The contaminants of emerging concerns (CECs) are defined by the United States
Geological Survey (USGS) as: “any synthetic or naturally occurring chemical or any
microorganism that is not commonly monitored in the environment but has the potential
to enter the environment and cause known or suspected adverse ecological and/or
human health effects.” In other words, CECs are any substances that can be suspected to
cause harm. Basically, they are substances used in human activities every day for all
kinds of purposes. Because they are so numerous, ubiquitous and chemically different,
they are frequently divided into categories that describe their purpose, use or other
characteristics. Some common categories are: pharmaceuticals, personal care products,
agricultural runoff (pesticides, pathogens and fertilizers), fuel-based-compounds,
chlorinated solvents, flame retardants (perfluorinatedalkyl compounds), plasticizers,
dyes and endocrine disruptors.
CECs are continuously entering water sources throughout the world because of their
widespread use. Conventional wastewater and recycled water treatments are only
partially effective in their removal or degradation, so they are discharged into the
environment with treated wastewater effluent, recycled water and wastewater plant
sludge. Other sources of CECs include industrial wastewater effluents, untreated
wastewater from manufacturing facilities, landfill leachates, effluents from poultry
farms and animal feeding facilities where veterinary drugs may be used.
Effects of CECs on human and ecosystem health are largely unknown and relatively
little is known about the ways they travel through the environment or how they may be
transformed or degraded during their residence time in the environment. Some studies
have shown that even very low exposure to certain CECs can have impacts on
biological systems. CECs are generally present at very low concentration in natural
waters.
Therefore, it is important on one hand to develop analytical methods able to detect these
molecules in accurate way to better evaluate their impact on the environment, and on
the other hand to enhance the efficiency of water remediation technologies in order to
decrease their potential adverse effects on biota. Several methods have been reported to
be suitable for both of these purposes; sorption based technologies were demonstrated
to be efficient and economical methods both for the enrichment of trace pollutants and
for their removal from water.
Among the large number of inorganic adsorbents, zeolites were proven to be efficient in
removing organic compounds from environmental matrixes. Recently, mesoporous
silica materials have also received increasing interest because their properties (high
surface area, high pore volume, controlled pore size) make them promising as
adsorbents in contaminant removal processes. Moreover, due to the thermal and
chemical stability of these siliceous adsorbents, they can be considered environmentally
compatible.
The aim of the present work is to study the adsorption properties of microporous (Beta,
ZSM-5, Y and ferrierite zeolites) and mesoporous (MCM-41 and HMS) siliceous
materials towards various organic compounds in aqueous solutions. This investigation
has the dual purpose of evaluating the application of the selected adsorbents both in
remediation technologies of natural waters and as medium for solid phase extraction
systems of pre-concentration.
For what concerns the studied contaminants, four compounds (ketoprofen,
hydrochlorothiazide, atenolol, erythromycin) have been selected as members of CECs.
These four drugs belong to different therapeutic classes and have been detected in
natural waters of several countries because they are not efficiently removed by
conventional wastewaters treatments plants. Moreover they are characterized by
different physico-chemical properties as molecular dimensions, acid/base behaviour,
hydrophobicity, ecc. Perfluorooctanoic acid (PFOA) was selected because, due to its
high water-solubility, it has been frequently found in aquatic environment; moreover it
is extremely persistent and toxic. The selected organic pollutants were toluene and
methyl-tert-butyl ether (MTBE): they were chosen as representative of BTEX (benzene,
toluene, ethylbenzene, xylene) and fuel oxygenate compounds, respectively. Because of
its high water solubility and hence high bioavailability, MTBE belongs to the class of
emerging contaminants too.
In this thesis, chromatographic, thermogravimetric and diffractometric techniques were
employed to study the adsorption process in order to: 1) investigate the adsorptive
properties of the siliceous materials; 2) characterise their structures after the adsorption
of the selected contaminants; 3) localise the organic species in the zeolites channel
systems; 4) probe the interactions between organic molecules and framework oxygen
atoms; 5) characterize the kinetic of the adsorption process.
In particular, the thermodynamics and kinetics of the adsorption process of
contaminants on hydrophobic zeolites were studied by using complementary
techniques: chromatography and thermogravimetry. Chromatography was mainly used
to measure the adsorption isotherms of the studied compounds. The adsorption isotherm
is useful for representing the capacity of a zeolite for adsorbing organics from water,
and in providing description of the functional dependence of capacity on the
concentration of pollutants. Experimental determination of the isotherm allows for
evaluating the feasibility of adsorption for treatment, in selecting a zeolite and in
estimating adsorbent dosage requirements. Moreover, from isotherm parameters it is
possible to evaluate the adsorption energy distribution of the process. The abovementioned
techniques were also employed to investigate the kinetics of the adsorption.
Kinetics deals with changes in chemical properties in time and is concerned especially
with rates of changes: hence it plays a fundamental role in determine the proper contact
time for the removal of pollutant components from wastewater. To investigate the
adsorption mechanism, diffraction techniques were employed to localize the organic
adsorbed into the zeolite structure. The information gathered by this last investigation –
in cooperation with the Earth Science Department UNIFE - allow to define the
interactions between organic molecules and zeolite framework.
The experimental data revealed that the amount of the organic pollutant embedded
inside the framework was influenced by the lattice structure, the hydrophobicity
(SiO2/Al2O3 ratio: SAR) and the thermal treatments of the adsorbent. For mesoporous
silica materials, an important role is also played by the procedure of template removal
(thermal treatment or solvent extraction).
Both hydrophobic and electrostatic interactions were demonstrated to contribute to the
adsorption process: in fact, hydrophobicity and dissociation constant of the solute
strongly affect the adsorption.
In many cases the selected molecule was proved to be adsorbed inside the framework of
the siliceous materials. Generally, it was also proved that the adsorption process was
very fast in all the studied zeolites towards several classes of pollutants. Also the
adsorption kinetic of PFOA on mesoporous silica materials was satisfying if compared
with literature data dealing with other types of adsorbents.
To investigate the possible competition of natural organic matter towards contaminants
adsorption, the effect of two lignin-derived phenolic monomers (caffeic acid and parahydroxybenzaldheyde)
with molecular dimensions comparable to those of the pores of
the adsorbent material on the adsorption properties of zeolites was considered. This last
part of the work of thesis is a fraction of a wider project whose purpose is to study the
interaction and mobility of groundwater pollutants adsorbed in zeolite pores in order to
improve the efficiency of permeable reactive barriers. This project involves Ferrara,
Bologna and Piemonte Orientale Universities with the financial and scientific support of
ENI Research Center of San Donato Milanese.
The results revealed that zeolites are selective adsorbents for organic pollutants. In fact,
it was demonstrated that toluene is preferentially and almost exclusively adsorbed from
mixtures of toluene and humic acid monomers in aqueous solutions.
In conclusion, favourable adsorption kinetics along with the effective and selective
adsorption of contaminants into zeolites and mesoporous siliceous materials make these
cheap and environmentally-friendly materials a tool with interesting applications for the
removal or enrichment of organic pollutants from contaminated waters