Synthesis and characterization of TiO2-pillared Romanian clay and their application for azoic dyes photodegradation

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Abstract

The synthesis and properties of metal oxide pillared cationic clays (PILCs) has been subject to numerous studies in the last decades. In order to obtain TiO2-pillared type materials, sodium montmorillonite from Romania–areal of Valea Chioarului, having the following composition (% wt): SiO2-72.87; Al2O3-14.5; MgO-2.15; Fe2O3-1.13; Na2O-0.60; K2O-0.60; CaO-0.90; PC-5.70 and cation exchange capacity, determined by ammonium acetate method, of 82 meq/100 g, as matrix, was used. Sodium form of the clay was modified, primarily, by intercalation of cetyl-trimethylammonium cations between negatively charged layers which will lead to the expantion of the interlayer space. For the preparation of the TiO2-pillared clay, the alkoxide molecules, as titania precursor, were adsorbed onto/into clay samples (1 mmol Ti/g clay), in hydrochloric acid environment, the resulted species being converted into TiO2 pillars by calcination. The as-prepared materials have been used as catalysts for Congo Red dye photodegradation, under UV. The photocatalytic activity of the pillared clays is a function of TiO2 pillars size, their increase leading to the enhancement of the contact areas between dye solution and photoactive species present in the interlayer space.

The structural characteristics and properties of the obtained materials were investigated by X-ray Diffraction, Thermogravimetry Analysis, UV–vis Diffuse Reflectance, Transmission Electron Microscopy and Energy Dispersive X-ray Analysis.

Introduction

The contamination of rivers and ground water by organic pollutants has acted as stimulus for numerous investigations focused on the effective pollution abatement methods. In aqueous environment, the presence of organic pollutants is of great concern. Most of the industrial and domestic waters are frequently contaminated with organic pollutants such as phenols, VOCs, pesticides, dyes, etc. [1]. The currently used approaches to remove the organic pollutants from the wastewaters are based on the adsorption or chemical oxidation processes. However, these processes have major drawbacks: the adsorption does not lead to pollutants degradation, while the chemical oxidation, in homogeneous phase, is not economically favorable, except the high concentrated pollutants [2].

The clays, such as montmorillonite, vermiculite, kaolinite, mica, hydrotalcite like compounds, etc., have attracted much attention in recent years for their applicability in wastewaters decontamination. These natural materials possess layered structures, large surface areas and high exchange capacity (CEC), which give them many potential applications in different fields [3].

For environmental uses, Romanian montmorillonite from Valea Chioarului areal, having a 2:1 layered structure, represents a promising material. Because of the isomorphic substitution within the layers (e.g., Al3+ replaced by Mg2+or Fe2+, or Mg2+ replaced by Li+ in the octahedral sheet; Si4+ replaced by Al3+ in the tetrahedral sheet), these layers are negatively charged, the charge neutrality being ensured by the presence of cations in the galleries formed by two adjacent sheets. These cations are exchangeable and the sum of these charges determines the cation exchange capacity value (CEC). Generally, the clays can adsorb organic substances either on their external surfaces or within their interlamelar spaces, by interaction with or by substitution of the cations presents in the interlayer spaces [4].

In the last decades, metal oxide pillared clays (PILCs) have been intensely investigated [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], their physical and chemical properties finding applications in different fields. These materials contain metal oxide pillars that sustain the clay sheets and lead to the formation of bi-dimensional porous networks. Several single and mixed oxide pillars have been prepared using the polycationic species of Al, Zn, Ti and Cr among others [5], [6], [7], [8], [9], [10]. Properties such as acidity, surface area and pore size distribution of PILCs lead to new shape-selective catalysts that are similar to zeolites [23].

TiO2 and TiO2-based materials [24], [25], [26], [27], [28], [29], [30] have attracted growing interest because of their low toxicity, such kind of materials being used as photocatalysts for efficient treatment of wastewaters containing toxic organic compounds [1], [2], [15], [16], [20], [21].

The Romanian montmorillonite from Valea Chioarului areal has the following chemical composition (wt%): SiO2-72.87; Al2O3-14.5; MgO-2.15; Fe2O3-1.13; Na2O-0.60; K2O-0.60; CaO-0.90; PC-5.70. Its mineralogical composition consists of kaolinite (K), illite (I), montmorillonite (M) and nontronite (N), besides small quantities of feldspar–albite (F) and quartz (Q). The determined cationic exchange capacity was 82 meq/100 g.

Romanian montmorillonite pillared with different metal oxides, possessing additional functions originated from both enhanced surface area and pillars identity, is interesting from both academic and industrial point of view [31], [32], [33]. The pillaring process can be realized by following two approaches. The first one, and the most intense used, is the direct pillaring which involves the treatment of Na-form of the clay with the pillaring solution, followed by the calcination of the resulted material [2], [33]. The second approach uses an intermediary step, the interlayer space being expanded with a swelling agent (indirect method) [31].

This paper reports the synthesis of TiO2-pillared Romanian clay, the size of pillars being controlled by expanding the clay's gallery using cetyl-trimethyammonium bromide (CTAB) as expanding agent, and a brief study of their application as photocatalyst for Congo Red (CR) dye degradation (Fig. 1). This dye is the sodium salt of benzidinediazo-bis-1-naphtylamine-4-sulfonic acid (C32H22N6Na2O6S2)2, its use in cellulose industries (cotton textile, wood pulp and paper) being diminished due to the high toxicity of residual wastewaters resulted from industrial processes.

Section snippets

Materials

The starting clay was obtained from Valea Chioarului areal, Romania. Chemicals like hydrochloric acid, titanium tetraisopropoxide (titania precursor) and cetyl-trimethylammonium bromide (swelling agent) were purchased from Merck.

Preparation of Ti-pillaring solution

1 mM of titanium tetraisopropoxide (titania precursor) was hydrolyzed with 1 M hydrochloric acid. Briefly, titanium alkoxide was dropped into hydrochloric acid solution, the resulted slurry being vigorously stirred for 3 h, at room temperature, with the obtaining of the

XRD analysis of materials

Table 1 shows the samples notations, chemical composition and XRD characteristic of these samples.

Fig. 2 shows the low angles XRD patterns of the surfactant-modified clay prepared using different amounts of CTAB. It can be observed that the (0 0 1) reflection, characteristic to the parent clay, from 2θ = 7.18°, is shifted towards lower 2θ values in case of the surfactant-modified samples. During the intercalative process, the basal spacing increase from 12.39 Å, corresponding to parent clay, to 15.03

Conclusions

The Romanian TiO2-pillared clay and the Romanian TiO2-pilarred organic modified clay were successfully obtained through the classic pillaring process of both Na-form and organic modified form of the clay, respectively. The size of TiO2 pillars vary as a function of CTA+ sample content, by using a higher amount of swelling agent the interlayer space being increased considerably (2.056 nm), comparative to that one corresponding to Na-form of the natural clay (1.239 nm), this increase being similar

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