The use of β-cyclodextrins to enhance the aqueous solubility of trichloroethylene and perchloroethylene and their removal from soil organic matter:: Effect of substituents

This paper describes a systematic study for the evaluation of different substituted beta-cyclodextrins (beta-CDs), as agents for the enhancement of the aqueous solubility of two major organic pollutants, trichloroethylene (TCE) and perchloroethylene (PCE). The aqueous solubility enhancement occurs through the formation of host-guest inclusion complexes between the CD molecule (host) and the polychloroethylene (guest) and is driven primarily by hydrophobic forces. The CDs evaluated are: methyl-beta-CD (Mbeta-CD), hydroxypropyl-beta-CD (HPbeta-CD), carboxymethyl-beta-CD (CMbeta-CD1, CMbeta-CD2), and sulfated-beta-CD (Sbeta-CD1, Sbeta-CD2); the degree of substitution was also varied. Using a 5% (w/v) aqueous CD solution, solubility enhancement factors (S-t/S-o) up to 5.5 and 14 were determined for TCE and PCE, respectively. Binding constants (K-11) for TCE with the substituted CDs were evaluated using an H-1 NMR technique; these were found to range from 3 to 120 M-1. It was shown that solubility enhancement, as well as the binding constant, is dependent on the type and degree of substitution of the beta-CD molecule; the determining factors are discussed. The CDs are also capable of effective removal of PCE and TCE retained by soil organic matter. Thus, a suitably substituted beta-CD may be a valuable additive in pump-and-treat protocols for site remediation of polychlorinated organics.