Equilibrium Study of Heavy Metals Adsorption on Kaolin - Industrial

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Equilibrium study of heavy metals adsorption on kaolin Magdalena Mat#ok, Roman Petrus, and Jolanta Karolina Warcho# Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b00880 • Publication Date (Web): 19 Jun 2015 Downloaded from http://pubs.acs.org on June 29, 2015

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Industrial & Engineering Chemistry Research

Equilibrium study of heavy metals adsorption on kaolin Magdalena Matłok1, Roman Petrus1 and Jolanta K. Warchoł2* 1

Rzeszów University of Technology, Department of Chemical and Process Engineering, 6 Powstańców Warszawy Str., 35-959 Rzeszów, Poland

2

Rzeszów University of Technology, Department of Water Purification and Protection, 6 Powstańców Warszawy Str., 35-959 Rzeszów, Poland

KEYWORDS: : kaolin, heavy metal adsorption, modeling of multicomponent equilibrium

ABSTRACT

The adsorption of Cu(II), Cd(II), and Zn(II) from one- and two-component systems, on raw and Na-form of kaolin was investigated under batch conditions. The transformation of the raw kaolin into its Na-form causes surface-to-surface arrangement of crystals, which leaded to quantitatively similar but certainly different metal ions isotherm curves. Subsequently, the identified homogenous Langmuirian adsorption on raw kaolin changed after sodium saturation into heterogonous one. The Langmuir constants obtained from one-component systems were used to test the applicability of the new isotherm equation for modeling of two-component equilibrium on raw kaolin. This model yielded quantification of the relative amounts of binding sites on the

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kaolin, showing partial specificity of the adsorbent to the individual metal ions. The validity of the model was confirmed by the alignment between the modeled equilibrium surface and 225 experimental points obtained for two different experimental sets.

INTRODUCTION Faced with more and more stringent regulations, water and wastewater pollution with heavy metals is a major concern in recent years. Among the main contamination sources are metal plating, petrochemical industries, painting, mining operations, tanneries, car radiator manufacturing, textile mill products, as well as fertilizers and fungicidal sprays.1, 2 Because of persistent and non-biodegradable nature of heavy metals, they have become a public health issue. Accumulation of heavy metals in living organisms, followed by bioamplification in the food chain, considerably increases their toxicity risk.3 Based on potential pollution impact on the environment, as well as on the industrial applications, among the metals of major environmental concern today are cadmium, zinc and copper.2 They commonly occur at raised concentrations in contaminated waters and soils.4 Adsorption of heavy metals on clay minerals is an important chemical process controlling their migration and bioavailability in soil and aquifer.5 Additionally, adsorptive properties of clays make them valuable as cost-effective material that can find practical application in treating metal-loaded sludge, soil remediation, and building a permeable barrier to slow or prevent the migration of toxic ions into groundwater.6,7,8 Many clay minerals have been studied for wastewater treatment, e.g. kaolinite 3, 9, montmorillonite 10, 11, bentonite 12, 13 and illite 14, 15. Kaolinite is a common phyllosilicate mineral, formed through the decomposition of feldspars by geological processes. It is a 1:1 sheet aluminosilicate composed of a (Si2O5)2-


2

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tetrahedral layer and an (Al2[OH]4)2+ octahedral layer bonded together by the sharing of oxygen atoms between the silicon and aluminum atoms in adjacent sheets. The layers are held together in the crystal by hydrogen bonding, thus ion migration in the interlayer region cannot occur.16 However, it is accepted that isomorphous substitution of Si4+ by Al3+ in surface tetrahedral sheets gives small negative charge of siloxane faces responsible for ion exchange. These are so called permanent active sites (P) with a pH independent charge. In contrary, the alumina faces and the crystal edges have a pH-depended (variable) active sites (V) caused by protonation and deprotonation of the silanol and aluminol hydroxyl groups.17 The aluminol sites are positively charged up to pH 9 and then (pH>9) turn into negatively charged, while the silanol sites contribute to the negative surface charge under neutral and basic pH conditions (at pH Cd (7.32 mmol/kg), while for Na-form the order changed to: Zn (8.60 mmol/kg) > Cu (8.10 mmol/kg) ≈ Cd (8.00 mmol/kg). The slight decrease in metal adsorption on NaCl saturated kaolin was observed only in the case of copper. Values of pHeq for concentrated copper solutions were too low (pHeq < 5.1) to ensure the presence of the various Cu hydroxyl species, which concentration increases only above pH ∼ 6.0.4 Furthermore, SEM/EDS elemental mapping of the copper loaded kaolin samples (Fig. 4) did not show any difference in distribution of copper on raw (Fig. 4B1) and on Na-form of kaolin external surface (Fig. 4B2). This may suggest preferential adsorption of Na and Cu on the same kind of active sites. Alkali cations can act as competitive ions and cause screening of surface negative charges leading to a drop in the adsorption of metal ions.40, 41 According to Sparks et al., Na+ competes more effectively for permanent negatively charged sites (Si-O) on the kaolinite faces than on the aluminol sites (Al-OH).42 Thus, the decrease in adsorption capacity observed may be caused by reduction of Cu2+ sorption on the permanent sites partially occupied by Na+. Data obtained for adsorption of Cu(II), Cd(II), Zn(II) were fitted to the Langmuir and the BiLangmuir equations. Representative models fitting for Zn(II) sorption on raw and Na-kaolin are


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presented in Fig. 5A and Fig. 5B, respectively. The isotherm parameters for each metal ion and both kaolin forms are listed in Table 3. NaCl activation has limited influence on Langmuir monolayer capacity for considered heavy metals ions adsorption on kaolin. This suggest that treatment with NaCl does not significantly increase the number of adsorption sites although it influences the strength of existing sites as evidenced by the equilibrium coefficient KL. Comparison between the raw kaolin and its Na-form revealed a big difference in the quality of approximation. Both models were unable to give a good fit to the Na-form equilibrium data, as evidenced by the values of statistical indicators. This confirms a heterogeneous adsorption of metal ions on NaCl pretreated kaolin that stems from the change of kaolin surface structure. On the contrary, both models appear to provide reasonable fit to the experimental data for raw kaolin. In line with statistical indicators (higher FT and lower ME and σ values), the isotherm curves predicted by the Bi-Langmuir model better coincide with the experimental points. It may result from the fact that this model has a greater number of constants, which allows to simulate the model variations more accurately.43 On the other hand, the Bi-Langmuir model generally overestimated the maximum adsorption capacity (qm,exp < qmBiL,1 + qmBiL,2, see Tab. 3). This value is the only equilibrium parameter that can be experimentally verified, and therefore acts as the most important criterion of equilibrium model fitting. Better approximation of qm,exp for all respective systems was obtained for the Langmuir model. The applicability of the Langmuir equation for modeling of heavy metals adsorption on raw kaolin was confirmed by other authors, who claimed that the adsorption of transition metals on permanent charge sites predominate up to pH 5.7 for Cu, pH 7.3 for Zn and 8.0 for Cd (compare with the pHeq depicted in section 2.3).4, 44, 45

Only above those pH values adsorption can occur on two different active sites which behave

independently and consequently meets the assumptions of the Bi-Langmuir model.21, 46 In related


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literature the fit of the Freundlich and the Langmuir models as well as hybrid models combining both of them (e.g. Lagmuir-Freudlich, Temkin, Toth, Redlich-Peterson) were assessed.47,

48

While the use of mentioned hybrid models would presumably yield a better fit of the salt (NaCl) specimen data, it would defeat our purpose, which was to show that the NaCl pretreatment influences sorption properties of kaolin towards metal ions, not to optimize data fitting.

Two-component adsorption The simultaneous adsorption of Cd(II), Cu(II), and Zn(II) ions on raw kaolin were conducted to check the interactive effect of concentration (C0) of given metal ions (M1) with respect to each other (M2). Representative adsorption data obtained for Cu-Cd system are presented in Fig. 6 in two different forms: 3D and 2D. On three-dimensional adsorption isotherm surfaces an equilibrium concentration of Cd2+ in solid phase (z-axis) is plotted as a function of the equilibrium concentrations of Cd2+ and Cu2+ in liquid phase (x- and y-axis). As can be seen in the 3-D graphs, there is a marked difference between the shapes of the two isotherm surfaces obtained for the experimental sets A and B, as illustrated in Fig. 6A and 6B, respectively. To make a meaningful comparison between both experimental sets, the obtained competitive isotherms qe,i=f(Ce,i) were presented on two 2-D graphs showing distribution of both competitive ions at equilibrium. On the 2-D graph of set A, the increase in the Cd2+ concentration (upward slope) causes a decrease in the uptake of Cu2+ (downward slope). Such evident inhibition effect is not visible on the 2-D graph representing experimental set B, where two distinct isotherms simultaneously arise and become parallel at plateau range. Their shape and location on the equilibrium graph depend on the molar ratio of competitive ions and their affinity for the surface. To analyze the competitive behaviour of the considered metal ions, the maximum uptakes of the one- and two-component systems (related to experimental set B) were compared (Fig.7). The


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given metal ion uptake is lowered by the presence of other metal ion existing in the system, probably due to displacement effect.4 The analysis of the maximum metal uptake obtained for all experimental sets representing competitive metals ratio equal 1:1 (Fig. 7), indicates that efficiency of Cu adsorption is higher than Cd (Cd-Cu set), Zn adsorption is higher than Cd (ZnCd set), and Cu adsorption is higher than Zn (Zn-Cu set). Summarizing, the isotherm curves obtained for the double-element systems show an overall Cu > Zn > Cd affinity sequence, which is in line with the data reported by other authors investigating multicomponent adsorption on kaolin.4,49 The adsorption isotherm surfaces of each two-component system (Fig. 8) were created by using the new model given by eqs 4-6, smoothed and fitted to the experimental adsorption data. Note that a given double-component system is represented on two 3-D diagrams. As can be seen, the predicted and experimental equilibrium data are quite consistent over a wide range of both ions concentrations. The scattering of experimental points is limited to isotherm range representing the high difference between the concentrations of the competitive ions. Since each equilibrium point corresponds to an individual experiment performed, an inaccuracy of metals concentration analysis can contribute to the observed discrepancy as well.50 The obtained model parameters along with statistical test results are presented in Table 4. As can be noticed, the deviations of the predicted data from the experimental points were much more pronounced in the two-component systems than in the one-component. This is most likely due to two facts: (1) an interaction of competitive ion increases the system’s nonideal behavior that may be inaccurately predicted by the empirical, Langmuir model type; (2) the estimation of model parameters in twocomponent systems requires the minimization of a sum of the ERRSQ objective function for both components:


16

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n

∑ (q i =1

2

e1,exp

n

− qe1,calc )i + ∑ (qe 2,exp − qe 2,calc )i

2

(8)

i =1

In case of high difference between the values of qe1 and qe2, one of the sum of square deviations in eq. 8 is very low. It should also be mentioned that the isotherms derived under the same approximation conditions (by using equilibrium constants (KL,i) using the extended Bi-Langmuir model were at least several percent less precise (data not shown). Analysis of the data in Table 4 indicates that metal ions are adsorbed on permanent (qmP) and variable (qmV) sites in different proportions, which depend on the composition of double-element system. It results from different pHeq of solutions as well as a different metal ion concentrations, which affect the amount of negative charge on both the edges (V) and the exposed basal hydroxyls (P).51 As stated previously, Cu suppresses the uptake of Cd and Zn by variable charge surfaces, while the presence of Cd and Zn decreases the uptake of Cu onto permanent sites.46 It complied with our results, where the amount of permanent sites available for competitive ions is higher for the Zn-Cd system (qmP = 7.06) than for the Cd-Cu and Zn-Cu systems (qmP = 6.93 and 6.69, respectively). Only for the Zn-Cu metal mixture, there is no variable sites available for Zn(II) (qmV = 0). Furthermore, the amount of variable sites obtained for Cu in both Cd-Cu (qmV = 0.63) and Cu-Zn (qmV = 0.71) systems is higher than the corresponding qmV obtained for Cd(II) (qmV = 0.61) or Zn(II) (qmV = 0.56). Despite the system complexity (a wide variety of both pH and metal ions concentrations), the proposed model allow to estimate the amount of permanent and variable active sites available for given metal ion. Such quantification could hardly be done via analytical method (e.g. titration) or via instrumental technique (e.g. spectroscopic). Thus, the new model, even if empirical, can facilitate the studying of kaolin selectivity towards given metal ions in different metal mixtures.


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CONCLUSIONS The results of this work demonstrate that sodium ions change the properties of kaolin and consequently its adsorption abilities. The shape of the isotherm curves depended on the kaolin form (raw or its Na-form), while their location depended on the identity of the metal. Apart from the statistical tests results and the visual inspection of modeling curves, the close approximation of the experimentally obtained maximum sorption capacity was used as a criterion for evaluation of the equilibrium models fit. It allows to clarify that the homogenous Langmuirian sorption of heavy metals on raw kaolin change after Na activation to heterogeneous one. Consequently, a new competitive Langmuir based model was developed to give insight into the selective uptake of metal ions from their mixture on raw kaolin. The equilibrium constants (KL,i) required for this purpose were obtained from modeling of one-component systems. The proposed mathematical description, provided the possibility to quantify the relative amounts of two kinds (V and P) ion exchange active sites, for which the couple of heavy metal ions have similar (Zn-Cd) or different (Zn-Cu, Cd-Cu) affinity. The validity of the physical assumptions which this model is based on, was confirmed by the alignment between the modeled equilibrium surface and 225 experimental points obtained for two different experimental sets.

Funding Sources This work was supported by a grant (NCN N209 760140) of the Polish National Science Center.

ACKNOWLEDGMENT


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We thank Dr. Eweryst Mendyk for his assistance in sorbent analysis.

REFERENCES

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FIGURES

A

B

Figure 1. SEM photomicrographs of (A) raw and (B) Na-kaolin surface.


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10 8

-1

qe [mmol kg ]

A 6 4 Cu Zn Cd

2 0 0,0

0,2

0,4

0,6

0,8

1,0

1,2

-1

Ce [mmol l ] 10

B B

8 -1

qe [mmol kg ]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 4 Cu Zn Cd

2 0 0,0

0,2

0,4

0,6

-1

Ce [mmol l ]

0,8

1,0

1,2

Figure 2. Single-element adsorption isotherms, for (A) raw kaolin and (B) its Na-form (pHinit 4.70 for Cu, 5.12 for Cd and 5.26 for Zn, T = 20°C, wt.= 1g, vol. = 40 mL).



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A

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B

Figure 3. (A) Pore size distribution of raw kaolin (grey line) and its Na-form (black) obtained by BJH and (B) X-ray powder diffraction pattern for raw kaolin.


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A1 Raw

A2 Na-form

B1 Cu

B2 Na Cu

Figure 4. Scanning electron micrograph (A) of raw kaolin (A1) and its Na-form (A2) along with ions mapping (B) after copper adsorption on raw kaolinite (B1, Cu-blue) and its Na-form (B2, Na-blue, Cu-green). Magnification 500x, map size: 74 µm x 54 µm.



10 Zn exp Langmuir BiLangmuir

A

8

-1

qe [mmol g ]

6

6

4

4

2

2

0 0,0

10

0 0,00

0,1

Zn exp Langmuir BiLangmuir

0,2

0,3

0,4 0,5 -1 C e [mmol l ]

0,02

0,6

0,04

0,7

0,06

0,8

0,08

0,9

B

8 -1

qe [mmol g ]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 38

8

6 6

4

4 2

2 0 0,00

0 0,0

0,1

0,2

0,3

0,4

0,5

0,02

0,6

0,04

0,7

0,06

0,8

0,08

0,9

-1

Ce [mmol l ]

Figure 5. Experimental (symbols) and calculated (lines) isotherms for sorption of Zn(II) by (A) raw kaolin and (B) its Na-form.


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A1

B1

7 6

7

A2

6

5 4 3 2 1 0

Cd:Cu = 2:3

B2

5

qe [mmol kg-1]

qe [mmol kg-1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,1

0,2

-1

0,3

Ce [mmol l ]

0,4

3 2 1

Cd Cu

0,0

4

0,5

0 0,0

Cd Cu

0,1

0,2

0,3

0,4

-1

0,5

0,6

0,7

Ce [mmol l ]

Figure 6. Experimental Cd-Cu adsorption isotherms. Experimental set (A) the initial concentration of Cd was fixed at 0.4 mmol/L, whereas the concentrations of Cu varied between 0.015 - 0.6 mmol/L; Experimental set (B) the initial concentration of both Cu and Cd was varied between 0.015 - 2.4 mmol/L so as to maintain the constant, molar ratio of Cd : Cu ions: 2:3 (pHinit 4.3 - 5.3, T = 20°C, wt. = 1g, vol. = 40 mL).



-1

Max metal uptake [mmol kg ]

9 8 6 5 4 3 2 1 0

-1


8

1:0 0:1 1:1 2:3 3:2 1:3 1:4 1:9 3:1 4:1 9:1 molar ratio Cd:Cu Zn Cu

7 6 5 4 3 2 1 0 9

-1

Cd Cu

7

9


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Page 32 of 38

8

1:0 0:1 1:1 2:3 3:2 1:3 1:4 1:9 3:1 4:1 9:1 molar ratio Zn:Cu Cd Zn

7 6 5 4 3 2 1 0

1:0 0:1 1:1 2:3 3:2 1:3 1:4 1:9 3:1 4:1 9:1 molar ratio Cd:Zn

Figure 7. Comparison of metal uptake in one- and two-component systems.


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A Cu-Cd

B Cu-Zn

C Cd-Zn

Figure 8. Experimental (symbols) and calculated (surface) isotherms for binary sorption of (A) Cd-Cu, (B) Cu-Zn, (C) Cd-Zn systems. Note: (1) and (2) are equilibrium concentration of M1and M2, respectively, in adsorbent phase (z-axis) plotted as a function of the equilibrium concentrations of the M1 and M2 in liquid phase (x- and y-axis).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

TABLES

Table 1. Chemical compositions (%) of raw kaolin and its Na-form obtained by XRF

Kaolin

SiO2 Al2O3 K2O Fe2O3 CaO

Na2O MgO

TiO2

P2O5

raw

46.4

37.9

1.68 0.665

0.064 0.030 0.243 0.484 0.067

Na-form 45.6

37.5

1.67 0.666

0.010 0.616 0.219 0.489 0.062


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Table 2. Textural parameters of raw kaolin and its Na-form obtained from N2 adsorption at 195.8 °C

Kaolin

Area (m2/g)

BJH average pore diameter (Å) Pore volume (cm³/g)

ABET t-Plot

Adsorption

Micropore 4V/A) Raw

Desorption

t-Plot

Total

(4V/A)

Micropore (pore < 2300 Å)

14.91 1.98

224.84

214.57

0.85 e-3

7.97 e-2

Na-form 15.10 2.91

210.87

199.25

1.32 e-3

7.55 e-2



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Page 36 of 38

Table 3. Langmuir (L) and Bi-Langmuir (BiL) constants for one-component systems Model

Metal ion

qm,exp [mmol/kg]

qmL or

KL or

qmBiL,1 + qmBiL,2

KBiL,1 and KBiL,2

ME [%]

FT

σ ·10-4

[mmol/kg] Raw kaolin Langmuir Cu

8.90

8.97

27.0

14.0

20.9

5.21

Zn

8.11

8.42

26.3

13.8

24.6

4.49

Cd

7.32

7.54

21.4

6.7

17.8

4.28

BiCu Langmuir Zn

8.90

6.33 + 3.54

7.55 and 250

7.59

112

2.34

8.11

2.09 + 7.17

1203 and 10.6

6.62

155

1.85

Cd

7.32

1.70 + 6.60

1173 and 9.69

6.26

305

1.18

Langmuir Cu

8.10

7.85

7602

18.0

1.78

13.9

Zn

8.60

8.35

5140

22.9

1.01

23.4

Cd

8.00

7.82

8504

38.1

0.86

25.1

BiCu Langmuir Zn

8.00

0.85 + 7.37

7.52 and 6826

17.3

1.61

14.6

8.60

1.06 + 7.77

5.62 and 6826

22.0

0.90

21.6

Cd

8.00

0.10 + 7.74

68.1 and 8190

38.2

0.75

26.8

Na-form of kaolin


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Table 4. Equilibrium model constants for two-component systems System

Cd-Cu

Cu-Zn

Cd-Zn

qmP [mmol kg-1]

6.93

6.69

7.06

qmV M1and M2 [mmol kg-1] 0.61 Cd and 0.63 Cu 0.71 Cu and 0.56 Zn 0.57 Cd and 0.00 Zn ME [%]

49.1

44.4

38.2

FT

22.6

21.9

28.9

σ ·103

1.17

1.24

1.17



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Page 38 of 38

Abstract Graphics


Equilibrium Study of Heavy Metals Adsorption on Kaolin - Industrial

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