Al layered double hydroxide nanoparticles in the removal of pharmaceutical pollutants

Effects of molar ratio and calcination temperature on the adsorption performance of Zn/Al layered double hydroxide nanoparticles in the removal of pharmaceutical pollutants.. Ala^aeddine[r]

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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

Rachid Elmoubarki, Hanane Tounsadi, Mohamed Abdennouri, Noureddine Barka

Laboratoire des Sciences des Materiaux, des Milieux et de la Modelisation (LS3M), FPK, Univ Hassan 1, B.P 145, 25000 Khouribga, Morocco

a r t i c l e i n f o

Article history:

Received 19 January 2018

Received in revised form

7 March 2018

Accepted 22 March 2018

Available online 29 March 2018

Keywords:

Layered double hydroxides

Reconstruction

Pharmaceutical pollutants

Regeneration

Wastewater treatment

a b s t r a c t

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 (r¼ 1, 3 and 5) Each synthesized material was calcined at 300, 400, 500 and 600C to increase their perfor-mance 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 3 and a calcination temperature of 300C Kinetics of the adsorption takes place in two steps; thefirst 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 (r¼ 3, T ¼ 300C) 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/)

1 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,

an-tibiotics, skin care products, hair styling agents and so forth[5] The

discharge of PPCPs or their metabolites into environment through the production process and daily consumption would pose long-term 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 conse-quence, 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]

* Corresponding author Fax: þ212 523 49 03 54.

E-mail address: elhalil.alaaeddine@gmail.com (A Elhalil).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.03.005

Journal of Science: Advanced Materials and Devices 3 (2018) 188e195

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Several technologies including photocatalytic degradation,

bio-logical 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 ofﬁllers on the surface,

and especially the tradability of interlayered anions[24,25]

The general formula of a LDH is: [MII1-xMIIIx(OH)2]xþ.(Anx/n)

mH2O, where MIIrepresents a divalent cation (Mg2þ, Zn2þ, Ni2þ,

Mn2þ, Fe2þ…), MIIIrepresents a trivalent cation (Al3þ, Cr3þ, Fe3þ,

Co3þ, Mn3þ…), An the compensating anion (Cl, NO3, ClO24,

CO23…), n the charge of the anion, and m is the number of water

molecules located in the interlayer region together with the anion

The coefﬁcient, x, is the molar fraction, [MIII/(MIIþ 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

co-precipitation method Zn-Al-CO3 materials were calcined at

different temperatures (300, 400, 500 and 600C) in a tubular

furnace Samples were characterized by different physicochemical

techniques The as-synthesized and calcined products were

inves-tigated in the removal of salicylic acid from aqueous solution

2 Experimental

2.1 Materials

The starting chemicals; zinc nitrate (Zn(NO3)2.6H2O), aluminum

nitrate (Al(NO3)3.9H2O), sodium carbonate (Na2CO3), sodium

hy-droxide (NaOH) and salicylic acid have been purchased from

SigmaeAldrich (Germany) Nitric acid, 65%, extra pure was

ob-tained 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-CO3layered double hydroxide materials were prepared by

co-precipitation method from metal salts, at different Zn/Al molar

ratios (r¼ 1, 3 and 5) A mixture solution of Zn(NO3)2.6H2O and

Al(NO3)3.9H2O with a total concentration of metal ions of 2 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 byadding suitable amounts

of NaOH solution (2 mol/L) The gel formed was stirred vigorously for

4 h and then transferred into an autoclave and hydrothermally

treated at 75C for 16 h Afterward, the suspension wasﬁltered and

washed with bidistilled water until reaching pH~7 and dried at

100C 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 600C) in

a tubular furnace for 6 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 2qrange from 5 to 70at 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 cm1

Elemental analysis for Zn/Al molar ratios was measured by an

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 3 mL of solution was extracted at different time intervals andfiltered by syringe filter (Minisart type NML, Membrane: A.C 0.2mm absolute) to remove the particles for analysis, and the concentrations of SA were analyzed using a double-beam scanning spectrophotometer (Shimadzu spectro-photometer, model biochrom) at the wavelength of 297 nm The adsorbed quantity and adsorption efficiency (Removal (%)) were calculated using the following equations:

q¼ðC0 CÞ

Removalð%Þ ¼C0 C

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

3 Results and discussion 3.1 Characterization 3.1.1 X-ray diffraction (XRD) study and ICP-AES analysis Fig 1illustrated 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 crys-tallinity 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 corre-sponds to three times the interlayer distance determined from the position of the (003) reflection The cell volume (V) was calculated according to the equation V¼pffiffiffi3

a2c=2 using the calculated cell parameters[27]

The lattice parameters (a and c) of different LDHs were calcu-lated according to Miller indices and Bragg equation 2d.sinq¼ nl, where d is interplanar spacing of certain crystal face,qis the Bragg diffraction angle, and l is the X-ray wavelength, and their relationships

Table 1shows the cell parameters (a and c), volume and the molar ratio Zn/Al of the samples, which was determined by ICP-AES 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 Al3þby Zn2þwith 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

A Elhalil et al / Journal of Science: Advanced Materials and Devices 3 (2018) 188e195 189

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angleqdecreased (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

(R2¼ 0.94)

After calcination, the lamellar solid collapsed and new peaks corresponding to ZnO oxide and ZnAl2O4 spinel phases were observed[28] At 300C the characteristic XRD peaks of ZnO oxide started to appear in all samples By increasing the temperature, Fig 1 X-ray diffractograms of the raw and calcined Zn-Al-CO 3 , (a): r ¼ 1, (b): r ¼ 3 and (c): r ¼ 5.

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characteristic reﬂections of the mixed composite ZnO-ZnAl2O4

appear at 600C The ratio ZnO/ZnAl2O4increases with increasing

Zn/Al molar ratio, because the amount of Al decreased

3.1.2 Fourier transform infrared spectroscopy (FTIR)

Fig 3shows the FTIR spectra of fresh and calcined Zn-Al-CO3at

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 cm1is 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 cm1indicates 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 cm1is assigned to theyCO3 of the carbonate

anions This band disappears during calcination The intensity of

this band rapidly decreases as the temperature increases above

300C, which is attributed to the decomposition of the carbonate

anion CO3in 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 cm1) and O-M-O

(430 cm1) vibrations [29] From theﬁgure it is noted that the

bands M-O increase progressively as the temperature rises from

300 to 600C

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 cm1for 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 inFig 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-3 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 ag-glomerates This observation was conﬁrmed by the low crystallinity observed in XRD patterns EDX spectrum of the materials is shown

inFig 4(b,d,f) The results conﬁrm the presence of Zn, Al, O and C

No peaks from other elements were detected, indicating high pu-rity 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 5 The result revealed that the removal takes place in two different steps; the ﬁrst step involves a rapid removal at 10 min The second one show a

Table 1

Zn/Al molar ratio, cell parameters (a and c) and volume cell.

cell (nm 3 )

LDH-r3-300-reconstructed

Fig 2 X-ray diffractograms of the raw LDHs: (a) close-up of crystal face (003),

(b) close-up of crystal face (110).

Fig 3 FTIR spectra of fresh and calcined Zn-Al-CO 3 at different temperature, (a): r ¼ 1, (b): r ¼ 3 and (c): r ¼ 5.

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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

3.2.2 Effect of calcination temperature For increasing the adsorption capacity of the LDHs, the materials were calcined at different temperatures (T¼ 300, 400, 500 and

600C).Fig 6shows 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 T¼ 300 and 400C, 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]

Fig 4 TEM-EDX images of Zn-Al-CO 3 at different Zn/Al molar ratio (a, b): r ¼ 1, (c, d): r ¼ 3 and (e, f): r ¼ 5.

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Fig 7shows 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

LDH-3-500 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 8 shows the effect of calcination temperature on the

adsorption performance of LDH-5 The LDH-5-600 material

mani-fests 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 3 h, with a large reconstruction step

It is evident that the calcination temperature has a great in

ﬂu-ence on adsorption of SA The capacity of the LDHs materials

in-creases with increasing calcination temperature until 400C This

result could be attributed to the decomposition of the carbonate

anion CO3 in the interlayer space (between 300C and 320C) and

the formation of mixed oxides, able quickly to be reconstructed by

SA At T¼ 600C, the characteristic peaks of spinel structure started

to appear, which is not beneﬁcial in the reconstruction process[31]

3.2.3 Reconstruction

Fig 9shows XRD patterns of LDH (r¼ 3 and T ¼ 300C) before

and after adsorption of SA Theﬁgure 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

an-ions and water into the interlayer spaces upon contact with

solu-tion 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

300-reconstructed indicate an increase of the parameters for

LDH-3-300-reconstructed due to the high volume of SA intercalated

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

inTable 2 It can be seen that obtained qmaxdata of the present study were found to be higher than those of the most corre-sponding 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 Ad-sorbates 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 300C and used again for the adsorption of SA The results are

Fig 6 Kinetics of SA removal by LDH-1 at different calcination temperatures Fig 8 Kinetics of SA removal by LDH-5 at different calcination temperatures.

Fig 9 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.

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given inFig 11 Theﬁgure 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

4 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 (r¼ 1, 3 and 5) were

syn-thesized by of co-precipitation method The LDHs were calcined at

different temperatures (300, 400, 500 and 600C) 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 3

and calcined at 300C 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

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Table 2

Comparison of the adsorption capacity of LDH for salicylic acid with literature.

Fig 11 Comparative SA adsorption kinetics for different cycles.

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