an investigation of the sorption of acid orange 7 from aqueous solution

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experimental parameters such as contact time, pH value (2-12) and temperature (20 ... One important class of dye widely used in the textile .... fewer and the rate of removal decrease (Mital et al., ... solution from 2 to 12. ... sorbent and sorbate (Vadivelan and Kumar, 2005). ..... Physics and Chemistry of Solids, 68, 818-823.
Environmental Engineering and Management Journal

November/December 2009, Vol. 8, No.6, 1391-1402

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

______________________________________________________________________________________________

AN INVESTIGATION OF THE SORPTION OF ACID ORANGE 7 FROM AQUEOUS SOLUTION ONTO SOIL Camelia Smaranda1∗, Dumitru Bulgariu2, Maria Gavrilescu1 1

“Gheorghe Asachi” Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection, Department of Environmental Engineering and Management, 71 Mangeron Blvd., 700050 Iaşi, Romania 2 “Al.I.Cuza” University of Iaşi, Faculty of Geography and Geology, Department of Geology and Geochemistry, 20A Carol I Blvd., 700506 Iaşi, Romania

Abstract The goal of the study was to investigate the sorption behavior of Acid Orange 7 (C.I. 15510) onto soil collected from Iasi area (Romania). The sorption isotherm, kinetic and thermodynamic studies were performed by batch mode. The effects of experimental parameters such as contact time, pH value (2-12) and temperature (20 - 400C) have been studied. Also the influence of initial dye concentration (10 - 100 mg/L) and sorbent dosage (from 5 g L-1 to 150 g L-1) were studied. It was found that equilibrium sorption amount increases with the increase in initial dye concentration, contact time, temperature and solution acidity. The experimental data were analyzed using four isotherm models, Langmuir, Freundlich, Temkin and Dubinin-Radushkevich. Sorption of Acid Orange 7 onto soil followed the Freundlich model. The pseudo first-order, pseudo second-order and intraparticle diffusion equations were selected to analyzed the sorption kinetic. Kinetic parameters, such as rate constants, equilibrium sorption capacities and correlation coefficients were calculated and discussed for each kinetic equation. It was shown that that sorption of Acid Orange 7 onto soil is well described by the pseudo second-order model. The thermodynamic study indicates that the sorption of Acid Orange 7 onto soil is spontaneous and endothermic.

Key words: Acid Orange 7, isotherm, kinetic models, soil, sorption, thermodynamics 1. Introduction In many industrial activities, such as manufacturing of textile, leather, rubber, plastics, paper, cosmetic, food etc. dyes are used for coloring. One important class of dye widely used in the textile industries is that of acid dyes. They are water soluble anionic dyes, containing one or more acidic groups along with one azo group. There are approximately 3000 azo dyes on the market. Discharging the industrial effluents containing dyes into water resources even in small amounts can contribute to water toxicity and correspond to an increasing risk for human beings and aquatic organisms, since these compounds are known to be toxic, carcinogenic, mutagenic, teratogenic (Abramian and El-Rassy, 2009; Morais et al., 2007; Noroozi et al., 2007).



It was found that about two percent of the manufactured dyes are discharged directly in the environment, especially in water and then can be transferred in soil and sediments. The behavior and persistence of dyes depend on their retention, transformation, transport and degradation in environmental compartments. AO7 is a monoazo acid dye currently used in tanneries, paper and textile industries and its presence in effluents may cause variuos environmental problems (Elizalde-González and HernándezMontoya, 2009). Sorption of this dye has been studied onto various adsorbents such as: spent brewery grains (Pedro Silva et al., 2004), highly porous titania aerogel (Abramian and El-Rassy, 2009), guava seed carbon (Elizalde-González and Hernández-Montoya, 2009), activated carbon (Aber et al., 2007; MetivierPignon et al., 2007; Qu et al., 2009), oxihumolite (Janos et al., 2007), sludge (Hsiu-Mei et al., 2009),

Author to whom all correspondence should be addressed: [email protected]

Smaranda et al./Environmental Engineering and Management Journal 8 (2009), 6, 1391-1402

macro algae (Padmesh et al., 2006), chitosan (Choiu et al., 2004), zeolite (Jin et al., 2008), bottom ash and de-oiled soya (Gupta et al., 2006), hydrotalcite (Géraud et al., 2007), montmorillonite treated with hexadecyltrimethylammonium (Bae et al., 2000), wood sawdust (Izadyar and Rahimi, 2007), but only few studies have been focused on sorption of AO7 on soil (Albanis et al., 2000). The present study investigates the sorption kinetics, equilibrium and thermodynamics of Acid Orange 7 (AO7) from aqueous solutions onto a soil. collected from Iasi area (Romania). The influence of experimental parameters such as: contact time, initial dye concentration, initial pH of aqueous solution, temperature and soil dose has been examined, in terms of sorption equilibrium, kinetics and efficiency.

2.1. Materials

Type / Subtype of soil Soil horizon Structure Anthrogenic modification Pollution pH Eh, mV CEC, mg-eq/100g soil

Protisoil / Entiantrosoil urbic associated with aluviosoil molic-gleic / pelic Apk / Bvk Medium / clay-sandy dusty Intense Weak- medium 5.88 580.13 10.61

Table 2. Chemical-mineralogical components of soil samples (%, w / w)*

2.1.1. Acid Orange 7 Acid Orange 7 is an anionic monoazo dye of acid class, with the molecular formula C16H11O4N2SNa (C.I. 15510). The molecular structure of the dye is shown in Fig. 1. Acid Orange 7 was provided by Laboratory of Organic Synthesis from Technical University of Iasi. The chemical was used in this study as received, without further purification. Dye was weighed and than dissolved in a proper deionized water volume to prepare the stock solution of 1000 mg L-1. HO N N

Fig. 1. Molecular structure of Acid Orange 7

2.1.2. Soil characteristics The soil samples were collected from an area of Iasi city, Romania (Technical University Campus) from the depth of 0-20 cm. The soil was classified as protisoil/entiantrosoil urbic associated with aluviosoil molic-gleic/pelic soil by Romanian System of Soil Taxonomy (SRTS-2003). The collected soil was dried in an oven at 105oC for a period of 3 hours. After drying the soil was milled for 30 minute at 800 rpm in order to obtain ≤ 0.02 mm size fraction. The soil pH value was 5.88, determined by potentiometric method (soil: bidistilled water, 1/5 w/v; size fraction < 0.01 mm; contact time – 30 minutes), using a multimeter Cornning Pinnacle Model 555. The redox potential was determined through direct method with a pair of platinumcalomel electrodes (Bulgariu et al., 2005). The

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Table 1. General characteristics of soil samples

Eh – redox potential (mV). CEC – total cationic exchange capacity (milligram equivalents/100 g soil).

2. Experimental

SO3Na

characteristics and chemical compositions of soil are presented in the Tables 1-3.

Smectite Illite Kaolinite Amorphous Total Total (%, w / w) Carbonates Iron oxides and Crystalline oxyhydroxides (%, Amorphous w/w) Total Crystalline Silica (SiO2) (%, w / w) Amorphous Total Other minerals** (%, w / w) Humus Humic Organic mater acids (%, w / w) Fulvic acids Humin Total Other organic compound# Total Vegetal material undecomposed, % (w / w) Fractions coarse ***, % (w / w) Clay minerals (%, w /w)

15.29 9.87 19.20 3.24 47.62 4.86 1.50 2.04 3.54 6.64 4.42 11.06 6.13 3.19 1.70 0.42 5.33 1.86 7.19 0.73 1.51

*Related to bulk soil sample. **Sulfates, phosphates, hard minerals and relicts (from parental materials). # Organic compounds unbounded to humus. ***Granulometric fraction Φ > 2.00 mm (including mineral fragments and non-altered parental rocks)

2.2. Method The sorption experiments were carried out in batch mode. For equilibrium studies fifty milliliters of Acid Orange 7 solution with known initial concentration, ranging from 10 to 200 mg L−1 were added to an accurately weighted mass of soil and agitated in a thermostatic shaker (IKA KS 4000 IC Control, Germany) at 150 rpm and 30oC. The experiments were carried out for 24 hours to ensure that equilibrium was obtained. At the end of the sorption time, the dye solution was separated from the sorbent by centrifugation at 6000 rpm for 20 minutes (Hettich EBA 20 Centrifuge, Germany).

An investigation of the sorption of Acid Orange 7 from aqueous solution onto soil

Table 3. Elemental chemical composition of soil samples Major elements (%, w / w) 53.95 20.21 2.85 0.06 1.69 4.20 1.26 8.06 0.37 0.22 4.83 97.70 Minor elements (µg /g) 1.13 17.02 78.05 1.37 93.74 48.26 217.85

Cd Cr* Pb As Zn Cu Mn

1,8 1,6 Co=10mg/L Co=50mg/L

The supernatant was filtered through 0.45 µm filter (OlimPeak) and the dye concentration in the residual solution was analyzed spectrophotometrically at 485 nm (CamSpec M 501, UK). The effect of pH on sorption performance was studied by adjusting the pH of dye solutions using dilute H2SO4 or NaOH solutions. pH was measured using a pH meter (Hanna Instruments). The effect of sorbent dosage was studied with different sorbent doses ranging from 1 to 150 g L-1, using two different initial concentrations 20 and 50 mg L-1 of dye. All the experiments were done in triplicate. For the kinetic study, dye solutions with different initial concentration (10 - 100 mg L-1) were agitated with 2.5 g of soil at natural pH of solution (6.8) and 30°C for predetermined intervals of time. The amount of dye sorbed on soil (q in mg/g) and efficiency of dye removed (P) were calculated using the relationships (1, 2):

q=

( Ci − C e ) x100 Ci

( Ci − Ce )V m

Co=100mg/L

1,2

*Total concentration.

P% =

Co=20mg/L

1,4

qt (mg/g)

SiO2 Al2O3 Fe2O3 TiO2 Na2O K2O MgO CaO P2O5 SO3 CO2 ∑

large quantity of dye has been sorbed onto soil after a relatively short contact time, where the uptake of more than 50% of the molecules was noticed within the first 60 minutes of the experiments (Fig. 2). The rate of removal and sorption capacity are higher in the beginning due to a larger number of vacant surface sites are available for the sorption of the Acid Orange 7 during the initial stage. After that, the remaining vacant surface sites are difficult to be occupied due to repulsive forces between dye adsorbed on the soil surface and solution phase. The two stage sorption mechanism with the first rapid slope and quantitatively significant efficiency has been observed.

(1)

(2)

where: Ci and Ce are the initial and equilibrium concentrations of dye (mgL-1), V is the volume of the dye solution (L) and m is the mass of adsorbent (g). 3. Results and discussions 3.1. Effect of contact time on sorption effectiveness The amount of dye sorbed per unit of soil mass (mg g-1) improves with contact time increasing and reached the equilibrium after 240 minutes. A

1,0 0,8 0,6 0,4 0,2 0,0 0

200

400

600

800

1000

1200

1400

1600

Time (min)

Fig. 2. Effect of contact time and initial AO7 concentration on sorption onto soil (sorbent dose 2.5g/50 mL; initial dye concentration in solution: 10 - 100 mg/L; temperature 300C).

3.2. Effect of initial dye concentration on sorption Four different concentrations, namely 10, 20, 50 and 100 mg L-1 were selected to investigate the effect of initial dye concentration (C0) on the sorption of AO7 onto soil. The experiments were performed at 300 C and natural pH of solution. The results obtained are shown in Fig. 2. The maximum sorption capacity increased from 0.132 mg/g to 1.6605 mg/g with the increase of dye concentration from 10 to 100 mgL-1. In addition, the percent of dye removal at equilibrium decreased when the dye concentration increased from 10 to 100 mg/L, probably as a result of the effect of increasing competition for active sites. It is evident that for lower initial concentration, the rate of dye removal is faster, while for higher concentrations the available sorption sites become fewer and the rate of removal decrease (Mital et al., 2006). Comparable results were obtained for sorption of Orange II on titania aerogel (Abramian and ElRassy, 2009). The removal curves are distinct, smooth and continuous indicating monolayer coverage of soil surface by dye (Namasivayam and Arasi, 1997).

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Smaranda et al./Environmental Engineering and Management Journal 8 (2009), 6, 1391-1402

When the amount of dose is 1 g L-1, the removal capacity is about 1 mg g-1, while a removal capacity of 0.09 mg g-1 results at the dose of 150 g L-1 (initial dye concentration 20 mg L-1). Furthermore, the amount of dye sorbed per mass unit is 6.2 mg g-1 (at a dose of 1 g L-1) and 0.2 mg g-1 (at the dose of 150 g L-1) for the initial dye concentration 50 mg L-1. The highest sorption capacity was attained at 50 g L-1 when 83.28 % of dye was sorbed on soil. The further increase of the sorbent dose did not produce any important change in adsorption capacity: for example increasing the dose to 150 g L-1 leads to 88 % of dye removed. In this way, 2.5 g adsorbent dose at 50 mL dye solution was found suitable for all experiments.

3.3. Effect of sorbent dose on AO7 sorption on soil One of the parameters that strongly affect the sorption capacity is the amount of the sorbent. The effect of the sorbent dose used for AO7 sorption was tested in the range from 1 to 150 g L-1. The sorbent dose influence was studied for 20 and 50 mgL-1 dye concentration, at natural pH and 30 0 C, the results are similar for both initial concentration (Fig. 3). 7 6 5

C0=50 mg/L

qe (mg/g)

4

C0=20 mg/L

3.4. Effect of pH on dye sorption capacity

3

The effect of pH on sorption behaviour of the dye on soil was studied by varying the pH of the dye solution from 2 to 12. The initial AO7 concentration was 20 mg L-1 (pH 6.27), the pH of the test solutions was adjusted by using dilute H2SO4 and NaOH solutions. As can be observed in Fig. 5, the maximum sorption capacity was obtained at pH 2 (qe=0.3229 mg g-1), while the sorption capacity was found to decreasing at high pH values, in basic domain.

2 1 0 0

20

40

60

80

100

120

140

160

Sorbent dose (g/L)

Fig. 3. Effects of sorbent dose on the amount of AO7 sorption (initial dye concentration 20 mgL-1 and 50 mgL-1; soil dose 1-150 g L-1)

0,35

0,30

qe (mg/g)

The percent of sorbed dye increases with increasing the amount of the sorbent as shown in Fig. 4. This can be the consequence of a greater availability of the exchangeable sites combined with a larger surface area at higher concentration of the adsorbent. It is understood that the removal capacity (q) increases with increasing sorbent doses, since the amount sorbed per mass unit decreases.

0,25

0,20

0,15

0,10 2

100

4

6

8

10

12

pH

1,0 80 0,8

qe(mg/g)

0,6

Amount of AO7 sorbed Efficiency (%)

40

0,4 20 0,2 0 0,0 0

20

40

60

80

100

120

140

160

Sorbent dose (g/L)

Fig.4. Effects of dose of sorbent on the amount of AO7 sorbed and sorption efficiency (initial dye concentration 20 mg L-1; sorbent dose 1-150 g L-1)

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Efficiency (%)

60

Fig. 5. Effect of pH on sorption of Acid Orange 7 on soil (initial dye concentration = 20 mg L-1; mass of sorbent 2.5 g; solution volume 50mL; temperature 303 K)

In basic medium, it can be observed that the adsorption capacity is very low: for example, at pH 12 the amount of dye sorbed on soil was 0.1404 mg per gram of soil. Low uptakes of the dyes at high pH would be the result of increased electrostatic repulsion between the functional groups of the AO7 (negatively charged) and the soil surface (Banerjee et al., 2006). Generally, the sorption process efficiency decreases with increasing pH for anionic dyes, while it increases with increasing pH for cationic dyes (Caliman and Gavrilescu, 2008; Wu et al., 2005).

An investigation of the sorption of Acid Orange 7 from aqueous solution onto soil

Similar results were reported by Abrahmian and ElRassy (2009) for adsorption of Acid Orange 7 on titania aerogel, by Lian et al. (2009) for adsorption of Congo red on bentonite, as well as by Wu et al., (2005) for adsorption of brilliant blue on mesoporous xerogel. 3.5. Effect of temperature on dye sorption on soil The effect of temperature on sorption capacity of soil was investigated at 293, 303, and 313 K, and the results are shown in Fig. 6. It can be observed a slightly increase of the amount of AO7 sorbed by soil with an increase of temperature from 293 K to 313 K, when the amount of dye sorbed per unit of sorbent increases from 0.251 mg/g to 0.275 mg/g (initial dye concentration 20 mg L-1). This indicating that increase of temperature may increase the mobility of the large dye molecules. 0,30

0,25

It is clear that the sorption of AO7 on the soil is an endothermic process and AO7 dye sorption may involve not only physical but also chemical sorption. 3.6. Sorption equilibrium studies Equilibrium data which result from sorption isotherms are basic requirements for the design of adsorption systems. Sorption equilibrium provides fundamental information about relationship between sorbent and sorbate (Vadivelan and Kumar, 2005). The equilibrium experimental data for sorption of AO7 onto soil were analyzed using the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models. 3.6.1. Langmuir isotherm Langmuir type isotherm is the most widely used isotherm for describe the sorption of pollutants from liquid phase. The Langmuir adsorption isotherm is based on the assumption that all sites possess equal affinity for the adsorbate. It may be represented in the linear form as follows (Eq. 3): Ce C 1 = + e qe Qm K L Qm

0,15

293K 303K 313K

0,10

0,05

0,00 0

100

200

300

400

500

Time (min)

Fig. 6. Effect of temperature on sorption of Acid Orange 7 on soil (initial dye concentration = 20 mg L-1; mass of sorbent 2.5 g; temperature 293, 303 and 313 K)

Also, the temperature may produce a swelling effect within the internal structure of sorbent, and this facilitates the entry of large dye molecules in soil (Acemioglu, 2004). Although this is an uncommon behaviour if sorption theory is considered, the enhancement in adsorption with temperature may be attributed to (Aksu and Tezer, 2000; Aksu et al., 2008): - increase in the number of active surface sites available for sorption, increase in the porosity and in the total pore volume of the sorbent; - the decrease in the thickness of the boundary layer surrounding the sorbent with temperature, so that the mass transfer resistance of adsorbate in the boundary layer decreases; - an increase in the mobility of the dye molecule with an increase in their kinetic energy; - enhanced rate of intraparticle diffusion of sorbate with the rise of temperature.

(3)

where Qm is the maximum AO7 uptake (mg g-1), KL the Langmuir adsorption constant, (L mg-1). The monolayer coverage is obtained from a plot of Ce/qe versus Ce. The slope and the intercept of the linear graph obtained from this plot give the values of Q0 and KL. The linear representations of Langmuir isotherm for AO7 sorption on Iasi soil, obtained at temperature 293, 303 and 313 K, are presented in Fig. 7. The calculated isotherms parameters for each temperature are summarized in Table 4. 3,0

2,5

2,0

Ce/qe (g/L)

qt (mg/g)

0,20

1,5

1,0

293 K 303 K 313 K

0,5

0,0 -2

0

2

4

6

8

10

12

14

16

18

20

22

Ce (mg/L)

Fig. 7. Langmuir isotherms of AO7 sorption on soil at different temperatures Table 4. Langmuir isotherm constants Tempe rature (K)

qe(exp) (mg g-1)

293 303 313

1.4753 2.0498 2.2754

Qm (mg g-1)

1.7310 3.4674 3.2786

Langmuir parameters KL RL (L mg-1)

0.00314 0.0130 0.0232

0.1272 0.1947 0.1651

R2

0.9718 0.9783 0.9200

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The values of KL are increasing with increase in temperature. The monolayer saturation capacity, Qm, were found to have values - 1.73, 3.46 and 3.27 mg g-1 for 293, 303 and 313 K, respectively. Generally, at all studied temperatures the calculated values of Qm are not very close to the experimental values, but the correlation coefficients values are relatively high, as can be seen in Table 4. The favorable nature of sorption can be expressed in terms of dimensionless constant called separation factor or equilibrium parameter (RL), which is defined by following equation:

1 1 + K L C0

0,4

0,2

0,0

(4)

where KL is the Langmuir constant and C0 is the initial concentration of dye in solution (mg L-1). The values of RL indicates the nature of adsorption isotherm; irreversible (RL= 0), favorable (0 < RL < 1), linear (RL = 1) or unfavorable (RL > 1) (Donia et al., 2009; Vadivelan and Kumar, 2005). The dimensionless separation factors calculated for sorption of AO7 on soil are found in the range zero and one, this indicating that the sorption of dye from aqueous solutions is favorable under the studied conditions.

0,0

log qe

RL =

above unit, which means the sorption process is favorable and is physical. The Freundlich isotherm assumes that there is a continuously varying energy of sorption as the most actively energetic sites are occupied first and the surface is continually occupied until the lowest energy sites are filled at the end of the process.

0,5

1,0

-0,2

1,5

2,0

293 K 303K 313 K

-0,4

-0,6

-0,8

log Ce

Fig. 8. Freundlich isotherm of AO7 sorption on soil at different temperatures Table 5. Freundlich isotherm parameters

3.6.2. Freundlich isotherm The Freundlich adsorption model was also applied for the adsorption of AO7 on soil. The Freundlich equation can be written as (Eq. 5): qe = K F Ce1 / n

(5)

where KF is a constant indicative of the relative adsorption capacity of the adsorbent (mg1−1/n) L1/n g−1) and n is a constant indicative of the intensity of the adsorption. The Freundlich expression is an exponential equation and therefore assumes that as the adsorbate concentration increases, the concentration of adsorbate on the adsorbent surface also increases. The linearized form of the Freundlich isotherm is shown in Eq. 6: 1 log qe = log K F + log Ce n

(6)

where qe is the amount of dye adsorbed at equilibrium time (mg g-1), Ce is the concentration of dye solution at sorption equilibrium (mg L-1). The equilibrium data were further analyzed using the linearized form of Freundlich equation using the same set experimental data, by plotting ln qe versus ln Ce (Fig. 8). The calculated Freundlich isotherm constants and the corresponding coefficient of correlation values were shown in Table 5. The correlation coefficients are relatively high (≥0.9606) showing a good linearity. The process is favorable because the values of n for all three studied temperatures are

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Temperature (K) 293 303 313

Freundlich parameters KF R2 1−1/n) 1/n −1 (mg L g ) 1.7352 0.0996 0.9606 1.4451 0.08342 0.9737 1.5371 0.1303 0.9914

n

3.6.3. Temkin isotherm The Temkin isotherm takes into accounts the effects of interaction of sorbate and sorbed species. The heat of sorption of all the molecules on the sorbent surface layer would decrease linearly with coverage due to sorbate–sorbate interactions (Oladoja et al., 2008). The linear form of the Temkin isotherm can be expressed as (Eq. 7): or

qe = B ln KT + B ln Ce

(7)

RT ln( K T Ce ) bT

(8)

qe =

The parameter B can be calculated using Eq. (9):

B=

RT bT

(9)

where KT (L mg-1) is the equilibrium binding constant, corresponding to the maximum binding energy, bT (J mol-1) is a constant related to the heat of sorption, R is the gas constant (8.314 J mol-1) and T is the absolute temperature (K). A plot of qe vs ln Ce is

An investigation of the sorption of Acid Orange 7 from aqueous solution onto soil

used to determine the isotherm constants bT and KT from the slope and intercept (Han et al., 2005). The values of the Temkin constants and correlation coefficients are presented in Table 6 and the plot of this isotherm is shown in Fig. 9. The correlation coefficient values did not show a good agreement with the experimental data.

E=

1 ( −2 BD )1 / 2

(12)

Dubinin-Radushkevich isotherm model is applied to the sorption data in the following liniarized form (Eq. 13):

ln q e = ln q D − BD ε 2

2,4

(13)

2,2

293 K 303 K 313 K

2,0 1,8 1,6

qe

1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 0

1

2

3

4

5

ln Ce

Fig. 9. Temkin sorption isotherm for Acid Orange 7 onto soil at different temperatures Table 6. Temkin isotherm parameters for AO7 sorption onto soil Temperature (K) 293 303 313

Temkin parameters bT (kJ mol-1) KT (L mg-1) 0.1755 7.4481 0.0563 4.8119 0.2595 5.2073

R2 0.9224 0.8766 0.8527

3.6.4. Dubinin-Radushkevich isotherm Dubinin-Radushkevich isotherm is another form of equilibrium correlation data, applicable to distinguish between chemical and physical types of sorption (Adamu, 2008). The Dubinin – Radushkevich model was chosen to estimate the characteristic porosity of the soil and the apparent free energy of sorption (Abdelwahab, 2007). The model is represented by Eq. (10):

qe = q D exp( − BD ε 2 )

(10)

where qD is the maximum sorption capacity (mg g-1), BD is the Dubinin - Radushkevich coefficient, ε is the Polanyi potential and can be correlated to the eq. (11):

ε = RT ln( 1 +

1 ) Ce

(11)

where R is the gas constant ant T is the temperature. The constant BD gives the mean free energy E (kJ g-1) of sorption per molecule of the sorbates when it is transferred to the surface of solid from infinity in the solution and can be calculated with the following relationship (Krishna Prasad and Srivastava, 2009):

The plots of ln qe against ε2 did not indicate a good fit of the isotherm to the experimental data (plot not shown). The isotherm parameters qD and BD, as well as the apparent energy (E) values of sorption are shown in Table 7. The higher the values of qD the higher sorption capacity. The values of qD from Table 7 are higher for the temperature 313 K, comparative to the other values of temperatures, showing that the studied soil exhibited higher sorption capacity at quite higher temperature. This result confirms the hypothesis from the paragraph 3.5, where the effect of temperature on dye sorption on soil is discussed. The porosity factors, BD for the soil toward the dye were 2.48, 5.13 and 9.34. The porosity factors were found to be high than unity, indicating that sorption of AO7 by soil may not be significant probably due to its large molecule size. The apparent free energies from the Dubinin – Radushkevich model for the sorption process are 0.44 kJ g–1 (293 K), 0.31 kJ g–1 (303 K) and 0.23 kJ g–1 (313K). The sorption energy values are less than 8 kJ mol–1 which indicate that sorption of AO7 on soil is physisorption. This results are in agreement with the data reported in the literature (Adamu, 2008; Igwe and Abia, 2007; Mittal et al., 2009). Table 7. Dubinin-Radushkevich isotherm parameters Temperature (K) 293 303 313

Dubinin-Radushkevich parameters qD BD E R2 -1 -1 –1 (g mg ) (mg g ) (kJ mol ) 0.8032 2.4853 0.4485 0.8621 1.1367 5.1336 0.3120 0.8293 1.0702 9.3408 0.2313 0.7925

By comparing the correlation coefficients R2 calculated to Langmuir, Freundlich, Temkin and Dubinin-Radushchevich isotherms can be deduced that the experimental equilibrium sorption data are well described by the Freundlich model, when the correlation coefficients were 0.9606, 0.9737 and 0.9914 at 20, 30 and 400C, respectively. Also, the Langmuir isotherm fits the experimental data relatively well. 3.7. Sorption kinetic studies The kinetic data can be processed to understand the dynamics of the sorption process in terms of the rate constant. Characteristics adsorption rate constants obtained form pseudo first order,

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pseudo second order and intraparticle diffusion models were used to investigate the mechanism of dye sorption on soil. Determination of best-fit kinetic model is the most common way to predict the optimum sorption kinetic expression. 3.7.1.Pseudo-first-order equation The pseudo-first-order equation of Lagergren is given by Eq. 14:

dqt = k1 ( qe − qt ) dt

(14)

(15)

(17)

where k2 (g mg-1 min-1) is pseudo second-order rate constant and h is the initial sorption rate. The plots of the linearized form of the pseudosecond-order kinetic model for the sorption of Acid Orange 7 on soil are shown in Fig. 11. The rate constants (k2), correlation coefficients of the plots together with the theoretical qe values are given in Table 8. 2000

1500

t/qt(min g/mg)

k1 t 2.303

A plot of log (qe-qt) against t was performed and values of k1 and qe were obtained from the slope and intercept, respectively. The plot of log (qe-qt) versus t gives a straight line for first order sorption kinetics and is presented in Fig. 10. Experimental and theoretically calculated qe values and the correlation coefficient of the plot (R2) are given in Table 8.

1000

T=293 K, Co=20mg/L T=303 K, Co=20mg/L

500

-0,4 -0,6

T=293 K, Co=20mg/L

-0,8

T=303 K, Co=20mg/L T=313 K, Co=20mg/L

-1,0

T=313 K, Co=20mg/L

0 0

100

200

300

400

500

t (min)

-1,2

log(qe-qt)

where k2 (g mg-1 min-1) is the rate constant of pseudosecond order adsorption, qe and qt (mg g-1) represent the amounts of dye adsorbed on soil at equilibrium and at any time t. The pseudo second-order rate constants can be determined by plotting t/q versus t, and k2 and qe can be calculated from the slope and intercept. The initial sorption rate, h (mg g−1 min−1), at t=0 is defined as (Maximova and Koumanova, 2007) (Eq. 17):

h = k2 qe2

where qt and qe are the amounts of AO7 sorbet at time t and equilibrium (mg g-1), and k1 is the pseudo firstorder rate constant for the sorption process (min-1). After integration and applying boundary conditions t = 0 to t = t and qt = 0 to qt = t, the integrated form of Eq. (14) results (Eq. 15):

log (qe − qt ) = log qe −

(16)

t 1 t = + qt k 2 qe2 qe

-1,4

Fig. 11. Plots of pseudo-second-order kinetic model of AO7 sorption onto soil

-1,6 -1,8 -2,0 -2,2 -2,4 0

100

200

300

400

500

t (min)

Fig. 10. Plots of pseudo first order kinetic of Acid Orange 7 sorption onto soil

The correlation coefficients for the model were reasonably high in some cases (at 293 and 303 K), however, all the intercepts of the straight line plots did not yield predicted qe values equal or reasonably close to the experimental qe values. From these may by concluded that the sorption of Acid Orange 7 on studied soil is not a first order reaction.

3.7.2. Pseudo-second-order kinetics The pseudo-second-order equation proposed by Ho and Mackay (1999) is expressed in the following form (Eq. 16): 1398

The correlation coefficients of all examined data were found very high (R2> 0.99) and experimental (qe,exp) and theoretical (qe,cal) values are approximately equal. These results confirming that the sorption of AO7 follows a pseudo second-order kinetic reaction at different temperature used in this study. It can be observed that an increase of the temperature leads to an increase in the sorption rate of the AO7 on soil from 0.2578 mg g-1 at 293 K to 0.2732 mg g-1 at 313 K.

3.7.3 Intra-particle diffusion model The adsorption mechanism of the dye consists of the following sequence of steps: transport of the dye molecules from the boundary film to the external surface of the sorbent (film diffusion), transport of the dye molecules from the surface of sorbent to intraparticle site and uptake of the dye molecules by the active sites of sorbent.

An investigation of the sorption of Acid Orange 7 from aqueous solution onto soil

Table 8. Sorption kinetic parameters for sorption of Acid Orange 7 on soil Temperature (K)

qe, exp (mg g-1) 0.2513 0.2540

Pseudo first-order kinetic equation k1 qe, calc R2 -1 -1 (min ) (mg g ) 0.0059 5.0745 0.9779 0.0069 5.405 0.9802

Pseudo second-order kinetic equation k2 h qe, calc R2 -1 -1 -1 (g mg min ) (mg g ) (mg g−1 min−1) 0.1481 0.2578 0.9952 0.0984 0.1948 0.2592 0.9952 0.0130

293 303 313

0.2758

0.0043

0.2336

6.7375

0.9021

The intra-particle diffusion model has been applied to identify diffusion mechanisms of dye sorption on soil. The intra-particle diffusion model is described by the following relationship (Eq. 18):

qt = kid t 1/ 2 + C

(18)

where: qt is the amount of dye sorbed on soil at various times t, kid is the intraparticle diffusion rate constant (mg g-1 min1/2) and C is the intercept (mg g-1). The values of intercept (C) give information about the thickness of the boundary layer, and for larger value of C, the boundary layer effect is greater. (Ayoob et al., 2008; Kannan and Meenakshisundaram, 2002). According to this model, the plot of qt versus the square root of time, t1/2 should be linear if intraparticle diffusion is involved in the adsorption process and if these lines pass through the origin, then intraparticle diffusion is the rate controlling step (Lian et al., 2009). If the line does not pass trough the origin, the process is very complex with more than one mechanism limiting the rate of sorption (Ayoob et al., 2008). Usually, the plot of qt versus t1/2 shows a multi-linearity that can be explained by a two or more steps process (Lorenc-Grabowska and Gryglewicz, 2007). The first stage is associated with the external mass transfer or instantaneous adsorption stage. The second stage illustrates the gradual adsorption stage, when the intraparticle diffusion is the rate controlling of the phenomena. The third stage is the final equilibrium one, where the intraparticle diffusion starts to slow down due to the low adsorbate concentration left in the solution (Noroozi et al., 2007). The relationship between qt and t1/2 for different temperature is presented in Fig. 12. Initially for all studied cases a linear relationship between q versus t1/2 with a zero intercept was found, indicating that the internal diffusion step dominates the sorption process before the equilibrium is reached. As can be observed in Fig. 12, the plots are not linear to the whole time range, resulting that more than one process affects the AO7 sorption onto soil. The sorption process tends to be followed by three phases. The initial curve portion between t = 2 and 10 minutes could be due to intraparticle diffusion effects. The intraparticle diffusion model constants were calculated. The correlation coefficients (R2) for the fist stage of the intraparticle diffusion model

0.2732

0.9927

0.0174

present values ranging from 0.9405 to 0.9934 which are lower than the pseudo-second-order model, but it indicates that adsorption of AO7 on soil may be followed by an intraparticle diffusion model. Similar results were reported by Wu et al., (2005) for adsorption of brilliant blue on mesoporous hybrid xerogel.

Fig. 12. Intraparticle diffusion model for adsorption of Acid Orange 7 on soil at different temperature

3.8. Thermodynamic study For better understanding the effect of temperature on the sorption, it is important to study the thermodynamic parameters such as standard Gibbs free energy change ∆G0, standard enthalpy ∆H0, and standard entropy ∆S0. The Gibbs free energy change represents the fundamental criteria of spontaneity (Han et al., 2005). The Gibbs free energy of sorption by using equilibrium constant is calculated from Eq. (19):

∆ G0 = −RT ln Kc

(19)

where Kc is Langmuir equilibrium constant (mol L-1) (Liu, 2006; Pedro Silva et al., 2004). Standard enthalpy, ∆H0 and standard entropy, 0 ∆S for sorption can be estimated from van’t Hoff equation given as relation (20): ln K c =

− ∆H 0 ∆S 0 + RT R

(20)

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Smaranda et al./Environmental Engineering and Management Journal 8 (2009), 6, 1391-1402

where Kc is the sorption equilibrium constant, R is the universal gas constant (8.314 J mol-1 K), T is the absolute temperature (K). The heat of sorption was calculated by plotting ln Kc versus 1/T, and ∆H0 and ∆S0 were calculated from slope and intercept. The Gibbs free energy change (∆G0) is related to the enthalpy and entropy change by the following equation (Wu et al., 2005):

∆ G0= ∆H0 – T ∆S0

(21)

The van’t Hoff plot for the sorption of AO7 onto soil is given in Fig. 13. The values of the thermodynamic parameters obtained by above mentioned equations are depicted in Table 9. 9,5

9,0

lnK

8,5

8,0

7,5

7,0

3,15

3,20

3,25

3,30 3

3,35

3,40

3,45

-1

1/Tx10 (K )

Fig. 13. Van’t Hoff plot for AO7 sorption onto soil

The negative values of free energy change ∆G0 for all studied values of temperature indicate the spontaneous nature of sorption of Acid Orange 7 onto soil. The positive value of ∆H0 suggests the endothermic nature of process. The endothermic nature is also indicated by sorption intensification with temperature, which is illustrated in Fig. 6. The enthalpy change agrees well with those reported in literature by Padmesh et al., (2006). Table 9. Thermodynamic parameters of the sorption of AO7 onto Iasi area soil Temperature (K) 293 303 313

∆G0 (kJ mol-1) -17.02 -21.22 -23.42

T∆S0 (kJ mol-1) 93.76 96.96 100.16

∆H0 (kJ mol-1)

∆S0 (J mol-1K-1)

77.06

322.16

Positive value of ∆S0 is due to increase randomness at the solid/solution interface during the sorption of the dye onto soil. The behavior similar to this was also observed by Pedro Silva et al., (2004) for the sorption of AO7 on spent brewery grains, Donia et al., (2009) for the sorption of Acid Orange 10 and Acid Orange 12 on amine modified silica, by Acemioglu (2004) for the

1400

sorption of Congo red onto fly ash, and by Wu et al., (2005) for the sorption on brilliant blue onto mesoporous xerogel. 4. Conclusions Sorption of Acid Orange 7 is dependent on initial concentration and contact time. The percent of dye sorbed on soil increased with decrease in pH values, the maximum capacity of sorption being attained at the temperature of 30°C and at pH 2. Also, the increase of temperature from 293 to 313 K reveals a very slightly increase of sorption capacity, indicating that the temperature did not have a major influence on sorption of AO7 onto soil from Iasi region, but is a proof that a complex physicochemical process occurs. A detailed analysis of experimental data was carried out to determine the best isotherm models for the sets of equilibrium data for AO7 on Iasi area soil. The experimental results were analyzed using four two-parameter adsorption isotherm models - the Langmuir, Freundlich, Temkin and DubininRadushkevich isotherms. The analytical data showed that the Freundlich isotherm offers the best fit of the data, and also the Langmuir isotherm described the data more appropriate than Temkin and DubininRadushkevich isotherms. The value of mean sorption energy (less than 8 kJ mol-1) indicates that sorption of dye on the investigated soil occurs through physisorption. The values of RL < 1 indicate the applicability of Langmuir sorption isotherm. Study of temperature based on Freundlich and DubininRadushkevich parameters reveals increasing trend in sorption capacity with increase in temperature. Based on the regression coefficient values, the sorption of AO7 onto soil can be well described by the pseudo second-order kinetic model. Initial sorption rate increases with the increase the temperature. The negative values of the Gibbs free energy (∆G0) indicate the spontaneity of the sorption of AO7 on soil. The positive value of enthalpy (∆H0) suggests the endothermic nature of process. Positive value of ∆S0 is due to increase randomness at the solid/solution interface during the sorption of the dye onto soil. Acknowledgement This research was financially supported by the Ministry of Education and Research of Romania, in the frame of the National Program of Research, Development and Innovation, PNCDI - II, Program IDEI, Project ID_595, Contract 132/2007.

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An investigation of the sorption of Acid Orange 7 from aqueous solution onto soil

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