Environ. Sci. Technol. 1993, 27, 889-894
Evaluation of Primary Photoproduct Quantum Yields in Fulvic Acid Aldo Bruccolerl, Bhuvan C. Pant, Devendra K. Sharma, and Cooper H. Langford'qt Department of Chemistry, Concordia University, 1455 de Maisonneuve West, Montreal, Quebec, H3G 1M8 Canada
Time-resolved photoacoustic spectroscopy in conjunction with magnetic circular dichroism (MCD)spectroscopy has been applied to determine energies and quantum yields for the formation of triplet states in aqueous solutions of two well-characterized (Laurentian and Armadale) fulvic acids. For the Laurentian sample, intersystem, crossing quantum yields range from 0.79 to 0.28 for the pH range 2.0-9.5. The average triplet energy is estimated as 1.8 X lo2kJ mol-1 from MCD spectra. For the Armadale sample, intersystem crossing quantum yields range from 0.82 to 0.35 for the pH range 2.0-9.5. The triplet energy is estimated as 1.7 X 102 kJ mol-l. With the primary photoproduct quantum yields, the overall photophysics and primary photochemistry of the fulvic acid may be described. The donor-acceptor model of humic spectra is also reinforced. Observed initial yields greatly exceed reported steady-state yields which appear to emphasize quenching and scavaging by humics themselves.
Introduction Humic substances are major organic light absorbers in natural waters. These colloidal organic materials have been implicated in a number of important photochemical pathways. To begin to unravel the contributing steps in such pathways, we require knowledge of the primary (as opposed to steady-state) photoproducts. This can be derived from modern time-resolved spectroscopy. This study concerns the photophysics and primary photochemistry of two well-characterized humic substances, the Armadale fulvic acid and the Laurentian fulvic acid. Earlier time-resolved absorption spectroscopy has identified the solvated electron, a corresponding radical cation, and triplet absorption in a broad excited-state absorption band (1, 2). Since the absorptivity of the solvated electron is known, the quantum yield for electron production and the corresponding radical cation can be evaluated. Luminescence yields have also been evaluated for the two fulvic acids (3). The present study is directed toward a description of the overall photophysics and primary photochemistry of the fulvic acids. The main tool complementing picosecond absorption spectroscopy in rendering this goal accessible is a time-resolved pulsed photoacoustic apparatus from which the quantum yields of triplets and the role of the radiationless decay of excitedstate singlets may be evaluated if an average triplet energy is available. An estimate of this energy value is obtained from the magnetic circular dichroism spectrum. The present report is a first step. It identifies the events which occur under ultraviolet irradiation in times between a few picoseconds and 2 pus. The humic substances chosen have been the subject of enough previous work to show that they are not atypical (1, 8). Nevertheless, these primary yields in the ultraviolet are large compared to steady-state values of expected subsequent products such Current address: The University of Calgary, Administration 131, 2500 University Drive N.W., Calgary, Alberta, T2N 1N4 Canada. 0013-936X/93/0927-0889$04.00/0
0 1993 American Chemlcal Society
as superoxide, hydrogen peroxide, and singlet oxygen. The next stage of our work will address the fate of these early time products. The two major contributions will be reaction with dissolved oxygen and reduction of steadystate yields by reaction with other components of the humic substances, Since typical concentrations of both these scavengers are no more than millimolar, reactions with these take times longer than microseconds even for diffusion-limited scavenging. There is one exception to this limit. Reaction with groups within the humic polymer may be quite fast.
Experimental Section Instrumentation. The apparatus for the photoacoustic measuredments is described in detail elsewhere (4,5). It is driven by a PRA LN 100 nitrogen laser (Photochemical Research Associates) which produces 10-60 pj pulses at 337.1 nm with a pulse width of approximately 300 ps. Repetition rates up to 100Hz are possible, The laser pulse is focused into the center of a sample cell consisting of a piezoelectric detector tube (Ai-4, Almax) with inner diameter 0.96 cm and length 1.25 cm. A quartz window was glued (with Norland 60 ultraviolet curing cement) to the bottom of the piezoelectric tube, and the sample solution was poured directly into the piezoelectric cell to a height of approximately 1 cm. The cell was mounted over a photodiode positioned at the center. The signal reaching the photodiode was monitored by a computer which controls the movement of two positioning motors to ensure that the laser beam passes directly along the longitudinal axis of the cell. The piezoelectric signal which occurs on arrival of each laser pulse is sent to the preamplifier and then to a boxcar integrator (Princeton Applied Research, Model 162-164). Recorded signals are the result of integration of 100pulses at each time delay. The boxcar had time gate strips of 50 ns so that successive points are 50 ns apart. The signal derived is a wave with frequency equal to the inverse of the traveling time of sound in the solution. The first maximum observed is the result of the first acoustic wave to arrive at the piezoelectric, and subsequent maxima are the result of echoes and detector resonances, if no other acoustic waves are produced. Because of the complexity of echoes and resonances, data are derived from the first peak. The height of this peak measures light energy converted to thermal energy, in times less than about 1ps. All steps of the experiment are controlled by a computer through an A/D interface (DT-2801, Data Translation). The data acquisition and analysis are implemented using the Asyst (Macmillan Software) package. The sensitivity is 7.5 X 10-lo V J-l. This results in a standard deviation of 10% in the calculated triplet quantum yields for the fulvic acid solutions. Time resolution is limited primarily by the traveling time of the acoustic wave in the cell, approximately 2 ps. The picosecond experiments were conducted in the Canadian Centre for Picosecond Laser Spectroscopy (ConcordiaUniversity) using a mode-locked Nd/YAG laser Envlron. Sci. Technol., Vol. 27, No. 5, 1993
889
producing a third harmonic pulse at 355 nm with a pulse width of 30 ps and a pulse energy of 2.5 pJ (1, 3). The conventional absorption spectra needed were recorded on a HP 8452 spectrophotometer. The wavelength resolution was 2 nm, and the absorbance resolution was 0.001. The magnetic circular dichroism (MCD) spectra needed were recorded by Dr. B. Hollebone at Carleton University in Ottawa using a locally fabricated instrument which is described elsewhere (6). Materials. The well-characteized fulvic acid known as Armadale fulvic acid was used. Sample preparation has been described previously (7). Schnitzer had determined the following distribution of functional groups in the sample: 3.3 mequiv g-1 phenolics; 7.7 mequiv g-1 carboxylate; 3.6 mequiv 8-l aliphatic alcohol; 0.6 mequiv g-' guinone. The elemental analysis is C = 49.52 % ; N = 0.58% ;H = 4.6%. Metal ion content is less than 10 pmol g-1. The Laurentian fulvic acid has been described by Wang et al. (8, 9). The elemental analysis is C = 41.14%; H = 4.11 % : N = 1.079%. This fulvic acid was fractionated by ultrafiltration. A 250-mL volume of fulvic acid stock solution containing 2.500 g of fulvic acid was passed sequentially through YM30 (nominal cutoff 30 000 Da) and YM2 (1000 daltons) filter membranes. The fractions were collected and lyophilized to yield subfraction fulvic acid solid samples. Tris[ (5-methyl-l,l0-phenanthroline-iron(II)1perchlorate (G.F. Smith Chemical Co.), called iron-phen below, was used as a photoacoustic standard. It has a quantum yield of unity for rapid radiationless decay (IO). No transient absorption of lifetime longer than 20 ps is observed (IO). The compound does not luminesce detectably. Thus, all light absorbed by this compound is converted to an acoustic signal which serves to calibrate the piezoelectric signal. All fulvic acid solutions were prepared and analyzed while open to the atmosphere and were air-saturated.
Met hod0 1ogy Time-Resolved P hotoacoustic Spectroscopy. This technique has emerged as a powerful tool for estimates of energetics of photophysical processes. A preliminary account of its application to fulvic acid has been given in ref 4. The signal obtained is the intensity of the acoustic pulse reaching the wall of the piezoelectric tube as a function of time. This is proportional to the energy converted from light to thermal energy in times less than the transient time of the acoustic wave ( E "
2. 5
H
a
E
tn
2.0
3
m 0 3
1.5
1 i
i,
m
3
0 0 0
0 i,
0 L
a Absorbance Figure 2. Photoacoustic signal as a function of the absorbance of solutions of Tris[(6methyi-l, 10-phenanthro1ine)-iron (I I)] perchlorate (iron-phen).
noise and background signals caused by absorption by the cell wall and base. As is now well-known (5, 111, the photoacoustic signal is proportional to optical absorbance for optically thin samples. The relation is linear up to an absorbance of approximately 0.1 at 337.1 nm. Therefore, all solutions were prepared with absorbance less than 0.1 at 337.1 nm. Fulvic acid solutions had concentrations less than 20 mg L-I. The dependence of signals in the present apparatus on absorbance was calibrated using iron-phen solutions of known absorbance. The linear regression coefficient for the line in Figure 2 is 0.99, and the slope is 2.32 X V with a standard deviation of about 4% for a pulse energy of 20 f 1 pJ. From these values, we can write an expression for the photoacoustic signal, PS, of fulvic acid
PS = (2.32 x lo-' VI20 X
J)E,A
(2)
where Ei is the input energy and A is the optical absorbance.
The energy output in the photoacoustic wave, Eps,is
E,, = PS/(1.16 X
V J-')
(3)
As opposed to present measurements, where the constant is the instrument sensitivity cited above, steadystate photoacoustic spectra measure all energy dissipated in thermal form and, hence, give a signal proportional to the difference of energy absorbed minus energy emitted as luminescence. For Armadale fulvic acid, luminescence yields are difficult to estimate precisely but are certainly less than about 0.03 at pH 7.0 (3, 41, so a steady-state signal for the fulvic acid is proportional to absorbance within experimental error. In contrast, the present pulsed experiment measures energy converted into thermal energy within the apparatus time constant of about 2 ps. Therefore, long-lived excited states that can store energy contribute to reduction of pulsed photoacoustic signals. Transient absorption spectra in the microsecond to millisecond time domain have revealed a broad excitedstate signal ( 1 , 2 ) . This was assigned as triplet excitedstate absorption. The transient photoacoustic signal allows estimation of the energy stored by these species. If a characteristic triplet energy can be assigned from MCD spectra, quantum yields for triplet formation can be estimated from the total energy stored for times longer than 2 p s and the average energy per triplet. The total energy stored by triplet states (longer than 2-ps lifetime), ETS,is the energy absorbed minus the photoacoustic energy released, Eps. If the intersystem crossing quantum yield (4) for the transfer of the excitedstate molecules to the triplet states is not 1,we will have The energy of one triplet state, ET, is the total energy divided by the number of moles in the triplet states, n T S :
from degenerate ground states, and A terms are a consequence of a transition to a degenerate excited state. Two of these can arise in the spectra of fulvic acids; A and B terms. A singlet-triplet absorption band is expected to produce an A term. An A term has a characteristic "dispersion band" shape with equal positive and negative regions. I t is from the center of this "dispersion band" that the average triplet energy is obtained. A B term resembles an ordinary absorption band with one region either all positive or all negative. Singlet-singlet transitions can produce B terms. A C term is characterized by distinctly unequal dispersion lobes. Picosecond Spectroscopy. Estimates of the quantum yield for the solvated electron and the corresponding radical at early times have been made directly from timeresolved absorption spectra recorded at 20 ps V J using the molar absorptivity for the solvated electron (15). At 675 nm (t = 25 "C), the value is 1.4 X lo4 M-l cm-l. We assume that the solvated electron is the only species contributing to transient absorbance at this wavelength (3). The appropriateness of the assumptions were established by the quantitative solvated electron scavenging experiments of Power et. al. (1,3). The path length of the cell is 0.200 cm. The volume of irradiation for the sample L based on the dimensions of was approximately 2 X the laser pulse at the cell. The concentration of the transient, solvated electron, at 20 ps, depends on the total number of fulvic acid molecules in the volume irradiated. The number of molecules excited depends on the number of photons absorbed. When the number of molecules in the volume irradiated is less than the number of photons incident, as was the case in these picosecond time-resolved experiments, then the quantum yield for the solvated electron, &, is given by relation 6 (3, 16): = [no. of solvated electrons produced]/
[no. of molecules excited] (7)
where = @&Ep and E, is the energy of a 337.1-nm photon which is equal to 5.9 X 10-19 J; therefore, the energy (in J mol-l) of the triplet state is equal to nTS
(5.9 X 1O-l' J) - (5.9 X 1O-l' J)(Ep,)/EiA
E, =
(6) 4z This equation gives a direct relation between the photoacoustic signal, the absorbance, the energy, and the quantum yield of the triplet state in the fulvic acid mixture (4,5). For all photoacoustic experiments, the laser frequency was set at 10 Hz for laser pulses of 20 pJ. At these settings there was no evidence for biphotonic processes (5). There was a linear relationship between the photoacousticsignal, PS, and pulse energy. MCD Spectroscopy. MCD spectra are useful for identification of spin-forbidden bands because transition intensities depend on the product of electric and magnetic dipole matrix elements and spin-forbidden bands are relatively more intense than in ordinary absorption spectroscopy (12-14). Moreover, line shape is very helpful to assignment. The three characteristic signal shapes are denoted A, B, and C terms (12-14). B terms arise from transitions between nondegenerate states. C terms arise
Results The MCD spectrum of the Armadale fulvic acid, Figure 3, shows an A term which is assigned as triplet absorption by Armadale fulvic acid. The center of this triplet region appears at approximately 14 000 cm-l or 1.7 X lo2kJ moll-l f 6 % . The main spectral feature is a B term at roughly 16 500 cm-I, which is attributed to singlet absorption. Similarly the MCD spectrum for the Laurentian fulvic acid, Figure 4, shows an A term which is assigned to triplet absorption. The center of this triplet region appears at approximately 15 500 cm-l or 1.9 X lo2 kJ mol-' f 5 % . Using the triplet energy of 1.7 X lo2 kJ mol-l for the Armadale fulvicacid, apparent triplet quantum yields were calculated from eq 6 and appear in Table I. These results confirm preliminary values in ref 4. Similarly, quantum yields were calculated for the Laurentian fulvic acid and Laurentian fulvic acid fractions at various pH's. The results are shown in Table 11.
Discussion Primary Yields. Fulvic acid samples absorb strongly in the near-ultraviolet region of the spectrum. The primary photophysical processes which may occur after excitation are luminescence, intersystem crossing, and internal conversion from the singlet to the ground state. The Envlron. Sci. Technol., Vol. 27, No. 5, 1993
881
3000 I I
Table 11. Apparent Triplet Quantum Yields Calculated for Various Solutions of Laurential Fulvic Acida
I
2000
1000 0 - 1000
-2000 t -3000 L+--,+++-I--c.-+l-4-~-- IJ 11.5 14.0 16.5 19.0 21.5 Wavenumbers ( X
-3
10
-1
ctti
a
,
0.15
4
pH
app quantum yield 4
ionic strength
2.0 5.0 9.5 2.0 5.0 9.5 2.0 5.0 9.5 2.0 5.0 9.5 2.0 5.0 9.5 2.0 5.0 9.5
0.79 0.60 0.28 0.26 0.21 0.15 0.85 0.73 0.31 0.32 0.27 0.18 0.88 0.88 0.86 0.90 0.91 0.90
