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|Title: ||Removal of adsorbing estrogenic micropollutants by nanofiltration membranes in cross-flow – experiments and model development|
|Authors: ||Semião, Andrea J.C.|
|Supervisor(s): ||Schäfer, Andrea|
|Issue Date: ||22-Nov-2011|
|Publisher: ||The University of Edinburgh|
|Abstract: ||Nanofiltration (NF) can be used in water and wastewater treatment as well
as water recycling applications, removing micropollutants such as hormones. Due to
their potential health risk it is vital to understand their removal mechanisms by NF
membranes aiming at improving and developing more effective and efficient
Although NF should be effective and efficient in removing small molecular
sized compounds such as hormones, the occurrence of adsorption onto polymeric
membranes results in performances difficult to predict and with reduced
effectiveness and efficiency. This study aims firstly at defining, understanding and
quantifying the relevant filtration operation parameters and, secondly, in identifying
the physical mechanisms of momentum and mass transfer controlling the adsorption
and transport of hormones onto polymeric NF membranes in cross-flow mode. The
hormones estrone (E1) and 17-b-estradiol (E2) were chosen as they have very high
endocrine disrupting potency. The NF membranes used and tested were the NF 270,
NF 90, BW30, TFC-SR2 and TFC-SR3 since they have a wide span of pore sizes.
The first step is to experimentally acquire the knowledge of how fluid flow
hydrodynamics and mass transfer close to the membrane affect hormone adsorption.
The focus will be particularly on the effect of operating pressure, circulating
Reynolds numbers (based on channel height, Reh) and hormone feed concentration.
These hydrodynamic parameters play an important role in concentration polarisation
development at the membrane surface.
A Reh increase from 400 to 1400 for the NF 270 membrane caused the total
mass adsorbed of E1 and E2 to decrease from 1.5 to 1.3 ng.cm-2 and 0.7 to 0.5
ng.cm-2, respectively. In contrast, a pressure increase from 5 to 15 bar yielded an
increase in the adsorbed mass of E1 and E2 from 1.0 to 1.8 ng.cm-2 and 0.5 to 0.7
ng.cm-2, respectively. Moreover, increasing hormone feed concentration caused an
increase in the mass adsorbed for both hormones. These observations led to the
conclusion that adsorption is governed by the initial concentration at the membrane
surface which, in turn, depends on the hormone feed concentration, operating Reh
and pressure. Membrane retention, however, depends on the initial polarisation modulus, defined as the ratio between the initial concentration at the membrane
surface and the initial feed concentration.
The same trends were obtained for the TFC-SR2 membrane. However, this
membrane has a much lower permeability compared to the NF 270 one (7.2 vs 17
L.h-1.m-2.bar-1, respectively) and concentration polarisation is less severe. The
experimental variations in mass adsorbed and retention as a function of the operating
filtration parameters (Reh and pressure) were therefore lower.
Based on these experimental results, a sorption model was developed. This
model predicts well both feed and permeate transient concentrations for both
hormones and membranes (NF 270 and TFC-SR2) in the common range of operating
pressures and Reh of spiral-wound membrane modules. The model was further
applied for E2 in the presence of background electrolyte, yielding good predictions.
These findings are an important advancement in determining which membrane
would be more suitable to effectively remove hormones with a substantial reduction
of experimental work.
The above-mentioned developed model does not give insight into the
phenomena occurring inside the membrane since it focuses on the feed conditions.
However, membrane characteristics, such as material and pore radius were found to
have an impact in adsorption and retention of hormones. It was found experimentally
that polyamide, from which the active layer of the NF membranes is made, adsorbs
three times more mass of hormone than any other polymers constituting the
membranes. Since this active layer is the membrane selective barrier of the
membrane that is in contact with the largest hormone concentration (due to
concentration polarization in the feed solution) it is concluded that the active layer
adsorbs most of the hormones. Further experimental work carried out in this thesis
showed that increasing the pore radius from 0.32 nm to 0.52 nm increased the E2
mass adsorbed from 0.17 ng.cm-2 to 1.1 ng.cm-2 and decreased the retention from
88% to 34%. These results show that the wider the pore, the larger the quantity of
hormone that penetrates (i.e. partitions) inside the membrane and, therefore, the more
the membrane adsorbs the hormone. For membranes of similar pore radius, the
membrane with larger internal surface area was found to adsorb more. All the previous results led to the establishment of a new model for the
hormone transport inside the membrane pore taking convection, diffusion and
adsorption into account. Since the differential equation describing the transport with
adsorption inside the pore has no analytical solution, a numerical model based on the
finite-difference approach was applied. With such a model, its validation against
experiments and parametric studies it was possible to understand the transport
mechanisms of adsorbing hormones through NF membranes. The results show that
for low pressures the hormone transport is diffusion dominated. In contrast, for
higher pressures (above 11 bar) the transport is convection dominated, showing that a
purely diffusion transport model does not describe well the actual transport
phenomena of hormones in NF membranes.
Furthermore, it was found that two similar molecules can behave very
differently in terms of adsorption on the membrane. E1, which adsorbs 20% more
than E2 in static mode, being slightly smaller than E2, partitions more inside the
membrane pore and adsorbs double under filtration conditions.
This study contributes to illuminating the adsorption mechanisms of
hormones onto NF membranes by understanding what parameters control adsorption
such as hydrodynamics, materials, structure, etc. This not only identifies a potential
problem in large scale applications, but it also provides an understanding of the
mechanisms involved in the removal of these hormones and a tool that can be used to
design future membranes for the improvement of micropollutant removal.|
fluid flow hydrodynamics
|Appears in Collections:||Engineering thesis and dissertation collection|
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