Aquatic Humic Substances - ACS Publications - American Chemical

23

Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch023

Aquatic Humic Substances as Sources and Sinks of Photochemically Produced Transient Reactants 1

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3

Jürg Hoigné, Bruce C. Faust , Werner R. Haag , Frank E. Scully, Jr. , and Richard G. Zepp 4

Swiss Federal Institute of Water Resources and Water Pollution Control (EAWAG), 8600 Dübendorf, Switzerland In sunlit surface waters aquatic humic substances and nitrate act as sensitizers or precursors for the production of photoreactants such as singlet oxygen, humic-derived peroxy radicals, hydrogen peroxide, solvated electrons, and ˙OH radicals. Lifetimes of the various reactants are controlled by their reactions with aquatic humic substances (˙OH radicals), by solvent quenching (singlet oxygen), by reactions with molecular oxygen (solvated electron), or by other processes (peroxy radicals). The steady-state concentration of each transient formed during solar irradiation was determinedfrom the apparent first-order disappearance rate of added organic probe compounds. The probe compounds used had selective reactivities with the individual transient species of interest. Effects of the photoreactants on the elimination of micropollutants and on chemical transformations of DOM are discussed.

1Current address: School of Forestry and Environmental Sciences, Duke University, Durham, NC 27706 Current address: SRI International, Menlo Park, CA 94025 Current address: Department of Chemical Sciences, Old Dominion University, Norfolk, VA 23508-8503 4Current address: Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, GA 30613

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3

0065-2393/89/0219-0363$07.00/0 © 1989 American Chemical Society

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

364

AQUATIC H U M I C SUBSTANCES


D I S S O L V E D ORGANIC MATERIAL IN SURFACE WATERS HAS A ROLE in pro

ducing or consuming different types of photoreactants. The conclusions stated in this chapter are based on data from a series of our recent publi cations. Extensive literature reviews are given in these publications and are not repeated here. This chapter will focus on the reactions for which humic materials act as sources or sinks. We make no attempt to include all other possible photochemical processes. For example, no discussion of hetero geneous processes is included, although there is evidence that they are important (e.g., in the redox cycling of metals). Moreover, photochemical processes mediated by superoxide and hydrogen peroxide are discussed in Chapter 22 by Cooper et al. During a cloudless summer noon hour, surface waters receive approx imately 1 k W / m of sunlight, or about 20 einsteins/m (20 mol of photons/m ) (Figure 1). Within 1 year about 1300 times this dose is accumulated (2). A large portion of these photons is absorbed by dissolved organic material ( D O M ) present in natural water. In addition, a rather small fraction of shortwavelength light is absorbed by nitrate (Figure 2). From a chemist's viewpoint, the resulting rate of interactions between photons and absorbers is very high. Assuming that most of the photons are absorbed in a well-mixed 1-m water column, we estimate that about 20 mmol/(L*h) of interactions occur between photons and absorbing sub2

2

2

Figure 1. Sofor radiation, (a) Mean dose intensity in a mixed 1-m water column in which all light is absorbed, (b) Monthly solarflux(280 < λ < 2800 nm) in Dubendorf(47.5° N\ 1985.



23.

365 Photochemically Produced Transient Reactants

HOIGNÉ ET AL.

λ (nm) Figure 2. Decadic molar absorptivities of Greifensee DOM and N0 ~ anion. The values for N0 ~ have been multiplied by a factor of 10. Typical DOC and [N0 ~] values for Greifensee are 4 mg/L (300 μΜ of carbon units) and 100 μΜ, respectively. Solar irradiation data are for sea level, after ref 1. 3

3

3

strates (Figure la). Assuming an average chromophore unit weight of 120 for D O M in water containing 4 mg of dissolved organic carbon (DOC) per liter, we arrive at a chromophore concentration of 0.033 m M . Thus, each chromophore is excited at a high rate of 600 times per hour. Some of these interactions lead to direct photochemical transformations of D O M and aqueous micropollutants to secondary products. But in addition, aquatic humic materials act as sensitizers or precursors for the production of reactive intermediates (so-called "photoreactants") such as singlet oxygen ( 0 ) (1, 3, 4), DOM-derived peroxy radicals ( R O C ) (5-7), hydrogen per oxide (8, 9), solvated electron (e ") (10-12), superoxide anion (0 ~) (13, 14) and humic structures excited to triplet states (IS). In addition, U V light absorbed by nitrate and nitrite produces O H * radicals (16, 17). Light-absorbing redox-active metal species may also be important sources of photoreactants, such as metals in lower valence states (18, 19). Of these photoreactants only H O , because of its relative inertness, accumulates and decomposes during extended illumination periods (hours). l

2

aq

2

2

z



366


A l l other species are highly reactive and short-lived; they are present only at very low concentrations and only during illumination. The role of D O M as a source and a sink of photoreactants is of interest because these photo reactants can chemically transform pollutants and, in many cases, the D O M itself. In principle the role of D O M as source and sink for photoreactants can be discussed without detailed knowledge of particular kinetic models (see Conclusions). However, a reaction kinetic approach is required for designing experiments that yield generalizable results. The main ideas of the model are summarized here (for details, see ref. 20). The steady-state concentration of relatively short-lived photoreactants ([X] ) is given by the rate with which these reactants are produced (r*), relative to the pseudo-first-order rate constant with which they become consumed k '): ss

x

[XL = r χ —

(1)

x

*χ'

The formation rate (τχ) is proportional to the rate of light absorption by the photochemical source substance (i.e., proportional to fc [A]) and to the quantum efficiency (Φ). As shown in equation 2, the rate of X consumption or quenching can be controlled by solvent quenching (k ), reaction with D O M acting as a scavenger (S) for the photoreactant (X) (fc [DOM]), re action with oxygen (k o [0 ]), reaction with other scavengers, and possibly by bimolecular reactions with itself (& , [X]). A

q

xs

Xt

2

2

X X

•solvent

DOM

Ό

or

•OH

NO ~

r = k [Α]·φ

2

+ DOM

kx. [D0M]

A

S

3

+ 02

02" ROO* A:photon absorber X:photo^ react ant

H 0 2

k

X,02Î02

2

*x,x

Ρ : probe molecule or micropollutant

k : specific light absorption rate-constant A

It

^transformed


(2)

23.

HOIGNÉ ET AL.

Photochemically Produced Transient Reactants

Rate constants of reactions controlling the fate of transient reactants cover a wide range. Therefore, in most cases only one of these reactions dominantly controls the lifetime of a specific photoreactant in a given system. To quantify production rates and steady-state concentrations of the main photoreactants ( 0 , Ό Η , R O O ' , and e "), rates of their selective reactions with added probe molecules (P) were determined (equation 2). Highly se lective probe molecules were chosen to discriminate between different types of photoreactants. Whenever possible, probe compounds with structures similar to those of the micropollutants of interest were applied. To probe for Ό Η , and R O O ' , experiments were performed in a way that produced a simple second-order rate law. The rate of transformation of Ρ was first order in concentration of both Ρ and X . 1


2

aq

-

^

= * P , X [ X L [P]

(3)

where k is the second-order rate constant for the reaction of X with P. For a closed parcel of water, and if the concentration of Ρ is low enough not to change [ X ] significantly, equation 3 integrates to ?x

ss

- l n j S ^ = *p, [X] IT Jo x

ss

X t

(4)

Therefore, the logarithm of the relative residual concentration of Ρ declines linearly with time (t) with a slope of Jfc [X] (apparent first-order kinetics). Then [ X ] can be calculated directly from the experimental elimination rate constants in cases where fc of a selected probe molecule is known. A l l kinetic experiments used in this overview for deducing rate constants yielded very good first-order plots. Transient production rate decreases with depth (z) in a surface water because of light screening by D O M (Figure 2). In waters that contain par ticles, light-scattering terms and absorption by particles must also be ac counted for. Assuming complete mixing, measured surface rates (r ) for production of photoreactants are normally converted to depth-averaged rates (r ) by multiplying by the light-screening factor S[ (25). PX

ss

ss

PX

(0)

(z)

z)

r

w = ω X Sf Γ

(5)

where

λ

2.3 Χ α

λ

Χ ζ


(6)

367

368


Here α is the deeadic absorption coefficient of the water at wavelength λ and ζ is the average water depth. In principle, the choice of wavelength for the corrections must account for the overlap (product) between the action spectrum (quantum efficiency times molar absorptivity) of the reaction con sidered and the spectrum of the light. Application of the screening factor to waters of medium depth may be highly complicated. However, for large depths (such that α x ζ > 1), the light-screening factor approximates λ


λ

= λ

(7) 2.3 Χ α

Χ ζ

κ

and the depth-averaged production rate becomes

r

Γ

«

=

(o)

1 2.3 Χ α

() 8

χ

Χ ζ

Given the assumption of vertical mixing, equation 8 corresponds es sentially to a dilution of the photochemical effect with increasing depth. For example, most U V light is absorbed within the top meter of even slightly eutrophic lakes (J). When most light is absorbed within the considered depth, z, r is independent of the concentration of D O M (DOC) if both α and r increase proportionally to D O C . However, it decreases proportion ally with the depth of the water body, because a higher absorbance at the surface is directly compensated by lower light penetration. If r is inde pendent of D O C (e.g., is proportional to N 0 " ) , then r decreases with D O C and z. Finally, diurnal and seasonal variations in light intensity (Figure lb) must be taken into account in any generalization of results. {z)

(0)

(0)

(z)

3

Characteristics of Various Photooxidants Singlet O x y g e n . It is possible to measure the steady-state concen tration of singlet oxygen by following the oxygenation of selective probe molecules such as dimethylfuran and furfuryl alcohol (I, 3). Furfuryl alcohol is less volatile and leads to products that are highly specific for * 0 reactions. Dimethylfuran, however, needs lower exposure times. From the literature and our own studies (I) we conclude that the for mation of * 0 from ground-state oxygen ( 0^ is sensitized by D O M , and that in natural waters its destruction rate is generally controlled by solvent (water) quenching. The sequence of reactions is given in equation 9. 2

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23.

H O I G N É ET AL.

SINGLET

369 Photochemically Produced Transient Reactants OXYGEN

FORMATION :


atmos.

Ρ: e.g. furfurylalcohol Ί

R}xid

At D O M concentrations typical for surface waters ( D O C < 20 mg/L), quenching of singlet oxygen by D O M can be neglected. Different types of aquatic D O M exhibit different quantum efficiencies for 0 production. Selected examples are presented in Table I. In sum marizing phenomenological studies using a variety of different humic sub stances, we conclude that D O M with higher specific light absorption exhibits somewhat lower quantum efficiencies (I). No significant relationship between quantum efficiency and molecular weight fraction was found (I). As an example, the steady-state concentration of singlet oxygen at the surface of the somewhat eutrophic pre-alpine Lake Greifensee in Switzerland ( D O C ~ 4 mg/L) during June, with summer midday sunshine (1 k W / m ) is 8 Χ 10" M . This number results from the observed furfuryl alcohol elimination rate of 3%/h. A light-screening factor can be estimated on the basis of action and absorption spectra typical for this lake water. Accounting for this, equations l

2

2

14

Table I. Half-Lives of Added Furfuryl Alcohol and Corresponding Steady-State Concentrations of Q Water DOC tie FF A* [Όι]. ' (M x I0 ) (M x JO ) Source (mg/L) (ft) 4.6 8 20 Greifensee 3.5 1.5 28 Etang de la Gruyère 6 13 Rhine, Basel 27 5.6 3.6 3.2 2.2 11 14 Secondary effluent** 15 l

2

β a

b

14

"Summer midday sunlight, 1 kW/m . ^Concentration at the surface. Concentration averaged over 1-m depth. ^Communal wastewater. 2


14


370

1 and 8 indicate that a depth-averaged steady-state concentration for an ideally mixed top meter of the lake ( [ 0 ] ) , within which most of the photoactive light is absorbed, is 5 Χ 10" M . At greater depth, the average [^Iss* simply be 5 Χ 10~ M divided by the depth in meters. Surface values of [ O ] increase proportionally with the rate of light absorption (i.e., with the D O C of the water) (Table II). For a water depth within which all light is absorbed, the area-based production of * 0 is independent of D O C (see last column in Table II). However, a comparison of very different types of surface waters must consider that quantum efficiences and action spectra for producing 0 vary somewhat with the type of D O M (J). The occurrence of singlet oxygen in sunlit waters can be important for the elimination of cyclic 1,3 dienes, polynuclear aromatic hydrocarbons, organic sulfides, and phenolic compounds when the latter are significantly dissociated into the reactive phenolate anions (e.g., chlorophenols) (Figure 3). For phenols with high phenolic p K values, correspondingly low reac tivities are found. (For comprehensive literature, see ref. 21). I

lm

2

ss

14

14

1

0

2

s s


2

l

2

a

O H Radical. Because O H radicals react with most organic substrates at nearly diffusion-controlled rates (H atom abstraction or *OH addition reactions), most organic substances can be applied as *OH probe molecules. Butyl chloride, in addition to other compounds, was often used as our probe substance of choice because of its easy analysis and its inertness against direct photolyis and other photoreactants (except e ")(20). In fresh surface waters a large part of Ό Η is produced from slow photolysis of nitrate (J 7, 20). The reaction is shown in equation 10. aq

•OH

RADICAL

FORMATION NO2

X~320nm NO3-

>

DOM

H* \^

0"

OH

• -Κ)

5

S

_ 1

*

I I

P: e.g. butylchloride

(10)

I I

φ *oxid *

for Greifenseewater ( D 0 O 4 m g / l )

The absorption of the relevant solar U V light by nitrate (at about 310 nm) is much smaller than that by D O M (Figure 2). As has been shown in preceding studies in which decomposed ozone has been used as an O H



c

2

4

P

10

1

9

8

m l

kp.l[Xi]u°,[XU° Probe Molecule, (%/h) Reactant k , (M-'s- ) Ό furfuryl alcohol (1.2 x 10 ) 3 OH' butyl chloride (3 x 10 ) 0.2 ROOtrimethylphenol (?) 15 CC1 (3 x 10 ) 0.13 Caq "Light screening by suspended sediments is neglected. ''Independent of [DOM]. Not determined for reasons given in the text. 'Only for DOC < 5 mg/L. 'Estimated for an assumed screening factor, S355 = 0.43. 0

e

8 χ 10 2 x 10 (?) 1.2 x 1 0

(M)

[ X L

14

17

16

m

m

J

a

52 χ

14 17

10-««

5 x 10 4 x 10

[X]

3

rf

2

3

2

M

6 d

6

0

Functionalities of [X] Surface 1-m Layer

Aquatic Humic Substances - ACS Publications - American Chemical

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