PREPRINT – ICPWA XY
Berlin, September 8-11, 2008
Sorption of Natural Organic Matter by Adsorber and Ion Exchange Resins –
Investigations with Starch and Phenylalanine as Model Substances
Madlen Pürschel and Volker Ender
University of Applied Sciences Zittau/Görlitz (FH)
Email: [email protected]
Demineralisation plants of power stations are not able to remove organics in all cases to a satisfied
degree. The present work focuses on natural organic matter (NOM) and its interaction with anion
exchanger and adsorber resins to optimize organics uptake. In this study, four different starches
(one of them 14C-labeled) with different molecular size distributions and L-phenylalanine (L-Phe)
were selected as model substances for the high-molecular-weight biopolymer fraction of NOM and
the low-molecular-weight neutral/amphiphilic one. Their uptake by various ion exchangers and
adsorbers was measured in column experiments. Results were discussed in terms of size exclusion,
anion exchange, adsorption, and hydrophilic/hydrophobic repulsion. In summary, at neutral pH,
starch has been removed preferably by size-exclusion followed by adsorption, whereas ion
exchange resins show higher uptake capacities than “pure” adsorber caused by stronger attraction
between starch and polar functional groups of the ion exchangers. At acidic pH, the uptake of
sulphate, as competitive adsorptive, leads to an earlier starch breakthrough at ion exchangers.
Therefore, adsorbers are more effective. For L-Phe, ion exchange is the main uptake mechanism. It
was found for both organics that the higher the water content of the resins, the more effective the
uptake is.
Introduction
Limited removal of natural organic matter
(NOM) from raw waters by demineralisation plants
leads to a certain rest content of organics in steam
water cycles of power stations. These organics are
considered to be a potential corrosion risk because
of their decomposition to low-molecular-weight
acids and carbon dioxide. Therefore, a limit of 200
ppb of total organic carbon (TOC) in the makeup
water is recommended (VGB guidelines [1]). Huber
[2] has presented 27 makeup waters of power
stations with a range from 100 to 300 ppb TOC. 60
% of these sites were within the limit of 200 ppb.
A lot of different methods and materials were
used to eliminate NOM, among them polymeric ion
exchangers [3 - 7], adsorber resins [8], magnetic ion
exchange resins [5], activated carbon [9], granular
ferric hydroxide [10], biologically active filter [11]
or reverse osmosis [12]. Ion exchange resins especially the anion exchangers – are of greatest
importance in makeup water treatment plants in
power stations, since they are able to remove NOM
to the above mentioned limit in the majority of
cases. Thereby, the uptake capacities were mostly
influenced by polymer composition (polystyrene or
polyacrylic), porosity, and charged functional
groups of the resins. However, it was difficult to
assess the impacts of raw water composition, NOM
characteristics, and ion exchanger and adsorber
properties on the NOM removal quantitatively. As a
pre-condition for that, more knowledge about the
mechanisms of NOM uptake (ion exchange or
adsorption) is necessary.
Deeper insights were achieved by introduction
of the liquid chromatography – organic carbon
detection (LC-OCD) method by Huber and
Frimmel [13]. So, investigations by Huber and
Gluschke [14] have shown that NOM with low
charge densities cannot be removed by ion
exchange resins in satisfied quantities. Following
the LC-OCD classification, these fractions are
hydrophobic organic carbon (HOC), biopolymers
and neutrals/amphiphilics as well as particulate
organic carbon (POC). Own investigations [15]
have revealed that about 50 % of the remaining
TOC in the makeup water comes from HOC/POC,
compared to one fifth in the input water. More than
80 % of the remaining chromatographic detectable
organic carbon (CDOC) consists of the fraction of
polysaccharides and neutrals/amphiphilics. Thus,
these NOM fractions have the highest potential for
further studies to increase the TOC uptake in water
treatment processes.
The present work continues previous
investigations [16] to the uptake mechanisms and
focuses on the relations between specific NOM
fractions (chemical type and size) and ion
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exchange/adsorption. For this purpose, four types of
starch (one of them 14C-labeled) with different
molecular sizes (analysed by ultrafiltration) were
selected as model substances for the highmolecular-weight biopolymer fraction, and their
uptake by ion exchange and adsorber resins was
measured in column experiments. The use of 14Clabeled starch – together with inactive starch –
allows to study the behaviour of both smaller and
larger starch particles in only one experiment.
Besides, the use of a radioactive tracer improves the
detection limit. Furthermore, the uptake of Lphenylalanine (L-Phe) as a model substance for the
low-molecular-weight neutrals/amphiphilics was
investigated.
Experimental Details
Adsorptives
Three types of inactive starch (Chemapol,
Riedel deHaën, Merck) and L-Phe (Roanal) were
used in analytical grade. 14C-labeled starch comes
from Biotrend as uniform C-labeled product (340
mCi/mmol = 12.58 GBq/mmol). All TOC solutions
were prepared with millipore water from Millipore
Elix/Milli-Q Academics (TOC < 20 ppb).
Analytical grade sodium hydroxide and sulphuric
acid were used for pH adjustment (pH 2.25 equals
to 400 ppm sulphate). The model substances were
checked with the LC-OCD method (Table 1).
Table 1 shows that, at neutral pH, all starches
are mainly identified as biopolymers. But, the
Chemapol starch presents higher amounts of other
LC-OCD fractions (especially POC), too, related to
a broader distribution of molecular weights. On the
other hand, Riedel deHaën and Merck starches have
dominant parts of the biopolymer fraction (96 and
94 %, respectively) at neutral conditions, indicating
that these starches have lower median molecular
weights than the Chemapol starch.
Compared to neutral conditions, the percentage
of the neutral/amphiphilic fraction of the Chemapol
starch arises from 7 to 19 % at pH 2.25. The same
result was obtained for Merck starch (arising from 0
to 19 %). These classification changes may be
related to an acid hydrolysis reaction following by a
degradation of high-molecular-weight starch
molecules to smaller ones.
Further, LC-OCD results verify that L-Phe
belongs - as expected - to the fraction of
neutrals/amphiphilics at all pH conditions.
Adsorbents
Ion exchange and adsorber resins were obtained
from Rohm and Haas Co. (Philadelphia, USA),
Bayer AG (Levercusen, Germany) and Purolite
(Bala Cynwyd, USA). Table 2 gives an overview of
the tested ion exchange resins. Water retention and
total volume capacity are obtained from product
data sheets of the manufacturers. Adsorber resins
characteristics are presented in Table 3 based on
product data sheets, too.
Procedures
Cleaning of the ion exchange and adsorber
resins
The cleaning procedure of the resins include
several steps of washing with millipore water,
rinsing with 1 N NaOH and 1.4 N HCl,
respectively, and final washing with millipore water
until the starch background value was less than 0.03
mg/L (details here not described).
For L-Phe experiments further cleaning steps
(threefold shaking (1 h) with 0.1 N NaOH,
treatment in a soxhlet reactor first with methanol,
second with acetonitrile (each for 24 h), rinsing
with millipore water) had to be applied, since L-Phe
background after the standard cleaning procedure
was still unsatisfying. Finally, the background value
was less than 0.1 mg/L.
Table 1: LC-OCD results of the model substances at different pH.
(for Chemapol starch and L-Phe median value ± standard derivation in % based on three measurements; for Riedel deHaën and
Merck starches single measuring)
Model solution
POC [%]
HOC [%]
Chemapol starch (pH 6)
Chemapol starch (pH 2.25)
Riedel deHaën starch (pH 6)
Merck starch (pH 6)
Merck starch (pH 2.25)
L-Phe (pH 6 and pH 2.25)
20 ± 12
26 ± 6
6
4
2
4±5
15 ± 10
0
0
0
Biopolymer [%] Neutrals/ Amphiphilics [%]
69 ± 17
39 ± 15
94
96
85
7±4
19 ± 7
0
0
13
about 100
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Berlin, September 8-11, 2008
Table 2: Properties of the tested ion exchange resins.
Resin
Water
Total capacity
retention [%]
[mol/l]
Matrix
Functional group
Structure
polystyrene-DVB
tertiary amine
macroporous
57-63
1.25
polyacrylic-DVB tertiary/quaternary amine macroporous
60-65
1.3
weak base anion exchanger
Amberlite IRA 96
Lewatit AP 246
strong base anion exchanger
Amberlite IRA 900
polystyrene-DVB quaternary amine, type 1 macroporous
58-64
1.0
Purolite A860
polyacrylic-DVB quaternary amine, type 1 macroporous
66-72
0.8
Table 3: Properties of the tested adsorber resins.
Resin
Matrix
Functional
group
Amberlite XAD 4
polystyrene-DVB
none
Amberlite XAD 1600
polystyrene-DVB
none
Amberlite XAD 7 HP
polyacrylic-DVB
carboxyl
Amberlite XAD 761 phenol-formaldehyde-DVB mainly phenol
Ultrafiltration
Chemapol, Riedel deHaën and Merck starch
solutions were ultrafiltered with an ultrafiltration
module from Pall, equipped with polyethersulfon(PES)-membranes with nominal molecular weight
cutoffs (nMWCO) of 0.45 μm, 0.3 μm, 0.16 μm,
50 kDa (all from Minisette) as well as 10 kDa and 5
kDa (Schleicher & Schuell). 10 litres of 2.1 ppm
TOC (5.62 mg/L) starch solution with pH 6 or 2.25
were filtered through the membranes beginning
with the membrane with the highest nMWCO (0.45
μm). Finally, filtrates were analysed by UV/VIS.
14
C-labeled starch was ultrafiltered with five
MicrosepTM-filters (Pall) of different pore sizes
(1000 kDa, 300 kDa, 50 kDa, 10 kDa and 1 kDa) in
filtration tubes. The filtration was followed by
centrifugation for 30 minutes (60 minutes for the 1
kDa filter) at 4000 rpm. Finally, the 14C-labeled
starch filtrates were quantified by liquid
scintillation counting (LSC).
Column experiments
Different model solutions (2.1 ppm TOC (5.62
mg/L starch), 2.5 ppm TOC (3.82 mg/L L-Phe) or
100 ppm TOC (152.83 mg/L L-Phe)) with pH 6 or
2.25 were given on a glass column (inner diameter
10 mm, filled with 5 ml of the selected resin, room
temperature) with a flow rate of 20 BV/h (bed
volume per hour) for ion exchangers and 4 BV/h
for adsorbers, respectively. The column effluent
Water
retention
[%]
54-60
66-73
61-69
62-70
Porosity
[ml/g]
0.5
1.4
0.5
1-1.2
Surface
Pore
area
diameter [Å]
[m²/g]
725-750
100
700-800
50-100
380-500
450
200
600
was collected at intervals and analysed by UV/VIS
spectrometry to monitor breakthroughs curves.
Analytical methods
LC-OCD analyses of the model substances and
millipore waters were carried out by DOC-LABOR
Dr. Huber [13]. Starch and L-Phe were measured
by UV/VIS (Lambda 2 from Perkin Elmer or
Varian, Cary 50 Bio) at 590 nm (after 1:1 starch
reaction with 1 mmol/l iodine-potassium-iodide
solution from Apolda and analytical grade
phosphoric acid, detection limit 0.03 mg/L) and at
257 nm (L-Phe, detection limit 0.1 mg/L),
respectively. The relative error due to input
solutions was determined as about 2.5 %. 14Clabeled starch was measured by LSC, using a
scintillation cocktail (Perkin Elmer, Ultima Gold)
and a counting system from Perkin Elmer, Wallac
Winspectral a/b (detection limit at 0.02 Bq/ml,
relative error due to the input solution (42 Bq/ml) at
0.75 %). Sulphate ions were indirectly determined
by titration of the hydrogen ions with sodium
hydroxide (relative error at 1.5 %). The overall
error of a single measure point of breakthrough
curves was about 5 %.
Results and discussions
Molecular size distribution of starches
Figure 1 shows the molecular size distribution
of the starches measured by ultrafiltration.
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PREPRINT – ICPWA XY
Berlin, September 8-11, 2008
percentage of starch in
filtrates [%]
100
80
60
40
20
0
< 1000 kDa
equals to about
< 0.45 µm
< 500 kDa
< 300 kDa
< 50 kDa
< 10 kDa
< 0.3 µm
< 0.16 µm
< 0.05 µm
< 0.02 µm
Chemapol starch (pH 6)
Merck starch (pH 6)
Chemapol starch (pH 2.25)
14C-labeled starch (pH 6)
< 5 kDa
< 1 kDa
< 0.0075 µm
< 0.005 µm
Riedel deHaën starch (pH 6)
Figure 1: Molecular size distribution of four starches investigated by ultrafiltration at neutral and acidic pH.
(for Chemapol starch (pH 6 and 2.25) median starch amounts in filtrates and standard derivation in % based on three
measurements, whereas for Riedel deHaën, Merck and 14C-labeled starches on double measuring; no results are available for
Chemapol, Riedel deHaën and Merck starches for fraction < 1 kDa as well as for 14C-labeled starch for fractions < 500 kDa
and < 5 kDa)
Figure 1 demonstrates that Chemapol starch
shows a different molecule size scale in relation to
the other starches. Riedel deHaën, Merck and 14Clabeled starches mainly consist of smaller particles
than 50 kDa related to the biopolymer LC-OCD
fraction, whereas Chemapol starch solutions have
higher amounts of larger particles. So, at neutral
pH, 78 % of the Chemapol starch particles belongs
to the fraction > 0.3 µm in comparison to less than
15 % of the other starches. Furthermore, an acidic
pH of the Chemapol starch solution leads to higher
amounts of smaller particles; nevertheless, they
consist of larger aggregates than Riedel deHaën,
Merck and 14C-labeled starches. These results are in
coincidence with the LC-OCD measures, not
surprisingly, because the LC-OCD method is based
on size exclusion chromatography. Summarized,
molecular size investigations show that the
molecule sizes of the starch particles decrease in the
following order: Chemapol, Riedel deHaën, Merck
and 14C-labeled starch.
Removal of starches with different molecular size
distributions by ion exchangers
Figure 2 illustrates the uptake of four starches
with different molecular size distributions by
polystyrene (IRA 900) and polyacrylic (A 860)
strong base ion exchangers at neutral pH (Fig. 2a/b)
and acidic pH (Fig. 2c/d). 14C-labeled starch was
used in combination with inactive Chemapol starch
(cactive/cinactive = 0.1) and was analysed by UV/VIS
(all starch molecules) and LSC (only active
molecules). In all following figures the x-axis refers
to the throughput in bed volume (BV) and the yaxis contains the output concentration divided by
the input concentration (ci/co).
Figure 2a/b demonstrates that starch uptake at pH 6
is definitely affected by size exclusion of starch
molecules. The smaller the particles, the higher
their removal is. So, the uptake arises in the order:
Chemapol with capacities of 0.2 mg/ml (IRA 900)
and 1 mg/ml (A 860), Riedel deHaën with 0.5
mg/ml (IRA 900) and 7 mg/ml (A 860), Merck with
4 mg/ml (IRA 900) and more than 6 mg/ml up to
1000 BV (A 860), and at last, 14C-labeled starch.
The size exclusion argument could be verified by
using larger particles (Chemapol -●-) and smaller
ones (14C-labeled -○-) in one experiment (Fig. 2a).
Large molecules are not able to diffuse into the
resin beads and are eluted after a short time,
whereas small starch molecules can reach the
adsorption sites inside the beads. In contrast, no
different uptake was observed between active and
inactive starch at polyacrylic resins (Fig. 2b). Here,
the pores are large enough to uptake also the
inactive Chemapol starch. Its large molecules block
adsorption sites and pores for the smaller 14Clabeled molecules. Then, inactive and active
molecules react in the same manner (Fig. 2b). As a
consequence, small 14C-labeled starch molecules
are even worse removed than Merck and Riedel
deHaën ones.
The comparison between polystyrene-DVB
(IRA 900, Fig. 2a) and polyacrylic-DVB (A 860,
Fig. 2b) starch uptake leads to the conclusion that a
resin with a higher water content and porosity (A
860) is able to uptake more starch molecules with a
broader size distribution (notice different scales of
the x-axis).
At acidic pH (Fig. 2c/d), starch uptake curves
are similar to the curves at pH 6 up to the sulphate
breakthrough, which occurs between 50 and 100
BV. The sulphate ions have a much stronger
affinity to the resin than starch molecules.
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PREPRINT – ICPWA XY
Berlin, September 8-11, 2008
IRA 900
b) 2
ci/co (starch; pH 6)
ci /co (starch; pH 6)
a) 2
1
1
0
0
0
50
100
BV
150
200
c) 4
0
250
IRA 900
3
2
1
0
200
400
BV
600
800
1000
d) 4
A 860
ci /co (starch; pH 2.25)
ci /co (starch; pH 2.25)
A 860
3
2
1
0
0
50
100
BV
150
200
250
0
50
100
BV
Chemapol starch
Riedel deHaën starch
14C-labeled + Chemapol starch - UV/VIS
14C-labeled + Chemapol starch - LSC
150
200
250
Merck starch
Figure 2: Removal of Chemapol, Riedel deHaën and Merck starches as well as 14C-labeled + Chemapol starch
(co = 2.1 ppm TOC) by two strong base anion exchangers (IRA 900 and A 860) at pH 6 and 2.25.
Consequently, the breakthrough point of starch is
mainly influenced by the ion exchange capacity due
to the sulphate ions. Starch will be eluted by the
latter ones indicating by values of the ratio ci/co > 1.
Low-molecular-weight 14C-labeled starch shows the
highest elution peak, which proves again that small
starch molecules have a higher affinity to anion
exchanger surfaces than larger ones. At last, starch
capacities were determined as 0.14-0.27 mg/ml
(IRA 900) and 0.28-0.45 mg/ml (A 860) at acidic
pH.
Removal of Chemapol starch by different ion
exchangers
Next Figure 3 includes two weak and two strong
anion exchange resins to compare both the
influence of resin’s structure and functional groups
on Chemapol starch removal at neutral and acidic
conditions. Fig. 3 demonstrates that polyacrylicDVB resins (AP 246 and A 860) remove Chemapol
starch to a significant higher degree than
polystyrene-DVB ones (IRA 96 and IRA 900) at
both neutral and acidic conditions. The total ion
capacities of polyacrylic-DVB resins are about
equal (see Tab. 2) to the polystyrene-DVB, so that
only the matrix properties can induce the different
uptakes. Both polystyrene-DVB resins have a water
retention of approximately 60-61 %, whereas the
polyacrylic-DVB resins show a water retention
ability of about 63 % and 69 %. High water
retention of resins increases the porosity of the
beads, so that starch molecules can permeate into
the resin to a greater extent. This result has also
been found for (median molecular) NOM fractions
by several authors (e.g. [3, 6, 8]).
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PREPRINT – ICPWA XY
Berlin, September 8-11, 2008
b) 2
IRA 96
ci /co (starch; pH 6)
IRA 900
AP 246
A 860
1
ci /co (starch; pH 2.25)
a) 2
0
IRA 96
IRA 900
AP 246
A 860
1
0
0
50
100
150
200
250
0
50
100
BV
150
200
250
BV
Figure 3: Removal of Chemapol starch (co = 2.1 ppm TOC) by IRA 96, IRA 900, AP 246 and A 860 anion
exchangers at pH 6 and 2.25.
At acidic pH, the breakthrough of starch was
again determined by the sulphate ions (Fig. 3b). Up
to the breakthrough point, polyacrylic-DVB resins
show higher uptakes than polystyrene-DVB ones
caused by different water contents (compare with
Fig. 2).
Further, Fig. 3a shows different starch uptakes
between weak and strong base polyacrylic
exchangers. At neutral pH, the weak base
exchanger is in the free base form. So, minor
electrostatic attraction should occur in comparison
to the strong base exchanger, leading to an earlier
breakthrough at the weak base exchangers. Kim et
al. [17] postulated that adsorption may take place
through interactions between the resin skeleton and
the non-ionic core of the NOM (hydrophobic
interaction) or through hydrogen bonds between
NOM and the nitrogen atom of the amine functional
groups. For hydrophilic starch molecules the latter
one is the more important adsorption mechanism in
the present case, which also explains that strong
anion exchangers with a high number of charge
sites showed advanced performance in starch
uptake than weak ones.
Removal of Chemapol starch by different adsorbers
Figure 4 shows the uptake of Chemapol starch
by several adsorber resins at pH 6 and 2.25. XAD 4
and XAD 1600 are “pure” adsorbers without any
active groups, whereas XAD 7 and XAD 761
possess weak acidic functional groups. For that
reason, latter ones have slightly weak cation
exchange properties.
2
XAD 4
XAD 4
XAD 1600
XAD 1600
XAD 7
XAD 761
1
ci /co (starch; pH 2.25)
ci /co (starch; pH 6)
2
XAD 7
XAD 761
1
0
0
0
20
40
BV
60
80
100
0
50
100
BV
150
200
250
Figure 4: Removal of Chemapol starch (co = 2.1 ppm TOC) by XAD 4, XAD 1600, XAD 7 and XAD 761
adsorbers at pH 6 and 2.25.
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Berlin, September 8-11, 2008
It was found that effectively no uptake occurs
for the polystyrene-DVB as well as for the weak
acidic adsorbers at neutral pH conditions, whereas
all resins adsorbed starch at acidic pH. Concerning
to the hydrophobic polystyrene-DVB adsorbers
without functional groups (XAD 4 and XAD 1600),
only porosity and pore diameters as well as the
molecular size distribution of the neutral starch
should affect starch capacities. Both polystyreneDVB resins have small pore diameters (0.005-0.01
μm). At neutral pH, the fraction of small Chemapol
starch molecules is too low (6 % < 0.05 μm), so that
size exclusion prevents starch removal. Contrary, at
acidic pH, 24 % of the Chemapol starch is smaller
than 0.05 μm. Thus, Chemapol starch may be
incorporated into the adsorber pores. A higher
starch uptake rate was found for XAD 1600 (1.21
mg/ml) in comparison to XAD 4 (0.02 mg/ml) at
acidic pH. The result is reasonable, since XAD
1600 possesses higher porosity (1.4 ml/g) and water
content (70 %) than XAD 4 (0.5 ml/g and 57 %).
In the same manner, porosity, pore diameter as
well as molecular size distribution of the starch
influence starch uptake by weak acidic polyacrylicDVB (XAD 7) and phenol-formaldehyde-DVB
(XAD 761) adsorbers, but for these resins also the
pH is important. At acidic pH, the weak acidic
functional groups are in poorly dissociated form, so
that a removal of neutral substances should be
increased in comparison to neutral conditions.
Anyway, at neutral conditions size exclusion is the
main effect, which prevents again starch removal.
At acidic pH, both adsorbers are able to retain a
considerable degree of Chemapol starch (about 1.07
mg/ml for XAD 7 and 0.69 mg/ml for XAD 761).
a) 2
Removal of L-Phe by ion exchangers
Figure 5 compares the removal of the L-Phe by
two weak (IRA 96 and AP 246) and two strong
(IRA 900 and A 860) anion exchangers as a
function of bed volume (BV) at two initial pH
conditions. Fig. 5a demonstrates two effects due to
the L-Phe uptake at neutral conditions.
First, weak base exchangers have shown lower
uptake rates than strong base exchangers. Their
calculated capacities based on integration of the
breakthrough curves were 0.07 mol/L (IRA 96) and
0.19 mol/L (AP 246) versus 0.30 mol/L (IRA 900)
and 0.42 mol/L (A 860) of the strong base
exchangers. At neutral pH, L-Phe should react as LPhe+/- (zwitterion) and L-Phe- (anion) mix, since its
isoelectric point is pH 5.48. Consequently, it should
be removed preferably by anion exchange and –
probably to a minor extent - by adsorption. At
neutral pH, weak base resins are in the free base
form, which minimizes ion exchange capacity,
whereas the strong base exchangers retain their ion
exchange capacities at all pH conditions.
Second, the respective polyacrylic-DVB resins
(AP 246 and A 860) cause higher uptakes than the
related polystyrene-DVB ones (IRA 96 and IRA
900), even though the ion exchange capacities show
no difference (as example IRA 900 versus A 860).
These higher capacities of the polyacrylic-DVB
resins may be explained by their higher water
content and porosity with higher specific inner
surface, so that adsorption effects are playing an
increasing role (see also [17]).
b)
4
IRA 96
ci /co (L-Phe; pH 6)
IRA 900
AP 246
A 860
1
0
ci /co (L-Phe; pH 2.25)
IRA 96
IRA 900
3
AP 246
A 860
2
IRA 900
Cl-Form
1
0
0
200
BV
400
600
0
50
100
BV
150
200
250
Figure 5: Removal of L-Phe (co = 100 ppm TOC) by IRA 96, IRA 900, AP 246 and A 860 anion exchangers
at pH 6 and 2.25.
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Berlin, September 8-11, 2008
b)
2
XAD 1600
XAD 1600
XAD 7 HP
XAD 7 HP
1
ci /co (L-Phe; pH 2.25)
ci /co (L-Phe; pH 6)
a) 2
0
1
0
0
20
40
BV
60
80
0
100
20
40
BV
60
80
100
Figure 6: Removal of L-Phe (co = 2.5 ppm TOC) by XAD 1600 and XAD 7 HP at pH 6 and 2.25.
More interactions between the molecules, which
were still adsorbed by ion exchange, and free
molecules in solution are possible, because of larger
distances between the functional groups.
At acidic initial conditions, L-Phe was also
uptaken by anion exchangers except for IRA 900
(Cl- form) (Fig. 5b). At pH 2.25, L-Phe reacts as LPhe+ cation and, hence, uptake by anion exchange
should not be expected. This is the case for IRA
900 in the Cl- form. Usually, anion exchange resins
in demineralisation plants are used in the OH- form.
These resins initially exchange sulphate ions (pH
adjustment by H2SO4) against OH- ions. This leads
to neutral conditions immediately in/at the resin.
Then again, L-Phe can react as L-Phe+/- and L-Phemix solution. If the resin`s capacity relating to the
sulphate ions is depleted, pH decreases to 2.25,
followed by a rapid L-Phe breakthrough with steep
peaks indicating elution of the previous adsorbed LPhe. Consequently, at acid initial conditions lower
L-Phe capacities for IRA 96 (0.04 mol/L), AP 246
(0.07 mol/L), IRA 900 (0.04 mol/L) and A 860
(0.05 mol/L) were found than at neutral initial
conditions without counterions.
Removal of L-Phe by adsorbers
The L-Phe breakthrough curves for a
polystyrene-DVB (XAD 1600) and a polyacrylicDVB (XAD 7) adsorber at neutral and acidic pH
conditions are shown in Figure 6.
Generally, no or much lower L-Phe uptake was
observed in contrast to the anion exchange resins.
Adsorption of L-Phe was somewhat higher on XAD
1600 resin than on XAD 7. Two facts may
contribute to this result: first, the differences in
porosity and surface area and second, the polarity
of resins relate to the adsorptive. As a precondition
for adsorption, corresponding interactions between
adsorber and the adsorptive are necessary. Even a
high surface area cannot promote adsorption, if the
precondition is not fulfilled. This can be seen for
pH 6: no adsorption of the L-Phe+/- (zwitterion) and
L-Phe- (anion) mix is occurring, although XAD
1600 has a much higher surface area than XAD 7
(700-800 m²/g versus 380-500 m²/g). In comparison
to pH 6, the L-Phe molecule should be more
hydrophobic because of its positive charge at pH
2.25, so that adsorption by the more hydrophobic
XAD 1600 will be possible, although in only
marginal quantities (Fig. 6b). This confirms the
results from Doulia et al. [18], who investigated the
adsorption of various acids on polystyrene-DVB
adsorbers.
Conclusions
-
-
The results of this work demonstrate:
Size-exclusion will be the main mechanism
for the removal of starch as model substance
of the biopolymer NOM fraction by anion
exchangers and adsorbers. Consequently,
starch uptake rises with decreasing molecular
size of the starch and increasing porosity,
pore diameter and water content of the resins.
Adsorption is postulated as the second
important factor for starch uptake both on ion
exchange resins and adsorbers. At neutral
pH, anion exchangers are able to uptake
starch in higher extent than “pure” adsorbers
based on intensive attractions between starch
molecules and the polar groups of the ion
8
PREPRINT – ICPWA XY
Berlin, September 8-11, 2008
-
-
exchangers. Hence, uptake occurs by weak
base anion also at pH 6, even though the free
nonionic base form exists. At acidic pH,
adsorbers are more effective than ion
exchangers, because sulphate, as competitive
adsorptive, leads to an earlier starch
breakthrough at ion exchangers.
L-Phe is preferably removed by anion
exchange and – probably only to a minor
extent - by adsorption.
Polyacrylic-DVB resins cause higher starch
and L-Phe uptakes than the related
polystyrene-DVB ones. This could be due to
their higher water content and porosity,
which easier allows penetration into the
resin.
As a consequence of this work, polyacrylic
anion exchangers should be an alternative in water
treatment
plants,
if
biopolymer
and
neutral/amphiphilic fractions of NOM make some
problems. For a better prediction of breakthrough
points of NOM fractions, further investigations are
necessary including equilibrium and kinetic
parameters.
[4]
[5]
[6]
[7]
[8]
Acknowledgements
This work was supported by the German
Federal Ministry of Education and Research
(BMBF) within the FH3 programme, code number
FKZ 17 57X 06. We are grateful to H. Heidenreich,
R. Illgen and C. Schmidt for extensive experimental
labworks. Further, we would like to thank G.
Bernhard, S. Sachs and K. Schmeide (Institute of
Radiochemistry, Forschungszentrum DresdenRossendorf (FZD)) for generous assistance in using
the 14C-technique and for valuable discussions.
Finally, we would like to express our gratitude to
the DOC-Lab Dr. Huber, Karlsruhe, for many LCOCD analyses and Vattenfall Europe Generation
AG & Co. KG for co-financing.
[9]
[10]
[11]
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