Membrane fouling by extracellular polymeric substances after
ozone pre-treatment: Variation of nano-particles size
Wenzheng Yua*, Dizhong Zhanga, and Nigel J. D. Grahama*
a Department of Civil and Environmental Engineering, Imperial
College London, South Kensington Campus, London SW7 2AZ, UK.
([email protected], [email protected], and
[email protected])
*Corresponding author: Tel: +44 2075946121, Fax: +44
2075945934
Abstract:
The application of ozone pre-treatment for ultrafiltration (UF)
in drinking water treatment has been studied for more than 10
years, but its performance in mitigating or exacerbating membrane
fouling has been inconclusive, and sometimes contradictory. To help
explain this, our study considers the significance of the influent
organic matter and its interaction with ozone on membrane fouling,
using solutions of two representative types of extracellular
polymeric substances (EPS), alginate and bovine serum albumin
(BSA), and samples of surface water. The results show that at
typical ozone doses there is no measurable mineralization of
alginate and BSA, but substantial changes in their structure and an
increase in the size of nano-particle aggregates
(micro-flocculation). The impact of ozonation on membrane fouling,
as indicated by the membrane flux, was markedly different for the
two types of EPS and found to be related to the size of the
nano-particle aggregates formed in comparison with the UF pore
size. Thus, for BSA, ozonation created aggregate sizes similar to
the UF pore size (100k Dalton) which led to an increase in fouling.
In contrast, ozonation of alginate created the nano-particle
aggregates greater than the UF pore size, giving reduced membrane
fouling/greater flux. For solutions containing a mixture of the two
species of EPS the overall impact of ozonation on UF performance
depends on the relative proportion of each, and the ozone dose, and
the variable behavior has been demonstrated by the surface water.
These results provide new information about the role of
nano-particle aggregate size in explaining the reported ambiguity
over the benefits of applying ozone as pre-treatment for
ultrafiltration.
Keywords: ultrafiltration; membrane fouling; ozone;
microflocculation; EPS; surface water
1 Introduction
Membrane technology has been used in water treatment for more
than 20 years, and will be one of the most important treatment
technologies in the future for drinking water, waste water and sea
water applications (Shannon et al., 2008; Elimelech and Phillip,
2011; Logan and Elimelech, 2012). However, the phenomenon of
membrane fouling is still a major limitation that affects the
selection and operation of membrane processes, and some fouling is
considered inevitable for situations such as large drinking water
treatment plants (Touffet et al., 2015). In many cases, some forms
of pre-treatment have been used to control membrane fouling, such
as coagulation (Liu et al., 2011a), adsorption (Pramanik et al.,
2015) and oxidation (de Velasquez et al., 2013), by removing
soluble and particulate contaminants. Among such contaminants,
bacteria and associated extracellular polymeric substances (EPS) /
biopolymers are particularly influential in causing reversible and
irreversible fouling after a long period of ultrafiltration (UF)
operation, and their removal or avoidance is an effective method of
controlling UF fouling (Yu et al., 2014).
The source of much of the EPS / biopolymers comes from the
substantial numbers of bacteria present in most surface waters,
which are impacted by effluent discharges and overflows, and land
runoff; for example the northwestern Mediterranean area (Gonzalez
et al., 2008). Such bacteria and associated biopolymers inevitably
exist on the surface of flocs or on the membrane surface in
drinking water treatment systems (Nguyen et al., 2012). There is a
strong adhesive force between the biopolymers and membrane/filter
cake (Myat et al., 2014a). Polysaccharides were identified as
dominant foulants in UF and nanofiltration (NF) treatment of
surface water (Amy and Cho, 1999), even though polysaccharide
concentrations in surface waters were comparatively low. The
presence of EPS may affect floc deposition on the membrane surface
and subsequently affect the biofouling propensity of the membrane
(Herzberg et al., 2009; Chen et al., 2014).
A range of specific oxidation methods have been widely studied
and applied in drinking water treatment, some of which have been
considered as a pre-treatment for membrane processes; these include
ozone (Liu et al., 2011b), chlorine (Yu et al., 2014), chlorine
dioxide, hydrogen peroxide, potassium permanganate (Lu et al.,
2015), and electrochemical oxidation (Qi et al., 2015). However,
most of these have undesirable secondary effects in their use; for
example, chlorine is associated with the production of halogenated
by-products, and permanganate increases sludge production and the
risk of elevated, residual Mn concentrations. In contrast, ozone
produces relatively less side-effects and can have beneficial
effects such as the destabilization of particles, polymerization of
dissolved organics and algae flocculation; the various potential
mechanisms of interaction by ozone have been reviewed elsewhere
(Jekel, 1994).
The application of ozone pre-treatment to membrane processes has
been considered by several researchers previously (Liu et al.,
2011b; Park et al., 2012) and it is evident from these studies that
there is no clear consensus that ozone mitigates membrane fouling.
Some researchers found that when applied solely as a pretreatment,
ozone was able to reduce membrane fouling (de Velasquez et al.,
2013). Among possible reasons for this was that ozonation caused a
significant degradation of biopolymers that led to a lower
reduction in flux for both UF and microfiltration (MF) filtration
systems (Filloux et al., 2012). Also, the combination of ozone with
ceramic membrane filtration was found to decrease fouling (by
around 25%) (Stylianou et al., 2015). Several researchers have also
explained that ozone treatment was effective at degrading colloidal
natural organic matter (or biogenic colloids) which were most
likely responsible for the majority of membrane fouling (Lehman and
Liu, 2009; Barry et al., 2014). A further study has shown that O3
oxidation caused a significant alleviation of membrane fouling for
all investigated NF membranes in drinking water treatment (Van
Geluwe et al., 2011). It was considered that this was caused by the
selective removal of unsaturated bonds and hydrophobic components
in the dissolved organic matter, and by the decomposition of
molecular chains into smaller fragments by O3 (Van Geluwe et al.,
2011; Barry et al., 2014). However, it is possible that the
degradation of organic matter can result in products (e.g.
bio-polymers) of a molecular size similar to the size of NF
membrane pores, which can cause significant membrane fouling. Also,
some researchers found that pre-ozonation may aggravate membrane
fouling (Zhu et al., 2010). Recently, problems (membrane fouling,
and safety) associated with applying ozonation before
ultrafiltration (UF) led to the processes being out of service for
an extended period of maintenance when treating surface water from
Lake Ontario (Siembida-Losch et al., 2015).
The results of previous studies cannot be generalized because
the effects of ozonation are likely to vary with the nature of the
source water DOC and other factors (Tobiason et al., 1990).
However, the application of ozone to raw water, prior to the
addition of coagulants and coagulant aides, was shown to reduce
coagulant and coagulant aid doses; small ozone doses significantly
improved effluent quality, and an increase in the flocs' settling
velocity due to a larger average size was found in wastewater
treatment (de Velasquez et al., 1998; Jasim et al., 2008).
Therefore, the characteristics of organic matter in water after
ozonation are likely to be altered and some products may act as
polymer/flocculation aids, and thus improve the coagulation
efficiency, as indicated previously (Jekel, 1994).
In this paper we examine the reasons for the variation in
membrane performance when ozone is applied as a pre-treatment. This
involved laboratory tests designed to elucidate the fundamental
interaction between two common types of EPS, polysaccharide
(specifically, alginate) and protein (specifically, bovine serum
albumin), and two representative UF membranes, with and without
ozone pre-treatment, and with the EPS separately and mixed. The
results, described subsequently, indicate that ozone has different
impacts on the EPS/UF interaction arising from the relative sizes
of the EPS products from ozonation and the UF pore size, which can
explain the contradictory behavior of ozonation on membrane
fouling, and the results were also confirmed by the surface
water.
2 Materials and methods
2.1 Test solutions
Sodium alginate (A18565, Alfa Aesar, UK) and bovine serum
albumin (BSA, Sigma, USA) were obtained as reagent grade chemicals.
Fresh solutions at a total concentration of 10 g/L were prepared
using deionized (DI) water. Sodium alginate was easy to dissolved
in the DI water, and the BSA solution was prepared by dissolving
BSA in 0.1 M phosphate buffer solution (PBS) (Ma et al., 2014). The
stock solution was stored in the dark at 4 oC and was brought to
room temperature prior to use in tests before working solutions
were made, and the stock solution was used within 3 days. The mixed
alginate/BSA solution was prepared with 5 mg/L alginate and 5 mg/L
BSA. Samples of surface water were obtained from a nearby
recreational lake in west-central London; the lake is of moderate
quality and subject to algal growth and contamination from aquatic
animals. All chemicals used in the tests were analytical regent
grade.
2.2 UF experiments
Short period (5-8 min), dead-end flow experiments were
undertaken using flat sheet UF membranes in a stirred cell (Amicon
8400, Millipore) with a constant upstream pressure (0.1 MPa) under
nitrogen gas. The fouling characteristics of the UF membrane were
studied by applying dilute (10 mg/L) solutions of alginate and BSA,
or samples of surface water. Before each experiment the alginate or
BSA solution was diluted in DI water (300 ml), with 5 mM NaHCO3, to
give the test solution. Then the pH of the final solution was
adjusted and maintained at 7.0 by adding either 0.1 M NaOH or 0.1 M
HCl. Subsequently, the alginate solution, BSA solution or surface
water were exposed to a range of ozone doses between 0 to 1 mg/L in
a close glass bottle. The ozone was added in gaseous form
(generated from air) and the dose was determined by the difference
on ozone concentration entering and leaving the test solution; the
ozone concentration was determined by passing the gaseous inlet and
outlet flows through potassium iodide solution, and employing the
potassium iodide/thiosulfate titration method, in accordance with
APHA Standard Methods (APHA, 2005).
Two representative types of PVDF ultrafiltration membrane (from
Millipore, USA, and Ande membrane separation technology &
engineering (Beijing) Co., Ltd, China) were used in these tests to
ensure consistency of the results, both with a nominal molecular
weight cutoff of 100 kDa (~ 10-20 nm). The zeta potential and
contact angle of the Millipore membrane and Chinese membrane were
determined as -56 mV and 35.2o±4.9o, and -42 mV and 56.3o±3.8o,
respectively. Prior to use each membrane was placed in DI water for
at least 24 h to remove impurities and production residues.
Immediately before the stirred cell test the DI water flux of the
membrane was determined by passing DI water through the membrane
until a stable permeate flux was reached. After filtration with
water samples by 1 bar for 300 mL, UF membrane was put at the
opposite side, and washed by DI water (25 mL) with 1 bar
(backwash). After that, the membrane was put as previous side and
then filtrated with 300 mL water samples (1 bar). In each test the
performance of the membrane was evaluated by recording the
variation of normalized flux, J/J0, as a function of time, where J0
is the initial membrane flux.
2.3 HPSEC
As indicated by previous researchers (Myat et al., 2014b), size
exclusion chromatography with UV detection (SEC-UV254) can be used
to identify biopolymers that are sensitive to UV absorbance, such
as BSA (but not alginate). SEC was carried out to determine the
apparent molecular weight (MW) distribution of UV-active substances
in the waters. High performance SEC (HPSEC) was performed using an
HPLC system (Perkin Elmer, USA) carried out with the following
components: a BIOSEP-SEC-S3000 column (Phenomenex, UK) (7.8 mm×300
mm), together with a Security Guard column fixed with a GFC-3000
disc 4 mm (ID), a Series 200 pump, a UV/VIS detector operated at a
wavelength λ=254 nm and auto-sampler. A solution of 10 mM sodium
acetate (Aldrich, USA) was used as the mobile phase with the flow
rate set at 1 mL/min, and the injection volume of water samples was
100 μL. Prior to operation, the mobile phase was purged at a
volumetric flow rate of 2 ml/min in order to clear any residual and
wash the column of any contaminants. Polystyrene sulfonate (PSS)
standards (American Polymer Standard Corp., U.S.) of molecular
weights 555000, 126700, 15450, and 1690 Dalton were employed to
calibrate the relationship between compound MW and the peak
retention time.
2.4 Other analytical methods
The UV absorbance at 254 nm, UV254, of 0.45 μm filtered
solutions was determined by an ultraviolet/visible
spectrophotometer (U-3010, Hitachi High Technologies Co., Japan).
Dissolved organic carbon (DOC) was determined with a total organic
carbon (TOC) analyzer (TOC-VCPH, Shimadzu, Japan). The nature of
the materials on the fouled membranes was analysed by fourier
transform infrared spectroscopy (FTIR, Spectrum 400, PerkinElmer,
USA) with Quest ATR Accessory (SPECAC Ltd, UK).
Hydrophilicity/hydrophobicity of the membranes was estimated by
measuring water contact angles with the help of a contact angle
system plus manufactured by Data physics using the software
DROPimage Advanced (Ramé-hart instrument Co., USA). Particle size
and zeta potential were measured by a Zeta Sizer instrument
(Nano-ZS90, Malvern Instruments Ltd, UK).
3 Results and Discussion
3.1 Membrane flux and backwash process
In order to investigate the impact of the ozonation of alginate
and BSA solutions on UF membrane fouling, different doses of ozone
were added to the feed water at pH 7.0 with 10 mg/L alginate or
BSA, respectively (Figure 1). Unreacted alginate has a nominal size
very similar to the UF pores, so a substantial extent of pore
blockage was expected and a corresponding severe flux decrease
(~70%) was duly observed. The application of ozone to the alginate
systematically increased the flux (Figure 1a), so that the decrease
of flux was only 23% with an ozone pre-treatment dose of 1 mg/L.
This behaviour is discussed subsequently by reference to changes in
the MW/physical sizes of the EPS as a consequence of ozone
oxidation.
For BSA the results were markedly different (Figure 1b). There
was little decrease of membrane flux in the absence of ozone
treatment. However, in sharp contrast to alginate, the addition of
ozone reduced the flux (J/J0 value) systematically with ozone dose.
As the indicative size of BSA (< 10 nm) was smaller than the
pore size of the UF membrane (10~20 nm), it was expected that the
unreacted solution of BSA would have little impact on membrane
flux, as was observed. It was further expected that the addition of
ozone to the BSA would degrade the protein to smaller MW species,
which would also have little effect on membrane fouling (or higher
flux). However, the results contradicted this expectation, as will
be discussed subsequently.
The validity of the results for the UF membrane was confirmed by
repeating the test after backwashing the membrane, and identical
results were found; an example of these is shown in Figure S1.
Similarly, the tests were undertaken with an alternative PVDF UF
membrane with the same nominal pore size (100 kDa MW cut-off). It
was found that the membrane fouling behaviour of the other PVDF
membrane (sourced in China) was very similar to the Millipore PVDF
membrane presented here, although the magnitude of the membrane
fouling was greater (Figures S1c and S1d). The Millipore membrane
demonstrated a much lower irreversible fouling, and some of the
difference in the irreversible fouling with alginate between the
Millipore membrane (-56 mV and 35.2o±4.9o) and the Chinese membrane
(-42 mV and 56.3o±3.8o) may be related to the higher negative zeta
potential and hydrophilicity of the Millipore membrane. The
mechanism will be discussed later.
3.2 Zeta potential
Previous results have shown that ozone is capable of degrading
or modifying proteins (Sharma and Graham, 2010) and polysaccharides
(Song et al., 2015); for the former, oxidation generally results in
changes in their folding ability and tertiary structures, and for
the latter breaking of the polymer strands and chemical
modification of the pyranose structure, such as for alginate
(Akhlaq et al., 1990). Figure 2 shows the measured zeta potentials
of alginate and BSA, individually and mixed, as a function of ozone
dose. The zeta potential of all solutions decreased as the ozone
dose increased, although the reduction for BSA was minor and much
less than for alginate. The zeta potential values of the mixed
solution (1:1 mass ratio) were, as expected, between those of the
individual alginate and BSA solutions. The results can be
attributed to changes in the chemical structure of the alginate and
BSA by the addition of ozone, for example C-OH to C=O moieties in
uronic acids, and polymer chain breakage. In principle, decreasing
the zeta potential of EPS nano-particles should enhance
electrostatic repulsion between the nano-particles and the membrane
surface (zeta potentials were -56 mV and -42 mV for the Millipore
and Chinese membranes, respectively), thereby reducing fouling
effects. The solution TOC concentration of alginate and BSA did not
change with increasing ozone dose (Figure 2), which showed that
there was no compound mineralisation during the ozone process.
These results are consistent with an earlier study by Pauls and
Thompson, who found that there was little TOC decrease in protein
resulting from the ozone treatment, for different representative
proteins (Pauls and Thompson, 1980).
3.3 Size distribution
The significance of ozone pre-treatment on the EPS types was
investigated by observing changes in the physical sizes of BSA and
alginate macromolecules with different ozone doses (Figure 3).
Molecular size is important since this determines, to a major
degree, the nature of the interaction between the influent EPS and
the UF membrane. The interaction can be in the form of a cake
layer, pore blocking or the adsorption/accumulation of organic
matter within the membrane pores. Where the size of the BSA or
alginate molecules, or ‘nano-particles’, is close to the diameter
of the membrane pores, it is likely to cause serious membrane
fouling. In contrast, nano-particles that are significantly smaller
than, or larger than, the pore size, are not likely to induce
significant membrane fouling. Figures 3a and 3b show the size
distributions of BSA nano-particles before and after membrane
filtration with ozone pre-treatment at different concentrations.
The peak value of the un-ozonated BSA size distribution was about
7.3 nm. The pre-membrane particle size increased with increasing
ozone dose, and the peak value of the particle size distribution
was 17.5 nm for the ozone dose of 1 mg/L. As the BSA particle size
distribution approaches to the nominal size of the membrane pores
(10~20 nm), a greater degree of membrane fouling would be expected
and this was observed (Figure 1b). Thus, pre-treatment with 1 mg/L
ozone caused approximately 45% reduction in membrane flux.
The size distribution of BSA after ultrafiltration was also
investigated to reveal which part of the BSA size distribution is
retained by the membrane pores. It is evident from Figure 3b that
the size distribution of BSA after ultrafiltration was in a narrow
range of 3.5 nm to 13 nm, for all ozone doses, and the ozonated
particles were only slightly greater than the un-ozonated BSA.
Comparing Figure 3a and 3b, there is little difference in the
un-ozonated BSA size distribution before filtration and after
filtration indicating no significant retention of BSA, and this is
consistent with only a slight reduction in membrane flux (Figure
1b). For 0.1 mg/L ozone, the larger size fraction of the BSA (12~15
nm) was retained in the membrane pores. As the size of BSA
nano-particles increased with ozone dose, the larger size fraction
of the BSA was retained by the membrane, especially for the size
fractions greater than 12 nm.
The majority of the fractions of BSA after 1 mg/L ozone were
larger than the membrane pores, and their retention correlated with
greater membrane fouling. This result may suggest some deformation
in the shape of BSA nano-particles as a consequence of ozonation
and/or physical compression when they enter the membrane pores. It
is speculated that the nano-particles may form an ellipsoid shape
with the smaller part able to enter and block the membrane pores,
thereby causing membrane fouling.
The effect of ozonation on the nano-particle size distribution
of alginate was also evaluated (Figure 3c). The average size of
un-ozonated alginate is approximately 25 nm, which is slightly
larger than the UF membrane pore size. As a consequence of this
alginate nano-particles would be expected to restrict and block the
membrane pores, leading to reduced membrane flux, as was observed
in the tests (Figure 1a). The application of ozone led to a
systematic increase in the size of alginate nano-particles with
ozone dose, both in terms of the mean size and distribution (Figure
3c), most likely through a process of micro-flocculation. The
formation of polysaccharide particles is an important pathway to
convert dissolved into particulate organic carbon during some
conditions, such as phytoplankton blooms, and can be described in
terms of aggregation kinetics (Engel et al., 2004). These larger
nano-particles, because of their size, have little impact on the
membrane pores and form a relatively porous cake layer on the
surface of the membrane. This reduces the trans-membrane pressure
difference and leads to a substantially increased flux, with the
porosity of the cake layer determining the membrane fouling.
The impact of ozonation on the changing nature of the alginate
nano-particles was also evident from the UV absorption of alginate
solutions, particularly between 190 nm and 240 nm. It can be seen
in Figure 3d that there was little UV absorption (> 200 nm) for
the un-ozonated solution, but UV absorbance increased with
ozonation. The systematic nature of the increase may be explained
by the effect of the increasing size of alginate aggregates (light
scattering), but also the increasing number of UV chromophores
(e.g. carbonyl groups) in the alginate products, with ozone dose.
Alginate has been shown to form hydrogel-like aggregates readily,
for example by adding Ca2+ (Zheng et al., 2016), and the presence
of carbonyl groups can induce the formation of aggregates
(Gomez-Ordonez and Ruperez, 2011).
3.4 MW variation
Further information about the changes in the nature of the
selected EPS with ozonation was obtained from the results of the
analysis by HPSEC. However, this was only possible for BSA as
alginate has a low UV absorbance at 254 nm (Figure 3d). For
un-ozonated BSA, a sharp MW peak near 60k Dalton was observed
(Figure 4a), which is similar to that reported elsewhere (67 kDa)
(Shiraiwa et al., 2011). After ozonation at the lowest dose, 0.1
mg/L, the peak absorbance substantially reduced and the MW
distribution broadened and shifted towards larger MW sizes. With
increasing ozone doses, the absorbance peak systematically reduced
and broadened, and moved to larger MW (Figure 4a). Additional peaks
were evident at 0.3 mg/L ozone dose, and a distinct peak
corresponding to a MW near 130k Dalton increased with ozone dose
between 0.5 and 1 mg/L. As the nominal pore size of the UF membrane
was ~ 100k Dalton, BSA aggregates of a size greater than this are
unable to pass through the membrane, and this was confirmed by the
SEC results of BSA products passing the membrane (Figure 4b).
Previous research showed that exposure of BSA to ozone induced the
formation of gel-phase products and increased their
viscosity,(Pauls and Thompson, 1980) which supports our results
that ozonation increased the size of BSA nano-particles. The
accumulation of BSA nano-particle aggregates equal to, or larger
than, 100k Dalton (Figure 4) is consistent with the physical size
measurements reported previously (Figure 3), and believed to be the
cause of the membrane fouling observed following ozonation.
In the 1980s, Fersht provided the first convincing experimental
evidence that hydrogen bonds contribute favorably to protein
stability (Fersht, 1987; Nisius and Grzesiek, 2012). Each N-HO
hydrogen bond can contribute about 5 kcal mol-1 to the stability of
a protein. However, for proteins in an aqueous environment the
effective energy of the system (proteins) with N-HO hydrogen bonds
with water molecules may be no more than about 2 kcal/mol. The OH
groups that are not hydrogen-bonded make a small favorable
contribution to protein stability (Pace, 2009). As hydrogen bonds
are readily broken and reformed, they determine alternative
conformations, and hence are also important for conformational
changes of proteins (Penner et al., 2014). In the tests reported
here, ozonation is probably the main cause of hydrogen bond
breakage, thereby inducing micro-flocculation of BSA
nano-particles.
Furthermore, the variation of the BSA structure could induce the
aggregation of protein (Tyedmers et al., 2010). There is a chance
that the exposed interaction surfaces are aggregation prone, thus
creating a risk of dysfunctional interactions (Gershenson et al.,
2014). The presence of unsaturated (double) bonds in the BSA, on
side branches, provides sites for ozonation to increase the number
of carbonyl groups through Criegee-type reactions (Criegee, 1975;
Geletneky and Berger, 1998):
(1)
In previous studies, the causative role of carbonylation in
inducing protein misfolding and aggregation was determined by
inducing carbonyl stress, which recapitulated the increased protein
aggregation observed (Schymkowitz and Rousseau, 2016; Tanase et
al., 2016). Carbonylated proteins accumulate progressively to form
visible aggregates (Erjavec et al., 2007).
Therefore, it can be concluded that the small dose of ozone
caused only minor changes in the dissolved organic matter (de
Velasquez et al., 2010), such as the amount and conformation of
organic substances (Becker and O'Melia, 2001), the conversion of
hydrophobic to hydrophilic fractions (Sadrnourmohamadi and
Gorczyca, 2015), and an increase in the concentration of oxygenated
functional groups, such as carboxylic acid. In addition, ozone may
reduce stabilizing organic polymer particles, and polymerize
meta-stable organics, leading to particle aggregation via bridging
reactions (Reckhow et al., 1986; Sadrnourmohamadi and Gorczyca,
2015).
3.5 FTIR analysis
Additional information regarding the impact of ozonation on
alginate and BSA solutions was provided by ATR-FTIR analysis.
Representative ATR-FTIR absorbance spectra of alginate and BSA
corresponding to the treatment with different ozone doses are
presented in Figure 5. For alginate, un-ozonated and ozonated,
a broad absorbance band centred at 3427.5 cm−1 was assigned to
hydrogen bonded O–H stretching vibrations. Weak signals around 2920
cm-1 and 2850 cm−1, assigned to C–H stretching
vibrations, were evident in the un-ozonated alginate but
disappeared as the ozone dose increased. All samples showed
asymmetric stretching of carboxylate O–C–O vibration at
1615.6 cm−1 (Leal et al., 2008; Salomonsen et al., 2008), and
the absorbance band at 1415.3 cm−1 may be due to C–OH
deformation vibration with a contribution of O–C–O symmetric
stretching vibration of the carboxylate group; these bands did not
change with ozonation. In contrast, with ozonation, an absorbance
band around 1690 cm−1 appeared and increased gradually
with ozone dose, caused possibly by oxidation of –C-H (2920 cm-1
and 2850 cm-1) to C-OH or COOH (1690 cm−1 (Salomonsen et al.,
2008; Xiao et al., 2014)), or C-OH to C=O, moieties by the addition
of ozone.
The alginate absorbance peak in the range of
1200–960 cm−1 shifted to a lower wavenumber (peak 1120
cm-1 to 1095 cm-1) with increasing ozone dose. The absorbance bands
at 1200–960 cm-1 are reported to be sensitive to skeletal
vibrations of the six membered (pyranose) ring of alginate
(Diaz-Visurraga et al., 2012). The band around
1120 cm−1 could be attributed to the C-OH stretching
vibration of alginate (Leal et al., 2008; Salomonsen et al., 2008;
Gomez-Ordonez and Ruperez, 2011) and breakage of the alginate C-O-C
chain links by ozonation may increase the formation of C-OH
groups.
In sharp contrast to alginate, the absorption peaks for BSA did
not display any detectable change as the ozone dose increased,
suggesting that the maximum ozone dose of 1 mg/L was unable to
cause significant changes to the chemical structure of the protein
(Figure 5b), although the complexity of the macro-molecular
structure may mask changes to its constituent compounds; thus, more
carbonyl groups are formed, but their peak is together with
carboxyl bonds (1680 cm-1). Therefore, the observed increase in the
size of BSA nano-particles with ozonation may be due to the
aggregation of these nano-particles through hydrogen bonding or
carbonyl stress.
3.6 Alginate and BSA mixture
As raw waters contain a variety of EPS, where the effect of
ozone on each may affect membrane performance differently, as
illustrated in this study, a mixture of alginate and BSA (in equal
5 mg/L concentrations) was investigated to observe their combined
effect on membrane fouling. Without ozonation the reduction in flux
(60%) of the mixture was similar to that for alginate (70%) (Figure
6a). As the size of alginate was similar to the pore size, pore
blockage should dominate the mechanism of membrane fouling firstly
for the mixture; after forming alginate fouling layer, BSA may be
trapped in alginate fouling layer, thus causing higher membrane
fouling. For the lowest ozone dose of 0.1 mg/L, the temporal flux
(J/J0) profile reduced slightly compared to the un-ozonated EPS
solution, but greater ozone doses increased the flux profile.
However, at the highest ozone dose of 1 mg/L the decrease of flux
was still substantial, approximately 40%, which was less than the
BSA alone but more than alginate alone (Figure 1). As discussed
above, the effects on the membrane flux are believed to be related
to the changes in the nano-particle size of alginate and BSA after
ozonation, and their size relative to the UF pore size. Figure 6b
shows the size distribution of the combined two species of EPS and
how they change with ozonation. It can be seen that there are
separate distributions for the two species of EPS, and the average
size of the membrane pores is between the size of un-ozonated BSA
and alginate. The quantity of EPS material within the pore size
region (10-20 nm) (i.e. region for pore fouling) is also determined
by the ozone dose. This is consistent with the observed changes in
flux (Figure 6a). From this it can be speculated what the impact on
fouling would be for different concentration ratios between
alginate and BSA. Thus, when the concentration of alginate is
greater than BSA, the membrane fouling will decrease with ozone
dose, while the opposite will apply if the concentration of BSA is
significantly greater than alginate.
Complementary information from SEC analyses was also used to
characterize the variation of combined alginate and BSA MW
distributions with different ozone doses (Figures 6c and 6d). As
alginate is not strongly UV absorbing at 254 nm, the SEC spectra
represents principally the BSA, and any UV absorbing products and
aggregates of BSA and alginate after ozonation. Comparing the
spectra in Figures 6c and 6d with those for BSA alone (Figure 4),
the respective spectra are very similar, but with lower absorbance
intensities for the former because of the lower concentration of
BSA (5 mg/L), which may indicate little interaction between the two
species of EPS as a consequence of ozonation. Overall, these
results show that the impact of ozonation on UF membrane fouling
may be either beneficial or detrimental depending on the relative
proportion of BSA and alginate, and their concentrations, with the
size of the EPS nano-particles after ozonation as the determining
factor.
3.7 Surface water
In order to validate the previous results obtained with model
compounds of proteins (BSA) and polysaccharides (alginate), similar
ozonation tests were undertaken with samples of surface water;
surface water contains many forms of organic matter, including a
range of proteins and polysaccharides, and humic substances. A
summary of the results is given in Figure 7, which showed that the
impact of ozonation was consistent with the model waters. In the
absence of ozonation the reduction in flux was about 75%, but for
the lowest ozone dose of 0.1 mg/L, the temporal flux (J/J0) profile
reduced slightly (77%) compared to the un-ozonated surface water
(75%), but with greater ozone doses the flux systematically
increased (Figure 7c). This flux variation was very similar to that
observed with the mixture of BSA and alginate (Figure 6a). To
consider this further the molecular weight distribution of the
organic matter in the surface water with different ozone
consumption was explored (Figure 7b). The results clearly show that
the surface water contains bio-polymers, humic substances and other
small MW organic matter, and a low dose of ozone (<0.1 mg/L)
increased the presence of large MW biopolymers. However, with
increasing ozone dose (>0.1 mg/L) the presence of large
bio-polymers and smaller MW organic matter, including humic
substances, decreased. Therefore, the effects on the membrane flux
are believed to be related to the changes in the size and contents
of bio-polymer/nano-particles after ozonation, and their size
relative to the UF pore size. It is concluded that the quantity of
biopolymer near 100k Dalton MW cut-off determined the membrane
fouling, which is consistent with the observed changes in flux
(Figure 7a). The results also indicate that the size of biopolymers
in the water determined the membrane fouling and that
representative proteins and polysaccharides could be useful
indicators of membrane behaviour.
When using UV254 to quantify the presence of organic matter in
the surface water, some constituent substances may be oxidized by
the ozone and their UV254 absorbance decreased. However, for
the case of humic-type substances these do not significantly affect
the membrane fouling, so only changes in the biopolymer fractions
are the main focus of discussion. While there may be some
uncertainty about changes in UV254 values following ozonation, the
size of biopolymers appeared to increase with ozone dose in the low
dose range (≤ 0.1 mg/L); this was evident by a slight shift in the
UV absorbance-MW distribution to higher MW (Figure 7b). At such low
ozone dose the ozone may break the polymers into small and large
products, and the larger products may then aggregate forming
micro-flocs with a size greater than the original biopolymers. It
is possible also that during this process, small molecular weight
substances could adsorb to the micro-aggregates, further increasing
their size. However, with increasing ozone dose it is assumed that
the breakdown of the organic matter under the effects of oxidation
would produce a general reduction in the MW distribution, as
suggested in Figure 7b.
Contrary to the general view among researchers that ozone
decreases the molecular weight of organic substances, we have shown
in this study that ozone reacts with biopolymer to produce
macromolecules/nano-particles of increased size with some dose.
Furthermore, the consequence of the increase in size makes the
biopolymer more likely to cause UF fouling. The implications of
these results are that similar effects are confirmed to occur in
real systems, which is why the reported performance of ozone has
been inconclusive or contradictory.
4 Conclusions
This study has investigated the impact of ozone on two
representative types of EPS and surface water in order to explain
inconsistencies in the literature concerning the benefits of ozone
pretreatment on ultrafiltration performance. The results have shown
that ozonation causes micro-flocculation of both BSA and alginate
(and some degradation), and revealed the importance of
nano-particle aggregate size as a critical factor that determines
the extent of pore blockage and fouling, and this phenomena was
confirmed by the tests with surface water. The specific findings of
this study are as follows:
1. Ozonation had measurable impacts on the BSA and alginate as
indicated by zeta potential (decreased), colloid size (increased)
and FTIR spectra. Since the ozone doses employed were relatively
small (≤ 1 mg/L), no mineralization was observed.
2. For BSA, membrane flux reduced systematically as the ozone
dose increased, while in contrast the opposite trend was observed
for alginate.
3. Both alginate and BSA nano-particle size increased with
increasing ozone dose, indicating a process of microflocculation.
The modal size of BSA nano-particles increased from 7.5 nm to 18.2
nm when the dose of ozone increased from 0 to 1 mg/L. The increased
modal size coincided approximately with (or was a little larger)
than the UF pore size, suggesting the strong likelihood of pore
blocking and fouling. For alginate the nano-particle size increased
from 20-30 nm (near the size of membrane pores) to much larger
sizes (≥ 80 nm) with ozone dose, indicating a diminishing
likelihood of pore fouling. In both cases the changing size
distributions were consistent with the observed changes in membrane
flux.
4. Changes in BSA nano-particle size observed by HPSEC were
consistent with the physical size measurements and confirmed the
formation of aggregates one order of magnitude greater in MW at the
highest ozone dose.
5. Ozone at low doses reacted with biopolymers in the surface
water to produce macromolecules/nano-particles of increased size
and this corresponded to a slight increase in UF fouling. At higher
ozone doses the MW of biopolymers reduced and this gave a
corresponding decrease in fouling. The implications of these
results are that the impact of ozone on UF fouling depends on both
the nature of the raw water and the ozone dose, and provide an
explanation as to why the previously reported performance of ozone
has been inconclusive or contradictory.
Acknowledgements
This research was supported by a Marie Curie International
Incoming Fellowship (FP7-PEOPLE-2012-IIF-328867) within the 7th
European Community Framework Programme for Dr Wenzheng Yu. This
work was also supported by the Engineering and Physical Sciences
Research Council from Great Britain (grant
number EP/N010124/1).
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Captions
Figure 1 Temporal variation of relative membrane flux with ozone
pretreatment for Alginate (a) and BSA (b)
Figure 2 Effect of ozone dose on the zeta potential and TOC of
10 mg/L alginate, BSA, and mixed alginate and BSA (1:1) solutions
(pH 7)
Figure 3 Size distribution of BSA before (a), and after (b),
membrane filtration with different ozone doses; variation of size
distribution (c), and UV absorbance (d), of alginate with different
ozone doses
Figure 4 MW distribution of BSA before (a), and after (b),
membrane filtration with different ozone doses
Figure 5 Variation of FTIR spectra of alginate (a) and BSA (b)
with different ozone doses
Figure 6 Variation of membrane flux (a) and size distribution
(b) of mixture (BSA and alginate (5 mg/L each)) and its MW
distribution before (c), and after (d), membrane filtration with
different ozone doses
Figure 7 Effect of ozone on the variation of molecular weight of
organic matter (a and b) and its influence on the Millipore
membrane fouling (c and d)
Figure 1 Temporal variation of relative membrane flux with ozone
pretreatment for Alginate (a) and BSA (b)
Figure 2 Effect of ozone dose on the zeta potential and TOC of
10 mg/L alginate, BSA, and mixed alginate and BSA (1:1) solutions
(pH 7)
Figure 3 Size distribution of BSA before (a), and after (b),
membrane filtration with different ozone doses; variation of size
distribution (c), and UV absorbance (d), of alginate with different
ozone doses
Figure 4 MW distribution of BSA before (a), and after (b),
membrane filtration with different ozone doses
Figure 5 Variation of FTIR spectra of alginate (a) and BSA (b)
with different ozone doses
Figure 6 Variation of membrane flux (a) and size distribution
(b) of mixture (BSA and alginate (5 mg/L each)) and its MW
distribution before (c), and after (d), membrane filtration with
different ozone dose
Figure 7 Effect of ozone on the variation of molecular weight of
organic matter (a and b) and its influence on the Millipore
membrane fouling (c and d)
Supporting Information
Figure S1 Temporal variation of relative membrane flux with
ozone pretreatment and backwash for: BSA (a) and Alginate (b) by
Millipore PVDF membrane; BSA (c) and Alginate (d) by Chinese PVDF
membrane
1
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