Influence of solute-solute interactions on membrane filtration
Abstract
An understanding of solute-solute interactions is essential for aquatic systems as this
can affect the fate and behaviour of micropollutants in the environment and
engineered systems. Despite the importance of solute-solute interactions there is a
general lack of understanding which may be attributed to the fact that many
engineering models overlook solute-solute interactions and that the quantification of
such interactions is inherently difficult. When solute-solute interactions are
considered, they are often studied at unrepresentative concentrations and do not
consider the influence of organic matter type or solution chemistry. Steroidal
hormones, such as estradiol and estrone, were selected as model micropollutants as
they are ubiquitous in the aquatic environment due to constant introduction of
wastewater effluent, and can have implications for growth and development of
organisms including impaired fertility and behavioural abnormalities. The purpose of
this study was to develop a methodology to quantify solute-solute interactions at
environmental concentrations, and to determine the implications of such interactions
in membrane filtration.
A solid-phase microextraction (SPME) technique was developed to quantify solutesolute
interactions at environmental (low) concentrations. Using SPME, organic
matter-water partition coefficients (log KOM) were measured for a range of steroidal
hormones including estradiol, estrone, progesterone and testosterone with different
organic matter types such as humic acid. The dominant mechanism of hormoneorganic
matter interactions was identified as hydrogen bonding. In the case of estrone
and progesterone the log KOM values were significantly influenced by organic matter
type and concentration, as well as solution chemistry. No difference was observed
for estradiol and testosterone due to generally weaker sorption to organic matter.
Previous studies have indicated that the presence of organic matter can alter
micropollutant retention in membrane filtration. Much of the current literature
focuses on solute-membrane interactions, as the influence of solute-solute
interactions are typically difficult to determine in membrane filtration. Therefore, hormone-organic matter interactions were studied to determine if this interaction had
an influence on hormone removal by ultrafiltration (UF) using a range of molecular
weight cut-off (MWCO) membranes. The results indicated increased retention of
estrone in the presence of humic acid, while organic matter concentration and
solution chemistry influenced retention by affecting solute-solute interactions. The
findings of this study indicate the importance of solute-solute interactions in
membrane filtration and experimental log KOM results were used to quantify the
findings and elucidate the influences of 1) membrane sorption, 2) solute-solute
interactions and 3) solute-foulant interactions. Further, the removal of steroidal
hormones using a magnetic ion exchange (MIEX®) resin with a range of MWCO UF
membranes was studied as such sorbents can be used to improve micropollutant
removal in wastewater treatment. Greater removal with IX-UF was observed
compared to UF alone and the main hormone removal mechanisms were sorption to
MIEX® and solute-fouling interactions.
The findings of this study indicate that it is indeed possible to quantify solute-solute
interactions at environmental concentrations using SPME, with hydrogen bonding
being the main mechanism of interaction for steroidal hormones and organic matter.
Further, micropollutant retention by membrane filtration can be influenced by solutesolute
interactions.