Environ. Sci. Technol. 1999, 33, 3027-3032
Removal of Natural Organic Matter by Coagulation-Flocculation: A Pyrolysis-GC-MS Study A S T R I D E V I L G EÄ - R I T T E R , † A R M A N D M A S I O N , * ,† T H I E R R Y B O U L A N G EÄ , † DANIELLE RYBACKI,‡ AND JEAN-YVES BOTTERO† Ge´osciences de l’Environnement, CEREGE, UMR 6635 CNRS-Universite´ Aix-Marseille III, BP 80, Europoˆle Me´diterrane´en de l’Arbois, 13545 Aix-en-Provence Cedex 4, France, and CIRSEE, Suez-Lyonnaise des Eaux, 38 rue du pre´sident Wilson, 78230 Le Pecq, France
The removal of the natural organic matter (NOM) in the Seine river water sampled at Le Pecq (France) was carried out by using three coagulants: ferric chloride, aluminum sulfate, and aluminum polychlorosulfate (PACS). The efficiency of the coagulation-flocculation process depended on the pH. The best results were obtained at pH ) 5.5 with ferric chloride and pH ) 7.0 with the Al based coagulants aluminum sulfate and PACS.The composition of NOM in the raw and clarified water as well as in the flocs was analyzed by pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). Using this technique on the flocs formed with ferric chloride, a selectivity of this coagulant for polyhydroxyaromatics (PHA) was shown. A preferential binding was also observed between PACS and polysaccharides (PS). No such trend could be observed with aluminum sulfate. These results were confirmed and refined by PyGC-MS analyses on the different mass fractions of the clarified water obtained by gel permeation chromatography.
Introduction The optimization of the drinking water treatment requires an enhanced removal of natural organic matter (NOM). NOM is partly responsible for the fouling of filtration membranes and may form disinfection byproducts. Moreover oxidation byproducts can favor bacterial regrowth in the distribution network. Enhanced removal of the organics implies a better knowledge of the chemistry of NOM. Surface waters include diverse types of NOM originating from various environmental systems (1). Surface water NOM is mainly composed of variable proportions of allochthonous and autochthonous compounds. Various analytical techniques such as 13C magnetic nuclear resonance (2, 3), transmission electron microscopy (4), and NOM fractionation on XAD resins (5, 6) have been used to characterize aquatic NOM. Organic compounds in water and sediments are a mixture of biopolymers such as carbohydrates, lipids, and proteins as well as complex and less well characterized macromolecules, generally referred to as humic substances (7). * Corresponding author phone: +(33) (0) 442 97 15 34; fax: +(33) (0) 442 97 15 40; e-mail:
[email protected]. † CNRS-Universite ´ Aix-Marseille III. ‡ CIRSEE. 10.1021/es981232p CCC: $18.00 Published on Web 07/16/1999
1999 American Chemical Society
TABLE 1. Characteristics of the Seine River Water at Le Pecqa LE PECQ TOC mg/L DOC mg/L UV at 254 nm THMFP µg/L chloroform µg/L pH at 20 °C turbidity NTU NO3- mg/L a
4.0 3.8 0.058 223 52 7.7 27 23
LE PECQ NO2- mg/L NH4+ mg/L SO42- mg/L Cl- mg/L Ca2+ mg/L Mg2+ mg/L Fe µg/L
0.4 0.3 27 15 103 6 10
THMFP: trihalomethane formation potential.
Pyrolysis-gas chromatography-mass spectrometry (PyGC-MS) has been used to characterize numerous NOM samples from a variety of sources, such as wood (8, 9), fossil objects (10), soil sediments (11, 12), and aquatic systems (7, 13-15). Only few investigators have compared the different water treatment processes using Py-GC-MS (16, 17). Gray (16) showed that Py-GC-MS is a powerful analytical technique to study the organic matrix and that the efficiency of the treatment processes changes according to the composition of NOM in the water. Using the same technique, Widrig (17) examined the role played by algal populations and water quality in DOC removal during ozonation and coagulation processes. They have shown that although the pyrolysis profile before and after coagulation of algal organic matter with FeCl3 is dominated by aromatic nitrogen-containing fragments, coagulation may have produced some preferential removal of the parent material producing the aliphatic nitrile pyrolysis fragments. While after ozonation, there was a dramatic change in the pyrolysis-GC-MS fingerprint of this organic matrix. Coagulation-flocculation is the most important process in the drinking water treatment industry considering the mass of removed matter. Coagulation has historically been employed in water treatment practice to primary decrease levels of turbidity. In the present paper, gel permeation chromatography (GPC) and Py-GC-MS are used to study the coagulationflocculation of the NOM in the Seine river at Le Pecq (France) by three common coagulants.
Materials and Methods All experiments described hereafter were done in duplicate or triplicate and were reproducible (standard deviation 5000 Daltons), G3 (1500 < apparent molecular weight < 5000 Daltons), and G5 (apparent molecular weight < 1500 Daltons). 3028
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 17, 1999
This GPC method concentrates the soluble salts in the G5 fraction (22). Since these salts may affect the Py-GC-MS analysis (23), results obtained by using this technique on fraction G5 were not taken into account in the present study. Since sorption effects on the gel may affect GPC, the data will only be used to analyze trends and not in a quantitative manner. Pyrolysis-Gas Chromatography-Mass Spectrometry. PyGC-MS was performed on the GPC fractions G1 and G3 of the clarified water samples and on the freeze-dried flocs using a pyroprobe 100 filament pyrolyzer (CDS, Oxford) connected with the split injection port of a Carlo Erba 4160 gas chromatograph interfaced with a NERMAG R-10-10 C quadrupole mass spectrometer. The GPC fractions were concentrated by vacuum rotary evaporation down to a few milliliters and reduced to 100 µL under a nitrogen stream. Fifty microliters of this concentrate was injected in quartz tube and dried slowly at room temperature. Freeze-dried floc samples (about 1 mg) were put directly in the quartz tube. The quartz tubes were submitted to a flash pyrolysis; the platinum filament was programmed to a final temperature of 640 °C at a heating rate of 20 °C/ms and held at this temperature for 20 s. The pyrolysis products were separated on a DB-WAX column programmed from 30 to 220 °C at a rate of 3 °C/min and held at this temperature for 20 min. Pyrolysis products were detected by the mass spectrometer operated at 70 eV and scanned from 20 to 450 amu at 1 scan/s. Interpretation of Pyrolysis Data. The peaks of the pyrochromatogram correspond to compounds which are identified by comparison with the built-in spectrum library or the literature data. The relative amount of the pyrolysis products is determined from the chromatogram peak areas. Interpretation of the pyrolysis data can done by the global description of the pyrolysis byproducts and the comparison of the pyrochromatograms or by grouping the thermal degradation fragments into categories. In our case, the shape of the pyrochromatograms (Figure 1) of the raw or clarified waters was similar; all the main thermal degradation byproducts listed by Bruchet et al. (7) were detected in the samples. Therefore the pyrolysis products were grouped into biopolymer classes to perform a semiquantitative analysis. The four major biopolymers usually found in natural water are polysaccharides (PS), proteins (PR), aminosugars (AS), and polyhydroxyaromatics (PHA). Attribution of the identified
TABLE 2. Distribution of DOC (%) in the GPC Fractions [Al] or [Fe] ×10-4 mol/L ferric chloride pH 5.5
raw percent water 0.5 G1 G3 G5
FIGURE 2. Evolution of turbidity versus the dose of coagulants.
FIGURE 3. Proportion of NOM remaining (UV and DOC measurements) versus added coagulant at different pH: (a) for ferric chloride, (b) for aluminum sulfate, and (c) for PACS. compounds to one of these categories and the calculation of their relative proportion were carried out according to the method developed by Bruchet et al. (7). Unidentified or unattributable compounds are listed as miscellaneous (Misc.).
Results and Discussion Removal of Organic Matter: Jar Test. With all three coagulants and at each pH value, the turbidity of the water was decreased from 27 NTU (Table 1) to less than 1 NTU when the coagulant dose was larger than 1 × 10-4 M. For a better readability of Figure 2, only one value of pH was presented. This figure shows a typical evolution of the residual turbidity versus the coagulant doses and pH. There is no regrowth of residual turbidity with the increase of coagulant concentration (Figure 2). The DOC and UV measurements on the clarified water showed that the efficiency of NOM removal depends on the pH of coagulation (Figure 3): for ferric chloride, the best results were obtained at pH 5.5 (about 60% of C removal), and the proportion of remaining C showed only little
35 36 28
2
pH 7.5 0.5
2
aluminum sulfate pH 6
pH 8
PACS pH 6
pH 8
0.32 1.3 0.32 1.3 0.35 1.4 0.35 1.4
19 17 29 28 30 26 32 28 31 36 55 50 43 41 34
20 33 45
15 20 65
21 12 12 11 16 25 30 22 28 22 54 58 66 61 62
dependence on the dose of Fe added (Figure 3a). This is in agreement with previous findings showing that the optimum coagulation pH values are 5.5 for iron chloride (18, 24). With the Al based coagulants (aluminum sulfate and PACS), the lowest levels of remaining C were observed at pH 7 (based TOC removal) (Figure 3b,c). On the other hand, in terms of UV absorbance decrease, the lowest levels of remaining organic matter were observed in a pH 6-7 range for aluminum sulfate (Figure 3b) and in a pH 7-8 range for PACS (Figure 3c). For Al salts, the optimum pH for removal of NOM is usually reported to be in the 5-6 range (25-27). This range of pH is lower than the optimum value found in the present work. At optimum pH, the highest efficiency in organic C removal was observed with ferric chloride (approximately 35% of remaining C at [Fe] ) 1 × 10-4 M vs 75-80% with the Al salts at the same molar concentration) (Figure 3). In all cases, the decrease of UV absorbance between raw and clarified water samples is stronger than the decrease of DOC (Figure 3). Similar results have been reported previously (28). Since the UV absorbance is due essentially to the presence of aromatic compounds, its stronger decrease compared to DOC indicates that, in our water samples, this category of organic molecules is preferentially removed by the coagulation-flocculation process. Selectivity of Coagulants for NOM. Characterization of NOM (Py-GC-MS and GPC) was performed on samples resulting from the coagulation by Al and Fe salts at low and high coagulant concentrations ([Fe3+] ) 0.5 × 10-4 M and 2 × 10-4 M, [Al3+] ) 0.32 × 10-4 M and 1.3 × 10-4 M for aluminum sulfate and 0.35 × 10-4 M and 1.4 × 10-4 M for PACS, corresponding for each coagulant to 20 mg/L and 80 mg/L) and pH values (5.5 and 7.5 for ferric chloride or 6 and 7 for aluminum sulfate and PACS). Influence of the Apparent Molecular Weight. The analysis of the GPC fractions on the raw and clarified water allows for determination of the role played by each mass fraction in the coagulation-flocculation process. DOC measurements on the raw water (Table 2) revealed that the G1 and G3 mass fractions of the Seine river water contain comparable amounts of organic C (approximately 35%), the G5 fraction representing a slightly lower proportion (28%). In the clarified water samples, the NOM in the different fractions is not evenly affected: in all cases, the proportions of DOC in the G1 and G3 fractions are lower than in the raw water (Table 2), which means that the coagulation primarily eliminates the molecules with an apparent molecular weight larger than 1500 D. This is in agreement with previous studies demonstrating that the predominant part of flocculated NOM consist of large biopolymers (29-31). In our case, the G1 fraction (>5000 D) always was the most affected by the coagulation, and in particular in the water clarified with PACS, where the G1 fraction never represented more than 16% of the DOC vs 35% in the raw water (Table 2). Selectivity of Coagulants for Biopolymers. Py-GC-MS analyses of freeze-dried raw water yielded the following NOM composition: PR correspond to 3% of NOM, AS 41%, PHA 14%, PS 21%, and miscellaneous compounds 21% (Table 3). VOL. 33, NO. 17, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3029
TABLE 3. Proportion (Percent of DOC) of the Different Types of Biopolymers in Freeze-Dried Raw Water and in Freeze-Dried Flocs [Al] or [Fe] ×10-4 mol/L ferric chloride pH 5.5
raw percent water 0.5 PS PR AS PHA MISC
21 3 41 14 21
2
pH 7.5 0.5
2
23 13 13 37 10 6 0 11 2 51 7 1 47 27 61 34 18 3 19 17
aluminum sulfate pH 6
PACS
pH 8
pH 6
pH 8
0.32 1.3 0.32 1.3 0.35 1.4 0.35 1.4 21 11 38 26 4
17 5 36 39 3
40 11 3 42 4
9 68 27 60 42 0 8 3 13 13 1 0 6 8 8 87 16 54 10 29 3 8 10 9 8
TABLE 4. Proportion (Percent of DOC) of the Different Types of Biopolymers in Raw Water and in Clarified Waters [Al] or [Fe] ×10-4 mol/L ferric chloride
aluminum sulfate
PACS
pH 5.5 pH 7.5 pH 6 pH 8 pH 6 pH 8 raw percent water 0.5 2 0.5 2 0.32 1.3 0.32 1.3 0.35 1.4 0.35 1.4
G1 PS PR AS PHA MISC
11 3 59 9 18
45 6 22 7 20
12 23 17 49 4 4 5 7 24 41 52 4 18 8 10 16 42 24 16 24
PS PR AS PHA MISC
28 4 28 30 10
39 62 12 22 9 3 1 7 12 5 56 16 22 14 10 8 18 16 21 47
43 7 14 16 20
39 7 26 13 15
35 9 7 4 8 5 3 3 3 2 21 45 55 66 51 15 6 10 4 11 24 37 25 23 28
40 5 23 20 12
35 5 6 8 46
36 4 9 17 34
G3 29 8 17 14 32
22 4 41 14 19
16 6 40 20 18
15 4 53 12 16
20 4 38 17 21
This distribution of NOM was considerably modified in the flocs resulting from the coagulation by the Al and Fe salts. In the following sections, the results and discussion are presented according to the behavior of coagulants toward the biopolymers. Selectivity of Coagulants for PS. PACS. All the flocs formed with PACS contain a higher proportion of PS than the raw water (Table 3). At low concentration ([Al] ) 0.35 × 10-4 M), the PS represent 60% or more of the NOM within the flocs (Table 3). At higher concentration ([Al] ) 1.4 × 10-4 M), their relative proportion (27-42%) remains higher than in the raw water (21%) but is decreased compared to the lower PACS dose. This probably results from the enhanced binding of other types of biopolymers due to the increased number of available Al sites. These high PS proportions and their evolution with the coagulant dose are indicative of a selectivity of PACS for this type of molecules. A strong decrease of the proportion of DOC in the G1 fraction of water clarified by PACS was observed (Table 2). An explanation of this strong decrease in the case of PACS was provided by a more detailed analysis of the NOM in the GPC fractions. Py-GC-MS data indicate that 11% of the G1 fraction and 28% of the G3 fraction of the raw water correspond to PS compounds (Table 4). These proportions drop respectively to 4-9% and 15-22% in the G1 and G3 fractions of the water clarified with PACS (Table 4). This confirms the preferential binding of PS molecules by PACS, which was already hypothesized from the analysis of the NOM in the flocs (Table 3). Furthermore, the comparison of the initial PS proportions in the fractions with those obtained after coagulation shows that the PS are removed to the same extent irrespective of the apparent molecular weight (Table 4). At pH 6.0 and [Al] ) 0.35 × 10-4 M, the proportion of PS in the clarified water is decreased by about 20% in both the 3030
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 17, 1999
G1 and the G3 fraction. In the other PACS samples, the decrease of the PS proportion in the G1 and G3 fractions is also almost identical: approximately 40% for pH 6.0 and [Al] ) 1.4 × 10-4 M, 50-60% for pH 8.0 and [Al] ) 0.35 × 10-4 M, and 30% for pH 8.0 and [Al] ) 1.4 × 10-4 M. Therefore, it can be assumed that the PS in the G1 and G3 fractions differ only in polymer molecular weight and not in chemical nature. Ferric Chloride and Aluminum Sulfate. The PS are not efficiently coagulated by these two coagulants. The flocs formed with aluminum sulfate or ferric chloride, except [Fe] ) 2 × 10-4 M at pH ) 7.5 and [Al] ) 0.32 × 10-4 M at pH ) 8, contain a proportion of PS comparable or lower than in the raw water (Table 3). Correlatively, the proportion of PS in the fractions G1 and G3 in the clarified water is usually significantly higher than in the raw water and does not follow any trend (Table 4). Unlike the case of PACS, no preferential binding or high affinity for PS could be observed with iron chloride and aluminum sulfate. Selectivity of Coagulants for PR. PR molecules correspond to a low proportion of DOC in the raw water (3%) (Table 3). The proportion of PR in the flocs formed with each coagulant are low and close to the initial value (Table 3). Moreover for all the coagulants (ferric chloride, aluminum sulfate, and PACS), the proportion of PS in the clarified water (fractions G1 and G3) is the same as in the raw water (Table 4). This suggests that PR molecules of Seine river water are not coagulated by Al or Fe salts. Selectivity of Coagulants for PHA. Ferric Chloride. All the flocs formed with ferric chloride were enriched in PHA: the proportion of this NOM category in the flocs ranged from 27% (pH 5.5, [Fe] ) 2.0 × 10-4 M) to 61% (pH 7.5, [Fe] ) 0.5 × 10-4 M), whereas the PHA represented only 14% of the NOM in the raw water (Table 3). Thus, ferric chloride seems to exhibit a selectivity for the PHA compounds. For a given coagulant dose, the proportion of PHA in the flocs increased between pH ) 5.5 and pH ) 7.5. Enhanced binding of the PHA with increasing pH can be easily explained by their higher reactivity due to the deprotonation of phenolic OH groups. Indeed, previous studies on natural organics showed that the titration of phenolic protons occurs quantitatively at pH values around 8, whereas this phenomenon is negligible at lower pH (32, 33). At each pH, the proportion of PHA within the flocs decreased with increasing ferric chloride dose (Table 3). This supports the hypothesis that ferric chloride selectively removes the PHA from the raw water: at low ferric chloride concentration (0.5 × 10-4 M), the high affinity of Fe for PHA compounds and the limited amount of Fe binding sites result in high proportions of PHA in the flocs. At higher ferric chloride concentration (2 × 10-4 M), the increased number of Fe binding sites enables the binding of other categories of NOM which translates to a lower relative proportion of PHA in the flocs. The analysis of the NOM in the clarified water also confirmed the selectivity of ferric chloride toward PHA compounds. However, in our water sample, this preferential binding is evident only in the G3 fraction in which the proportion of PHA is decreased by 20% (pH 5.5, [Fe] ) 0.50 × 10-4 M) to 75% (pH 7.5, [Fe] ) 2.0 × 10-4 M) with respect to the G3 fraction of the raw water (Table 4). The proportion of PHA in the G1 fraction remains relatively unaffected. This strongly suggests that, contrary to the PS, the G1 and G3 fractions contain PHA compounds of different chemical nature and reactivity and thus presumably of different origin, the more reactive molecules being in the G3 fraction. Aluminum Sulfate. Large proportions of PHA (26-87%) are also present in the flocs formed by coagulation with aluminum sulfate (Table 3). Unlike ferric chloride, the proportion of PHA in the flocs formed with aluminum sulfate increased with both pH and coagulant dose. As stated above,
the increase of the PHA proportion at higher pH is consistent with a higher reactivity of this type of molecules due to deprotonation. The interpretation of the increase of the PHA proportion with the aluminum sulfate dose at a same pH is not straightforward. It may denote a differential reactivity among the PHA molecules (e.g. COOH vs OH groups), the less stable complexes being formed only at higher Al concentration. Therefore, although the high proportion of PHA in the flocs indicates a certain affinity, aluminum sulfate may not be considered as selective toward PHA. The analysis of the NOM in the clarified water also confirmed the better affinity of PHA for aluminum sulfate. Similarly to the case of ferric chloride, the proportions of PHA in the G1 fractions remains relatively unaffected by coagulation with aluminum sulfate (9% in the raw water and >7% in the clarified water). Only the PHA of the G3 fraction are removed (30% in the raw water and
