The Efficiency of Different Oxidation Methods in Total ...

Chem. Anal. (Warsaw), 48, 243 (2003)

The Efficiency of Different Oxidation Methods in Total Organic Carbon Analysis by U. Raczyk-Stanis³awiak, J. Œwietlik, B. Kasprzyk and J. Nawrocki* Faculty of Chemistry, Adam Mickiewicz University, ul. Drzyma³y 24, 60–613 Poznañ, POLAND Key words:

total organic carbon, TOC, TOC apparatus

TOC analysis is required for the quality of drinking water and sewage assessment. The analysis is thought to be questionable, since there are many commercially available TOC analysers, employing different oxidation methodologies. The authors have made an attempt to compare the efficiency of three TOC analysers: TOC 1010, LABTOC, and GO–TOC 100 utilising different oxidation methods, such as: persulfate/100°C, persulfate/UV and hightemperature oxidation, respectively. Humic acid, polyacryloamide flocculants and perfluorooctanoic acid were used as model compounds. None of the examined apparatus led to their complete oxidation. The influence of pre-oxidation with ozone and chlorine dioxide on the efficiency of the subsequent TOC analysis was investigated and was found to cause better combustion of the organic carbon. The storage time of the samples treated with phosphoric acid did not influence the efficiency of TOC analysis. Ogólny wêgiel organiczny (OWO) jest jedynym, dobrze zdefiniowanym parametrem okreœlaj¹cym sumê organicznych zanieczyszczeñ wód i œcieków. Do oznaczania OWO stosuje siê ró¿ne metody: katalityczne utlenianie wysokotemperaturowe i utlenianie niskotemperaturowe (wykorzystuj¹ce metody chemiczne i utlenianie wspomagane promieniowaniem UV). Wiele firm oferuje aparaty do oznaczania OWO wykorzystuj¹ce ró¿ne metody utleniania. Autorzy przebadali trzy analizatory TOC 1010, LABTOC i GO–TOC 100 wykorzystuj¹ce ró¿ne metody spalania: nadsiarczan/100°C, nadsiarczan/UV i wysokotemperaturowe spalanie. Oznaczano efektywnoœæ utleniania kwasów huminowych, poliakryloamidowych flokulantów i kwasu perfluorooktanowego. ¯aden z badanych aparatów nie zapewnia³ ca³kowitego utleniania badanych zwi¹zków. Zastosowanie wstêpnego ozonowania i wstêpnego utlenienia dwutlenkiem chloru zapewnia³o wy¿sz¹ efektywnoœæ spalania zwi¹zków podczas analiz. Czas przechowywania próbek utrwalonych kwasem fosforowym nie mia³ wp³ywu na efektywnoœæ oznaczenia OWO.

* Corresponding author. E-mail: [email protected]. Tel.: +48-618293430; fax: +48-618293400

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U. Raczyk-Stanis³awiak, J. Œwietlik, B. Kasprzyk and J. Nawrocki

Total organic carbon (TOC) analysis is a fast method, suitable for the determination of both: total organic contaminants and water and sewage contamination level. Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) are other indices that allow us to determine the level of water contamination with organic compounds. However, these procedures are inconvenient and time-consuming. Total organic carbon (TOC) is thought to be the only sufficiently well defined index for the measurement of the total amount of organic contaminants in water and sewage. Different oxidation procedures including high-temperature catalytic oxidation and low-temperature oxidation involving chemical oxidation and UV-promoted oxidation are applied to the TOC determination. Various TOC analysers utilising different oxidation methods were available. Van Hall reported the method of the organic carbon oxidation in 1963 for the first time [1]. High-temperature oxidation of the organic compound, leading to CO2 emission with its subsequent infrared detection (IR) was applied [2]. There are three methods for TOC determination described in Standard Methods for the Examination of Water and Wastewater [2]: high-temperature oxidation (‘Combustion – Infrared Method’); persulfate oxidation (‘Wet – Oxidation Method’) and UV-promoted persulfate oxidation (‘Persulfate – Ultraviolet Oxidation Method’). Hightemperature combustion is the most convenient approach towards samples containing more than 1 mg L-1 [2]. Table 1. Advantages and disadvantages of TOC analysis [3] Method of oxidation High-temperature oxidation

Persulfate oxidation

Advantages - speed of process - particulate organic carbon can also be determined - relatively low level of interference

UV-promoted persulfate oxidation Ultraviolet oxidation -

high sensitivity high CO2 recovery high level of precision low maintenance cost high sensitivity high CO2 recovery high level of precision fast oxidation rate low maintenance cost no specific chemical reagents are required - low cost of maintenance

Disadvantages - low sensitivity - difficulties with background determination - low concentration of salts present in a sample is required, - the possibility of CO2 loss in the atmosphere of condensing water vapour - possible problems with organic carbon recovery out of some aromatic compounds - high maintenance cost - the possibility of catalyst poisoning - slow oxidation rate - non quantitative recovery in the case of high concentration of TOC - possibility of interferences derived from chloride during detection stage in the atmosphere rich in oxygen

-

relatively low level of oxidation large volume of sample required low accuracy for the range below 5 ppm long response time

Oxidation methods in total organic carbon analysis

245

In order to determine the TOC concentration in the sample, which is expected to contain less than 1 mg L-1, persulfate as well as UV-promoted persulfate oxidation method is recommended. Basic advantages and disadvantages of various oxidation methods are outlined in Table 1. Potassium hydroxyphthalate (KHP) is the most commonly used standard for the TOC analysis. It has a characteristic simple chemical structure and easily undergoes oxidation. In contrast, it is usually much more difficult to oxidise the natural organic matter present in water. Najm et al. [9] points at two factors responsible for this difference: natural humic substances are of diverse composition, as well as their oxidation potentials are varied and differ from this of KHP, water contains natural organic matter of complex composition and at various concentration levels, which lead to the differences in the determined TOC content; these differences become more pronounced for bigger molecules. Many research groups have made an attempt to compare different methods for TOC determination in water [4–10]. High-temperature oxidation method is regarded to be the most convenient one for the sea water determination. It is capable of determining the higher amount of organic carbon than persulfate method [6,7,8,11]. It has been reported [5] that the best results can be obtained for the filtered water, free from particulate organic matter (only DOC was measured) in a short analysis time. The storage of unfiltered samples, after their prior freezing and acidifying, did not provide one with reliable results. The influence of large quantities of chloride on the efficiency of persulfate method (assisted with ultraviolet light or temperature) was not taken into consideration. However, Aiken [7] has observed that chloride tends to compete with organic carbon for persulfate ion. The efficiency of various methods for TOC determination was also examined for non-saline water [8–10]. Six different oxidation methods were employed for KHP and natural organic matter [8]. High-temperature catalytic oxidation, UV-promoted persulfate oxidation and potassium dichromate oxidation in the presence of H2SO4/ H3PO4 were investigated and they resulted in comparable results when KHP was examined. Yet, the results differed significantly when natural water samples were examined. A high-temperature oxidation method was found to give results higher by 20–24% than persulfate and dichromate methods, which was attributed to the incomplete oxidation of complex humic substances of high molecular weight [8]. Two oxidation methods: high-temperature catalytic combustion and UV-promoted persulfate oxidation were compared by Najm et al. [9] and the effect of water sample filtration preceding the analysis was investigated. The incompatibility between the two examined methods was observed. In particular, UV-promoted persulfate oxidation of raw water samples resulted in lowered TOC concentration levels. The efficiency of particulate organic carbon oxidation was also investigated. Neither the dis-

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solved or suspended inorganic carbon, nor the sample turbidity affected the efficiency of the analysis. Moreover, the higher the concentration of the suspended organic carbon in the sample, the stronger deviations among the results were observed. The high-temperature method was considered to give better results, since it is capable of oxidising not only dissolved organic carbon but also particulate organic carbon and therefore is regarded as being more useful for the total organic carbon determination. Chemical methods are efficient for low levels of organic carbon. Moreover, they are more efficient only in the case of dissolved organic carbon analysis. Kaplan compared the efficiencies of high-temperature combustion method, chemical oxidation and UV-promoted persulfate oxidation towards dissolved organic carbon present in natural water samples originating from several places in North America. The removal of inorganic carbon from the examined sample had to be undertaken before analysis [10]. It is commonly observed during the routine laboratory TOC analysis that TOC content increases after ozonation of water. Thus, we believed that the combustion of NOM is not complete and may be increased after the pre-oxidation of the water sample. The aim of this work was to assess the efficiencies of various oxidation methods for TOC determination performed in the apparatus, which is currently commercially available. Humic substances and organic flocculants were examined, since they are of great importance in water treatment technology. TOC determination was also carried out for perfluorooctanoic acid, which was found to be particularly resistant to the oxidation process. The purpose of this work was to investigate the influence of the pre-oxidation processes, such as ozonation and oxidation with chlorine dioxide on the determined TOC content in water samples. The effect of the storage time was also investigated.

EXPERIMENTAL Materials The following model organic compounds were used throughout: humic acid, sodium salt (Aldrich), polyacrylamide flocculants Magnafloc LT24 and Magnafloc LT22 (Ciba, acrylamide concentration, 0,05%; molecular mass, > 7 × 106); perfluorooctanoic acid (Aldrich). Prior to the solution preparation, the elemental analysis of humic acid and flocculants was performed with Perkin Elmer 240 Analyser, USA in order to establish the carbon, nitrogen and hydrogen content in the sample. According to these results, humic acid and flocculants solutions of increasing concentration were prepared: 2, 5, 10, 15 mg L-1C for humic acid and 5, 10, 15 mg L-1C for flocculants. Perfluorooctanoic acid solutions were prepared by dissolution of 0.2, 0.04 and 0.02 g of the substrate in 1 L of deionized water. All the prepared samples were preserved with phosphoric acid.

Oxidation methods in total organic carbon analysis

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Apparatus Three different apparatus for TOC analysis employing different oxidation method were used: TOC 1010 (I.O.Analytical), oxidation method: persulfate/100°C, detection: IR absorption, detection level: 2 ppb–125 ppm C, sample volume: 10 mL. LABTOC (Pollution & Process Monitoring Ltd), oxidation method: persulfate/UV, detection: IR absorption, determination range: 0–10 ppm C, detection level: 1% of determination range, sample volume: a few mL. GO TOC 100 (Gröger & Obst GmbH), oxidation method: high-temperature oxidation, detection: IR absorption, detection level: >1.67 ppm C, analyser for industrial purpose – on-line. Their calibration was carried out according to the manufacturer’s instructions. The potassium hydroxyphthalate (KHC8H4O4) solution served as a standard. Size exclusion chromatography (SEC) The molecular mass distribution of humic acid was determined with high-performance size exclusion chromatography (SEC), on a DIONEX DX–500 Chromatography System equipped with TosoHaas TSK gel G 3000 SWXL column and TosoHaas TSK gel SW guard column (Tosoh Corporation, Japan), and UV–detection at 254 nm (AD 25 detector, Dionex, USA). 0.01 mol L-1 phosphate buffer of pH 7.00±0.05 was used as an eluent. The samples were injected without the buffer addition. The analyses were performed at 30°C. All samples were filtered through the 0.45 µm membrane. Peak maximum calibrations were performed with sodium polystyrene sulfonate standards (30900 D, 13400 D, 4850 D, 1120 D, 172 D) (PSS Polymer Standards, Germany) (Perminova et al., 1998). Molecular mass, which is related to the size of the solute molecules, was plotted against the retention time employing the logarithmic ordinate scale (Fig. 1). 5,5

5

Log Mw

4,5

4

3,5

3

2,5

2 6

7

8

9

10

11 Time, min

Figure 1. SEC – calibration curve

12

13

14

15

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U. Raczyk-Stanis³awiak, J. Œwietlik, B. Kasprzyk and J. Nawrocki

RESULTS AND DISCUSSION The characteristics of the substances investigated Elemental analysis was applied in order to investigate the percentage content of carbon, nitrogen and hydrogen. The results are presented in Table 2. The found carbon content was used to prepare the samples of known organic carbon contents. In other words, the elemental analysis served as a reference procedure for the studies on the TOC analysers’ efficiency. Table 2. Elemental analysis of humic acid and magnafloc LT24 and LT22S

Humic acids, sodium salts Magnafloc LT24 Magnafloc LT22S

%C

av. %C

%N

av. N%

%H

av. % H

40.20 39.93 42.92 42.68 45.09 44.81

40.07

0.65 0.67 15.38 15.40 14.06 13.82

0.66

3.95 3.93 7.34 7.40 7.44 7.38

3.94

42.80 44.95

15.39 13.94

7.37 7.41

Size exclusion chromatography – characteristics and molecular size distribution of humic acids, sodium salts The molecular size distribution of humic acids before and after its treatment with ozone and chlorine dioxide is presented in Figure 2. It can be observed that molecules of molecular mass of 1870Da are predominant, while those of approximately 250Da share the smaller part. Ozone treatment affects the molecular size distribution of humic acids, since a number of peaks indicating lower-mass molecules (1500, 1120, 870, 600, 350 and 250Da) can be observed (Fig. 2). A significant decrease of the absorbance of the molecules lighter than 500Da was noticed. It is proposed that these molecules are the most reactive fractions of humic acids. Molar extinction of humic acid increases in the presence of chlorine dioxide, which leads to the formation of highly absorbing groups. The molecular size distribution of untreated humic acids and humic acids treated with chlorine dioxide does not differ significantly. Yet, some changes are obvious and several weak peaks referring to 1580, 1200, 870, 600 and 250Da can be observed, as well as one single peak of 350Da, which was not found for the untreated humic acids (Fig. 2).

Oxidation methods in total organic carbon analysis

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Aldrich humic acid

1,75x10-2 1,50x10-2

Aldrich humic acid + O3

-2

1,25x10

Aldrich humic acid + ClO2 AU

1,00x10-2 7,50x10-3 5,00x10-3 2,50x10-3

0 7,00

8,00

9,00

10,00

11,00

12,00

13,00

14,00

15,00

Minutes

Figure 2. SEC chromatogram of the sodium salt of humic acid

TOC of humic acids, flocculants and perfluorooctanoic acid aqueous solution determination A series of experiments were carried out in order to determine the oxidation efficiency of humic acids and polyacrylamide flocculants in aqueous solution – Magnafloc LT24 and Magnafloc LT22. The average results from three sets of apparatus: TOC 1010, LABTOC, and GO–TOC 100 were compared and given in Figures 3 and 4. 100 90

Efficiency of oxidation, %

80 70 60 50 40 30 20 10 0

TOC

LABTOC 2 ppmC

5 ppmC

10 ppmC

GOTOC 15 ppmC

Figure 3. The comparison of the efficiency of TOC analysers – humic acid determination

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U. Raczyk-Stanis³awiak, J. Œwietlik, B. Kasprzyk and J. Nawrocki

120

Efficiency of oxidation, %

100 80

60 40 20 0 LT22TOC

LT22LABTOC 5 ppmC

LT24TOC 10 ppmC

LT24LABTOC

15 ppmC

Figure 4. The comparison of the efficiency of TOC analysers – polyacrylamide flocculants determination

The efficiency of TOC determination in humic acid aqueous solution performed with GO–TOC, TOC1010 and LABTOC was found to be 73–87%, 72–82% and 62–74%, respectively. The analysis of flocculants’ aqueous solution resulted in better efficiency of 79–98% for TOC1010. The concentration of flocculants did not affect the efficiency of the analysis. It can be concluded that the best results were obtained for the GO–TOC apparatus utilising a high-temperature oxidation method. A relatively low efficiency characterises the LABTOC apparatus based on the persulfate/ UV method. The results indicate that no complete oxidation of either humic acid or flocculants occur. However, flocculants provide much higher efficiency of analysis than humic acid, which suggest that the efficiency of TOC determination depends mainly on the structure of the molecule than on its molecular mass. Experiments were also carried out for perfluorooctanoic acid aqueous solution. As presented in Table 3, the results of TOC determination are unsatisfactory for both analysers: 6.1–8.2% and 2.6–12.1% for TOC 1010 and LABTOC, respectively. There is a tendency of better results for lower concentrations of humic acids observed for all methods of combustion. The results obtained for the LABTOC analyser indicate rather low precision of analysis of the samples containing perfluoroorganic compounds. Table 3. TOC apparatus efficiency for perfluorooctanoic acid determination Concentration of perfluorooctanoic acid 0.2 g L-1 ( 46.4 mg L-1C) 0.04 g L-1 (9.28 mg L-1C) 0.02 g L-1 ( 4.64 mg L-1C)

TOC, mg L-1

Efficiency, %

TOC 1010

LABTOC

TOC 1010

LABTOC

2.83 0.76 0.38

1.20 0.88 0.56

6.1 8.2 8.2

2.6 9.5 12.1

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251

The effect of TOC of humic acids aqueous solution determination after preoxidation with ozone and chlorine dioxide As it was mentioned in the previous paragraph, complex organic molecules, such as humic acids, are resistant to chemical decomposition to some extent. Therefore, it was essential to examine the influence of oxidation with ozone and chlorine dioxide on the efficiency of organic carbon determination with various examined TOC analysers. We suspected that humic acids would be only partially oxidized and compounds of lower molecular mass would be formed, which are possible to undergo degradation more easily than the initial substrate. This might result in higher oxidation efficiency, which in turn might depend on the TOC analyser used. The comparison between TOC 1010, LABTOC and GO–TOC 100 analysers was performed again. Three stages of the experiment could be distinguished: TOC determination in initial solution; TOC determination in the solution treated with ozone (ozone dosage: 0.5 mgO3/mgC); TOC determination in solution treated with chlorine dioxide (chlorine dioxide dosage: 0.2 mgClO2/mgC). The results presented in Figure 5 exhibit that the application of ozone or chlorine dioxide did not provide one with the complete oxidation of organic matter, yet ozonation led to better results. Regardless of the equipment used, the efficiency of TOC analysis was improved in comparison to the results obtained without pre-oxidation, especially for low concentrations of humic acids (2 mg L-1); for comparison see Figure 3. Chlorine dioxide is a weaker oxidant than ozone and thus it did not reveal the same tendency. 100 90

Efficiency of oxidation, %

80 70 60 50 40 30 20 10 0

O3TOC

O3LABTOC 2 ppmC

O3GOTOC 5 ppmC

ClO2TOC

10 ppmC

ClO2LABTOC ClO2GOTOC

15 ppmC

Figure 5. The comparison of the efficiency of TOC analysers – humic acids determination in the presence of ozone and chlorine dioxide as oxidant

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TOC determination in polyacrylamide flocculants (Magnafloc LT24) solution after oxidation with chlorine dioxide The oxidation efficiency of polyacrylamide flocculants (Magnafloc LT24) aqueous solutions of various concentrations was investigated with TOC 1010 and LABTOC, and in the presence of chlorine dioxide (Fig. 6). Both apparatus provided one with similar results. However, the higher the dose of the chlorine dioxide was the higher the oxidation and TOC analysis efficiency became. The most concentrated flocculants samples encountered the most pronounced influence of pre-oxidation with chlorine dioxide. It may be anticipated that chlorine dioxide, though as less powerful oxidant than ozone, destroys the structure of flocculants and causes the increase of TOC analysis efficiency as a result. TOC 1010 140

Efficiency of oxidation, %

120

100

80

60

40

20

0

2.14

without oxidant

4.28

+0,2 ppmClO2

+0,4 ppmClO2

8.56

-1

mg L TOC

+0,8 ppmClO2

+1,2 ppmClO2

(b) LABTOC

Efficiency of oxidation, %

140

120

100

80

60

40

20

0

2.14 without oxidant

4.28 +0,2 ppmClO2

+0,4 ppmClO2

8.56

-1

mg L TOC

+0,8 ppmClO2

+1,2 ppmClO2

Figure 6. The comparison of the efficiency of TOC analyser–magnafloc LT24 determination in the presence of chlorine dioxide as oxidant

Oxidation methods in total organic carbon analysis

253

The effect of the time of storage on TOC analysis The effect of the storage time on humic acids and Magnafloc LT24 and LT22S aqueous solutions was studied. Analysis was conducted with TOC 1010 immediately after the preparation of the samples and 2 and 6 days later (Fig. 7). The best efficiency of TOC analysis was obtained for fresh samples; however, longer storage time did not significantly influence the quality of analysis. Figure 7 confirms much better efficiency of TOC analysis of flocculants in comparison to humic acid. 100 90

Efficiency of oxidation, %

80 70 60 50 40 30 20 10 0 fresh sample

after 2 days

after 6 days

humic acid 5 ppm

humic acid 10 ppm

magnafloc 24 5 ppm

magnafloc 24 10 ppm

magnafloc 22 5 ppm

magnafloc 22 10 ppm

Figure 7. The effect of the storage time on humic acids and magnafloc 24 and 22 TOC determination

CONCLUSIONS The studies on high molecular mass organic compounds, such as humic acids, polyacryloamid flocculants and perfluorooctanioc acid indicate that: high molecular mass humic acids and polyacrylamide flocculants were not completely oxidized, regardless of the apparatus employed. Higher oxidation efficiency was observed for low concentrations of humic acids (2–5 mg L-1C). Perfluorooctanoic acid was observed to be resistant to oxidation. The TOC analysers examined did not allow determining its content efficiently. The structure of the compound and not its high molecular mass is responsible for its incomplete oxidation, since flocculants can be oxidised to a higher extent. It has been reported that the temperature oxidation method is regarded to be the most efficient for humic acid determination [1,7]. Wet oxidation method (persulfate/

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U. Raczyk-Stanis³awiak, J. Œwietlik, B. Kasprzyk and J. Nawrocki

100°C) demonstrated an insignificantly lower efficiency. Persulfate/UV method is characterised by even lower efficiency. Pre-ozonation enhances the efficiency of TOC determination for humic acids, while there is no enhancement observed in the presence of chlorine dioxide. However, the improvement of TOC determination was found for flocculants pre-oxidised with chlorine dioxide. The best results were obtained for freshly prepared samples. These results are even better than those already published [4]. The storage time of 2–6 days did not affect the efficiency of TOC analysis of the samples previously treated with phosphoric acid.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

V Hall C.E., Safranko J. and Stenger V.A., Anal. Chem., 35, 315 (1963). Standard Methods for Examination Water and Wastewater 1993. Namieœnik J., Chem. Anal., 33, 835 (1988). Sharp J.H., Marine Chemistry, 1, 211 (1973). Sugimura Y. and Suzuki Y., Marine Chemistry, 24, 105 (1988). Wagnersky P.J., Marine Chemistry, 41, 61 (1993). Aiken G.R., Envir. Sci. Technol., 26, 2435 (1992). Koprivnjak J.F., Blanchette J.G., Bourbonniere R.A., Clair T.A., Heyes A., Lum K.R., McCrea R. and Moore T.R., Wat. Res., 29, 91 (1995). 9. Najm I., Marcinko J., and Oppeheimer J., JAWWA, 92, 84 (2000). 10. Kaplan L.A., JAWWA, 92, 149 (2000). 11. Spyres G., Nimmo M., Worsfold P.J., Achterberg E.P. and Miller A.E.J., Trends in Analytical Chemistry, 19, 498 (2000). Received June 2002 Accepted September 2002