BTEX and MTBE adsorption onto raw and thermally modified diatomite

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Abstract

The removal of BTEX (benzene, toluene, ethyl-benzene and xylenes) and MTBE (methyl tertiary butyl ether) from aqueous solution by raw (DR) and thermally modified diatomite at 550, 750 and 950 °C (D550, D750 and D950 respectively) was studied. Physical characteristics of both raw and modified diatomite such as specific surface, pore volume distribution, porosity and pHsolution were determined, indicating important structural changes in the modified diatomite, due to exposure to high temperatures. Both adsorption kinetic and isotherm experiments were carried out. The kinetics data proved a closer fit to the pseudo-second order model. Maximum values for the rate constant, k2, were obtained for MTBE and benzene (48.9326 and 18.0996 g mg−1 h−1, respectively) in sample D550. The isotherm data proved to fit the Freundlich model more closely, which produced values of the isotherm constant 1/n higher than one, indicating unfavorable adsorption. The highest adsorption capacity, calculated through the values of the isotherm constant kF, was obtained for MTBE (48.42 mg kg−1 (mg/L)n) in sample D950.

Introduction

In recent decades, water pollution phenomena have become more and more frequent and acute. Petroleum hydrocarbons represent one of the most common categories of groundwater pollutants that are found at many contaminated sites, making surface water and/or groundwater unsuitable for many uses (including drinking), due to their toxic and/or carcinogenic properties.

BTEX are typical hazardous organic compounds that are present in almost any petrol or gasoline spill on the soil's surface or in the subsurface. These pollutants have been found to cause many serious health side effects to humans (e.g. skin and sensory irritation, central nervous system depression, respiratory problems, leukemia, cancer, as well as disturbance of kidney, liver and blood systems) and therefore their removal from groundwater and surface waters is essential.

MTBE is also a very persistent and hazardous water contaminant that can be found at many gasoline-contaminated sites, given that it represents a very common oxygen additive to gasoline, which has been used since the middle 1970s to increase its burning efficiency and thereby reduce emissions of carbon monoxide and other organic compounds. MTBE is highly soluble in water (∼23,200–54,000 mg/L) [1], [2], [3] and can be carried far away from the contamination source very quickly. Its high mobility can be verified by the fact that MTBE has been detected in the drinking water [4], rivers [5] and snow samples [6] (with no proximate apparent contamination source) of different European cities. MTBE has been associated with several health side effects (e.g. nausea, headaches, dizziness and breathing difficulties) and has been classified by U.S. EPA as a potential/suspected carcinogenic substance.

BTEX removal from groundwater has been widely studied and several processes have been successfully applied, including bioremediation, volatilization, oxidation, as well as adsorption. However, in practice, the utilization of these removal processes on a large scale presents certain advantages and disadvantages, as far as applicability, site dependence, efficiency and cost parameters are concerned. MTBE has also been widely studied and the main mechanisms that have been proposed for its removal from water include bioremediation/natural attenuation, volatilization (air stripping/air sparging), chemical oxidation, as well as adsorption.

Adsorption is a process that can be applied either in situ (with permeable reactive barriers) or ex situ, is relatively simple (compared to others) and can achieve quite satisfactory removal efficiencies. Activated carbon is perhaps the most widely used adsorbent for organic compounds, due to its high adsorption capacity. Yet, activated carbon is a relatively expensive material that also has a high regeneration/reactivation cost. Generally, in order to evaluate and select an adsorbent, other parameters have to be determined and taken into account, apart from its adsorption capacity, such as production and regeneration cost, availability, environmental compatibility and energy consumption during its production and regeneration. Therefore, research has been focused on finding new, preferably natural, abundant and cheap materials for replacing activated carbon as an organic compound adsorbent. BTEX removal from water by adsorption on resins [7], surfactant modified zeolites [8] and organo-clays [9] has already proved to be an interesting perspective. Moreover, silicalite, mordenite, zeolite-β [10], as well as polymer [11], synthetic [12] and carbonaceous [13] resins and high silica zeolites [14], [15] have also been successfully tested for MTBE removal from water.

In this study, raw and thermally modified diatomite have been tested for their adsorption potential for BTEX and MTBE from aqueous solutions. Diatomite, also known as diatomaceous earth or kieselguhr, is a fine sedimentary rock of biogenetic origin, which mainly consists of amorphous silicon (SiO2·nH2O) that derives from skeletons of aquatic plants, called diatoms. Diatomite is abundant in many areas of the world and has unique physical characteristics, such as high permeability (0.1–10 mD) and porosity (35–65%) [16], small particle size, low thermal conductivity and density [17] and high surface area [18]. The properties of diatomite's surface, such as hydrophobia, solubility, charge, acidity, ion exchange and adsorption capabilities, are highly governed by the presence of water, which is partially structurally connected to the crystal mesh of the diatomite, forming active hydroxyl groups on it [19].

Diatomite has already been used for the adsorption of different elements and substances from water and wastewaters, either in its natural form (raw) or modified (chemically or thermally) (Table 1), presenting very promising and positive results. Therefore, its potential capability to adsorb common petroleum contaminants, such as BTEX and MTBE, under specific conditions, has been explored in this paper.

Section snippets

Adsorbent

The diatomite used in this study was obtained from the Kozani area in Northern Greece. According to the material provider, its mineralogical composition has been found to be: 22% Quartz, 13% Plagioclase, 1% Pyroxene (augite), 2% Hematite and 62% Total Clays (Muscovite: main phase 50–60%, Vermiculite: intermediate phase 20–30% and Smectite: small phase 5–20%). Its chemical composition mainly consists of 67% SiO2, 14.7% Al2O3, 5.1% Fe2O3, 3.3% MgO, 1.9%CaO, 1.5% K2O and 1.04% TiO2.

The diatomite

Adsorbents characteristics

The results obtained for the diatomite samples’ specific surface (BET) and pore volume distribution are presented in Table 2.

As can be concluded from Table 2, thermal treatment of the diatomite causes important changes in its specific area, as well as its pore volume distribution. More specifically, thermal treatment at 550 °C caused an increase in the specific, as well as the external surface, area of the diatomite of about 12.76% and 20.47%, respectively. This is probably attributed to the

Conclusions

The goal of this paper was to explore and present diatomite's capability to adsorb certain common petroleum contaminants. Therefore raw and thermally modified diatomite was tested for the adsorption of BTEX and MTBE from aqueous solutions. The results showed that thermally modified diatomite at 550 °C present the highest adsorption capacity for the majority of the examined contaminants. Diatomite samples thermally modified at 750 and 950 °C presented lower adsorption capacity, which is directly

Acknowledgement

Author M. Aivalioti would like to thank the Public Benefit Foundation “Alexander S. Onassis” for its financial support.

References (45)

  • B. Gao et al.

    Studies on the surface modification of diatomite with polyethyleneimine and trapping effect of the modified diatomite for phenol

    Appl. Surf. Sci.

    (2005)
  • P. Yuan et al.

    The hydroxyl species and acid sites on diatomite surface: a combined IR and Raman study

    Appl. Surf. Sci.

    (2004)
  • R.A. Shawabkeh et al.

    Experimental study and modeling of basic dye sorption by diatomaceous clay

    Appl. Clay Sci.

    (2003)
  • Z. Al-Qodah et al.

    Adsorption of methylene blue by acid and heat treated diatomaceous silica

    Desalination

    (2007)
  • A.A.M. Daifullah et al.

    Impact of surface characteristics of activated carbon on adsorption of BTEX

    Colloid. Surf. A

    (2003)
  • W.T. Tsai et al.

    Removal of herbicide paraquat from an aqueous solution by adsorption onto spent and treated diatomaceous earth

    Bioresour. Technol.

    (2005)
  • W.T. Tsai et al.

    Adsorption of bisphenol-A from aqueous solution onto minerals and carbon adsorbents

    J. Hazard. Mater.

    (2006)
  • M.A.M. Khraisheh et al.

    Effect of OH and silanol groups in the removal of dyes from aqueous solution using diatomite

    Water Res.

    (2005)
  • P.A. Quinlivan et al.

    Effects of activated carbon characteristics on the simultaneous adsorption of aqueous organic micropollutants and natural organic matter

    Water Res.

    (2005)
  • A. Ridha et al.

    Aqueous silver (I) adsorption on a low density moroccan silicate

    Ann. Chem. Sci. Mater.

    (1998)
  • M.A. Al-Ghouti et al.

    The removal of dyes from textile wastewater: a study of the physical characteristics and adsorption mechanisms of diatomaceous earth

    J. Environ. Manage.

    (2003)
  • Y. Yang et al.

    Adsorption properties for urokinase on local diatomite surface

    Appl. Surf. Sci.

    (2003)
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