Water Disinfection and Natural Organic Matter - ACS Publications

Chapter 12 Use of UV Spectroscopy To Study Chlorination of Natural Organic Matter

Downloaded by UNIV ILLINOIS URBANA on May 3, 2013 | http://pubs.acs.org Publication Date: November 19, 1996 | doi: 10.1021/bk-1996-0649.ch012

Gregory V. Korshin, Chi-Wang Li, and MarkM.Benjamin Department of Civil Engineering, Box 352700, University of Washington, Seattle,WA98195-2700

Effects of chlorination on the width and intensity of absorbance of the electron-transfer band in the U V spectra of NOM are studied. For λ>250 nm, chlorination causes the absorbance to decrease. For a wide range of chlorine doses and chlorination times, the decrease in absorbance is linearly related to the amount of CHCl generated. The contraction of the ET band caused by chlorination is suggested to correspond to selective removal of activated aromatic rings and breakdown of NOM molecules into smallerfragments.For conditions typically encountered in drinking water treatment, chlorination of NOM substantially increases the intensity of fluorescence. This result is consistent with the oxidation of the NOM into smallerfragments.It is proposed that the width of the ET band is an indicator of the concentration of aromatic carbon in NOM, in both unaltered and chlorinated samples. This hypothesis is supported by data from C solid-state NMR and U V absorbance measurements. It is concluded that the U V spectroscopy has significant capabilities to probe the structure and reactions of ΝΟΜ. 3

13

Natural organic matter (NOM) dissolved in water both absorbs light and fluoresces (7). The bulk ofNOM (typically,from90% of the total organic carbon) is comprised of polydisperse, polymeric humic substances. These substances may be subdivided into several sub-fractions, the most important of which are humic, fulvic and hydrophilic acids (2). The rest of the organic carbon is comprised of carbohydrates, simple organic acids, proteins and amino acids. The contribution of NOMfractionsother than humic substances to the absorbance andfluorescenceof NOM is generally negligible (3). Since this communication deals primarily with optical properties of NOM, references to NOM in this paper refer specifically to the humic fraction of the ΝΟΜ. In the UV region, and especially at λ>250, light absorption is 0097-6156y96/0649-0182$15.00/0 © 1996 American Chemical Society In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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KORSHIN ET AL.

UV Spectroscopy To Study Chlorination of NOM 183

due predominantly to aromatic units in the NOM structure (4,5). To date, no quantitative theory of UV absorbance of NOM has been proposed, and the relationships among molecular conformation and reactivity and the spectral parameters ofNOM have not been adequately explored. Most researchers have limited their data collection to monitoring the absorbance at 254 nm (A^), using these values as a rough indicator of the overall NOM concentration. This wavelength has been chosen primarily because it is the wavelength of the brightest line in the emission spectrum of low-pressure emission mercury lamps (6), and at this wavelength the absorbance of NOM normally is high enough to be measured reliably. A254 is often a good surrogate parameter both for DOC and for the trihalomethane formation potential (THMFP) (7-9). The value of A ^ prior to chlorination has also been used in multi-parametric statistical models used to predict THM formation (10,11), but the term has not shown to provide any direct insight into the chemistry of interaction between chlorine-based oxidants and ΝΟΜ. Also, although chlorination of NOM dramatically decreases light absorption (12-15), no formal analysis of the change in A254 induced by chlorination has been offered. Semi-empirical kinetic models of DBP formation (e.g., 16-19) generally make no attempt made to use spectral information, other than using the initial value of A ^ as noted above. There is a tremendous, unrealized potential for using absorption spectroscopy to understand the chemistry of ΝΟΜ. The technology for measuring absorbance is both highly sensitive and experimentally simple. Measurements usually do not require sample preparation (except, generally,filtrationto remove particles), and the range of linearity between absorbance and the concentration ofNOM (measured as DOC) typically extendsfrombelow 0.1 mg/L to several tens or even hundreds of mg/L, depending of the chosen wavelength (20-21). This paper represents an attempt to partially bridge the gapfromgeneral statements acknowledging the utility of UV spectrophotometry to quantitative measurements describing the relationship between the electronic spectra of NOM and the properties of NOM, including its behavior in reactions with halogens. A Hypothesis of the Compound Electron-Transfer Band in the UV spectra of Natural Organic Matter The absorbance of light by NOM is due primarily to the presence of aromatic structures incorporated into the molecules of humic substances. Depending on the NOMs origin, aromatic carbon may constitutefrom30% of the total organic carbon (4, 5,15, 22, 23). Carboxylic groups are also prominent in the structure of NOM and define its largely acidic character. The aromatic rings found in NOM are not uniform and may be substituted with a variety of activating and nonactivating functional groups. Given the range of average molecular weight typical for NOM (depending on the origin and experimental techniques, the estimates vary from 5000K (20, 21, 23-26)), there is a virtually countless number of possible combinations of aromaticringssubstitution patterns in the molecules. In the electronic spectra of aromatic compounds, there is always an absorption band (often referred to as the electron-transfer (ET) band) centered at 240i

A (E) =

Σ

BT

ε^βχρ|

(1)

2

Δ;

all ET binds

where % is the molar extinction coefficient for the ET bandfroma particular chromophore /, q is the concentration of that type of chromophore, and E and Δι are the position of the maximum and the width (measured at wavelengths where the absorbance is 50% of the maximum) of the corresponding band of light absorption. Since the energy of light quanta is related to the wavelength of the light as follows: 0>i

where Ε is expressed in electron-volts (eV) and λ in nanometers, Equation 1 could be modified to express absorbance as a function of wavelength, but the function would not be Gaussian. We have postulated that, when the absorbancesfromall the aromatic chromophores in NOM are superimposed, the spectrum can be modeled as having a composite ET band, which also has a Gaussian dependence on energy, i.e., we have postulated that the composite ET band may be described as: A (E) = 8 c exp BT

0

41n(2)(E-E . )

2

0 ET

(3)

0

where A^, E^ and Δ are parameters analogous to the individual ET bands, but calculated as the best-fit for the composite band. Similarly, ε and c are best-fit values based on the chromophore mixture in the sample. The values of c and ε cannot be determined independently by analysis of UV spectra; only their product Ao= c
)= f (( i

i

E

--

E

»-> -( ,

E

-

E

->')

4

E ^ is an arbitrary energy chosen as the reference point for the calculation. The quadratic expression in brackets on therightside of the equation will be called the modified Gauss coordinate. According to Equation 4, the logarithm of the ratio of absorbance at any energy to that at the reference energy is proportional to the value of the modified Gauss coordinate, with a slope equal to 4(Ιη2)/Δ . For a given experimental spectrum, the values of all the parameters in Equation 4 are known, so the hypothesis that the spectrum is Gaussian and that it can be characterized by a angle set of Gaussian parameters can be tested. In this communication, an attempt is made to apply this conceptual approach to interpreting the changes in the values of A and Δ caused by chlorination of a water sample containing NOM, as well as to demonstrate possible practical application of UV spectrophotometry in the studies ofNOM chlorination. 2

0

Changes of UV Spectra of NOM upon Chlorination Water used in the experiments wasfroma water supply reservoir in Mt. Vernon, WA, or was NOM that was concentratedfromthis water by reverse osmosis or by adsorption on the surface of iron oxide (30). DOC concentrations in the raw water rangedfrom3.2 to 4.0 mg/L. Reverse osmosis retained virtually 100% of the UVabsorbing species; adsorption by iron oxides retained, on average, 95% of the UV absorption. DOC retention efficiency by the two techniques was approximately 90% and 85%, respectively. The samples were buffered at pH 7.0 with phosphate (concentration 0.03 M), dosed with 0.5 to 40 mg/L chlorine, and incubated in the dark at 25°C for up to 168 hours. Chlorine consumption, DOC, UV absorbance, fluorescence, and the concentration of CHCI3 were monitored. A high precision, high dynamic range Perkin-Elmer Lambda-18 spectrophotometer was used to measure the absorbance of light (typically, a 5-cm quartz cell was used). Fluorescence spectra were recorded using a Perkin-Elmer LS-50Bfluorometer.The concentration of CHC1 3

In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

186

WATER DISINFECTION AND NATURAL ORGANIC MATTER


0.4

Wavelength, nm

Figure 1. UV spectra of Mt. Vernon NOM subjected to chlorination. Reaction time 24 hours. Initial DOC 10 mg/L, pH 7.00. Chlorine dose variesfrom0 to 30 mg/L. 800 600 J2 400

8 200 0.00

0.05

0.10

0.15

0.20

0.25

Absorbance change @ 254 nm

Figure 2. Relationship between the decrease of UV absorbance at 254 nm caused by chlorination and the concentration of CHC1 generated. NOM wasfromuntreated or pre-concentrated Mt.Vernon water (initial DOC 3.6 and 10 mg/L, respectively). Reaction times were 2 to 168 hours, chlorine dosesfrom0 to 40 mg/L. pH 7.00, phosphate buffer. 3


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UV Spectroscopy To Study Chlorination of NOM 187

was measured either by the purge-and-trap or the extraction method (37). Volatile DBPs other than CHC1 were not detected, apparently because of the absence of bromide in this water (analysis for bromide was done by ionic chromatography, detection limit was ca. 25 μ ^ ) . Prior to recording the UV and fluorescence spectra and measuring the concentration of CHC1 ,freechlorine was quenched with sodium sulfite at a dose approximately 20% in excess of the stoichiometric requirement. A set of UV spectra of the chlorinated NOM is presented in Figure 1. At λ>230 nm, addition of chlorine always causes UV absorbance to decrease. The relative decrease of absorbance becomes more pronounced with increasing λ. As a result, both the intensity of the band at its maximum (A*,), le. at an energy close to 4.90 eV and the bandwidth (Δ) decrease. The decrease of absorbance is not surprising per se, since it is widely accepted that chlorine attacks activated aromatic rings which, in turn, constitute the predominant UV-absorbing chromophore in NOM (32, 33). Because of the relationship between UV absorbance and the concentration of aromatic carbon in NOM and because aromatic groups are widely believed to be DBP precursors, the change of the UV absorbance caused by chlorination might be correlated with the formation of DBPs (exemplified by CHC1 in this communication). For the sake of conformity with previous research, this hypothesis is evaluated here using absorbance data at a wavelength of254 nm (4.88 eV), though other wavelengths (for example, 270 and 300 nm) have been used as successfully. Based on the premise that chlorine attacks activated aromatic rings, and DBPs such as CHC1 are formed as a result, it follows that the more aromatic rings are destroyed, the more DBPs accumulate in the solution. As afirstapproximation, we use ΔΑ254, defined as the decrease in absorbance at 254 nm in the chlorinated sample compared to the unchlorinated one, as a measure of the destruction of such rings. If the reactions between chlorine and the aromatic rings it attacks have a constant yield for a particular type of DBP, then a given decrease in should correspond to a fixed amount of that DBP generated, regardless of whether the decrease in A ^ occurs in a short period of time in a solution containing high concentrations of NOM and CI or a much longer time in a less concentrated solution. Figure 2 shows values of [CHC1 ] for the Mt.Vernon NOM as a function of ΔΑ254 for chlorine dosesfrom0.5 to 40 mg/L, reactiontimesfrom2 to 168 hours, and for two DOC concentrations. Over this wide range of parameters, a very good linear correlation exists between Δ Α ^ and the concentration of CHC1 (R*=0.95). The bestfit line does not pass through the origin: at ΔΑ2 250 nm, chlorination causes the absorbance to decrease. The decrease of absorbance is in a very good linear correlation with the generation of CHC1 . It is concluded that direct observation of absorbance changes permits to estimate the generation of DBPs and to monitor chlorination reactions in situ. Chlorination causes the ET band to contract. This effect is suggested to correspond to selective removal of activated aromatic rings, breakdown of NOM molecules into smallerfragmentsand decrease of inter-chromophore interactions. Fluorescence spectra provide direct confirmation of this hypothesis: the intensity of emission in chlorinated NOM samples is considerably increased which typically takes place for low molecular weight NOM fractions. Based on the results for chlorination, it is proposed that the width of the ET band is sensitive to the concentration of aromatic carbon in all NOM samples including 3


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those unaffected by oxidation with chlorine. Calculations based on the independent data of C solid-state NMR and UV absorbance confirm this. It is concluded that the UV spectroscopy has significant capabilities to probe the structure and reactions of ΝΟΜ. 13


Acknowledgments This work has been funded by AWWA Research Foundation (grant # 159-94). Supportfromthe engineering companies HDR Engineering, Inc. (Omaha, NE), SAUR (Maurepas, France) and DYNAMCO (West Sussex, UK) is greatly appreciated. Personal thanks to Steve Reiber (HDR Engineering) for keen interest and encouragement of this study. References 1. MacCarthy, P.; Rice, J.A. In Humic Substances in Soil, Sediment and Water; Aiken, G.R. et al., Eds.; John Wiley & Sons: New York, NY, 1985. 2. Leenheer J.A. Env.Sci. Technol., 1984, 15, 578-587. 3. Laane, R.W.P.M., Koole, L.. Neth J. Sea Res., 1982,15, 217-227. 4. Traina, S.J., Novak, J., Smeck, N.E. J. Environ. Qual., 1990, 19, 151-153. 5. Novak, J.M., Mills, G.L., Bertsch, P.M. J. Environ. Qual., 1992, 21, 144-147. 6. Handbook of Chemistry and Physics. Lide, D.P., Ed. 71st Edition. CRC Press: Boca Raton, 1990. 7. Edzwald, J.K., Becker, W.C., Wattier K.L. J. Amer. Water Work Assoc., 1985, 77, April, 122-132. 8. Singer, P.C.; Chang, S.D. J. Amer. WaterWorksAssoc., 1989, 81, August, 61-65. 9. Reckhow, D.A.; Singer P.C. J. Amer. Water Works Assoc., 1990, 82, April, 173180. 10. Engelholm, B.A.; Amy, G.L. J. Amer. Water Works Assoc., 1983, 75, August, 418-423. 11. Amy, G.L,. Chadik, P.A., Chowdhury, Z. J. Amer. Water Works Assoc., 1987, 79, July, 89-97. 12. Gjessing, E.T. Physical and Chemical Characteristics ofAquatic Humus. Ann Arbor Science Publishers, Inc.: Ann Arbor, MI, 1976. 13. Van Breemen, A.N.; Nieuwstad, T. J.; van der Meent-Olieman, G.C. Water Res., 1979, 13, 771-779. 14. Jensen, J.N.; Johnson, J.D.; St.Aubin, J.; Christman R.F. Org. Geochem., 1985, 8, 71-76. 15. Hanna, J.V.; Johnson, W.D.; Quezada, R.A.; Wilson M.A.; Xiao-Qiao, L. Environ. Sci. Technol., 1991, 25, 1160-1164. 16. Urano, K.;Wada, H.; Takemasa, T. Water Res., 1983,17,1797-1802. 17. Adin, Α.; Katzhendler, J.; Alkaslassy, D.; Rav-Acha, Ch. Water Res., 1991, 25, 797-805. 18. Kavanaugh, M.C.; Trussell, A.R.; Cromer, J.; Trussell,R.R.J.Amer. Water Works Assoc., 1980, 72, October, 579-582. 19. Peters, C.J.; Young, R.J.; Peny, R. Env. Sci. Technol., 1980, 14, 1391-1395.



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