IN-SITU SPECTROSCOPIC INVESTIGATIONS OF MOLECULAR MECHANISMS ENABLING SORPTION OF DIOXINS AND PCBS BY SMECTITE CLAYS

Description

<p>Dioxins and poly-chlorinated biphenyl (PCB)
compounds are high priority organic pollutants which are similar in structure
and well known for their toxicity, bioaccumulation and persistence in the
environment. Dioxins and PCBs have a high affinity for certain types of clay
minerals. However, the molecular mechanism for the observed high affinity of
these compounds to clay minerals is not well understood and has been the main
focus of this research work. The mechanisms
that govern dioxin-clay and PCB-clay interactions were investigated from two
perspectives. First, the influence of selected properties clay minerals on
dioxin sorption was investigated via in-situ spectroscopic techniques (ATR-FTIRand
Raman) structural (XRD) and macroscopic batch sorption methods using
dibenzo-p-dioxin (DD) as a model solute. Second, the influence of solute
properties, especially position and degree of chlorination and molecular
planarity, on sorption was investigated.</p>

<p> Smectites,
especially, Cs-saponite effectively adsorbs dibenzo-p-dioxin (DD) from water
with values reaching 10,000 mg kg^-1, or one weight percent which
greatly exceeds that by other naturally occurring sorbents such as soil organic
matter. Adsorption was promoted by clay interlayer exchangeable cations with
low hydration energies, and by smectites in which negative charge in the
smectite originate from the tetrahedral siloxane sheets. IR-active bands of DD
sorbed to saponite in the 1280 to 1500 cm^-1region were perturbed
compared to the ‘reference’ IR spectra. Combined batch sorption, XRD and
spectroscopic data confirm that the intercalation of DD occurred in the clay interlayer
and site specific interactions occur between DD molecule and Cs⁺
cation. </p>

<p> Sorption
of 1-chloro-dibenzo-p-dioxin (1-ClDD) and 2-chloro-dibenzo-p-dioxin (2-ClDD) on
homoionic (Na⁺, K⁺, Rb⁺, and Cs⁺) smectites
was evaluated to explore the effect of chlorine substitution position (and
steric hindrance) of dioxin on sorption mechanisms. Similar to DD, adsorption
was influenced by the hydration energy of exchangeable cations and the origin
of negative charge in the smectite. XRD measurements revealed that 1-ClDD
molecules were oriented nearly parallel to the siloxane surface of the clay
while 2-ClDD adopted a tilted orientation, similar to DD. The location of the
chlorine constituent in 1-ClDD prevents the molecule from its apparent
energetically more favorable orientation. In-situ ATR-FTIR spectra revealed
that sorption of 1-ClDD to Cs-saponite resulted in the loss of interfacial H₂O
and suggested that the sorption 1-ClDD displaces interlayer H₂O and
2-ClDD is less sterically restricted in the clay interlayer. </p>

<p> Sorption
of three dioxins (DD, 1-ClDD and 2-ClDD) was compared to three PCBs (PCB-1,
PCB-4, and PCB47) with similar octanol-partition coefficients (log K_ow)
but varying molecular planarity and degree of chlorination onto Cs-saponite,
which was shown to be representative of other smectites, revealed that despite
having similar structure and hydrophobicity, dioxins have higher affinity for
smectites than PCBs. Sorption studies also showed that sorption of PCBs is
influenced by molecular planarity and hydrophobicity. Polarizability and dipole-moment
were identified as important solute properties that affect the sorption
behavior of dioxins and PCBSs. Linear relationships between these properties
and log K_f’(subcooled liquid solubility normalized Freundlich sorption coefficient) values
suggest that high sorption affinity of planar dioxins could be due to a
combination of <i>Van der Waals</i>
interactions with the siloxane surface, steric effects, and site-specific
interactions between dioxin and exchangeable cations. In contrast, the sorption
of PCBs was highly influenced by their molecular orientation.</p>

Links & Downloads

1. 10.25394/pgs.8048189.v1

Tags

Agronomy

smectite clay minerals

dioxin concentrations

spectroscopy examination

FTIR methodology

Raman spectroscopy

Additional Fields

Identifer	oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8048189
Date	10 June 2019
Creators	Kiran R. Bangari (5929499)
Source Sets	Purdue University
Detected Language	English
Type	Text, Thesis
Rights	CC BY 4.0
Relation	https://figshare.com/articles/IN-SITU_SPECTROSCOPIC_INVESTIGATIONS_OF_MOLECULAR_MECHANISMS_ENABLING_SORPTION_OF_DIOXINS_AND_PCBS_BY_SMECTITE_CLAYS/8048189