Environ. Sci. Technol. 2010, 44, 4142–4148
Photoreductive Dissolution of Iron Oxides Trapped in Ice and Its Environmental Implications K I T A E K I M , † W O N Y O N G C H O I , * ,† MICHAEL R. HOFFMANN,‡ HO-IL YOON,§ AND BYONG-KWON PARK§ School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea; W. M. Keck Laboratories, California Institute of Technology, Pasadena, California, United States of America; and Korea Polar Research Institute, Incheon, South Korea
Received December 14, 2009. Revised manuscript received April 15, 2010. Accepted April 20, 2010.
The availability of iron has been thought to be a main limiting factor for the productivity of phytoplankton and related with the uptake of atmospheric CO2 and algal blooms in fresh and sea waters. In this work, the formation of bioavailable iron (Fe(II)aq) from the dissolution of iron oxide particles was investigated in the ice phase under both UV and visible light irradiation.Thephotoreductivedissolutionofironoxidesproceeded slowly in aqueous solution (pH 3.5) but was significantly accelerated in polycrystalline ice, subsequently releasing more bioavailable ferrous iron upon thawing. The enhanced photogeneration of Fe(II)aq in ice was confirmed regardless of the type of iron oxides [hematite, maghemite (γ-Fe2O3), goethite (R-FeOOH)] and the kind of electron donors. The iceenhanced dissolution of iron oxides was also observed under visible light irradiation, although the dissolution rate was much slower compared with the case of UV radiation. The iron oxide particles and organic electron donors (if any) in ice are concentrated and aggregated in the liquid-like grain boundary region (freeze concentration effect) where protons are also highly concentrated (lower pH). The enhanced photodissolution of iron oxides should occur in this confined boundary region. We hypothesized that electron hopping through the interconnected grain boundaries of iron oxide particles facilitates the separation of photoinduced charge pairs. The outdoor experiments carried out under ambient solar radiation of Ny-Ålesund (Svalbard, 78°55′N) also showed that the generation of dissolved Fe(II)aq via photoreductive dissolution is enhanced when iron oxides are trapped in ice. Our results imply that the ice(snow)-covered surfaces and ice-cloud particles containing iron-rich mineral dusts in the polar and cold environments provide a source of bioavailable iron when they thaw.
* Corresponding author phone: +82-54-279-2283; fax: +82-54279-8299; e-mail:
[email protected]. † School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH). ‡ W. M. Keck Laboratories, California Institute of Technology. § Korea Polar Research Institute. 4142
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Introduction The role of bioavailable iron in aquatic and marine environments is critical in biota (1-4). Martin’s (5, 6) iron-fertilization hypothesis has been tested in mesoscale field experiments that yield enhanced primary productivity (7), although the extent of the CO2 uptake (8, 9) triggered by iron addition is the subject of much debate (10). The dominant input of iron to the open ocean is supplied by aeolian dusts that are transported mainly from the desert area (11-13). The bioavailable iron largely comes from the light-induced dissolution occurring on iron (ferric) (oxyhydr)oxide (a component of mineral dust and ambient aerosol) (1, 14-16). For example, Finden et al. (16) reported that particulate iron oxides provide bioavailable iron via photoreductive dissolution for phytoplankton growth. Previous reports on the photoreductive dissolution of iron (oxyhydr)oxides in the aqueous phase indicate that the rates of reaction depend on the incident light intensity, wavelength range, concentration and specific nature of the electron donors in solution, pH, ionic strength, and the crystalline phase of the iron (oxyhydr)oxides (e.g., goethite, hematite) (17-19). In this work, we focus our attention on the photochemical reaction of iron (oxyhydr)oxides trapped in ice and its possible contribution to the redox cycle of iron under sunlight. The iron-containing dust particles trapped in ice, snow, and frozen cloud aerosols may undergo photoreductive dissolution under solar light and release more bioavailable iron upon thawing. The heterogeneous photochemical reactions taking place in polycrystalline ice may differ dramatically from those in the aqueous phase. Most chemical reactions in aqueous solution are generally slowed at lower temperature. However, many bimolecular chemical reactions are reported to be accelerated by freezing above the eutectic point, in which the solutes are concentrated as they are excluded from crystalline ice into a liquid-like grain boundary region within the polycrystalline ice matrix (freeze concentration effect) (20-24). For example, Takenaka et al. (20, 21) found that the oxidation of nitrite ion (NO2-) with dissolved oxygen to form nitrate (NO3-) was accelerated by a factor of 105 during freezing of aqueous nitrite solutions. It has also been reported that the photochemical processes taking place in ice can be very different from those in the aqueous phase and might play an important role in the chemical transformation in cold environments (25, 26). This study investigated the photoreductive dissolution of various iron (oxyhydr)oxides in ice (typically around pH 3.5 that represents cloudwater condition) under UV or visible light irradiation and demonstrated that the heterogeneous ice photochemical process is significantly different from the aqueous counterpart. The experimental parameters affecting the ice-enhanced photochemical process, the mechanism, and the environmental implications are discussed.
Experimental Section Materials. Optically transparent R-Fe2O3 (hematite) colloids were prepared from the hydrolysis of FeCl3 following the method described by Faust et al. (27). The as-prepared aqueous colloid (800 mg · Fe2O3/L) was diluted to 8-16 mg/L in most experiments. The diluted hematite colloid with pH 3.5 and 600 µM of chlorides was stable without coagulation for 10 days. The synthesized hematite phase was confirmed by X-ray diffraction analysis (PANalytical X’Pert diffractometer with an X’Celerator detector) using Cu KR radiation (see Supporting Information, Figure S1(a)). The electronic bandgap transition of the synthesized hematite was observed 10.1021/es9037808
2010 American Chemical Society
Published on Web 05/06/2010
at the onset of 550 nm in the UV-visible absorption spectrum (see Supporting Information, Figure S1(b)). The primary colloidal particle sizes were in the range of 10-40 nm as observed by high resolution field emission transmission electron microscopy (HR-FETEM). Other commercially available iron oxides of hematite (Aldrich, BET surface area 8 m2/g), maghemite (Aldrich, BET surface area 36 m2/g), and goethite (Aldrich, BET surface area 185 m2/g) were also used and compared with the colloidal hematite for their photodissolution reactions. Commercial iron oxides were ground into fine powder before preparing the stock aqueous suspension to increase the dispersion. Suwannee River fulvic and humic acids were purchased from the International Humic Substances Society (http://www.ihss.gatech.edu). Photolysis. Aqueous suspension/colloid of iron oxides (10 mL) with a desired concentration (8-16 mg/L) was placed in a quartz tube (12 × 125-mm), sealed with septa, and solidified in an ethanol bath cooled at -20 °C. The temperature of the freezing bath was gradually lowered from -5 to -20 °C within 30 min to prevent the breakage of the tubes. After freezing, the colloidal hematite was uniformly dispersed along the ice column without aggregation while the commercial iron oxide powder was slightly concentrated in the lower part of the ice column because of the gravitational settling during freezing. The whole ice column was irradiated by the lamp after the freezing process. Sample tubes (maximum 16 tubes) were located in a merry-go-round photolysis reactor that was rotated at a constant speed (0.8-1.0 rpm) around a 100-W mercury lamp (Ace Glass Inc.) for uniform irradiation. Light was filtered by a pyrex jacket (transmitting λ > 300 nm; see Supporting Information Figure S1(b)) surrounding the mercury lamp that was immersed in the ethanol bath. Visible light irradiation experiments used a 100-W halogen lamp (Phillips). A 300-W Xe arc lamp with vertical beam turning assemblies was also used for natural sunlight simulation. In this case, samples were placed in Petri dishes covered with quartz tops and solidified using a Peltier device (Ace-Tec, Korea). All irradiated ice samples were thawed for sampling and the subsequent analysis for Fe2+. Aqueous photochemical experiments of iron oxides dissolution were also carried out as a control at 25 °C using the same experimental setup. Analysis. Fe2+ concentrations were measured spectrophotometrically by the 1,10-phenanthroline method (28). One-milliliter aliquots of the sample solution were withdrawn from the quartz reaction tube after thawing the ice sample. The liquid aliquot was filtered with a 0.45-µm filter to remove iron oxides particles. Because of their aggregated state, no particles passed through the 0.45-µm filter. The filtrate was then added to an amber vial containing 2 mL of 0.2% 1,10phenanthroline, 1.5 mL of an buffer, which was diluted with water to the total volume of 10 mL. The vial was mixed vigorously and kept for 1 h in the dark before the analysis. The absorbance measurements at 510 nm (εm ) 2.5 × 104 L mol-1 cm-1) were done using a UV-visible spectrophotometer (UV-2401PC Shimadzu). The absorbance calibration for the determination of [Fe2+] was carried out using a freshly prepared ferrous solution (Fe(ClO4)2 · 6H2O). The total dissolved iron (Fe2+ and Fe3+) concentration was determined by atomic absorption spectroscopy (AAS, SpectrAA-800). Ferrioxalate actinometry was used for comparing the UV light intensity absorbed by aqueous and ice samples (29). The ferrioxalate solution was solidified prior to irradiation, irradiated under the same conditions as the iron oxide photolysis, and then thawed in the dark for the analysis of Fe2+. The optical images of ice were obtained with a Zeiss JENALAB-pol polarizing microscope equipped with a Linkam LTS 350 thermal stage (temperature range of -196 to 350 °C) and a Linkam LNP94 liquid nitrogen pump. One or two
droplets of sample solutions were dropped on the cover glass and put on the stage. The desired temperature (-20 °C) of the airtight stage containing sample solution was controlled by the liquid nitrogen pump. It took 2 min to cool the stage at -20 °C. Outdoor Experiments. The photodissolution experiments were also carried out under ambient solar radiation. Since the freezing temperature is not attained and the solar flux is highly varying in our geographical location and season, the outdoor experiments were conducted in the arctic region, Ny-Ålesund (78°55′N, 11°56′E, sea level) from May 14 to 28, 2009 where sunlight is available 24 h a day. Quartz tubes containing the various iron oxides with electron donors were frozen in a refrigerator before being exposed to sunlight. The solidified ice samples were placed horizontally on the surface of ambient snow for exposure to incident solar radiation. The irradiated samples remained solid during the entire exposure to sunlight. Control photolyses of aqueous samples containing iron oxides were carried out simultaneously under the same irradiation conditions. In order to prevent the freezing of the aqueous samples under ambient exposure, samples were placed on an electrically heated mat on the snow. The concentrations of photogenerated Fe2+ were immediately determined after solar irradiation in the Korea Polar Research Institute (KOPRI) DASAN Station (Ny-Ålesund). The ambient temperature ranged between -11 and -1 °C. The integrated solar irradiance as measured at the Koldewey Station in Ny-Ålesund varied from 1.8 to 34.1 W/m2 for UV band of 300 < λ < 370 nm, depending on the angular position of the sun and the weather condition, with a daily average of 11.0 W/m2 (corresponding to about 31 µEinstein m-2sec-1 assuming 335 nm photons).
Results and Discussion Formation of Fe(II)aq via Photoreductive Dissolution of Iron Oxides. We conducted a series of heterogeneous photochemical experiments with hematite (R-Fe2O3: nanosized colloidal sol synthesized in the laboratory) to quantify soluble Fe2+ generated in ice under conditions characteristic of ambient solar radiation. We found that the photodissolution of hematite under UV irradiation (mercury lamp) of λ > 300 nm, which occurs very slowly in aerated aqueous solutions of pH 3.5, is significantly accelerated in the polycrystalline ice phase both in the absence and presence of an added electron donor, as shown in Figure 1. The addition of various electron donors markedly enhanced the dissolution of hematite in ice, while their presence little influenced the photodissolution in water except for oxalic acid (Figure 1b). In all cases, the dissolution of hematite was enhanced in the ice phase compared to the aqueous counterpart reactions that were performed under the same irradiation conditions. In the absence of light (i.e., dark control), there was no detectable generation of Fe2+ in both ice and aqueous phases. The photodissolution of hematite in water and ice was carried out under Xe-arc lamp irradiation (solar simulating condition) as well and the same trend was observed. The concentrations of Fe2+ generated from hematite after 12 h irradiation (Xe lamp) with formic acid, citric acid, and oxalic acid (600 µM each) were 5, 23, and 8 µM, in aqueous solution; 24, 52, and 41 µM, in ice, respectively. To test whether there is the ice-induced dissolution of iron oxides to generate Fe3+ ions in the dark, which can be followed by their photoreduction to Fe2+, the following control experiment was done. First, the hematite solution was frozen then thawed in the dark, which was immediately followed by UV irradiation. The production of Fe2+ was as little as that of the aqueous photodissolution. Furthermore, we compared the concentration of Fe2+ (determined by the 1,10-phenanthroline method) with the total dissolved iron (Fe2+ plus Fe3+ determined by AAS) and found little difference VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Photoreductive dissolution of colloidal hematite under visible light (λ > 400 nm, 100-W halogen lamp). (a) [r-Fe2O3] ) 8 mg/L, [formic acid] ) 600 µM, pHi ) 3.2-3.5. (b) Comparison of Fe2+ production after 10 days of visible light irradiation in the presence of various organic acids as ED in aqueous solution at 25 °C (black bar) and ice at -20 °C (gray bar). [r-Fe2O3] ) 16 mg/L, [ED] ) 600 µM, pHi ) 3.2-3.5.
FIGURE 1. Production of Fe(II)aq via photoreductive dissolution of iron oxides under UV light (λ > 300 nm). (a) Hematite colloid in water at 25 °C and hematite trapped in ice at -20 °C. The formic acid was added as an electron donor (ED). [r-Fe2O3(colloid)] ) 8 mg/L. (b) Comparison of Fe2+ production after 48 h of UV irradiation in the presence of various organic acids as ED in aqueous solution at 25 °C (black bar) and ice at -20 °C (gray bar). Experimental conditions were as follows: [r-Fe2O3(colloid)] ) 16 mg/L, [ED] ) 600 µM, [humic or fulvic acid] ) 1 ppm, pHi ) 3.2-3.5. (c) Formation of Fe(II)aq via photoreductive dissolution of various types of iron oxides (commercially available powder) under 48 h UV irradiation in aqueous solution (black bar) and ice phase (gray bar), [Iron oxide] ) 0.2 g/L, [formic acid] ) 6000 µM, pHi ) 3.5. between them. This excludes the possibility of the ice-induced (photo)dissolution of Fe3+. Therefore, the mechanism of the ice-induced dissolution (in the dark) of iron oxides and the subsequent photoreduction of Fe3+ to Fe2+ can be dismissed. The photodissolution reactions were also investigated using commercially available iron oxide powders having different particle size and crystallinity [hematite, maghemite (γ-Fe2O3), goethite (R-FeOOH)]. The dark dissolution was negligible for all samples. The enhanced photoactivities in ice were also confirmed regardless of the type of iron oxides trapped in ice (Figure 1c). The photodissolution of iron oxides in water and ice was also carried out under visible light 4144
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irradiation (λ > 400 nm) because the majority of sunlight is in the visible region. Although the dissolution rates were much slower than under UV irradiation as previously reported (30), the trend remained the same. Figure 2 shows that the visible light-induced formation of Fe2+ from the dissolution of colloidal hematite was clearly enhanced in ice. The light intensities absorbed by the aqueous and ice samples were measured using the ferrioxalate actinometry. The intensities of the UV light absorbed by the aqueous and ice samples were 3.2 × 10-4 and 1.5 × 10-4 einstein min-1 L-1 (λ > 300 nm), respectively, although the incident light flux should be the same (see Supporting Information, Figure S2). The light intensity absorbed by the ice sample should be taken only as a rough estimate because the actinometry in ice phase has some uncertainty. The light intensity absorbed in the ice sample was about half of that in aqueous water because the turbid (white) ice sample scatters out the incident light. Therefore, the photodissolution rate of iron oxides trapped in ice is even higher than that in water despite much reduced light intensity absorbed by the ice sample. The enhanced photodissolution of iron oxides in ice was also confirmed from the TEM images. The hematite colloid particles were compared before and after the UV irradiation (Figure 3). The sizes of the primary particles in ice were significantly reduced after the UV exposure as a result of the photodissolution, while those in the aqueous phase were little changed under the same irradiation condition. We estimate that the dissolved amount of Fe2+ after 48 h of UV irradiation in ice corresponds to the photodissolution of 30 to 40% of the mass of the hematite particles. The enhanced photodissolution of iron oxides trapped in ice may be related with the singular chemical process (e.g., freeze concentration effect (20-24)) occurring in the ice grain
FIGURE 4. Effect of oxygen on the photoreductive dissolution of hematite under UV irradiation. [r-Fe2O3] ) 16 mg/L, [formic acid] ) 600 µM, pHi ) 3.5, 48 h irradiation.
FIGURE 3. TEM images of hematite colloid particles before and after UV light irradiation with [r-Fe2O3] ) 8 mg/L, formic acid (600 µM), pHi ) 3.5. (a) initial sample, (b) after 48 h irradiation in the aqueous phase (25 °C), (c) after 48 h irradiation in the ice phase (-20 °C). boundary region. In this respect, the change in the oxygen availability in ice should be considered. Takenaka et al. (21) suggested that oxygen is accumulated in the grain boundary region during the freezing process. The presence of oxygen generally inhibits the photoreductive dissolution of iron oxides through either reoxidizing the surface-bound Fe(II) or scavenging conduction band electrons on the photoexcited iron oxide particle (17, 31). This actually implies a retarded photodissolution of iron oxides in ice, contrary to the
observations of this study. To investigate the role of oxygen in the photochemical process, we carried out the photodissolution experiments in both aerated and deaerated conditions, and the result is shown in Figure 4. The dissolution of iron oxides was retarded in the presence of oxygen as expected but the enhanced production of Fe2+ was observed in both aerated and deaerated ice samples. This indicates that the enhanced photodissolution of iron oxides in ice cannot be related with the role of oxygen. The microenvironment in the liquid-like grain boundary region is characterized by a low pH, which also needs to be considered. Even though the present experiments were carried out by freezing a solution initially at pH 3.5, the pH in the liquid-like grain boundary region is predicted to be even lower due to the freeze concentration effects of the ice-excluded protons. It was previously estimated that the local concentration of acids in the ice grain boundary region is increased by 2-3 orders of magnitude in contrast to the aqueous solution (24, 32). The photoreductive dissolution of hematite should be enhanced at lower pH (14, 18, 19), as confirmed in this study: [Fe2+] after 48 h UV irradiation of hematite in aqueous solution was 5 µM at pH 3.50 and 31 µM at pH 1.15 (under the condition of Figure 1b with formic acid). Even at pH 1.15, the dark reductive dissolution of hematite was insignificant. Therefore, the lower pH in the ice grain boundary should be an important factor responsible for the enhanced photodissolution in ice. The ice-enhanced photodissolution effect rapidly decreased with increasing pH. The concentration of Fe2+ generated after 48 h UV irradiation (with formic acid under the condition of Figure 1b) was about 83 µM at pHi 3.5, 16 µM at pHi 4.0, and negligible above pH 5. The fact that the reoxidation of the photogenerated Fe(II) to Fe(III) exponentially increases with raising pH from 3 to 7 (33) should also limit the photogeneration of ferrous ions to acidic conditions. Outdoor Experiments under Solar Radiation. The photodissolution of iron oxides was studied in the arctic region (Ny-Ålesund, Svalbard, 78°55′N) in order to confirm the laboratory-observed phenomenon under natural environmental conditions. The outdoor experimental results are summarized in Table 1. The production of Fe2+ from the photodissolution of iron oxides in ice was found to be consistently higher than that in the corresponding aqueous phase, which confirms the laboratory results. Incidentally, the polar environment (where most surface is covered with ice and snow) in which the outdoor experiments were carried out provides an ideal natural condition where the studied process can actually occur. Photochemical Mechanisms. The observed photodissolution of iron oxides should be a result of the reductive process that occurs through the photoinduced electron transfer upon VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Production of Fe2+ under Arctic Sunlight (Outdoor Experiments in Ny-Ålesund)
various ED (after 72 h irradiation)a no ED formic acid acetic acid citric acid oxalic acid various iron oxides (after 48 h irradiation)b synthesized colloidal hematite (R-Fe2O3) hematite (R-Fe2O3) maghemite (γ-Fe2O3) goethite (R-FeOOH)
aqueous (µM)
ice (µM)
15 13 13 33 18
38 73 51 111 100
13
82
6 17 35
19 389 517
a [colloidal R-Fe2O3] ) 16 mg/L, [ED] ) 600 µM. oxide] ) 0.2 g/L, [formic acid] ) 6 mM.
b
[Iron
absorbing photons. The photoreductive transformation of Fe(III) to Fe(II) in the oxide lattice is initiated by either the ligand-to-metal charge transfer (LMCT) from the adsorbed organic reductant to Fe(III) species on the surface (eq 1) or the bandgap excitation of iron oxide semiconductor (Eg ) 2.2 eV or 565 nm) followed by the trapping of conduction band electron in the lattice Fe(III) sites (eqs 2,3). Photochemically formed Fe(II) at the surface of iron oxides (Fe(II)surf) then desorbs into the solution (eq 4). The adsorption of Fe(II) on iron oxides should be negligible under the acidic condition where Fe2+ ions are electrostatically repelled from the positively charged surface (pHzpc ) 7.8-8.3 for hematite).
≡Fe(III)-L + hv f ≡ Fe(II)-L·+(photoinduced LMCT)
(1)
+ + hvb (bandgap excitation) FeIII2O3 + hv f ecb
(2)
Fe(III)(at lattice or surface site) + ecb f Fe(II)surf
(3)
Fe(II)surf(at lattice or surface site) f Fe(II)aq
(4)
The present results imply that the above photoinduced electron transfer processes are accelerated in ice. Freeze concentration effects increase the concentration of iron oxide particles and organic ligands in the grain boundary region and may enhance the surface complexation (eq 1) and the subsequent dissolution of iron oxides. Oxalic acids that form strong surface complexes on iron oxides (31, 34) exhibited the highest photodissolution rates in not only the aqueous but also the ice phase (Figure 1b). However, whether the LMCT mechanism is working in the very acidic ice boundary layer (where the ligand complexation is not very favored) is uncertain. However, the enhanced photodissolution in ice was also observed in the absence of added electron donors, which implies that the enhanced LMCT mechanism cannot be the sole reason. The iron oxide particles confined/concentrated in liquid-like boundary domains in ice might be extensively aggregated. The presence of boundary domains at the edge of ice crystals and the extensive aggregation of hematite particles in the boundary edge are clearly observed in the optical images of Figure 5. Iron oxide particles should be excluded from the ice lattice into the ice crystal boundaries upon freezing, which concurs with the drastic change of the environmental properties around the particles such as pH, ionic strength, and the concentration of solutes and electron donors. Such conditions force the iron oxide particles to aggregate. Within the agglomerates of semi-
FIGURE 5. Optical images of polycrystalline ice showing the highly aggregated iron oxide particles trapped in ice veins (grain boundary region) at -20 °C. (a) pure ice, (b) ice with maghemite (γ-Fe2O3) (1 g/L), (c) ice with colloidal hematite (0.8 g/L), and (d) schematic illustration of concentrated hematite particles (orange circles) and formic acids (ball-and-stick models) in ice grain boundary region. 4146
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conductor nanoparticles, the charge-pair separation can be facilitated by the electron hopping through the interconnected grain boundaries and the following interfacial electron transfer reactions can be enhanced compared with those occurring on an isolated particle (35, 36). Environmental Implications. The present finding that the photodissolution of iron oxides can be significantly enhanced in the ice phase has many implications in the cold environmental condition where ice and snow containing mineral dusts are exposed to sunlight. Since the proposed phenomenon is particularly prominent in the acidic condition, acidic ice and snow should be of interest. One example is related to the fate of cloud droplets containing iron dusts. The pH of cloudwater is typically 3.5-5.5 or lower (12). Atmospheric deposition of mineral dusts supplies much of the nutrient iron to the ocean. The Aeolian mineral dust particles containing iron form cloud condensation nuclei and undergo freeze-thaw cycles in the upper troposphere, bringing a more soluble iron fraction upon deposition. For example, one study reported that the soluble fraction of iron in aerosols was ∼1% near sources, but often 10-40% farther away, producing a significant increase in soluble Fe deposition on remote ocean region (12, 13). The ice-enhanced photodissolution of iron oxides may contribute to the improved solubility of iron in acidic aerosols during the upper atmospheric transport. Another example is acidic snow/ice containing iron dusts. A snow/ice layer on sea ice under solar radiation may contribute to the iron supply into the polar ocean through the proposed ice-enhanced photochemical process. This may be partly responsible for ice-edge algal blooms that are observed during polar springtime (37, 38). Since the photogenerated ferrous ions would be rapidly reoxidized in circumneutral natural waters, the implications of the ice-induced photodissolution on aquatic biota would depend on how fast microorganisms can assimilate the dissolved ferrous ions. In general, the proposed phenomenon can be operating wherever there is acidic snow/ice and the iron supply through the photochemical path is important. However, the actual dissolution of iron oxides should sensitively depend on various experimental conditions such as pH, crystallinity and surface area of the minerals, and electrolyte/organics concentrations. Therefore, extrapolating any laboratory results to natural environmental conditions requires careful confirmation, which can be a subject of further studies.
Acknowledgments Funding for this work was provided by KOSEF NRL program (No. R0A-2008-000-20068-0), KOSEF EPB center (No. R112008-052-02002), KCAP (Sogang Univ.) funded by NRF (2009C1AAA001-2009-0093879), and Korea Polar Research Institute (KOPRI). K.K. thanks J. Klanova and P. Klan for their kind support of his visit and training at Masaryk University, Czech Republic. We gratefully acknowledge the contributions of D. Hrazdira and Y. Ahn.
Supporting Information Available X-ray diffractogram/UV-visible absorption spectrum of synthesized hematite, and the ferrioxalate actinometry for ice and aqueous samples. This information is available free of charge via the Internet at http://pubs.acs.org/.
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