Environ. Sci. Technol. 1986,2 0 , 943-948
Photoinduced Reductive Dissolution of a-Fe,O, by Bisulfite Bruce C. F a d and Michael R. Hoffmann”
Environmental Engineering Science, W. M. Keck Engineering Laboratories 138-78, California Institute of Technology, Pasadena, California 9 1125 and-to-metal charge transfer from the adsorbed organic reductant to an Fe(II1) center on the metal oxide. Frank and Bard (20)have reported that S(1V) is photochemically oxidized in suspensions of a-Fe203,Ti02, and CdS, although they provided little kinetic data and no mechanistic detail. There are at least two potentials chromophores in surface photochemical reactions. They are the bulk solid and surficial complexes. Consequently, a knowledge of the primary photochemical steps is essential to an understanding of interfacial photochemical reactions. Furthermore, primary photochemical reactions and subsequent secondary reactions involving Fe(II1) and S(1V) may significantly alter the speciation of iron and sulfur and enhance the rate of S(1V) oxidation in cloudwater or haze aerosol (15, 16). In light of the above information, the principal objectives of this study were to elucidate the role of light in the reductive dissolution of hematite by S(1V) and to interpret the results in terms of existing theories (11, 12) of the photochemical activity of semiconductorsuspensions. The kinetic details of the Fe,03/S(IV) system in the presence of oxygen will be the subject of a future communication.
w The kinetics of the photoinduced reductive dissolution of a-Fe203by bisulfite in anoxic colloidal suspensions have been investigated. Quantum yields, @PFe(II),as a function of wavelength for Fe(II)*,-, production were obtained. Photochemical reactivity has been discussed in terms of the excitation of the 02-to Fe3+ charge-transfer band of a-Fe203and in terms of photoassisted charge-transfer reactions of Fe(II1)-S(1V) surficial complexes. ~~~
Introduction
The role of photochemical reactions at solid interfaces in natural waters (1-5) and atmospheric water droplets (6) is a topic of current interest. Early research on surface photo-redox reactions was motivated by the possibility of using the visible and near-W portion of the solar spectrum to assist in the “splitting” of water into Hz and 02. However, oxygenated aqueous suspensions of semiconductors, such as ZnO, when illuminated with visible light gave H202 as the product; they did not yield hydrogen and oxygen from the disproportionation of water. Calvert et al. (7) reported that low concentrations of a variety of organic compounds such as glycerol, acetone, ethanol, formate, phenol, oxalate, and toluene increased the yield of hydrogen peroxide. Similar results were reported by Markham and Laidler (8). Calvert and co-workers (7, 9, 10) demonstrated by isotopic labeling that H202resulted from reduction of oxygen and that the quantum yield for H202 production increased near the band gap energy of ZnO. In the absence of suitable oxidants, semiconductorssuch CdS and ZnS have been found to undergo photoinduced dissolution (11, 12). Hoffmann and co-workers (13-16) have established that total iron concentrations in urban fogs and clouds are unusually high. For example, they have measured total iron concentrations that exceeded M. However, they (14, 16) have also shown that the majority of the iron present is operationally classified as soluble or microcolloidal. This observation was surprising in light of the fact that the major source of iron in dry aerosol appears to be in the form of highly insoluble iron oxides and oxyhydroxides. Foster (17) found that a-Fe203comprised 5-15 w t % of fuel ash from power stations, while Taylor and Flagan (18)reported that Fez03 comprised a significant fraction of the fine aerosol discharged during coal combustion. Iron(II1) oxides also constitute substantial fractions of photochemically active sand found in various desert environments (19). Given the relative abundance of Fe(II1) oxides in aerosols and surface waters, a thorough investigations of their photo-redox behavior is warranted. Waite and Morel ( 3 ) have studied the photoreductive dissolution of lepidocrocite (y-FeOOH) in the presence of citrate, while Cunningham et al. (4) have studied the photoassisted reduction of goethite (a-FeOOH) in the presence of adsorbed ethylene glycol. Both groups have interpreted their results in terms of a photoinduced lig_
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* Author to whom correspondence should be addressed.
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Present address: Federal Institute for Water Resources and Water Pollution Control (EAWAG), Swiss Federal Institute of Technology (ETH), CH-8600Dubendorf, Switzerland. 0013-936X/86/0920-0943$01.50/0
Experimental Section
~
All solutions were prepared from analytical grade reagents and high-purity water (“Q-H20”, resistivity 1 18 Mohm-cm). Stock solutions of Na2SO3were standardized with 103-/1-(21). All S(1V) stock solutions were stored under prepurified N2 in an unpressurized glovebox. Hematite (a-Fe203),which was synthesized according to the procedures described by Matijevii. et al. (22a) as modified by Faust (22b),precipitated in the form of hexagonal platelets with approximate edge dimensions of 0.12 X 0.015 hm (batch 1). The fresh suspensions were washed a minimum of 4 times, with 0.01 M HC104,for periods of at least 1day per wash. X-ray diffraction patterns, which were obtained with a Norelco vertical diffractometer using Cu (Ka)radiation, were found to be identical with a-Fe203 standards (22b). Characteristic peaks for other Fe(II1) oxides were not observed. Reflectance spectra were obtained with a Beckman UV 5240 spectrophotometer equipped with an integrating sphere. The total exchange capacity of hematite, which had previously been equilibrated in a 0.05 M NaF solution at pH 3.6 (HC104),was determined by fluoride desorption (23). This gave a ratio of [total exchange sites]/[a-Fe203] = 0.09 (i.e., -4.5% of the total iron was present as fluoride accessible surficial sites (batch 2)). On the basis of these measurements the apparent reactive surface area was 38 m2/g. Batch adsorption experiments, at constant pH, were carried out in the dark for 90 min. Sample aliquots were filtered and preserved for S(1V) analyses. Monochromatic light was generated by focusing light from a 450-W Xe light source (Osram) through a water filter into a 100-mm focal length holographic grating monochromator (Instruments SA Inc.) and through an appropriate order-sorting filter. The collimated beam was then used to illuminate a tubular Pyrex reactor (10-cm path length, 0.37 L volume) equipped with optical Pyrex
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 9, 1986 943
(nM/rnin) o LIGHT, F e ( l I ) h
5 [a-Fep03]= 7.5 x ~ O M- ~
, I'
1
b n
p H = 2.92-2.96
5
pH
I
0
0
0.5
1 .o
I .5
AQUEOUS [S(E)],(mM)
glass faces at both ends. The bandwidth, with 4-mm slit widths, was approximately 25 nm. All experiments were performed at T = 25.0 f 1.0 "C and p = 0.1 M in NaC104. Oxygen-free solutions were prepared by purging with N2 for a minimum of 1 h immediately before the initiation of any experiment. Residual O2 in the N2 stream was scrubbed with an acidified solution of V(I1) (24). Reagents were transfered under N2 in a glovebox. In addition, the headspace of the reactor was purged continuously with N2 during the course of all experiments. Light intensity was quantified before each experiment with Reinecke's salt, KCr(NH,)2(NCS)4,as the chemical actinometer (25). Acidified (0.1 M HC104) solutions of KCr(NH3)2(NCS)4were photolyzed, and the rate of thiocyanate release was followed colorimetrically. During the course of each kinetic experiment, samples were taken periodically and filtered through 0.2-pm Nuclepore filters. S(1V) in the filtrate was preserved by addition of NaH2PO4/Na2HPO4,NaOH, and disodium ethylenediaminetetraacetate (Na2EDTA). The decrease in [S(IV)], in samples preserved by this method, was less than 7% over a period of 34 days. Final concentrations of total orthophosphate and EDTA after dilution were 0.05 and 0.001 M, respectively, with a final pH of 6.8-8.0. Samples were preserved for Fe(II),, analyses by acidification to pH N 1with H2S04and by purging with N2 to remove all S(1V) (26). All samples were stored in the dark at 5 "C until analysis. The method of Humphrey et al. (27),which uses 5,5'dithiobis(2-nitrobenzoic acid) (DTNB) as a colorimetric reagent, was used for S(1V) determinations. Stock solutions of DTNB were prepared by dissolving the solid in 95% ethanol. Analyses for Fe(II),, used the modified 1,lOphenanthroline technique of Tamura et al. (28), with fluoride added to sequester Fe(II1). For measurements of total aqueous iron ([Fe(II),] + [Fe(III)aq]),hydroxylamine sulfate was substituted for the NH4F to reduce Fe(I1Qaq to Fe(II)aq, which was then measured by using 1 , l O phenanthroline. Results A binary-site isotherm for S(1V) adsorption on a-Fe203 is presented in Figure 1. Both pH and [a-Fe203]for the adsorption study were identical with those used in kinetic runs. The data shown in Figure 1 suggest the existence of two independent binding sites on the surface of a-FezO3 with substantially different adsorption constants. The Environ. Sci. Technol., Vol. 20,
No. 9, 1986
2.86-2.95
[ a - Fe,03] = 7.5x 10-4M
a, LL
U
Figure 1. Adsorption of S(IV) onto hematite. Experimental results are presented together with calculations from a binary-site Langmuir adsorption isotherm.
944
(X=504nm) 42.5 DARK, F e W 1.0 + DARK,FeT(oq) 1.0
[ s( IV)], =2.0x I O - ~ M .+ 0
-
w-+ 100
+ +
-+
' 200
TIME (rnin) Figure 2. Photochemical and thermal dissolution of hematite in anoxic suspensions containing S(1V).
time required to achieve equilibrium was several minutes in all cases. The chemical speciation of S(1V) over the pH range of interest is dominated by the following species: S02.H20, HOS02-, SOZ-, S2052-, FeOSOOH2+,FeOS02+,and =Fe(111)-S(1V) surficial complexes. The quantum yield for a unimolecular reaction in a cylindrical reactor is defined as follows (29): @F~(II)
(d[Fe(II),,]/dt)/[Io(A/V)(l - 10-fCL)l (1)
where @ F ~ ( I I )is the experimental quantum yield, d[Fe(II),,]/dt is the average rate of Fe(II)aqproduction (M/ min), Io is the intensity of light incident to the reactor cell [einstein/(min.cm2)], A is the area of the reactor cell exposed to light (cm2),V is the reactor volume (liter), t and C are the molar absorptivity (M-l cm-l) and concentration (M), respectively, of the photochemically reactive Fe(II1) species, and L is the light path length (cm). If the photochemically reactive Fe(II1) species absorbs virtually all of the incident light (tCL > l),eq 1 simplifies to @F~(II)= (d[Fe(II),,]/dt)/[I,(A/V)1 (2) Results of a representative set of kinetic data used to determine quantum yields in the anoxic Fe(III)/S(IV) system are shown in Figure 2. In this case, at X 504 nm the measured values of d[Fe(II), ]/dt and Io (504 nm) were used to evaluate @Fe(II) accor%ing to eq 1. For each wavelength of interest an experiment of this type was performed. The thermal (dark) reaction rate was subtracted from measured rates of illuminated reactions to obtain the true photochemical reaction rates. The data shown in Figure 2 indicate that the reductive dissolution of hematite is enhanced significantly by visible light. However, in the absence of S(IV), no detectable Fe(1Uaq was released in anoxic suspensions over 8 h of irradiation at X 375 nm. Quantum yields for Fe(II)aqproduction are presented in Table I and illustrated in Figure 3. The observed quantum yields are lower than the intrinsic quantum yields because of inefficiencies due to light scattering by the colloidal suspension. As shown in Figure 3 the observed quantum yields increase sharply from 420 to 350 nm. This sharp increase occurs in a region of the spectrum dominated by the lowest energy charge-transfer band (Amax 375 nm for O*- Fe3+)of hematite (30). In addition, Figure 3 illustrates that the apparent increase in quantum yield also overlaps the region of maximum
-
Table I. Experimental Quantum Yields for Fe(II)aqProduction in Deoxygenated Hematite Suspensionsa A, nm
lo*
@Fe(IIl
2.70 1.00 0.95 0.59
350 375 392 416 452 504 585 676 676
0.40
0.33 0.22 0.12 0.13
x 10-3
ODIC
2.55 2.93 3.03 2.72 2.13 1.19 0.12 0.05 0.05
19.1 22.0 22.7 20.4 16.0 8.9 0.9 0.4 0.4
ezd
x 10-3 4.44 5.00 4.31 3.43 2.22 1.32 0.69 0.54 0.54
ODze 1.33 1.50 1.29 1.03 0.67 0.40 0.21 0.16 0.16
OD2/ CODi
0.07 0.06 0.05 0.05 0.04
@Fed
0.390
0.170 0.190 0.120 0.100
0.04
0.080
0.19 0.29 0.29
0.010
0.006 0.006
[a-Fez03]= 0.75 mM, batch 2; [S(IV)lO= 0.20 mM; pH 2.86-2.95. b e l values, the molar extinction coefficients for Fez03,were taken from Marusak et al. (33). OD, is the solid optical density for [a-Fez03]= 0.75 mM and 1 = 10 cm. ez values, the apparent molar extinction coefficients for the Fe(II1)-S(1V) complex, were taken from the data of Faust (22b) and were scaled to the extinction coefficient for iron(II1) thiocyanate (22a). 'ODz is the optical density of Fe(II1)-S(1V) at 30 WMand 1 = 10 cm. f$'FelII)is the quantum efficiency based on Fe(II1)-S(1V) absorption. SPECTRAL RESPONSE vs QUANTUM YIELD 6 0
0 05
4 8
0 04
36
0 03
2 4
0 02
12
0 01
w
2 m
a
g
QFe
00 200
300
400
500
600
700
(E)
0 00 800
WAVELENGTH ( n m l
Flgure 3. (i) Experimental quantum yields for Fe(II)aqproduction in anoxic hematite suspensions with [a-Fe,03] = 0.75 mM and [s(Iv)]T = 0.20 mM at pH 2.86-2.95. (ii) Charge-transfer absorption band of 367 nm) with [S(IV)], = 1.0 mM, Fe(II1)-S(IV) aqueous complex (A, [Fe(III)a,]T 0.11 mM, and pH 2.94 (10-cm path length). This was obtained by subtraction of the spectra for solutions of equal concentration: Fe(III)aq(0.11 mM) at pH 2.90 and of S(IV) (1.0 mM) at pH 2.94; from the spectrum of the Fe(II1)-S(IV) solution. (ill) Absorption coefficient of a-Fe,03 thin films (1lo), after Marusak et al. (32). The peaks at 315 and 375 nm correspond to 0'Fe3+ charge-transfer bands (44).
-
absorption of aqueous-phase Fe(II1)-S(1V) complexes (e.g., Fe(III)S03+,Fe(OH)HS03+,and Fe(III)HS0,2+)that have apparent absorption maxima near 367 nm. Childs and Ollis (31) have pointed out that, in addition to surface photo-redox reactions, three other general processes may contribute to the net redox activity. They are (i) surface thermal (dark) reactions, (ii) aqueous-phase thermal (dark) reactions, and (iii) aqueous-phase photochemical reactions. Aquated SOz, HOSOZ-, S032-, and SzOz- have absorption maxima at X
