Hybrid Membranes of Polyamide Applied in Treatment of Waste Water

In this work, it was prepared hybrid membranes of polyamide6 (PA6) with montmorillonite (MMT) and porogenic agent (CaCl2). The hybrid membranes with CaCl2 were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), porosimetry by mercury intrusion (PMI), flux measurements and rejection. By means of X-ray diffraction, it was revealed that the hybrid membranes with CaCl2 have an exfoliated and/or partially exfoliated structure. For FTIR and DSC of hybrid membranes with CaCl2, it was found that the spectra and the crystalline melting temperature remained virtually unchanged compared to PA6 membrane. From the SEM images, it was observed that the addition of the MMT and the CaCl2 in the membrane of PA6 caused an increase in the amount of pores the surface and cross section of these membranes. By PMI, it was observed that the presence of MMT and CaCl2 in the membrane caused an increase in the average diameters of pores. The water-oil separation tests, indicated a significant reduction of oil in the permeate, allowing treatment of wastewater contaminated with oil.


Introduction
From the beginning of the 70s, classical separation processes such as distillation, filtration, adsorption, ion exchange, centrifugation, solvent extraction, crystallization and others, are substituted by a new class of processes called membranes separation processes (MSP) by using synthetic membranes as selective barriers 1 .
The porous synthetic membranes have pores of various sizes.These pores are responsible for all the properties that make the membranes useful in various applications, to separate particles and to fractionate molecules of different molecular weights 2 .
Water treatment is a crucial field closely related to environmental, economic, and social issues.Several water sources for our daily lives and industries typically contain oil, which has become the most common pollutant worldwide.
In recent years, it is possible to find a large amount of research work about membranes for the treatment of oily wastewater in the literature, not only in experimental but also in theoretical terms [3][4][5][6][7] .
One of the major problems of the industries is the treatment of the effluent before being discharged in the environment or in a public sewage system.A large portion of this effluent is composed of oil in water emulsions 8,9 .The MSP are relatively simple and easy to operate, presents low consume energy and produces an effluent with good quality, facilitating reuse in the industrial process.These processes are presented as an alternative for the treatment of oily waste, mainly in the form of stable emulsions 1,2 .
Scholars reported the existence of several studies using MSP for the treatment of oily waste, including in the form of stable emulsions, most focusing on the use of ultrafiltration (UF) and microfiltration (MF) membranes in rejection of the oil droplets.In this study the drops of oil are completely rejected and the continuous phase is permeated [9][10][11] .
The phase inversion method is used to obtain polymeric membranes which are produced by precipitation from a polymer solution, allowing morphological modification from changes made in the parameters used during the membranes preparation 12 .
The synthetic polymers most widely used in the preparation of membranes are polyamide, polysulfone, polyacrylonitrile, polycarbonate, polyetherimide, poly (vinylidene fluoride), among others.Membranes prepared from these polymers have improved chemical, thermal and mechanical resistance compared with traditional polymers.In addition, they exhibit good resistance to chlorinated compounds (despite the low resistance to mechanical compression) and can be used with non-aqueous solvents 13,14 .
Polyamides are materials of high tensile strength, abrasion and fatigue resistance, low friction coefficient and good toughness.These properties were improved by the nanocomposite made with this polymer when compared to pure polymer 15 .
The combination of materials and processing allowed obtaining hybrids with special properties that do not exist in the original material.These properties depend on many factors, including the aspect ratio of the load, degree of dispersion, orientation of the load in the matrix and adhesion in load-matrix interface [22][23][24][25][26] .
The addition of nanoparticles of inorganics salts in hybrid materials provides significant changes in the morphological structure of the membranes.Furthermore, the use of these additives gives a wide applicability ranging from microfiltration to reverse osmosis 27 .This work examined the feasibility of the hybrid flat membranes with the addition of an inorganic salt for use in the treatment of wastewater.

Materials
For this study, it was used the following materials: montmorillonite (MMT) Brasgel PA supplied by Bentonit União Nordeste (BUN), located in Campina Grande-PB/ Brazil.The polymer matrix used was the polyamide6 (PA6), named Technyl C216 and supplied by Rhodia, São Paulo -SP, Brazil.The formic acid manufactured by Vetec/ SP, Brazil, was used as solvent to dissolve the polymeric matrix.It was used calcium chloride dihydrate (CaCl 2 .2H 2 O) PA as an additive, with average molar mass of 147.01 g.mol -1 , manufactured by Vetec/SP, Brazil.

Preparation of hybrid materials
The hybrid materials were prepared by the melt intercalation method in high speed homogenizer, MH-50H model of the brand MH Equipment.Before starting preparation, PA6 was dried in a circulating air oven for two hours, at 80 °C and, subsequently remained in a vacuum oven, at 80 °C for 48 h.Then, the hybrid materials were prepared with 1, 3 and 5% w/w of MMT.The procedure for mixing the hybrid materials were as follows: approximately 40 g of pure polyamide was placed in the chamber of the MH-50H homogenizer and the equipment was turned on for 1 minute, under these conditions the pure polyamide is melted by friction.Then, the MMT was placed and, the equipment was turned on again for 5 seconds.After, the pure polyamide or hybrid material was removed and, then crushed in a knife mill.

Preparation of polymeric and hybrid membranes
The pure PA6 or hybrid material with 1, 3, 5% w/w of MMT was dissolved in formic acid for a fixed time of 24 hours and added 10% w/w of CaCl 2 to obtain a dispersion for the preparation of membranes, using the phase inversion technique by immersion precipitation.These dispersions were spread by means of glass rods with spacing of approximately 0.2 mm on glass plates and remained for a time of 300 seconds exposure in the atmosphere before the precipitation in a bath.This time of exposure of the dispersions before precipitation was suitable for obtaining flat microporous selective barriers.After, the dispersions were immersed in a non-solvent bath (with distilled water and room temperature).The formed membranes remained in the bath until its complete precipitation.Afterwards, the membranes were washed with distilled water and kept in a aqueous solution with 20% w/w of glycerin.The membranes were stored in a solution with glycerin to prevent pore collapse, and before being subjected to the permeability test, they were washed with distilled water to eliminate the glycerin.

Characterizations of polymeric and hybrid membranes
The polymeric and hybrid membranes were characterized by X-ray diffraction (XRD), using a Shimadzu XRD 6000 equipment, with λ = 1.541Å, operating at 40 kV and 30 mA and scanning angle from 1.5 to 30°.The analysis of Fourier transform infrared spectroscopy (FTIR) was performed on a Perkin-Elmer Spectrum 400 spectrometer with scan from 4000 to 650 cm -1 .The membranes were characterized by Differential Scanning Calorimetry (DSC) on a Shimadzu DSC-50 equipment, from room temperature to 300 °C at a heating rate of 10 °C.min -1 and under nitrogen atmosphere.The images from scanning electron microscopy were obtained in SSX 550 Superscan -Shimadzu equipment.For analysis of the cross section, the samples were fractured in liquid nitrogen to prevent a plastic deformation.The membranes were coated with gold.The porosity of the membranes was estimated by the mercury porosimeter Altopore IV, model 9500 of Micromeritics.All membranes were characterized in the form of thin films.

Water permeability and oil retention
For the tests of flux measurements with distilled water and water-in-oil emulsion, it was used a dead end filtration system in a cell with 13.0 cm 2 of effective area.The membranes were submitted to permeation tests at 1.0 bar pressure.The membrane performance was evaluated by the permeate flux and selectivity with an emulsion of oil/water present in the feed solution at the concentrations of 50, 100 and 200 mg.L -1 and presenting oil droplets with diameters varying from 3 to 8 µm, as commonly used in microfiltration processes 28 .
The selectivity of the membranes was estimated by the rejection coefficient (R%), calculated based on the ratio of the concentrations of oil in the permeate (C P ) and the power (C 0 ) expressed by Equation 1: 21.8° of the crystalline plan (001), which can be attributed to phase γ (gamma) of PA6 36,37 .The addition of 10% w/w of CaCl 2 in the solution of membrane preparation did not cause significant changes with regard to the crystalline behavior and the main crystalline phase is α, as illustrated by the X-ray diffraction curve of pure PA6 membranes and their hybrids.
The results of FTIR analysis of the pure PA6 membranes and their hybrids 1, 3, 5% w/w of MMT, adding CaCl 2 as additive, can be seen in Figure 2.

Results and discussion
Figure 1 illustrates the results from X-ray diffraction of pure PA6 membranes and the hybrid materials with 1, 3, 5% w/w of MMT, adding 10% w/w of CaCl 2 .In Figure 1 it was observed the presence of two peaks at 20º and 24º, for all compositions (PA6 pure and hybrid materials), which can be attributed to the formation of crystalline phase characteristic of polyamides, called phase α 1 and α 2 and related to the crystal diffraction planes (100) and (002)/(202), respectively 24 .Polyamides are semicrystalline polymers and the presence of peak broadening indicates the existence of amorphous regions.Furthermore, the introduction of clay, regardless of the amount, changes slightly the shape and intensity of the peaks at 20º and 24º of the polyamide6 [30][31][32][33][34][35] .
It was found that the addition of 5% w/w of MMT in PA6, cause the appearance of the new reflection at 2θ = From Figure 2, it is noted that the general profile of FTIR spectra of the PA6 remained virtually unchanged, except for a small increase in the intensity of the peaks at wavelengths of 800, 1033 and 1095 cm -1 .These peaks can probably be related to the stretch vibration of the Si-O-Si group of the clay, since the contributions of PA6 in this particular area are very small.In addition, it was noticed a slight increase in the intensity of these peaks, which can be related to the increase in clay content, used in the preparation of hybrid membranes [38][39][40][41][42] .
Also in Figure 2, it was observed typical characteristic bands of formic acid: a broadband at 3300 cm -1 , related to the axial deformation of the O-H group; at 2940 and 2860 cm -1 there are bands related to axial strain of C-H group, which is superimposed with the band of O-H group; and at 1640 cm -1 , there is the axial deformation of the C=O group; the region of 1420 cm -1 is in agreement with the angular deformation of the C-O-H group; at 1256 cm -1 there is a band related to the axial strain of the C-O group and also involves interactions with C-O-H group.However, these bands are not associated only with the formic acid used in preparing the membranes, but also are superimposed to the bands inherent of the PA6 43 .In the solution preparation step, a chemical reaction occurs between the -OH group of formic acid (H-COOH) and Cl -ion from CaCl 2 , promoting the formation of the chloride acid (H-COCl) in the solution 44 .In this way, occurs an increase in the intensity of a band at 935 cm -1 , which can be related to angular deformation of the O-H group from formic acid, produced in the step of solvent exchanging by the non-solvent, specific to the reaction between the chloride acid and water 45 .
Table 1 shows the melting temperature (T m ), melting enthalpy (ΔH F ) and degree of crystallinity (X c ) for pure PA6 membranes and their hybrids 1, 3, 5% w/w of MMT, adding 10% w/w of CaCl 2 .For all the analyzed membranes, there are two endothermic peaks corresponding to the two crystalline phases existing, in approximately, 220 °C related to the α phase and in 210 ºC related to the γ phase.The adding of clay improves the barrier properties of the films, however induces the appearance of γ phase [46][47][48][49] .
In Table 1, there are variations in the degree of crystallinity of the nanocomposite membranes when compared with the membrane PA6.Probably, the presence of the clay crystalline behavior altered of PA6.However, when small percentages of clay are added to polymer matrices, the degree of crystallinity Xc, increases the nanocomposite which may demonstrate a nucleating effect of the clay 34,35,49 .
The SEM images of the top surface and cross section of pure PA6 membranes and their hybrids 1, 3, 5% w/w of MMT, adding 10% w/w CaCl 2 , are shown in figures from 3 to 6.In all SEM images it was observed asymmetric microporous membranes with a selective layer (filter skin) on the top and a porous layer on the bottom.In cross section, it is still possible to observe a variation in pore size throughout its thickness, with pores of spherical shape and evenly distributed.This difference in morphology on the cross section of the membrane is responsible for the permeability and selectivity of the membrane 50,51 .
The addition of 10% of CaCl 2 in the preparation of pure PA6 membrane caused an increase in the amount of the pores and uniformity distribution of them on the surface and cross-section of these membranes.This behavior occurred due to the polar character and hydrophilicity of CaCl 2 and also the physical-chemical interactions existing between CaCl 2 and formic acid.
The exchange of the solvent by the non-solvent accelerates the solubility of CaCl 2 during the formation of the membrane, promoting their solvation in formic acid and its hydration in water.Thus, the CaCl 2 present in aqueous phase, accelerates the process of formation and distribution of pores on the membrane surface 52 .Furthermore, in preparation of the solution occurs the formation of the acid chloride, which in the exchange step of solvent by the non-solvent, reacts rapidly with the water.
The addition of 10% w/w of CaCl 2 to prepare the hybrid membranes with 1, 3, 5% w/w of MMT, promoted a greater absorption of water vapor present in the atmosphere and favored the formation of pores in these regions where MMT and CaCl 2 were dispersed on the surface of the membrane.In this step, the acid chloride reacts with water, contributing to the formation of a structure with a greater number and larger pores on the top surface 53 .
For all cross-sections analyzed it was observed the formation of macrovoids interconnected internally.The formation of these pores is attributed to the presence of a larger volume of trapped gases near the selective layer due to the concentration of PA6 54 .It was also observed almost the same thickness of the selective layer and support of pure PA6 membrane/CaCl 2 compared to their hybrids 1, 3, 5% w/w of MMT.
In Figure 7 is shown the pore size distribution characterized by the mercury intrusion porosimetry of pure PA6 membranes and their hybrids 1, 3, 5% w/w of MMT, adding 10% of CaCl 2 .It was observed a similar behavior of the curves for all membranes.The peak on the left side covers a range of diameters with an average of approximately 180 μm which corresponds to the larger pores, which would be the porous support membrane 55 .It was noted that from this peaks is not possible to make an precise analysis of the pores in the support, since they have very large pore sizes above detection range mercury porosimetry.The region of the curve, which is the rightmost, is related to the pores on the selective layer of the membrane, which is responsible for the phase separation.It was observed that the presence of clay and of CaCl 2 in the hybrid membranes had a direct influence on increasing pore size, in relation to the pure PA6 membrane.Furthermore, this increase was directly proportional to the increase in the percentage of clay.

Membranes
Tm γ / Tm α (ºC) ΔH F (J.g -1 ) X c (%) 1 T m = Melting temperature; ΔH F = Melting enthalpy; X c = Degree of crystallinity, ΔH F /ΔH F100 x 100%; Δ HF100 = Melting enthalpy of PA6 with 100% of crystallinity, 188 J.g -128 ; The estimation of the degree of crystallinity was normalized in relation to the PA6.Also according to the curves of Figure 7, it is clear that pure PA6 membrane show an medium pore diameter of 1.0 µm, with low porosity of 29%, enabling a significant reduction in the permeate flux.Therefore, the clay is incorporated into the membrane in order to act as porogenic agent promoting the increase of the pores and porosity of the obtained membrane.
Moreover, the hybrid membranes exhibited medium pore diameters in the range between 1.4 and 1.9 µm, with porosity ranging from 34-42%.The values obtained for average pore diameters and porosity is appropriate for hybrid asymmetric microporous membranes, classifying for applications in microfiltration processes.The permeate flux with distilled water to all membranes (Figure 8) initially presented a decrease and stabilized after 40 minutes.This decreases was due to a mechanical compaction promoted by the applied pressure.This process uses the pressure gradient as the driving force, and the permeate flux is directly proportional thereto.When it comes to a pure solvent and without interaction with the material of the membrane, the permeate flux for a given operating pressure should be constant, since the membrane present a mechanical stability.The PA6 is a hydrophilic polymer, facilitating the absorption of water by reducing the permeate flux rate and size of pores existing on the surface of these membranes 51,[55][56][57][58] .
The permeated flux (J) of the membranes for oil/ water emulsion was plotted in the relation to stabilized distilled water flux (J 0 ) and can be observed in Figure 9.It was observed for all membranes an initial decline and a stabilization of the permeated flux after a certain time.The  membranes with MMT (1, 3 and 5%) presented higher J/ J 0 compared with PA6 pure membrane and this behavior is due to the formation of larger pores at surface of these membranes 17,18,55 .
In general, J/J 0 tends to be lower when using high concentration emulsions.The increase in oil concentration provides clogging of the pores of the membranes and also contributes to a concentration polarization on the membrane surface, leading to a decrease in the J/J 0 19,20 .Table 2 shows the oil concentration in the permeate (C P ), in the feed suspension (C 0 ) and the rejection coefficient, using synthetic emulsion, for the pure PA6 membranes and their hybrids 1, 3, 5% w/w of MMT, adding 10% w/w of CaCl 2 .For all membranes the rejection coefficient was higher for the feed oil concentrations of 100 and 200 mg.L -1 .These behavior can be explained by the mean droplet size dispersion of oil for 50 and 200 mg.L -1 of oil concentrations, and these sizes are in the range of the average pore size of the selective layer of the membranes [57][58][59] (see Table 2).Also due to the formation of oil polarization concentration close to the membranes surface, leading to clogging and obstructing thus the passage of oil, mainly at the highest concentration (200 mg.L -1 ).The polarization concentration is inherent in any selective transport process and contributes to the formation of a layer of polarization which provides additional resistance to mass transfer of solvent through the membrane, causing a drop in the flux permeated [60][61][62] .
All the studied membranes in the separation process of oil/water emulsions at the concentrations (50, 100 and 200 mg.L -1 ) achieved a significant reduction of oil in the permeate, meeting the requirements set by the resolution 430 of the National Environment Council, in Brazil, which states that the content of oils and greases in water must have a maximum concentration of 20 mg.L -1 to be discarded in the environment 63 .

Conclusions
Hybrid membranes of polyamide6 (PA6) with 1, 3 and 5% of untreated clay (MMT) and addition of porogenic agent (CaCl 2 ) was prepared with success.The hybrid membranes presented crystalline phases α 1 and α 2 (alpha) feature of polyamide and an exfoliated and / or partially exfoliated structure.The infrared spectra of the hybrid membranes with CaCl 2 remained unchanged, except for the slight increase in the intensity of the peaks at 800, 935, 1033 and 1095 cm -1 , due to the octahedral layer and the vibration of the stretch of Si-O-Si group from clay.There was no change in the crystalline melting temperature of the hybrid membranes with CaCl 2 when compared with pure PA6 membrane.From the SEM images it was observed that the addition of clay and CaCl 2 in the membranes caused an increase in the amount of pores and in the distribution uniformity of them, on the surface and in cross-section of the membranes when compared with pure PA6 membrane.The presence of clay and additive in the membrane caused an increase in average pore diameters of the selective layer, in the range from 1.2 to 2.7 µm.In general, the permeate flux with distilled water in membranes decreases due to a swelling and/or compression of the membranes.The water-oil separation tests, with feed concentration from 50 to 200 mg.L -1 of oil in water, indicated a significant reduction of oil in the permeated with results below 20 mg.L -1 , meeting the requirements set by the National Environment Council, in Brazil, and with potential for the treatment of wastewater contaminated with oil.

Figure 7 .
Figure 7. Pore size distribution for pure PA6 membranes and their hybrids with 1, 3, 5% of MMT, adding the additive CaCl 2 , by the mercury intrusion porosimetry.

Figure 8
Figure8shows the curves of the distilled water flux to the pure PA6 membrane and their hybrids 1, 3, 5% w/w of MMT, adding 10% w/w of CaCl 2 .The permeate flux with distilled water to all membranes (Figure8) initially presented a decrease and stabilized after 40 minutes.This decreases was due to a mechanical compaction promoted by the applied pressure.This process uses the pressure gradient as the driving force, and the permeate flux is directly proportional thereto.When it comes to a pure solvent and without interaction with the material of the membrane, the permeate flux for a given operating pressure should be constant, since the membrane present a mechanical stability.The PA6 is a hydrophilic polymer, facilitating the absorption of water by reducing the permeate flux rate and size of pores existing on the surface of these membranes51,[55][56][57][58] .The permeated flux (J) of the membranes for oil/ water emulsion was plotted in the relation to stabilized distilled water flux (J 0 ) and can be observed in Figure9.It was observed for all membranes an initial decline and a stabilization of the permeated flux after a certain time.The

Table 2 .
The oil concentration in the feed suspension (C 0 ), in the permeate (C P ) and the rejection coefficient for all membranes at a pressure of 1.0 bar using synthetic emulsion