Volatile Compounds Trapping

VOCs are widely used in industry as solvents, dry-cleaning agents, degreas-ers, and intermediates in the production of plastics, synthetic resins or pharmaceuticals. From an environmental point of view, it is necessary to limit and control their emissions because they affect the change of climate, the growth and decay of plants, and the health of human beings and all animals (Khan and Ghoshal 2000).

New regulations regarding volatile organic compounds emissions demand more efficient and less costly technologies (Liebscher 2000). There are many techniques available to control VOCs emission (destruction based and recovery based) with many advantages and limitations. Sorption is used to remove VOCs from gas streams by contacting the contaminated air with a solid (adsorption) or a liquid solvent (absorption) with high removal efficiency. CDs were used as solid support in order to enhance the adsorption capacity of sorbents or as aqueous solution to improve the transfer between gaseous and aqueous phase during the absorption process.

One of the first studies investigating the use of solid support modified by CDs in the sorption of organic contaminants in air showed that the trapping of low-molecular weight organic vapors by b-CD polyurethane resins were improved compared to polyurethane resins containing no CD units (Mizobuchi et al. 1981).

Butterfield et al. (1996) have shown that solid b-CD spread over a glass fiber filter could extract volatile PAHs from air by the formation of inclusion complexes. b-CD has been also immobilized on PEGylated Merrifield resins to study the trapping capacity of this new material for a mixture of VOCs (Siu et al. 2005). The resin modified by b-CD absorbs 60.1% of the VOCs against 29.3% for the unmodified. Considering relatively low % loading of CD, these values indicate a high efficiency of trapping.

Szejtli (19895 suggests the use of aqueous solutions of CD to scrub gaseous effluent, but very few studies were done on this subject (Uemasu et al. 1996; Blach et al. 2008) . Uemasu et al. used aqueous solutions of branched b-CD to remove volatile chlorinated hydrocarbons. Blach et al. used a-CD, b-CD and b-CD derivatives e.g. RAMEB, CRYSMEB, HPBCD, SBE to trap toluene and showed that temperature, concentration of the waste gas and CD concentration have an influence

By pass

By pass

Fig. 2.7 Experimental setting for the study of dynamic absorption (Blach et al. 2008)
Table 2.5 Henry's law constants (Hc, dimensionless) and formation constants (Kf, M ') of toluene at 30°C determined by static headspace gas chromatography

h2o

[CD] M

a-CD

b-CD

HPBCD

CRYSMEB

RAMEB

4-SBE-b-CD

H

0.268

0.01

0.198

0.086

0.107

0.115

0.090

0.131

0.05

-

-

0.031

0.035

0.025

0.048

Kf

-

-

38

142

163

165

171

155

on the absorption capacities of the absorbent. In both studies, production of solid support was not observed and regeneration of used absorbent can be achieved by heating the solution. The experimental setting used by Blach et al. is presented in Fig. 2.7.

The trapping of gaseous radioactive iodine, from the fission of uranium 235 isotope, was investigated by using aqueous solution of a-, b- and g-CD as well as their methylated analogs and insoluble crosslinked polymers (Szente et al. 1999). The methylated a-CD leads to a 200-fold increase in the aqueous solubility of iodine, the methylated b-CD to a 100-fold increase while the methylated g-CD does not provide iodine binding.

The immobilization of VOCs in aqueous CDs solutions were also investigated by using static headspace gas chromatography analysis for single or multiple contaminants (Saito et al. 2003; Yang et al. 2006; Fourmentin et al. 2007; Blach et al. 2008; Gao et al. 2009) . Henry's law constants (Hc) as well as stability constants were determined for different pollutants. In all cases, Hc decrease with increasing CD concentration (see Table 2.5). This behavior should be directly linked to the formation of an inclusion complex between the CDs and toluene, improving the apparent solubility of toluene. These data have demonstrated that the volatility of toluene could be greatly reduced in the presence of b-CDs, in comparison with a-CD. The use of RAMEB at a concentration of 0.05 M leads to a reduction of volatility up to 95%.

More recently, soluble and insoluble crosslinked CD polymers were synthesized by using a low methylated-P-CD (CRYSMEB) and epichlorohydrin with and without guest as template and were used to trap toluene in the gas phase (Mallard-Favier et al. 2011) . Insoluble imprinted polymers trapped more efficiently toluene than insoluble polymers in gas phase and in water, showing that polymerization in the presence of toluene limits the blocking of CD cavities.

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