[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Aquatic Humic Substances as Sources and Sinks of Photochemically Produced Transient Reactants

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

    Jrg Hoign, Bruce C. Faust1, Werner R. Haag2, Frank E. Scully, Jr.3, and Richard G. Zepp4

    Swiss Federal Institute of Water Resources and Water Pollution Control (EAWAG), 8600 Dbendorf, 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 reac-tants 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 (per-oxy radicals). The steady-state concentration of each transient formed during solar irradiation was determined from the apparent first-order disappearance rate of added organic probe compounds. The probe compounds used had selective reactivities with the individual tran-sient species of interest. Effects of the photoreactants on the elimi-nation of micropollutants and on chemical transformations of DOM are discussed.

    1Current address: School of Forestry and Environmental Sciences, Duke University, Durham, NC 27706

    2Current address: SRI International, Menlo Park, CA 94025 3Current 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

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

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

  • 364 AQUATIC HUMIC SUBSTANCES

    DISSOLVED 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 publications. Extensive literature reviews are given in these publications and are not repeated here. This chapter wil l 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 heterogeneous 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 approximately 1 k W / m 2 of sunlight, or about 20 einsteins/m 2 (20 mol of pho-tons/m 2) (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 (DOM) present in natural water. In addition, a rather small fraction of short-wavelength 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 sub-

    Figure 1. Sofor radiation, (a) Mean dose intensity in a mixed 1-m water column in which all light is absorbed, (b) Monthly solar flux (280 < < 2800 nm) in

    Dubendorf(47.5 N\ 1985.

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

  • 23. HOIGN ET AL. Photochemically Produced Transient Reactants 365

    (nm)

    Figure 2. Decadic molar absorptivities of Greifensee DOM and N03~ anion. The values for N03~ have been multiplied by a factor of 10. Typical DOC and [N03~] values for Greifensee are 4 mg/L (300 of carbon units) and 100 , respectively. Solar irradiation data are for sea level, after ref 1.

    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 mM. 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 (l02) (1, 3, 4), DOM-derived peroxy radicals ( R O C ) (5-7), hydrogen peroxide (8, 9), solvated electron (eaq") (10-12), superoxide anion (02~) (13, 14) and humic structures excited to triplet states (IS).

    In addition, U V light absorbed by nitrate and nitrite produces OH* 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 2 O z , because of its relative inertness, accumulates and decomposes during extended illumination periods (hours).

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

  • 366 AQUATIC HUMIC SUBSTANCES

    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 photoreactants 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] s s) 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 kx'):

    [XL = rx * '

    (1)

    The formation rate () is proportional to the rate of light absorption by the photochemical source substance (i.e., proportional to fcA[A]) and to the quantum efficiency (). As shown in equation 2, the rate of X consumption or quenching can be controlled by solvent quenching (kq), reaction with D O M acting as a scavenger (S) for the photoreactant (X) (fc x s [DOM]), reaction with oxygen (kXto2[02]), reaction with other scavengers, and possibly by bimolecular reactions with itself (&X,X[X]).

    DOM or

    NO3~ r = kA [ ]

    A:photon absorber X:photo^ react ant : probe molecule or

    micropollutant kA : specific light absorption

    rate-constant

    2 OH

    02" ROO*

    H 2 0 2

    solvent

    + DOM kx.S[D0M] + 02 k X , 0 2 0 2

    It

    *x,x

    ^transformed

    (2)

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

  • 23. HOIGN ET AL. Photochemically Produced Transient Reactants 367

    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 ( 1 0 2 , , R O O ' , and e a q"), rates of their selective reactions with added probe molecules (P) were determined (equation 2). Highly selective 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 , and R O O ' , experiments were performed in a way that produced a simple second-order rate law. The rate of transformation of was first order in concentration of both and X .

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