In situ DRIFTS-MS study of EDTA photocatalytic degradation

Journal Pre-proof In situ DRIFTS-MS study of EDTA photocatalytic degradation Reem Al Sakkaf, Giovanni Palmisano, Thomas Delclos, Mar´ıa Jose´ ´ ˜ Lopez-Mu noz, Gabriele Scandura

PII:

S0920-5861(19)30555-3

DOI:

https://doi.org/10.1016/j.cattod.2019.10.002

Reference:

CATTOD 12507

To appear in:

Catalysis Today

Received Date:

29 July 2019

Revised Date:

2 October 2019

Accepted Date:

7 October 2019

´ ˜ MJ, Scandura Please cite this article as: Al Sakkaf R, Palmisano G, Delclos T, Lopez-Mu noz G, In situ DRIFTS-MS study of EDTA photocatalytic degradation, Catalysis Today (2019), doi: https://doi.org/10.1016/j.cattod.2019.10.002

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

In situ DRIFTS-MS study of EDTA photocatalytic degradation Reem Al Sakkaf1,2, Giovanni Palmisano1,2, Thomas Delclos1, María José López-Muñoz3, Gabriele Scandura1,2* 1

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Department of Chemical Engineering, Khalifa University of Science and Technology - Masdar Institute, PO BOX 54224, Abu Dhabi, United Arab Emirates. 2 Center for Membrane and Advanced Water Technology, Khalifa University of Science and Technology - Masdar City, PO BOX 54224, Abu Dhabi, United Arab Emirates. 3 Department of Chemical and Environmental Technology, ESCET, Rey Juan Carlos University, C/ Tulipán s/n, 28933 Mostoles, Madrid, Spain. * [email protected]

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Graphical abstract

Highlights

EDTA degradation over TiO2 was studied through an in-situ DRIFTS-MS analysis Gas, relative humidity, irradiation source and EDTA loading used were investigated In anaerobic conditions EDTA degradation is minimal and reduced species develop In aerobic conditions oxygenated products besides CO2 and H2O were produced EDTA degrades under visible light (>410 nm) via ligand-to-metal charge transfer 1

Abstract Ethylenediaminetetraacetic acid (EDTA) is often detected as contaminant in domestic wastewater

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(DWW) and liquid nuclear waste (LNW). Unfortunately, the existence of EDTA in DWW and LNW was proven to have an adverse impact on the conventional wastewater treatment efficiency.

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In this work, photocatalytic degradation of EDTA over commercial TiO2 (Evonik P25) has been investigated by using an irradiated environmental chamber with real-time diffuse reflectance

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infrared Fourier transform spectroscopy (DRIFTS) and mass spectroscopy (MS) analysis. The

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degradation mechanism has been studied by varying different parameters, namely carrier gas, relative humidity, irradiation source and the loading of EDTA adsorbed on TiO2. Results showed

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a minimal degradation of EDTA under anaerobic conditions with production of ammonia and short-chained organic molecules (e.g. acetaldehyde). On the contrary, EDTA photooxidation took

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place under aerobic conditions with carbon dioxide and water as the main products. It is important to note that EDTA forms a visible absorbing complex with P25, which allowed photodegradation of EDTA under pure visible light (wavelength > 410 nm). Remarkably, the reaction mechanism was different when the system was irradiated with simulated solar light and near UV light.

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Keywords: EDTA, in situ analysis, photocatalysis, LMCT mechanism

1. Introduction

Ethylenediaminetetraacetic acid (EDTA, C10H16N2O8) is an organic acid and chelating ligand used as decontamination agent in agricultural, industrial and pharmaceutical applications as well as in

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nuclear power plants.1, 2 Therefore, the amount of EDTA in both domestic and industrial waste water (WW) has been increasing over the last decades.3 This compound can complex metal cations, which can also be found in WW, hampering their removal by ordinary treatment processes.4 Hence, including an effective non-conventional treatment step is highly desirable as concerns environmental protection. Indeed, degradation of EDTA is difficult: biological methods5 and chlorine based treatments6 are inefficient, whereas this chemical is barely absorbed by activated

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carbon filters.6 In the past the biodegradability of EDTA has been studied extensively by

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researchers using different tests and methods7, 8 and all of them showed that EDTA is highly recalcitrant.9

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Chemical transformation pathways involving hydrolysis, reaction with solvated electrons, organic

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peroxyradicals, or singlet oxygen are all irrelevant to degrade EDTA and its metal chelates in river waters with short residence time.10 Similarly, Frank and Rau concluded that photodegradation is

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the only abiotic process that can contribute significantly to the degradation of EDTA.11 On the other hand, advanced oxidation processes (AOP) such as ozonation,12 UV coupled to oxidants (e.g.

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H2O2)13 and radiolysis,14 appear to be more promising. Particularly, heterogeneous photocatalysis,15-17 which does not require the presence of additional oxidants in water but O2, is an encouraging alternative in the treatment of select recalcitrant species,18 enabling a complete mineralization of the compounds by using a semiconductor together with light irradiation at the

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appropriate wavelengths (typically ranging from UVA to visible).19, 20 Several studies have proved that radicals generated on solid photocatalysts are effective to degrade adsorbed EDTA upon light irradiation. Commercial titanium oxide (Degussa Evonik P25, TiO2) is the most reported photocatalyst in literature because of its high photoactivity, low price and easy availability.21-24

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Babay et al. irradiated P25 aqueous suspensions containing EDTA under near UV light. They reported a 90% EDTA conversion with a corresponding decrease in TOC (total organic carbon) of 9% when bubbling O2, which was crucial for the reaction to take place efficiently. Moreover, TOC decrease was 32% when Fe(III) was added in the reaction system.25 In their proposed mechanism, EDTA was oxidized directly by hydroxyl radicals, while oxygen was reduced to superoxide by

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conduction band electrons, forming additional HO•, through sequential steps. Seshadri et al. investigated the photocatalytic degradation of EDTA in radioactive liquid waste by

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applying UV irradiation along with TiO2 in the presence of H2O2 and O2.2 According to them, a photocatalytic step could be applied to remove EDTA from liquid waste before the conventional

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chemical treatment since the degradation products of EDTA do not interfere with the latter.

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Generally speaking, (photo)catalysts respond dynamically to the actual environment where they have been applied. Thus, in situ methods can deliver instantly significant information on the

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‘working state’ of a heterogeneous catalyst and allow for structure-function relations to be inferred.26 Combining mass spectroscopy (MS) technique with diffuse reflectance infrared Fourier

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transformed spectroscopy (DRIFTS) allows to probe the catalyst surface while reactions are taking place and to detect light and volatile reaction products through MS.27, 28 The objective of this research is to investigate the degradation mechanism of EDTA under simulated solar, pure visible, or near UV light by applying in situ DRIFTS-MS analysis. EDTA

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has been adsorbed on P25 surface (P25-EDTA complex) with different loadings and placed in the environmental DRIFTS chamber, which was fed with wet or dry oxidant (O2) or inert (Ar) gas.

2. Experimental 2.1 Preparation of materials

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Titanium dioxide-ethylenediaminetetraacetic acid complexes (P25-EDTAx) were prepared as follows. A certain amount x of EDTA (x = 2, 11, 55 mg), purchased from Sigma-Aldrich, was fully dissolved in 100 ml of DI water at 50 °C (the concentration of EDTA was below its solubility value). Afterwards 80 ml of water containing 1 g TiO2 (Evonik P25) were added. Then the suspension was alternately sonicated and mechanically stirred for 1 hour, heated under stirring at

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70 °C until all the liquid evaporated. The powder was then kept in oven at 70 °C overnight and it underwent freeze-drying at 45 °C using a Labconco 4.5L Cascade for 3 hours. According to the

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estimated molecular surface for EDTA obtained through ChemDrawOffice, 11 mg of EDTA form a monolayer over 1 g of P25 having a surface area equal to 50 m2.

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2.2 Characterizations

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Diffuse reflectance spectra (DRS) were recorded by a UV−vis spectrophotometer (Shimadzu UV2600) in the wavelength range of 200−800 nm. Raman spectra were obtained by using a Witec

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Alpha 300R equipment, with an excitation wavelength of 532 nm and a laser power of ca. 75 mW. Scans were taken over an extended range (1000–10 cm−1) with 2 s integration time and 100

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accumulations. Before DRIFTS-MS runs, samples were treated under vacuum (0.25 mbar) at 45 °C for 12 h, by using a Labconco 4.5L Cascade Freeze Dryer, in order to cleanse the surface from all volatile impurities. DRIFTS analysis was performed using a Bruker VERTEX 80/80v equipped with a liquid nitrogen (LN) cooled MCT detector. Each spectrum was acquired in the 4000–800

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cm−1 range with a spectral resolution of 2 or 4 cm−1 and 64 accumulations. 2.3 Photocatalytic degradation of EDTA with in-situ DRIFTS/MS spectra analysis Photocatalytic runs (see table 1) were conducted in a DRIFTS-environmental chamber (model HVC-DRC, Praying Mantis™ Reaction Chamber, by Harrick Scientific Product, Inc.), located in the above mentioned Bruker VERTEX 80/80v and equipped with one borosilicate window for

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UV-vis irradiation and two KBr windows for the IR beam (Figure S8). Two irradiation sources were used: a 500 W mercury-xenon lamp and a 300 W xenon lamp with a broad UV-visible and solar emission spectrum, respectively (Figures S6-S7 report the emission spectra of the lamps and all the radiation intensities reaching the catalyst surface can be found in Table S1, measured by using DeltaOhm 9721 radiometer and the matching probes). Note that the Xenon lamp was used

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at a power of 270 W. In both cases, the chamber was physically illuminated by means of an optical fiber connected to the irradiation source. For few runs, a UV-cutoff filter was placed in between

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the chamber and the optical fiber connected to the xenon lamp to block photons below 410 nm (in runs under visible light). Gas (oxygen or argon, 20 ml/min) was always flowed through the

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chamber. The relative humidity (RH) of the gas was controlled by a NETZSCH humidity

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generator. RH = 0% refers to the runs in which the humidity generator was not used, namely, the gas cylinder was directly connected to the environmental chamber. A small fraction of the gas

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exiting the chamber was examined by using a NETZSCH QMS 403 D mass spectrometer (MS) whereas the exceeding gas was bubbled in a flask containing glycerol in order to ensure a complete

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isolation of the system from atmosphere. The current intensity vs. time curves were collected for all mass-to-charge ratio (m/z or mass) from 1 to 80. The discussion on the MS results refers to the mass

spectra

of

chemicals

available

in

the

NIST

Chemistry

WebBook

(http://webbook.nist.gov/chemistry). In the following, only the trends of the current intensities

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corresponding to the significant masses selected in the range m/z = 1 to 80 will be discussed. During every run, the solid sample was left inside the chamber under gas flowing for 12 h enabling thermodynamic equilibrium between the catalyst surface and the chamber atmosphere (this time period is indicated as ‘BI’ or ‘before irradiation’ or ‘under dark condition’). Subsequently, the lamp was switched on (time t = 0 refers to this moment). After at least 4 h of irradiation (this time

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period is indicated as ‘DI’ or ‘during irradiation’ or ‘under irradiation’), when the spectra system reached an equilibrium point, the lamp was switched off. One hour later, the sample was removed from the chamber (this time period is indicated as ‘AI’ or ‘after irradiation’) Both MS and DRIFTS spectra were recorded during these 3 time periods. In all MS figures, relative intensity is calculated

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as the current intensity at time t minus the one at time 0.

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Table 1. List of the photocatalytic runs. RH is the relative humidity in the DRIFTS chamber where the gas was flowed at 20 ml/min. EDTA RH Run Gas Lamp UV cut-off [mg/gcat] [%] 1 11 0 Ar Hg-Xe 500W no 2 11 0 O2 Hg-Xe 500W no 11

50

O2

Hg-Xe 500W

4

11

80

O2

Hg-Xe 500W

5

0

50

O2

Hg-Xe 500W

6

2

0

O2

Xe 270W

no

7

2

30

O2

Xe 270W

no

8

2

80

9

55

80

10

2

30 80

no no

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55

no

O2

Xe 270W

no

O2

Xe 270W

no

O2

Xe 270W

yes

O2

Xe 270W

yes

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3. Results and discussion

The DRIFTS spectra (recorded at room conditions) of bare P25, EDTA and different loadings of EDTA on P25 are shown in Figure 1. The broad band at 1628 cm–1 (

OH)

corresponding to the the

bending vibration mode of adsorbed water molecules, is observed for both bare P25 and P25EDTA complexes. A second broad band at 1409 cm–1 is ascribed to the symmetric stretching of the carboxylate group (νsym (COO–)) and indicates that the binding mode for P25-EDTA complex is 7

bidentate.29 At high concentration of EDTA (55 mg/gcat), the intensity ratio νsym (COO-) /

OH

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greater than for the other 2 investigated concentrations (2 and 11 mg). This could be due to the high loading of EDTA, although there are other possible explanations. In the P25-EDTA55 complex, EDTA could also bind in the monodentate mode and, consequently, the C=O stretching mode of EDTA (νC=O)), usually at 1675 cm–1, appears shifted to 1724 cm–1 (Figure 1). The latter

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band can also be ascribed to the C=O stretching vibration in carboxylic groups as dimer.30 More EDTA adsorbed on the surface reduces the possibility of water adsorbing on the same sites OH)

and increasing the intensity ratio νsym(COO–)/

OH.

Indeed, the broad

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lowering the intensity of (

band at 2700-3700 cm–1, owing to stretching vibrational mode of clusters of water molecules

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bound through hydrogen bonds to TiO2, remarkably fades in the most loaded sample (P25OH

in P25-EDTA2 and P25-EDTA11.

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EDTA55). However, νsym(COO–) could be overlapped with

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by the adsorption of EDTA.

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Finally, the peak at 3700 cm–1, assigned to stretching vibrations of free OH– groups, is unmodified

Figure 1. DRIFTS spectra of bare P25, EDTA, P25-EDTA complexes.

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Raman spectroscopy was not very informative in studying the P25-EDTA complexes, since only TiO2 peaks were detected (Figure S1). The strong signals of rutile and, in particular, anatase did not allow to detect bands ascribable to EDTA. Despite P25 and EDTA, individually, do not absorb in the visible region, once EDTA is adsorbed on P25, a ligand-to-metal complex arises being able to absorb visible radiation as shown in DRS

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(Figure 2). Previously literature studies do agree with this observation.29,31

Figure 2. UV-vis DRS of P25 and P25-EDTA complexes. Absorbance spectrum of 0.5 M EDTA

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aqueous solution.

Hence, it is important to investigate the photocatalytic properties of such complex, which has been achieved by carrying out reactivity tests (run) in the DRIFTS-environmental chamber and analyzing the gas phase composition in real-time with MS. Indeed, Kim and Choi reported formerly that P25-EDTA complex can absorb visible light thanks to a ligand-to-metal charge transfer (LMCT) mechanism: electrons are directly photoexcited from the ground state (also 9

known as HOMO, highest occupied molecular orbital) of the adsorbed EDTA to the conduction band (CB) of TiO2 because of the strong electronic coupling between the adsorbate orbitals and Ti d orbitals. These CB electrons, excited through the LMCT mechanism, can trigger redox reactions and the latter ones are clearly distinguished from the truly photocatalytic reactions taking place due to the excitation of TiO2 itself. Thus, both photocatalytic mechanisms (band gap excitation

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3.1 EDTA photooxidation under wide range UV-vis radiation

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and LMCT) should be always considered in the following.

Charge carriers generated under irradiation on P25 surface can be transferred to the HOMO of

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EDTA initiating redox reactions or dissipative processes involved by charges recombination

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(Scheme 1). At the same time, P25-EDTA complex absorbs light and, consequently, electrons located at the HOMO of adsorbate are injected into the CB of the photocatalyst. In turn, they

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recombine with holes in the valence band (VB) or react with suitable electron acceptors. However, in anaerobic conditions (run 1), there are no electron acceptors in the reaction system, since O2 is

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absent and EDTA, which tends to be an electron donor rather than acceptor, undergoes a reduction though. The reason can lie on the higher availability of electrons to trigger reductions rather than holes to oxidize EDTA. In fact, the FTIR spectra in Figure 3 show the disappearance of the band centered at 1409 cm–1 AI, implying that EDTA was no longer adsorbed on P25 as BI. Masses (m/z)

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44 and 18 increased (Figure 4), indicating mainly CO2 and H2O formation resulting from organic carbon oxidation (Scheme 2), and a likely desorption of water from the surface of the catalyst. Yet, the signals of CO2 and H2O were significantly smaller than in the similar run where O2 has been flowed instead of argon (see run 2). Reaction 1 should also lead to nitrate formation from nitrogen (in the EDTA molecule) oxidation. Simultaneously, the reduction of EDTA’s amine groups

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(Scheme 2) produced ammonia as highlighted by the facts that m/z = 15 upped (Figure 4), by the color of the solid surface which turned into brownish from white, and by the typical smell of ammonia noticed once the environmental chamber had been opened. Conceivably, the reduction of carboxylic groups produced acetaldehyde as corroborated by peaks in m/z = 43 (Figure 4), which is not due to CO2 formation. Nonetheless, EDTA oxidation pathway could be also triggered

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by traces of oxygen (O2) in the cell, coming as impurities of argon gas (indeed, m/z = 32 signal,

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the parent peak of molecular O2 mass spectrum, started decreasing at t = 0, not shown).

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Figure 3. Run 1. DRIFTS spectra of P25-EDTA11 BI and AI.

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literature.29,

32-34

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Scheme 1. Energy diagram of P25, which contains rutile and anatase, and EDTA, drawn based on CBA: anatase conduction band; VBA: anatase valence band; CBR: rutile

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conduction band; VBR: rutile valence band. The continuous blue arrows refer to the electron

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under visible radiation.

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transitions under UV light, whereas the continuous red arrows refer to the electron transitions

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Scheme 2. Photocatalytic reactions during run 1.

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Figure 4. Run 1, 2, 3, 4. m/z relative intensity vs. irradiation time curves. The legend refers to the carrier gas and relative humidity used in the reaction chamber. Irradiation source: Hg-Xe lamp.

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From this point onward, all the runs were performed with O2 as carrier gas through the reaction chamber, unless otherwise specified. Run 2, 3 and 4 were performed by varying the percentage of relative humidity in the chamber. Photocatalytic degradation of EDTA was evident since the band at 1409 cm–1 disappeared AI, whereas the overlapped peaks at 1631 and ca. 1700 cm–1 were less intense, which might indicate the photocatalytic conversion of the C=O group (Figure 5). Moreover, new peaks appeared in the 13

carboxyl stretching region at 1575 cm–1, and C-O stretching region at 1320 and 1255 cm–1 (for the latter, when RH = 0%) AI, probably due to Ti-EDTA complexes since they do not correspond to free EDTA. Nevertheless, the band at 1320 cm–1 can also be indicative of the formation of nitrates which remain adsorbed on the surface. The flattening of the broadband between 2700 and 3700 cm–1 is worth noting, as it can be ascribed to the dehydroxylation of P25 surface after

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photodesorption of adsorbed water taking place DI.

Figure 5. Run 2, 3, 4. DRIFTS spectra of P25-EDTA11 BI and AI. The legend refers to relative humidity used in the reaction chamber.

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MS analysis corroborated the presence of CO2 and H2O as main products of EDTA oxidation. Mass spectrum peaks of CO2 are 12, 16, 22, 28, 29, 44 (base peak), 45, 46. However, m/z = 16 (and 32) cannot be used in this discussion because the mass is related to O2 which is the flowing gas. Likewise, mass 28 and 29 refer to N2 which is always present in the mass spectrometer as impurity. The trend of all other mass signals corresponding to CO2 (m/z = 12, 22, 44, 45, 46) was consistent along irradiation time (Figures 4 and S2). Similarly, m/z = 17 and 18 (the 2 main peaks 14

related to water) followed the same trend (Figure S2). The production of water was more evident when RH = 0% because MS can barely detect the increased water production during the reaction in the presence of humidity. On the other hand, the desorption of water from the solid during irradiation was demonstrated by the disappearing of the δOH absorption band. However, other reaction products can generate peaks with the same m/z, but their intensities are lower than those

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related to CO2 signals. Masses 45 and 46 might also indicate the presence of formic acid deriving from the decarboxylation of EDTA, which comes with the release of electrons, making them

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available to complex titanium centers. Notable, mass 43 did not appear because photogenerated electrons combined with O2 and therefore the EDTA reduction pathway was hampered (Figure 4).

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When the level of RH increased from 0 to 80% (run 2, 3, 4), EDTA degraded faster and,

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accordingly, more CO2 was produced during the first 20 minutes of irradiation (Figure 4). In fact, it is well-known in photocatalysis that more oxidative radical species are generated in the presence

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of H2O.35

3.2 EDTA photooxidation under simulated solar radiation

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The runs described so far were performed by using the 500 W mercury-xenon lamp, which is very powerful (high power and energy of UV photons) and it caused P25 to lose its hydroxyl groups early during the degradation process (see DRIFTS spectra in Figure 6). Consequently, the number of oxidative radicals generated under irradiation decreases, in turn affecting the EDTA oxidation.

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Thus, that lamp was replaced by a 300 W Xenon one, which simulates the solar spectrum. Furthermore, the EDTA concentration was lowered by 5 times because a lesser loading of EDTA allows for lower coverage of TiO2 surface, resulting in an efficient formation of reactive radicals from hydroxyl groups with eventual oxidation of EDTA.

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Figure 6. Run 5. DRIFTS spectra of TiO2 P25 BI and AI.

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In the absence of humidity in the chamber (run 6), negligible amounts of CO2 and H2O were detected by MS (Figure 7). However, DRIFTS spectra BI and AI were different. After irradiation,

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the intensity of νsym(COO–) band decreased suggesting a partial degradation of EDTA. Moreover, two new peaks arose at 1560 cm–1 and 1300 cm–1 that can be both assigned to the formation of

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NO3– (figure 8).30,36

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When water (as humidity in the carrier gas) was introduced in the chamber (run 7 and 8), EDTA photooxidation produced CO2 and H2O as highlighted by mass 44 and 18, respectively (Figure 7). Nevertheless, the conversion of EDTA in run 7 was lower than in run 8 after 4 hours of irradiation, given that less CO2 was detected and, moreover, the band at 1560 cm–1 did not disappear

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completely (Figure 8). At high level of RH (run 8), the area under the curves of masses 12, 22, 44, 45 (these masses are ascribable to the CO2 mass spectrum) were larger than those relative to run 7

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(RH = 30%) (Figures 7 and S3). On the other hand, the area corresponding to mass 46 (also part of CO2 mass fragmentation spectrum) was not smaller in run 7, pointing to the possible formation

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of an additional reaction product with a mass fragment m/z = 46. This result confirmed that partial

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degradation of EDTA took place during 4 hours of irradiation at mild levels of RH (run 7). It is worth to note that oxalic acid and formic acid (two possible fragments of EDTA) have 46 as the

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second most abundant peak in their mass spectra. In run 8, the signal m/z = 43 was quite higher than run 6 and 7, such signal cannot be due to either oxalic acid or formic acid since these

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compounds do not have mass 43 in their spectra. Thus, the signal m/z = 43 can be tentatively

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identified as acetic acid, whose base peak is 43 (Figure 7).

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Figure 8. Run 6, 7, 8. DRIFTS spectra of P25-EDTA2 BI and AI. The legend refers to relative

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humidity used in the reaction chamber. The black arrows mark the bands at 1560 cm–1 and 1300

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cm–1, as discussed in section 3.2.

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Run 9 was analogous to run 8 but with more EDTA adsorbed on TiO2 surface. The amounts of CO2 and H2O produced under irradiation were significantly higher, as expected (Figure S4).

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Moreover, the signal relative to mass 43 reached its maximum intensity at earlier time, with respect to run 8, because of the higher initial concentration of EDTA (Figure S4). DRIFTS spectrum AI showed a weak band around 1318 cm-1 which can be ascribed to NO3– adsorbed on P25 (Figure

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9).

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Figure 9. Run 7, 9, 10, 11. DRIFTS spectra BI and AI. RH, relative humidity used in the reaction

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chamber. SOL refers to Xe lamp, VIS refers to Xe lamp+UV cut-off.

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3.3 EDTA photooxidation under pure visible radiation

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As discussed earlier, P25-EDTA complex is able to absorb light at wavelengths (λ) greater than 400 nm, whereas both EDTA and bare P25 cannot. In order to investigate whether or not such complex can in fact react under pure visible light, without any oxidant added to the reaction system,

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runs 10 and 11 were performed in the same conditions as runs 7 and 9, respectively, but inserting a glass filter blocking radiation below 410 nm. It can be stated that both complexes (P25-EDTA2 and P25-EDTA55) are photocatalytically active since several peaks were found during MS analysis

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in the irradiation period. Signal at mass 44 (Figure 10), which refers to CO2, showed that EDTA photooxidation under pure visible light took place at lower reaction rates (when comparing run 7 to 10 or run 9 to 11). Under solar light (run 7 and 9), mass 44 hit a maximum after 10 minutes, then it lowered along irradiation time. On the contrary, under pure visible light (run 10 and 11), the same curves fell down very slowly after reaching a maximum. Moreover, several masses (including 13, 15, 25, 26, 30, 31, 41, 42 and 43) in run 10 showed a considerable intensity at ca. 20

50 minutes, whereas the band at 1409 cm–1 increased and broadened DI and AI. Probably, few reaction intermediates which are not overoxidized under visible light, comprising a COO– group, adsorbed on P25 surface, whereas others would be responsible for those MS peaks (Figure 10 and Figure S5). A medium intensity band at 1410-1405 cm–1 is also characteristic of the group -CH2CO- deformation vibration (Figure 9).30 In other words, the energy of the electrons,

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produced through the LMCT mechanism after the absorption of the visible radiation, is not sufficient. Thus, these electrons can easily recombine with holes in the EDTA HOMO (Scheme

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1).

Enhancing EDTA concentration and RH fostered mineralization of EDTA when comparing run

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11 with 10. Run 11, by contrast with run 9, showed much less CO2 production but almost the same

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amount of the product(s) referring to mass 43 peak (see area below the curve in Figure 10). The band at 1409 cm–1 still appeared AI in run 11, although with a lower intensity than BI, due to both

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EDTA and by-products (Figure 9). At λ > 410 nm P25 is inactive: electrons from VB cannot be promoted to CB. Nevertheless, P25-EDTA complex absorbs the light and electrons are injected

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from EDTA HOMO to P25 CB, in turn reacting with O2. Nevertheless, the oxidation of the reaction intermediates to CO2 was significantly slower under pure visible light and RH = 30%. The slope of the mass 44-curve at the beginning of the reaction (from t = 0 to t = 10 min, roughly), in fact, was lower than the same run under solar light and, as a consequence, the reaction intermediates

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(see for instance mass 43) accumulated in the reaction system (Figure 10). This effect was less evident in run 11 (compare to run 10) because of the higher level of RH.

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Figure 10. Run 7, 9, 10, 11. m/z relative intensity vs. irradiation time curves. The legend refers to the relative humidity used in the reaction chamber, the amount of EDTA adsorbed on P25 and irradiation conditions. SOL refers to Xe lamp, VIS refers to Xe lamp+UV cut-off. The legend is

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the same for the 4 graphs.

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Scheme 3. Summary of the photocatalytic reactions during runs 2-4, 6-11. The most significant

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masses (m/z) referred to other oxygenated products are indicated.

4. Conclusions

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The degradation mechanism of EDTA has been investigated through an in situ DRIFTS-MS

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analysis by using different experimental conditions. EDTA photooxidation reaction pathways was affected significantly by the following parameters: the carrier gas and level of relative humidity (in the environmental DRIFTS chamber), the wavelength and the intensity of the light source.

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EDTA degradation was minimal under anaerobic conditions resulting in production of ammonia and short-chained organic molecules such as acetaldehyde. On the contrary, the main products were CO2 and H2O when O2 was used as carrier gas. P25-EDTA complex was still photoactive

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under pure visible light due to the LMCT mechanism. In the latter case, the EDTA oxidation rate was lower compared to the run with solar light. In addition, few reaction intermediates could not be overoxidized because electrons produced through the LMCT mechanism, under visible radiation, are not sufficiently energetic and can easily recombine with holes in the EDTA HOMO.

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In situ DRIFT-MS analysis can provide new qualitative insights into photocatalytic degradation mechanisms of organic molecules, as shown in this work for the case study of EDTA, which is a pollutant of environmental concern. Future studies will test more complex photocatalysts under the best experimental conditions to further develop this research. The material could be studied by in situ DRIFTS-MS and also by

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other surface-sensitive techniques.

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Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Acknowledgments

Masdar Institute of Science and Technology (now merged into Khalifa University of Science and Technology) is acknowledged for funding (SSG2017-000016). MJLM acknowledges the research

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project CTM2015-69246-R (MINECO/FEDER).

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References

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