Journal of Colloid and Interface Science 416 (2014) 44–53

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Photo-redox reactions of dicarboxylates and a-hydroxydicarboxylatesat the surface of Fe(III)(hydr)oxides followed with in situ ATR-FTIRspectroscopy

0021-9797/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcis.2013.10.030

⇑ Corresponding authors.E-mail addresses: paul.borer@eawag.ch (P. Borer), stephan.hug@eawag.ch (S.J.

Hug).

Paul Borer ⇑, Stephan J. Hug ⇑Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland

a r t i c l e i n f o

Article history:Received 30 August 2013Accepted 17 October 2013Available online 28 October 2013

Keywords:Photo-reactivityPhoto-oxidationCarboxylic acidsa-Hydroxycarboxylic acidsIron(III)(hydr)oxidesATR-FTIR

a b s t r a c t

Colloidal mineral-phases play an important role in the adsorption, transport and transformation oforganic and inorganic compounds in the atmosphere and in aqueous environments. Artificial UV-lightand sunlight can induce electron transfer reactions between metal ions of the solid phases and adsorbedcompounds, leading to their transformation and degradation. To investigate different possible photo-induced oxidation pathways of dicarboxylates adsorbed on iron(III)(hydr)oxide surfaces, we followedUV-A induced photoreactions of oxalate, malonate, succinate and their corresponding a-hydroxy ana-logues tartronate and malate with in situ ATR-FTIR spectroscopy in immersed particle layers of lepidocro-cite, goethite, maghemite and hematite at pH 4. UV-A light (365 ± 5 nm) lead to fast degradation ofoxalate, tartronate and malate, while malonate and succinate were photo-degraded at much slower rates.Efficient generation of OH-radicals can be excluded, as this would lead to fast and indiscriminate degra-dation of all tested compounds. Rapid photo-degradation of adsorbed oxalate and the a-hydroxydicarb-oxylates must be induced by direct ligand-to-metal charge transfer (LMCT) or by selectively oxidizingvalence band holes, both processes requiring inner-sphere coordination with direct ligand-to-metalbonds to enable efficient electron-transfer. The slow photo-degradation of malonate and succinate canbe explained by low-yield production of OH-radicals at the surface of the iron(III)(hydr)oxides.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Photo-induced reactions between adsorbed organic compoundsand mineral surfaces are important in many environmental andtechnical systems. In oceans, lakes and rivers, light-induced pro-cesses involving colloids can increase the bioavailability of metals(e.g., iron) and of carbon [1–4]. Organic compounds present in air-borne mineral oxide particles play important roles in photo-in-duced heterogeneous reactions in the atmosphere [5–7]. Inphotochemical water treatment with mineral oxide catalysts, dis-solved and adsorbed organics influence the photo-degradationand transformation of toxic inorganic and organic pollutants [8–12]. Among organic compounds, carboxylic acids are abundant inthe environment and are used in many engineered systems. Car-boxylate groups are also responsible for much of the adsorptionbehavior and photo-reactivity of larger organic compounds, suchas fulvic- and humic acids and of siderophores. [13–16]

Dicarboxylic and polycarboxylic acids have a high affinity toFe(III) and form photo-active complexes with monomeric and

polymeric Fe(III) in solution and with Fe(III)-sites on the surfacesof iron(hydr)oxides at acidic to circumneutral pH values. DissolvedFe(III)-complexes absorb light from the UV-C to the blue visible re-gion (200–450 nm) and many of them are photo-reactive [17],such that light-absorption leads to reduction of Fe(III) to Fe(II)and the formation of an oxidized organic radical in the initial step.While the structure, photo-reactivity and the photoproducts of dis-solved Fe(III)-complexes have been investigated and quantified forseveral carboxylic and polycarboxylic acids [16,18–24], informa-tion about the structure and the photo-reactivity of the corre-sponding surface complexes is still limited [14,21,25–29]. Fe(III)at pH values from 3 to 7 is typically present as polymeric Fe(III)and as amorphous or crystalline iron(hydr)oxides. A significantfraction of the polycarboxylates in Fe(III)-containing waters inthe mildly acidic to neutral pH-range is thus expected to be com-plexed at the surfaces of Fe(III)(hydr)oxides and their photo-con-version is of broad interest. Fe(III)-carboxylate complexes canalso enhance the photochemical degradation or transformation ofother compounds that do not adsorb strongly to surfaces, by effi-cient production of Fe(II) and H2O2, for example reduction of Cr(VI)to Cr(III) [30], oxidation of As(III) to As(V) [31], oxidation and deg-radation of atrazine [32], bisphenol [33], and pentachlorophenol[11].

P. Borer, S.J. Hug / Journal of Colloid and Interface Science 416 (2014) 44–53 45

In this study, we address the photo-reactivity of oxalate, malon-ate, succinate, tatronate and malate adsorbed on lepidocrocite,maghemite, goethite and hematite with in situ Attenuated TotalReflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR).ATR-FTIR in a suitable setup is surface sensitive and photo-oxida-tion reactions taking place at or near the surface can be followedin situ, which is not possible with batch experiments. Additionally,vibrational spectra provide information about the structure of ad-sorbed compounds, which can be linked to the observed photo-reactivity.

The goals were to assess the relative photo-reactivities of ad-sorbed dicarboxylates and a-hydroxydicarboxylates and to assessthe importance of different possible degradation pathways, suchas oxidation by light-induced ligand-to-metal charge transfer(LMCT), oxidation by valence band holes, and oxidation by OH-radicals.

2. Materials and methods

2.1. Chemicals

Analytical grade of oxalic-, malonic-, succinic-, tartronic-,malic- and glyoxylic acids or their salts were purchased from Sig-ma–Aldrich and Fluka and used as received. Samples of lepidocro-cite (c-FeOOH), goethite (a-FeOOH) and hematite (a-Fe2O3) usedin this study have been characterized previously [25,34–36]. Thespecific BET surface area of lepidocrocite, goethite and hematitewere 130, 38 and 24 m2/g, respectively. The pH at the point of zerocharge (PZC) was 7.4 for lepidocrocite and 9.3 for hematite. Theisoelectric point (IEP) of goethite was at pH 8.3. Maghemite (c-Fe2O3) with a specific surface area of 43 m2/g was purchased fromMKnano (MK Impex Corp.), Mississauga, Toronto, Canada.

2.2. FTIR Measurements

Spectra were recorded on a Biorad FTS 575C instrumentequipped with a mercury cadmium telluride (MCT) detector anda 9-reflection diamond ATR unit with KRS-5 optics (SensIR Tech-nologies, 15 Great Pasture Road, Danbury, CT 06810-9931). Thediameter of the round diamond disk was 4 mm. Scans from 4000to 400 cm�1 were collected at 2 cm�1 resolution versus the appro-priate background spectrum as explained below. Data analysis wasperformed with Matlab (The MathWorks, Inc.).

2.3. Preparation of mineral oxide layers on the ATR-FTIR crystal

The diamond ATR disk was coated with 30 lg of the respectivemineral oxide by spreading 2–3 lL of suspensions containing be-tween 10 and 15 mg oxide/mL over the disk. The suspension filmswere gently dried under an N2 stream, rinsed with H2O and driedagain. Comparison of spectra of the dried layers collected beforeand after the rinsing procedure showed that usually less than 5%of the applied oxide was removed by rinsing and that the layerswere stable in contact with aqueous solutions. Assuming an aver-age density of loosely packed oxide particles of �1.0–2.0 g/cm3 inthe deposited layer (covering approximately 70–90% of the dia-mond ATR disk surface), we estimate an oxide layer thickness ofbetween 1 and 4 lm.

Residuals of adsorbed carbonate (on hematite) and sulfate(maghemite) were removed by adding and removing 1 mL vol-umes of aqueous solutions of HCl and NaOH (1 mM) to the oxidelayers several times for a total duration of about 30 min.

2.4. ATR-FTIR photoirradiation experiments

Experiments were conducted as follows: A single-beam back-ground spectrum was collected with washed layers covered with1 mL Millipore water adjusted to pH 4 with HCl. Absorbance spec-tra were subsequently continuously collected against this back-ground spectrum. Adsorption of the dicarboxylates was followedafter exchanging the water with 1 mL of a 200 lM solution ofdicarboxylic acid (pH 4). After 30 min, a stable adsorption plateauwas in general reached and no significant pH shifts (<0.1 pH unit)were observed during adsorption. Then the 1 mL solution was re-moved and replaced with 20 lL of fresh 200 lM solution of pH4.0 and covered with a UV-A transparent glass plate to avoid evap-oration. After additional �10 min of equilibration in the dark, theimmersed oxide layer was illuminated with a UV-LED lamp at3.25 cm distance for 30 min. The UV-LED lamp (Dr. Groebel UV-Elektronik GmbH, Ettlingen, Germany) had an emission maximumat 365 nm with a bandwidth of 10 nm (365 ± 5 nm). For the givensetup, an irradiance of 960 W/m2 was measured by ferrioxalateactinometry [37], corresponding to a photon flux of �2 lmol pho-tons/min (365 nm) at the oxide layer. The irradiance was approxi-mately 9-fold stronger than the irradiance of natural sunlight inthe range of 300–450 nm, which can be assumed to be primarilyresponsible for the observed photoreactions. After illumination,1 mL of fresh 200 lM dicarboxylate was added and the re-adsorp-tion was followed during approximately 30 min. Spectra weremeasured continuously during adsorption, illumination and re-adsorption by recording 64 co-added scans, resulting in one spec-trum every 73 s.

2.5. Optimization of the photoirradiation setup

In experiments conducted as described, ATR-FTIR is surface sen-sitive, as solution species in the concentration range used in thisstudy (200 lM) do not contribute to measured spectra. Thus, themeasured spectra during irradiation are composed of adsorbeddicarboxylates (parent compounds) and potential degradationproducts. In order to limit re-adsorption of parent compounds fromthe solution reservoir and exchange with formed photoproducts atthe surface, the solution volume above the oxide layer was reducedto 20 lL prior to irradiation. By doing so, minor depletions of par-ent compounds can be followed and photoproducts accumulate atthe surface of the oxides to observable concentrations. This is par-ticularly important for less photo-reactive compounds, where re-adsorption of parent compounds and ligand-exchange with formeddegradation products at the surface can outcompete the kinetics ofphoto-oxidation. As shown by the example of malonate (see resultsand discussion), 20 lL was a sufficiently small solution volume andfurther volume reduction was not required.

3. Results and discussion

3.1. Photoirradiation of adsorbed oxalate

Fig. 1 shows successive spectra during UV-A illumination(365 nm) of oxalate adsorbed on lepidocrocite (A) and maghemite(B) at pH 4. Adsorbed oxalate is rapidly and almost completely oxi-dized at the surface of both oxides and carbonate formed duringphoto-degradation of oxalate accumulates at the surface of lepido-crocite, as indicated by the small absorbance increase at 1350 and1470 cm�1. On maghemite, carbonate formed during illuminationapparently does not accumulate at the surface of maghemite.Accumulation of photoproducts other than carbonate was not indi-cated by the spectra. The negative absorbance at 1020 cm�1 inFig. 1A is due to partial detachment of the lepidocrocite layer from

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Fig. 1. Irradiation of oxalate on lepidocrocite (A) and maghemite (B) at pH 4. Insets show the absorbance changes at 1409 cm�1 (A) and 1422 cm�1 (B) during adsorption of200 lM oxalate in the dark (empty circles), irradiation during 30 min (filled circles) and subsequent re-adsorption of oxalate in the dark after renewal of the oxalate solutionabove the oxide film (empty circles). Spectra were collected every 73 s.

46 P. Borer, S.J. Hug / Journal of Colloid and Interface Science 416 (2014) 44–53

the ATR-diamond crystal prior to illumination and minor photo-reductive dissolution during illumination.

Experiments performed with hematite and goethite (Support-ing Information, Figs. S1 and S2) show also rapid oxidation of oxa-late at the surface and the formation of adsorbed carbonate asindicated by the increase of spectral bands at 1485 and1355 cm�1 on hematite and at 1495 and 1335 cm�1 on goethite.Due to the high photo-reactivity of surface complexes formed byoxalate, re-adsorption of fresh oxalate from a larger solution vol-ume (10 mL) during irradiation does not outcompete the kineticsof photo-oxidation at the surface of lepidocrocite at pH 4, as clearlyshown in Fig. S3 in the Supporting Information.

3.2. Photoirradiation of adsorbed malonate

In contrast to oxalate, only minor fractions of adsorbed malon-ate are photo-oxidized during the same time of illumination(Fig. 2). From the decrease in absorbance near the peaks of thesymmetric stretch vibrations of the carboxylate groups (mCOO�)at 1400 and 1410 cm�1 (positions where the largest changes wereobserved) on lepidocrocite and maghemite, we estimate that 18%and 7%, respectively, of adsorbed malonate on lepidocrocite and

maghemite were photo-oxidized during 30 min of illumination.The importance of keeping the solution volume above the oxidelayer as minimal as possible is illustrated by leaving the solutionvolume at 1 mL during irradiation. No detectable spectral changeswere observed in Fig. S4 (Supporting Information). On the otherhand, a further decrease of the solution volume above the oxidelayer from 20 lL to 2 lL – thus further reducing the extent of pos-sible re-adsorption of parent compounds during irradiation – didnot increase the extent of overall photo-oxidation of adsorbed mal-onate under otherwise identical conditions (see Fig. S5 in the Sup-porting Information). Therefore, 20 lL solution volume issufficiently small to determine initial oxidation rates of less-reac-tive dicarboxylates such as malonate. In contrast to the resultswith lepidocrocite and maghemite, no clear spectral changes indic-ative of photo-oxidation of adsorbed malonate were observed ongoethite and hematite (Figs. S6 and S7).

3.3. Photoirradiation of adsorbed succinate

As was observed for malonate, only small spectral changes ofadsorbed succinate were observed during illumination on lepido-crocite and maghemite (Fig. 3). Based on the absorbance changes

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Fig. 2. Irradiation of malonate adsorbed to lepidocrocite (A) and maghemite (B) at pH 4. Insets show the absorbance changes at 1400 cm�1 (A) and 1410 cm�1 (B) duringadsorption of 200 lM malonate in the dark (empty circles), irradiation during 30 min (filled circles) and subsequent re-adsorption of malonate in the dark after renewal of themalonate solution above the oxide film (empty circles). Spectra were collected every 73 s. In Figs. A and B, difference spectra (dashed lines) illustrate the spectral changesduring the irradiation period.

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indicated in the insets of Fig. 3A and B, approximately 14% and 6%of adsorbed succinate on lepidocrocite and maghemite werephoto-oxidized during 30 min of illumination. In the case ofmaghemite, the formation of a photoproduct residing at the sur-face is clearly indicated by the increase in spectral intensity at1431 cm�1. On hematite, photo-oxidation of adsorbed succinatewas not observed (Fig. S8 in the Supporting Information).

3.4. Photoirradiation of adsorbed tartronate

In order to assess the photo-reactivity of a-hydroxy analogues,in situ photoirradiation experiments were conducted with tartro-nate and malate. In the case of tartronate, rapid photo-oxidationon lepidocrocite and maghemite was observed. Fig. 4 shows thedecrease in absorbance of adsorbed tartronate at the surface of lep-idocrocite and maghemite with no clear indications of spectrallydistinguishable photoproducts other than carbonate (in the caseof lepidocrocite) accumulating at the surface. According to the de-crease in spectral intensity in the region of the symmetric carbox-ylate stretch vibrations, approximately 70% and 80% of adsorbedtartronate were photo-degraded during 30 min of illumination onlepidocrocite and maghemite, respectively. The uniform decrease

in the spectral intensity in the investigated spectral range (maghe-mite) suggests that photoproducts are further oxidized at a higherrate than tartronate.

3.5. Photoirradiation of adsorbed malate

In contrast to tartronate, where a more or less uniform decreasein spectral intensity was observed during illumination, the spectralchanges arising during illumination of adsorbed malate on lepido-crocite and maghemite point towards a partial photo-oxidation ofmalate and formation of a photoproduct other than carbonate(Fig. 5). As shown by the difference spectra (dashed lines inFig. 5) obtained during the illumination period, the spectral inten-sity of bands assigned to symmetric stretch vibrations of the car-boxylate groups (mCOO�) of adsorbed malate on lepidocrocite andmaghemite at 1396 and 1399 cm�1 decreased while the spectralintensity of vibrational bands at 1427 and 1430 cm�1 remained un-changed or even increased. Similarly, the decrease in intensity ofasymmetric stretch vibrations of the malate carboxylate groups(mCOO�) above 1500 cm�1 was accompanied by the appearance ofnew peaks at 1580 and 1576 cm�1 on lepidocrocite and maghe-mite, respectively. Although carbonate formation was observed

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Fig. 3. Irradiation of succinate adsorbed to lepidocrocite (A) and maghemite (B) at pH 4. Insets show the absorbance changes at 1400 cm�1 (A) and 1397 cm�1 (B) duringadsorption of 200 lM succinate in the dark (empty circles), irradiation during 30 min (filled circles) and subsequent re-adsorption of succinate in the dark after renewal of thesuccinate solution above the oxide film (empty circles). Spectra were collected every 73 s. In both Figs. A and B, difference spectra (dashed lines) illustrate the spectralchanges during the irradiation period.

48 P. Borer, S.J. Hug / Journal of Colloid and Interface Science 416 (2014) 44–53

at the surface of lepidocrocite by the evolving peak at 1350 cm�1, itdid not contribute significantly to the spectral features of the dif-ference spectrum obtained during illumination. Carbonate doesnot accumulate at the surface of maghemite, as clearly illustratedduring photo-oxidation of oxalate, malonate and succinate (Figs. 1–3B). Thus, the comparison of spectral features of adsorbed malateand the difference spectra obtained during illumination suggestthat the spectral contributions at 1576–1580 and 1427–1430 cm�1 originate from one or more photoproducts. Vibrationsat the above mentioned positions are typical for carboxylates. Infact, the peak positions fit perfectly to the spectra of adsorbed mal-onate (Fig. 2), which shows strong vibrations at 1576 cm�1 (mCOO�asym) and 1422–1429 cm�1, the latter consisting of contributionsof the symmetric stretch vibrations of the carboxylate groups(mCOO� symm) and CAH bending modes [38,39].

To track the degradation of malate and the formation of malon-ate at the surface, the spectra in Fig. 5 were fitted by referencespectra of adsorbed malate and malonate (from Figs. 2 and 5) inthe spectral range of 1250–1470 cm�1 by a simplex routine in Mat-lab. Fig. 6 shows the calculated fractions of adsorbed malate andmalonate at the surface of lepidocrocite during adsorption,

photo-degradation and subsequent re-adsorption of malate afterrenewal of the solution above the oxide film. Fig. 6 indicates thatwhile approximately 78% of adsorbed malate is degraded duringthe illumination period, the photoproduct malonate accumulatesat the surface of lepidocrocite. Subsequent to the renewal of themalate solution, adsorbed malonate is exchanged by fresh malatefrom the replenished larger solution reservoir. That the spectra inFig. 5A can be entirely explained by the presence of malate andmalonate is indicated by the small residuals of the fitted spectrato the measured spectra (Fig. S9), based on the calculated fractionsof malate and malonate. Other photoproducts than malonate donot contribute significantly to the measured spectra.

Similar results were found for malate photo-degradation onmaghemite. Approximately 83% of adsorbed malate was degradedduring the illumination period, leading also to the accumulation ofmalonate at the surface (Fig. S10 in the Supporting Information).

In the case of hematite (Fig. S11), malate photo-oxidation wasaccompanied by a significant accumulation of carbonate at the sur-face, as observed by the intense bands at 1350 and 1485 cm�1. Inaccordance with the observations with lepidocrocite and hematite,the peaks evolving at 1580, 1437 cm�1 (difference spectrum in

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Fig. 4. Irradiation of tartronate adsorbed to lepidocrocite (A) and maghemite (B) at pH 4. Insets show the absorbance changes at 1315 cm�1 (A) and 1382 cm�1 (B) duringadsorption of 200 lM tartronate in the dark (empty circles), irradiation during 30 min (filled circles) and subsequent re-adsorption of tartronate in the dark after renewal ofthe tartronate solution above the oxide film (empty circles). Spectra were collected every 73 s.

P. Borer, S.J. Hug / Journal of Colloid and Interface Science 416 (2014) 44–53 49

Fig. S11) can be clearly assigned to malonate accumulating at thesurface.

3.6. Comparison of photo-degradation kinetics

Fig. 7 illustrates the kinetics of photo-degradation of the inves-tigated compounds at the surface of lepidocrocite, based on thespectral decrease at the location of the corresponding symmetricstretch vibrations of the carboxylic acids (mCOO�). For malate,the calculated fractions in Fig. 6 were considered. Photo-oxidationof malonate and succinate occurred at a rate of �0.5% of the ad-sorbed compounds per minute. The corresponding a-hydroxydi-carboxylates were oxidized at much higher rates reachingapproximately 5%/min of adsorbed malate and 10%/min of ad-sorbed tartronate during the first 5 min of illumination. For ad-sorbed oxalate, an initial oxidation rate of 9%/min was observed.Similar initial rates for the investigated compounds were observedon maghemite and goethite. The initial photo-oxidation rate is pri-marily dependent on the photo-reactivity of the surface com-plexes, while for later stages additional processes, such asoxidation of photoproducts, photo-dissolution of the iron(III)(-hydr)oxides, reactions with reactive oxygen species formed duringirradiation at the surface and in solution, may determine the

apparent oxidation rates. In the following, we will focus on the ini-tial photo-oxidation rates, which reflect the photo-reactivity ofsurface complexes and on the potential photo-oxidation pathways.

3.7. Photo-degradation mechanisms on Fe(III)(hydr)oxides

Dicarboxylic acids adsorbed to Fe(III)(hydr)oxide surfaces canbe photo-transformed and degraded according to the followingmechanisms (M1–M4).

M1: Absorption of light by Fe(III)-dicarboxylate surface com-plexes into ligand-to-metal charge transfer transition (LMCT)bands, leading to direct oxidation of the adsorbed dicarboxylatewithin the surface complex. M2: Absorption of light in the bulkof the iron(hydr)oxides leading to O2�? Fe3+ LMCT transitions[40,41], with the formation of conduction band electrons (e�)and valence band holes (h+). These charge carriers can recombinein the bulk [42,43], but may also travel to the surface where theyare trapped in energetically shallower states. Surface trapped h+

can oxidize adsorbed dicarboxylates if they are sufficiently oxidiz-ing, while surface trapped electrons can be described as Fe(II)-spe-cies [41] and can be re-oxidized to Fe(III) by dissolved oxygen. M3:Sufficiently oxidizing surface trapped h+ can oxidize surface hydro-xyl groups or adsorbed water to surface bound or free OH-radicals,

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Fig. 5. Irradiation of malate adsorbed to lepidocrocite (A) and maghemite (B) at pH 4. Insets show the absorbance changes at 1396 cm�1 (A) and 1399 cm�1 (B) duringadsorption of 200 lM malate in the dark (empty circles), irradiation during 30 min (filled circles) and subsequent re-adsorption of malate in the dark after renewal of themalate solution above the oxide film (empty circles). Spectra were collected every 73 s. In both Figs. A and B, a difference spectrum (dashed line) illustrates the spectralchanges during the irradiation period.

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Fig. 6. Calculated fractions of adsorbed malate and malonate on lepidocrocite fromthe irradiation experiment with malate on lepidocrocite (Fig. 5A). Fractions of thesecompounds are shown during the adsorption phase in the dark (empty symbols),irradiation during 30 min (filled symbols) and subsequent renewal of the malatesolution above the oxide film (empty circles).

Fig. 7. Photo-degradation of succinate, malonate, malate, tartronate and oxalate onlepidocrocite as a function of time, based on the decrease in spectral intensity at thepeak locations of the corresponding symmetric stretch vibrations of the carboxylategroups (mCOO�). For malate, the fractions of adsorbed malate from Fig. 6 wereconsidered.

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which indiscriminately oxidize inner- or outer-spherically ad-sorbed dicarboxylates. Mechanisms M2 and M3 are related to thesemiconducting properties of the iron(hydr)oxides. M4: Photolysisof surface Fe(III)-hydroxyl groups in LMCT transitions can lead tothe formation of OH-radicals and reduced metal sites [34,40,44].

What is common in M1 and M2 is that these reactions seemonly possible when the ligand is adsorbed as an inner-sphere com-plex, with a direct coordination of ligand atoms to a surface metalatom and sufficient electronic overlap to enable fast electron trans-fer between surface and ligand. Electron back-transfer can lead tolow reactivity, but when follow-up reactions are fast, degradationand transformation can proceed quickly. For mechanisms M1 andM2, we would expect no immediate direct photo-reactivity for out-er-spherically (electrostatically) bound ligands or for ligandsbound to surface hydroxyl groups through hydrogen bonds[45,50]. In contrast, OH-radicals produced in mechanisms M3and M4 can react indiscriminately with compounds adsorbed onor close to the surface and inner-sphere coordination with directmetal-to-ligands bonds is not required. Also, back-transfer ofcharge between surface and ligand is not likely after H-abstractionor oxidation of the ligand with OH-radicals, and the products candiffer from those that are formed after oxidation by M1 or M2. Ithas been found by many groups (as reviewed by Henderson [46])that such mobile OH-radicals are formed efficiently at aqueousTiO2 surfaces and are able to degrade most organic ligands ad-sorbed on or close to the surface. Fast photo-degradation of malon-ate adsorbed to TiO2 and to TiO2/Au was observed by Hu and Bürgiand was ascribed to strongly oxidizing h+ [47]. In the case ofFe(III)(hydr)oxides, the slow degradation of malonate and succi-nate shows that the production of strongly oxidizing h+ and/orOH-radicals is not efficient. Recent work on photo-reductive disso-lution of lepidocrocite and ferrihydrite in the absence of ligandsother than H2O and OH� suggests that OH-radicals can be formedin low yields at acidic pH (pH 2–6) at the surface of these phases[34,48]. These studies investigated the formation and fate of reac-tive oxygen species ðOH;O�2 ;H2O2Þ and of Fe(II) formed duringphoto-reductive dissolution of Fe(III)(hydr)oxide phases with scav-engers and kinetic modeling. It was suggested for lepidocrocite,that OH-radicals are formed upon irradiation of wavelengths be-low 515 nm with low yields, and that they reside at or near theoxide surface and do not diffuse away from the oxide surface. Fromone of these studies [48], the quantum yield for the formation ofFe(II) surface sites on lepidocrocite was estimated at 0.002 in thewavelength range of 300–475 nm (pH 3). This value likely reflectsan upper limit of the quantum yield for the formation of surfacetrapped h+ and surface bound or mobile OH-radicals under theseconditions. Due to re-oxidation of surface Fe(II) sites with reactiveoxygen species, only part of the reduced surface sites (>Fe(II))eventually dissolves into solution at acidic pH [34]. By quenchingmobile OH-radicals formed at the surface with the radical scaven-ger POHPAA (para-hydroxyphenylacetic acid), the formation ofFe(II) at pH 5 increased [34]. If we use the determined formationrate of dissolved Fe(II) in the presence of the radical scavenger toassess an apparent quantum yield for the formation of mobileOH-radicals, then the lower estimate at pH 5 under light of 300–475 nm is �1.5 � 10�5. Extrapolating this lower-bound estimateto the conditions of the 20 lL ATR photoirradiation setup, approx-imately 1 nmol of OH-radicals could form during the 30 min irradi-ation period with light of 365 nm. If we assume surfaceconcentrations of adsorbed ligands of 1 lmol/m2 on lepidocrocite(values typical for dicarboxylic and a-hydroxydicarboxylic acidsadsorbed on Fe(III)(hydr)oxides at comparable pH values) [25,49],approximately 4 nmoles of dicarboxylic and a-hydroxydicarboxy-lic acids are adsorbed at the surface of lepidocrocite at pH 4. Inthe solution reservoir of 20 lL, additional 4 nmoles of carboxylicacids are present, which can adsorb to the surface during the

photoirradiation period. The estimated formation of mobile OH-radicals on lepidocrocite is thus sufficient to induce a measurabledegradation of up to 25% of adsorbed carboxylates on lepidocrocitewith this setup.

3.8. Surface structures and photo-degradation mechanisms

The photo-degradation of the investigated compounds bymechanisms M1–M4 is related to the type and reactivity of surfacecomplexes (inner-sphere, outer-sphere and hydrogen-bondedcomplexes). Mobile OH-radicals (M3 and M4) oxidize inner-spher-ically as well as outer-spherically adsorbed or hydrogen-bonded li-gands. In the case of lepidocrocite, the observed decrease of 18%and 14% of adsorbed malonate and succinate, respectively, within30 min. is in the range that can be explained by M3 and M4.

Compound-specific and potentially much higher photo-degrada-tion rates according to mechanisms M1 and M2 are expected for li-gands that form inner-sphere complexes on surfaces, and this isobserved for oxalate and a-hydroxy-dicarboxylates. In a previousATR-FTIR study on the adsorption of oxalate, malonate and succinateon TiO2 phases and lepidocrocite [39], it was proposed that oxalateforms surface complexes with structures that are different frommalonate and succinate. The most distinct features for adsorbedoxalate are the two coupled C@O bond vibrations at around 1680and 1710 cm�1 which are not present in the other adsorbed dicar-boxylates [39]. It was suggested by several research groups[26,50,51] that oxalate forms both outer-sphere complexes (electro-static and hydrogen-bonded) and more strongly adsorbed inner-sphere surface complexes at the surface of Fe-oxides. In the inner-sphere complexes the deprotonated hydroxyl groups of the two car-boxyl-groups together form a 5-membered bidentate mononuclearchelate structure with two non-binding C@O double bonds. The ab-sence of clear C@O double bond vibrations for malonate and succi-nate suggests that the two more distant carboxyl groups in thesecompounds each form independently 5-membered bidentate-bridging structures or mixed monodentate hydrogen-bonded struc-tures involving several surface Fe(III) sites. Thus, in the case of oxa-late, we expect that the inner-sphere complexes show similarphoto-reactivity via mechanism M1 as the structurally similar solu-tion Fe(III)-complexes and our experiments confirmed that ad-sorbed oxalate is photo-degraded at much higher rates than whatwould be expected from degradation with OH-radicals (M3 andM4). Dissolved Fe(III)-complexes of malonate are much less photo-reactive [18,19,24,27], with a quantum yield for Fe(II) formationfrom mononuclear Fe(III)(malonate)2 of only 0.027 at 366 nm, 40times lower than for Fe(III)(oxalate)2 (�1.17) [19,52]. These differ-ences may even be larger for surface complexes, if malonate formsless reactive surface complexes with carboxylate groups coordi-nated at separate surface iron sites, in contrast to dissolved com-plexes where coordination in mononuclear Fe(III)-complexes islimited to one Fe(III). The non-reactivity of malonate and succinateon hematite and goethite (Figs. S6–S8 in the Supporting Informa-tion) might be due to the higher stability of these oxides and to evenlower yields of OH-radicals by photolysis of surface FeAOH groups(M4) or photo-dissolved Fe(III). The very similar degradation ratesof oxalate and the a-hydroxydicarboxylates on the different oxidesseem to support direct photolysis of surface-complexes according toM1. While the similar rates do not exclude M2, one would expect lar-ger differences between the various oxides if a semiconductor mech-anism with oxidation by h+ was important. The very low degradationrates of malonate and succinate clearly discount efficient productionof OH-radicals. Inefficient production of OH-radicals on iron(hydr)-oxides was also reported in a previous study, where fast photo-deg-radation of 4,40-Bis(2-sulfostyryl)biphenyl (optical whiteningagent) was observed on the surfaces of rutile and anatase, but noton hematite and lepidocrocite [53].

52 P. Borer, S.J. Hug / Journal of Colloid and Interface Science 416 (2014) 44–53

In the a-hydroxydicarboxylates, the hydroxyl group is known tofacilitate the photo-induced decarboxylation, by stabilization ofthe decarboxylated cation-radical and rapid formation of the corre-sponding oxo-acid [18]. These follow-up reactions are not possiblein unsubstituted dicarboxylic acids. The high photo-reactivity ofthe a-hydroxydicarboxylates compared to the correspondingdicarboxylates was observed at the surface of all investigatedFe(III)(hydr)oxides. The extent of photo-oxidation of adsorbedtartronate and malate during 30 min of irradiation at 365 nmwas >70% on lepidocrocite and close to the photo-reactivity of oxa-late. This exceeds by far the rates that can be explained by the lowyield of OH-radicals. Surface ligand-to-metal charge transfer (M1)and oxidation by h+ (M2) must therefore be the main pathwaysfor the photo-oxidation of the a-hydroxydicarboxylates at the sur-face of the investigated Fe(III)(hydr)oxides. Thus, the significantphoto-reactivity of a-hydroxydicarboxylates on iron(III)(hydr)ox-ides supports inner-sphere coordination.

3.9. Photoproducts

In a previous ATR-FTIR study, the products formed duringphoto-degradation of citrate (a a-hydroxytricarboxylate) at thesurface of lepidocrocite were investigated [25]. The first photo-product of citrate was identified as a keto carboxylic acid (3-oxo-glutaric acid), which itself is further oxidized, but at lower ratesthan citrate and is not exchanged at the surface at too high ratesby adsorbing citrate. In analogy to citrate, we hypothesize thatthe first photo-degradation products of malate and tartronate are3-oxopropanoate (a b-keto carboxylate) and glyoxylate (an a-ketocarboxylate), respectively. While the first product is commerciallynot available for product identification or analysis of its photo-reactivity, we can test the photo-reactivity of the speculated pho-toproduct of tartronate. Fig. S12, shows that glyoxylate on lepido-crocite and maghemite is photo-oxidized at slightly faster ratesthan tartronate (�11%/min in the first 5 min of illumination) with-out spectral indications of stable photoproducts other than carbon-ate (on lepidocrocite). Thus, if glyoxylate is formed during theoxidation of tartronate, it is degraded more quickly than it isformed, which is in line with the observation that no stable photo-products other than carbonate were detected by ATR-FTIR spec-troscopy (see Fig. 4).

In the case of malate, our results suggest that malonate wasformed during illumination and accumulates on the surface dueto its low photo-reactivity. The formation of malonate can be ex-plained by the oxidation of the speculated first photoproduct 3-oxopropanoic acid with hydroxyl radicals [54]. Efficient productionof hydroxyl radicals is expected during the initial rapid photo-transformation of malate: photo-induced decarboxylation leadsto production of O�2 in aerobic solutions, formation of H2O2 andFe(II), and subsequent production of OH-radicals in Fenton reac-tions. The formation of hydroxyl radicals is in accordance with pre-vious observations from photolysis experiments with aqueousFe(III)-citrate complexes [55]. With the depletion of malate, theefficient formation of reactive oxygen species and Fe(II) ceasesand the formed malonate is further degraded only slowly. The for-mation of 3-oxopropanoic acid has been recently also suggested asa transient photoproduct during malate oxidation in irradiatedTiO2 suspensions, where malonate was also observed as an inter-mediate [56].

4. Conclusions and significance

The objectives of this study were to (1) determine the photo-reactivity of adsorbed dicarboxylates on different important iron(-hydr)oxides, (2) distinguish between different hypothesized degra-

dation pathways and (3) distinguish between inner-sphere andouter-sphere coordination of dicarboxylates on the surface of ir-on(hydr)oxides. We found that adsorbed malonate and succinatewere not or only slowly photo-oxidized at the surface of all inves-tigated iron(III)(hydr)oxides, at rates far below those of the corre-sponding a-hydroxydicarboxylates (tartronate and malate) andoxalate. The slow photo-oxidation of malonate and succinatemay proceed through low-yield production of OH-radicals by di-rect photolysis of surface hydroxyl groups or traces of photo-dis-solved Fe(III)-species in the surrounding solution. The fastdegradation and photo-transformation of adsorbed oxalate, tartro-nate and malate on all investigated iron-phases must occur bymore efficient pathways, most likely by absorption of light intoLMCT-bands of inner-sphere coordinated surface complexes, fol-lowed by rapid decarboxylation and subsequent reactions withoxygen. Photo-oxidation by selectively oxidizing h+ in a semicon-ductor mechanism can also contribute, but we would expect differ-ences in the yields and reactivities for the different oxides. Fastphoto-transformation other than by OH-radicals provides strongsupport for inner-sphere surface complexation, as LMCT chargetransfer bands are only present in surface complexes with direct li-gand-to-iron bonds and electron transfer to surface trapped h+ isalso expected to be more efficient for inner-sphere than for out-er-sphere coordination. Our results thus provide strong supportfor inner-sphere complexation of oxalate, tartronate and malate,in addition and independent of a still open discussion over differ-ent interpretations of spectral shifts that are observed betweendissolved and adsorbed carboxylates [39,45,50]. The low reactivityof malonate and succinate neither supports nor discounts inner-sphere coordination: The reason for the low reactivity could be fastback-electron transfer and recombination due to lack of suitablefollow-up reactions in the absence of a-hydroxy or a-carboxylgroups.

The strength of the applied ATR-FTIR method is its ability toselectively observe surface processes directly and in situ. Batchexperiment with sample collection and filtration can measure onlystable subsequent photoproducts in the aqueous phase afterdesorption. By focusing on the initial phase of illumination, weare able to relate the photo-reactivity of adsorbed compounds topossible photo-oxidation mechanisms. We plan similar experi-ments with compounds with different functional groups and withcompounds that form different photoproducts by reaction withOH-radicals and valence band holes. Observation of processesand products on the surface and in solution are complementaryand can lead to a better understanding of fundamental processes.The different photo-reactivities of adsorbed compounds on varioussurfaces can help to explain which products are sufficiently stableto be observed in the solution phase of UV-exposed suspensions orin particle layers in environmental and technical systems.

Acknowledgments

We thank Thomas Rüttimann (Eawag) for experimental andtechnical support. Furthermore, Prof. Ruben Kretzschmar and Dr.Yves Brechbuehl (ETH Zürich) are kindly thanked for providing asample of hematite.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.030.

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