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Specific Adsorption of Chromium Specieson Kaolinite SurfaceDr. Feng Rao a , Shaoxian Song a & Alejandro Lopez-Valdivieso aa Instituto de Metalurgia, Universidad Autónoma de San Luis Potosí,San Luis Potosí, México

Available online: 29 Jun 2011

To cite this article: Dr. Feng Rao, Shaoxian Song & Alejandro Lopez-Valdivieso (2012): SpecificAdsorption of Chromium Species on Kaolinite Surface, Mineral Processing and Extractive MetallurgyReview: An International Journal, 33:3, 180-189

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SPECIFIC ADSORPTION OF CHROMIUM SPECIESON KAOLINITE SURFACE

Feng Rao, Shaoxian Song, and Alejandro Lopez-ValdiviesoInstituto de Metalurgia, Universidad Autonoma de San Luis Potosı,San Luis Potosı, Mexico

Specific adsorption of hydrolyzed chromium species on kaolinite surface was studied.

Electrokinetic behavior of kaolinite particles in aqueous solutions in the presence of CrCl3showed that the hydrolyzed chromium species can adsorb on kaolinite surfaces and reverse

its charge. This adsorption reached the maximum near the precipitation pH (ppt) of

chromium hydroxide. The adsorption followed Langmuir isotherm and was irreversible.

It might be resulted from hydrogen bonding between the hydrolyzed metal species and

kaolinite surfaces. A novel chemical adsorption model was proposed between kaolinite

surfaces and the hydrolyzed multivalent metal species.

Keywords: chromium salts, hydrogen bond, specific adsorption, zeta potential

1. INTRODUCTION

Kaolinite particle is consisted of layers, which are held together with hydrogenbonds and van der Waals interaction. Each layer consists of a SiO4 tetrahedral sheetand an octahedral sheet with Al3þ as the octahedral cation (Hu, Liu, and Xu 2003).It is plate-like mineral and the aspect ratio (particle diameter=particle thickness) isabout 5–15 (Solomon and Hawthorne, 1983). Its basal planes are composed of amixture of Al–OH sites from the octahedral sheet and Si–O sites from the tetrahedralsheet, respectively. Its edge planes present the hydroxyl sites (Al–OH or Si–OH) afterhydrating in aqueous solution. The basal planes of kaolinite are believed to carry apermanent charge due to the isomorphous substitution of Si4þ by Al3þ, whereasthe charge on the edges depends on the solution pH (Moon and Fuerstenau 2003;Bergaya, Theng, and Lagaly 2006).

The adsorption of multivalent metallic cations on oxides and silicate mineralshas been greatly concerned in mineral processing (Eisele, Kawatra, and Ripke 2005;Miller et al. 2007). For instance, Fuerstenau and Palmer (1967) have systematicallystudied the adsorption of multivalent cations, such as Fe3þ, Al3þ, Pb2þ, Mg2þ,Mn2þ, and Ca2þ, at quartz surfaces and their influence on quartz flotation responsewith anionic surfactant as collector. They found that the activation of quartz occursin the pH range in which the cations hydrolyzed to hydroxyl species. Similarly,multivalent cations were found to modify oil-silicate mineral flocculation in aqueous

Address correspondence to Dr. Feng Rao, Instituto de Metalurgia, Universidad Autonoma de San

Luis Potosı, Av. Sierra Leona 550, San Luis Potosı, C. P. 78210, Mexico. E-mail: raofeng1@163.com

Mineral Processing & Extractive Metall. Rev., 33: 180–189, 2012

Copyright # Taylor & Francis Group, LLC

ISSN: 0882-7508 print=1547-7401 online

DOI: 10.1080/08827508.2011.562949

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solution and thus influence oil contaminated soil remediation and bitumenextraction from oil sand (Gan, Crozier, and Liu 2009). In addition, multivalentcations are used as highly effective destabilizer in dewatering clay dispersions ofmineral tailings, which have become an important technological and environmentalproblem confronting the mineral industry worldwide (Johnson, Dixon, and Scales1999; Mpofu, Addai, and Ralston 2003).

Modi and Fuerstenau (1957) first reported the specific adsorption of multivalentions (Ba2þ; SO2�

4 ; S2 O2�3 ) on oxide particles (corundum) in aqueous solution. They

realized that these ions could function as surface-active ions and reverse the charge ofcorundum particles. Then, James and Healy (1972a, 1972b, 1972c) studied theadsorption of multivalent metal ions on oxide particles comprehensively. They foundthat only the hydrolyzed species adsorbed on a negative charged oxide surface toreverse the sign, but not the multivalent ions. There were many works that alsoreported the charge reversal of clay particles in aqueous solution through the adsorp-tion of multivalent metal ions, such as Co2þ, Cd2þ, Cu2þ, Mn2þ, Ca2þ, and Al3þ.These ions affected the stability and dewatering behavior of clay dispersions a lot(Hunter and James 1992; Alkan, Demirbas, and Dogan 2005). In the present work,we studied the adsorption of chromium species on kaolinite particle surface. Anadsorption model of hydrolyzed multivalent metal species on kaolinite surfaces hasbeen deduced through the measurements of zeta potentials and adsorptions.

2. EXPERIMENTAL

2.1. Materials

The kaolinite sample used in this work was collected from the Silicatos yDerivados plant in the state of Jalisco, Mexico. It was purified first by using ahigh-intensity magnetic separator to remove iron mineral particles and then byscreening to remove coarse particles. Next, the <18 mm fraction was treated with1mol=L NaCl solution that was shaken in a bottle, kept overnight, and then centri-fugated at 3000 rpm for 2 h. This cycle of agitation=centrifugation was repeatedsix times to achieve close to 99% Naþ exchange. After that, the cation-exchangedkaolinite was washed with distilled water. The washing procedure was the same asthat for Naþ exchange but more drastic centrifugation (7000 rpm for 3 h) was usedbecause the stability of the colloidal dispersion had increased.

The particle size distribution of the purified kaolinite sample was determinedusing a Shimadzu SALD-1100 laser diffraction particle size analyzer (Japan). Asshown in Figure 1, the kaolinite sample has a particle size distribution of the D50

(particle size at 50% of cumulative undersize) of 4.0mm and the D85 of 9.2mm.X-ray measurements were performed on the kaolinite by using a Rigaku 2200 X-rayreflection meter (Japan) with Cu Ka radiation. As shown in Figure 2, the X-rayreflection pattern of the kaolinite sample had sharp and narrow peaks, indicating thatthe kaolinite sample had well ordered crystal structure and high purity. Also thekaolinite sample was assayed by chemical analysis as 38.51% Al2O3, 45.48% SiO2,0.23% Fe, and 0.04% Ti. An Autosorb gas adsorption analyzer from QuantachromeInstruments (USA) was used to determine the surface area of the kaolinite samplethrough N2 adsorption, of which the specific surface area is 24.58m2=g.

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Sodium chloride and chromium chloride with analyzed purity were obtainedfrom Sigma-Aldrich Corporation (USA). Sodium hydroxide and hydrochloric acidfrom the same company were used for adjusting pH value of the dispersions. Thewater was first distilled then passed through resin beds and a 0.2-mm filter.

2.2. Measurements

A Colloidal Dynamics ZetaProbe Analyzer (Australia) with electroacoustictechnology was used to determine the zeta potentials of the kaolinite particles inaqueous solutions. The detailed description for the instrument and the measurementcan be found in our previous paper (Rao, Song, and Lopez-Valdivieso 2009).

Figure 2 X-ray reflection pattern of the purified kaolinite.

Figure 1 Particle size distribution of the purified kaolinite.

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Adsorption densities were determined through a batch depletion method at22�C. First, 250mL aqueous kaolinite dispersion was prepared at a weight concen-tration of 2%. Then, a given concentration of chromium salt was added into thedispersion while the pH was controlled at a given value. Next, the dispersion wasconditioned on a magnetic stirrer for 30min. After that, the dispersion was centri-fuged at 5000 rpm for 30min. The solution from the centrifugation was sampledfor determining the chromium concentration in the solution through chemical analy-sis. The adsorption density of chromium on kaolinite surfaces is calculated as follows:

C ¼ ðC0 � CÞVW

; ð1Þ

where C is the adsorption density; Co and C are the concentrations of chromium inkaolinite dispersion before and after the adsorption; V is the volume of the kaolinitedispersion, in our case, V¼ 250mL; andW is the weight of the kaolinite sample used,in our case, W¼ 5 g.

3. RESULTS AND DISCUSSIONS

The electrokinetic behavior of the kaolinite particles in aqueous sodium salt(NaCl) and chromium salt (CrCl3) solutions is shown in Figure 3. In the case ofsodium salt, zeta potential of kaolinite increased linearly as increasing the NaCl con-centration from 0 to 12� 10�3mol=L. It approached zero as a limiting value, but keptnegative. However, in the case of chromium salt, the zeta potential sharply increasedwith low addition of CrCl3. The sign reversed from negative to positive at a CrCl3concentration of 1� 10�3mol=L. Over this critical concentration, the kaoliniteparticles became more positive charged. Then the increase was mild at a CrCl3concentration higher than 1.8� 10�3mol=L. The charge reversal was resulted from

Figure 3 Zeta potentials of kaolinite in aqueous NaCl and CrCl3 solutions.

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the specific adsorption of chromium species on kaolinite surfaces. Monovalentelectrolytes (e.g., NaCl and KNO3) compress the electrical double layer of particles(kaolinite) in aqueous solutions and thus reduce the absolute value of zeta potential,but do not reverse the sign of charge (Hiemenz and Rajagopalan, 1997). Otherwise,multivalent electrolytes (e.g., CrCl3) or their hydrolyzed species could reverse the signof charged particles through specific adsorption (Fuerstenau, Lopez-Valdivieso, andFuerstenau 1988).

The zeta potential of kaolinite in aqueous solutions as a function of pH at vari-ous chromium salt (CrCl3) concentrations is presented in Figure 4. As it is noted, thezeta potential was negative in the range of pH 2–12 and no isoelectric point (IEP)appeared without CrCl3. This result corresponded well with that as reported else-where (Siffert and Kim 1992), the negative charge of kaolinite particles is originatedfrom isomorphous substitutions. In the presence of CrCl3, all the curves shifted upand showed similar behavior except that the more the CrCl3 addition, the higherthe curve. The zeta potentials increased as increasing pH from pH 2.5 to around6; thereafter, it decreased with pH and approached the value that without CrCl3at the same pH. Thus the curves of zeta potentials versus pH showed a positive peakaround pH 6 in the presence of CrCl3. A series of IEPs from pH 3 to 8.5 wereobserved as increasing CrCl3 concentrations from 1.5� 10�3 to 8� 10�3mol=L.

The adsorption density of chromium on kaolinite particles in aqueous solu-tions as a function of pH at various chromium salt concentrations is shown inFigure 5. As it is noted, the adsorption of chromium species started at a low pH.The higher the CrCl3 concentration, the lower the beginning of the adsorption.The adsorption reached the maximum value at around pH 6, which was the precipi-tation pH (ppt) of chromium hydroxide at the corresponding CrCl3 concentrationand then kept a plateau up to pH 6.3–6.5.

The equilibrium diagram of chromium species in aqueous solutions is shown inFigure 6 (Bulter 1998). The Cr3þ dominates at pH 1–3 and then decreases withincreasing pH. The concentration of hydrolyzed species increases as increasing pH

Figure 4 Zeta potentials of kaolinite as a function of pH in aqueous CrCl3 solutions.

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from pH 2 to the precipitation pH (ppt) of Cr(OH)3, which is at pH 6.3. Chromiumhydroxide (amorphous) is the main species in the solution at pH 6.3–12. The positivezeta potentials shown in Figure 4, therefore, arise from the adsorption of hydrolyzedchromium species. This adsorption of the positively charged chromium speciesreached maximum near the ppt of Cr(OH)3 (see Figure 5).

If the concentration of chromium species in the aqueous dispersions is highenough to coat completely the kaolinite surface, the particle would behave asCr(OH)3 particles. The IEP of kaolinite particles in the presence of 8� 10�3mol=LL CrCl3 was pH 8.5 (see Figure 4), which is very close to the IEP of the Cr(OH)3particles. These results demonstrate that the kaolinite particles were coated com-pletely by the hydrolyzed species.

Figure 5 Adsorption of chromium on kaolinite as function of pH at various CrCl3 concentrations.

Figure 6 Equilibrium diagram of chromium species as a function of pH in aqueous solution.

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Figure 7 shows the adsorption and desorption of chromium on kaolinitesurfaces as a function of the equilibrium concentration of chromium salts in thesolutions at a given pH. As it is noted, the adsorption density increased as increasingthe equilibrium chromium concentration until reached a plateau. Obviously, theadsorption followed the Langmuir isotherm (Hiemenz and Rajagopalan 1997), indi-cating a monolayer adsorption. And the adsorption depended on the pH: the higherthe pH, the higher the adsorption density.

According to the Langmuir isotherm, the adsorption density can be representedby equation

C ¼ CmKC

1þ KCð2Þ

where C is the equilibrium concentration of chromium (�10�3mol=L); C and Cmax

are the adsorption density (mmol=m2) and the maximum adsorption density(mmol=m2), respectively, at an equilibrium chromium concentration; andK is the con-stant (�103 L=mol). Table 1 presents the Langmuir constants Cmax and K calculatedfrom Figure 7. As it is noted, the maximum adsorption density increased about

Table 1. Langmuir constant of the adsorption of hydrolyzed

chromium species on kaolinite

Langmuir constants

pH T(�C) Cmax (mmol=m2) K (�103 L=mol)

5 25 6.361 2.91

5.8 25 10.174 22.56

Figure 7 Adsorption and desorption of hydrolyzed chromium species on kaolinite surfaces at various

chromium concentrations.

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3.8 mmol=m2 as increasing the pH from 5 to 5.8, while the constant K increased19.65� 103 L=mol. This indicates that the adsorption of chromium species onkaolinite surface was strengthened as increasing pH from 5 to 5.8.

As seen in Figure 7, there was no desorption of chromium from kaolinitesurface when diluting the solutions with water at the given pH. It is one of thecharacteristics of the chemical adsorption (Hunter 2001). As increasing pH from 5to 5.8, the chromium adsorbates change into higher positively charged species:Cr3ðOHÞ5þ4 (see Figure 6). This might result the increasing of the adsorption energy(see Table 1).

The similar adsorption and desorption phenomenon of other multivalentmetal species on clay particle surfaces was found elsewhere (Johnson, Dixon,and Scales 1999). Thus a chemical adsorption nature between hydrolyzed metalspecies and kaolinite surfaces is proposed as following. As is shown in Figure 8,the Si–O sites from the tetrahedral sheet of kaolinite layers are hydrated intoSi–OH sites in aqueous solution. Then, the hydrolyzed metal species react withthese sites through hydrogen bond. A covalent bonding between the metal atomof the hydrolyzed metal species and the oxygen atom on the Si–OH sites wasformed as a Si–O–M. Similarly, a covalent bond of Al–O–M between the atomof the hydrolyzed metal species and the oxygen atom on the Al–OH sites (octa-hedral sheets) was formed.

4. CONCLUSIONS

The charge of kaolinite was reversed through the adsorption of chromiumspecies. Only the hydrolyzed chromium species specifically adsorbed on kaoliniteparticle surfaces.

This adsorption followed the Langmuir isotherm. It was monolayer adsorp-tion and irreversible. Therefore, the interaction between hydrolyzed metal speciesand kaolinite surfaces might be chemical. A chemical adsorption model wasproposed.

Figure 8 Schematic presentation of the chemical adsorption of hydrolyzed metal species on kaolinite

surface.

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ACKNOWLEDGMENTS

The financial supports for this work from the Consejo Nacional de Ciencia yTecnologıa (CONACyT) of Mexico under the grants nos. 48436-Y and 92506are gratefully acknowledged. Also, F. Rao would like to thank the CONACyT foroffering him the scholarship under the grant no. 213624 during his Ph.D. study.

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