GeoJournal 40.1-2: 213-217. © 1996 (October) Kluwer Academic Publishers. Printed in the Netherlands.

Fractionation and chlorination of organic carbon in water from Yinluan River, Tianjin, China*

Tao, Shu, Department of Geography, Peking University, Beijing, P.R. China

Received xxx; accepted xxx

Abstract: Total organic carbon in unpolluted water from Yinluan River, Tianjin, China was fractionated into five operational defined fractions of particulate organic carbon, hydrophobic compounds, humic substances, non-humic anions, and other hydrophilic compounds using a multi-step filtration and adsorption scheme. The organic carbon contents and the trihalomethane formation potential of these fractions were determined. It was demonstrated that about half of the dissolved organic carbon in the sample is humic substances which is quite active in term of chlorination reaction. Particulate organic carbon is also an important precursor of the volatile halogenated hydrocarbons. All other fractions, which account for another half of the dissolved organic carbon, contribute to a relatively small percentage of the trihalomethanes produced during the chlorination experiment. It was concluded that humic substances, dissolved or suspended, are the most important precursor of halogenated hydro- carbons during chlorination, owing to their high abundance and trihalomethane formation potential.

1. Introduction

One of the major sources of drinking water contam- inates is the by-products which result during disin- fection (Rook 1974). The organic compounds found in natural waters, particularly surface water, react with chlorine during the disinfection and hundreds of chlorination by-products, mainly trihalomethane (THMs), are then produced. It has been demonstrated that among the organic compounds identified in drinking water in the United States, over 30% are halogenated (USEPA 1976). Some of these are toxic and apparently carcinogenic (National Cancer Institute Carcinogenesis Program 1976).

Of many factors influencing the rate of THMs formation and the total THMs yield, the type of precursor is critical due to the difference in both abundance and reaction activity of various natural organic compounds. Rice (1980) has presented an extensive list of organic compounds that yield chlo- roform when chlorinated. The majority of studies on formation of THMs have focused on humic material which is the most common organic precursor occur-

* This study was supported by the National Excellent Young Scientist Fund of China.

ring in natural waters. Meanwhile, there is limited information available on the chlorination activities of non-humic substances which accounts for half of the dissolved organic carbon (DOC) in surface water (Thurman 1986; Tao et al. 1990a). An in-depth study on the relative importance of all potential precursors is, therefore, necessary for a better understanding of the formation of THMs in drinking water supply.

Separation and identification of each individual organic precursor in natural wate¢ is extremely dif- ficult, if not impossible. The total organic carbon (TOC) of natural water, however, can be separated into a series of homogenous fractions based on their physical and chemical properties which may relate to their chlorination reactivity. A suitable combination of separation techniques into a comprehensive analytical procedure can result in the quantitative isolation of most organic solutes from water. The first fractionation procedure for determining organic solutes in various river waters was developed by Sirotkina et al. (1974), who sequentially used freeze concentration, adsorption upon ion exchange cellulose, and Sephadex-gel filtration procedures. Leenheer (1981) has developed another analytical procedure referred to DDC fractionation analysis, based on resin-adsorbent columns and aqueous

214 S. Tao

reagents, to quantitatively classify organic solutes in water into hydrophilic and hydrophobic acids, bases, and neutrals. While Leenheer's procedure was designed to characterize the complex process of sorption of various organic solutes upon various adsorbents, a similar procedure with necessary modification might be adopted to separate naturally occurring aquatic organic compounds into more homogeneous fractions which have similar proper- ties regarding their chlorination reactivities.

The objective of this study was to establish a fractionation procedure for separation of aquatic organic compounds from unpolluted river water and to demonstrate its application on quantitative com- parison of the chlorination reactivity of various frac- tions of the organic compounds.

2. Experimental

The surface water from Yinluan River was sampled in July, 1990 at the Yixinbu pump station, 110 km downstream from Yuqiao Reservoir. Content of TOC and DOC of the sample was 7.25 mgC/L and 4.75 mgC/L, respectively. The sample was transported to the laboratory in a glass bottle. The fractionation started within 3 hours of sampling.

A 0.45 gm filter and four synthetic resin-adsor- bent columns were combined for the fractionation. The sample of river water passed five steps in the separation. The flow chart of the fractionation scheme is presented in Figure 1. Macroporous resin (XAD-8) and anion-exchange resin (201 x 7) were purified by 24-h sequential Soxhlex extractions with acetone and hexane. Slurry resin with double distilled water was packed into 200 mm glass columns of 8

El(B1)

12. 451= filter

XAD-8 res/n [

- - E2 (B2 )

HCI(pH2) l :- E3(B3)

I I I HCI(pH2) I ~ ES(B5) E4(B4) [ XAD-Sres/n ]

E6(B6)

Figure 1. Fractionation scheme showing the five steps of filtration and column adsorption.

mm diameter. All columns were then rinsed with double distilled water until the DOC of the effluent from every step decreased to 0.01 mg/L or less. The effluent collected from each step of the scheme just before the application of the sample was used as a blank for both organic carbon measurement and the chlorination experiment. The samples from various steps of the separation were designated as E1 to E6, while B1 to B 6 were used for blanks.

Organic carbon contents of various samples were measured using a Shimadzu TOC-10B analyzer. The organic carbon contents of various fractions were cal- culated based on the measured differences between the separation steps by subtraction among measured organic carbon contents in effluent from relevant steps (El to E6). Compared to direct measurement of the organic carbon content in elutriates from resin columns, the sample contamination during the elution, which could cause a serious interference, could be totally avoided. The measured organic carbon contents of the effluent during the cleanup of the filter and columns with double distilled water (B 2 tO B6) served as blanks.

For the chlorination experiment, 50 mL of the sample (B1 to B 6 and E~ to E6) was introduced into a 250 mL flask, to which 4 mL of aqueous solution saturated with chlorine was added (equivalent to 8 mg C 12/L) as the chlorinating agent and the flask was immediately sealed. The samples were thoroughly mixed and allowed to stand for 5 hours at room tem- perature to ensure complete reaction. At the end of the reaction, the THMs formation reaction was ter- minated by injecting excess sodium thiosulfate. Three duplicates for each sample or blank were tested to insure the accuracy of the experiment.

THMs analysis was carried out using a Shimadzu GC-9A gas chromatography instrument incorporating an electron capture detector, with direct injection of 100 gL gas samples from the flask after shaking for 10 minutes. Separation was achieved on a glass column, 3.2 mm x 4600 mm, coated with OV-1 by using N 2 as the carrier gas at a flow rate of 50 m/min. The injection port temperature was 150 °C and the column temperature was isothermal at 80 °C.

3. Results and discussion

3.1. Fractionation of organic carbon in the river water

Organic carbon contents of the effluent from various steps of the fractionation procedure are tabulated in Table 1. For organic carbon data listed in the table, blank values (Bi) have been subtracted from the directly measured organic carbon contents in the sample effluent.

The contents of five operational defined fractions of particulate organic carbon (POC), hydrophobic

Fractionation and chlorination of organic carbon 215

Table 1. Organic carbon contents of the effluent from various steps of the fractionation procedure. E~ and B~ are samples and blanks respectively

Effluent Component Organic carbon (mg C/L)

E~ Total organic carbon (TOC) 7.25 E 2 - B 2 Dissolved organic carbon (DOC) 4.75 E3 - B3 Hydrophilic 4.58 E 4 - B 4 Hydrophilic/non-humic 2.33 E5 - B5 Hydrophilic/non-anion 2.73 E6 - B6 Hydrophilic/non-anion/non-humic 1.83

compounds, humic substances, non-humic anions, and other hydrophilic compounds were calculated based on data listed in Table 1. For instance, the POC content of the sample was calculated by subtracting E2 from E1 and the content of hydrophobic com- pounds was calculated by subtracting E 3 from E 2. Because part of the humic substances was able to be adsorbed on anion exchange resin, the level of non- humic anions was derived by subtracting E6 from E4 rather than subtracting E5 from E3. The carbon contents of the five fractions of natural organic com- pounds in water from Yinluan River (F1 to Fs) are given in Table 2.

The TOC of the sample was 7.25 mgC/L, in which 4.75 mgC/L was DOC. This result is very close to previously reported 4.56 mgC/L for the river system (Tao et al. 1990a). The POC value of the sample may fluctuate from very low to extremely high, depending on season, precipitation, flow rate, water depth, sampling location, and many other factors. The breakdowns of the four dissolved organic carbon fractions are relatively constant and are shown in Figure 2. A low level of hydrophobic compounds (F2) was observed (3.5%) since the river has generally not been contaminated and hydrophobic organic com- pounds have a strong tendency of being removed from the water column. Similar to results in the literature (Thurman 1986; Tao et al. 1990a), humic and fulvic acids (F3) of the sample contributed to around half of the sample DOC (47.7%). The rest of the DOC was either non-humic anions (F4) or other hydrophilic compounds (Fs).

3.2. Trihalomethane j~rmation potential of various fractions of the TOC

A common approach of fractionation study involves preoperative fractionation procedure, during which the organic compounds adsorbed on columns will be elutriated and used in subsequent experiments. For the chlorination study, however, this practice may suffer a series of problems, particularly incom- plete recovery and sample contamination (Leenheer and Farrier 1979; Tao et al. 1990b). For this reason, the chlorination experiments in this study were carried out using effluent directly from the fraction- ation procedure (El to E6). The trihalomethane formation potential of each fraction (F1 to Fs) was then derived by subtraction between measured results using the relevant effluent (same as organic carbon calculation shown on the third column of Table 2). The results from parallel experiments using blank effluent from the fractionation pro- cedure (B1 to B6) were adopted for background correction.

Volatile halogenated compounds detected by GC after chlorination were chloroform (CHC13), bromodichloromethane (CHBrC12), dibromochloro- methane (CHBr2C1), and bromoform (CHBr3), which have often been reported as most commonly formed THMs during the reaction of chlorine with aquatic organics (USEPA 1976) and happen to be those regulated by the U.S. Environmental Protection Agency in finished drinking water delivered to the consumer (Fed. Regist 1985). The level of these com-

Table 2. Concentrations and percentages of the five operational defined fractions of organic compounds in water from Yinluan River, Tianjin, China. Data for the five fractions were derived from subtraction among measured organic carbon concentrations of relevant effluent from the fractionation procedure presented in Table 1

Fraction Component Derivation OC (mgC/L) Percentage (%)

Fi POC E l - E 2 2.50 F2 Hydrophobic compounds E 2 - E 3 0.13' F 3 Humic and fulvic acids E 3 - E 4 2.25; F4 Non-humic anions E4 - - E 6 0.50 F5 Other hyudrophilic compounds E 6 1.83

Dissolved DOC F2 + F3 + F4 + F5 4.75 Total TOC Ej 7.25

34.6 2.3

31.0 6.9

25.2

216 S. Tao

Humic Acids (47.4%) ~ , : , : . : . : . : . : . . . . . . . ,

Hydrophobic Anions (13.5%) (lO.5%)

O t h e r s (38.6%)

Hydrophobic (4.4%)

Suspended (50.6%)

Anions (1.4%)

Humic (43.4%)

Figure 2. Breakdown of four fractions of DOC (F 2 to Fs listed in Table 2) in water from Yinluan River, Tianjin.

200

pounds originating from the five fractions of organic compounds (F1 to F 5) are presented in Figure 3.

The most abundant trihalomethane was CHC13 (Figure 3) mainly produced during chlorination of particulate organic compounds (F~) and dissolved humic substances (F3). A relatively large quantity of CHBrC12 primarily from humic substances (F3) has also been detected. The levels of the other two tri- halomethanes, CHBr2C1 and CHBr 3, were quite low and appeared to be generated from humic substances (F 3) as well.

Figure 4 shows the total trihalomethanes con- tributed by various fractions of the aquatic organic compounds. The majority (over 90%) of the THMs detected were either from POC or dissolved humic substances. Humic substances including both humic and fulvic acids were considered to be a major pre- cursor of halogenated compounds in drinking water (De Leer et al. 1985; Cloirec et al. 1990). Most THMs generated from POC was probably due to the chlorination reaction of humic material in suspended solids as well.

To demonstrate the THMs formation potential of various precursors during chlorination, the quantities of THMs produced were divided by the organic

320 ~ CHCIs

CHBrCI2

20 ~ ~ CHBr2CI

12 ~ CHBrs

FI F2 1=3 i=4 1=5

Figure 3. THMs produced from the five fractions of TOC during chlorination.

Figure 4. Breakdown of chlorinated compounds gener- ated from various fractions of TOC during chlorination.

120

9O

¢O

0 ~ ,. ...... :a__ Fe F4 F5

Figure 5. Comparison of THMs formation potential among various fractions of TOC.

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carbon content of each fraction and the results are shown in Figure 5. The chlorination potentials of POC (F1) and humic substances (F3) were signifi- cantly higher than those of the other fractions sepa- rated during the fractionation procedure.

4. Conclusion

The naturally occurring aquatic organic compounds in water from Yinluan River were fractionated into five operational defined fractions, of which the most important precursors of THMs during chlorination reaction appeared to be particulate organic carbon and humic substances owing to their relatively high abundance and trihalomethane formation potential.

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Fractionation and chlorination of organic carbon 217

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