laser flash photolysis of dissolved aquatic humic material and the sensitized production of singlet...

Laser Flash Photolysis of Dissolved Aquatic Humic Material and the Sensitized Production of Singlet Oxygen

Frltz H. Frlmmel,* Herbert Bauer, and Joachlm Putzlen

Institut fur Wasserchemie und Chemische Balneologie der Technischen Universitat Munchen, D-8000 Munchen 70, FRG

Patricia Murasecco and Andr6 M. Braun

Institut de Chimie Physique, Ecole Polytechnique FQd6rale de Lausanne Ecublens, CH-10 15 Lausanne, Switzerland

Aqueous solutions of isolated humic substances were investigated by laser flash photolysis. Kinetics of the decay of excited states were measured in emission and absorption. From the absorption data at least two transients with different lifetimes (order of and s) could be de- duced. Quenching effects were determined for oxygen and Ni(II), Co(II), and Cd(I1). The quenching efficiency of oxygen is similar to the efficiency of the paramagnetic metal ions. The quantum yield of the singlet oxygen production sensitized by humics was determined to be 1-370 with the furfuryl alcohol method. The results are discussed with respect to the photochemistry of aquatic ecosys tems.

Introduction Awareness of the potential of photochemical reactions

in the photic zone of aqueous systems has increased in the past decade (1,2). The photolysis of defined chromophores like methyl iodide, tryptophan, vitamin BI2, etc. was investigated under natural aquatic conditions. Fairly little is known about the photochemistry of complex chromophores with unknown structure like humic substances (HUS). These substances however play an important role in the aquatic ecosystems (3) and are the main light-absorbing compounds dissolved in natural water.

The photophysical and photochemical properties of soil and aquatic humic materials have been reviewed recently by Choudhry (4 ) . Zepp et al. (5) have investigated the photosensitizing effect of humic substances on various trace organic chemicals in water. The authors present evidence that an energy transfer from electronically excited humic substances to molecules like pentadiene or di- methylfuran might occur. In cases where oxygen is involved in photosensitized reactions, energy transfer can lead to singlet oxygen production.

Singlet oxygen (lo,) is an efficient oxidant for a variety of unsaturated organic compounds. The reaction of photochemically produced ‘02 with furfuryl alcohol as a trapping agent was introduced by Braun and co-workers (6,7) to measure the sensitizing efficiency of photoreactive synthetic chromophores. The method was applied to aquatic organic materials as sensitizers (8,9). As an ap- plication of these results, IO2 production in sunlight-irra-

Table I. Origin of Aquatic HUS Isolated from Freshwaters

freshwater date of code origin system sampling ref

BAN 13 Bansee lake Jan. 1985 12 BM 4 Brunnensee bog lake Feb. 1983 BM 7 Brunnensee bog lake Nov. 1984 ZIL 1 Zillhamer See lake Dec. 1984 REF Suwannee stream Jan. 1985 13

diated Swiss surface waters was calculated by Haag et al. (10).

This work is based on humic substances (HUS) extracted from aqueous and terrestrial samples. Spectro- scopic methods and the furfuryl alcohol method were used to obtain information about the photosensitizing efficiency of the HUS and some of their metal complexes for lo2 production.

Experimental Section Humic Substances Preparation. Aquatic humic

material was isolated from different freshwaters in Bavaria, FRG (Table I). A modification of Mantoura and Riley’s XAD-2 method (11) was used to obtain HUS as described previously (12). The eluted HUS were separated into humic acid (HA) and fulvic acid (FA) fractions by pre- cipitating the HA at pH 2.0 (HC1; 24 h in the dark). Up to 500 mg of dissolved organic carbon (DOC) of dissolved FA (<0.45 pm; membrane filtration) was adsorbed on a small XAD-2 column (25 cm; 70 g). After being rinsed with

M HC1 and bidistilled water, FA was eluted with 0.2 M NaOH. The solution (ca. 100 mL) was passed through a column filled with 3 g of Lewatit S 1080 (0.1-0.25 mm particles) cation-exchange resin (Merck) in the H+. form. The solutions of the recovered protonated FA were used directly. FA was also purchased from the International Humic Substances Society as “reference fulvic acid’’ (Su- wannee River).

Terrestrial humic material (ERD) was extracted from a soil sample (cultivated rendzina; Miinchen-Grosshadern, FRG). A total of 5 kg of the soil was taken from 5 to 25 cm below the soil surface. Five portions of 1 kg were put in five bottles (2 L). Each soil sample was treated with 700 mL of 1 M NaOH for 18 h, maintaining efficient ag-

0013-936X/87/0921-0541$01.50/0 @ 1987 American Chemical Society Environ. Sci. Technol., Vol. 21, No. 6, 1987 541

Table 11. General Characterization of Aqueous HUS Solutions0

A(254 A(366 A(436 A(532

BAN 13 (FA) 3.0 0.0406 0.0073 0.0018 0.0004 BAN 13 (FA) 7.0 0.0413 0.0082 0.0022 0.0005 BAN 13 (FA) 10.5 0.0438 0.0100 0.0031 0.0008 BAN 13 (HA) 7.0 0.0427 0.0123 0.0047 0.0016 BAN 13 (HA) 13.0 0.0442 0.0151 0.0065 0.0022 BM 4 (FA) 8.0 0.0507 0.0104 0.0032 0.0009 BM 4 (FA/l)* 7.0 0.0604 0.0127 0.0042 0.0012 BM 4 (FA/2)* 7.0 0.0463 0.0089 0.0026 0.0007 BM 7 (FA) 3.0 0.0680 0.0144 0.0042 0.0010 REF (FA) 3.0 0.0378 0.0070 0.0016 0.0003 REF (FA) 7.0 0.0377 0.0082 0.0022 0.0005 ERD (FA) 3.0 0.0204 0.0038 0.0013 0.0004 ZIL 1 (FA) 3.0 0.0555 0.0086 0.0022 0.0005 ZIL 1 (FA) 7.0 0.0470 0.0093 0.0025 0.0006

code pH nm) nm) nm) nm)

"Absorbance (cm-l) is normalized to 1 mg of DOC/L. *FA/I = fraction 1; FA/2 = fraction 2.

Table 111. Characterization of HPLC Fractions of FA (BM 4)

fraction retention areal % corg, p~ no. time, min UV RI 90 (21 O C )

1 5.5 83 21 68.4 6.8 2 10.2 17 79 31.6 7.4

itation. After sedimentation (24 h) the aqueous phase was filtered and centrifuged a t 4000 rpm for 30 min. The solution was acidified with HC1 to pH 2.0 and left for 12 h in the dark. The precipitated HA was separated from the dissolved FA by decantation and filtration with a 0.45-pm membrane. The concentrated FA solution was adsorbed on a XAD-2 column (25 cm; 70 g), eluted, and protonated in the same manner as the aquatic samples.

Aqueous solutions of HUS, FA, and HA were prepared with bidistilled water. The pH was adjusted with 0.1 M aqueous HC1 and NaOH (s.P. grade, Merck). Character- istic data of the solutions are shown in Table 11. Metal ions were added as chlorides (p.a. grade, Merck) in aqueous solutions. The optical density of all solutions for the laser flash photolysis experiments was about 0.05 at X = 532 nm. Experiments in the absence of oxygen were performed with solutions purged with argon for 30 min. After that, no dissolved oxygen could be detected (detection limit I 0.02

Fractionation of FA. Fractionation of FA was done by HPLC with a Spherogel-TSK 2000 SW column (300 X 7.5 mm i.d.; 10-wm particles), connected to a solvent de- livery module (Beckman 114 M), a refractive index (RI) detector (Beckman), or a UV detector (Perkin-Elmer LC- 15). The fractograms were registered on a Chromatopac C-R2A integrator (Shimadzu), . and the fractions were collected with a Gilson Model 201 fraction collector. FA solutions of ca. 2400 mg/L (as DOC) and pH 7.0 were injected (injection volume 100 pL) and chromatographed with bidistilled water at a flow rate of 1 mL/min. The two main fractions (Table 111) were collected and concentrated in a rotary film evaporator a t 35 "C with a water pump vacuum.

Photometric and Photochemical Characterization. Spectrophotometric measurements were made with a Pye Unicam Sp 8-100 UV/vis spectrophotometer. Laser photolysis experiments were performed in the optical system shown in Figure 1. Pulses of the frequency-dou- bled (532 nm; 15 ns) Nd-YAG laser (J. K. Lasers, Series 2000) were monitored by a reference diode in order to correct for intensity variations. Unless stated differently,

542

mg/L).

Environ. Sci. Technol., Vol. 21, No. 6 , 1987

LASER SEMI TRANS - PARENT MIRROR I _ _ _ REFERENCE \ DIODE

X e SAMPLE MONO DIODE DATA LAMP CHROMA TOR SYSTEM

S = SHUTTER

F = FILTER

Figure 1. Apparatus for laser flash photolysis.

f ,

j . i'\,

I : , I t

I

700 800 900 1000 1100 1200 1300 lL00 1iOO WAVELENGTH lnm l

Figure 2. Uncorrected emission spectrum of BM 7 (FA): c(D0C) = 60 mg/L; Ar, A, =: 532 nm; a.u. = arbitrary units.

the pulse energy has been selected to be approximately 25 mJ. Filters permitting transmission at wavelengths greater than 590 nm have been inserted into the analytic beam before and after the sample. Signals of the transients have been amplified (Tektronix 7A13) and recorded on a digital storage oscilloscope (Tektronix 7D20).

The experiments with furfuryl alcohol as a specific trapping agent for IO2 were performed in a spectrophotometric cell (optical path 2 cm; volume 16 mL) by holding an oxygen electrode (Beckman, modified) on top. The cell is part of an apparatus used for the determination of quantum yields (a) of sensitized photooxidations (9 ,14) . Solutions of HUS with an optical density of ca. 1 at the wavelength of excitation, and containing M furfuryl alcohol, were irradiated in a bubble-free filled cell. The solution was thermostated a t 25 "C and rapidly stirred.

The pH values were determined with an Ingold 405 electrode connected to a Philips digital pH meter, PW 9408. The DOC values were determined with a UV-DOC analyzer, Tocor 3 (Maihak), together with a MA1 3 integrator.

R e s u l t s a n d Discussion Laser Flash Induced Emission Spectra. Emission

spectra from 700 to 1450 nm were drawn from the maxima of the time-dependent emission curves at the given wavelength. The resulting spectra have been corrected for the laser intensity but are not corrected for the spectral response of the detector and the light-absorbing characteristics of HzO.

Emission lifetimes were smaller than 500 ns and have not been analyzed. Different HUS show similar emission

l z 6 10 l i r e ' TIME Iusec I TIME IpsecI

Flgure 8. Decay of the translent absorption of BM 7 (FA) under (a) Ar and (b) normal atmosphere monitored at A = 900 nm.

characteristics, in particular an increase of signal intensity toward wavelengths below 700 nm. A typical spectrum is shown for BM 7 (FA) (Figure 2). The emission spectra were not altered by variation of the concentration of dissolved oxygen.

Laser Flash Induced Absorption Spectra. In addition to the short-lived emitting states of HUS, light- absorbing transients can be observed (15). Similar decay curves measured at different wavelengths imply that an irradiated sample contains a t least two transients of significantly differerent lifetimes. Figure 3a shows an exam- ple for the decay curve where the two transients can be differentiated optically. For the determination of the rate constants, the evaluated time was chosen for the short- lived transient up to 10 ps and for the long-lived transient up to 70 ps. Lifetimes of the transients were calculated with a computer program that analyzes the decay curves according to first- and second-order kinetics.

In Ar-saturated solutions both transients decay with first-order kinetics. Rate constants for the fast decaying transients are on the order of lo5 s-l; those for the slow decaying transients have been found to be on the order of lo4 s-l (Table IV). It has been shown that the observed transients are generated by a monophotonic process by varying the laser pulse energy from 2.7 to 28 mJ. There was a linear correlation of the energy with the absorption of both transients.

Quenching of Excited States. Oxygen is generally an efficient quencher of triplet states. On the basis of the data shown in Table IV, BM 7 (FA) was chosen to determine the quenching effect of oxygen on both transients. Their initial intensity is significantly decreased in the presence of dissolved oxygen (Figure 3). The same effect was also found for the other HUS. In air-saturated solution [c(Op) = 2.5 X M] the long-lived transient could not be observed (Figure 3b). Therefore, transient B has to be quenched by oxygen with a pseudo-firsborder rate constant of a t least (3.4 f 1.1) X lo5 s-l. This is the rate constant for the decay of the transient in an air-saturated solution, which is about the same as that for the decay of transient A under Ar. So it is obvious that the rate of oxygen quenching of transient A is smaller than its decay rate under Ar. This may be expected since the rate constant for the decay of transient A is higher than the calculated pseudo-first-order rate constant for diffusion-controlled quenching by oxygen, which is 2.5 X lo5 s-l, assuming a rate constant of 1 X lo9 s-l for the diffusion of oxygen. In case of a solution containing substances of macromolecular structure, the rate of oxygen diffusion may be even smaller. This would consequently lead to a smaller rate of oxygen quenching.

Laser flash photolysis experiments on natural waters and a commercially available humic sample also show the quenching effect of oxygen according to a diffusion-controlled reaction (16). A transient with a lifetime of 5 ps

Cd IIIl

35 7 0

co lIIl

35 70

0 NO METAL 0 O l r 1 0 - i 10 ?mol Me/rng DOC(HUS1 IMeI ADDED

Figure 4. Quenching effect of NI(II), Cd(II), and Co(I1) on laser-excited BAN 13 (FA): A,, = 532 nm; A,, = 700 or 1270 nm.

0.5 0.1 0.5 10 mmolN NdII )

Figure 5. Stern-Volmer analysis of emlssion quenching (Aex = 532 nm; A,, = 700 nm) of BAN 13 (FA) by Ni(I1) at pH 3.5: c(D0C) = 125 mg/L; I o / I = ratio of intensities of the emissions without (I,) and with ( I ) metal addition.

was reported, which is in the range of the short-lived transient A of this work (Table IV). However, an excited state with a lifetime T > 5 ps was not observed, probably due to a shorter experimental time scale and/or wavelengths of excitation and detection different from the experiments reported here.

Paramagnetic metal ions have been found to be good quenchers for transients of HUS (15). We made a more quantitative investigation on the reaction of a FA sample (BAN 13) with Ni(II), Co(II), and diamagnetic Cd(I1). The variation of the intensity of the emission was measured as a function of metal concentration and pH value. I t is interesting to note that the complexation capacity of BAN 13 (FA) for Cu(I1) a t pH 6.8, which is close to the total complexation capacity for bivalent metal ions, was determined by polarographic titration (17) to be 1.6 pmollmg of DOC. The decrease of emission is summarized in Figure 4. At pH 3.5 there is a significant quenching effect for Ni(I1) and Co(I1) and none for Cd(I1). The relatively high decrease of the emission in the case of the paramagnetic ions at a concentration of 0.1 pmollmg of DOC can be explained by the limited availability of metal-complexing functional sites in HUS at low pH values. This hypothesis is supported by the nonlinear Stern-Volmer analysis (Figure 5) indicating static quenching caused by coordi- native bonds between metal and HUS (18). An increase of relative complexation for a total metal ion concentration of 1.0 and 10 pmol/mg of DOC appears at pH 7.0 and is overcompensated at higher pH values by the competitive formation of hydroxo compounds as shown for Ni(I1) at pH 10.5. Precipitation of metal compounds at higher pH values causes a severe limitation for complexation reactions, especially if only labile and relatively unstable complexes are present. A fairly small quenching effect

543 Environ. Sci. Technoi., Vol. 21, No. 6, 1987

Table IV. First-Order Rate Constants (k) and Lifetimes (7,

of Two Light-Absorbing Transients of Different HUS

BM 7 (FA) 3.5 5.3 f 1.7 1.4-2.8 1.7 * 0.5 45-83 BAN 13 (FA) 3.5 1.5 f 0.7 4.5-12.5 1.6 f 0.8 42-125 BAN 13 (FA) 7.0 1.5 f 0.4 5-9 1.2 f 0.3 67-111 BAN 13 (HA) 7.0 1.2 f 0.5 5.9-14.3 5 f 2 14-33 REF (FA) 7.0 1.2 f 0.4 6.3-12.5 5.5 f 1.3 15-23

~~

Table V. Calculated Quantum Yields for HUS-Sensitized '02 Production Derived from the Furfuryl Alcohol Method

HUS pH DOC (HUS), mg/L @(*O,)

BAN 13 (FA) 3.5 144 0.030 BAN 13 (FA) 7.0 144 0.021 BAN 13 (FA) 10.5 144 0.015 BAN 13 (HA) 13.0 68 0.020 BM 4 (FA) 8.0 100 0.016 BM 4 (FA/1) 8.0 92 0.016 BM 7 (FA) 3.5 80 0.010 ZIL 1 (FA) 3.5 100 0.026 ZIL 1 (FA) 7.0 100 0.017 REF (FA) 3.5 145 0.018 REF (FA) 7.0 130 0.010 ERD (FA) 3.5 250 0.030

appears at pH 7.0 for higher Cd(I1) concentrations. Similar quenching effects can be observed when light-absorbing transients of the HUS are measured in the presence of Co(II), Ni(II), and Cd(I1). Total quenching occurs for Co(I1) and Ni(I1) a t 10 pmol/mg of DOC.

The quenching due to metal ions coordinatively bound to HUS obviously occurs a t a much shorter time scale compared with the quenching by 02. Static emission quenching by metal ions is far more effective, especially on short-lived transients. This may be of importance in aquatic systems, in the case of heavy metal pollution, where a decrease in lo2 production can be predicted due to the much faster quenching of excited states by com- plexed paramagnetic metal ions.

Furfuryl Alcohol Reaction. For the evaluation of the sensitizing effect of HUS, the photochemical oxidation of furfuryl alcohol was applied. This method is suited for the determination of the quantum yield of '02 production in aqueous solutions containing triplet sensitizers (6). Fur- furyl alcohol reacts specifically with lo2 (19), the decrease of the total concentration of dissolved oxygen being measured continuously. The quantum yield of lo2 production was determined at h = 366 nm since the system shows the highest efficiency at that wavelength. I t has been demonstrated that lo2 production is sensitized by HUS absorbing light also a t h = 546 nm (10). This sug- gests that the same mechanism is valid for the whole range.

The efficiency of lo2 production is probably much lower than the corresponding efficiency of intersystem crossing (ISC), which has not yet been measured.

The number of absorbed photons is measured and in- tegrated over the irradiation time. Oxygen consumption and the number of photons absorbed are necessary in order to calculate the apparent quantum yield @R for the pho- tooxidation of furfuryl alcohol and from that the quantum yield for the lo2 production @(lo,) according to eq 1 and 2, where c ( O , , ~ ) and c(Oz,,) are the total dissolved oxygen

concentrations at the beginning (i) and the end (e) of the irradiation in moles per liter, n is the amount of photons absorbed in einsteins, V is the reaction volume in liters, f (= 0.8) is an experimentally based correction taking into account thermal secondary reactions (9,20), and c(R-OH) is the initial concentration of furfuryl alcohol (10 mmol/L). ,6 was determined (9,20) to be 2.2 mmol/L, and that value was also taken in this work. At the chosen c(R-OH) the influence of P on @(IO2) is fairly small.

The results are shown in Table V. Even though the @(IO2) values include an uncertainty of ca. f30%, it is obvious that 1-3% of the radiation (366 nm) absorbed leads to the formation of IO2. This is in good agreement with the results from other works (2). A t pH 3.5 the soil FA shows the highest quantum yield. The same oxygen- saturated FA solution was irradiated as a system control without furfuryl alcohol. There was no oxygen consumption detectable that exceeded the precision of the mea- surement (i0.05 ppm) within a period of 2 h.

With increasing pH the quantum yield for lo2 production decreases (Figure 6). Zepp et al. (21) found a similar pH dependence for samples from the Aucilla River. This can only be partially explained by the increase of absorption of HUS with increasing pH (Table 11). According to the interpretation of Zepp et al., the anionic chromophores formed by deprotonation do not play an important role in singlet oxygen production. In addition, at high pH, quenching of the singlet oxygen by hydroxide ions has to be taken into account. Also, the increase of anionic sites in the macromolecule and the influence of the polar effect on its configuration might lead to a higher local concentration of potential physical quenchers of singlet oxygen.

Gel permeation chromatographic fractions of a FA sample show that the apparently lower molecular size fraction has a significantly higher sensitizing effect than the higher molecular size fraction and the gross sample as well. Obviously the low molecular size fraction contains the more effective photosensitizing structural regions whereas the higher molecular size fraction has more absorbing but photochemically inactive parts. I t is interesting to note that the first fraction shows a much higher refraction index but a much lower specific absorption a t 254 nm than the second one.

Conclusions

The singlet oxygen production of aqueous HUS with a quantum yield of 1-3 % was measured under conditions that are not too far from natural aquatic systems. The results from laser flash photolysis suggest an energy transfer from a long-lived excited triplet state (lifetime ca.

s) of HUS to oxygen. However, other excited states of HUS, e.g., radicals, and their interaction with oxygen should also be considered. Paramagnetic metal ions do not quench only the long-lived excited states but also the short-lived ones (lifetime s) found in emission experiments. Whereas oxygen quenching is controlled by diffusion, metal ion quenching works via complexation.

Since the quenching effects for similar molar concentrations of oxygen and metal ions are in the same order of magnitude, the influence on singlet oxygen production in aquatic systems would be only influenced by severe metal pollution.

lo2 does not react chemically with HUS within the detection limit of the method applied. This leads to the

544 Environ. Sci. Technol., Vol. 21, No. 6, 1987

lv I '021

0 03

002

001

..BAN

35 7 105 pH - VALUE

Figure 6. Quantum yields of aquatic fulvic acids at different pH values.

conclusion that the amount of C atoms involved in HUS structures that can be oxidized by lo2 (e.g., furans, dienes, reactive alkenes, etc.) can be neglected, since these structures would anyway be expected to be oxidized in the photic zone of aquatic systems as part of the genesis of HUS.

In dealing with the influence of HUS on the photochemistry of natural aquatic systems, it has to be kept in mind that the IO2 production is only one possibility for the reaction pathway and may play even a minor roll compared to the direct transfer of energy to other reactants. For a detailed understanding of the lo2 production in natural systems, it is necessary to investigate the influence of the major constituents of the water. Especially Fe(I1) and Fe(III), which occur together with HUS in most freshwaters, have to be included in further studies. As information on the structure of HUS becomes increasingly available, the identification of the photochemically active regions should be possible.

A c k n o w l e d g m e n t s

We thank Angelika Immerz and Helmut Niedermann for their experimental assistance for the isolation and fractionation of the HUS samples and K.-E. Quentin for his support of these studies. We thank M. Gratzel for the possibility to use the laser photolysis equipment; P. P. Infelta wrote and updated the computer programs for the kinetic analysis.

Registry No. 02, 7782-44-7; Ni, 7440-02-0; Co, 7440-48-4; Cd, 7440-43-9.

L i t e r a t u r e Cited (1) Roof, A. A. M. In T h e Handbook o f Environmental

Chemistry; Hutzinger, O., Ed.; Springer-Verlag: Berlin, 1982; Vol. 2, Part B, pp 43-72.

(2) Zafiriou, 0. C.; Joussot-Dubien, J.; Zepp, R. G.; Zika, R. G. Enuiron. Sci. Technol. 1984, 18, 358A-371A.

(3) Christman, R. F.; Gjessing, E. T. Aquatic and Terrestrial Humic Materials; Ann Arbor Science: Ann Arbor, MI, 1983.

(4) Choudhry, G. G. Residue Rev. 1984,92, 59-112. (5) Zepp, R. G.; Schlotzhauer, P. F.; Sink, R. M. Environ. Sei.

(6) Graf, G. A.; Braun, A. M.; Faure, J. Chimia 1980, 34,

(7) Gassmann, E.; Braun, A. M. Ecole Politechnique FBdBrale de Lausanne, Switzerland, unpublished data, 1983.

(8) Frimmel, F. H.; Trampisch, H. (Technische Universitat Munchen, FRG); Gassmann, E.; Braun, A. M. (Ecole Politechnique FBdGrale de Lausanne, Switzerland), unpublished data, 1983.

(9) Haag, W. R.; HoignB, J.; Gassmann, E.; Braun, A. M. Chemosphere 1984, 13, 631-640.

(10) Haag, W. R.; HoignB, J.; Gassmann, E.; Braun, A. M. Chemosphere 1984,13,641-650.

(11) Mantoura, R. F. C.; Riley, J. P. Anal. Chim. Acta 1975, 76,

(12) Frimmel, F. H.; Immerz, A.; Niedermann, H. In t . J . En- uiron. Anal. Chem. 1983, 14, 105-115.

(13) Water Resources Division, Denver Federal Center, Inter- national Humic Substances Society (IHSS), Denver, CO, 1983.

(14) Braun, A. M.; Gassmann, E.; Curchod, J.-M., unpublished data, 1985.

(15) Langford, C. H.; Sharma, D. K.; Power, J.; Joussot-Dubien, J.; Bonneau, R. Presented in part a t the 189th National Meeting of the American Chemical Society, Miami, FL, April 1985.

(16) Fischer, A. M.; Kliger, D. S.; Winterle, J. S.; Mill, T. Chemosphere 1985,14, 1299-1306.

(17) Frimmel, F. H.; Immerz, A.; Niedermann, H. In Com- plexation of Trace Metals in Natural Waters; Kramer, C. J. M.; Duinker, J. C., Eds.; Martinus Nijhoff/Dr. W. Junk The Hague, 1984; pp 329-343.

(18) Abdel-Kader, M. H.; Braun, A. M. J . Chem. SOC., Faraday Trans. 1 1985,81, 245-253.

(19) Maurette, M.-T.; Oliveros, E.; Infelta, P. P.; Ramsteiner, K.; Braun, A. M. Helv. Chim. Acta 1983, 66, 722-733.

(20) Gassmann, E. Ph.D. Theses, Ecole Polytechnique FBdGral de Lausanne, Switzerland, 1984.

(21) Zepp, R. G.; Baughman, G. L.; Schlotzhauer, P. F. Che- mosphere 1981,10, 119-126.

Technol. 1985,19,74-81.

234-236.

97-106.

Received for review October 24, 1985. Revised manuscript received September 24,1986. Accepted February 13,1987. F.H.F. gratefully acknowledges the financial support by the Deutsche Forschungsgemeinschaft Bonn-Bad Godesberg (Grant Fr 536-8). A.M.B. thanks Schweizerischer Nationalfonds (Grant 2.824-0.83) and Schweizerischer Schulrat for financial support.

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laser flash photolysis of dissolved aquatic humic material and the sensitized production of singlet...

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