layered double hydroxide adsorptive membranes for efficient removal of pharmaceutical environmental contaminants

Carbohydrate Polymers 214 (2019) 204–212

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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Cellulose acetate/layered double hydroxide adsorptive membranes for efficient removal of pharmaceutical environmental contaminants

T

Matei D. Raicopola, Corina Andronescue, Stefan I. Voicub,c, Eugeniu Vasiled, ⁎ Andreea M. Pandeleb,c, a

University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, Polizu 1-7, Bucharest, Romania Advanced Polymer Materials Group, University Politehnica of Bucharest, Polizu 1-7, Bucharest, Romania c Department of Analytical Chemistry and Environmental Engineering, University Politehnica of Bucharest, Polizu 1-7, Bucharest, Romania d Univerisity Politehnica of Bucharest, Romania e Chemical Technology III, University Duisburg-Essen and NETZ – Nano Energie Technik Zentrum, CENIDE Center for Nanointegration, Carl-Benz-Straße 199, D-47057 Duisburg, Germany b

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanocomposite membrane Cellulose acetate Layered double hydroxide

The increasing amount of residual pharmaceutical contaminants in wastewater has a negative impact on both the environment and human health. In the present study, we developed new cellulose acetate/Mg-Al layered double hydroxide (Mg-Al LDH) nanocomposite membranes as an efficient method to remove pharmaceutical substances from wastewater. The morphology, porosity, surface properties and thermal stability of nanocomposite membranes containing various amounts of nanofiller were evaluated by scanning electron microscopy (SEM), X-ray microtomography (μCT), contact angle measurements and thermogravimetric analysis (TGA). The Mg-Al LDH nanofiller showed a high degree of exfoliation in the polymer matrix, evidenced by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The hydrodynamic properties and adsorption capacity were evaluated with pure water and aqueous solutions of two common drugs, diclofenac sodium (DS) and tetracycline (TC), and the nanocomposite membranes showed an improved permeability compared with neat cellulose acetate. The membrane prepared with 4 wt.% Mg-Al LDH loading exhibited the highest water flux compared with the pure polymer one (529 vs 36 L·m−2·h-1) and a tenfold increase in adsorption capacity for DS. This enhancement is attributed to electrostatic interactions between the negatively charged drug molecule and positively charged Mg-Al LDH layers. Conversely, in the case of TC, the increase in adsorption capacity was smaller and was assigned to hydrogen bonding interactions between the drug molecule and the nanofiller.

1. Introduction Pharmaceutical substances are an important class of emerging environmental contaminants, which lately led to increased concerns as large quantities of these substances end up both into surface waters and wastewaters (Miege, Choubert, Ribeiro, Eusebe, & Coquery, 2009; Wang & Wang, 2016). Among the frequently encountered pharmaceutical contaminants are antibiotics such as tetracycline (TC) and antiinflammatory drugs such as diclofenac sodium (DS), whose environmental concentration can be as high as 100–500 mg/L in the case of TC and over 28 μg/L in the case of DS (Fu et al., 2015; Qi, Zhihao, Xiuwen, Pu, & Huiling, 2015) and their elimination rate by conventional treatments is rather low (Pojana, Fantinati, & Marcomini, 2011). Polymer membrane technology is considered a promising method

⁎

for the removal of pharmaceutical contaminants from wastewater (Wang et al., 2018). This is due to the fact that membrane processes provide high removal efficiency at a lower energy cost. Although cellulose acetate (CA) is a common polymer frequently employed for the fabrication of porous polymeric membranes it has some disadvantages such as low water permeability, poor mechanical properties and susceptibility to both chemical and microbial attack. So far, many studies aimed at solving these issues and also increasing membrane selectivity and permeability by employing CA blends with other miscible polymers (El-Gendi, Abdallah, Amin, & Amin, 2017; Munirah, Mimi Mazira, Ramlah, & Sharifah, 2014; Riaz et al., 2016) or by adding nanofillers such as layered clays (Corobea et al., 2016), the most common being montmorillonite as it possesses cation-exchange properties and can be easily modified (Dehkordi, Pakizeh, & Namvar-Mahboub, 2015;

Corresponding author. E-mail address: [email protected] (A.M. Pandele).

https://doi.org/10.1016/j.carbpol.2019.03.042 Received 14 January 2019; Received in revised form 1 March 2019; Accepted 12 March 2019 Available online 14 March 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.

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Rodríguez, Galotto, Guarda, & Bruna, 2012; Zhang et al., 2016). In contrast, layered double hydroxides (LDHs) are class of lamellar compounds with a brucite-like structure having the general formula [M2+ 13+ x+ x2+ [An− is a divalent cation like Mg, x/n-·mH2O] , were M xMx (OH)2] Ni, Cu; M3+ is a trivalent cation such as Al, Cr, Fe, V and Ga. The value of x determines the charge density and anionic exchange capacity and ranges between 0.2 and 0.4 (Desigaux et al., 2006). Due to the multiple layers with opposite charges, more specifically an excess of positive charge (M3+) balanced by anions (An−) in the interlayers, LDHs have the ability to participate in anion exchange reactions (Lin, Fang, & Chen, 2014). Also, it is well known that LDHs are active adsorbents and have large surface areas, high thermal stability and high porosity (Sepehr, Al-Musawi, Ghahramani, Hazemian, & Zarrabi, 2017). These remarkable properties allow the use of LDH in wastewater treatment (Wiyantoko, Kurniawati, Purbaningtias, & Fatimah, 2015). Current approaches for obtaining CA-LDH composite membranes include the synthesis of CA/LDH hybrid films through solvent evaporation (De Castro et al., 2018) or LDH incorporation into CA and poly(acryloyl morpholine) blends in order to increase the thermo-mechanical stability (Yoshitake et al., 2013). Also, commercial CA membranes were modified through layer-by-layer deposition of sepiolite, LDH and graphene oxide in order to obtain 3D hierarchical micro-structures for oil/ water separation (Li, Gao, Wu, & Li, 2017). The aim of this study is to obtain highly versatile CA membranes through the phase inversion approach, by incorporating Mg-Al LDH intercalated with sodium dodecyl sulfate (SDS) within the polymer matrix, in order to increase their permeability and adsorption capacity for pharmaceutical contaminants. The structure of CA/Mg-Al LDH nanocomposite membranes was investigated using scanning electron microscopy (SEM), X-ray microtomography (μCT), X-ray diffraction (XRD) and contact angle measurements. The hydrodynamic and adsorption characteristics of the nanocomposite membranes were further evaluated using pure water and aqueous solutions of two commonly pharmaceutical contaminants, DS and TC. We demonstrate that the addition of the nanofiller leads to a considerable increase in porosity, hydrophilicity, water permeability and improves the mechanical properties of the resulting nanocomposite membranes. 2. Experimental 2.1. Chemicals and reagents CA with a molecular weight of 34,000 Da and an acetylation degree of 67%, HPLC grade N,N’-dimethylformamide (DMF) and acetone were purchased from Sigma-Aldrich. Mg(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH, sodium dodecyl sulfate (SDS), diclofenac sodium (DS) and tetracycline (TC) were of analytical grade and purchased from SigmaAldrich. All solutions were prepared with distilled water. The Mg-Al LDH intercalated with SDS was synthesized according to the protocol described by Andronescu, Garea, Vasile, and Iovu, (2014).

blotted with filter paper, pressed to obtain a smooth surface and dried in an oven at 40 °C for 24 h. 2.3. Nanocomposite characterization Thermogravimetric analysis (TGA) was performed using a TA Instruments Q500 thermogravimetric analyzer, from room temperature to 600 °C, at a heating rate of 10 °C/min, under both nitrogen and air atmospheres. Differential Scanning Calorimetry (DSC) measurements were carried out on a Netzsch DSC 204 F1 Phoenix equipment. Samples were heated from 20 to 300 °C at a heating rate of 10 °C/min under a flow of nitrogen (20 mL/min). The glass transition temperature (Tg) was evaluated using the intersection of the tangents at the DSC cooling curves. The morphology of CA/Mg-Al LDH nanocomposite membranes was evaluated using a FEI QUANTA INSPECT F scanning electron microscope (SEM) and images were taken at 500x magnification for all the samples. Transmission electron micrsocopy (TEM) images were recorded on a Philips CM120STHRTE microscope. Small membrane pieces were embedded in epoxy resin and, after complete curing of the resin, thin sections were cut with an ultramicrotome and transferred to copper grids covered with holey carbon films. X-Ray microtomography (μCT) was performed on a Bruker SkyScan 1272 microCT. Rectangular specimens (˜5 mm length and ˜3 mm width) were cut from the middle of each membrane. Image acquisition was made with a resolution (pixel size) of 2 μm, a rotation step of 0.2° and 10 average frames per capture. The Bruker NRecon1.7.1.6 software package was used to reconstruct the raw images and then the total porosity, structure thickness and specific surface area were quantified using the Bruker CTAn analysis software. For that, the 3D reconstructed images obtained from μCT were subjected to a sequence of steps including the selection of a region of interest, image binarization after thresholding and morphometric 3D analysis. After thresholding, the binarized images are black and white, without intermediary grey tones. White is associated to the sample itself, while the empty space (pores) is depicted in black. Structure thickness is a term used to describe the width of the sample walls as a function of the product between the number of white pixels and the scanning resolution (pixel size). X-Ray diffraction was performed on a Panalytical X’Pert Pro diffractometer using CuKα radiation (λ = 1.5418 Å) from 2° to 70° (2θ) at a scanning rate of 1° per min. The membrane porosity was also estimated using the water intrusion method (Zhang, Lu, & Zhao, 2014). Membranes were weighed, immersed in distilled water for 24 h at RT, then carefully taken out, wiped with filter paper and weighed again. Porosity was calculated using the following formula (Eq. (1)):

Ɛ=

(Ww − Wd ) ƍ w (Ww − Wd ) Wd ƍw + ƍd

(1)

where ε is the membrane porosity (%), Ww and Wd are the mass of the wet and dry membranes, ƍw is the water density (1.0 g/cm3) and ƍd is the membrane density (1.28 g/cm3) (Guler, Elizen, Vermaas, Saakes, & Nijmeijer, 2013). Three measurements were performed for each sample and porosity is reported as average value. The contact angles (θ) were measured on the active surface of dry membranes with a Kruss DSA100S Drop Shape Analyzer using the sessile drop method. Advancing (θa) and receding (θr) contact angles were recorded by increasing and decreasing the drop volume (Uscar et al., 2010). At least five contact angle measurements were performed in different locations on the sample and results are reported as average values. The contact angle hysteresis (Δθ) was calculated as the difference between θa and θr. The surface free energy was estimated using the Owens, Wendt, Rabel and Kaelble (OWRK) method after measuring the contact angles for water and ethylene glycol (Eqs. (2) and (3)):

2.2. Synthesis of the composite membranes 12 g of CA powder was dissolved at room temperature in 93 mL DMF under magnetic stirring for 24 h at 20 °C, in order to obtain a homogenous solution. The dried Mg-Al LDH/SDS powder was dispersed into 25 mL of polymer solution at several concentrations (0; 0.03 g; 0.06 g and 0.12 g Mg-AL LDH) by ultrasonication for 1 h. The neat CA and the nanocomposite membranes were obtained through the phase inversion method (Pandele et al., 2017). The polymer solutions were cast on a glass plate using a doctor blade and the plate was gently immersed in a coagulation bath (100 mL of distilled water at 20 °C) for 2 min., in order to complete the formation of the membranes without allowing the solution to evaporate. The membranes were taken out of the coagulation bath and thoroughly washed with distilled water and ethanol to remove residual DMF. After that, the wet membranes were

σs = σsl + σl · cosθ 205

(2)

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Fig. 1. TGA curves recorded in nitrogen (A) and air (B) for CA and CA/Mg-Al LDH composite membranes.

σsl = σs + σl − 2 ( σsD⋅σlD +

σsP + σlP )

3. Results and discussions

(3)

where σl is the surface tension of the liquid, σs is the surface free energy of the solid, σsl is the interfacial tension between the liquid and solid, σD is the disperse part and σP is the polar part of the surface free energy. The membrane performance in terms of water flux and drug retention was assessed at RT using a dead-end filtration setup (laboratory pressure ultrafiltration cell, Millipore XFUF04701). Before performing the experiments, each membrane was washed with 10 mL ethanol and twice with 20 mL of distilled water. Water flux measurements were carried out using 60 mL of distilled water, at several transmembrane pressures between 2.0 and 3.0 bar. The drug retention was evaluated using 60 mL of 0.01 g∙L−1 DS or TC aqueous solutions as feed solutions, at a transmembrane pressure of 2.0 bar. Each test was performed on three different membranes having the same composition, the permeate was recirculated five times and the water flux and drug retention values were averaged. The concentration of drugs in the feed solutions and permeate was determined spectrophotometrically at λ = 280 nm for DS and λ = 360 nm for TC, respectively, using a Shimadzu UV-3600 UV-VIS spectrometer fitted with a quartz cell having a light path of 10 mm. The water flux (Wf) expressed in L·m−2·h-1and the percentage drug retention (R) were calculated by the following formulas (Eqs. (4) and (5)):

Wf =

R=

V A∙t

CA/Mg-Al LDH nanocomposites were synthesized using the phase inversion method, starting from stable dispersion of CA and different amounts of Mg-Al LDH intercalated with SDS anions. These particular nanofiller were chosen because we expected that an LDH intercalated with organic anions assures a better compatibility with the organic polymer matrix (Zhidong, Xinke, Yue, Zhengquan, & Peng, 2014). Furthermore, the intercalation of large anions such as SDS increases the distance between the LDH layers and improves the dispersibility in the polymer matrix, leading to intercalated and/or exfoliated nanocomposites (Andronescu et al., 2014). In order to see the influence of the amount of Mg-Al LDH on the structural changes of the CA membranes, composites containing 1, 2 or 4% wt. Mg-Al LDH were synthesized.

3.1. Thermal characterization In order to investigate the thermal stability of the composite membranes, TG analysis was performed under both nitrogen and air atmospheres. The thermal degradation profile of CA recorded in a nitrogen atmosphere (Fig. 1A) exhibits two degradation steps, the first being assigned to the evaporation of the solvent and the second step is attributed to the degradation of the CA polymer chains (Pandele et al., 2017). CA/Mg-Al LDH composite membranes display a similar behavior, independent on the amount of filler (Azzaoui et al., 2014). Contrary to our expectations, the thermal stability enhancement of the nanocomposite materials is modest, and it is tentatively assigned to the cooling effect brought by the release of water from the LDH nanofiller. A similar behavior was observed by Andronescu et al. (2014) who investigated several types of LDHs as reinforcing agents for polybenzoxazine resins. The TGA curves recorded in air display a different degradation profile compared with the one recorded in nitrogen, showing a new degradation step ˜460 °C, assigned to the thermo-oxidative carbonization amorphous carbon residue. The composite membranes show a slightly increased thermostability in oxidative atmosphere. In the presence of 4% Mg-Al LDH, the 10% mass loss achieved by the membrane is registered at 12 degrees higher temperature than for the neat polymer. The mass residue of pure CA is 8% while in the case of the nanocomposite membranes increases, reaching a value of 20% in the case of the CA/Mg-AL LDH 4 wt.%. The mass residue in the case of the nanocomposite membranes is attributed to the formation of oxides and oxydydroxides derived from Mg-Al LDH, as well as to the condensed carbon structures formed during CA degradation (De Castro et al., 2018) (Castro et al., 2018) (Table 1).

(4)

CF − CP * 100 CF

(5) −2

where Wf is the water flux (L·m ·h ), V is the volume (L) of the feed solution, A is the area (m2) of the membrane, t is the time (h), R is the drug retention (%), and CF and CP are the concentration (g∙L-1) of drugs in the feed and the permeate, respectively. The membrane hydraulic permeability (Rm) was calculated from the slope of the water flux (Wf) versus transmembrane pressure (ΔP) plot, according to Eq. (6) (Arthanareeswaran, Thanikaivelan, Sirinivas, Mohan, & Rajendran, 2004).

Rm =

ΔP Wf

-1

(6)

Mechanical tests were performed on wet samples using a universal testing machine (Instron, Model 3382) at a relative humidity of 45–50 % and a speed of 0.5 mm/min. The size of the samples was 10 cm × 1 cm. At least six specimens were tested for each membrane composition and the average values are reported. Young’s Modulus was calculated from the slope of the linear portion of the stress-strain curve. 206

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the total porosity, structure thickness (i.e. the average width of the sample walls) and specific surface area (i.e. surface area divided by volume) of the synthesized materials, the values being summarized in Table 2. It is apparent that the total porosity increases and the structure thickness decrease in case of the nanocomposite membranes. The lower structure thickness value corresponding to the nanocomposite membranes indicates that pore walls are thinner in this case, which confirms the SEM results previously discussed. Also, the structure thickness can provide valuable information regarding the materials’ mechanical properties, lower values being associated with an increased elasticity (Ionita et al., 2015). Fig. 4 shows the XRD patterns for the Mg-Al LDH intercalated with SDS, neat CA and CA/Mg-Al LDH composite membranes. The aim of the XRD investigation was to study the exfoliation of the Mg-Al LDH nanofiller within the polymer matrix. The XRD pattern of neat CA displays two peaks located at 2θ = 8.36° and 17.56°, which correspond to the ordered and less ordered regions of the polymer, as it is known that the CA structure consists of blocks of pyranose rings (ordered region) and domains of acetyl pendant groups (discontinuous region) (Das, Ali, & Hazarika, 2014). The diffraction pattern of Mg-AL LDH intercalated with SDS displays five peaks located at 2θ = 3.11°, 6.92°, 20.93°, 35.38° and 61.24°, consistent with both the JCPDS File Card No. 22-700 of rhombohedral hydrotalcite and other values reported in the literature (Chuang, Tzou, Wang, Liu, & Chiang, 2008; Li, Luo, Xiao, Wang, & Meng, 2016). The XRD patterns corresponding to the composite membranes are similar with the XRD pattern of neat CA, as no additional peaks are observed. The fact that the Mg-Al LDH diffraction peaks are missing in this case is an indication that exfoliated structures are obtained (Ferfera-Harrar & Dairi, 2013). These results are supported by a TEM analysis. As can be seen in the TEM micrographs (Fig. 5), the Mg-Al LDH nanofiller display a high degree of exfoliation. Also, the nanofiller is well dispersed in the CA matrix at lower concentration (2 wt. %) but the nanocomposite containing the largest amount of nanofiller (4 wt.%) appears inhomogeneous due to the agglomeration of Mg-Al LDH.

Table 1 The TGA data for CA and CA/ Mg-Al LDH composite membranes. Sample

CA CA/Mg-Al LDH 1 wt. % CA/Mg-Al LDH 2 wt. % CA/Mg-Al LDH 4 wt. % *

Total mass loss at 600 °C (%)

Td10% (oC)*

Tmax (o C)**

N2

Air

N2

Air

N2

Air

90 90

92 92

293 292

297 301

334 356

335 340

456 461

90

89

306

302

357

340

460

87

80

310

310

359

347

459

Temperature at which the weight loss is.10% Maximum degradation temperature.

**

3.2. Structural characterization Scanning electron microscopy (SEM) was employed to assess the influence of the Mg-Al LDH nanofiller on the polymer membrane morphology. For each membrane composition, micrographs were taken both on the active and porous layers and also on the cross-section (Fig. 2). The cross-section images for all the samples display an asymmetric sponge-like structure with two distinct layers -an active and dense layer, and a porous layer. The incorporation of inorganic nanofiller within the polymer matrix leads to a notable modification of the internal structure of the nanocomposite membranes. While pure CA displays a heterogeneous structure with irregular pores, the formation of macrovoids in the porous layer of the CA/Mg-AL LDH composite membranes could be observed. Also, at a close examination of the images, the pore wall thickness increases in case of the composite membranes with 1 and 2 wt. % Mg-Al LDH. The formation of the macrovoids is attributed to the high affinity of Mg-Al LDH to water; a similar behavior being observed when graphene oxide was used as nanofiller for CA membranes (Vetrivel, Saraswathi, Rana & Nagendra, 2018). Conversely, the composite membrane with the highest concentration of Mg-Al LDH (4 wt.%) also exhibits an ordered structure but the macrovoid walls are thinner in this case. Furthermore, the cross-section SEM images reveal a significant change in membrane thickness, which decreases with increasing Mg-Al LDH concentrations (Table 2). An explanation for this fact can be the high hydrophilicity of Mg-Al LDH, which leads to a significant increase of the membrane coagulation rate during the precipitation of the membrane, when the non-solvent starts to extract the solvent from the polymer solution (Vetrivel, Saraswathi, Rana, & Nagendran, 2018). Although the amount of polymer is the same for all the samples, by increasing the Mg-Al LDH content the active layer of the membranes becomes more compact, resulting in a thinner membrane. Considering the top view of the membranes, the active surface shows a porous structure with open pores for all the samples. In case of the porous layer, the images reveal changes in terms of pore size and density for the composite membranes with high Mg-Al LDH loadings. The average pore diameter decreases from 75 ± 4 μm to approx. 40 ± 2 μm for the composite with 4 wt.% Mg-Al LDH. This can also be explained by the presence of the inorganic nanofiller, which increases the coagulation rate. In order to have a general view on the internal structure and estimate the porosity of the synthesized materials, μCT was performed as a complementary characterization to SEM. Fig. 3 illustrates the 3D reconstruction models of both the active and porous layer for each membrane composition. All membranes display a porous structure with open and interconnected pores, and the presence of inorganic nanofiller seems to have an impact on the pores density and uniformity. By increasing the Mg-Al LDH content within the polymer matrices the pores become more regular and their average diameter decreases. Furthermore, the analysis of μCT images allowed an estimation of

3.3. Surface properties It was expected that the amount of Mg-Al LDH incorporated in the nanocomposites, as well as its degree of exfoliation, have a high impact on the membrane hydrophilicity. Obviously, an increased hydrophilicity is important from a technological point of view, especially in wastewater treatment, because it leads to an increase of the water flux which can be processed through the membrane. Therefore, in order to evaluate the surface characteristics of the nanocomposite membranes, contact angle measurements were conducted and results are summarized in Table 3. It can be seen that the incorporation of Mg-Al LDH nanofiller within the polymer matrix leads to a decrease of the water contact angle compared with neat CA, which indicates an increased hydrophilicity (Awaja, Gilbert, Kelly, & Fox, 2009). Further, dynamic water contact angle measurements were performed and the values of the advancing and receding contact angles as well as the contact angle hysteresis are reported in Table 3. As expected, the Δθ values corresponding to the nanocomposite membranes are higher than the value for pristine CA and increase with increasing LDH content. Since Δθ depends on the surface roughness and chemical heterogeneity of the material, this enhancement is attributed on one hand to the increased membrane porosity and on the other hand to the contrasting hydrophobicity/hydrophilicity of CA and LDH nanofiller (Zhang, Wahlgren, & Sivik, 1989). With the contact angle values measured for distilled water and ethylene glycol (EG) we also calculated the surface free energy of the samples using the OWRK method. In this approach, the surface free energy is divided into a polar part and a disperse (non-polar) part (Amelia & Winnicka, 2017). The surface free energy of CA membranes 207

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Fig. 2. SEM micrographs of the CA and CA/Mg-Al LDH nanocomposite membranes.

Mg-Al LDH nanocomposite membranes have also been evaluated and results are presented in Fig. 6A and Table 4. The pristine CA membrane exhibits an average flux of 36 L·m−2·h-1. The incorporation of Mg-Al LDH in the nanocomposites brings a spectacular improvement of hydrodynamic properties in terms of water permeability, the average flux reaching 529 L·m−2·h-1 for the nanocomposite membrane containing 4 wt. % Mg-Al LDH. This can be explained by taking into account the combined effects of an enhanced hydrophilicity of the nanocomposite membranes as evidenced by water contact angle measurement, the decrease in membrane thickness and the increase in total porosity (Amelia & Winnicka, 2017) as evidenced by the SEM analysis. Assessing the performance of the nanocomposite membranes for the

was found to be 42 mN/m, and this value increases with the addition of nanofiller. From the data presented in Table 4 it appears that the enhancement of surface free energy is due to an increase of the polar part, which is to be expected after the incorporation of the charged Mg-Al LDH nanofiller. Somewhat surprising however is the fact that the nanocomposite containing 2 wt.% Mg-Al LDH has the highest surface free energy, 55 mN/m, and this was also explained by the nanofiller agglomeration at higher concentration (i.e. 4 wt. %). 3.4. Membrane performance The water flux and hydraulic permeability for pristine CA and CA/ 208

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Table 2 The membrane thickness, total porosity, structure thickness and specific surface area for neat CA and nanocomposite membranes. Sample

Membrane Thickness (μm)

Total Porosity* (%)

Structure Thickness (μm)

Specific Surface Area (1/μm)

Membrane Porosity** (%)

CA CA/Mg-Al LDH 1 wt. % CA/Mg-AL LDH 2 wt. % CA/Mg-Al LDH 4 wt. %

338 162 215 134

97 98 98 99

18.79 13.99 11.69 12.41

0.17 0.23 0.27 0.27

97 97 98 99

computed from μCT data for the entire volume of the sample; estimated using the water intrusion method. The membrane porosity was also estimated using the water intrusion method (Zhang et al., 2014), the obtained values ranging from 97% to 99%, in good agreement with the values computed from μCT data. *

**

adsorption of pharmaceutical contaminants is very important from a technological standpoint. Diclofenac sodium and tetracycline were chosen in this study since they have different functional groups, so different interactions with the membrane surface are to be expected. The adsorption capacity of the neat CA and composite membranes was investigated with feed solutions containing 0.01 g/L DS (pH = 7) and 0.01 g/L TC (pH = 6.9), respectively. The solution pH is a very important factor for the adsorption performance because it has a high impact on the degree of ionization for both drug and membrane. In the case of DS (pKa = 4.2), the CA membrane retained only 2.7% of the drug, due to a lack of electrostatic interaction between the neutral polymer and the negatively charged DS molecule. After the introduction of LDH nanofiller, the adsorption capacity of the membranes increases remarkably, reaching 21% in case of the composite with 4 wt.% Mg-Al LDH (Fig. 6B). The increase in adsorption capacity can be explained by the fact that at pH = 7, DS molecules are mainly in anionic form (−COO−), which allows the electrostatic attraction with the positively charged LDH layers (Li et al., 2018). Conversely, in the case of TC which contains hydroxyl, amine and amide groups (pKa1 ≈ 3.4; pKa2 ≈ 7.5; pKa3 ≈ 9.3), the neat CA membrane retained about 17% of the drug. This rather high value can be attributed to hydrogen bonding interactions with the hydroxyl groups of CA (Kesting & Eberlin, 1966). With the addition of LDH into the polymer, the adsorption capacity also increases (Fig. 6B). Because the TC molecule is negatively charged only

Fig. 4. The XRD diffractograms of Mg-Al/LDH-SDS, CA and nanocomposite membranes.

Fig. 3. μCT tomograms of CA and CA/Mg-Al LDH nanocomposite membranes. 209

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Fig. 5. TEM micrographs corresponding to the CA/Mg-AL LDH nanocomposite membranes containing 2 and 4 wt.% nanofiller. Table 3 Contact angle data for pristine CA and CA/Mg-Al LDH composite membranes. Sample

Water θr (º)

θa (º) CA CA/Mg-Al LDH 1 wt. % CA/Mg-Al LDH 2 wt. % CA/Mg-Al LDH 4 wt. %

65 59 57 54

± ± ± ±

1 1 1 1

46 23 22 15

± ± ± ±

EG θa (º)

Surface free energy (mN/m)

Polar part (mN/m)

Dispersive part (mN/m)

40 43 44 45

42 44 55 44

33 38 53 40

8 6 1 4

Δθ (º) 3 1 1 2

19 36 35 39

± ± ± ±

1 1 1 1

at pH values greater than 9.3, the enhancement of adsorption capacity was assigned in this case to the increased hydrophilicity of the nanocomposite membranes. Another explanation would be the van der Waals interaction between TC and SDS or hydrogen bonding with the hydroxyl groups on the surface of LDH (Wu et al., 2013). These results show very clearly that the interactions between drugs and the membrane surface are the determining factors which ultimately affect the adsorption performance, and the CA/Mg-Al LDH nanocomposite membranes developed in this study show promising properties for the removal of various pharmaceutical contaminants. Membrane performance was also assessed in terms of mechanical properties, and results are summarized in Table 4. The values show a decrease of Young’s Modulus and an increase of elongation at break after incorporation of inorganic filler, which means that the nanocomposite membranes have an increased elasticity. These results are also supported by DSC measurements, where a decrease of Tg values

Table 4 The hydraulic permeability, Young’s Modulus, elongation at break and Tg of the CA and CA/Mg-Al LDH composite membranes. Sample

Rm (L/m2·h·bar)

Young’s Modulus (kPa)

Tensile strain (%)

Tg (ºC)

CA CA/Mg-Al LDH 1 wt. % CA/ Mg-Al LDH 2 wt. % CA/ Mg-Al LDH 4 wt. %

20 102

694 ± 15 441 ± 9

11.4 ± 1.0 13.7 ± 109

188 176

192

420 ± 4

15.8 ± 1.5

177

268

290 ± 1

17.8 ± 1.2

173

Fig. 6. The water flux A) and drug retention B) on CA/Mg-Al LDH nanocomposite membranes measured at a transmembrane pressure of 2 bar. 210

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(Table 4) can be observed for the nanocomposite materials. Therefore, the inorganic filler acts as plasticizer, presumably due to the presence of SDS in its structure, and thus enhances the membranes’ mechanical properties.

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4. Conclusions Using a phase inversion method we successfully obtained nanocomposite membranes based on cellulose acetate and Mg-Al LDH intercalated with SDS. The membranes were characterized both structurally and morphologically using various methods. A SEM examination revealed that nanocomposite membranes are thinner and have a more homogenous structure with regular pores, an observation confirmed by X-ray microtomography. The decrease in membrane thickness was attributed to the hydrophilic nature of the nanofiller, leading to an increased coagulation rate during the precipitation step. The exfoliation of Mg-Al LDH in the nanocomposite was demonstrated using XRD and TEM. In terms of surface properties, the nanocomposite membranes are more hydophilic, as indicated by contact angle measurements. Furthermore, the hydrodynamic properties and adsorption capacity of the nanocomposite membranes were assessed using aqueous solutions of two commonly encountered pharmaceutical environmental contaminants. The membrane prepared with 4 wt.% Mg-Al LDH loading exhibited the highest water flux (529 L·m−2·h-1) and a tenfold increase in adsorption capacity for diclofenac sodium compared with neat cellulose acetate. This enhancement was attributed to electrostatic interactions between the negatively charged drug molecule and positively charged Mg-Al LDH layers. Conversely, in the case of tetracycline, the increase in adsorption capacity was smaller and was assigned to hydrogen bonding interactions between the drug molecule and Mg-Al LDH. The membranes were also tested in terms of mechanical properties and the nanocomposite materials demonstrated an increased elasticity, judging from the lower Young’s Modulus and higher tensile strain values. The improvement of mechanical properties was further confirmed by DSC results, which showed a decrease in Tg for the nanocomposites. Acknowledgement This work has been funded by University Politehnica of Bucharest, through the “Excellence Research Grants” Program, UPB – GEX 2017. Identifier: UPB- GEX2017, Ctr. No. 72/25.09.2017” . Contact angle and μCT measurements were performed on equipment aquired through the European Regional Development Fund, Competitiveness Operational Program 2014-2020, Priority axis 1, Project No. P_36_611, MySMIS code 107066, Innovative Technologies for Materials Quality Assurance in Health, Energy and Environment - Center for Innovative Manufacturing Solutions of Smart Biomaterials and Biomedical Surfaces – INOVABIOMED. References Amelia, A., & Winnicka, K. (2017). Polymers in pharmaceutical taste masking applications. Polymery, 62, 417–496. Andronescu, C., Garea, S. A., Vasile, E., & Iovu, H. (2014). Synthesis and characterization of polybenzoxazine/layered double hydroxides nanocomposites. Composites Science and Technology, 95, 29–37. Arthanareeswaran, G., Thanikaivelan, P., Sirinivas, K., Mohan, D., & Rajendran, M. (2004). Synthesis, characterization and thermal studies on cellulose acetate membranes with additive. European Polymer Journal, 40, 2153–2159. Awaja, F., Gilbert, M., Kelly, G., & Fox, B. (2009). Adhesion of polymers. Progress in Polymer Science, 34, 948–968. Azzaoui, K., Lamhamdi, A., Mejdoubi, E. M., Berrabah, M., Hammouti, B., Elidrissi, A., et al. (2014). Synthesis and characterization of composite based on cellulose acetate and hydroxyapatite application to the absorption of harmful substances. Carbohydrate Polymers, 111, 41–46. Chuang, Y. H., Tzou, Y. M., Wang, M. K., Liu, C. H., & Chiang, P. N. (2008). Removal of 2Chlorophenol from aqueous solution by Mg/Al layered double hydroxide (LDH) and modified LDH. Industrial & Engineering Chemistry Research, 47, 3813–3819. Corobea, M. C., Muhulet, O., Miculescu, F., Antoniac, I. V., Vuluga, Z., Florea, D., et al.

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