Adsorption studies of Cr(III) ions from aqueous solutions by DEHPA impregnated onto Amberlite XAD7 – Factorial design analysis

chemical engineering research and design 9 0 ( 2 0 1 2 ) 1660–1670

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Adsorption studies of Cr(III) ions from aqueous solutions by DEHPA impregnated onto Amberlite XAD7 – Factorial design analysis M. Ciopec a , C.M. Davidescu a , A. Negrea a , I. Grozav b , L. Lupa a,∗ , P. Negrea a , A. Popa c a

Faculty of Industrial Chemistry and Environmental Engineering, University “Politehnica” Timisoara, 2 Piata Victorie, 300006 Timisoara, Romania b Faculty of Mechanics, University “Politehnica” Timisoara, 2 Piata Victoriei, 300006 Timisoara, Romania c Romanian Academy, Institute of Chemistry, 24 Mihai Viteazul Blv., 300223 Timisoara, Romania

a b s t r a c t The present paper investigates the adsorption of Cr(III) ions using the SIR, prepared by impregnation of Amberlite XAD7 with di-(2-ethylhexyl)-phosphoric acid (DEHPA), which has been chosen as an extractant for the purpose of this study. The Amberlite XAD7–DEHPA resin was impregnated with DEHPA and ethylic alcohol as solvent trough dynamic column impregnation method. The influence of different physicochemical parameters (pH, resin dosage, initial concentration of Cr(III) ions, contact time and temperature) upon the adsorption capacity of XAD7–DEHPA, in the Cr(III) ions removal process from aqueous solution, has been investigated. The pH for Cr(III) ions adsorption was found as 3.0 for this material. The results showed that the adsorption equilibrium was reached after 45 min. The adsorption process is best described by the pseudo-second order kinetic model. Langmuir adsorption isotherm gave a satisfactory fit of the equilibrium data. The maximum adsorption capacity is ∼3 mg Cr(III) ions/g SIR. The thermodynamic studies allowed us to determine the thermodynamic parameters G◦ , H◦ and S◦ . In this paper the factorial design of experiments was used to study the performance of the adsorption process. © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Amberlite XAD7; DEHPA; Solvent impregnated resin (SIR); Cr(III); Sorption kinetics; Factorial design

1.

Introduction

Industrial processes generate waste that if disposed untreated would have a detrimental effect on the environment and human health. Heavy metals like Cr, Cu, Pb, Mn, Hg and Cd are common pollutants in soil as well as in water. Due to their greater stability they cannot be degraded and removed from the environment (Mustafa et al., 2008). Chromium is found in two oxidation states, Cr(III) and Cr(VI), the latter being the more toxic (Hosseini-Bandegharaei et al., 2010; Mustafa et al., 2008; Saha et al., 2004). It is extensively used in pigments and paints, leather tanning, fungicides, ceramic and glass industries. Cr(III) ion is an essential nutrient for human health also. However, the presence of strong oxidants can change it to harmful Cr(VI) (Mustafa et al.,

∗

2008; Tadesse et al., 2006). A lot of work is present in literature on Cr(VI) ions removal but very limited research work is done on the removal of Cr(III) ions from aqueous solutions (Deepa et al., 2006; Lazaridis et al., 2005; Lazaridis and Charalamous, 2005; Mustafa et al., 2008; Taeyoon et al., 2003). Conventional techniques of metal ions removal from environmental matrices include the following processes: precipitation, solvent extraction, electrochemical recovery, and membrane separation (Belkhouche and Didi, 2010; Chabani et al., 2007; Chun-hua et al., 2009; Kocaoba and Akcin, 2005; Mustafa et al., 2008; Saha et al., 2004). Most of these processes may be ineffective, extremely expensive, or generate secondary pollution. In recent years, adsorption process has been widely practiced for metal ions removal, because of its competitive and effective process for the above purpose. The

Corresponding author. Tel.: +40 256 404192; fax: +40 256 404192. E-mail address: [email protected] (L. Lupa). Received 15 December 2011; Accepted 27 January 2012 0263-8762/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2012.01.016

chemical engineering research and design 9 0 ( 2 0 1 2 ) 1660–1670

use of macro porous organic polymer supports, with a high surface area and good mechanical stability, is found more suitable for the removal of toxic elements from dilute solution, due to their faster kinetics, ease of regeneration and high adsorption capacity (Belkhouche and Didi, 2010; Chabani et al., 2007; Hosseini-Bandegharaei et al., 2010; Mustafa et al., 2008; Saha et al., 2004). Impregnating appropriate solid supports, such as Amberlite XAD series, is one of the well-known and effective solid sorbent preparation methods for treatments purposes (Belkhouche and Didi, 2010; Hosseini-Bandegharaei et al., 2010; Mustafa et al., 2008; Narin et al., 2008). There are four methods available for the impregnation of the desired extractant into the polymeric supporting structure: the dry method, wet method, modifier addition method and dynamic column method (Mendoza et al., 2000; Muraviev et al., 1998; Saha et al., 2004). The dynamic column method has the advantages of short impregnation time and high efficiency, which can be obtained not only in laboratory test but on an industrial scale (Juang, 1999). The aim of this present work is to study the adsorption performance of Amberlite XAD7 resin impregnated with DEHPA as organophosphorus extractant trough dynamic column impregnation method in the process of Cr(III) ions removal from aqueous solutions. In many process development and manufacturing applications, the number of potential process or design (factors) is large. Screening design is used to reduce the number of factors or design parameters by identifying the key ones that affect product quality or process performance. This reduction allows one to focus process improvement efforts on the few really important factors, or the “vital few” (Montgomery, 2001; Stas et al., 2002). From these reasons, in this paper, the factorial design of experiments was used to study the adsorption performance of XAD7–DEHPA in the removal process of Cr(III) ions from aqueous solutions.

2.

Materials and methods

2.1.

Reagents

The di(2-ethylhexyl)phosphoric acid (DEHPA) ∼98.5% used as extractant, was supplied by BHD Chemicals Ltd. Poole England and used as received. Amberlite XAD7 resin (supplied by Rohm and Hass Co.), size 0.3–0.9 mm, was used as support. The mean particle diameter ro was taken equal to 0.6 mm. The mean pore size is ∼9 nm with a surface area of 450 m2 /g. As organic solvent was used ethanol from Chimopar Romania. A stock solution of 1 g/L Cr(III) ions was prepared by diluting an appropriate amount of Cr(NO3 )3 in 0.5 mol/L HNO3 solution (Merck Standard Solution). Other solutions of Cr(III) ions were prepared from the stock solution by appropriate dilution. All other chemicals used for experiments were of analytical reagent grade, and were used without further purification. Distilled water was used in all experiments.

2.2.

Preparation of impregnated resins

The Amberlite XAD7 resin was impregnated with DEHPA and ethylic alcohol as solvent by dynamic column impregnation method (Juang, 1999). A certain amount of polymer fully swollen by the solvent was packed in a glass column of 4 cm diameter and 15 cm height. Then the extractant solution was fed into the column with a 0.1 L/h flow rate until the

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extractant concentration in the outlet was equal to the feed one. The polymeric beads were separated through a porous filter using a vacuum pump, washed with water and dried at 50 ◦ C for 24 h (Benamor et al., 2008). The resulting SIR were finally washed with distilled water.

2.3.

Adsorption experiments

The influence of different physicochemical parameters (pH, resin dosage, contact time, initial concentration of Cr(III) ions and temperature) upon the adsorption capacity of XAD7–DEHPA, in the Cr(III) ions removal process from aqueous solution, has been investigated. The initial pH of the sample solutions were adjusts in the range of 1–5 by using HNO3 or NaOH solution. In each experiment 0.1 g of sorbent was suspended in 25 mL of 10 mg/L Cr(III) ion solution. The samples were stirred for 2 h. A pH-meter CRISON Multimeter MM41 was used to measure the pH of all solutions. In order to determine the effect of S:L ratio on adsorption, experiments were conducted with 10 mg/L Cr(III) ion concentration and samples having different XAD7–DEHPA dosage ranging from 0.05 to 0.2/25 mL Cr(III) ion solution at different contact times (1, 2, 3, 4, 5, 15, 30, 45, 60, 90 and 120 min) and at the room temperature 25 ± 1 ◦ C. To investigate the effect of the temperature (298, 308 and 318 K) on Cr(III) adsorption, the experiments were conducted at constant concentration of Cr(III) ions (10 mg/L) and different stirring time (5, 15, 30, 45, 60, 90 and 120 min). Adsorption isotherm were carried out with different initial concentrations varying from 5 to 50 mg/L Cr(III) ions while keeping the resin amount at constant value (0.1 g in 25 mL Cr(III) ion solution) at room temperature (25 ± 1 ◦ C) at the stirring time of 45 min. All the samples were stirred using a shaker bath MTA Kutesz, Hungary. After stirring, the samples were separated trough filtration. The residual concentration of Cr(III) ions from filtrate was analyzed trough atomic adsorption spectrophotometer using an atomic adsorption spectrophotometer Varian SpectrAA 280 Fast Sequential Atomic Absorption Spectrometer with an airacetylene flame at wavelength  = 357.9 nm. The amount of adsorbed Cr(III) ions per unit of XAD7–DEHPA was determined by using the following equation (Abdel Raouf and El-Kamash, 2006; Davidescu et al., 2011; Gode and Pehlivan, 2006; Uysal and Ar, 2007; Yu et al., 2009):

q=

(C0 − Ce )v m

(1)

where qe is the adsorption capacity (mg/g) and C0 and Ce are the concentration of the Cr(III) ions in the solution (mg/L) before and after adsorption, respectively. v is the volume in aqueous solution (L) and m is the amount of the resin (g).

2.4.

Kinetic models for the adsorption

In order to investigate the mechanism of adsorption, the pseudo-first-order and the pseudo-second-order adsorption models were used to test dynamical experimental data. The pseudo-first-order rate expression of Lagergren is generally described by the following equation (Abdel Raouf and ElKamash, 2006; Davidescu et al., 2011; Deepa et al., 2006;

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Hosseini-Bandegharaei et al., 2010; Lazaridis et al., 2005; Shek et al., 2006; Uysal and Ar, 2007; Yu et al., 2009):

where values of H◦ and S◦ can be determined from the slope and intercept of the plot between ln Kd versus 1/T.

ln(qe − qt ) = ln qe − k1 t

2.6.

(2)

where qt and qe represent the amounts of the Cr(III) ions adsorbed on the resin at time t and at equilibrium time, respectively, mg/g; k1 is the specific adsorption rate constant, min−1 . The rate k1 was obtained from slope of the linear plots of ln(qe − qt ) versus t. The pseudo-second order model is based on the assumption that the rate limiting step may be chemical sorption or chemical sorption involving valence forces trough sharing or exchange of electrons between sorbent and sorbate. The linear form of the pseudo-second-order rate expression of Ho and McKay is given by (Davidescu et al., 2011; HosseiniBandegharaei et al., 2010; Lazaridis et al., 2005; Yu et al., 2009): 1 t t = + qt h qe

(3)

where h = k2 · q2e ; k2 is the pseudo-second-order constant, min−1 (mg/g)−1 . Other terms have their usual meanings. If second-order kinetics is applicable, the plot of t/qt versus t should give a linear relationship from which the constants qe and k2 can be determined.

2.5.

Thermodynamics of the adsorption

In general, the experimental conditions such as metal ion concentration and temperature have strong effects on the equilibrium distribution coefficient value (Kd ); so it can be used as a comparative measure to the efficiency of various adsorbents. Equilibrium distribution coefficient value (Kd ) is the amount of removed chromium per gram of XAD7–DEHPA divided by its concentration in the liquid phase: Kd =

qe Ce

(4)

Temperature dependence of the adsorption process is associated with several thermodynamic parameters. Thermodynamic considerations of an adsorption process are necessary to conclude whether the process is spontaneous or not. Thermodynamic parameters such as Gibbs free energy change (G◦ ), enthalpy change (H◦ ) and entropy change (S◦ ) can be estimated using equilibrium constant changing with temperature. The Gibbs free energy change of the adsorption reaction is given by the following equation (Abdel Raouf and El-Kamash, 2006; Chabani et al., 2007; Hosseini-Bandegharaei et al., 2010; Kyzas et al., 2009; Mustafa et al., 2008; Yu et al., 2009): ◦

G = −RT ln Kd

(5)

where R is universal gas constant (8.314 J/(mol K)), T is absolute temperature (K) and Kd is the distribution coefficient. Relation between G◦ , H◦ and S◦ can be expressed by the following equations: ◦

◦

G = H − TS◦ ◦

ln Kd =

(6)

Determination of the activation energy

The rate constant (k) were determined from Eq. (3) at different temperature and were used to estimate the activation energy of Cr(III) ions onto XAD7–DEHPA The rate constant is expressed as a function of temperature according to the well known Arhenius equation (Ada et al., 2009; Khezami and Capart, 2005; Malkoc and Nuhoglu, 2007; Mustafa et al., 2008; Yu et al., 2009): ln K2 = ln A −

E RT

where K2 is the rate constant value for chromium adsorption, E is the activation energy in kJ/mol, T is temperature in K and R is the gas constant and A is constant called the frequency factor. Value of E can be determined from the slope of ln K2 versus 1/T.

2.7.

Adsorption isotherms in a batch system

Langmuir and Freundlich isotherm studies were undertaken in order to determine the maximum adsorption capacity qm (mg/g) of XAD7–DEHPA toward Cr(III) ions. The model parameters can be construed further, providing understandings on adsorption mechanism, surface properties and an affinity of the adsorbent. The Freundlich equation may be written as: ln qe = ln KF +

1 ln Ce n

(7)

(9)

while the linear form of the Langmuir isotherm may be written as: 1 Ce Ce = + qe KL qm qm

(10)

where Ce corresponds to the residual concentration of Cr(III) ions in the solution at equilibrium (mg/L), qm is a measure of monolayer adsorption capacity (mg/g) and KL is a constant related to the free energy of adsorption, KF and 1/n are characteristic constants that can be related to the relative adsorption capacity of the adsorbent and the intensity of adsorption, respectively (Abdel Raouf and El-Kamash, 2006; Chabani et al., 2007; Davidescu et al., 2011; Deepa et al., 2006; HosseiniBandegharaei et al., 2010; Lazaridis et al., 2005; Mustafa et al., 2008; Saha et al., 2004; Yu et al., 2009).

2.8.

Factorial design study

For any process is important to know the influence of different physicochemical parameters (named also control factors) upon the results of the process. For finding parameters rank of influence was used a statistical software MINITAB.1 This software helped us to design the experiments, for finding the most influents parameters and also to make a first mathematical model of the process (a linear one). Using this linear model was also possible to make a first optimization of the chemical process (Miron et al., 2004).

◦

S H − R RT

(8)

1

MINITAB 15 Statistical Software.

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3.00

2.50

2.50

2.00

2.00

q, mg/g

q, mg/g

1.50

1.00

1.50

1.00 0.50

S:L=0.05:25 S:L=0.1:25 S:L=0.2:25

0.50 0.00 1.5

2

2.5

3

pH

3.5

4

4.5

5

Fig. 1 – Effect of pH on XAD7–DEHPA adsorption capacity. T = 25 ± 1 ◦ C, m = 0.1 g SIR, v = 25 mL, C0 = 10 mg/L Cr(III), t = 2 h. The MINITAB software has given the possibility to have the best results with minimum of the effort (minimize time and financial resources).

3.

Results and discussion

3.1.

Effect of pH on adsorption process

0.00

5.5

The removal of metal ions from aqueous solution by adsorption is highly depending on the pH of the solution which affects the surface charge of the adsorbent and the degree of ionization and speciation of the adsorbate (Belkhouche and Didi, 2010; Chabani et al., 2007; Gundogan et al., 2004; Kula et al., 2008; Kumar and Bandyopadhyay, 2006; Saha et al., 2004). Most research has been conducted on heavy metal sorption indicated that the decrease in sorption at acidic pH may be due to the increase in competition with protons for active sites. At the alkaline pH values, however, other effects may arise from some processes, such as the predominant presence of hydrated species of heavy metals, changes in surface charge and the precipitation of the appropriate salt (Banerjee et al., 2008; Kula et al., 2008; Sarin and Pant, 2006). To verify the effect of pH on Cr(III) ions adsorption onto XAD7–DEHPA, experiments were conducted modifying the initial solutions pH from 1 to 5. The obtained results are shown in Fig. 1. The results show that the pH value is very significant for the performance of XAD7–DEHPA in the process of Cr(III) ions adsorption from solution. The adsorption capacity of the SIR increase sharply at pH range 1–3. At pH > 3 the adsorption capacity increase slowly with pH increasing. The optimum initial pH of the Cr(III) ion solution in the process of Cr(III) ions adsorption onto XAD7–DEHPA is 3, where almost 90% of chromium is removed. The further experiments were conducted at an initial pH of the solutions of 3.

0

20

40

60

80

100

The resin amount is an important parameter to obtain the quantitative uptake of metal ion. Fig. 2 shows the adsorption capacity versus time at three different resin dosages. The adsorption capacity increase rapidly during the first 45 min of stirring with the stirring time increasing at all three S:L ratio studied, then increased slowly until the equilibrium

140

Fig. 2 – Effect of time on XAD7–DEHPA adsorption capacity at three different resin dosages. T = 25 ± 1 ◦ C, C0 = 10 mg/L Cr(III), pH 3. state was reached. It can be noticed that the adsorption capacity degrease with the resin dosage increasing. We consider that the optimum S:L ratio is 0.10:25 when at the equilibrium time of 45 min is achieved an adsorption capacity of 2.14 mg/g, therefore this dosage was selected for the all further experimental studies.

3.3. Effect of temperature on adsorption at different contact time To attain the adsorption equilibrium time the results of the effect of stirring time on the adsorption capacity for all the studied temperatures are presented in Fig. 3. It can be seen that the equilibrium lie in the range of 45–120 min at all of studied temperature. The adsorbent amount of Cr(III) ions slightly increase with temperature increasing from 298 to 318 K. Additionally, it was seen that the equilibrium time was independent from the temperature. For subsequent experiment, an equilibrium time of 90 min was chosen for the sake of convenience.

3.4.

Kinetic studies

The first order and the pseudo-second order kinetic models are employed in this work. The slopes and intercept of the plots of 2.25 2.15 2.05 1.95 1.85 1.75 1.65

298 K 308 K 318 K

1.55

3.2. Effect of resin dosage on adsorption at different contact time

120

t, min

q, mg/g

1

1.45 1.35 1.25 0

20

40

60

80

100

120

140

t, min

Fig. 3 – Effect of time on XAD7–DEHPA adsorption capacity at three different temperature. m = 0.1 g, v = 25 mL, C0 = 10 mg/L Cr(III), pH 3.

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Table 1 – Kinetic parameters for Cr(III) ion sorption onto XAD7–DEHPA. Parameter

qe , exp (mg/g)

Temperature (K)

Pseudo-first-order model qe , calc (mg/g)

298 308 318

2.15 2.18 2.19

k1

0.524 0.375 0.325

(min−1 ) 0.0565 0.0326 0.0362

ln(qe − qt ) versus t for Cr(III) ions adsorption onto XAD7–DEHPA (figure omitted for the sake of brevity), were used to estimate the pseudo-first-order rate constant (k1 ) and the equilibrium adsorption capacity (qe ), respectively for all studied temperatures. A plot of t/qt versus t should yield a straight line. From the intercept and slop (Fig. 4) are calculated the second-order rate constant (k2 ) and the equilibrium adsorption capacity (qe ) for all the studied temperatures. The obtained parameters for both kinetic models are given in Table 1. It was observed that the correlation coefficient for the pseudo-first-order model was much lower than for the pseudo-second-order rate. Furthermore, the calculated equilibrium sorption capacity values for the first order model, at all temperatures, qe,calc , are not close to the experimental values qe,exp , while, the theoretically predicted equilibrium sorption in the case of the pseudo-second-order model is close to that determined experimentally, at all temperatures. This shows that the kinetics of Cr(III) ions removal by XAD7–DEHPA is described by a pseudo-second-order expression instead of a pseudo-first-order. These suggest that the adsorption system studied belong to the second-order kinetic model based on the assumption that the rate determining step may be chemical adsorption or chemisorptions involving valence forces through sharing or exchange of electrons between adsorbent and adsorbate (Hosseini-Bandegharaei et al., 2010; Yu et al., 2009). The rate constant k2 increased with temperature increasing (Table 1) shows that the process is endothermic.

3.5.

Thermodynamic studies

The thermodynamic parameters were determined from the slope and intercept of the plot between ln Kd versus 1/T (figure omitted from the sake of brevity). The values of G◦ , H◦ and S◦ are given in Table 2. The magnitude of G◦ decreased with rising of temperature indicating that the adsorption is favorable at high temperature. The negative value confirm the feasibility of the 60

50

t/qt

40 298 K 308 K 318 K

30

20

Pseudo-second-order model R2

qe , calc (mg/g)

k2 (min−1 (mg/g)−1 )

0.9306 0.9254 0.8586

2.20 2.22 2.22

0.22 0.24 0.27

R2 0.9999 0.9999 0.9999

process and the spontaneous nature of Cr(III) ions adsorption onto XAD7–DEHPA. The value of H◦ was positive, indicating that the adsorption reaction is endothermic. This is also supported by the increase in value of adsorption capacity of the adsorbent with rising of temperature (Abdel Raouf and El-Kamash, 2006; Chabani et al., 2007; Hosseini-Bandegharaei et al., 2010; Kyzas et al., 2009; Mustafa et al., 2008; Yu et al., 2009). The positive value of S◦ show the increasing randomness at the solid/liquid interface during the adsorption of Cr(III) ions on XAD7–DEHPA. Obviously it is shown from the results reported in Table 2 that the temperature affects the adsorption process of chromium ion adsorption onto the XAD7–DEHPA in which the higher the temperature provided more energy to enhance the adsorption rate.

3.6.

Determination of activation energy

Value of E can be determined from the slope of ln K2 versus 1/T (figure omitted from the sake of brevity). If the value of E is between 8 and 16 kJ/mol then the adsorption process follows by chemical ion-exchange and if E < 8 kJ/mol the adsorption process is of physical nature (Ada et al., 2009; Banerjee et al., 2008). The activation energy for the adsorption of Cr(III)ions onto XAD7–DEHPA was calculated and its value was 8.05 kJ/mol. This value is of the same magnitude as the activation energy of activated chemisorption. The positive value of E also suggests that rise in temperature favors the adsorption and adsorption process is an endothermic process in nature.

3.7.

Equilibrium studies

It has been seen that the temperature increasing has a positive effect on Cr(III) ions adsorption onto XAD7–DEHPA, but this influence is not very significant, so from the economical point of view the temperature choose for the equilibrium studies was 298 K. Adsorption isotherms are very powerful tools for the analysis of adsorption process. Adsorption isotherms establish the relationship between the equilibrium pressure or concentration and the amount of adsorbed by the unit mass of adsorbent at a constant temperature. The adsorption isotherm of Cr(III) ions is presented in Fig. 5. The adsorption capacity increased with increasing equilibrium concentration of chromium. Then, it approached

Table 2 – Thermodynamic parameters evaluated for Cr(III) ion sorption on XAD7–DEHPA.

10

Temperature (K)

0 0

20

40

60

80

100

120

t, min

Fig. 4 – Pseudo-second-order kinetic plot.

140

298 308 318

G◦ (kJ/mol) −1.06 −1.11 −1.15

H◦ (kJ/mol)

S◦ (J/mol K)

0.22

4.31

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Table 3 – Parameters of different isotherm for the Cr(III) ions adsorption on XAD7–DEHPA. Freundlich isotherm

Langmuir isotherm

KF (mg/g)

1/n

R2

1.58

0.2483

0.8093

3.50

qm (mg/g)

1.102

3.2

R2 0.9993

The value of RL indicates the shape of the isotherm to be unfavorable, RL > 1; linear, RL = 1; favorable 0 < RL < 1; and irreversible, RL = 0 (Abdel Raouf and El-Kamash, 2006; Chabani et al., 2007; Hosseini-Bandegharaei et al., 2010; Mustafa et al., 2008; Saha et al., 2004; Yu et al., 2009). RL values were found to be between 0 and 1 for all the concentration of Cr(III) ions showing that the adsorption is favorable.

3.00

2.50

qe, mg/g

KL (L/mg)

2.00

1.50

3.8.

1.00

0.50 0.00

5.00

10.00

15.00

20.00

25.00

30.00

Ce, mg/L

Fig. 5 – The adsorption isotherm of Cr(III) ions. 10 9 8 7

Ce/qe

6 5 4 3 2 1 0 0.00

5.00

10.00

15.00

20.00

25.00

30.00

Ce, mg/L

Fig. 6 – Langmuir plot of Cr(III) ions adsorption on XAD7–DEHPA. a constant value at the high equilibrium concentration. The maximum adsorption capacity of chromium determined experimental is 3 mg/g. The linearized Langmuir plot of Cr(III) ions adsorption on XAD7–DEHPA is given in Fig. 6 (the linearized Freundlich plot was omitted from the sake of brevity). The Langmuir and Freundlich constants evaluated from isotherms and their correlation coefficients are presented in Table 3. The Freundlich plot has a correlation coefficient very low; this suggests a restriction on the use of Freundlich isotherms. The numerical value of 1/n < 1, which provides information about surface heterogeneity and surface affinity for the solute, indicates a very high affinity of the XAD7–DEHPA for Cr(III) ions. It is clear that the Langmuir isotherm model provide an excellent fit to the equilibrium adsorption data giving correlation coefficients of 0.9993 and a maximum adsorption capacity close to that determined experimental. The essential feature of the Langmuir equation can be expressed in terms of a dimensionless separation factor, RL defined as: RL =

1 1 + KL C0

(11)

Factorial design study

The targets of the factorial design was to find the parameters of the process which have a significant influence upon the process and to find the setting values of the principal parameters for setting the results of the process on the desired values. All these targets have to be reach with minimum of the resources. That was possible by using the MINITAB software to design a full factorial experiment for 5 parameters (named control factors in MINITAB) of the process. The design of the full factorial for 5 control factors is presented in Table 4. The order of the experiments was randomize, for minimizing the effect of the noises factors. In the same table was inscribed the responses of the process, respectively adsorption capacity and efficiency of the chemical process. The data from Table 4 have been analyzed with MINITAB software and the rank of the control factors influence upon adsorption capacity can be seen in Fig. 7. Fig. 7 shows that significance effect upon adsorption capacity have the control factors: pH, concentration, S:L ratio and time. The same control factors and in the same order has a significant effect upon process efficiency (figure omitted from the sake of brevity). The main effects of the control factors upon adsorption capacity can be seen in Fig. 8a and upon chemical process efficiency in Fig. 8b. In the figure before, can be seen that the parameters pH have a strong positive influence upon both adsorption capacity and process efficiency. The S:L ratio has a strong negative effect upon adsorption and light positive influence upon efficiency. Concentration has a strong positive influence upon adsorption and strong negative effect upon efficiency. Time has a mild positive effect upon both responses and temperature has a very weak positive effect upon responses. These conclusions are in a strong correlation with the results obtained from the adsorption studies. The interactions among different control factors, for adsorption capacity and for efficiency are presented in Fig. 9. The interactions are when the lines, in Fig. 9, are not parallel. A strong deviation from the parallelism put in evidence a strong interaction among control factors. Fig. 9 shows following significance interaction: pH and S:L ratio, pH and concentration, for adsorption capacity, Fig. 9a, respectively pH and concentration for efficiency of the chemical process. That means when will draw conclusion must be very carefully, because the interaction can mask the main effects of the control factors.

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Table 4 – The design of the full factorial for 5 control factors. pH

Time

−1 −1 −1 1 1 1 −1 1 1 1 −1 −1 −1 1 1 −1 −1 1 −1 −1 1 1 −1 −1 1 1 −1 −1 −1 1 1 1

−1 −1 1 −1 1 1 1 1 −1 −1 1 1 1 1 −1 −1 −1 −1 1 −1 −1 1 −1 −1 1 −1 1 −1 1 1 −1 1

S:L ratio

Concentration

−1 1 −1 −1 1 1 1 1 1 1 −1 −1 1 −1 1 −1 1 −1 −1 1 −1 −1 1 −1 −1 1 1 −1 1 1 −1 −1

Temperature

−1 1 −1 −1 1 −1 1 1 1 −1 1 1 1 1 −1 −1 −1 1 −1 −1 −1 −1 1 1 −1 1 −1 1 −1 −1 1 1

Adsorption capacity

−1 −1 1 1 1 1 −1 −1 1 1 −1 1 1 1 −1 1 −1 −1 −1 1 −1 −1 1 −1 1 −1 1 1 −1 −1 1 −1

These screening designs have made a mathematical model, a linear one, for both, adsorption and efficiency. In Table 5 are presented the estimated coefficients for mathematical model in the case of adsorption capacity. In the same way have been computed the coefficients for chemical process efficiency. Using these mathematical models, can be made a lot of graph, that give us the direction in which have to set the control factors for having the desired values of the chemical

0.29 0.16 0.33 1.33 2.25 0.62 0.56 2.05 1.24 0.57 0.86 1.40 0.97 5.54 0.56 0.34 0.11 4.39 0.23 0.13 1.28 2.04 0.20 0.23 2.12 1.14 0.14 0.43 0.13 0.62 4.49 5.33

11.76 3.83 13.58 53.58 45.28 100.00 11.25 41.20 24.98 90.98 4.47 7.24 19.39 28.70 90.34 13.76 17.96 22.72 9.58 20.22 51.58 84.34 4.75 1.20 86.96 22.83 21.94 2.23 21.44 100.00 23.22 27.63

process results. In Fig. 10 is presented one of the contour plots, respectively surface plot for two control factors (pH and concentration), in the case of adsorption capacity. In Fig. 10 can be observed that for high values of adsorption capacity can be set high values for pH and concentration. In the case of chemical process efficiency (figure omitted for the sake of brevity), for having high values for efficiency, is needed to set high values for pH and S:L ratio.

Pareto Chart of the Effects (response is Ads_Cap, Alpha = .05, only 30 largest effects shown)

Term

0.114 A D C AC AD CD B ACD BD AB E ABCD BC DE ABC BE BCD BDE AE CE ADE ABD ABE A BDE ACDE ACE BCDE BCE ABCDE CDE

0.0

F actor A B C D E

0.5

1.0 Effect

Efficiency

1.5

Lenth's PSE = 0.0514529 Fig. 7 – Effect upon adsorbtion capacity.

2.0

N ame pH Time Ratio S /L C oncentration Temp

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chemical engineering research and design 9 0 ( 2 0 1 2 ) 1660–1670

a

Main Effects Plot for Ads_Cap Data Means pH

Time

Ratio S/L

2.0 1.5

Mean

1.0 0.5 -1

1

-1

Concentration

1

-1

1

Temp

2.0 1.5 1.0 0.5 -1

1

-1

1

Main Effects Plotfor Efficiency

b

Data Means pH

Time

Ratio S/L

50 40 30

Mean

20 10 -1

1

-1

Concentration

1

-1

1

Temp

50 40 30 20 10 -1

1

-1

1

Fig. 8 – Main effects for adsorption capacity (a) and for process efficiency (b).

In the same way have been made all the contours and surfaces plots, for all the control factors and for all process responses. So have been seen the direction of setting of the all control factors, for obtaining the desired results. The software MINITAB gives us the possibility to make a first optimization, using this screening design. The goals for this optimization were to maximize both adsorption capacity and process efficiency. The goals of optimization, global solution, the composite desirability and the graph with optimal values, are presented in Fig. 11. This first optimization estimate that chemical process will assure an adsorption capacity about 0.65 in proportion of 35%

and an efficiency of the process about 99% in proportion of 75%. The composite desirability for both responses will be about 51%. For result these values is necessary that the pH, time, temperature and approximately S:L ratio have to be at the high level of the instigated domain, respectively concentration has to be al the low level (see Fig. 11). This screening design model is a linear one. Because in reality the processes are mostly nonlinear, may be these values for process responses cannot be reached. Also because the optimal values for processes results for the limits of the domains of the control factors, we will develop our research making in the future a new experimental design, one nonlinear, using the most important control factors and restraining the domain of

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Fig. 10 – Contour plot and surface plot for adsorption. Fig. 9 – Interactions of the control factors.

Table 5 – Estimated coefficients for adsorption capacity. Term

Coefficient

Constant pH Time S:L ratio Concentration Temp pH × time pH × S:L ratio pH × concentration pH × temp Time × S:L ratio Time × concentration Time × temp S:L ratio × concentration S:L ratio × temp Concentration × temp pH × time × S:L ratio pH × time × concentration pH × time × temp pH × S:L ratio × concentration Ph × S:L Ratio × temp pH × concentration × temp Time × S:L ratio × concentration Time × S:L ratio × temp Time × concentration × temp S:L ratio × concentration × temp pH × time × S:L ratio × concentration pH × time × S:L ratio × temp pH × time × concentration × temp pH × S:L ratio × concentration × temp Time × S:L ratio × concentration × temp pH × time × S:L ratio × concentration × temp

0.199929 0.126767 −0.00454285 −26.5053 −0.0402975 0.00120631 0.00254760 −3.11155 0.0303882 0.000370575 0.646008 0.000147452 1.26111E−05 4.76277 −0.0310357 0.000311429 −0.343641 −3.43545E−05 1.04554E−06 −3.44919 −0.0826864 −5.22355E−05 −0.0277473 −0.00574010 3.86393E−06 −0.0415525 0.0107060 0.000501990 −6.38368E−07 0.0105789 0.000211672 −2.78070E−05

Fig. 11 – Optimization of the chemical process responses.

chemical engineering research and design 9 0 ( 2 0 1 2 ) 1660–1670

the control factors around the optimal values, that have been result from the screening design.

4.

Conclusion

The present investigation shows that XAD7–DEHPA is an effective adsorbent for Cr(III) ions removal. The effects of process parameters such as contact time, pH, temperature and resin dosage on process equilibrium were studied. The optimum pH obtained for Cr(III) ions adsorption onto XAD7–DEHPA is 3. It was also noted that an increase of temperature and resin dosage resulted in higher Cr(III) ions adsorption and the equilibrium was obtained within 45 min. The optimum S:L ratio was chose 0.1:25, and because the temperature increasing had not a significant influence, from the economical point of view, the optimum temperature was chose the 298 K. By applying the kinetic model to the experimental data it was found that the adsorption of Cr(III) ions onto XAD7–DEHPA follows the pseudo-second-order rate kinetics. The negative G◦ values showed that the adsorption was feasible and spontaneous. The positive H◦ depicted endothermic nature of the adsorption. The positive S◦ value revealed the increased randomness at the solid–solution interface. The activation energy of adsorption evaluated with the second-order rate constant according to the Arhenius equation also showed that the adsorption process was endothermic. The value of the activation energy and the fact that the adsorption is best described by the pseudo-second order model indicates the fact that the adsorption mechanism of Cr(III) ions onto XAD7–DEHPA involves besides physical adsorption also chemical reaction or ion-exchange. The linear Langmuir and Freundlich isotherm models were used to represent the experimental data and these could be relatively well interpreted by the Langmuir isotherm. RL values between 0 and 1.0 further indicate a favorable adsorption of Cr(III) ions onto XAD7–DEHPA. The monolayer adsorption capacity of Cr(III) ions calculated from Langmuir model was 3.2 mg/g. Using a screening design of the chemical process, in the first stage of our research, have been define the most significant parameters, that influence chemical process and was possible to make a first optimization of the process. The research will be carry on, making a new experimental design, a nonlinear one, with less control factors (parameters) “vital few”.

Acknowledgment This work was partially supported by the strategic grant POSDRU/89/1.5/S/57649, Project ID 57649 (PERFORM-ERA), cofinanced by the European Social Fund – Investing in People, within the Sectoral Operational Programme Human Resources Development 2007–2013.

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