Research papersVOC adsorption on raw and modified clay materials
Introduction
The emissions of volatile organic compounds (VOCs) are becoming one of the most stringent environmental challenges in many industrial processes. In fact, VOCs are known to be among the major contributors to the formation of photochemical ozone and secondary organic aerosol (SOA), which would result in health damage and serious environmental problems.
The abatement techniques of VOCs can be divided into two categories. The first one is the destructive techniques such as thermal oxidation and catalytic oxidation, which eliminate the undesirable compounds, and the second is the recuperative techniques such as adsorption, absorption, and condensation. In general, condensation is more effective than adsorption when VOC concentrations are relatively at higher levels (> 1%) (Khan and Ghoshal, 2000). Therefore, adsorption which is a reliable chemical engineering method is widely used thanks to the flexibility of the system, low energy and cheap operation costs (Serrano et al., 2004). This technique has been found to be effective at low concentration levels (ppm or sub-ppm) (Gupta and Verma, 2002). Up to now, only 10% of the industrial abatement units are based on adsorption. This percentage should, nevertheless, increase in the near future given the strict control over VOCs releases set by the regulations. This explains the current growing interest in the development of highly efficient industrial adsorbents for the treatment of VOC-loaded waste gases (Zaitan et al., 2008).
Activated carbon is also the most widespread alternative to eliminate the volatile organic compounds due to its developed microporosity which ensures good adsorption capacities (Manjare and Ghoshal, 2006). However, its application is limited by the sensitivity to high temperature and the difficulty of regeneration because of its thermal and chemical instability, causing significant safety problems. On the other hand, hydrophobic zeolites are on average ten times more expensive and they are sensitive to the presence of humidity (Zaitan et al., 2008). Hence, different alternative adsorbents are desirable to be developed to overcome these kinds of problems (Hu et al., 2009).
In addition to their use in various fields of the human life like agriculture and industry (pharmaceutical, cosmetic, ceramic, painting, rubber, plastic, etc.), the natural clays are very abundant materials with a low cost and porous texture. Such characteristics make them of a capital interest for a possible use in the adsorption of VOCs for their removal from waste gases.
The material exploited in the present study is taken from the Douiret formation of Tataouine in the south of Tunisia. The layer of such area has a lower Aptian age and belongs to the Merbah el Asfer group.
The objective of this work is to improve the adsorbent properties of the clay material by intercalation of organic molecules; the didodecyldimethyl ammonium bromide, in the porous texture. Indeed, cyclohexane, toluene and chlorobenzene were selected for the study of VOC adsorption since they are known to be among the most harmful VOCs.
Section snippets
Materials
The clay material used in this study is taken from deposits located in the south of Tunisia from the area of Douiret; which belongs to the Merbah el Asfer group. Such a group is settled in a basin limited by the Tebaga of Mednine mole in Southern Tunisia from the North and the Garian mole in Tripoli, Lybia, from the South.
It is situated in the heart of the Chotts anticlinal megastructure and along the large cretaceous cliff bordering the Dahar plate. Besides, it varies from few meters of
Chemical analysis
The crude clay composition is presented in Table 1. SiO2 and Al2O3 are the major constituents of the clay with other oxides present in trace amounts. The percentage of iron is relatively high which is typical of Tunisian clays (Benzina, 1990).
From the data in Table 1, the structural formula of Douiret clay is: .
Thermal analysis
Coupled diagrams of differential thermal analysis (DTA) and thermogravimetric analysis (TG) of the raw clay material are presented in Fig. 2. They reveal the
Conclusion
The adsorption isotherms were of type II of the B.E.T. classification therefore the adsorption was multi-layered. These isotherms prove that the raw and the Na-clay materials have a little affinity for the studied VOC adsorption. The adsorption isotherms of these VOCs on the organoclay show that the intercalation of didodecyldimethyl ammonium bromide can significantly improve the toluene, cyclohexane and chlorobenzene adsorption to reach 2, 2.5 and 3.5 mg g− 1 respectively, indicating that this
Acknowledgment
The authors would like to express their thanks to the Organic laboratory of Synthesis and Environment, University of the Littoral Coast of Opal, France, for the technical support.
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