TiO₂ pillared montmorillonite as a photoactive adsorbent of arsenic under UV irradiation

Abstract

A TiO₂ pillared montmorillonite (TiO₂/MMT) adsorbent was used to remove As(III) and As(V) from aqueous solution with or without UV irradiation. The adsorption kinetics, role of pH, effect of UV, and column experiments are discussed herein. The Ti-polyoxocations in pillared montmorillonite (MMT) were crystallised to anatase in the interlayer and on the external surface with hydrothermal treatment. The anatase TiO₂ exhibited not only an improved arsenic adsorption capacity, but also enhanced photocatalytic efficiency. The TiO₂/MMT adsorbent was characterised by X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photo-electron spectroscopy (XPS) and nitrogen adsorption–desorption techniques. The interlayer d-spacing and BET-specific surface area increased after intercalation of TiO₂ in montmorillonite. Without UV irradiation, the adsorption capacities of As(III) and As(V) were 4.58 and 4.86 mg g⁻¹. With UV irradiation, the adsorption capacities of As(III) and As(V) were 5.19 and 5.16 mg g⁻¹, respectively, and the equilibrium concentrations of both As(III) and As(V) were below 10 μg L⁻¹. In addition, column experiments of arsenic-contaminated groundwater removal were conducted using granular TiO₂/MMT packed in a 10-cm-ID column with initial arsenic concentrations of 120, 220 and 410 μg L⁻¹. The results indicated that TiO₂/MMT is a promising adsorbent for As(III) and As(V) removal from arsenic-contaminated water.

Introduction

High arsenic concentrations in drinking water resource are a major environmental problem and occur in many parts of the world, especially in Bangladesh and West Bengal in India [1], [2]. Long-term exposure to arsenic could cause acute and chronic adverse health effects, including cancer [3]. Due to its high toxicity, the World Health Organization (WHO) recommended to lower arsenic levels in drinking water to 10 μg L⁻¹ in 1993, and the United States Environmental Protection Agency (US-EPA) published a new 10 μg L⁻¹ standard for arsenic levels in drinking water in 2001 [4]. Arsenic is found in a variety of forms in groundwater, including arsenite (As(III)), arsenate (As(V)), methylarsenic (MMA) and dimethylarsenic (DMA), but the main source of arsenic in groundwater is in inorganic forms [5]. Classical technologies such as adsorption, membrane processes, reverse osmosis and ion exchange have been developed to remove arsenic from aqueous systems. Adsorption is one of the most commonly used methods for arsenic removal from aqueous solutions due to its high efficiency and economic value.

Mohan and Pittman compared most of the arsenic adsorbents in a critical review [6]. Iron oxides and activated alumina are the most widespread and effective adsorbents for inorganic arsenic adsorption from aqueous solutions [7], [8]. TiO₂ is also an attractive arsenic adsorbent because it is inexpensive and environmentally friendly [9]. TiO₂ nanoparticles are an effective adsorbent for both As(III) and As(V), especially for the adsorption of As(III) under UV irradiation due to its ability to oxidise As(III) to As(V) in the presence of UV light [10], [11], [12]. However, the use of TiO₂ nanoparticles is limited to simple applications because the suspensions need to be filtered after the adsorption process. Natural materials, such as kaolinite, montmorillonite and red mud, are used to remove arsenic from water because of their attractive price [13], [14]. However, the adsorption capacity of these materials is not satisfactory.

Modified and pillared clays are new possibilities of adsorptive media and are characterised by their favourable microporous structures, economical value and high cation exchange capacity; in addition, these materials are environmentally friendly [15]. The modification of clay is predominantly performed by pillaring various polyoxy cations, such as Al³⁺, Fe³⁺, Mn⁴⁺ and Ti⁴⁺. Titania pillared clay has a mesoporous structure due to its small TiO₂ particles acting as pillars between silicate layers, and shows high adsorption ability due to its large specific surface area [16]. TiO₂ pillared montmorillonite has a considerably larger interlayer d-spacing in comparison to other TiO₂ modified clay and shows high adsorption and efficient photocatalytic ability [17], [18]. Additionally, the interlayer surface of TiO₂ pillared montmorillonite is generally hydrophobic, which is an advantage in adsorbing and degradation organic compounds in water [19]. Ding et al. reported that TiO₂ pillared montmorillonite showed a high performance for photocatalytic degradation of dimethyl phthalate ester in water [20]. Ooka et al. reported that the hydrothermal treatment TiO₂ pillared montmorillonite could enhance the photocatalytic degradation rate of endocrine disruptors [21]. Furthermore, TiO₂ pillared montmorillonite also show photocatalytic activity owing to nanocrystalline TiO₂ [22]. Thus, this would be one of the suitable materials for the photocatalysis of arsenite in aqueous solution.

The aim of the present work was to prepare TiO₂/MMT by a hydrolysis method and comprehensively evaluate the arsenic adsorption on TiO₂/MMT with and without UV irradiation. After a general characterisation of TiO₂/MMT, the adsorption kinetics data were analysed. The effects of pH and the role of UV light on arsenic adsorption were investigated in order to understand the mechanism of arsenic adsorption under UV irradiation. A column study was carried out to evaluate the application of TiO₂/MMT for arsenic-contaminated groundwater removal.