www.elsevier.com/locate/apsusc

Applied Surface Science 253 (2007) 5756–5761

Removal of aliphatic amino acids by hybrid organic–inorganic

layered compounds

Fabiano Silverio *, Marcio Jose dos Reis, Jairo Tronto, Joao Barros Valim

Departamento de Quımica, Faculdade de Filosofia Ciencias e Letras de Ribeirao Preto, Universidade de Sao Paulo,

Av. Bandeirantes, 3900, 14040-901 Ribeirao Preto, CEP, Brazil

Available online 27 December 2006

Abstract

Amino acids have been extensively used in several processes of the pharmaceutical and food industries. Treatments for the recovery and reuse of

the wastewaters generated from these processes are few and little known. This work aims at studying the influence of variables like temperature, pH

and ionic strength on the adsorption of the amino acids Asp and Glu, contained in aqueous solutions, on layered double hydroxides of the Mg–Al–

CO3–LDH system. The adsorption experiments were performed at two different temperatures (298 and 310 K), two different pH values (7.0 and

10.0), and two ionic strength conditions (with or without the addition of NaCl). The adsorption isotherms exhibited similar profiles under the

various conditions studied: an increase in temperature as well as an increase in the pH value decreased the amount of adsorbed amino acid while an

increase in the ionic strength increased Asp and Glu adsorption. The PXRD analysis showed that the diffractograms obtained before and after the

adsorption of amino acids have a similar pattern. The FT-IR spectra of the adsorbed material presented specific bands, which are related to the

amino acids. The concentration range varied up to the anion solubility product and the extraction rate lay between 2.7 and 23.4% at higher

equilibrium concentrations, showing that Mg–Al–CO3–LDH is efficient at removing the amino acids from the aqueous medium.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Hydrotalcite; Layered double hydroxides; Aspartic acid; Glutamic acid; Adsorption

1. Introduction

Layered double hydroxides (LDHs) have received con-

siderable attention during last years owing to their properties

and applications [1–7]. These materials can be structurally

described starting from the structure of Brucite, Mg(OH)2. This

mineral consists of octahedra sharing their edges, being the

magnesium cations located in the centers of such octahedra and

the hydroxyl anions placed on their vertices, which results in

plane and neutral layers (see Fig. 1a) stacked together by

hydrogen-bonds. If some of the bivalent cations are iso-

morphically substituted by trivalent ones in this structure, the

resulting layers display a residual positive charge that needs to

be neutralized. Neutralization is possible due to anions and

water molecules localized between the layers, which stacks the

double hydroxide layers and leads to the hydrotalcite-like

structure (Fig. 1b). These compounds are represented by the

* Corresponding author. Tel.: +55 16 3602 3881; fax: +55 16 3602 4838.

E-mail address: jobvalim@usp.br (J.B. Valim).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.12.040

general formula: ½M1�x2þMx

3þðOHÞ2�Ax=mm� � nH2O, where

M2+ is a bivalent cation, M3+ is a trivalent cation, Am� is the

interlayer anion with m� charge. Several organic or inorganic

anions can occupy the interlayer domain of LDHs. Due to their

high porosity and high specific surface area, LDHs are

extensively used as catalysts and adsorbents [8,9]. Hydro-

talcite-like compounds can remove anions from aqueous

solution by three different processes: (i) adsorption, (ii) anion

exchange, and (iii) regeneration of a calcined precursor [10–

16].

Because they have high specific surface area, hydrotalcite-

like compounds have been used in the removal of different

anionic species from aqueous solutions and for gas adsorption.

Adsorption from aqueous solution can occur when the

interlayer anion presents low exchange tendency, as in the

case of carbonate [17]. Our research group has demonstrated

the efficacy of LDHs of the [Mg–Al–CO3] system, calcined or

not, in the sorption and adsorption of surfactants and organic

compounds from aqueous solutions [13,15,16,18,19].

Amino acids like aspartic acid (Asp) and glutamic acid (Glu)

are used in several sections of chemical, pharmaceutical, and

Fig. 1. Schematic structural representation: (a) brucite; (b) hydrotalcite.

F. Silverio et al. / Applied Surface Science 253 (2007) 5756–5761 5757

food industries, where the treatment of industrial wastewaters is

little practised [20–24]. Both amino acids have two carboxylic

groups in their structure, and they differ just by the presence of

an extra –(CH2)– group in the Glu aliphatic chain, which

enhances its hydrophobic character. The aim of this work is to

study the effects of temperature, pH, and ionic strength on the

adsorption of the amino acids Asp and Glu, contained in

aqueous solutions, on layered double hydroxides.

2. Experimental methods

2.1. Preparation of Mg–Al–CO3–LDH—the adsorbent

The LDH was prepared by the pH variable coprecipitation

method proposed by Reichle [25]. The reactants were all

purchased from Merck (>99% assay). All the solids were

characterized by powder X-ray diffraction (PXRD), Fourier-

transform infrared spectroscopy (FT-IR), and specific surface

area (SBET).

2.2. Aspartic acid and glutamic acid—the adsorbates

The amino acids Asp and Glu were obtained from Merck

(>99.5% assay) and used without further purification. All the

amino acid solutions and their dilutions were prepared with

deionized water, and the pH was adjusted to 7 (with HCl) and/or

10 (with NaOH), with good control of the involved variables.

2.3. Adsorption experiments

The adsorption isotherms were obtained by the batch

method, which consisted in adding 50 cm3 aliquots of the

amino acid solution at different initial concentrations (in the

range 1.0 � 10�4 to 6.0 � 10�2 mol dm�3) to a constant mass

(500 mg) of the adsorbent LDH previously dried under vacuum

at room temperature, contained in erlenmeyers with stoppers.

The amino acid solutions were prepared at pH 7 or 10, with or

without the addition of 0.1 mol dm�3 NaCl. The obtained

suspensions were submitted to ultrasonication for 10 min, in

order to homogenize particle size. The isotherms were obtained

at 298 and 310 K.

The samples were kept in a thermostatic bath with orbital

shaking at a constant temperature (T, �0.5 K) for 70 h, to

ensure that the adsorption equilibrium was reached. After this

time, each sample was divided into two parts: one was

centrifuged at 10,000 � g for 20 min, separating the super-

natant (in which the amount of the amino acid was quantified by

UV–vis spectrophotometry) from the solid (which was dried

and characterized); the other part of the sample was kept in

aqueous suspension, for determination of the electrokinetic

potential. After a decanting time of 10 min, the electrokinetic

potential of the suspensions was measured at the same

temperature of the adsorption experiments.

3. Results and discussion

On the basis of the data of elemental and TG analyses, the

adsorbent LDH prepared herein was assigned the empirical

formula ½Mg0:7Al0:3ðOHÞ2�0:3þðCO3Þ0:15

2� � 0:7H2O, which

corresponds to a 2.5:1 Mg/Al ratio. This data is extremely

important because the adsorption capacity depends on the

charge density and is directly related to this ratio.

The adsorption isotherms for Asp and Glu are shown in

Fig. 2. The experiments were carried at two temperatures (298

and 310 K), two different pH values (7 or 10), and two ionic

strength conditions (with or without addition of 0.1 mol dm�3

NaCl) at 298 K.

According to the Giles classification for adsorption

isotherms [26], the adsorption of Asp (Fig. 2a) leads to L-

type 1 isotherms for the following conditions: 298 K, pH 7,

with or without addition of 0.1 mol dm�3 NaCl; and 310 K,

pH 10, with addition of 0.1 mol dm�3 NaCl and where no

Fig. 2. Adsorption isotherms under different conditions: (a) Asp; (b) Glu.

F. Silverio et al. / Applied Surface Science 253 (2007) 5756–57615758

adsorption plateau is observed. This is because the concentra-

tion of the amino acid solutions is limited by the amino acid

solubility products (Ksp) under these conditions. The other

isotherms are L-type 2. Adsorption isotherms obtained for Glu

(Fig. 2b) without the addition of NaCl are L-type 2, while the

others are S-type 1. The S-type isotherms give evidence of

cooperative adsorption, which takes place because of the van

der Waals interactions between the hydrophobic part of the

amino acid (–CH2 groups of Glu molecule).

All isotherms indicate that an increase in temperature seems

to have limited influence on adsorption below certain

equilibrium concentrations, after which the increase in

temperature causes a decrease in the amount adsorbed. This

behavior can be explained considering that the adsorption

process reduces the entropy of the system (DS < 0). Therefore,

the higher the temperature of the system, the higher the positive

entropic contribution to the Gibb’s free energy (DG =

DH � T DS) of the process. This decreases the contribution

of the exothermic enthalpy to the negative free energy. Thus,

the organization of compact aggregates with a larger number of

amino acid molecules in/on the LDH should become more

difficult at higher temperatures. This behavior has been

reported for several systems involving the adsorption of

surfactants on mineral oxides [12,13,27–29].

Although a decrease in adsorption with the increasing pH

can be seen for both amino acids, this effect can be more clearly

observed by comparing the isotherms of Glu at 298 K obtained

at pH 7 and pH 10, both without the addition of NaCl. This

decreased adsorption upon the increasing pH can be explained

by considering two factors: (i) the increase in the negative

charge of Asp and Glu (�1 at pH 7 and �2 at pH 10), and (ii)

the decrease in the surface charge of the adsorbent with the

increase of pH. The isotherm at pH 10 displays a plateau

beginning at an equilibrium concentration that is almost half of

that of the corresponding isotherm at pH 7. Moreover, at pH 10,

the amino acid has two charged extremities and can be adsorbed

in two different orientations: perpendicular to the layers of the

LDH (which is more probable at pH 7), and parallel to them,

occupying two sites per molecule of amino acid (which is more

probable at pH 10). So, considering that adsorption occurs

predominantly by electrostatic interactions, i.e., it depends on

the LDH surface charge, the amount of amino acid removed at

pH 10 should be lower than that removed at pH 7 [28,30–34].

The adsorption of Asp generally increases with the addition

of NaCl, except for low concentrations at pH 10, when the

adsorbed amounts are similar for the experiments carried out

with and without the addition of NaCl. The isotherms for Glu

adsorption at pH 10 show that the adsorption increases a lot

with the ionic strength for equilibrium concentrations higher

than 0.02 mol dm�3; below this concentration, a small variation

can be noticed. At pH 7, a similar behavior is observed, but the

adsorption starts to increase with the ionic strength at

equilibrium concentrations higher than 0.04 mol dm�3, which

is almost twice that observed at pH 10. This fact shows that the

effect of the ionic strength is more pronounced at concentra-

tions near the saturation of the adsorption. So the increased

adsorption at higher ionic strength should be explained in terms

of a decrease in the repulsion between the polar groups of the

amino acid molecule.

The electrokinetic potential curves are very similar for both

amino acids, with high and positive values at low equilibrium

concentrations (see Fig. 3). These values decrease quickly as

the amino acid concentration increases, reaching negative

values for the isotherms at pH 10. Isotherms obtained at pH 7

present positive electrokinetic potential values only, which

means that the saturation of the positive charge of the particles

does not take place in this condition. These results are in

agreement with the isotherm profiles. The rate of extraction of

Asp and Glu by the LDH is shown in Table 1. The material is

highly capable of removing these amino acids, especially when

one considers that the LDH had not been pre-calcined and that

the plateau had not been completely achieved under all the

conditions used here. At low Asp and Glu initial concentrations,

approximately 22 and 29% of the amino acid is removed,

respectively, whereas the extraction rate is as high as 16% in the

last points of the isotherms, where the initial amino acid

concentration is in the limit of its respective solubility. Bearing

in mind that the amino acids limit concentration is almost

0.06 mol dm�3, the material is potentially applicable to the

treatment of wastewater containing these species [35,36].

Fig. 3. Electrokinetic potential related to the adsorption: (a) Asp; (b) Glu.

Table 1

Extraction rate (ER, %) of Asp and Glu by Mg–Al–CO3–LDH under different conditions

Aspartic acid Glutamic acid

pH 7 pH 10 pH 7 pH 10 pH 7 pH 10 pH 7 pH 10

298 K,

I � 0 M

310 K,

I � 0 M

298 K,

I � 0 M

310 K,

I � 0 M

298 K,

I = 0.1 M

298 K,

I = 0.1 M

298 K,

I � 0 M

310 K 298 K,

I � 0 M

310 K,

I � 0 M

298 K,

I = 0.1 M

298 K,

I = 0.1 M

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

22.0 8.4 12.0 9.6 18.2 15.4 23.7 24.1 28.8 14.0 25.6 15.0

7.7 4.6 26.3 14.1 12.3 16.4 18.1 21.2 20.4 17.6 22.4 7.1

6.0 4.5 23.8 14.1 12.7 14.7 16.6 20.1 22.3 15.1 20.6 22.5

4.5 3.1 16.6 5.7 12.1 16.0 13.7 17.0 11.9 12.5 16.8 16.3

8.2 3.8 22.4 10.9 13.0 13.7 11.9 12.4 7.6 9.8 10.9 10.1

3.7 2.2 18.4 6.2 27.1 20.0 8.8 12.8 7.0 8.9 7.8 8.3

5.5 4.9 15.3 5.7 28.1 15.5 9.6 10.5 7.8 9.2 4.8 6.3

9.2 5.5 13.9 5.0 22.4 15.6 8.6 9.4 7.9 7.8 5.3 3.8

10.2 7.2 12.6 3.7 23.8 16.2 10.2 10.4 7.4 7.1 5.2 5.4

11.9 6.4 10.5 4.0 23.0 15.3 10.2 10.5 7.6 6.1 4.2 4.5

12.3 5.9 8.3 3.9 18.0 15.0 9.6 10.3 7.9 5.9 5.4 11.0

12.1 6.3 6.8 3.6 23.2 15.4 10.2 10.4 7.1 5.3 5.7 15.9

13.7 6.6 5.9 3.1 20.3 14.4 10.5 10.8 6.8 4.6 6.2 18.7

14.4 6.8 5.6 2.8 18.4 12.6 11.3 9.7 6.1 4.4 6.3 18.9

12.8 5.6 5.0 3.0 17.1 12.4 10.7 9.3 6.1 4.2 7.2 17.6

14.0 6.3 4.8 2.7 16.7 12.0 9.1 7.8 4.8 3.5 10.1 15.4

Table 2

General parameters observed for the pure LDH and LDH after adsorption of Asp and Glu under different conditions

Sample Medium particle

size (A)

SBET (m2 g�1) Pores’ total

volume (cm�3 g�1)

Pores’ medium

diameter (A)

Pure LDH 507.9 34.5 0.127 147.4

Asp 298 K; pH 7; I � 0 M 597.5 38.1 0.117 122.6

Asp 310 K; pH 7; I � 0 M 690.8 40.9 0.170 166.3

Asp 298 K; pH 7; I � 0.1 M 655.6 41.0 0.163 159.6

Asp 298 K; pH 10; I � 0 M 690.8 40.8 0.246 241.1

Asp 310 K; pH 10; I � 0 M 690.8 41.6 0.222 213.2

Asp 298 K; pH 10; I = 0.1 M 655.6 43.1 0.108 100.4

Glu 298 K; pH 7; I � 0 M 655.6 41.6 0.110 105.4

Glu 310 K; pH 7; I � 0 M 896.4 40.0 0.170 170.0

Glu 298 K; pH 7; I = 0.1 M 655.6 45.1 0.134 118.9

Glu 298 K; pH 10; I � 0 M 690.7 1.6 0.006 151.1

Glu 310 K; pH 10; I � 0 M 690.7 2.3 0.009 163.1

Glu 298 K; pH 10; I = 0.1 M 690.6 42.7 0.220 205.9

F. Silverio et al. / Applied Surface Science 253 (2007) 5756–5761 5759

Fig. 4. Left: PXRD; right: FT-IR (a) clean LDH; and LDH adsorbed with (b) Asp at 298 K, pH 7, I � 0.0 M; (c) Asp at 298 K, pH 10, I = 0.1M; (d) Glu at 298 K, pH

7, I � 0.0 M; (e) Glu at 298 K, pH 10, I = 0.1 M.

F. Silverio et al. / Applied Surface Science 253 (2007) 5756–57615760

The solids were characterized by PXRD and FT-IR analyses

before and after the adsorption experiments (Fig. 4). The PXRD

analysis shows there is no difference in the basal spacing of

7.6 A, which corresponds to intercalation of carbonate anions.

The fact that the basal spacing does not change gives evidence

that the there is no anionic exchange with the amino acid

anions. The FT-IR analysis shows the presence of organic

species in the LDH (Asp and Glu), as seen by the increase in the

two bands related to the asymmetric and symmetric stretches of

the carboxylate group at 1590 and 1400 cm�1, respectively.

The average particle size of the materials was calculated

from the PXRD data by means of the Debye–Scherer method

[37], and the results are shown in Table 2. The average particle

sizes of the adsorbed solids are close, and they are larger than

that of the pure LDH, which suggests an improvement in the

stacking of the LDH after adsorption. This is probably because

the adsorption process can organize and aggregate small

particles, thus producing larger particles.

The specific surface area (SBET) of the solids adsorbed with

Asp under different conditions is larger than that of the clean

LDH. The solids adsorbed with Glu also present larger SBET

than the clean adsorbent, except for the samples adsorbed at pH

10 and lower ionic strength, where the plateau is reached at a

much lower equilibrium concentration than the others,

producing solid aggregates with very small pores. The same

can be observed for the total volume of the pores.

4. Conclusions

This work presents a new alternative for the removal of

amino acids from aqueous solution with potential application in

the treatment of wastewater from industrial processes. Our

results have shown that adsorption occurs at a very efficient

extraction rate, even at high amino acid concentrations. The

removal of Asp and Glu by adsorption is greatly dependent on

the studied variables. Our results suggest that an increase in

temperature reduces the amount of removed amino acid, mainly

at higher concentrations. An increase in the pH value, from 7 to

10, results in an increase in the negative charge of the amino

acid and a reduction in the positive charge of the adsorbent,

consequently leading to a decrease in the amount of adsorbed

Asp and Glu. On the other hand, an increase in the ionic

strength, by addition of NaCl, results in a large increase in the

amount of removed Asp and Glu, except in the case of Glu at pH

10. This effect is probably due to the higher hydrophobic

character of Glu, which makes its adsorption less influenced by

the ionic strength, thus reducing the electrostatic character of

the adsorption, if compared with the electrostatic character of

Asp under the same condition.

Acknowledgements

The authors thank Coordenacao de Aperfeicoamento de

Pessoal de Nıvel Superior, CAPES, Conselho Nacional de

Desenvolvimento Cientıfico e Tecnologico, CNPq, and

Fundacao de Amparo a Pesquisa do Estado de Sao Paulo,

FAPESP for the financial support.

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