Journal of Science: Advanced Materials and Devices (2018) 188e195 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Effects of molar ratio and calcination temperature on the adsorption performance of Zn/Al layered double hydroxide nanoparticles in the removal of pharmaceutical pollutants ^eddine Elhalil*, Meryem Farnane, Aicha Machrouhi, Fatima Zahra Mahjoubi, Alaa Rachid Elmoubarki, Hanane Tounsadi, Mohamed Abdennouri, Noureddine Barka Laboratoire des Sciences des Mat eriaux, des Milieux et de la Mod elisation (LS3M), FPK, Univ Hassan 1, B.P 145, 25000 Khouribga, Morocco a r t i c l e i n f o a b s t r a c t Article history: Received 19 January 2018 Received in revised form March 2018 Accepted 22 March 2018 Available online 29 March 2018 This work focuses on the development of zinc/aluminum layered double hydroxides (LDHs) phases intercalated by carbonates ions (Zn-Al-CO3) and their use in the removal of pharmaceutical pollutants The materials were synthesized by the co-precipitation method at different Zn/Al molar ratios (r ¼ 1, and 5) Each synthesized material was calcined at 300, 400, 500 and 600 C to increase their performance Samples were characterized by various physicochemical techniques including XRD, FTIR, ICP-AES and TEM-EDX The as-synthesized and calcined products were used for the removal of salicylic acid (SA) as a model of pharmaceutical pollutants The results obtained show that the Zn/Al molar ratios and calcination temperatures have a great influence on the adsorption capacity The optimum adsorption efficiency was found to be 94.59% for Zn/Al molar ratio of and a calcination temperature of 300 C Kinetics of the adsorption takes place in two steps; the first fast rapid step can be interpreted by the adsorption on the external surface of the crystallites, while the second slow step could be due the reconstruction phenomenon of LDHs structure “memory effect” After the adsorption processes, XRD patterns show that the calcined product (r ¼ 3, T ¼ 300 C) was reconstructed by a salicylic acid The adsorption performance was slightly decreased with regeneration cycles © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Layered double hydroxides Reconstruction Pharmaceutical pollutants Regeneration Wastewater treatment Introduction Environmental contamination has reached a stage where it must be seriously examined Among the various types of pollution, water pollution has attracted the most attention of researchers The main sources of water contamination include industrial activities (food, textile, paper, rubber, leather, plastics, coal, petrochemical, pharmaceutical, etc.), agricultural activities (the use of pesticides and herbicides in agriculture, forestry, as well as veterinary and aquaculture drugs), and domestic activities [1e3] During the last decades, there has been a rising concern about the pharmaceuticals and personal care products (PPCPs) discovered in various surface and ground waters across the world [4,5] These chemicals include a wide variety of substances such as painkillers, tranquilizers, antibiotics, skin care products, hair styling agents and so forth [5] The * Corresponding author Fax: ỵ212 523 49 03 54 E-mail address: elhalil.alaaeddine@gmail.com (A Elhalil) Peer review under responsibility of Vietnam National University, Hanoi discharge of PPCPs or their metabolites into environment through the production process and daily consumption would pose longterm adverse effects, such as gene modification and resistance to drugs, on the aquatic microorganisms and human bodies, even at trace concentrations [6,7] Besides, due to the continuous usage and release into aquatic environment, the pollution caused by PPCPs usually exhibits the pseudo-persistent behavior [8] As a consequence, effective removal of such hazardous substances from water system needs to be given priority to avoid any potential toxicity to living organisms Salicylic acid (SA) is largely employed worldwide in many pharmaceutical formulations such as aspirin, lopirin, fenamifuril, diflunisal, salicylamide, and benorylatum [9e11] In spite of that, SA is a typical pollutant in the industrial wastewater, capable of causing serious environmental problems Also, SA is toxic to the human being, it can induce headache and nausea and even affects the normal functions of liver and kidney For these reasons, efficient removal and recycling of SA from aqueous solution is a pressing problem and has attracted numerous attentions in recent years [12,13] https://doi.org/10.1016/j.jsamd.2018.03.005 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) A Elhalil et al / Journal of Science: Advanced Materials and Devices (2018) 188e195 Several technologies including photocatalytic degradation, biological processes, membrane separation and adsorption have been used for the treatment of wastewater [14e21], among which adsorption is proved to be one of the most attractive and effective techniques [22,23] Layered double hydroxides (LDHs) or even anionic clays are the subject of a lively interest since these last years, because of their high anionic exchange capacity (2e5 mmol/g), their high specific surface area (20e120 m2/g), the presence of fillers on the surface, and especially the tradability of interlayered anions [24,25] The general formula of a LDH is: [MII1-xMIIIx(OH)2]xỵ.(Anx/n) mH2O, where MII represents a divalent cation (Mg2ỵ, Zn2ỵ, Ni2ỵ, Mn2ỵ, Fe2ỵ), MIII represents a trivalent cation (Al3ỵ, Cr3ỵ, Fe3ỵ, Co3ỵ, Mn3ỵ), An the compensating anion (ClÀ, NOÀ3, ClO2À4, CO2À3…), n the charge of the anion, and m is the number of water molecules located in the interlayer region together with the anion The coefcient, x, is the molar fraction, [MIII/(MII ỵ MIII)] [26] Our work focuses on the elaboration of LDH materials, based on zinc and aluminum metals and interspersed by carbonates ions (Zn-Al-CO3) Several Zn/Al molar ratios were synthesized by coprecipitation method Zn-Al-CO3 materials were calcined at different temperatures (300, 400, 500 and 600 C) in a tubular furnace Samples were characterized by different physicochemical techniques The as-synthesized and calcined products were investigated in the removal of salicylic acid from aqueous solution 189 inductively coupled plasma-atom emission spectrum (ICP-AES, JobinYvon Ultima2.) after dissolving the materials in HNO3 acid Transmission electron microscopy coupled to the energy-dispersive X-ray spectroscopy (TEM/EDX) images were collected on a TEM TECNAI G2/FEI instrument, at an accelerating voltage of 120 kV 2.4 Adsorption test A solution containing SA with an initial concentration of 30 mg/L was prepared by dissolving the desired quantity in bidistilled water The adsorption performances of different materials were carried out by mixing 250 mg of each sample in 250 mL of the above cited solution in beaker under stirring mL of solution was extracted at different time intervals and filtered by syringe filter (Minisart type NML, Membrane: A.C 0.2 mm absolute) to remove the particles for analysis, and the concentrations of SA were analyzed using a double-beam scanning spectrophotometer (Shimadzu spectrophotometer, model biochrom) at the wavelength of 297 nm The adsorbed quantity and adsorption efficiency (Removal (%)) were calculated using the following equations: qẳ C0 Cị R Removal %ị ẳ (1) C0 À C Ã100: C0 (2) Experimental 2.1 Materials The starting chemicals; zinc nitrate (Zn(NO3)2.6H2O), aluminum nitrate (Al(NO3)3.9H2O), sodium carbonate (Na2CO3), sodium hydroxide (NaOH) and salicylic acid have been purchased from SigmaeAldrich (Germany) Nitric acid, 65%, extra pure was obtained from Scharlau chemie (Spain) All the used chemicals were of analytical grade and were used without further purification Bidistilled water was used as the solvent throughout this study 2.2 Preparation of LDHs and their calcined products Zn-Al-CO3 layered double hydroxide materials were prepared by co-precipitation method from metal salts, at different Zn/Al molar ratios (r ¼ 1, and 5) A mixture solution of Zn(NO3)2.6H2O and Al(NO3)3.9H2O with a total concentration of metal ions of mol/L and Na2CO3 (1 mol/L) was added drop-wise in a backer containing 50 mL of bidistilled water The pH of the mixture was adjusted and kept constant at 8.5 ± 0.2 during the synthesis by adding suitable amounts of NaOH solution (2 mol/L) The gel formed was stirred vigorously for h and then transferred into an autoclave and hydrothermally treated at 75 C for 16 h Afterward, the suspension was filtered and washed with bidistilled water until reaching pH~7 and dried at 100 C for 24 h The resulting products (Zn-Al-CO3) were ground into fine powder and stored in sample bottles for further use Samples were calcined at different temperature (300, 400, 500 and 600 C) in a tubular furnace for h They were labeled as LDH-r-T, where r represents the Zn/Al molar ratio and T the calcination temperature 2.3 Characterization Powder X-ray diffraction (XRD) patterns of all samples were recorded in 2q range from to 70 at room temperature on a D2 PHASER diffractometer, using CuKa radiations with 30 KV and 10 mA FTIR spectra in KBr pellets were collected on a Perkin Elmer (FTIR-2000) spectrophotometer, in the range of 4000-400 cmÀ1 Elemental analysis for Zn/Al molar ratios was measured by an where q (mg/g) is the quantity of SA adsorbed per unit mass of adsorbent, C0 (mg/L) is the initial SA concentration, C (mg/L) is the SA concentration after adsorption and R (g/L) is the mass of adsorbent per liter of aqueous solution Results and discussion 3.1 Characterization 3.1.1 X-ray diffraction (XRD) study and ICP-AES analysis Fig illustrated the XRD patterns of LDHs and their calcined products The XRD patterns of the fresh materials showed a layered double hydroxide type structure in all the samples Typical peaks at (003), (006), (012), (104), (015), (018), (110) and (113) diffraction plans have been observed The (003) reflection is typical of hydrotalcite-type materials and its intensity is related to the crystallinity degree of the material If a hexagonal packing is assumed, the cell parameters (a and c) can be calculated by means of the (003) and (110) reflection values, where parameter a represents the average metalemetal distance in the interlayer structure calculated from the position of the (110) reflection and parameter c corresponds to three times the interlayer distance determined from the position of the (003) reflection p The ffiffiffi cell volume (V) was calculated according to the equation V ¼ a2 c=2 using the calculated cell parameters [27] The lattice parameters (a and c) of different LDHs were calculated according to Miller indices and Bragg equation 2d.sinq ¼ nl, where d is interplanar spacing of certain crystal face, q is the Bragg diffraction angle, and l is the X-ray wavelength, and their relationships Table shows the cell parameters (a and c), volume and the molar ratio Zn/Al of the samples, which was determined by ICPAES The table indicates a slightly increasing of parameters a, c and volume cell with increasing molar ratio This result could be attributed to the substitution of Al3ỵ by Zn2ỵ with the ionic radii for Zn (0.74 nm) which is larger than that of Al (0.53 nm) The d value increases steadily with increasing average radii of metallic cations, which depends directly on the angle q The value of diffraction 190 A Elhalil et al / Journal of Science: Advanced Materials and Devices (2018) 188e195 Fig X-ray diffractograms of the raw and calcined Zn-Al-CO3, (a): r ¼ 1, (b): r ¼ and (c): r ¼ angle q decreased (Fig 2) when the interplanar spacing d increased according to Bragg equation There are a great dependence between the volume cell and the molar ratio with the correlation coefficient (R2 ¼ 0.94) After calcination, the lamellar solid collapsed and new peaks corresponding to ZnO oxide and ZnAl2O4 spinel phases were observed [28] At 300 C the characteristic XRD peaks of ZnO oxide started to appear in all samples By increasing the temperature, A Elhalil et al / Journal of Science: Advanced Materials and Devices (2018) 188e195 191 Table Zn/Al molar ratio, cell parameters (a and c) and volume cell Sample Zn/Al ratio a (nm) c (nm) Volume cell (nm3) LDH-r1 LDH-r3 LDH-r5 LDH-r3-300reconstructed 1.170 3.860 5.710 e 0.3055 0.3076 0.3085 0.3082 2.2515 2.2775 2.2862 2.2831 18.1981 18.6622 18.8432 18.7811 Fig X-ray diffractograms of the raw LDHs: (a) close-up of crystal face (003), (b) close-up of crystal face (110) characteristic reflections of the mixed composite ZnO-ZnAl2O4 appear at 600 C The ratio ZnO/ZnAl2O4 increases with increasing Zn/Al molar ratio, because the amount of Al decreased 3.1.2 Fourier transform infrared spectroscopy (FTIR) Fig shows the FTIR spectra of fresh and calcined Zn-Al-CO3 at different Zn/Al molar ratio and calcination temperatures For the sake of clarity only the main absorption bands were listed Broad and intense band centered on 3400 cmÀ1 is attributed to the O-H stretching vibration in the brucite-like layers and the interlamellar water molecules The broadening of this band is attributed to hydrogen-bond formation [29] The band at approximately 1617 cmÀ1 indicates the O-H bending vibration of the interlayer water molecules Even after calcination, some H2O in the air would have dissolved into the mixture during the storage The band observed at 1368 cmÀ1 is assigned to the yCO2À of the carbonate anions This band disappears during calcination The intensity of this band rapidly decreases as the temperature increases above 300 C, which is attributed to the decomposition of the carbonate anion CO2À in the interlayer space The bands in the low-frequency region correspond to the lattice vibration modes such as the translation vibrations by M-O (590 and 670 cmÀ1) and O-M-O (430 cmÀ1) vibrations [29] From the figure it is noted that the bands M-O increase progressively as the temperature rises from 300 to 600 C For the band characteristic of carbonate ions, the wave number varies slightly with Zn/Al molar ratio It moves toward the low frequencies with the increase of Zn/Al ratio (1371, 1368 and 1366 cmÀ1 for LDH-1, LDH-3 and LDH-5 respectively) This result due to the difference in the atomic mass between zinc (65.40 g/ mol) and aluminum (26.98 g/mol) 3.1.3 TEM/EDX observation The TEM images of the samples are shown in Fig 4(a,c,e) As can be seen, LDH-1 and LDH-3 were well crystallized with typical hexagonal structure morphology, as reported previously [30] LDH- Fig FTIR spectra of fresh and calcined Zn-Al-CO3 at different temperature, (a): r ¼ 1, (b): r ¼ and (c): r ¼ shows platelet particles confirmed the perfect lamellar structure obtained by XRD analysis and the average particle size distribution was around 100 nm For LDH-1, the image display hexagonal platelet like particles, the darker lines indicate the presence of aggregate crystallites which probably obtained from a dense agglomeration of particles The average diameter of dispersed particles is around 180 nm In the case of LDH-5, we can observe the disappearance of the hexagonal platelets and appearance of agglomerates This observation was confirmed by the low crystallinity observed in XRD patterns EDX spectrum of the materials is shown in Fig 4(b,d,f) The results confirm the presence of Zn, Al, O and C No peaks from other elements were detected, indicating high purity of the products 3.2 Adsorption of salicylic acid 3.2.1 Effect of Zn/Al molar ration The effect of contact time on SA adsorption by Zn-Al-CO3 at different Zn/Al molar ratio was illustrated in Fig The result revealed that the removal takes place in two different steps; the first step involves a rapid removal at 10 The second one show a 192 A Elhalil et al / Journal of Science: Advanced Materials and Devices (2018) 188e195 Fig TEM-EDX images of Zn-Al-CO3 at different Zn/Al molar ratio (a, b): r ¼ 1, (c, d): r ¼ and (e, f): r ¼ subsequent removal until equilibrium is reached The adsorption efficiency of SA reached 5.12, 15.22 and 25.53% for LDH-3, LDH-5 and LDH-1, respectively Fig Adsorption kinetics of SA onto Zn-Al-CO3 at different Zn/Al molar ratios 3.2.2 Effect of calcination temperature For increasing the adsorption capacity of the LDHs, the materials were calcined at different temperatures (T ¼ 300, 400, 500 and 600 C) Fig shows the effect of calcination temperature on the adsorption performance of LDH-1 The adsorption capacity of SA by LDH-1-500 and LDH-1-600 is lower (33.8 and 16.3% respectively) At T ¼ 300 and 400 C, the samples show the highest adsorption capacity compared to the other calcination temperatures The curves can be divided into two steps, a first one due to the adsorption on the surface of LDH and a second may be due to reconstruction phenomenon [31,32] A Elhalil et al / Journal of Science: Advanced Materials and Devices (2018) 188e195 Fig Kinetics of SA removal by LDH-1 at different calcination temperatures Fig shows the adsorption kinetics of SA on LDHs-3 and their calcined products It can be seen that the removal efficiency increased rapidly with time, and then reached the equilibrium constant value The SA is weakly adsorbed (25.5 and 9.1%) on LDH3-500 and LDH-3-600, respectively In the presence of LDH-3-300 and LDH-3-400 the removal of SA is important reach: 66.5 and 74.4%, respectively Fig shows the effect of calcination temperature on the adsorption performance of LDH-5 The LDH-5-600 material manifests the lowest percentage of elimination of SA (12.7%) followed by LDH-5-500 (56.3%) In the presence of LDH-5-300 and LDH-5-400, around 81.6 and 82.3% respectively of SA was removed within a contact time of h, with a large reconstruction step It is evident that the calcination temperature has a great influence on adsorption of SA The capacity of the LDHs materials increases with increasing calcination temperature until 400 C This result could be attributed to the decomposition of the carbonate anion CO2À in the interlayer space (between 300 C and 320 C) and the formation of mixed oxides, able quickly to be reconstructed by SA At T ¼ 600 C, the characteristic peaks of spinel structure started to appear, which is not beneficial in the reconstruction process [31] 3.2.3 Reconstruction Fig shows XRD patterns of LDH (r ¼ and T ¼ 300 C) before and after adsorption of SA The figure shows the reappearance of the typical peaks of LDHs structure corresponding to (003), (006), (012), (104), (015), (018), (110) and (113) reticular plans This result confirms the reconstruction phenomenon After calcination, the interlayer space is removed The obtained material can retake anions and water into the interlayer spaces upon contact with solution After adding LDHs in SA solution, they can uptake new anions (SA) into their interlayer spaces as a result of “memory effect” A comparison of the parameters (a and c) of LDH-3 and LDH-3-300reconstructed indicate an increase of the parameters for LDH-3300-reconstructed due to the high volume of SA intercalated Fig Kinetics of SA removal by LDH-3 at different calcination temperatures 193 Fig Kinetics of SA removal by LDH-5 at different calcination temperatures Fig X-ray diffractograms of the LDH-3-300 and LDH-3-300-reconstructed Fig 10 Schematic illustration of the adsorption phenomenon of SA onto LDH structure Proposed mechanism for the adsorption of SA is shown in the Fig 10 The maximum adsorption capacity obtained in this study was compared to previous records of various adsorbents as summarized in Table It can be seen that obtained qmax data of the present study were found to be higher than those of the most corresponding adsorbents in the literature [33e37] 3.2.4 Regeneration The regeneration of adsorbents is the most difficult and expensive part of an adsorption technology It may account for >70% of the total operating and maintenance cost for an adsorption system [38] A successful regeneration process should restore the adsorbent similar to its initial properties for effective reuse Adsorbates can be recovered either for reuse or for proper disposal, depending on their market demand The best adsorbent (LDH-3-300) was regenerated by calcination at 300 C and used again for the adsorption of SA The results are 194 A Elhalil et al / Journal of Science: Advanced Materials and Devices (2018) 188e195 Table Comparison of the adsorption capacity of LDH for salicylic acid with literature Adsorbents qmax (mg/g) References Inorganic-organic clays Y-zeolites Pinewood biochar Banana peel MIM LDH-1 (T ¼ 400 C) LDH-3 (T ¼ 300 C) LDH-5 (T ¼ 300 C) 6.7 9.3 10.7 9.8 22.3 21.99 28.37 24.5 [33] [34] [35] [36] [37] This work This work This work Fig 11 Comparative SA adsorption kinetics for different cycles given in Fig 11 The figure shows that the adsorbent remove a large amount of reaches 78.77%, with little loss of SA adsorption capacity 16.72% Regenerated adsorbent can be used in several cycles The results suggest that the LDH-3-300 material may have practical application potential as an effective and stable adsorbent for removal of different pharmaceuticals Conclusion Our work focuses on the development of LDHs phases based on zinc and aluminum metals and interspersed by carbonates ions (Zn-Al-CO3) Several Zn/Al molar ratios (r ¼ 1, and 5) were synthesized by of co-precipitation method The LDHs were calcined at different temperatures (300, 400, 500 and 600 C) LDHs materials were characterized by several physicochemical techniques (XRD, FTIR, ICP-AES and TEM/EDX) During the calcination, the materials transformed into mixed metal oxides (ZnO-ZnAl2O4) The best removal rate of SA (94.59%) was obtained by Zn/Al molar ratio of and calcined at 300 C The LDH-3-300 has been reconstructed by SA The adsorbent showed high stability after two regeneration cycles Finally the results showed that these synthetic anionic clays present a remarkable performance to be used as economic and efficient adsorbents for the removal of pharmaceutical pollutants from an aqueous solution References [1] N Uma Sangari, B Jothi, S Chitra Devi, S Rajamani, Template free synthesis, characterization and application of nano ZnO rods for the photocatalytic decolourization of methyl orange, J Water Process Eng 12 (2016) 1e7 [2] N Singh, C Balomajumder, Simultaneous removal of phenol and cyanide from aqueous solution by adsorption onto surface modified activated carbon prepared from coconut shell, J Water Process Eng (2016) 233e245 [3] Q Yang, Y Liao, L Mao, Kinetics of 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