Effects of Physical Parameters onto Adsorption of the Borderline

Article pubs.acs.org/jced

Effects of Physical Parameters onto Adsorption of the Borderline Amino Acids Glycine, Lysine, Taurine, and Tryptophan upon Amberlite XAD16 Resin Vincenza Ferraro,†,‡ Isabel B. Cruz,†,‡ Ruben Ferreira Jorge,‡ Manuela E. Pintado,† and Paula M. L. Castro*,† †

CBQF/Escola Superior de Biotecnologia, Centro Regional do Porto da Universidade Católica Portuguesa, Rua Dr. António Bernardino de Almeida, 4200-072 Porto, Portugal ‡ WeDoTechCompanhia de Ideias e Tecnologias, Lda./CiDEB, Escola Superior de Biotecnologia, Centro Regional do Porto da Universidade Católica Portuguesa, Rua Dr. António Bernardino de Almeida, 4200-072 Porto, Portugal ABSTRACT: The adsorption of the borderline amino acids glycine, lysine, taurine, and tryptophan on the commercial resin Amberlite XAD16 was investigated. The effect of amino acid hydrophobicity/hydrophilicity and effects of environmental conditions upon adsorption equilibrium were evaluated. Optimal settings for a maximum recovery of a selected amino acid, namely, taurine, were obtained by the response surface methodology. Changes in temperature, pH, ionic strength, percentage of ethanol added to the amino acid solution, amino acid concentration, and adsorbent dose showed effects on the recovery of each amino acid upon the uncharged, nonfunctionalized, and hydrophobic matrix of the Amberlite XAD16 resin. Adsorption was favored at the lower temperature investigated, 10 °C; a pH decrease down to 2 favored adsorption of glycine, lysine, and tryptophan; the addition of ethanol allowed an increase in amino acid recovery except for lysine. The addition of sodium chloride up to the value of 1.5 M showed a positive effect on adsorption of all amino acids. A dose of 10 g Amberlite XAD16 for 100 mL of solution was the most adequate at the amino acid concentrations tested. Optimal conditions for the maximum recovery of taurine were achieved at a temperature of 13.5 °C and at a ionic strength of 1.32 M NaCl.

1. INTRODUCTION Adsorption is a separation process in which certain components of a fluid phase selectively migrate to the surface of a solid by preferential partitioning.1 It depends not only on adsorbent characteristics but also on fluid phase composition and environmental conditions such as temperature, pH, ionic strength, solvent species, fluid dynamics, and adsorbent dose.2,3 When efficiency, selectivity, and costs are taken into account, adsorption can be considered a convenient technique for the recovery of amino acids from aqueous or organic solutions; over the last decades several studies on amino acids purification and recovery by adsorption upon various materials, such as activated carbon, silica, ion-exchange, alumina, and polymeric resins, have been reported.4−7 Amberlite XADs are commercial macroreticular synthetic adsorbents, either based on styrene-divinylbenzene or acrylic esters copolymers, and have a variety of surface areas and surface polarities and diversified average pore-size distributions. Under the brand Amberlite XADs a number of resins are included such as the Amberlite XAD2, Amberlite XAD4, Amberlite XAD8, and Amberlite XAD16, among others.8 These resins are characterized by higher surface areas and easier regeneration than ionexchange adsorbents or activated carbon and have been considered among the most advantageous resins for recovery of amino acidsand other organic compounds like peptides, © 2013 American Chemical Society

proteins, phenolsfrom diluted liquid solutions and also from air streams.5,9 The synthesis of polymeric adsorbents has been driven by the need to overcome restrictions associated with the use of ion-exchange resins, which show lower diffusion kinetics and hydraulic properties, and lower chemical stability when compared with Amberlite XAD analogues. For both ionexchange and Amberlite XAD resins, the particle size of spherical beads is approximately 0.5 mm; however each 0.5 mm bead of Amberlite XADs consists of many small microspheres whose diameter is as small as 10−4 mm, a feature which allows us to speed up the adsorbate diffusion through each bead and also to improve hydraulic properties when column equipment are employed. Furthermore, because of the highly cross-linked nature, Amberlite XAD resins have good physics durability.10 The regeneration of ion-exchange resins is more costly since it is more energy consuming and leads to the undesirable use of acids and bases, while less expensive organic solvents and/or water are required for regenerating synthetic polymeric adsorbents.4 The easy of regeneration can be considered the most important characteristic of Amberlite XAD resins, since Received: November 9, 2012 Accepted: January 29, 2013 Published: February 11, 2013 707

dx.doi.org/10.1021/je301206r | J. Chem. Eng. Data 2013, 58, 707−717

Journal of Chemical & Engineering Data

Article

39 % in resin weight was observed due to water retention, and this phenomenon has been taken into account in adsorption calculation. Resin pretreatment was carried out before each adsorption experiment. SEM pictures of the resin before and after washing are reported in Figure 1.

for an economical point of view, the success of a sorption process usually depends on the adsorbent regeneration.8 This work investigated the adsorption behavior of borderline amino acids onto Amberlite XAD16 resin, where the term “borderline” is to be referred to the hydrophobicity/hydrophilicity character of the amino acid. Amberlite XAD16 was selected among other Amberlite XAD resins because it is the most apolar and has the higher surface area and porosity. Adsorption data were collected for tryptophan, which is the most hydrophobic, for taurine and glycine that are both amphiphilic (i.e., neutral) amino acids, but with different acidic groups, and for lysine which is the most water-soluble although it is not the most hydrophilic. For each amino acid, effects of environmental conditions such as temperature, pH, ionic strength, percentage of ethanol added into solution, initial concentration of amino acids, and adsorbent dose upon adsorption equilibrium were studied. The main objective of the research reported here was assessing how the recovery of an amino acid from an aqueous solution by sorption can be significantly affected by some physical parameters apart from the affinity toward the selected adsorbent, here determined by the hydrophobic/hydrophilic character of the amino acid. To our knowledge, no similar studies were reported for the effects of physical parameters on the adsorption of selected amino acids on the Amberlite XAD16 resin. This information is useful in some real situations, where, for instance, purification of amino acids from an industrial wastewater containing salt or organic solvents could be required.

Figure 1. Scanning electron microscopy images of Amberlite XAD16 along the sorption process: (a) Amberlite XAD16 before washing; (b) Amberlite XAD16 after washing.

2.2. Amino Acids. Amino acids glycine, lysine, taurine, and tryptophan were of analytical grade (> 99.9 %). In Table 1 are listed the physicochemical characteristics of these amino acids, such as the molecular weight (MW), isoelectric point (IP), solubility in water at 25 °C (Sw,25°C), hydrophobicity index at pH 2 and 7 (HI2 and HI7, respectively) and charge at neutral pH. 2.3. Determination of Adsorption Isotherms. Experimental equilibrium adsorption data (adsorption isotherms) were obtained in a batch mode for each amino acid. A fixed amount of pretreated Amberlite XAD16 was added to a 250 mL Erlenmeyer flask containing 100 mL of distilled water at a known initial concentration of amino acid, C0 (g·L−1), and placed onto a stirring hot plate; mixing was achieved using a magnetic bar. Continuous sampling showed that approximately 5 h of continuous agitation were required for reaching equilibrium. The equilibrium concentration of amino acid in solution, Ceq (g·L−1), was determined by HPLC-UV−vis, using the methods reported in next section. The amount of amino acid retained onto the resin at equilibrium, Cad (g·g−1), was calculated as follow:

2. MATERIALS AND METHODS 2.1. Adsorbent. Amberlite XAD16 resin has a polystyrene cross-linked with divinylbenzene matrix and is hydrophobic, uncharged and no-functionalized. It has a high surface area (≥ 800 m2·g−1), an average pore size of 100 Ǻ , a porosity ≥ 0.55 mL·mL−1, a moisture holding capacity of 62 % to 70 %, a dipole moment of 0.3 D, and is useful for the adsorption of neutral to hydrophobic compounds of low and medium molecular weight [(100 to 350 000) Da] from aqueous (i.e., polar) solutions.10 Before adsorption, Amberlite XAD16 was washed according to the resin manufacturer10 to eliminate residual monomers and the preservative agents sodium chloride (NaCl) and sodium carbonate (NaCO3), with which the resin was rinsed before packaging to control bacteria and mold growth during storage. Residual monomers must be eliminated because of the wide band ultraviolet absorbancedue to the π-electrons of conjugated bond structurewhich could interfere with spectrophotometric determination of compounds to be adsorbed. Water and acetone were used for resin washing, since water removes salts and acetone mainly removes residual monomers. Different volumes of solvents and different times were tried before implementing the final procedure described below. For the water phase, when no more NaCl and NaCO3 were present in the renewed water the resin was considered clean. When UV absorbance of the renewed acetone phase was the same of acetone alone the resin was clean since there were no more monomers absorbing UV radiation. The ratios and the time reported below ensured resin cleaning. Briefly, for each adsorption experiment, 1 volume of a fixed amount of resin was placed in an Erlenmeyer flask with 10 volumes of distilled water and rinsed under continuous stirring for 30 min at ambient temperature. Then, the resin was filtered with a paper filter, put back in the Erlenmeyer flask with 10 volumes of acetone, and rinsed under the same conditions. After washing, an increase of

Cad =

C0·V0 − Ceq ·(V0 + ΔV ) 1000·md

(1)

where V0 is the initial volume of amino acid solution (ml) and ΔV is the dilution brought into amino acid solution by addition of conditioned wet resin and is expressed as ΔV = (mw − md)/ ρH2O, where mw is the wet resin weight (g), md is the dry resin weight (g), and ρH2O is the density of water (g·mL−1) at the experimental temperature. For each amino acid, the effect of six parameters on adsorption equilibrium was evaluated, namely, temperature, ionic strength, ethanol added to solution, pH, amino acid initial concentration, and adsorbent dose. Ten solutions of each amino acid were used to study each parameter, and experiments were carried out by modifying one variable at each run. Initial concentrations, C0, were in the range (0.05 to 0.5) g·L−1 and with the values:( 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, and 0.50) g·L−1. The effect of temperature was assessed at (10, 20, and 30) °C. The effect of pH was tested at 20 °C for the neutral value of pH 6, for the acidic value of pH 2, and for the alkaline value of pH 11; sodium hydroxide NaOH and chloridric acid HCl were used for alkalizing and acidifying, 708



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Table 1. Physicochemical Properties of Amino Acids

a

n.a.: not available.

where R is the gas constant (8.314 J·mol−1·K−1), T is the absolute temperature (K), and Keq is the dimensionless thermodynamic adsorption equilibrium constant expressed as Keq = (C0 − Ceq)/Ceq, where C0 is the initial concentration of compound in solution, (C0 − Ceq) represents the solid-phase equilibrium compound concentration (g·L −1 ), and C eq represents the equilibrium compound concentration in solution (g·L−1). The integration of eq 4 (Van’t Hoff equation) by separating variables and combination with eq 3 leads to the following expression

respectively, amino acid solutions to the desired value. The effect of ionic strength was studied at 20 °C by adding an amount of sodium chloride, NaCl, to the amino acid solutions up to different molarities: 4.3 M NaCl, 1.5 M, and the null value 0 M. The effect of ethanol on adsorption equilibrium was tested at 20 °C by comparing the normal (nonethanolic) solutions of amino acids with two mixtures composed of 25 % (v/v) and 50 % (v/v) food-grade ethanol 99.5 % purity. Adsorbent dose effects was tested at 20 °C for the values (5, 10, and 20) g of Amberlite XAD16 resin added to 100 mL of normal (without ethanol and without salt) amino acid solution. All of the experiments done were summarized in Table 2. The percentage of amino acid (R(%)) retained on the resin has been calculated as follow: Cad·md R(%) = ·100 (C0/1000) ·V

ln Keq =

Table 2. Experiments for Adsorption Equilibrium Study pH temperature (°C) ethanol (% v/v) ionic strength (M NaCl) adsorbent dose (gresin·100 mLsolution−1) range of initial concentration (g·L−1)

value tested 2 10 0 0 5

6 20 25 1.5 10 0.05 to 0.5

11 30 50 4.3 20

2.4. Determination of Variation of Standard Gibbs Energy (ΔG°), Enthalpy (ΔH°), and Entropy (ΔS°). Thermodynamic parameters ΔG°, ΔH°, and ΔS° were determined by the following equations:

ΔG° = ΔH ° − T ·ΔS°

(3)

⎛ ∂ ln K ⎞ ΔH ° eq ⎟ ⎜ ⎜ ∂1 ⎟ = − R ⎝ ⎠ T

(4)

or

Keq = e−ΔG ° / RT

(5)

Therefore, a plotting of the natural logarithmic of the equilibrium constant versus the reciprocal temperature gives a straight line whose slope is the minus enthalpy variation divided by the R constant (−ΔH°/R), and the intercept is equal to the entropy variation divided by the R constant (ΔS°/R). 2.5. HPLC-UV−vis Chromatographic Analysis of Amino Acids. Chromatographic analysis were performed using a Beckman and Coulter 168 series HPLC system interfaced to a Photo Diode Array UV−vis detector (PDA (190 to 600) nm). All eluents were filtered through a 0.45 μm cellulose membrane and degassed in an ultrasound bath for 15 min prior to use as mobile phases. All determinations were carried out in triplicate. Taurine was analyzed using a Waters Nova-Pack RP-HPLC, C18 column (150·3.9 mm, 4 μm). Taurine analysis was carried out using the method described by Orth,11 with a slight modification in the gradient elution and in the column. Standard solutions for calibration, covering the range of (10 to 80) μg·mL−1, were prepared from a stock solution containing 100 μg·mL−1 taurine dissolved in ultrapure water. The gradient of the mobile phases was as follows: 100 % of acetonitrile from 0 % to 50 % in 15 min and constant at 50 % for the subsequent 5 min. The other mobile phase was composed of 1.3 g of NaHPO4·H2O and 0.11 g of NaHPO4·2H2O in 1 L of ultrapure water at pH 6.0. The elution flow rate was 1 mL·min−1. The

(2)

parameter

ΔS° ΔH ° − R R·T

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Figure 2. Amino acid adsorption isotherms at 20 °C (mean values ± standard deviation bars, which are not visible), 0 M NaCl, 0 % (v/v) ethanol, 10 g of resin/100 mL solution, and theoretical model data set (Toth).

resin at a given condition, and KL (L·g−1) is the Langmuir parameter. Toth isotherm:

taurine peak was detected at 360 nm, with a retention time of 9.20 min. For glycine, lysine, and tryptophan the method reported by Alonso et al.12 was selected, which use a RP-HPLC, C18 Ultrasphere 5-ODS (25·4.6, 5 μm) column. Standards used for the calibration (Sigma-Aldrich) were 25 mM of glycine, lysine, and tryptophan, all dissolved in ultrapure water. Amino acids were detected at 254 nm, and retention times were 8.93 min for glycine, 33.49 min for tryptophan, and 36.94 min for lysine. 2.6. Scanning Electron Microscopy of Amberlite XAD16. Amberlite XAD16 resin has been scanned by electron microscopy before and after washing. The following scanning conditions were used: accelerating voltage 15 kV, vacuum 10 Pa, magnification 50×. 2.7. Experimental Data Modeling. Experimental isotherms were compared with the theoretical isotherms proposed by Freundlich,2 Langmuir,2 and Toth,13 whose models are reported below: Freundlich isotherm: 1/ n Cad = KF·Ceq

Cad =

(6)

1 ln Ceq + ln KF n

(7)

where Cad and Ceq have meanings reported previously, and KF (L·g−1) and n are the Freundlich parameters. Langmuir isotherm: Cad =

Cad,max ,L·KL ·Ceq 1 + KL ·Ceq

(8)

Linear form of the Langmuir isotherm: Ceq Cad

=

Ceq Cad,max ,L

+

1 Cad,max ,L·KL

[1 + (KT·Ceq)t ]1/ t

(10)

where Cad and Ceq have meanings reported previously, Cad,max,T (g·g−1) is the maximum amount that can be adsorbent on the resin at a given condition, and KT (L·g−1) and t are the Toth parameters. Parameters of the Toth isotherm and Cad,max,T were obtained by a nonlinear regression, minimizing the sum of the squares of the errors. Parameters of the Freundlich and Langmuir isotherm and Cad,max,L were obtained either by a linear regression (by plotting eqs 7 and 9, respectively) or by a nonlinear regression with the same error function as above. Adequacy of the models was evaluated by two goodness-of-fit criteria, namely, the coefficient of determination (R2) and standard error of the regression (SER). One-way analysis of variance (ANOVA) on the percentage of amino acid recovered on the resin was carried out setting a confidence level of 95 % (p ≤ 0.05). The response surface for taurine was obtained by a 32 fractional factorial design (two variables at three levels); the variables studied were temperature and ionic strength, and levels were (10, 20, and 30) °C and (0, 1.5, and 4.3) M NaCl, respectively. According to factorial design symbology, the symbol “+” indicates a parameter combination whose response (R(%)) is known from the one-variable-at-a-time approach, while the symbol “−” indicates a parameter combination whose response must be tested.

Linear form of the Freundlich isotherm: ln Cad =

Cad,max ,T·KT·Ceq

3. RESULTS AND DISCUSSION 3.1. Adsorption of Amino Acids. Results obtained provided evidence of amino acid adsorption as well as evidence of differences in adsorption extent depending on operating conditions (temperature, pH, ionic strength, ethanol, initial concentration, and adsorbent dose), and differences in amino

(9)

where Cad and Ceq have meanings reported previously, Cad,max,L (g·g−1) is the maximum amount that can be adsorbent on the 710



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Table 3. Freundlich Parameters n and KF [(L·g−1)n] and Statistical Parameters R2 and SER for Tested Amino Acids amino acid parameter pH 2 6 11 Temp. (°C) 10 20 30 EtOH (% v/v) 0 25 50 Ionic Strength (M) 0 1.5 4.3

lysine n

KF

glycine

R

2

SER −5

n

KF

taurine

R

2

SER −5

n

KF

tryptophan

R

2

SER −5

n

KF

R2

SER

1.27 1.30 1.33

0.016 0.012 0.009

0.96 0.98 0.98

2·10 1·10−5 8·10−6

1.16 1.19 1.19

0.038 0.031 0.026

0.97 0.95 0.98

3·10 3·10−5 2·10−5

1.17 1.18 1.09

0.041 0.040 0.036

0.98 0.99 0.97

4·10 7·10−6 5·10−5

1.36 1.37 1.34

0.244 0.197 0.194

0.98 0.99 0.97

2·10−5 8·10−6 3·10−5

1.28 1.32 1.36

0.015 0.012 0.008

0.98 0.99 0.98

3·10−5 9·10−6 2·10−5

1.20 1.19 1.25

0.034 0.031 0.021

0.99 0.98 0.96

7·10−6 8·10−6 5·10−5

1.20 1.18 1.24

0.045 0.040 0.025

0.98 0.99 0.99

6·10−5 3·10−6 7·10−6

1.40 1.37 1.30

0.204 0.197 0.179

0.99 0.98 1

7·10−6 6·10−6 3·10−6

1.30 1.28 1.29

0.012 0.013 0.013

0.98 0.98 0.99

9·10−6 1·10−5 7·10−6

1.19 1.18 1.17

0.031 0.038 0.046

0.99 1 0.98

9·10−6 5·10−6 2·10−5

1.18 1.15 1.21

0.040 0.047 0.048

0.97 0.98 0.99

5·10−5 9·10−6 9·10−6

1.37 1.38 1.38

0.197 0.222 0.269

0.98 0.98 0.99

4·10−5 4·10−5 1·10−5

1.30 1.31 1.40

0.012 0.011 0.006

0.99 0.98 0.98

9·10−6 8·10−6 8·10−6

1.19 1.18 1.21

0.031 0.038 0.019

0.98 0.99 0.96

8·10−5 5·10−6 5·10−5

1.18 1.23 1.26

0.040 0.043 0.023

0.99 0.97 0.98

3·10−5 3·10−5 3·10−5

1.37 1.41 1.36

0.197 0.241 0.089

0.98 0.99 1

4·10−5 6·10−6 5·10−6

Table 4. Langmuir Parameters Cad,max,L (g·g−1) and KL (L·g−1) and Statistical Parameters R2 and SER for Tested Amino Acids amino acid parameter pH 2 6 11 Temp. (°C) 10 20 30 EtOH (% v/v) 0 25 50 Ionic Strength (M) 0 1.5 4.3

lysine Cad,max,L

KL

glycine R

2

SER −5

Cad,max,L

KL

taurine R

2

SER −5

Cad,max,L

KL

tryptophan R

2

SER −5

Cad,max,L

KL

R2

SER

0.0088 0.0074 0.0062

4.27 4.00 3.92

0.97 0.99 0.98

7·10 6·10−5 8·10−5

0.0163 0.0132 0.0123

4.48 4.71 4.37

0.99 0.99 0.98

3·10 4·10−5 7·10−5

0.0134 0.0144 0.0146

7.18 5.58 3.98

0.99 0.98 0.98

4·10 7·10−5 1·10−4

0.0170 0.0183 0.0181

87 62 53

0.98 0.99 0.98

1·10−4 8·10−6 9·10−5

0.0084 0.0074 0.0055

4.49 4.00 3.63

0.99 0.96 0.98

4·10−5 8·10−5 2·10−4

0.0130 0.0132 0.0147

5.74 4.71 2.86

0.99 0.98 0.98

3·10−5 8·10−5 5·10−4

0.0141 0.0144 0.0160

7.04 5.58 3.19

0.99 0.99 0.98

8·10−5 2·10−5 4·10−5

0.0117 0.0183 0.0144

133 61 54

0.99 0.98 0.99

7·10−5 4·10−5 8·10−5

0.0074 0.0078 0.0079

4.00 4.11 4.07

0.98 0.99 0.99

5·10−5 3·10−5 8·10−6

0.0131 0.0142 0.0150

4.71 5.40 6.20

0.99 0.99 0.99

9·10−6 7·10−5 2·10−5

0.0141 0.0196 0.0156

7.08 4.38 6.66

0.99 0.99 0.99

7·10−5 4·10−5 6·10−5

0.0183 0.0154 0.0108

61 93 193

0.99 0.98 0.98

9·10−6 7·10−5 3·10−5

0.0074 0.0068 0.0049

4.00 3.95 4.10

0.99 0.99 0.99

4·10−5 8·10−5 8·10−6

0.0132 0.0141 0.0104

4.71 5.37 3.63

0.96 0.99 0.99

1·10−4 8·10−5 5·10−5

0.0144 0.0127 0.0102

5.58 8.27 5.47

0.99 0.98 0.99

2·10−5 1·10−4 7·10−5

0.0183 0.0097 0.0096

61 231 48

0.99 0.99 0.98

4·10−5 6·10−5 1·10−4

order of magnitude of the condensation process, (0.5 to 10) kcal·mol−1, and free energy changes are in the range (−5 to 0) kcal·mol−1.16 Results showed that the orientation of adsorption was with the hydrophobic tail of the amino acids toward the resin surface and the hydrophilic tail toward the aqueous solution. In the case of amino acid tryptophan, in addition to the abovementioned mechanisms, adsorption by polarization of π electrons should occur due to the electron-rich aromatic nuclei of its molecule. Thus, the interaction developed between the longer hydrophobic and aromatic chain of the tryptophan molecule, consisting of 10 carbon atoms, and the Amberlite XAD16 surface was stronger than those developed between the shorter taurine, glycine, and lysine molecules and the resin. The addition of an aromatic group to the hydrocarbon chain should have an effect on the hydrophobicity equivalent to about three and one-half methylene groups.17

acid recovery depending on the hydrophobic/hydrophilic character of each amino acid. In Figure 2 it can be observed that adsorption increased in the order tryptophan > taurine > glycine > lysine, which is also the order of decrease in the carbon number of amino acid chain, that is, a decrease in the amino acid hydrophobicity. At the initial concentration C0 = 0.5 g·L−1, T = 20 °C, 0 M NaCl, and 0 % ethanol, the amino acid recovery was 98 % for tryptophan, 85 % for taurine, 81 % for glycine, and 63 % for lysine. The phenomenon of a compound capture onto Amberlite XAD16 resin is a physisorption process characterized by van der Waals forces, hydrophobic interactions and hydrogen bonding by CH group.10,14 Forces attracting the molecules of the fluid to the solid surface are weak when compared to chemical adsorption and ion-exchange.15 The energy of activation is usually not higher than 1 kcal·mol−1, and the binding process is exothermic. Heat evolves with the same 711



4·10−5 6·10−5 1·10−4 0.99 0.99 0.99 65 74 30 0.79 0.87 0.56 0.0201 0.0235 0.0207 8·10−5 1·10−5 7·10−5 0.99 0.99 0.99 6.00 6.07 5.92 0.78 0.81 0.78 0.0159 0.0187 0.0115 1·10−4 8·10−5 8·10−5 0.99 0.99 1 5.17 5.46 3.03 0.0147 0.0156 0.0129

0.72 0.72 0.68

0.72 0.77 0.68

0.0088 0.0093 0.0089

0.0088 0.0075 0.0059

4.46 4.65 4.43

0.98 0.98 0.98

1·10−4 1·10−4 8·10−3

0.77 0.81 0.87

9·10−6 7·10−5 7·10−5 0.99 0.99 0.99 67 71 76 0.83 0.87 0.91 0.0191 0.0206 0.0224 7·10−5 4·10−5 6·10−5 0.99 0.99 0.99 7.04 7.30 7.64 0.80 0.83 0.85 0.0140 0.0148 0.0157 1·10−4 9·10−5 4·10−5 0.99 1 0.99 5.11 5.42 5.75 0.0147 0.0160 0.0172

0.69 0.79 0.72 0.0101 0.0085 0.0067

4.47 4.58 5.34

0.99 0.99 0.99

1·10−4 3·10−5 1·10−4

0.77 0.82 0.86

1·10−5 9·10−6 8·10−5 0.99 0.99 0.99 146 66 60 0.75 0.77 0.73 0.0136 0.0204 0.0167 3·10−5 5·10−5 4·10−5 0.99 0,99 0.99 7.7 6.00 3.46 0.75 0.78 0.76 0.0160 0.0159 0.0179 1·10−4 1·10−4 3·10−5 0.99 0.99 1 5.46 4.55 2.88 0.96 0.96 0.98 0.0138 0.0138 0.0147 1·10−4 1·10−4 1·10−4 0.99 0.99 0.98

0.69 0.62 0.59 0.0100 0.0090 0.0086

5.25 4.22 4.06

0.99 0.99 1 87 67 53 0.93 0.70 0.88 0.0174 0.0131 0.0188 1·10 1·10−4 7·10−5

SER R2

2 6 11 Temp. (°C) 10 20 30 EtOH (% v/v) 0 25 50 Ionic Strength (M) 0 1.5 4.3

5.42 5.07 4.94

0.99 0.99 0.98

1·10 1·10−4 1·10−4

0.0181 0.0147 0.0144

0.83 0.83 0.78

4.48 4.70 4.37

1 0.99 0.99

8·10 1·10−4 1·10−4

0.0155 0.0159 0.0129

0.76 0.78 0.70

7.94 6.00 3.54

0.99 0.99 0.99

−4

KT KT KT

−5

tryptophan

t Cad,max,T SER R2 KT

taurine

t Cad,max,T parameter

t Cad,max,T pH

lysine

R2

SER

Cad,max,T

t

glycine

R2

SER

amino acid

Table 5. Toth Parameters Cad,max,T (g·g−1), KT (L·g−1), and t, and Statistical Parameters R2 and SER for Tested Amino Acids 712

−4

Taurine and glycine are both amphiphilic and neutral amino acids, and taurine is more soluble in water than glycine; nevertheless, a higher extent of adsorption occurred for taurine. Taurine and glycine molecules have the same terminal hydrophilic groups OH and NH2; however, the acidic group SO2OH of taurine exhibits a higher inductive effect and electron-acceptor character than the acidic group COOH of glycine, which per contra shows both electron-donor and acceptor behavior. This resulted in a higher affinity of taurine with Amberlite XAD16 since this resin exhibits strong electron donor properties due to the rich π electrons nuclei.2 Also, since the SO2OH group is bigger than the COOH group of glycine, the electron-acceptor behavior of taurine was still more evident on adsorption.18,19 Regarding lysine, the lower extent of adsorption is attributable mainly to the presence of the hydrophilic terminal groups OH and NH2 and also to the middle NH2 group. It should be noticed that strong negative effects onto adsorption from aqueous solutions upon hydrophobic adsorbents could be attributed either to the presence of an additional hydrophilic group or to its position. When the hydrophilic group is positioned at the end of the hydrocarbon chain the negative effect is more pronounced in comparison with a hydrophilic groups present at a central position.19 Modeling of experimental data by nonlinear regression showed that Freundlich, Langmuir, and Toth isotherms are all suitable; coefficients of determination were very near to the perfect fit (R2 near to 1, and in some case equal to 1), and the standard error of the regression was close to zero and in any case not greater than 10 % of the value estimated. Data from nonlinear fitting are reported in Tables 3, 4, and 5. For the Freundlich model, either the linear or the nonlinear regression provided a goodness-of-fit and the same adsorption parameters. For the Langmuir model, on the contrary, only the nonlinear regression provided a goodness-of-fit; isotherm linearization was inappropriate for this system since coefficients of determination, R2, were always < 0.85, and SER was high. For all of the adsorption equilibrium isotherms, parameters varied depending on environmental conditions. The Freundlich model is applicable to real surfaces; it describes multilayer adsorption with nonuniform distribution of adsorption heat and affinity on heterogeneous surfaces.20 The parameter KF denotes the adsorption capacity of the adsorbent and represents the quantity of amino acid adsorbed on the resin for a unit equilibrium concentration. The coefficient 1/n denotes the intensity of the adsorption and the heterogeneity of the adsorbent. The greater the value of KF the greater the adsorption while the greater the value of n the stronger the bond between the amino acid and the adsorbent.15 According to the Freundlich theory, the value of n can be used to determine whether adsorption is favorable, since the parameter is a measure of the binding energy between adsorbent and adsorbate. According to Freundlich, a number of layers of adsorbate can be formed on each adsorbent site, depending on the binding energy, which decreases exponentially with the number of layers.16 Therefore the greater the n the greater the number of layers and then, the greater the amount of a compound adsorbed on a resin and the more favorable the adsorption. Values for n in the systems studied were always > 1, indicating that adsorption has been favorable.21,22 The term “favorable” or “unfavorable” is referred to the fact that the amount of adsorbate increases with equilibrium concentration in a nonlinear way. In the Freundlich model, favorable

8·10−5 8·10−6 3·10−5

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conditions occur for n > 1; when n = 1, adsorption is linear; when n < 1, adsorption is unfavorable; for n less than 0.5 a compound is difficult to adsorb. Generally, the Freundlich isotherms have value of n in the range 1 < n ≤ 10.21 The capacity factor KF of the systems studied showed a higher variability that n being dependent on the amino acid nature2 (Table 3). The Langmuir model is applicable for ideal surface since assumes monolayer adsorption on homogeneous surfaces where all sites have equal affinity for the adsorbate and where no lateral interactions and steric hindrance between adsorbed molecules occur. The parameter KL is a measure of the binding energy, like n in the Freundlich equation, and is related to the separation factor of the adsorption as RL = 1/(1 + KL·C0), where C0 is the highest initial concentration of amino acids in solution. When the separation factor is 0 < RL < 1 adsorption is favorable, when RL > 1 adsorption is unfavorable, when RL = 1 adsorption is linear and when RL = 0 adsorption is irreversible.20 In all of the adsorption experiments, that factor was always 0 < RL < 1 (Table 4). The Toth model was developed to improve Langmuir isotherm fitting by introducing the factor t which takes into account the heterogeneity of the adsorption system, like n in the Frendlich isotherm. The more the t deviates from 1 the more the heterogeneous the system is, and for value of t = 1 the Toth equation reduces to the Langmuir isotherm.20 When environmental conditions of adsorption were changed, the extent of the process itself was modified; however the order of adsorption of the tested amino acids remained the same, as discussed in the following paragraphs. 3.2. Effects of Temperature upon Adsorption Equilibrium. An increase in temperature corresponded to a decrease in adsorption. In Table 6 thermodynamics parameters for each

Table 7. Percentage Recovery of Amino Acids Depending on Temperature, Ionic Strength, pH, and Ethanol at C0 = 0.5 g·L−1 and at 10 g Resin/100 mL of Solutiona

Table 6. Thermodynamic Parameters of Adsorption ΔG°, ΔH°, and ΔS° for Each Amino Acid and for Each Temperature

Generally, it was observed that, by increasing ionic strength, adsorption from aqueous solutions onto nonpolar hydrophobic adsorbent, and probably onto polar adsorbents without strongly charged sites, increases as well. The influence of salt on adsorption is attributable to an increase of the hydrophobic effect, which leads to the segregation of water molecules, nonpolar, and amphiphilic substances, a phenomenon also known as “salting-out”.3 Apart from affecting the solvent power (i.e., modification in H-bonding between water molecules and amino acids) ionic strength also influences interactions at the amino acid−resin interface, such as electrostatic forces, Hbonding forces, repulsive forces between electron-rich aromatic nuclei of adsorbate and adsorbent, as well as interactions between amino acids themselves.23 The more the electronic density in aromatic ring of an adsorbate, the more pronounced the effect of salt, since the reduction in repulsive forces is higher.17 At the higher ionic strength 4.3 M adsorption diminished, most probably due to the excessive Na+ and Cl− ion concentration, which reduced the mobility of amino acids in solution as well as the access of amino acids to the resin surface.23 3.4. Effects of pH upon Adsorption Equilibrium. Changes in pH of solution affected the mechanism and the extent of adsorption of tryptophan, glycine, and lysine. Regarding taurine, no changes were observed by decreasing pH, but a decrease in adsorption occurred at alkaline pH values as in the case of other amino acids. In Table 7 the amino acid recovery yields depending on solution pH are reported, while in Figure 3c taurine isotherms (at 20 °C, 0 % ethanol, and 0 M

ΔG°/(kcal·mol−1)

ΔH°

recovery of amino acid at 0 M NaCl, pH 6, and 0 % v/v ethanol temperature (°C)

amino acid

10 °C

20 °C

30 °C

kcal·mol

tryptophan taurine glycine lysine

−2.85 −1.25 −1.03 −0.63

−2.62 −1.12 −0.96 −0.51

−2.31 −0.93 −0.81 −0.30

−4.92 −4.95 −4.17 −4.82

glycine

tryptophan

taurine

glycine

98a,A 85b,A a,B 100 89b,B 94a,C 75b,C recovery of amino acid at 20 °C, 0 M 0 % (v/v) ethanol

0 1.5 4.3

pH

taurine

lysine

100a,A 87b,A 83b,A 69d,A a,B b,B b,B 98 85 81 63d,B 96a,C 79b,C 76c,C 52d,C recovery of amino acid at 20 °C, pH 6, and 0 % v/v ethanol

ionic strength (M of NaCl)

tryptophan

taurine

lysine

81c,A 63d,A c,B 84 60d,B 71c,C 47d,C NaCl, and

glycine

100a,A 85b,A a,B 98 85b,A 94a,C 81b,B recovery of amino acid at 20

2 6 11

lysine

84b,A 81c,B 78c,C °C, pH 6, and

69c,A 63d,B 57d,C 0 M NaCl

ethanol (% v/v)

tryptophan

taurine

glycine

lysine

0 25 50

98a,A 99a,A 100a,B

85b,A 87b,B 88b,C

81c,A 84c,B 85c,C

63d,A 64d,A 64d,A

a

For each parameter, values in the same line that are not followed by the same lowercase superscript letter are significantly different (p ≤ 0.05). Values in the same columns that have not the same capital superscript letter are significantly different (p ≤ 0.05).

ΔS° −1

tryptophan

10 20 30

kcal·mol−1·K−1 −0.0103 −0.0133 −0.0110 −0.0169

amino acid are reported. Parameters were calculated taking into account the mean Keq value over the range of amino acid initial concentration (0.05 to 0.5) g·L−1. Values for ΔG° in the range (−2.85 to −0.30) kcal·mol−1 and values for ΔH° in the range (−4.92 to −4.92) kcal·mol−1 indicated that adsorption was spontaneous, physical, and exothermal.15,16,21 The optimal adsorption temperature was 10 °C. In Table 7 amino acid recovery is reported, while in Figure 3a taurine isotherms (at 0 M NaCl, 0% v/v ethanol, and pH 6) are shown as an example of the effect of temperature upon adsorption. 3.3. Effects of Ionic Strength upon Adsorption Equilibrium. An increase in amino acid adsorption occurred by increasing the ionic strength of solution up to 1.5 M NaCl. A decrease in adsorption was noticed by increasing ionic strength from 1.5 M up to 4.3 M. In Table 7 amino acid recovery depending on ionic strength is reported, while in Figure 3b taurine isotherms (at 20 °C, 0 % v/v ethanol, and pH 6), as an example of the effect of NaCl concentration upon adsorption, are shown. 713



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Figure 3. Adsorption isotherms for taurine depending on temperature (a), ionic strength (b), and pH (c), and adsorption isotherms for lysine depending on percentage of ethanol in solution (d) (mean values ± standard deviation bars, which are not visible, and theoretical model data set), at C0 = 0.5 g·L−1 and 10 g resin/100 mL of solution.

naturally tends to loss a proton in aqueous solutions, in the entire pH range.26 3.5. Effects of Ethanol upon Adsorption Equilibrium. Addition of ethanol into solution resulted in an increase of adsorption for the neutral amino acids taurine and glycine; no significant differences were observed for lysine, and only a slight increase was noticed for tryptophan. Results for recovery are reported in Table 7, while in Figure 3d lysine isotherms (at 20 °C, pH 6 and 0 M NaCl) are reported, as an example of the effect of ethanol upon adsorption depending on amino acid nature. The addition of ethanol to an aqueous solution exerts a similar “salting-out” effect of neutral electrolytes onto neutral and hydrophobic species, that is, reduces their solubility, while solubility of hydrophilic amino acids remains substantially constant.27 This phenomenon resulted in a higher affinity of taurine, glycine, and tryptophan for the resin, while results for lysine remained essentially unchanged. Also, among the foodgrade solvents, ethanol is able to precipitate the greater amount of NaCl, allowing the amino acids to be kept in solution,28 and this feature can be taken into account in a real situation when recovery of amino acids after salt precipitation could be required. 3.6. Effects of Initial Concentration on Amino Acid Recovery. The effect of initial concentration of amino acids was assessed at 20 °C, 0 % v/v ethanol in solution, pH 6, and 0 M NaCl. Results showed that adsorption decreases by increasing initial concentration; however, that decrease was

NaCl) are reported, as an example of pH effects upon adsorption. The pH, as well as the ionic strength, exerts a major effect on the resin surface. Since amino acids have both an acidic and basic site, and Amberlite XAD16 copolymers are aromatic and possess good electron-donor properties, it is expected that polar and electrostatic forces occur together with dispersion forces. The interaction between the nonionizable amino acid R group and the Amberlite XAD16 surface is considered to be constant throughout the entire pH range.2 Therefore, the variations in adsorption efficiency should be the result of ionization of the COOH and/or NH2 groups. Generally, adsorption is favored by acidic conditions, where the COOH group exists as the undissociated form and the NH2 is ionized. In basic solution, where the COOH group is ionized and the NH2 group is un-ionized, and at intermediate pH, where both groups are ionized, adsorption is lower. Also, if any additional ionizable sites are present on the amino acids side chain, ionization at these sites will also contribute to the overall charge of the species as a function of pH and to the extent of adsorption.24 At acidic pH, when the electron acceptor COOH group is uncharged, amino acids can interact with resin, taking advantage of the electron donor properties of its surface, apart from the normal interaction by the R chain.25 In general, the pH has no or little effect on the adsorption of strong acids, either because they are almost completely dissociated in aqueous solutions or due the higher stability of the respective conjugated bases.7 That was apparently the case of taurine. Being a sulfonic acid, taurine is a strong acid since 714



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3.7. Effects of Amberlite XAD16 Dose upon Amino Acid Recovery. Results for recovery of amino acids depending on adsorbent dose, at 20 °C, 0 % v/v ethanol in solution, pH 6, and 0 M NaCl, showed that optimal amount was 10 g of dry Amberlite XAD16 for 100 mL of solution. As represented in Figure 4b and c, the lower value of 5 g/100 mL resulted in a significant reduction of adsorption, but a further increase above 10 g/100 mL did not have any significant effect upon amino acid recovery. The percentage of amino acids adsorbed upon resin is determined by the sorption capacity of Amberlite XAD16 itself;30 therefore adsorption augmented by increasing the adsorbent dose. However, at high resin dosages the mobility of amino acids in solution is reduced,31 which results in any significant increment in recovery, or worse, in a decrease in adsorption, as in the case of tryptophan and taurine. 3.8. Effect of Parameter Interaction upon Adsorption of Taurine. Results for taurine adsorption yield depending on pH and ionic strength, at the temperature of 10 °C and with a resin dose of 10 g/100 mL solution (Table 8), showed that the

less significant for tryptophan (Figure 4a). The recovery of taurine decreased from 90 % down to 85 % by increasing

Table 8. Recovery of Taurine by Combining pH with Ionic Strength, at 10 °C and 10 gresin/100 mL Solutiona pH ionic strength (M)

2

6

11

0 1.5 4.3

87a,A 90a,B 79a,C

87a,A 90a,B 79a,C

81b,A 80b,A 78b,B

a

Values in the same line that are not followed by the same lowercase superscript letter are significantly different (p ≤ 0.05). Values in the same columns that have not the same capital superscript letter are significantly different (p ≤ 0.05).

effect of ionic strength was stronger than the effect of pH. This result may not be specific for taurine but due to the fact that adsorption onto uncharged and not functionalized resins is generally highly influenced by a pH change at a low ionic strength (≤ 1 M), while the pH factor is not significant at higher ionic strength values (> 1 M) as reported by Kyriakopoulos et al.23 A 32 factorial design experiment was then performed considering just the most significant factors, that is, temperature and ionic strength. Levels considered were (10, 20, and 30) °C for temperature and (0, 1.5, and 4.3) M for ionic strength. A matrix of experiments and matrix of data for factorial design are reported in Tables 9 and 10, respectively. In Figure 5 the Table 9. Matrix of Experiments for Factorial Design Figure 4. Effect of initial amino acid concentration (a) and effect of adsorbent dose depending on amino acid initial concentration (b, c) on recovery (mean values ± standard deviation bars, which are not visible) (⧫, tryptophan; gray ■, taurine; ○, glycine; gray ▲, lysine).

temperature (°C)

concentration from (0.05 to 0.5) g·L−1; same reduction of recoveryfrom 86 % to 81 %was observed for glycine, and even more for lysinefrom 76 % to 63 %. A reduction of just 2 % (from 100 % to 98 %) was observed in the case of tryptophan. Results obtained are in accordance with amino acids affinity for the resin. The initial concentration is important since a given mass of adsorbent can only adsorb a fixed amount of a specific compound. Therefore, the more concentrated the solution, the smaller the recovery of a compound.29

ionic strength (M) (Factor 2)

(Factor 1)

0 (Level 1)

1.5 (Level 2)

4.3 (Level 3)

10 (Level 1) 20 (Level 2) 30 (Level 3)

+ + +

− + −

− + −

second-order fitted surface of response is reported (R% of taurine). The maximum recovery of taurine was 90.1 % at the temperature of 13.5 °C and at an ionic strength of 1.3 M NaCl. A fitted function for the recovery of taurine, obtained by the factorial design, was given by the following equation: 715



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Table 10. Matrix of Data for Factorial Designa temperature (°C) (Factor 1)

ionic strength (M) (Factor 2) 0 (Level 1)

10 (Level 1) 20 (Level 2) 30 (Level 3)

a,A

87 85a,B 79a,C

1.5 (Level 2) b,A

91 89b,B 79a,C

4.3 (Level 3) 76c,A 75c,A 65c,B

a

Values in the same line that are not followed by the same lowercase superscript letter are significantly different (p ≤ 0.05). Values in the same columns that have not the same capital superscript letter are significantly different (p ≤ 0.05).

R(%) = 80.27 + 4.45·Is − 1.52·Is2 + 1.03·T − 0.04·T 2 − 0.03·T ·Is

(11)

where R(%) represents taurine recovery expressed in percent, Is represents ionic strength expressed in molarities of NaCl, and T represent temperature expressed in Celsius degrees. As can be noticed from eq 11 and in the Pareto graph (Figure 6), recovery of taurine was significantly dependent upon a linear and quadratic effect of each factor (p > 0.05); however, the linear effect of ionic strength was the most significant. 3.9. Regeneration of the Amberlite XAD16 Resin. Amberlite XAD16 was regenerated, that is, cleaned from amino acid adsorbed, using acetone; ethanol was tested for solutions of amino acids in the water. These solvents were chosen since they are of food-grade and because in the range of concentration tested amino acids have the same solubility in both of them. However, despite the similar polarity (acetone ε = 21, ethanol ε = 24, where ε is the dielectric constant), acetone is an aprotic liquid and for this reason has a higher affinity for the aromatic structure of Amberlite XAD16 than ethanol, which has a protic nature.32 Other nonfood-grade solvents like methanol and isopropanol are also suitable for Amberlite XAD16 regeneration from amino acids.10

Figure 6. Pareto plot of the standardized effects of the two factors on the response (R% of taurine). “L” stands for “linear”, and “Q” stands for “quadratic”. The vertical line defines significance limit expressed by the p-value at a confidence level of 95 %.

4. CONCLUSION Adsorption of glycine, lysine, taurine, and tryptophan upon Amberlite XAD16 resin was favorable and in accordance with the specific nature of each amino acid. A decrease in temperature and an increase in ionic strength (up to 1.5 M) favored adsorption of all amino acids; the addition of ethanol (up to 50% v/v) favored adsorption of glycine, taurine, and tryptophan; a decrease in pH (down to 2) favored the adsorption of glycine, lysine, and tryptophan. The optimal adsorbent dose for the highest recovery of amino acids was 10 g for 100 mL of solution, within a range of concentration of (0.05 to 0.5) g·L−1. When the recovery of taurine must be maximized, adsorption must be carried out at the temperature of 13.5 °C and at the ionic strength of 1.32 M NaCl. This study showed how physical parameters modify adsorption interactions such as van der Waals forces,

Figure 5. Second-order fitted surface of response (R% of taurine) for 32 factorial design. 716



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(19) Dutta, M.; Baruah, R.; Dutta, N. N. Adsorption of 6-aminopenicillanic acid on activated carbon. Sep. Purif. Technol. 1997, 12, 99− 108. (20) Foo, K. Y.; Haamed, B. H. Insight into modeling of adsorption isotherm system. Chem. Eng. J. 2010, 156, 2−10. (21) Qiu, T.; Zeng, Y.; Ye, C.; Tian, H. Adsorption thermodynamics and kinetics of p-Xilene on activated carbon. J. Chem. Eng. Data 2012, 57, 1551−1556. (22) Skopp, J. Derivation of the Freundlich adsorption isotherm from kinetics. J. Chem. Educ. 2009, 86 (11), 1341−1343. (23) Kyriakopoulos, G.; Doulia, D.; Hourdakis, A. Effect of ionic strength and pH on the adsorption of selected herbicides on Amberlite. Int. J. Environ. Anal. Chem. 2006, 86 (3−4), 207−214. (24) Myers, M., Ed. Surfactant Science and Technology, 3rd ed.; Wiley: Hoboken, NJ, 2006. (25) Kroeff, E. P.; Pietrzyk, D. J. Investigation on the retention and the separation of amino acids, peptides and derivatives on porous copolymer by high performance liquid chromatography. Anal. Chem. 1978, 50 (3), 502−511. (26) Hamborg, E. S.; Niederer, J. P. M.; Versteeg, G. F. Dissociation constant and thermodynamic properties of amino acids used in CO2 adsorption from (293 to 353) K. J. Chem. Eng. Data 2007, 52, 2491− 2502. (27) Ji, P.; Zou, J.; Feng, W. Effect of alcohol on the solubility of amino acids in water. J. Mol. Catal. B: Enzym. 2009, 56, 185−188. (28) Ferraro, V.; Braga Cruz, I.; Ferreira Jorge, R.; Pintado, M. E.; Castro, P. M. L. Solvent extraction of sodium chloride from codfish salting processing wastewater. Desalination 2011, 287, 42−48. (29) Meško, V.; Markowska, L.; Minčeva, M. Two resistance mass transfer model for the adsorption of basic dyes from aqueous solutions on natural zeolite. Maced. J. Chem. Chem. Eng. 1999, 18 (2), 161−169. (30) Kumar, P. S.; Ramakrishnan, K.; Kirupha, S. D.; Sivanesan, S. Thermodynamic and kinetic studies of cadmium adsorption from aqueous solution onto rice husk. Braz. J. Chem. Eng. 2010, 27 (2), 347−355. (31) Argarval, H.; Sharma, D.; Shindu, S. K.; Tyagi, S.; Ikram, S. Removal of mercury from wastewater using green adsorbents-A Review. Elec. J. Env. Agric. Food Chem. 2010, 9 (9), 1551−1558. (32) Manin, N. G.; Belichenko, S. Y.; Korolev, V. P. Thermochemistry of benzene solutions in alcohols, aprotic solvents, and mixtures of aprotic solvents with methanol. Russ. J. Gen. Chem. 2003, 73 (1), 9− 16.

hydrophobic interactions, dipole−dipole interactions, and hydrogen bonding, and how this can be exploited in real situations, such as the treatment of industrial effluents either for compounds recovery or for purification.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Telephone/fax: +351 225580059/225090351. Funding

V.F. thanks Marie Curie Actions (European Research Area) for a doctoral grant (ref. InSolEx-RTN under FP6). This work was supported by National Funds from FCTFundaçaõ para a Ciência e a Tecnologia through project PEst-OE/EQB/ LA0016/2011. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Sivasankar, B., Ed. Engineering Chemistry; McGraw-Hill: New York, 2008. (2) Doulian, D.; Rigas, F.; Gimouhopoulos, C. Removal of amino acids from water by adsorption on polystyrene resins. J. Chem. Technol. Biotechnol. 2001, 76, 83−89. (3) Fujita, T.; Hirofumi, W.; Shigenori, T. Effects of salt addition on strength and dynamics of hydrophobic interactions. Chem. Phys. Lett. 2007, 438, 42−48. (4) Grzegorczyk, D. S.; Carta, G. Adsorption of amino acids on porous polymeric adsorbents-I. Equilibrium. Chem. Eng. Sci. 1996, 51 (5), 807−817. (5) Lee, J. W.; Nguyen, T. P. B.; Moon, H. Adsorption and desorption of phenyalanine and tryptophan on a non-ionic polymeric sorbent. Korean J. Chem. Eng. 2006, 23 (5), 812−818. (6) Nagai, H.; Hasegawa, K. Divalent amino acids adsorption on cation exchange resin. Sep. Sci. Technol. 2009, 44 (13), 3075−3087. (7) Pletrzyk, D. L.; Chu, C. H. Separation of organic acids on Amberlite XAD compolymers by reverse-phase high pressure liquid chromatography. Anal. Chem. 1977, 49 (6), 860−867. (8) Yang, W. C.; Shim, W. G.; Lee, J. W.; Moon, H. Adsorption and Desorption Dynamics of Amino Acids in a nonionic polymeric sorbent XAD16 Column. Korean J. Chem. Eng. 2003, 20 (5), 922−929. (9) Shim, W. G.; Lee, J. W.; Yang; Moon, H. Multicomponent adsorption equilibria of amino acids on polymer resins. In Adsorption, Chromatography, Ion-Exchange, Proceeding of 4th ICSST; Zhanfga, T., Sung Hyun, H., Eds.; World Scientific Publishing: Singapore, 2004. (10) Rohm & Haas Company. Amberlite XAD resins, Technical Data Sheet; Philadelphia, 2010. (11) Orth, D. L. HPLC determination of taurine in sport drinks. J. Chem. Educ. 2001, 78 (6), 791−792. (12) Alonso, M. L.; Alvarez, A. I.; Zapico, J. Rapid analysis of free amino acids in infant foods. J. Liq. Chromatogr. 1994, 17, 4019−4030. (13) Kumar, K. V.; Monteiro de Castro, M.; Martinez-Escandell, M.; Molina-Sabio, M.; Rodriguez-Reinoso, F. A site energy distribution function from Toth isotherm for adsorption of gases on etherogenerous surfaces. Phys. Chem. Chem. Phys. 2011, 13, 5753−5759. (14) Gao, S.; Hahn, J. R.; Ho, W. Adsorption induced hydrogen bonding by CH group. J. Chem. Phys. 2003, 119 (12), 6232−6236. (15) Smith, J. M., Ed. Chemical Engineering Kinetics; McGraw-Hill: Singapore, 1981. (16) Chen, G. H. Application of Physical Chemistry; Chemical Industry Press: Beijing, 2008. (17) Koleskin, Y. A.; Kozlov, V. V. Structure of organic oxygen compounds of sulphur. Russ. Chem. Rev. 1968, 37 (7), 519−532. (18) Yang, Q. Catalytic application of sulfonic acid functionalized mesoporous benzene-silica with crystal-like pore wall structure in esterification. J. Mol. Catal. A: Chem. 2005, 230, 85−89. 717


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