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Removal of hydrophilic pollutants from water with organic adsorption polymers
Chemical Engineering and Processing 38 (1999) 601 – 610 www.elsevier.com/locate/cep
Removal of hydrophilic pollutants from water with organic adsorption polymers Part I. Adsorption behaviour of selected model compounds Fritz H. Frimmel *, Marcus Assenmacher, Martin So¨rensen, Gudrun Abbt-Braun, Gudrun Gra¨be Engler-Bunte-Institut, Waterchemistry, Uni6ersita¨t Karlsruhe, Engler-Bunte-Ring 1, D-76131 Karlsruhe, Germany Received 15 April 1999; accepted 16 April 1999
Abstract The adsorbability of 2-aminonaphthalene-1-sulfonate, diuron, 1-naphthol and natural organic matter (NOM) onto an organic polymer resin and onto activated carbon was investigated. Isotherms with the substances alone and in the presence of dissolved NOM were measured. There was a good adsorbability of diuron and 1-naphthol on both sorbents. At low initial concentrations of the compounds the activated carbon showed higher adsorptivity, whereas for high initial concentrations the polymer resin showed an equal or better adsorption behaviour. 2-Aminonaphthalene-1-sulfonate and the NOM showed favorable adsorption behaviour to activated carbon but was only poorly adsorbed on the polymer resin. In the presence of NOM, the adsorbability of the single compounds decreased significantly on activated carbon. Nearly no influence was found for the adsorption of the pollutants on the resin. For the polymer resin, additionally, the breakthrough behaviour of the substances was investigated. The results obtained in the batch experiments for the single substances were confirmed. However, in the presence of NOM the breakthrough occurred at shorter times for all three substances. Regeneration of the resin with isopropanol proved to be a good cleaning method. A recovery of 92–96% of the substances was reached. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Adsorption; Organic adsorber polymer resin; Hydrophilic pollutants; Humic substances; Drinking water treatment
1. Introduction During the last years, the concentrations of anthropogenic organic pollutants has increased in most groundwaters and in surface waters. Since most of these substances are considered to be toxic or even carcinogenic, they have to be eliminated during water treatment processes to guarantee safe drinking water. The adsorption of organic substances on activated carbon is well known in drinking water treatment. Micropollutants like pesticides or halogenated hydrocarbons can be eliminated to a great extent by application of adsorbing filters. It has also been shown that natural
Dedicated to Professor Em. Dr-Ing. Dr h.c. mult. E.-U. Schlu¨nder on the occasion of his 70th birthday. * Corresponding author. Tel.: + 49-721-6082580; fax: + 49-721699154. E-mail address: [email protected] (F.H. Frimmel)
organic matter (NOM), often called humic substances (HS) and described as natural polyelectrolytes are adsorbed, and by this, decrease the adsorption capacity for the pollutants [1,2]. As a consequence, the adsorption efficiency for the trace substances and the breakthrough times for filters decrease [3,4]. The resulting disadvantages are high energy demands and consequently high costs for regeneration or renewal of the adsorbents. Hydrophilic organic compounds which are poorly or not biodegradable often occur in the raw water of drinking water treatment plants. Due to their hydrophilic properties they can only be partly eliminated during the drinking water treatment process and therefore can be found in drinking water [5]. The use of synthetic adsorber polymers in drinking water treatment has been investigated by several authors [6–8]. During the last few years, attempts were made to improve these adsorber polymers which origi-
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nally were developed on the basis of ion exchange resins and have a specific surface of 200 – 800 m2/g [9]. The use of new technologies allowed the production of highly porous polymers with a specific surface of 800–1500 m2/g which is similar to the surface of activated carbon [10]. Lewatit EP 63, a styrene-divinyl benzene copolymer, has been used for the elimination of substances with MW B200 g/mol and of chlorinated hydrocarbons [11–15]. In addition, this polymer has been used in water analysis for the selective enrichment of organic trace pollutants [9]. The aim of this work was to compare the equilibrium adsorption behaviour of organic substances on the synthetic adsorber polymer with that on activated carbon. Isotherms with the single compounds were determined in the presence of HS. In addition, the breakthrough behaviour of the compounds was investigated in filter columns in a laboratory plant. Finally, the regenerability of the polymer resin with isopropanol was tested based on the published results on regeneration with hot water, water vapour or with organic solvents like methanol or isopropanol [16]. For the experiments, organic substances showing different hydrophilic characters were used as model compounds. 2-Aminonaphthalene-1-sulfonate was selected as a representative of aromatic sulfonic acids which are used as precursors in the dye industry [17]. Diuron is a widely used herbicide, and 1-naphthol was chosen as a representative metabolite of polycyclic aromatic hydrocarbons (PAH).
2.2. Adsorbents The adsorption polymers LiChrolut EN (Bayer) and Lewatit EP 63 (Merck) were used. Both resins consist of a styrene–divinyl benzene copolymer. Lewatit EP 63 has a specific surface of 1000–1400 m2/g [18]. LiChrolut EN is ground to finer particles with a specific surface of 1200 m2/g. The activated carbon (F 300) used had a specific surface of 1000 m2/g (BET).
2.3. Adsorption isotherms Isotherms were obtained by variation of the ratio of solution volume to sorbent mass at constant initial concentration of the sorptive. Calculations were done according to the Freundlich equation which is an empirical two-parametric power function (Eq. (1)): qi,GG = KF × cni,GG
(1)
where qi,GG is the equilibrium solid-phase concentration of i adsorbed from solution per unit mass of adsorbent (mg/g); KF is the Freundlich constant in (mg/g)/(mg/l)n; ci,GG is the equilibrium concentration of i in solution (mg/l); and n is the Freundlich exponent. A minimum contact time of 48 h was chosen to reach equilibrium. The isotherms were either run in distilled water or in tap water (Karlsruhe, DOC=0.6 mg/l) without or in the presence of NOM. All experiments were run at ambient temperature.
2.4. Laboratory plant for filter experiments 2. Experimental set-up
2.1. Adsorpti6es Natural organic matter (NOM) was taken from two bog lakes (Wildsee, Hohlohsee, northern Black Forest, Germany). The content of dissolved organic carbon (DOC) was analysed by wet chemical oxidation (DC 80, Dohrmann) and also by catalytic oxidation (TOC 5000 Total Organic Carbon Analyzer, Shimadzu). The model compounds 2-aminonaphthalene-1-sulfonate, diuron and 1-naphthol were quantified by UV/vis spectroscopy with a UV/vis-spectrophotometer (Lambda 5, Perkin Elmer). The absorption maxima used for calibration and determination were l= 282 and 341 nm for 2-aminonaphthalene-1-sulfonate, l= 248 nm for diuron, and l=215 nm for 1-naphthol. In the presence of NOM the compounds were analysed by a diode-array detector after liquid chromatographic separation (HPLC 1090, Hewlett Packard).
The flow scheme for the laboratory plant is shown in Fig. 1. Three parallel glass columns (chromatography columns) with an inner diameter of di =2.5 cm and di = 1 cm and a length of 45 cm were used (in Fig. 1 only one column is shown). Two precision pumps with adjustable flow between 0.1 and 50 ml/ min (Type P-50, Pharmacia) and 0.5–100 ml/min (Type P-6000, Pharmacia) fed the columns with the solution. The resulting filter velocities were between 0.01–6 and 0.06–12 m/s (empty bed velocity). The columns were made of a solvent resistant material as the regeneration of the polymer resin should be made with methanol. The length of the filters could be varied between 15 and 45 cm. For the experiments it was decreased additionally by glass beads to 5–10 cm. The columns were filled with Lewatit EP 63. To reach a filter velocity of 3.7 m/h, the large column (2.5 cm diameter) was run with a flow rate of 30 ml/min. The small column (1 cm diameter) was run with a flow rate of 4.8 ml/min. The UV absorbing substances were detected online in the outlet with an UV-detector (Uvicord VW 2251, Pharmacia).
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3. Results and discussion
3.1. Adsorption equilibrium of model compounds Adsorption isotherms of model compounds on F 300 and LiChrolut EN were investigated for different initial concentrations (10, 1, 0.5 and 0.09 mg/l). The adsorption equilibria of diuron on activated carbon and on LiChrolut EN with an initial concentration of 10 mg/l are shown in Fig. 2. Two ranges with different adsorption behaviour are clear for the two types of adsorbent.
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However, as activated carbon is a hydrophobic material, only a low adsorption capacity was expected in the case of the relatively hydrophilic diuron. Since the data show a relatively high adsorption capacity, diuron, however, seems to be able to move into the micropores of the activated carbon. Higher adsorption capacities were found for activated carbon masses below 1.6 mg/l, in comparison to LiChrolut EN which showed higher adsorption capacities for diuron in the higher concentration range (\ 1.6 mg/l). The adsorbent load increased with increasing initial concentration of diuron
Fig. 1. Flow scheme of the laboratory plant.
Fig. 2. Adsorption equilibrium of diuron on activated carbon (F 300) and LiChrolut EN, measured values (, ") and Freundlich calculation (–). The data were calculated by two fittings (low and high concentration range).
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Fig. 3. Adsorption isotherms of diuron on activated carbon (F 300) and Lewatit EP 63 for different initial concentrations c0 (measured values and Freundlich calculation).
much more rapidly for the organic polymer than for activated carbon. This behaviour is reflected in the higher Freundlich exponent for Lichrolut EN (LiChrolut EN: n=0.42; F 300: n =0.13). In the case of high concentrations of polar compounds in the liquid phase, it seems that the organic polymer resin is more suitable for their effective removal. In the case of 2-aminonaphthalene-1-sulfonate and 1-naphthol the loading on activated carbon is much higher than on LiChrolut EN (not shown). Almost no adsorption on LiChrolut EN was found for 2-aminonaphthalene-1-sulfonate. As the polymer resin did not have any ionic functional groups, it was expected that compounds with ionic character would show little adsorbability. The adsorption isotherms of diuron on activated carbon (F 300) and on Lewatit EP 63 for the initial concentrations of 1, 0.5 and 0.09 mg/l are shown in Fig. 3. For both sorbents different adsorption isotherms were found for various initial concentrations. This behaviour is characteristic for competing adsorption of several substances. It was not expected for the model compounds. An explanation can be seen in different adsorption mechanisms for the various concentration ranges. In the lower concentration range the sorbent surface will be covered by a monomolecular layer, whereas with increasing concentration of the substances the covering can be made increasingly of several layers. In general the adsorption of diuron, 2-aminonaphthalene-1-sulfonate and 1-naphthol on activated carbon was higher than on Lewatit EP 63 in the low concentration range.
3.2. Influence of natural organic matter on the adsorption equilibrium The adsorption isotherms of dissolved NOM (Wildsee) were determined for different pH values
(pH= 7 and 2) and different DOC concentrations. At pH= 7, the adsorption of DOC on LiChrolut EN was fairly poor, as already observed for the hydrophilic model compounds (Fig. 4). For both sorbents the adsorption of DOC was significantly higher at pH 2 compared to pH 7. This is due to the formation of unassociated acidic sites as a consequence of protonation of the anionic functional groups at pH 2, which leads to a decrease in the molecular size of the polyelectrolytic humic substances and to an increase of their hydrophobic character [19]. However, the adsorption capacity of DOC on LiChrolut EN is lower compared to that on activated carbon (not shown). This agrees with the data of Boenig et al. [20] who also found a poor adsorption of humic substances on ionic polymer resins of polystyrene type. Detailed investigations on the adsorption of humic substances from different origin on activated carbon are given by Abbt-Braun et al. [21]. The influence of humic substances on the adsorption equilibrium of diuron on activated carbon F 300 is shown in Fig. 5. No influence on the adsorption of diuron was observed for high masses of sorbents or low equilibrium concentrations. Obviously there are enough sites for the adsorption to avoid competing effects. However, the sorption of diuron decreases in the presence of humic substances with decreasing masses of sorbents (increasing equilibrium concentration). Carbon-fouling was observed for high DOC concentration of the bog lake water (3 and 10 mg/l DOC, Wildsee) [22]. No significant influence on the adsorption of diuron on F 300 in the presence of low DOC concentrations (tap water) was found. The sorption behaviour of a mixture compared to a single compound isotherm can be described by the Freundlich exponent n [1,23]. As
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there is only a small decrease of n, we suggest that part of the humic substances have an adsorption behaviour similar to diuron and that there is no strong interaction between the substances. The adsorption isotherms of diuron on F 300 are similar in the presence of 3 and 10 mg/l DOC. This suggests that the sorbability of NOM is decreasing for higher initial DOC concentrations. It is known that humic substances can form aggregates with increasing concentration [2]. Whereas the polar functional groups are orientated to the liquid phase, the hydrophobic building blocks are positioned in the core of the aggregates, resulting in a decreased adsorbability. In addition, the interaction of diuron and humic substances will also influence the adsorption. Electron donor–acceptor complexes and hydrogen bonds between humic substances and triacine herbicides were postulated by Senesi and Testini [24]. Schnitzer and
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Kahn [25] presumed covalent bonding between pesticides and humic substances, and sorption of the organic compounds on solid humic substances. Therefore, humic substances will have a multifunctional influence on the distribution of diuron. The attempt to separate the associates of NOM and diuron from the free diuron with HPLC/DAD was not successful. It can be deduced that associates, if they really exist, have a fairly weak interaction of the components. The adsorption of diuron onto LiChrolut EN was not highly influenced by NOM (not shown). The Freundlich constants decreased to 98% (DOCHS,0 =3 mg/l) and to 94% (DOCHS,0 = 10 mg/l). For tap water (DOC= 0.6 mg/l), no effect could be observed. This is in agreement with the poor adsorption of humic substances on the polymer resin. The small decrease of the
Fig. 4. Sorption isotherms of NOM (Wildsee, pH 7) on activated carbon (F 300) and on LiChrolut EN.
Fig. 5. Influence of humic substances (HS) on the adsorption of diuron on activated carbon (F 300) at pH 7.
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Fig. 6. Comparison of the breakthrough curves of the model compounds in the laboratory columns filled with Lewatit EP 63.
Freundlich constants can again be explained by the formation of weak associates of diuron and NOM which do not adsorb. The fact that there is no pronounced influence of the NOM on the selective adsorption of polar compounds on LiChrolut EN is an advantage in its practical application to remove organic micropollutants in the presence of an organic background. In drinking water treatment, it may be a goal to remove specific toxic compounds while the non-toxic NOM can remain in the water phase. In this respect, the polymer resin may be preferable to activated carbon.
3.3. Filter experiments The breakthrough curves of the model compounds on Lewatit EP 63 are given in Fig. 6. The different adsorption behaviour of the components is well reflected in the different breakthrough times. The breakthrough of diuron was very slow; 20% of the initial concentration (co) was found after 10 000 bed volumes and even after 80 000 bed volumes (not shown in Fig. 6), only 60% of the feed concentration was found in the outflow. The good adsorbability of 1-naphthol on Lewatit EP 63 could be confirmed by the filter experiments. In the case of technical applications, the low concentration of 1-naphthol in the outflow at the beginning would be an advantage. The extremely low adsorbability of 2-aminonaphthalene-1-sulfonate and NOM on the polymer resin is also reflected in the short breakthrough times. If the feed concentration of 2-aminonaphthalene-1-sulfonate increased from 1 to 10 mg/l, the breakthrough took place in half the time. The same results were observed for NOM (not shown).
3.4. Influence of NOM on the breakthrough of model compounds In the presence of NOM, the breakthrough time of diuron decreased (Fig. 7). Even after a few bed volumes, the concentration in the outlet reached 40% of the feed concentration. Since NOM have a low adsorbability on the polymer resin it is unlikely that a preloading of humic substances caused this decrease. It is interesting to note that Baldauf [26] and Sontheimer et al. [2] showed that, for activated carbon, humic substances have a strong influence on adsorption processes. The breakthrough of 1-naphthol in the resin columns is also faster in the presence of NOM (Fig. 8). It is very likely that again interactions between naphthol and the humic substances are responsible for a higher mobility of naphthol. The shapes of the two breakthrough curves are quite similar and start from the baseline after several bed volumes. Therefore Lewatit EP 63 can be considered for the removal of 1-naphthol from polluted water. The ionic character of 2-aminonaphthalene-1-sulfonate and of NOM are responsible for a fairly rapid breakthrough in the Lewatit EP 63 column. There is a mutual influence on the breakthrough behaviour (Fig. 9). A decrease of the pH-value had a significant influence on the breakthrough of 2-aminonaphthalene-1-sulfonate in the presence and absence of NOM. A similar pH-dependent behaviour was found for the humic substances alone. Two reasons may be considered. First the pH-value has an influence on the dissociation equilibrium of the functional groups and so on the ionic character. Second, the pH-value has an influence on the size of the molecules of polyelectrolytes. At lower pH values, the molecules are smaller as the repulsive forces between the functional groups decrease with protona-
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tion. Consequently, more pores and adsorption sites are available for the protonated substances, leading to their retardation. The outflow concentrations of more than 100% of the feed concentration (overshoot) in the presence of NOM suggest its displacement as a weakly absorbing component by the sulfonate (chromatographic effect) [27]. For technical applications, the complete breakthrough is of minor interest, as filters are regenerated at a certain outlet concentration which is normally lower than the feed concentration. It could be shown that for synthetic resins also, the pH value of the feed and the concentration of NOM have an essential influence on the running time of the filter up to the maximal tolerable outlet concentration.
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3.5. Regeneration of the polymer resin One of the main advantages in technical applications of polymer resins compared to activated carbon is the regeneration at low costs. Therefore we investigated what quantity of the adsorbed substances could be desorbed by means of an organic solvent. The loaded sorbent in the filter was flushed with isopropanol. Fig. 10 shows the desorption behaviour of diuron and 1naphthol. For the strongly adsorbable substances diuron and naphthol, about 40–60 bed volumes of the regeneration solvent were needed to reach complete desorption. The maximum concentration was in the g/l range. The regenerate was enriched with the sorptives for three
Fig. 7. Influence of NOM on the breakthrough behaviour of diuron at pH 7.
Fig. 8. Influence of NOM on the breakthrough behaviour of 1-naphthol at pH 7.
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Fig. 9. Influence of NOM on the breakthrough behaviour of 2-aminonaphthalene-1-sulfonate.
Fig. 10. Desorption of diuron and 1-naphthol with isopropanol. Table 1 Recovery of the adsorbed substances after regeneration with isopropanol
Adsorbed mass Desorbed mass Desorbed amount (%)
NOM (as C)
2-Aminonaphthalene-1-sulfonate
1-Naphthol
Diuron
1.72 mg 1.22 mg 84.3
13.71 mg 13.09 mg 95.5
2.695 g 2.568 g 95.9
2.04 g 1.886 g 92.3
orders of magnitude which is a big advantage with regard to the further treatment or disposal of the regenerate. In addition the sorptive can be reused if it is a valuable substance. This may reduce the overall costs of treatment enormously. The weakly adsorbable substances 2-aminonaphthalene-1-sulfonate and NOM could be completely desorbed within 10 bed volumes of the solvent. The
maximum concentration obtained was about three times the initial feed concentration. On the basis of the breakthrough curves, mass balances for the sorbed substances were calculated. The recovery of the sorptives are shown in Table 1. For the single compounds, recoveries between 92 and 96% were found. This is in good agreement with recoveries \ 90%, found by Dedek et al. [9] for the enrichment of
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samples for the analysis of trace substances. Only about 84% of the NOM were desorbed. Possibly there is an irreversible loading of the resin as it can also be found for activated carbon. It has to be investigated in further experiments if this leads to a decrease of the adsorption capacity within the number of loading and regeneration cycles.
4. Conclusions For the application in drinking water treatment technology where low concentrations of pollutants are typically found, the activated carbon F 300 showed better adsorption characteristics than the polymer resins LiChrolut EN or Lewatit EP 63. However, in the higher concentration range (c0 =7 – 10 mg/l), the loads on the polymer resin were similar or even higher than on activated carbon. So the use of the resin is convenient at higher concentrations, e.g. in heavily polluted waters. An advantage of the use of polymer resin in comparison to activated carbon was the poor adsorbability of NOM like humic substances. Practically no influence of NOM on the isotherms of the investigated substances was found. As a consequence, toxic substances can be removed from the water phase while the non-toxic NOM present remain in solution. A comparable effect to ‘carbon-fouling’ — which leads to a decrease of the adsorption capacity caused by the irreversible pre-loading of activated carbon by humic substances—does not exist for the synthetic resins applied. Nevertheless, in the presence of humic substances the breakthrough times on Lewatit EP 63 decreased. This can be explained mainly by interactions between the humic substances and the single compounds. We presume that the power of the polymer resin is the selective removal of pollutants in complex matrix solutions. The validity of this assumption has to be verified in further experiments.
Acknowledgements The authors thank Bundesministerium fu¨r Bildung und Forschung (BMBF) for financial support (BMBF 02WT9612/7) and Professor Dr R. Nießner and Priv. Doz. Dr habil. W. Ho¨ll for the collaboration within the joint research program ‘Entfernung von organischen hydrophilen Schadstoffen mit Hilfen neuartiger Adsorberpolymere bei der Wasseraufbereitung — Grundlagen zur technischen Anwendung’. We gratefully acknowledge the experimental work and discussions with Elly Karle, Christian Specht and Andreas Schindeln.
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Removal of hydrophilic pollutants from water with organic adsorption polymers Part I. Adsorption behaviour of selected model compounds Fritz H. Frimmel *, Marcus Assenmacher, Martin So¨rensen, Gudrun Abbt-Braun, Gudrun Gra¨be Engler-Bunte-Institut, Waterchemistry, Uni6ersita¨t Karlsruhe, Engler-Bunte-Ring 1, D-76131 Karlsruhe, Germany Received 15 April 1999; accepted 16 April 1999
Abstract The adsorbability of 2-aminonaphthalene-1-sulfonate, diuron, 1-naphthol and natural organic matter (NOM) onto an organic polymer resin and onto activated carbon was investigated. Isotherms with the substances alone and in the presence of dissolved NOM were measured. There was a good adsorbability of diuron and 1-naphthol on both sorbents. At low initial concentrations of the compounds the activated carbon showed higher adsorptivity, whereas for high initial concentrations the polymer resin showed an equal or better adsorption behaviour. 2-Aminonaphthalene-1-sulfonate and the NOM showed favorable adsorption behaviour to activated carbon but was only poorly adsorbed on the polymer resin. In the presence of NOM, the adsorbability of the single compounds decreased significantly on activated carbon. Nearly no influence was found for the adsorption of the pollutants on the resin. For the polymer resin, additionally, the breakthrough behaviour of the substances was investigated. The results obtained in the batch experiments for the single substances were confirmed. However, in the presence of NOM the breakthrough occurred at shorter times for all three substances. Regeneration of the resin with isopropanol proved to be a good cleaning method. A recovery of 92–96% of the substances was reached. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Adsorption; Organic adsorber polymer resin; Hydrophilic pollutants; Humic substances; Drinking water treatment
1. Introduction During the last years, the concentrations of anthropogenic organic pollutants has increased in most groundwaters and in surface waters. Since most of these substances are considered to be toxic or even carcinogenic, they have to be eliminated during water treatment processes to guarantee safe drinking water. The adsorption of organic substances on activated carbon is well known in drinking water treatment. Micropollutants like pesticides or halogenated hydrocarbons can be eliminated to a great extent by application of adsorbing filters. It has also been shown that natural
Dedicated to Professor Em. Dr-Ing. Dr h.c. mult. E.-U. Schlu¨nder on the occasion of his 70th birthday. * Corresponding author. Tel.: + 49-721-6082580; fax: + 49-721699154. E-mail address: [email protected] (F.H. Frimmel)
organic matter (NOM), often called humic substances (HS) and described as natural polyelectrolytes are adsorbed, and by this, decrease the adsorption capacity for the pollutants [1,2]. As a consequence, the adsorption efficiency for the trace substances and the breakthrough times for filters decrease [3,4]. The resulting disadvantages are high energy demands and consequently high costs for regeneration or renewal of the adsorbents. Hydrophilic organic compounds which are poorly or not biodegradable often occur in the raw water of drinking water treatment plants. Due to their hydrophilic properties they can only be partly eliminated during the drinking water treatment process and therefore can be found in drinking water [5]. The use of synthetic adsorber polymers in drinking water treatment has been investigated by several authors [6–8]. During the last few years, attempts were made to improve these adsorber polymers which origi-
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nally were developed on the basis of ion exchange resins and have a specific surface of 200 – 800 m2/g [9]. The use of new technologies allowed the production of highly porous polymers with a specific surface of 800–1500 m2/g which is similar to the surface of activated carbon [10]. Lewatit EP 63, a styrene-divinyl benzene copolymer, has been used for the elimination of substances with MW B200 g/mol and of chlorinated hydrocarbons [11–15]. In addition, this polymer has been used in water analysis for the selective enrichment of organic trace pollutants [9]. The aim of this work was to compare the equilibrium adsorption behaviour of organic substances on the synthetic adsorber polymer with that on activated carbon. Isotherms with the single compounds were determined in the presence of HS. In addition, the breakthrough behaviour of the compounds was investigated in filter columns in a laboratory plant. Finally, the regenerability of the polymer resin with isopropanol was tested based on the published results on regeneration with hot water, water vapour or with organic solvents like methanol or isopropanol [16]. For the experiments, organic substances showing different hydrophilic characters were used as model compounds. 2-Aminonaphthalene-1-sulfonate was selected as a representative of aromatic sulfonic acids which are used as precursors in the dye industry [17]. Diuron is a widely used herbicide, and 1-naphthol was chosen as a representative metabolite of polycyclic aromatic hydrocarbons (PAH).
2.2. Adsorbents The adsorption polymers LiChrolut EN (Bayer) and Lewatit EP 63 (Merck) were used. Both resins consist of a styrene–divinyl benzene copolymer. Lewatit EP 63 has a specific surface of 1000–1400 m2/g [18]. LiChrolut EN is ground to finer particles with a specific surface of 1200 m2/g. The activated carbon (F 300) used had a specific surface of 1000 m2/g (BET).
2.3. Adsorption isotherms Isotherms were obtained by variation of the ratio of solution volume to sorbent mass at constant initial concentration of the sorptive. Calculations were done according to the Freundlich equation which is an empirical two-parametric power function (Eq. (1)): qi,GG = KF × cni,GG
(1)
where qi,GG is the equilibrium solid-phase concentration of i adsorbed from solution per unit mass of adsorbent (mg/g); KF is the Freundlich constant in (mg/g)/(mg/l)n; ci,GG is the equilibrium concentration of i in solution (mg/l); and n is the Freundlich exponent. A minimum contact time of 48 h was chosen to reach equilibrium. The isotherms were either run in distilled water or in tap water (Karlsruhe, DOC=0.6 mg/l) without or in the presence of NOM. All experiments were run at ambient temperature.
2.4. Laboratory plant for filter experiments 2. Experimental set-up
2.1. Adsorpti6es Natural organic matter (NOM) was taken from two bog lakes (Wildsee, Hohlohsee, northern Black Forest, Germany). The content of dissolved organic carbon (DOC) was analysed by wet chemical oxidation (DC 80, Dohrmann) and also by catalytic oxidation (TOC 5000 Total Organic Carbon Analyzer, Shimadzu). The model compounds 2-aminonaphthalene-1-sulfonate, diuron and 1-naphthol were quantified by UV/vis spectroscopy with a UV/vis-spectrophotometer (Lambda 5, Perkin Elmer). The absorption maxima used for calibration and determination were l= 282 and 341 nm for 2-aminonaphthalene-1-sulfonate, l= 248 nm for diuron, and l=215 nm for 1-naphthol. In the presence of NOM the compounds were analysed by a diode-array detector after liquid chromatographic separation (HPLC 1090, Hewlett Packard).
The flow scheme for the laboratory plant is shown in Fig. 1. Three parallel glass columns (chromatography columns) with an inner diameter of di =2.5 cm and di = 1 cm and a length of 45 cm were used (in Fig. 1 only one column is shown). Two precision pumps with adjustable flow between 0.1 and 50 ml/ min (Type P-50, Pharmacia) and 0.5–100 ml/min (Type P-6000, Pharmacia) fed the columns with the solution. The resulting filter velocities were between 0.01–6 and 0.06–12 m/s (empty bed velocity). The columns were made of a solvent resistant material as the regeneration of the polymer resin should be made with methanol. The length of the filters could be varied between 15 and 45 cm. For the experiments it was decreased additionally by glass beads to 5–10 cm. The columns were filled with Lewatit EP 63. To reach a filter velocity of 3.7 m/h, the large column (2.5 cm diameter) was run with a flow rate of 30 ml/min. The small column (1 cm diameter) was run with a flow rate of 4.8 ml/min. The UV absorbing substances were detected online in the outlet with an UV-detector (Uvicord VW 2251, Pharmacia).
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3. Results and discussion
3.1. Adsorption equilibrium of model compounds Adsorption isotherms of model compounds on F 300 and LiChrolut EN were investigated for different initial concentrations (10, 1, 0.5 and 0.09 mg/l). The adsorption equilibria of diuron on activated carbon and on LiChrolut EN with an initial concentration of 10 mg/l are shown in Fig. 2. Two ranges with different adsorption behaviour are clear for the two types of adsorbent.
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However, as activated carbon is a hydrophobic material, only a low adsorption capacity was expected in the case of the relatively hydrophilic diuron. Since the data show a relatively high adsorption capacity, diuron, however, seems to be able to move into the micropores of the activated carbon. Higher adsorption capacities were found for activated carbon masses below 1.6 mg/l, in comparison to LiChrolut EN which showed higher adsorption capacities for diuron in the higher concentration range (\ 1.6 mg/l). The adsorbent load increased with increasing initial concentration of diuron
Fig. 1. Flow scheme of the laboratory plant.
Fig. 2. Adsorption equilibrium of diuron on activated carbon (F 300) and LiChrolut EN, measured values (, ") and Freundlich calculation (–). The data were calculated by two fittings (low and high concentration range).
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Fig. 3. Adsorption isotherms of diuron on activated carbon (F 300) and Lewatit EP 63 for different initial concentrations c0 (measured values and Freundlich calculation).
much more rapidly for the organic polymer than for activated carbon. This behaviour is reflected in the higher Freundlich exponent for Lichrolut EN (LiChrolut EN: n=0.42; F 300: n =0.13). In the case of high concentrations of polar compounds in the liquid phase, it seems that the organic polymer resin is more suitable for their effective removal. In the case of 2-aminonaphthalene-1-sulfonate and 1-naphthol the loading on activated carbon is much higher than on LiChrolut EN (not shown). Almost no adsorption on LiChrolut EN was found for 2-aminonaphthalene-1-sulfonate. As the polymer resin did not have any ionic functional groups, it was expected that compounds with ionic character would show little adsorbability. The adsorption isotherms of diuron on activated carbon (F 300) and on Lewatit EP 63 for the initial concentrations of 1, 0.5 and 0.09 mg/l are shown in Fig. 3. For both sorbents different adsorption isotherms were found for various initial concentrations. This behaviour is characteristic for competing adsorption of several substances. It was not expected for the model compounds. An explanation can be seen in different adsorption mechanisms for the various concentration ranges. In the lower concentration range the sorbent surface will be covered by a monomolecular layer, whereas with increasing concentration of the substances the covering can be made increasingly of several layers. In general the adsorption of diuron, 2-aminonaphthalene-1-sulfonate and 1-naphthol on activated carbon was higher than on Lewatit EP 63 in the low concentration range.
3.2. Influence of natural organic matter on the adsorption equilibrium The adsorption isotherms of dissolved NOM (Wildsee) were determined for different pH values
(pH= 7 and 2) and different DOC concentrations. At pH= 7, the adsorption of DOC on LiChrolut EN was fairly poor, as already observed for the hydrophilic model compounds (Fig. 4). For both sorbents the adsorption of DOC was significantly higher at pH 2 compared to pH 7. This is due to the formation of unassociated acidic sites as a consequence of protonation of the anionic functional groups at pH 2, which leads to a decrease in the molecular size of the polyelectrolytic humic substances and to an increase of their hydrophobic character [19]. However, the adsorption capacity of DOC on LiChrolut EN is lower compared to that on activated carbon (not shown). This agrees with the data of Boenig et al. [20] who also found a poor adsorption of humic substances on ionic polymer resins of polystyrene type. Detailed investigations on the adsorption of humic substances from different origin on activated carbon are given by Abbt-Braun et al. [21]. The influence of humic substances on the adsorption equilibrium of diuron on activated carbon F 300 is shown in Fig. 5. No influence on the adsorption of diuron was observed for high masses of sorbents or low equilibrium concentrations. Obviously there are enough sites for the adsorption to avoid competing effects. However, the sorption of diuron decreases in the presence of humic substances with decreasing masses of sorbents (increasing equilibrium concentration). Carbon-fouling was observed for high DOC concentration of the bog lake water (3 and 10 mg/l DOC, Wildsee) [22]. No significant influence on the adsorption of diuron on F 300 in the presence of low DOC concentrations (tap water) was found. The sorption behaviour of a mixture compared to a single compound isotherm can be described by the Freundlich exponent n [1,23]. As
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there is only a small decrease of n, we suggest that part of the humic substances have an adsorption behaviour similar to diuron and that there is no strong interaction between the substances. The adsorption isotherms of diuron on F 300 are similar in the presence of 3 and 10 mg/l DOC. This suggests that the sorbability of NOM is decreasing for higher initial DOC concentrations. It is known that humic substances can form aggregates with increasing concentration [2]. Whereas the polar functional groups are orientated to the liquid phase, the hydrophobic building blocks are positioned in the core of the aggregates, resulting in a decreased adsorbability. In addition, the interaction of diuron and humic substances will also influence the adsorption. Electron donor–acceptor complexes and hydrogen bonds between humic substances and triacine herbicides were postulated by Senesi and Testini [24]. Schnitzer and
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Kahn [25] presumed covalent bonding between pesticides and humic substances, and sorption of the organic compounds on solid humic substances. Therefore, humic substances will have a multifunctional influence on the distribution of diuron. The attempt to separate the associates of NOM and diuron from the free diuron with HPLC/DAD was not successful. It can be deduced that associates, if they really exist, have a fairly weak interaction of the components. The adsorption of diuron onto LiChrolut EN was not highly influenced by NOM (not shown). The Freundlich constants decreased to 98% (DOCHS,0 =3 mg/l) and to 94% (DOCHS,0 = 10 mg/l). For tap water (DOC= 0.6 mg/l), no effect could be observed. This is in agreement with the poor adsorption of humic substances on the polymer resin. The small decrease of the
Fig. 4. Sorption isotherms of NOM (Wildsee, pH 7) on activated carbon (F 300) and on LiChrolut EN.
Fig. 5. Influence of humic substances (HS) on the adsorption of diuron on activated carbon (F 300) at pH 7.
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Fig. 6. Comparison of the breakthrough curves of the model compounds in the laboratory columns filled with Lewatit EP 63.
Freundlich constants can again be explained by the formation of weak associates of diuron and NOM which do not adsorb. The fact that there is no pronounced influence of the NOM on the selective adsorption of polar compounds on LiChrolut EN is an advantage in its practical application to remove organic micropollutants in the presence of an organic background. In drinking water treatment, it may be a goal to remove specific toxic compounds while the non-toxic NOM can remain in the water phase. In this respect, the polymer resin may be preferable to activated carbon.
3.3. Filter experiments The breakthrough curves of the model compounds on Lewatit EP 63 are given in Fig. 6. The different adsorption behaviour of the components is well reflected in the different breakthrough times. The breakthrough of diuron was very slow; 20% of the initial concentration (co) was found after 10 000 bed volumes and even after 80 000 bed volumes (not shown in Fig. 6), only 60% of the feed concentration was found in the outflow. The good adsorbability of 1-naphthol on Lewatit EP 63 could be confirmed by the filter experiments. In the case of technical applications, the low concentration of 1-naphthol in the outflow at the beginning would be an advantage. The extremely low adsorbability of 2-aminonaphthalene-1-sulfonate and NOM on the polymer resin is also reflected in the short breakthrough times. If the feed concentration of 2-aminonaphthalene-1-sulfonate increased from 1 to 10 mg/l, the breakthrough took place in half the time. The same results were observed for NOM (not shown).
3.4. Influence of NOM on the breakthrough of model compounds In the presence of NOM, the breakthrough time of diuron decreased (Fig. 7). Even after a few bed volumes, the concentration in the outlet reached 40% of the feed concentration. Since NOM have a low adsorbability on the polymer resin it is unlikely that a preloading of humic substances caused this decrease. It is interesting to note that Baldauf [26] and Sontheimer et al. [2] showed that, for activated carbon, humic substances have a strong influence on adsorption processes. The breakthrough of 1-naphthol in the resin columns is also faster in the presence of NOM (Fig. 8). It is very likely that again interactions between naphthol and the humic substances are responsible for a higher mobility of naphthol. The shapes of the two breakthrough curves are quite similar and start from the baseline after several bed volumes. Therefore Lewatit EP 63 can be considered for the removal of 1-naphthol from polluted water. The ionic character of 2-aminonaphthalene-1-sulfonate and of NOM are responsible for a fairly rapid breakthrough in the Lewatit EP 63 column. There is a mutual influence on the breakthrough behaviour (Fig. 9). A decrease of the pH-value had a significant influence on the breakthrough of 2-aminonaphthalene-1-sulfonate in the presence and absence of NOM. A similar pH-dependent behaviour was found for the humic substances alone. Two reasons may be considered. First the pH-value has an influence on the dissociation equilibrium of the functional groups and so on the ionic character. Second, the pH-value has an influence on the size of the molecules of polyelectrolytes. At lower pH values, the molecules are smaller as the repulsive forces between the functional groups decrease with protona-
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tion. Consequently, more pores and adsorption sites are available for the protonated substances, leading to their retardation. The outflow concentrations of more than 100% of the feed concentration (overshoot) in the presence of NOM suggest its displacement as a weakly absorbing component by the sulfonate (chromatographic effect) [27]. For technical applications, the complete breakthrough is of minor interest, as filters are regenerated at a certain outlet concentration which is normally lower than the feed concentration. It could be shown that for synthetic resins also, the pH value of the feed and the concentration of NOM have an essential influence on the running time of the filter up to the maximal tolerable outlet concentration.
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3.5. Regeneration of the polymer resin One of the main advantages in technical applications of polymer resins compared to activated carbon is the regeneration at low costs. Therefore we investigated what quantity of the adsorbed substances could be desorbed by means of an organic solvent. The loaded sorbent in the filter was flushed with isopropanol. Fig. 10 shows the desorption behaviour of diuron and 1naphthol. For the strongly adsorbable substances diuron and naphthol, about 40–60 bed volumes of the regeneration solvent were needed to reach complete desorption. The maximum concentration was in the g/l range. The regenerate was enriched with the sorptives for three
Fig. 7. Influence of NOM on the breakthrough behaviour of diuron at pH 7.
Fig. 8. Influence of NOM on the breakthrough behaviour of 1-naphthol at pH 7.
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Fig. 9. Influence of NOM on the breakthrough behaviour of 2-aminonaphthalene-1-sulfonate.
Fig. 10. Desorption of diuron and 1-naphthol with isopropanol. Table 1 Recovery of the adsorbed substances after regeneration with isopropanol
Adsorbed mass Desorbed mass Desorbed amount (%)
NOM (as C)
2-Aminonaphthalene-1-sulfonate
1-Naphthol
Diuron
1.72 mg 1.22 mg 84.3
13.71 mg 13.09 mg 95.5
2.695 g 2.568 g 95.9
2.04 g 1.886 g 92.3
orders of magnitude which is a big advantage with regard to the further treatment or disposal of the regenerate. In addition the sorptive can be reused if it is a valuable substance. This may reduce the overall costs of treatment enormously. The weakly adsorbable substances 2-aminonaphthalene-1-sulfonate and NOM could be completely desorbed within 10 bed volumes of the solvent. The
maximum concentration obtained was about three times the initial feed concentration. On the basis of the breakthrough curves, mass balances for the sorbed substances were calculated. The recovery of the sorptives are shown in Table 1. For the single compounds, recoveries between 92 and 96% were found. This is in good agreement with recoveries \ 90%, found by Dedek et al. [9] for the enrichment of
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samples for the analysis of trace substances. Only about 84% of the NOM were desorbed. Possibly there is an irreversible loading of the resin as it can also be found for activated carbon. It has to be investigated in further experiments if this leads to a decrease of the adsorption capacity within the number of loading and regeneration cycles.
4. Conclusions For the application in drinking water treatment technology where low concentrations of pollutants are typically found, the activated carbon F 300 showed better adsorption characteristics than the polymer resins LiChrolut EN or Lewatit EP 63. However, in the higher concentration range (c0 =7 – 10 mg/l), the loads on the polymer resin were similar or even higher than on activated carbon. So the use of the resin is convenient at higher concentrations, e.g. in heavily polluted waters. An advantage of the use of polymer resin in comparison to activated carbon was the poor adsorbability of NOM like humic substances. Practically no influence of NOM on the isotherms of the investigated substances was found. As a consequence, toxic substances can be removed from the water phase while the non-toxic NOM present remain in solution. A comparable effect to ‘carbon-fouling’ — which leads to a decrease of the adsorption capacity caused by the irreversible pre-loading of activated carbon by humic substances—does not exist for the synthetic resins applied. Nevertheless, in the presence of humic substances the breakthrough times on Lewatit EP 63 decreased. This can be explained mainly by interactions between the humic substances and the single compounds. We presume that the power of the polymer resin is the selective removal of pollutants in complex matrix solutions. The validity of this assumption has to be verified in further experiments.
Acknowledgements The authors thank Bundesministerium fu¨r Bildung und Forschung (BMBF) for financial support (BMBF 02WT9612/7) and Professor Dr R. Nießner and Priv. Doz. Dr habil. W. Ho¨ll for the collaboration within the joint research program ‘Entfernung von organischen hydrophilen Schadstoffen mit Hilfen neuartiger Adsorberpolymere bei der Wasseraufbereitung — Grundlagen zur technischen Anwendung’. We gratefully acknowledge the experimental work and discussions with Elly Karle, Christian Specht and Andreas Schindeln.
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