Functionalization and characterization of clay mineral halloysite for environmental application

E. A. Santos1, A. M. Ferreira1,2

1Centro Federal de Educação Tecnológica de Minas Gerais, Departamento de Engenharia de Materiais, MG, Brazil 2Instituto Nacional de Ciência e Tecnologia em Recurso Mineral, Água e Biodiversidade - INCT-ACQUA e-mail: [email protected]

Abstract: The present work was aimed at synthesizing nanostructured hybrid materials derived from the functionalization of halloysite with ligand containing amino (-NH2) group for application in adsorption processes. The modification routes were based on the grafting reaction between hydroxyl groups present on clay surface and the hydrolysable alkoxy group of the 3-aminopropyltriethoxysilane (APTES) in dry toluene under reflux. The clays in their natural form and after structural modification were characterized by X-ray diffraction (XRD) and specific surface area (BET method). The XRD results shown the structure of halloysite has been preserved after acid activation and functionalization process. The specific surface areas of clay samples decreased significantly upon grafting. This study demonstrates that the surface chemistry of halloysite nanotubes is readily modified, enabling environmental application. Keywords: Halloysite, functionalization, adsorption.

Introduction

Environmental contamination with heavy metals and dyes has increased throughout the world due to disposal of hazardous effluent into receiving waters. Precipitation, coagulation, membrane separation, ion exchange and adsorption have been extensively exploited for dyes and heavy metals contaminated wastewater treatment. Among them, adsorption has received increasing attention due to high efficiency, low cost, environmental friendliness and the possibility of the materials recycling [1]. Halloysite is a naturally occurring 1:1 dioctahedral aluminosilicate clay mineral chemically with a nanotubular structure. It has huge specific surface area, plentiful micropores, abundant hydroxyl groups, environmental friendliness, and biocompatibility, making them potentially useful as adsorbents. In each halloysite nanotube, the external surface is composed of siloxane (Si-O-Si) groups, whereas the internal surface consists of aluminol (Al-OH) groups [2]. The modification of oxide surfaces by coupling with functionalized organosilanes is applicable to the fields of catalysis, adsorption, electrochemistry, chromatography and nanocomposite materials. Functionalization with organosilane involve grafting reactions that occur by establishing covalent bonds between the reactive groups of the layer, normally hydroxyl groups, and silane molecules, which ensure high chemical structural, and thermal stability for the compound [3]. These reactions can be restricted to the crystal surface and the internal surface of the lumen or to the layer surface, in which case an interlayer expansion occurs. The resulting material can be defined as a hybrid material. Functionalized clays may provide specific sites for the adsorption of specific adsorvates.

Experimental Procedure

The halloysite (H4Al2O9Si2.2H2O) (H_NATURAL) and APTES 98% were purchased from Sigma Audrich. The toluene 99% also provided byAlfa Aesar and HCl 37% by Alphatec. The functionalization of halloysite as adsorbent was performed as follows: (i) 10 g of halloysite was slowly added to 100mL (0.15, 0.30 and 0.45 mol/L) of HCl with magnetically stirring for 12 h at 298 K. Afterward, the samples solution were then filtered and washed sequentially with distilled water three times and dried at 383 K for 12 h. The resultant samples are here referred to as H_ACID_0.15, H_ACID_0.30 and H_ACID_0.45. (ii) 5 g of pure halloysite or acid halloysite were ground and mixed with 125 ml toluene and 5 ml of APTES. The flask with the containing the magnetically stirring mixture was evacuated using a vacuum pump and a slight fizzing of the suspension was observed as the air was removed. After the fizzing stopped, the flask was sealed for 30 min to reach equilibrium. The evacuated suspension was then transferred to the refluxing system for modification at 393K for 10h under constant stirring. The solid phase in the resultant mixture was filtered and extensively washed with toluene for three times before drying in an oven at 298K overnight. The resultant samples were referred to as H_APTES, H_APTES_ACID_0.15. (iii) The X-ray diffraction patterns of modified and unmodified clay samples were obtained with a Shimadzu, model XRD-7000 diffractometer, fitted with a Cu tube (λ=1.54 Å, step size 0.06° 2q, 5 s/step). The isotherms of adsorption/desorption of N2 were obtained at 77 K (liquid nitrogen). The specific surface area was obtained by a multipoint BET method.

Results and Discussion

Nitrogen adsorption and desorption analyses were conducted to investigate the surface area. The results are shown in Table 1. The halloysite acidified (H_ACID_0.15) had no change in their specific surface area in relation to natural halloysite. This can be explained by the low concentration of acid that does not alter the structure of the halloysite. The acid activation improves the grafting efficiency by increasing the specific surface area and the density of superficial hydroxyl groups. The specific surface area (H_ACID_0.30) increases from 66 m2/g. Subsequently, however, it decreases to 53.76 m2/g (H_ACID_0.45) attributed to disaggregation of silica layers. The funcionalized samples showed a smaller specific surface area than the natural halloysite due to the reduction of microporous contribution. Decreasing the surface area of halloysite after modification confirmed the successful modification of halloysite by APTES.

Table 1-Specific surface area of halloysite samples Samples Specific surface area (m2/g) H_NATURAL 50.31 H_ACID_0,15 49.51 H_ACID_0,30 66.00 H_ACID_0,45 53.76 H_APTES 14.02 H_APTES_ACID_0.15 40.96

Figure 1 shows the X-ray diffraction patterns of samples of raw halloysite, acidified and funcionalized halloysite. The crystallographic structure of this clay is preserved even after the functionalization as no significant changes were observed in the characteristic basal spacing (d001: 7.51 Å to 7.40 Å) after functionalization. This result indicates that most of the interlayer inner-surface AlOH groups of halloysite were unavailable for grafting, since they were blocked by the strong hydrogen bonds between layers, suggesting that the vast majority of grafting occurred on the AlOH groups at the internal surface of the lumen, which are accessible by APTES.

Figura 1: XRD patterns of halloysite samples.

Conclusions

Functionalization of natural halloysite nanotubes can be achieved by modification with organosilane APTES. Grafting occurs between hydrolyzed APTES and the surface hydroxyl groups, including the aluminol groups at internal surface of lumen and the aluminol group at edges or external surface defects. The XRD results showed that the acidification and functionalization process did not affect the crystallographic structure of the halloysite. The analysis of the surface area of the halloysite showed that acidification of the sample tends to increase the specific surface area while the functionalization tends to decrease surface area. Expected to see that despite the functionalization halloysite reduce its surface area, reducing the active sites amenable to adsorption, it proves more efficient and selective for the adsorption of heavy metals and dyes.

References

[1] ZHAO, Y. et al. Halloysite nanotubule clay for efficient water purification. J. Colloid and Interface Sci., 406, 121-129, 2013. [2] YUAN, P. et al. Functionalization of halloysite clay nanotubes by grafting with γ- aminopropyltriethoxysilane. J. Phys. Chem., v.112, p.15742–15751, 2008. [3] FERREIRA, A. M. et al. Smectite organofunctionalized with thiol groups for adsorption of heavy metal ions. Appl. Clay Sci., 42, 410–414, 2009.