of the values of the correlation coefficient (R2), it was found that the pseudo-second-order kinetic model has the highest matching with the experimental data than the other models for interpretation of adsorption mechanism of Cr(VI) for pure PA, PA/mTiO2 NC5%, PA/m-TiO2 NC10% and PA/m-TiO2 NC15% (Fig. 8).
qt = k intra (t)1/2 + C
3.5.1. Effect of concentrations on the adsorption of Cr(VI) In order to investigate the behavior of adsorption isotherms, 7 tests was done at different initial Cr(VI) concentrations levels (from 5 to 700 ppm) with 0.001 g of adsorbent at the optimum of time and pH (Fig. 9a). To investigate the adsorption isotherm of adsorption, three models, Langmuir adsorption isotherm, Freundlich adsorption isotherm and Dubinin–Radushkevich (D-R) isotherms were used.
−1
Where kintra is the intra-particle diffusion rate constant (mg g min −1/2 ) which can be calculated from the slope between qt vs. t 1/2 and C is a constant. The kinetic model with a higher correlation coefficient R2 was selected as the best model. To estimate of sorption performance of sorbents the distribution coefficient (kd) of metal ions, was calculated Table 3 Comparison of the kinetic parameters for the PA and NCs in Cr (VI) adsorption. Sorbent
PA NC5% NC10% NC15%
qe(exp (mg g)
71.818 76.363 80.909 85.909
Pseudo first order
Elovich kinetic
Intra-particle diffusion
Pseudo second order
qe,cal (mg/g)
Kad (g/mg min) × 10−3
R1 2
β (mg/g min)
R2 2
Kintra (mg/g min)
C
R3 2
K1 × 10−3
R4 2
75.187 79.360 84.033 90.909
1.4 1.5 1.75 1.7
0.9995 0.9996 0.9995 0.9980
0.1324 0.1324 0.1317 0.1463
0.9411 0.9411 0.9231 0.9256
1.546 1.546 1.547 1.401
50.904 55.449 60.138 62.531
0.843 0.843 0.819 0.841
2.7 34.5 1.27 1.84
0.948 0.857 0.950 0.936
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Fig. 9. The effect of concentration (a); Langmuir adsorption isotherm (b); Freundlich adsorption isotherm (c); Dubinin–Radushkevich (D-R) isotherms (d) for the adsorption of Cr(VI) in the pure PA and NCs of PA with 5%, 10% and 15% of modified TiO2.
1) Langmuir adsorption isotherm: In this model, there is no interaction among adsorbed molecules and adsorption process happens on homogeneous surfaces. The Langmuir model showed in below equation [49]:
logq e = logkf +
That Kf (the adsorption capacity) and n (intensity of a given adsorbent) are the Freundlich isotherm constant (Fig. 9c). The values of the constants in both models are obtained from the slope and the position (Fig. 9b and c). Table 4 shows the results of the fit and of the constants of both models for four adsorbents including pure PA, PA/m-TiO2 5%, PA/m-TiO2 10% and PA/m-TiO2 15%. The values of n for pure PA, PA/m-TiO2 5%, PA/m-TiO2 10% and PA/mTiO2 15% were 9.26, 9.42, 9.37 and 9.97, respectively. The values between 1 and 10 for n in the adsorption process are favorable [50]. All the correlation coefficients and parameters obtained for the isotherm models from Table 4 reveal that the Langmuir isotherm is the best model to demonstrate the adsorption of Cr(VI) onto all adsorbents.
Ce/qe = 1/KLqmax + Ce/qmax Where Ce (mg/L), qe (mg/g) and qmax (mg/g) signify the equilibrium concentration, the adsorption capacity and the maximum adsorption capacity of the adsorbents in the aqueous solution, respectively. KL is a constant related to binding energy of the sorption system (L/mg) (Fig. 9b). 2) Freundlich adsorption isotherm: This model can be explained the multilayer adsorption of an adsorbate onto a heterogeneous surface of an adsorbent. The linear form of Freundlich isotherm model expression is given as:
3) Dubinin–Radushkevich (D-R) isotherms: For investigated the nature of adsorption, this model is used. The linear form of this model expressed by the following equation:
Table 4 Isotherm parameters for PA and PA/m-TiO2 in Cr(VI) adsorption.
Lnqe = lnqm − βε2
Models
Parameters
PA
NC 5%
NC 10%
NC 15%
Langmuir
qm/mg g−1 KL/L mg−1 R2
86.84 0.1241 0.9996
96.15 0.1139 0.9994
97.08 0.1307 0.9997
98.03 0.1992 0.9999
Freundlich
KF/mg g−1 N R2
48.82 9.26 0.9228
50.35 9.49 0.9180
51.27 9.37 0.9085
55.38 9.98 0.8147
89.09
89.10
91.03
95.87
4.7
4.6
4.7
4.8
1.041 0.9232
1.052 0.9356
1.041 0.9277
1.030 0.9910
DubbininRadushkevich
qm/mg g
−1
-BD × 10−7/ mol2 kj−2 ED/kJ mol−1 R2
1 logCe n
Where β is the activity coefficient related to mean sorption energy (mol2/kJ2), and ε is the Polanyi potential, that can be calculated from bellow equation:
ε = RTln(1 + 1/Ce) where R and T are the ideal gas constant (8.3145 J/mol K) and absolute temperature (K), respectively (Fig. 9d). Ea is the free energy change of adsorption (kJ/mol), which required transferring 1 mol of ions from solution to the adsorbent surface, that it can be calculated from bellow equation [51]:
Ea = 1/(−2β)1/2 33
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In this model, if Ea < 8 kJ/mol, the mechanism of adsorption is Physical adsorption, if 8 > Ea < 20 kJ/mol adsorption is dominated by chemical ion exchange and if Ea > 20 kJ/mol, mechanism of adsorption is chemical adsorption [52]. The E values 1.041, 1.052, 1.041 and 1.030 for PA, PA/m-TiO2 5%, PA/m-TiO2 10% and PA/mTiO2 15%, respectively. This suggests that metal adsorption onto this adsorbents is physical adsorption. 4. Conclusions In this study, new PA containing s-triazine ring was successfully synthesized via direct polycondensation of 4,4-(6-morpholine-1,3,5triazine-2,4-diyl)bis(azanediyl)dibenzoic acid and 1,3-phenylenediamine in the presence of molten TBAB and TPP in good yield and moderated inherent viscosity. Although, dispersion of nanoparticles is a key property to obtain organo-inorgano hybrid composite, so by effective surface modification of TiO2 nanoparticles using 1,3,5-triazine containing silane coupling agent, they were uniformly dispersed weight different percentages into PA matrix. Morphology study of resulting PA/m-TiO2 NCs showed well-dispersed m-TiO2 nanoparticles in the polymer matrix by FE-SEM and TEM analyses. TGA data indicated that thermal stability of the NCs have enhanced with increasing m-TiO2 nanoparticles content. Then, this polymer and NCs were used for Cr(VI) ions removal by optimizing the parameters including pH, contact time and concentration. The maximum uptakes of Cr(VI) at pH = 4.0 is about 76.25%, 80.83%, 83.75% and 86.25% for pure PA, PA/m-TiO2 NC5%, PA/m-TiO2 NC10% and PA/m-TiO2 NC15%, respectively. With due attention to, Ea obtained from Dubinin–Radushkevich isotherm, adsorption mechanism for these adsorbents is physical adsorption. Also, the four adsorption kinetics including pseudo-first-order, pseudo-second-order, Elovich, and intra-particle diffusion model, were discussed briefly to understand the adsorption procedure. Due to the proximity of R2 values of the investigated four kinetic models, pseudo second-order model could be consider as a suitable model for interpretation of adsorption mechanism of Cr(VI). Therefore, the polymer and it’s NCs could be considered as a good adsorbent for removal of Cr(VI) from aqueous solutions due to the presence of different functional group such as nitrogen atoms on the triazine ring and NH as a good platform for the absorption process. Notes The authors declare no competing financial interest. Acknowledgements We wish to express our gratitude to the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, for partial financial support. References [1] M.A. Hashim, S. Mukhopadhyay, J.N. Sahu, B. Sengupta, J. Environ. Manage. 92 (2011) 2355–2388. [2] Z. Lin, Y. Zhang, Y. Chen, H. Qian, Chem. Eng. J. 200 (2012) 104–112.
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