Facile production of HNTs\PDA\PF nanocomposites by unique and environment-friendly method for the removal of phenolic pollutants in water as an environmental adsorbent

Journal of the Taiwan Institute of Chemical Engineers 108 (2020) 1
15

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Facile production of HNTs\PDA\PF nanocomposites by unique and environment-friendly method for the removal of phenolic pollutants in water as an environmental adsorbent Mehdi Hatami*, Mohammadreza Yazdan Panah, Manzar Mahmoudian Department of Polymer Science and Engineering, University of Bonab, P.O. Box 5551761167, Bonab, Iran

A R T I C L E

I N F O

Article History: Received 13 September 2019 Revised 19 December 2019 Accepted 4 January 2020

Keywords: Halloysite nanotube Nanocomposite Surface modification Polydopamine Adsorption of phenols

A B S T R A C T

A new kind of hybrid nanocomposites (NC)s consisted of phenol-formaldehyde (PF), polydopamine (PDA) and modified halloysite nanotubes (HNT)s had been prepared by sonochemical synthesis. The surface modifications of HNTs were achieved by mussel inspired sonochemistry using dopamine (DA) as an active intermediate. In this process bio-safe DA could self-polymerized to adhere to the surface of HNTs and formed an active PDA species. The PF/PDA/HNT NCs had been fabricated by the in-situ polymerization of phenol, formaldehyde in alkaline condition in the presence of PDA modified HNTs through polycondensation reaction. Characterizations with FT-IR, powder X-ray diffraction, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) confirmed the synthesis of NCs with well dispersion properties. In continuation of the research the possiblity of application of prepared nanomaterials for removal of phenolic compounds were inspected. Obtained results showed that the presence of nanotubular filler, existance of different polar functional units in PDA structures, H-bonding and hydrophobic interactions of PF matrix with phenols are the key factors for improving the adsorption capacity for phenol by PF/PDA/HNTs NCs. © 2020 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.

1. Introduction Nanoscience comprises the modifying of different inorganic and organic structures at atomic nanometers scale to attain exclusive properties, which can be appropriately exploited for the desired applications [1]. Nano-sized organic and inorganic particles are finding lots of applications in different industries [2
4]. Salavati et al. [5,6] have investigated the synthesis and characterization of different types of nanostructures and have also employed these nanostructures to remove environmental pollution [7
17]. In recent years, there has been a powerful focus on the development of polymer based nanocomposites (NCs). Polymer NCs are fabricated commercially for new diverse applications such as sporting goods, flying instruments, electronical processors, etc. [18]. Different types of nanocomposites have been prepared and used to eliminate environmental pollution [19
24]. These creative ideas apparently include polymer engineering and polymer technology and in this field the investigations cover a broad range of topics [3,4,25
30]. Among the different kind of polymer matrices the polycondensation polymers as engineering polymers showed great interests [31
34]. Phenol

* Corresponding author. E-mail address: [email protected] (M. Hatami).

formaldehyde (PF) resins are of considerable industrial importance and have wide applications in new era [35
39]. In composite science and technology application of tubular filler showed great attentions due to the enhancement in chemical and physical properties of structures. Among the different tubuluar fillers the new nanoscale tubuluar clay structure showed great interest because of its wounderful optical, mechanical, physicochemical, and thermal properties for application in reinforcement in composite technology [40
43]. Halloysite nanotubes (HNT)s consists of alumino silicate nanotubes with emperical structure of Al2Si2O5(OH)4¢nH2O, which shows a preponderant appearance of cylindrical hollow structure. Halloysite showed 50
80 nm. in outter surface, 10
15 nm. in inter surface and longitude about 1000 nm. [44
46]. HNTs have the unique elemental features due to the many hydroxyl units on the structure surfaces [47]. Treatment of the inorganic material surface with diverse organic or inorganic materials could be verily helpful for different applications. Our investigations in nanostructures functionalization and characterizations oriented us to the synthesis of new surface adjuster of nanomaterials for the treatment of inorganic nano-structures to produce organic modified materials [48-57]. In recent years enormous amounts of studies have concentrated on mussel inspired coatings. Mussel coatings are bio-degradable, environmental friendly nontoxic materials, that make these materials sorely fascinating for therapeutic aims [58]. Nevertheless, the

https://doi.org/10.1016/j.jtice.2020.01.001 1876-1070/© 2020 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.

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greatest interesting feature of mussel coating is the muscles could bond to diverse surfaces like plastics, metal oxides, and metals in aqueous environments [59]. These mussel adhesive properties depend on the existence of the different functional units, and also the presence of the active chemical materials on the mussel structures. Among mussel adhesive compounds, polydopamine (PDA), the dopamine (DA) macromolecule, is a vital element necessary to attain mussel adhesive characteristics. DA was polymerized in alkaline aqueous solution to create the active thin polymeric film [60,61]. The concept of ultrasonic technology is lay on the application of acoustic waves. Many researchers have been utilized sonochemical process in the fabrication of nanostructure hybrid materials. Many chemical and physical benefits of ultrasonic irradiations can assumed in preparation stages of different nanomaterials. Ultrasonic waves showed great potential applications in polymer technology. Many publications were published based on using ultrasonic process in the preparation of polymer NCs. For example, Hatami et al. [30,32] have described the ultrasound assisted fabrication of NCs based on cerium oxide and polymer matrix with superior properties. Mallakpour et al. [26,33] have designated the surface modification of nanoparticles by ultrasonic irradiations for application in different NCs. The sonication process was effectively reduced the agglomeration of nanoscale ingredients in NC structures [62
64]. Therefore, the homogeneous dispersion of nanoscale components in NCs were achieved by assistance of ultrasonic waves. Recently, adsorption treatment has appeared as a most powerful treatment option for the removal of sevral types of polutions [65]. Adsorption process using solids as adsorbents are employed widely for water and wastewater purification in industrial applications. In the past few years, variuos clasess of adsorbents have been used for the elimination of a wide range of pollutants in aqueous medium. HNT and its composites with various inorganic and organic substances have been largley utilized as adsorbent for the elimination of various types of polutions [66]. Wastewaters comprising phenol moieties showed a severe ecological footprint, and management of release of these waters into the environment must be considered. Different phenolic structures are regularly present in the sewages [67]. Investigations of many phenolic compounds in low concentrations revealed the carcinogenic nature of these compounds. Therefore the importance of removal of phenol is very clear to everyone. Numerous adsorbents such as activated carbon, polymeric materials, and zeolites have been applied to eliminate the phenolic pollutants from wastewater [68
72]. The captivating advantage of PDA, the polyfunctional support is placed in effectively interactions with PF functional units, and significantly compatibilized PF polymer composite for environmental applications. In addition, owing to the mussel-inspired nanostructured surface, the prepared PDA-modified HNT frameworks could support the attachment and linkage by chemical interactions through polymer functional units, and significantly facilitated the formation of new NCs under ultrasonic treatment. Therefore, the prepared HNT/PDA nanolayer surfaces as modified nanotubular fillers could effectively adhered to the matrix and effectively enhance the biocompatibility of NC, and showed great potential for industrial outdoor usage in PF industrial applications. Therfore in this studdy, the robust ultrasonic process was used for surface alteration of HNTs by in-situ process related to the polymerization of DA in alkaline solution. After that, PF/PDA/HNTs NCs were prepared. Various thermal and morphological aspects of prepared samples were deeply investigated by different instuments. Also the applications of these NCs in removal of phenols in wastewater were examined.

2. Experimental 2.1. Materials The nanostructured HNTs were prepared by Sigma-Aldrich chemical company. Tris(hydroxymethyl)aminomethane (Tris buffer), dopamine hydrochloride (3-hydroxytyraminium chloride), formaldehyde (37% aqueous solution), phenol and ammonia were provided from Merck chemical company. 2.2. Characterization Fourier transform infrared (FT-IR) spectroscope was engaged to examine the chemical interactions in the modified nanostructures; matrix and NCs. FT-IR spectra of the prepared materials were recorded between 4000
400 cm
1 by using IRAffinity-1S spectrophotometer of Shimadzu, Japan. The FT-IR spectra were acquired with application of KBr powder. Different bands were assigned as wavenumber unit (cm
1). The morphological investigation of the nanotubes and nanostructures onto and into the matrix was illustrated by applying the field emission scanning electron microscopy [FE-SEM, HITACHI (S-4160)]. A transmission electron microscope (Zeiss-EM10C-100 KV) was applied to show the morphology of NCs. Thermogravimetric analysis (TGA) was accomplished with a Sanaf TGA under air atmosphere at a heating rate of 20 °C/min from 25 °C to 800 °C. The AFM topographic pictures were found by using digital multimode apparatuses Compact Frame, Bruker. The XRD peaks was obtained by using a D8ADVANCE, Bruker. The diffractograms in the range of 5
80° for 2u were evaluated using Cu Ka incident beam  (λ = 1.51418 A). The absorption treatment of the samples were tested by the use of Shimadzu UV-1800 (Japan) UV
visible spectrophotometer. 2.3. Modification of the surface of the HNTs by PDA HNTs modified with PDA were fabricated by the following procedure. 0.1 g of HNTs were dispersed in 6 mL Tris-HCl buffered solution (0.0012 mgL
1, pH = 8.5) and sonicated for 30 min. After that, 0.01 g of DA was inserted into the suspension sample of HNTs and was sonicated for 1.5 h. The ultimate construction was stirred for 24 h to find coated HNTs with light-dark insoluble macromolecule. The subsequent light-dark material was washed with hot deionized water three times and then filtrated. Later the separated sample was dried at 85 °C for 20 h under vacuum. 2.4. Preparation of PF resole matrix 3 g of phenol, 6 mL of formalin solution and 2 mL of 2 M ammonia were inserted into a three-necked flask with well-appointed by a magnetic stirrer. To avoid the vaporization of mixture reflux condenser was used onto the system. The blend was stirred for 8 h. PF was exposed by sonochemical process by application of ultrasonic probe at power of 85 W for 50 min and frequency of 20 kHz. For curing, the prepared PF was heated in an aluminum foil in vacuum condition for 3 h at 140 °C. 2.5. Synthesis of modified PDA/HNTs/PF NCs PDA/HNTs/PF NCs were prepared in four dissimilar primary feed of HNTs content to resole weight (2, 4, 8 and 12 wt%) and the corresponding PF/PDA/HNTs NCs were named as NC1, NC2, NC3 and NC4, respectively. At each process at the first step, the proper amounts of surface modified nano-halloysite was dispersed in 10 mL water for 0.5 h and sonicated for 10 min in a three-necked flask. After that the

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Fig. 1. (a) Mechanism of formation of PDA, and (b) surface modification of HNTs by mussel inspired sonochemical synthesis.

required amounts of phenol, formalin and ammonia were dissolved and poured into the mentioned three-necked flask. The value of pH of mixture was adjusted to 8.5 and the sample was sonicated for 2 h. Then the provided sample was stirred for 8 h.

Ultrasound with power of 85 W and frequency of 20 kHz for 50 min. was applied in the preparation procedure. The obtained resin was cured in an aluminum plate in vacuum condition at 120 °C until the mixture was hardened.

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2.6. Experimental procedure for evaluation of removal of phenolic compounds The standard solutions of 4-amino phenol, phenol, and salicylic acid were prepared in our laboratory using analytical reagent grade of salicylic acid, phenol and, 4- amino phenol by dissolving 300 mg of these compounds in 1 L of pure water. The proton concentration of samples was established to the desired value using sodium hydroxide or sulfuric acid. Batch adsorption studies were achieved using the batch contact method. Typically 0.1 g of adsorbent was inserted into a 10 mL of phenolic solution at room temperature in constant pH. The mixture was agitated using mechanical shaker during 12 h to attain constant condition. After allowing to this effective interaction time, adsorbent was removed by centrifuge and then the remaining solution was analyzed by UV-visible spectroscopy at wavelength of 270 nm to determine the phenolic compounds and salicylic acid concentration. The phenolic compounds and salicylic acid removal efficiency (RE%) was obtained using following equation: RE % ¼ ½ðCo
C Þ =Co  100

ð1Þ

Co and C are the concentrations of phenolic compound before and after exposure to the adsorbent respectively. 3. Results and discussion 3.1. Surface modification of HNTs by mussel inspired sonochemical method

Fig. 2. FT-IR spectra recorded by KBr pellets of (a) pure HNTs, (b) surface modified HNTs using PDA (DHNTs), and (c) PF matrix.

Mussel-inspired chemistry is a new approach that has been used to modify organic and inorganic compounds surfaces. At present, it has established as an adaptable and simplistic surface modification technique to diverse materials irrespective of their size, shape, and composition. With the development of bio-based methods, DA became progressively employed in the surface engineering of materials. DA is a neurotransmitter found in the animal/humans. DA comprises a catechol unit and one amine functional group, and can be self-polymerized in weak alkaline solution to form a macromolecular structure as PDA. PDA can strongly adhere to the surface of materials. The formation of the PDA is schematically illustrated in Fig. 1 part (a). It is understandable that when the DA is added to tris buffer solution containing HNTs, PDA is constructed through self-polymerization of DA in the alkaline condition by sonication process. Originally by sonication of nano halloysite, the dispersed nano tubes were alignments, and after addition of DA into the mixture, a black coating of PDA appears on the surface of yellowish color HNTs, due to the precipitation and adhesion of PDA in the buffer solution on the surface of inorganic clay nanotubes. The sonication process was effectively reduced the agglomeration of nanoscale ingredients in NC structures. Wang et al. [60] have used the PDA nanoparticles in accomplished by hydroxyapatite (HA) nanorods as an active layer for surface modification of titanium atructure for biomedical applications. This research group reported the excellent biocompatibility and adhesion propetties for PDA in modification process. Fig. 1 part b illustrates the surface modification process of HNTs by mussel inspired ultrasonic process based on to the suggested mechanism for fabrication of PDA on the surface of HNTs. Sonication process was accomplished by direct applying of ultrasonic probe horn in the solution mixture for avoiding of the accumulation of HNTs before and after the PDA coating. 3.2. Characterization of HNTs surfaces

Fig. 3. FE-SEM micrographs in the range of 200 nm
5 mm resolution of PDA modified halloysite nanotubes (a, b), and PF matrix (c, d).

FT-IR spectra of HNTs and modified HNTs by PDA are displayed in Fig. 2. The broad peak at around 3600 cm
1 is associated to the stretching vibration of hydroxyl unit which belongs to interlayer of HNT. The hydroxyl stretching and bending vibrations of molecular

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Fig. 4. Schematic for preparation of PF/PDA-HNTs NCs by ultrasonic assisted synthesis.

water were assigned at 3450 and 1630 cm
1 respectively. The characteristic absorption bands of Al-O-Si and Al-OH were observed at 537 and 910 cm
1 respectively. These results were confirmed by the results obtained by Feng et al. [61]. The surface coating of PDA on the surface of halloysite sample is observed by the analysis of newly prepared peaks such as the many catechol moieties represented by stretching of O-H bands around 3200 cm
1, aromatic C-H stretching vibration band around 3015 cm
1, asymmetric methylene stretching vibration band about 2930 cm
1 and the stretching C=O vibration band about 1714 cm
1. Also characteristic absorption peaks of selfprovided PDA seem in the range of 1610 to 1290 cm
1 (nitrogen single bond hydrogen bending and phenolic carbon single bond carbon stretching), 1612 cm
1 (carbon double bond carbon) significance vibrations of aromatic kind and nitrogen single bond bending vibrations) and 1511 cm
1 (nitrogen single bond hydrogen vibrations). The occurrence of distinctive absorbance bands of self-polymerizing DA leads that DA has effectively converted to PDA in the reaction media. By the way, to expansion of new information about the morphology of HNTs, PDA and surface modification process, the FE-SEM analysis was utilized. The FE-SEM pictures of PDA-HNTs are shown in Fig. 3 (a, b). The smooth edges and unbroken shapes were observed in Fig. 3 (a, b) for PDA-HNTs. The morphological structure did not change after alteration with ultrasonic process and the PDA-HNTs displayed cylindrical like structures and bundled construction. 3.3. Preparation of PF/PDA/HNTs NCs PDA as a member of catechol moieties are a commanding generation of functional heterocyclic compounds. Furthermore, PDA shows an

imperative role in bio-systems. Therefore, the fabrication of PDA and its derivatives has attracted much attention [42]. PDA contains amino catechol, heterocyclic conjugated aromatic benzene ring moiety, in which the catechol units also may act as a cohesive part of the surface modification process. Although the author research group in some previous studies [49,50] has used 4-H pyran structures for modification of silica nanoparticles, but due to the difficulties in preparation process of these heterocyclic compounds, new functional materials were searched for surface modification process. PDA was showed many advantages. The more important factor is the simplicity in fabrication process. PDA structure was shown in Fig. 1. The similarity in reactivity of catechol and phenol in reaction with formaldehyde in alkaline solution bring new idea in the preparation of novel NCs in this study. Therefore in this study, in the first step phenol and formaldehyde were reacted in alkaline solution with excess amounts of formaldehyde by ultrasonic assisted method. Ultrasonication process delivered the suitable mechanical and thermal conditions for preparation step [11,13]. In the second step by addition of surface modified HNTs by PDA in the presence of excess amounts of formaldehyde in alkaline condition the condensation and crosslinking processes were started. Therefore the chemical bonds between polymer matrix and nano-tubular halloysite surfaces components were established. By creation of chemical linkages and also trapping of nano tubular inorganic ingredients in PF matrix, it could be expected that the thermal nature and physical properties of these nanostructures were dramatically enhanced. The preparation reaction of NC has been illustrated in Fig. 4. PF and PF/PDA/HNTs NCs were exposure to the ultrasonic cavities. Sonication process supported the crosslinking phenomena of phenol, formaldehyde and PDA. Also ultrasonic voids were affected

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Fig. 5. FT-IR spectra recorded by KBr pellets of (a) PF/PDA-HNTs NC 2 wt% (NC1), (b) PF/PDA-HNTs NC 4 wt% (NC2), (c) PF/PDA-HNTs NC 8 wt% (NC3) and (d) PF/PDA-HNTs NC 12 wt% (NC4).

to the particle sizes of materials by prevention of aggregation of nanostructures in NCs. 3.4. FT-IR investigations of PF/PDA/HNTs NCs Unadulterated PF and prepared NCs were analyzed by FT-IR spectroscopy and were displayed in Fig. 2(c) and Fig. 5. The phenolic hydroxyl units stretching broad bands were positioned from 3500 to 3100 cm
1. The carbon single bond hydrogen stretching vibration bands are detected at 3000 and 2923 cm
1. The vibration band of phenolic C-O units was assigned as a strong band around 1245 cm
1. The bands at 1644, 1610, 1510 and 1101 cm
1 are distinguished as stretching of O-H, blending of molecule water, carbon double bond carbon stretching vibration bands in aromatic ring and the C-O linkages, respectively. For NCs the characteristic bands are positioned around to the neat PF and PDA parents with diminutive movement in absorption bands. The spectra of NCs revealed that the band at the region from 700
400 cm
1 was attributed to Al-O. The absorption bands of the phenolic matrix diminshed the unique vibrations of surface modified nanotubular halloysite. Obtained information from FT-IR specroscopy confiremed the successful fabrication of the PF/PDA/HNTs NCs. Obtained results showed that the robust interfacial connections between the hydroxyl functional units of PF with catechol units of PDA, bridging structure of methylene between phenolic units and PDA or other phenols caused to well trapping of the nano-tubular halloysites in the structure of final matrix.

Fig. 6. FE-SEM micrographs with 200 nm and 500 nm resolution of PF/PDA-HNTs NC 8 wt%.

2u around 10
25° which confirmed the amorphous structure. Also observation of the patterns of NCs showed that a wide peak in the assortment of 2u = 10
30° was detected for all samples. This broad peak observed in XRD patterns of all NCs was assigned for presence of amorphous structure in samples.

3.5. X-ray diffraction patterns analysis Applied pure halloysite displays a sharp peak at 2u = 12° related to the basal spacing of 0.72 nm obtained by braggs equation which categorizes as dehydrated HNT [26]. PF exhibited a wide-ranging peak at

3.6. FE-SEM analysis Fig. 6 (a-d) displays the FE-SEM descriptions of the PF/PDA/HNTs NCs over two different magnifications (200 and 500 nm). The images

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Fig. 9. TEM images with 300 nm resulation of NC2.

Fig. 7. FE-SEM micrographs in the range of 200 nm-1 mm resolution of (a) NC1, (b) NC2, (c) NC3, and (d) NC4.

of PF/PDA/HNTs NCs illustrated that the nanoscale structures were clearly detectable in the pictures. This fact is depended to pure polymeric matrix morphology and also surface modified nanofillers. Incorporation of nano fillers in polymer matrix can manifestly alternated the morphological features of the NCs surfaces. The micrographs of PF/PDA/HNTs NCs samples with different surface modified nano halloysite have been shown in Fig. 6 (a-d). Investigation of pictures of Fig. 6 (a
d) associated to the NCs shown that with increase in the number of particles in NC, structural features was changed. The very tiny cracks on the structure of composites were observed. Based on prepared structures, there are no empty spaces were observed on the FE-SEM images and these findings confirmed the good interfacial adhesion between organic layer of fillers and

functional units of polymer matrix. The dispersions of nano fillers in NCs are uniform. These dispersions were established due to the stabilization effect of interaction of polymer functional groups with surface adapted nano structures and also the role of ultrasonication process. Sonication was produced a uniform morphology, in which it guarantees a creation of powerful interaction between nano-fillers and the polymer matrix. With enhancement in the nano fillers content the physical and chemical bonds between PDA functional units and functional units of PF matrix were increased and based on these interactions the morphological discrepancies were observed. On the other hands, the contributions of hydroxyl units of polymer matrix and PDA functional units significantly restricted the chain mobility and fixed the structure. Many interactions did not allow the structure to have reasonable flexibility. The deployment of nano halloysites in the structure of the final nanocomposites was also evaluated by investigation of the fracture surfaces of NCs (Fig. 7). Good dispersion of modified nano halloysites can endorse the effective prevention from aggregation between nanotubes by chemical and physical designed interactions. The organic-inorganic networks were well established in NCs based on linkages between phenolic functional units and PDA ones. For NC4 the FE-SEM results offered the dispatched nanostructure fillers dispersed in polymer matrix with crashed outside from 40
90 nm. Thoroughly, FE-SEM pictures proven that nanoscale fillers were healthy dispersed and consistently spread in organic media. The EDS analysis was used to identify the chemical composition of prepared nanostructure of NC4. This result showed the presence of carbon, nitrogen, oxygen, aluminium and silicone elements in the composition of fabricated nanocomposite. The lines of C Ka, N Ka, O Ka, Al Ka, Si Ka with weight percentage of 77.11, 0.16, 26.25, 1.21, and 0.27 were detected form the EDS analysis pattern. It is implicit that the fabricated organic- inorganic networks between PF and PDA modified inorganic halloysite nanotubes can be considered based on chemical and physical robust interactions. Reactive positions of phenolic moieties and PDA reactive sites can react by excess formaldehyde to create the methylene bridges between organic and inorganic structures. As a consequence, increase in the methylene bridges and hydrogen bondings interactions were developed the network structures in fabricated NCs. 3.7. TEM study

Fig. 8. TEM images with 200 nm resolation of surface modifed HNT by PDA.

Direct evidence of the construction of nanotubular halloysite coated with biomimetic PDA was observed by considering TEM analysis. TEM micrograph of surface modified HNT with PDA as the

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Fig. 10. The two and three dimensional AFM topography images of PF (a, b) and NC3 (c, d).

mussle inspired technology was illustrated in Fig. 8. By coating with PDA the surface hydroxyl units of HNTs were modified. Fig. 8 clearly shows the HNT core with PDA shell in nanometric scale. TEM pictures of NC2 were offered in Fig. 9. The uniform spreading of surface modified nanotubular HNTs were shown in bio-nanostructure. The brighter color stratums were interrelated to the polymer structures and the darker color sections were associated to the HNTs. By observation of TEM pictures, the nanotube diameter without coating is below than 30 nm and after coated with PDA is below than 60 nm. Also in bionanovcomposite the diameter of nano HNTs are below than 70 nm. Obtained results indicated that the PDA modified surfaces of HNTs were well distributed in the NCs. The existence of the PDA functional units on the surfaces of the amended HNTs and organic environment of the PDA biomacromolecule resulted in the respectable interfacial connection concerning to the phenolic matrix and PDA modified HNTs in the NCs. 3.8. AFM studies Designations of different macromolecules for use in nanotechnology are nowadays imperative activities to fabricate enormous NC structures with defined tasks especially in surface science. The atomic force microscope (AFM) is suitable instrument for evaluation of soft condensed matters. AFM provided good statistics based on study the fine morphology, chemical and physical properties of surface at the nanometer scale. The ability of AFM to provide the two and three dimensional measurements data as well as generous software performance for analysis of materials, introduced this tool as effective

instrument for evaluation of topography of materials surfaces. In this work, the AFM delivers the evidence about surface morphologies via 2- and 3-dimensional investigations. AFM pictures were delivered in 6.8 £ 6.9 mm2 and 6.1 £ 6.4 mm2 scan area for polymer and NC3. Fig. 10 demonstrates the two dimensional AFM images for PF (a) and for NC 3 (c). Also the three dimensional AFM images of parts (a) and (c) were shown in part (b) and (d) of Fig. 10. Three images of (a) showed the two dimensional topographic images for polymer matrix with different pictorial art works. It seemed that by introducing the surface modified halloysite nanotubes in PF as polymer matrix the peaks in some places of surface were created. The bump and projection of nano-tubular halloysites on the surface of NC in three dimensional images for NC3 was clearly observed. The heights of protruded tubes on the surface of NC3 in the selected area were below than 50 nm. Analysis of the two and three dimensional images revealed that the HNTs are good spread in the matrix based on the bulged positions. Abundant procedures have been applied to quantize the fractal dimension (D) values based on AFM results. In this research for evaluation of the D value of PF/PDA/HNTs NC films the Gwyddion software was used. Three practical methods including the power spectrum, partitioning, and triangulation were applied to analyze D value from AFM statistics (Fig. 11 (a
c)). The D values for NC3 were calculatd 2.12, 2.29 and 2.14 from power spectrum, partitioning and triangulation approaches respectively. Fractal investigation has displayed that NC3 is moderately sensitive to the nano filler content. The Minkowski utilities are applied to define universal geometric features of constructions. 2-dimensional detached alternatives of surface (S),

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Fig. 11. Fractal analysis: (a) power spectrum, (b) partitioning, (c) triangulation, and the Minkowski functions: (d) volume, (e) surface, (f) connectivity.

Fig. 12. (a) Selected gates from two dimensional AFM image of NC3 and (b) the height profiles related to measuring length for NC3.

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The asymmetrical peaks associated to the dissimilar topography of the exteriors for NC3 were detected. For the drown lines, the outlines were displayed alike designs for four gates but line 1, and 4 showed different protruded valleys. The maximum profile for the valuation measurements were distinguished to smaller than 35 nm for NC3 sample. The height distribution function displayed great interest among the different numerical functions. The height values are considered as regularized histograms. The height distribution amounts fit in to the first-order geometric amounts, relating to the numerical properties of the discrete themes. These utilities are specifically called the height-height correlation, the autocorrelation (ACF), and the power spectral density functions (PSDF). Fig. 13 was demonstrated the ACF, height distribution outline, and PSDF for NC3. The median values of total point’s roughness graph though a mean gate upper and lower the evaluation length was estimated as Ra. Rp is defined as the advancement of the highest peak in the roughness segment over the inspected longitude. The Rp for the NC3 was obtained from AFM image. The Rtm and other statistical graphs were obtained and illustrated in Fig. 10. The ADF (amplitude distribution function) is the applied factor may afford measurable data related to the irregularity and smoothness of the evaluated media. By investigating of the roughness curvatures and ADF assumed in Fig. 14, it might be detected that the surface roughness features and the height distribution graphs presented the distinctive impressions for this work. So ADF graph obviously displayed the height distribution for nominated entry of the appearance. 3.9. Thermal analysis of NCs

Fig. 13. The first-order statistical quantities, (a) height distribution profile, (b) autocorrelation function (ACF), (c) the height-height correlation function (HHCF), and (d) the power spectral density function (PSDF) for NC3.

volume (V) and connectivity (x) are embodied as geometrical descriptors. One of the morphology descriptors which were applied to define several morphological and geometrical assets of structures is Minkowski connectivity. Minkowski functions related to the fabricated NC3 were demonstrated in Fig. 11 parts (d
f). Four dissimilar lines with magnitudes slighter than 4.5 mm were consigned in separate dimension part for NC3 sample and shown in Fig. 12.

Thermogravimetric analysis (TGA) data deliver respectable data about the ingredients and thermal strength of final formulation of products. The TGA curvatures of PF, and NCs are presented in Fig. 15. The polymeric models were warmed from 30 to 800 °C with warming speed of 20 °C/min. Decomposition of NC1, NC3 and pure PF takes place in two steps. Phenolic resin TGA analysis result provided that the weight loss of ten wt% in the range of 30
200 °C was related to the dehydration process and also some other specious elimination phenomenon of free hydroxyl groups and methylol units in the structure of PF. The other mass loss was happened as a result of curing process, which was related to the crosslinking reaction. In continuance of this stage the thermal decomposition of the organic linkages were observed. Insertion of PDA-HNTs intensifies the thermal durability of PF matrix. It was established that by growing the PDA-HNTs content in the PF/PDA-HNTs NCs their thermal strength appeared to be enhanced. The barrier effects of tubular clays were hindered the diffusion of the deprivation products from the provided network of the PF matrix. Additional clarification was established by attention to the interactions of the PDA-HNTs and PF, which amended the degradation properties. The NCs considerably loses their weights between 400 °C to 800 °C. Furthermore, the rise of the values of the maximum decomposition temperature (Tmax) value displays that the thermal strength could be enhanced by the assimilation of surface modified nanotubes. Besides that, the movement of the thermal firmness is in agreement with the nano structure contents. The inorganic nanotubes can provide adequate areas as connection sites and facilitate the crosslinking process by extra reactive functional units based on PDA sites. However, everyone know that by increase in the filler loading the aggregation phenomena could become serious problems, but here the occurrence of organic PDA layer on the surface of HNTs relatively resolved this problem. The limit oxygen index (LOI) is applied in order to calculate the flammability of a solid. By application of Van Krevelen and Hoftyzer equation, Char yield (CY) could be used for calculation of LOI [58]. LOI ¼ 17:5 þ 0:4 CR

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Fig. 14. The statistical graphs for NC3: (a) minimum, (b) median, (c) mean, (d) maximum, (e) kurtosis, (f) Ra, (g) Rq, (h) Rt, (i) Rz, and (j) skewness.

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Table 2 Adsorption capacity of phenol by the investigated materials. Material

Phenol removal efficiency (RE%)

The recovery and reuse (3th)

HNT DA NC1

6.5% 1% 93.7%

2.1
83%

Table 3 Adsorption capacity of compounds by the PF/PDA/HNTs.

Fig. 15. TGA thermographs of (a) PF, (b) NC1 and (c) NC3 at heating rate of 20 °C/min.

Where CR = char yield The values of LOI of PF/PDA/HNTs NC1 and PF/PDA/HNTs NC3 have been considered from their CY at 800 °C. Consequently, NCs can be categorized as self-extinguishing compounds. The inorganic nanoscale tubular clay structures as fillers, aromatic backbone, crosslinked nature of matrix, and additional PDA reactive sites in the construction of NCs improved the thermal permanency parameters of the prepared nanostructures from thermal environments. 3.10. Phenol adsorption investigations Phenol is an organic compound, commonly soluble in water and the solution of phenol in water is colourless. Ultraviolet visible spectrophotometer is a common instrument used for the determination of phenol concentration. This process was achieved by drawing the calibration curves based on different concentration of phenol and related absorbance. For each experiment the phenol solution is placed into the sample cuvette to get a graph of the ultraviolet absorption of the solution over the range of characteristic wavelength λmax=270 nm. Based on the evaluation of decrease in concentration of phenol by different adsorbent, the adsorption blend grasped balance after 720 min. so, for all tests of this study the 720 min. agitation time was applied. Shen [73] experimental results on using the organobentonite as an adsorbent clearly showed that the approval of phenol adsorption by bentonite is the same as observed by activated carbon. Table 1 shows the comparison of different mechanisms of pollutants removal by different active specious. Table 2 shows the comparison of adsorption capacity of phenol by the HNTs, PDA, and PF/PDA/ HNTs. From the test result of adsorption in Table 2, this research group can see that when 0.1 g of DA, HNTs, and PF/PDA/HNTs was added to phenol solution, the removal efficiency increased from 1% to 96.7%. In other words, the adsorption capacities of NCs were enhanced by the impregnation of cross-linked phenolic resin with

Material

Phenol

4-Amino phenol

Salicylic acid

NC1 NC2 NC3 NC4

93.7 93.3 94.2 94.5

76.5 75.8 76.3 75.6

51.4 52.6 54.6 53.6

surface modified nanotubes. Though crosslinking technique had been used to increase physical properties of adsorbent in numerous reports, but outcome revealed that a decrease had been observed in adsorption capacity due to the decrease in effective interaction sites of complex. From this point, presence of nanotubular filler modified by PDA in PF matrix is a more proper contender for enhancing the adsorption capacity for phenol. Table 2 shows the comparison of adsorption capacity of phenol by the HNTs, PDA, and PF/PDA/HNTs. In this study two major types of interactions were assumed. The hydrogen bonds interactions and physical hydrophobic interactions were occurred between NC structure and phenolic units. The enhanced adsorption capacity of the PF/PDA/HNTs NC is possibly caused by the subsequent issues. First, the surface modified HNTs with a high surface area have outstanding adsorption capacity for organic compounds. Next, the NC can adsorb phenol by a synergistic way of electrostatic force interactions and also hydrogen bonds. The recovery and reuse studies reveal that the adsorption capacity of phenol by PF/PDA/HNTs in the third cycle decreases slightly from 93.7% to 83.0%. The influence of presence of acidic and basic groups in phenolic structure in solution on performance of nanocomposite adsorbent efficiency was investigated. Experimental results show that the adsorption efficiency of nano adsorbent for 4-amino phenol and salicylic acid in solution has a little decrease in comparison to pure phenol. The adsorption capacity for phenol, 4-amino phenol and salicylic acid were reported in Table 3. The PF/PDA/HNTs nanocomposites used in this research are heterogeneous materials consisting of inorganic clay minerals and organic PDA and PF. These nanocomposites are estimated to comprise large amounts of polar functional units such as -OH, -NHCO, -O, -NH2, and -N and exhibit generally polar

Table 1 Comparisons of provided materials by mechanism and active spacious in environmental remediation. Active specious

Media

Removal process

Model contamination

References

CoFe2O4@SiO2@Dy2Ce2O7 Nd2Sn2O7 Iron-functionalized reduced graphene oxide silver/reduced graphene oxide halloysite-derived mesoporous silica nanotube silica modified TiO2 Synthetic opolymers Zeolites powdered activated carbon polystyrene and polymethacrylate cross-linked with divinylbenzene zeolite/PU organobentonite PF/PDA/HNTs

Water Water Water Water Water Water Water Water Water Water Water Water Water

Photocatalytic Photocatalytic adsorption Photo
catalytic adsorption Photo
catalytic Adsorption Adsorption Adsorption Adsorption Adsorption Adsorption Adsorption

Rhodamine B erythrosine fulvic acid methyl orange methylene blue Methylene blue Phenol Phenol Phenol Phenol Phenol Phenol Phenol

6 9 20 24 41 48 68 69 70 71 72 73 This study

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Fig. 16. Mechanism of phenol removal by NC.

performance. Up to now, the influences of inorganic mineral materials in combination with two kinds of organic polar polymeric materials on the adsorption were not investigated. Phenol adsorption on pure carbons was determined by the well-known p-p interactions and “donor-acceptor system”. As stated in the literature [53], the most acidic, carboxylic unit, and some of the most basic groups decreased phenol adsorption at small concentrations. Obtained results in this study are in good accordance with findings of other researchers. Adsorption efficiency increased in NC compare to HNTs, PDA due to the coefficient of inorganic mineral materials and organic polar polymeric materials. Also physical adsorption of the non-polar aromatic phenol onto PF/PDA/HNTs nanocomposites was clearly happened and the suggested mechanism was shown in Fig. 16. 4. Conclusions The objectives of this research have been focused on the preparation, investigation and application of PF/PDA/HNTs NCs as an adsorbent for phenolic removal pollutants. The PDA-HNTs were prepared by the insertion of PDA as an exterior converter onto the halloysite surfaces by mussel inspired sonochemical technique which enhances its biocompatibility. By in-situ polymerization of DA on the surface of HNTs by assistance of ultrasonic process agglomerations of nanotubular structure were condensed and the spreading of nanomaterials in the polymeric matrix was boosted. At the final step, the PF/ PDA/HNTs NCs were produced by introduction of different amounts of PDA/HNTs (2, 4, 8, and 12 wt%) into polymerization media of phenol and formaldehyde using sonochemical method. The special geometrical features of PDA/HNTs on surface sinuate, thermal characteristics, and morphological microstructure of the PF NCs were inspected by TEM, FT-IR, XRD, AFM, TGA and FE-SEM. The morphology investigations were proven by the uniform distribution of the PDA-HNTs in the PF matrix functional linkages between HNTs and PDA, also PDA and PF in network. TGA results showed that increasing in the PDA/HNTs content in the PF/PDA/HNTs NCs cause to the improve in thermal stability. The AFM investigation results indicated that the irregular surfaces of NCs comprise stands with dissimilar heights associated to the nano-tubular structure. PF/PDA/HNTs NC is a more suitable adsorbent of phenol in wastewater treatment

systems. Adsorption efficiency increased in NC compare to HNTs, PDA due to the coefficient of inorganic mineral materials and organic polar polymeric materials. Declaration of Competing Interests None. Acknowledgments We wish to express our thankfulness to the Research Affairs Division University of Bonab, Bonab for partial financial support. Further financial support from Iran Nanotechnology Initiative Council (INIC) is appreciatively acknowledged. References [1] David JP, Charitidis CA. Nanocomposites: materials, manufacturing and engineering. 1st ed. Berlin: De Gruyter; 2013. [2] Huang X, Zhi C. Polymer nanocomposites. Springer; 2016. [3] Idumah CI, Hassan A, Ogbu J, Ndem J, Nwuzor IC. Recently emerging advancements in halloysite nanotubes polymer nanocomposites. Compos Interfaces 2019;26(9):751–824. [4] Dastan D, Chaure N, Kartha M. Surfactants assisted solvothermal derived titania nanoparticles: synthesis and simulation. J Mater Sci: Mater Electron 2017;28 (11):7784–96. [5] Zinatloo-Ajabshir S, Salavati-Niasari M. Preparation of magnetically retrievable CoFe2O4@Sio2@dy2Ce2O7 nanocomposites as novel photocatalyst for highly efficient degradation of organic contaminants. Compos Part B: Eng 2019;174:106930. [6] Zinatloo-Ajabshir S, Morassaei MS, Salavati-Niasari M. Facile synthesis of Nd2Sn2O7-SnO2 nanostructures by novel and environment-friendly approach for the photodegradation and removal of organic pollutants in water. J Environ Manag 2019;233:107–19. [7] Salavati-Niasari M, Salemi P, Davar F. Oxidation of cyclohexene with tert-butylhydroperoxide and hydrogen peroxide catalysted by Cu(ii), Ni(ii), Co(ii) and Mn(ii) complexes of N,N0 -bis-(a-methylsalicylidene)-2,2-dimethylpropane-1,3-diamine, supported on alumina. J Mol Catal A: Chem 2005;238(1):215–22. [8] Zinatloo-Ajabshir S, Salavati-Niasari M, Hamadanian M. Praseodymium oxide nanostructures: novel solvent-less preparation, characterization and investigation of their optical and photocatalytic properties. RSC Adv 2015;5 (43):33792–800. [9] Zinatloo-Ajabshir S, Morassaei MS, Salavati-Niasari M. Eco-friendly synthesis of Nd2Sn2O7
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