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1 Supporting Information Imidazolium-based ionic liquids in water: Assessment of photocatalytic and photochemical transformation Paola Calza a, Davide Vione a,* Debora Fabbri a, Riccardo Aigotti b, and Claudio Medana b a Department of Chemistry, University of Torino, via P. Giuria 5, Torino, Italy b Department of Molecular Biotechnology and Health Sciences, University of Torino, via P. Giuria 5, Torino, Italy * Corresponding author. Phone: ; davide.vione@unito.it There are 19 pages, 2 Tables and 15 Figures in this Supporting Information.

2 Table S1. List of [M+H] + and MS 2 product ions obtained from HMIM and its TPs. Values labeled with an asterisk were acquired at low resolution in the linear ion trap only. [M+H] + Δmmu t R MS 2 Δmmu (HMIM) C 4 H 7 N min (117-B) C 4 H 9 O 2 N (117-A) C 4 H 9 O 2 N (60) C 2 H 6 ON (88) C 3 H 6 O 2 N (72) C 3 H 6 ON min [100] C 4 H 7 ON [10] C 3 H 6 ON [40] C 2 H 4 ON min [100] C 4 H 7 ON [20] C 3 H 6 ON [30] C 2 H 4 ON min 42* [100] C 2 H 4 N * [20] H 2 NO min min 44* [100] CH 2 NO S 2

3 Table S2. List of [M+H] + and MS 2 product ions obtained from EMIM and its TPs. Values labeled with an asterisk were acquired at low resolution in the linear ion trap only. [M+H] + Δmmu t R, min MS 2 Δmmu (EMIM) C 6 H 11 N (145-B) C 6 H 13 O 2 N (145-A) C 6 H 13 O 2 N (127) C 6 H 11 ON (125) C 6 H 9 ON (74) C 3 H 8 ON (102) C 4 H 8 O 2 N (60) C 2 H 6 ON (88) C 3 H 6 NO (72) C 3 H 6 ON [30] C 4 H 7 N [10] C 6 H 11 N 2 O [30] C 4 H 8 NO [20] C 3 H 6 NO [20] C 6 H 11 N 2 O [80] C 4 H 8 NO [100] C 3 H 6 NO [100] C 3 H 6 NO [50] C 4 H 8 NO [100] C 5 H 9 N [20] C 3 H 6 N 46* [100] CH 4 NO [50] C 4 H 6 NO [30] C 3 H 8 NO * [100] C 2 H 4 N * [20] NH 2 O * [100] CH 2 NO S 3

4 Figure S1. Transformation products formed from HMIM over time in the presence of TiO 2 -P25. Panels on the top show the most abundant species, while in the bottom the least abundant species are shown. Figure S2. MS 2 spectrum for the species 117-B. S 4

5 Figure S3. HRMS 2 spectrum of the species 111 S 5

6 Figure S4. Transformation products formed from EMIM over time in the presence of TiO 2 -P25. Panels on the top and middle show the most abundant species, while that in the bottom reports the least abundant ones. S 6

7 Figure S5. MS 2 spectrum for the species 145. Figure S6. MS 2 spectrum for the species 127. Figure S7. MS 2 spectrum for the species 125. S 7

8 Direct photolysis EMIM (initial concentration 20 µm) was irradiated under the TL K05 lamp (emission maximum at 365 nm, see Figure S8) at ph 7. A control run was also carried out in the dark, by wrapping the cylindrical cells containing the solutions in double aluminium foil, and by placing them under the same lamp used for the irradiation experiments. In this way, comparable temperature and stirring conditions as for the irradiation experiments are achieved. The transformation of EMIM in the dark was negligible. Under the adopted conditions, irradiated EMIM followed a pseudo-first order transformation kinetics with R EMIM = (1.90±0.16) M s 1. The photon flux absorbed by EMIM can be EMIM ε EMIM ( λ) b[ EMIM ] expressed as: P = p ( λ) [1 10 ] dλ = Einstein L 1 s 1, where p (λ) is a λ the incident spectral photon flux density of the lamp, ε EMIM (λ) the molar absorption coefficient of EMIM (see Figure S8), b = 0.4 cm the optical path length in solution and [EMIM] = 20 µm. From these data it is possible to obtain the polychromatic photolysis quantum yield of EMIM between 300 and 450 nm, where the spectra of the lamp and EMIM overlap, as Φ EMIM = R EMIM (P a EMIM ) 1 = 0.117± Reaction with OH The reaction rate constant between EMIM and OH was determined upon competition kinetics with 2-propanol, using the UVB photolysis of H 2 O 2 as the OH source. Figure S9 reports R EMIM as a function of the concentration of 2-propanol, upon UVB irradiation of 1 mm H 2 O µm EMIM at ph 7, adjusted with NaOH. The maximum adopted concentration of 2-propanol was 10 mm. The trend of R EMIM vs. [2-Propanol] shows a decrease with increasing alcohol concentration. The main reactions that would be involved in EMIM degradation are the following: H 2 O 2 + hν 2 OH [Φ 1 = 0.5] (S1) 2-Propanol + OH R [k 2 = M 1 s 1 ] (S2) EMIM + OH Products [k 3 ] (S3) S 8

Emission spectrum (spectral photon flux density in solution) of the UVA lamp (Philips TL K05).

9 Figure S8. a) Absorption spectra (molar absorption coefficients) of EMIM and of antraquinone-2- sulphonate (AQ2S). Emission spectrum (spectral photon flux density in solution) of the UVA lamp (Philips TL K05). b) Absorption spectrum of Rose Bengal. Incident spectral photon flux density of the lamp Philips TL D 18W/16 Yellow. c) Absorption spectra of nitrate and H 2 O 2. Incident spectral photon flux density of the lamp TL 01. S 9

10 Figure S9. Initial transformation rates (R EMIM ) of 20 µm EMIM upon UVB irradiation of 1 mm H 2 O 2, as a function of the concentration of added 2-propanol. The solution ph was 7, adjusted with NaOH. The dashed curve is the fit function (equation S4), the dotted ones represent the 95% confidence bands of the fit. The error bounds to the rate data represent ±σ. Upon application of the steady-state approximation to OH, one gets the expression (S4) for the initial transformation rate of EMIM in the presence of 2-propanol, where R is the formation rate of OH in reaction (S1): OH R R k [ EMIM ] OH 3 EMIM = (S4) k3 [ EMIM ] + k2 [2 Propanol] From the fit of the experimental data with equation (S4) we obtained R OH.= (2.82±0.10) 10 9 M s 1. Most importantly, the fit yielded k 3 = (2.1±0.3) M 1 s 1 as the reaction rate constant between EMIM and OH. The value of k 3 indicates that the reaction between EMIM and OH is near diffusive control in aqueous solution. 1 S 10

11 Reaction with CO 3 The assessment of the reactivity between organic compounds and CO 3 can be carried out with a semi-quantitative screening method, which makes use of nitrate and bicarbonate under UVB irradiation. 2 The rationale of the method is that the photolysis of nitrate yields photogenerated fragments inside a cage of solvent molecules ([ OH + NO 2 ] cage ) that can either recombine back to nitrate or diffuse into the solution bulk, where OH can react with dissolved substrates. 2-4 The ions HCO 3 and CO 3 2 can react with bulk OH to yield CO 3, which is considerably less reactive than the hydroxyl radical, but they could also react with cage OH. This reaction would produce CO 3 and inhibit photofragment recombination to nitrate (reactions S5-S11). NO 3 + hν + H + [ OH + NO 2 ] cage (S5) [ OH + NO 2 ] cage OH + NO 2 (S6) [ OH + NO 2 ] cage NO 3 + H + (S7) [ OH + NO 2 ] cage + HCO 3 CO 3 + H 2 O + NO 2 (S8) [ OH + NO 2 ] cage + CO 2 3 CO 3 + OH + NO 2 (S9) OH + HCO 3 CO 3 + H 2 O OH + CO 3 2 CO 3 + OH (S10) (S11) Because HCO 3 and CO 3 2 inhibit the reaction (S7) of photofragment recombination, the formation rate of CO 3 in the presence of nitrate + bicarbonate under irradiation would be significantly higher than the formation rate of OH with nitrate alone. 2 The formation of a higher amount of a less reactive species has variable effects depending on the reactivity of a dissolved substrate with CO 3 vs. OH. The degradation of compounds that are poorly reactive toward CO 3, and that would also undergo insignificant transformation by CO 3 in surface waters, would be inhibited by bicarbonate. Conversely, the degradation of compounds that react with CO 3 to a significant extent would be enhanced by bicarbonate. 2 An important issue is that the bicarbonate addition to nitrate also modifies the solution ph, with potential impacts on nitrate photolysis and substrate reactivity because of e.g. acid-base equilibria. For this reason, the effect of bicarbonate addition should be compared to the behaviour of the substrate in the presence of a phosphate buffer (NaH 2 PO 4 + Na 2 HPO 4 ) at the same concentration (comparable ionic strength) and ph of NaHCO 3. Figure S10 reports the initial transformation rate of 20 µm EMIM upon UVB irradiation in the presence of: (i) variable NaHCO 3 concentrations; (ii) 10 mm NaNO 3 and variable NaHCO 3 concentrations; (iii) 10 mm NaNO 3 and a phosphate buffer, at the same concentration of NaHCO 3 and same ph (within ±0.1 units). S 11

12 The experimental data allow the following inferences to be made: (a) the transformation rate of EMIM without nitrate was lower than with nitrate but it was not negligible. Therefore, the direct photolysis of the substrate would be significant under the used experimental conditions; (b) the transformation rate of EMIM slightly increased with increasing phosphate (and ph). However, the extent of the increase with ph was comparable to the level of the experimental uncertainty; (c) compared to phosphate at equal ph, bicarbonate inhibited the transformation of EMIM. The inhibition by bicarbonate, compared to phosphate, of EMIM transformation upon nitrate photolysis suggests that EMIM is little reactive toward CO 3. Therefore, reaction with CO 3 is expected to be a minor transformation process of EMIM in surface waters. Figure S10. Initial transformation rates under UVB irradiation of ( ) 20 µm EMIM and 10 mm NaNO 3, as a function of the concentration of NaHCO 3 ; () 20 µm EMIM and 10 mm NaNO 3, as a function of the concentration of added phosphate buffer (same concentration as NaHCO 3 and same ph, within 0.1 units); ( ) 20 µm EMIM, without nitrate, as a function of NaHCO 3 concentration. The solution ph varied from 7 to 9, depending on the concentration of NaHCO 3 or the phosphate buffer. The error bounds to the rate data represent ±σ. S 12

13 Reaction with 1 O 2 Figure S11 reports the initial transformation rate of EMIM, as a function of its initial concentration, upon irradiation at ph 7 of 10 µm Rose Bengal (RB), used as a source of 1 O 2 (reaction S12). From the linear trend of the plot one derives R EMIM = (3.59±0.96) 10 6 [EMIM]. Figure S11. Initial transformation rates of EMIM (R EMIM ) upon irradiation of 10 µm Rose Bengal (RB) under the yellow lamp (Philips TL D 18W/16), as a function of the initial EMIM concentration. The solution ph was 7, adjusted with NaOH. The fit line is dashed, the dotted ones represent the 95% confidence limits of the fit. The error bounds to the rate data represent ±σ. The reaction (S13) between EMIM and 1 O 2 would be in competition with the thermal deactivation of singlet oxygen (reaction S14): 5 RB + hν + O 2 RB + 1 O 2 (S12) EMIM + 1 O 2 Products [k 13 ] (S13) 1 O 2 O 2 [k 14 = s 1 ] (S14) S 13

14 Upon application of the steady-state approximation to 1 O 2, one gets the following expression for the initial transformation rate of EMIM (R EMIM ): R EMIM R1 k13 [ EMIM ] O2 = (S15) k + k [ EMIM ] where R is the formation rate of 1 O 1 O 2 by 10 µm RB under the used irradiation device. For very 2 low [EMIM] one gets (k 13 [EMIM] «k 14 ): lim 1 { R } = R1 k k [ EMIM ] EMIM [ EMIM ] 0 O (S16) Equation (S16) is consistent with the linear trend of R EMIM vs. [EMIM] reported in Figure S11. The measurement of R was carried out upon irradiation of 10 µm RB mm furfuryl alcohol 1 O 2 (FFA), which reacts with 1 O 2 with a rate constant k FFA = M 1 s 1. 6 The time evolution of FFA under irradiation is reported in Figure S12. Figure S12. Time trend of FFA (initial concentration 0.1 mm) upon irradiation of 10 µm RB under the used yellow lamp (Philips TL D Yellow) at ph 7. The initial transformation rate of FFA was R FFA = (4.86±0.42) 10 8 M s 1. Photogenerated 1 O 2 could undergo deactivation or reaction with FFA, and upon application of the steady-state approximation to [ 1 O 2 ] one obtains: S 14

15 R 1 O2 k14 + kffa [ FFA] = RFFA (S17) k [ FFA] FFA From equation (S17) one gets R = (1.06±0.09) 10 6 M s 1. From equation (S16) and Figure S11 1 O one derives REMIM [ EMIM] = R1 k13 k14 = (3.59±0.96) From the known values of O 2 k 14 one gets k 13 = (8.5±3.0) 10 5 M 1 s 1 as the reaction rate constant between EMIM and 1 O 2. R and 1 O 2 Reaction with irradiated AQ2S. Figure S13 reports the initial transformation rate of EMIM (R EMIM ) as a function of its initial concentration, upon UVA irradiation of 0.1 mm AQ2S at ph 7. The direct photolysis of EMIM (irradiation without AQ2S) was negligible at the adopted irradiation time scale (up to 4 h). The triplet state 3 AQ2S*, which is the main reactive species of AQ2S under irradiation, has a formation quantum yield Φ 3AQ2S* = 0.18 and a first-order deactivation rate constant k 3AQ2S* = s The formation rate of 3 AQ2S* would be R 3AQ2S* = Φ 3AQ2S* P a AQ2S (where P a AQ2S is the photon flux absorbed by AQ2S, in units of Einstein L 1 s 1 ) and the 3 AQ2S* transformation or deactivation would be in competition with the reaction with the substrate S (having rate constant k 3AQ2S*,S, see Scheme below, with ISC = inter-system crossing). 9,10 Reaction between 3 AQ2S* and ground-state AQ2S can be minimised by adopting an AQ2S initial concentration of 0.1 mm or lower. 11 Figure S13. Initial transformation rates of EMIM upon UVA irradiation of 0.1 mm AQ2S, as a function of the initial EMIM concentration. The solution ph was 7, adjusted with NaOH. The fit line is dashed, the dotted ones represent the 95% confidence limits of the fit. The error bounds to the rate data represent ±σ. S 15

16 Upon application of the steady-state approximation to 3 AQ2S*, the transformation rate of EMIM by irradiated AQ2S can be expressed as follows: R EMIM k 3 [ EMIM ] AQ2S AQ 2S*, EMIM = Φ 3 P 2 * a (S18) AQ S k + k [ EMIM ] 3 AQ 2S* 3 AQ2S*, EMIM Under the hypothesis that k 3AQ2S*,EMIM [EMIM] «k 3AQ2S*, one gets: R EMIM k 3 * AQ2S AQ2S, EMIM = Φ 3 * P [ EMIM ] AQ2S a (S19) k 3 * AQ2S EMIM absorbs a negligible fraction of the lamp radiation compared to AQ2S. For instance, at 365 nm (the emission maximum of the TL K05 lamp) the absorbance of 0.1 mm AQ2S would be over 700 times higher compared to 30 µm EMIM (the highest adopted EMIM concentration). Therefore, AQ2S can be considered with an excellent approximation as the only light-absorbing species in solution under the used lamp. The photon flux absorbed by AQ2S can thus be expressed as P = o ( A S λ [ p λ AQ ( )) ) (1 10 ] dλ (, where p o (λ) is the incident spectral photon flux density of AQ2S 2 a λ the lamp and A AQ S ( ) is the absorbance of AQ2S in the irradiated system. In particular, with 2 λ [AQ2S] = 0.1 mm under the Philips TL K05 lamp, one gets AQ S P 2 a = Einstein L 1 s 1. Figure S13 shows that R EMIM follows a linear trend vs. EMIM concentration that is consistent with equation (S19). In particular, from the experimental data one gets R EMIM = (1.53±0.09) 10 5 [EMIM] (with R EMIM in M s 1 and [EMIM] in molarity) and, upon comparison with equation (S19), S 16

one derives Φ Φ (0.18), 3 * 3 * AQ2S k AQ 2 S *, EMIM P k AQ2S 1 3 * 3 * ( 3 * ) AQ2 S a AQ 2S, CLO AQ 2S k (5.0 10 6 s 1 ) and AQ2S 3 = (2.7±0.2) 10 8 M 1 s 1. k = (1.53±0.09) 10 5.

17 one derives Φ Φ (0.18), 3 * 3 * AQ2S k AQ 2 S *, EMIM P k AQ2S 1 3 * 3 * ( 3 * ) AQ2 S a AQ 2S, CLO AQ 2S k ( s 1 ) and AQ2S 3 = (2.7±0.2) 10 8 M 1 s 1. k = (1.53±0.09) From the know values of AQ S P 2 a ( Einstein L 1 s 1 ), one gets: Figure S14. Half-life time in SSD (summer sunny days equivalent to 15 July at 45 N latitude) of EMIM, as a function of water depth and dissolved organic carbon (DOC). Other water conditions: 0.1 mm nitrate, 1 µm nitrite, 1 mm bicarbonate, 10 µm carbonate. S 17

18 Figure S15. Time evolution of EMIM (initial concentration C o = 20 µm) upon irradiation under the Philips TL K05 lamp, in Milli-Q (ultrapure) water, and in water from lakes Candia (CA) and Avigliana (AV). Main water parameters: dissolved organic carbon (DOC) = 6.9 mg C L 1 (CA), 5.8 mg C L 1 (AV); [NO 3 ] = 15 µg L 1 (CA), 1730 µg L 1 (AV); inorganic carbon (IC) = 11.8 mg C L 1 (CA), 32.0 mg C L 1 (AV); ph = 8.3 (CA), 9.6 (AV). S 18

19 References 1. G.V. Buxton, C.L. Greenstock, W.P. Helman, A.B. Ross, critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals ( OH/ O ) in aqueous solution, J. Phys. Chem. Ref. Data 17 (1988) D. Vione, S. Khanra, S. Cucu Man, P.R. Maddigapu, R. Das, C. Arsene, R.I. Olariu, V. Maurino, C. Minero, Inhibition vs. enhancement of the nitrate-induced phototransformation of organic substrates by the OH scavengers bicarbonate and carbonate. Wat. Res. 43 (2009) G. Mark, H.G. Korth, H.P. Schuchmann, C. von Sonntag, The photochemistry of aqueous nitrate ion revisited, J. Photochem. Photobiol. A: Chem. 101 (1996) R.C. Bouillon, W.L. Miller, Photodegradation of dimethyl sulfide (DMS) in natural waters: Laboratory assessment of the nitrate-photolysis-induced DMS oxidation. Environ. Sci. Technol. 39 (2005) M.A.J. Rodgers, P.T. Snowden, Lifetime of 1 O 2 in liquid water as determined by timeresolved infrared luminescence measurements, J. Am. Chem. Soc. 104 (1982) F. Wilkinson, J. Brummer, Rate constants for the decay and reactions of the lowest electronically excited singlet-state of molecular oxygen in solution, J. Phys. Chem. Ref. Data 10 (1981) I. Loeff, A. Treinin, H. Linschitz, Photochemistry of 9,10-anthraquinone-2-sulfonate in solution. 1. Intermediates and mechanism, J. Phys. Chem. 87 (1983) A.E. Alegría, A. Ferrer, G. Santiago, E. Sepúlveda, W. Flores, Photochemistry of watersoluble quinones. Production of the hydroxyl radical, singlet oxygen and the superoxide ion, J. Photochem. Photobiol. A: Chem. 127 (1999) P.R. Maddigapu, A. Bedini, C. Minero, V. Maurino, D. Vione, M. Brigante, G. Mailhot, M. Sarakha, The ph-dependent photochemistry of anthraquinone-2-sulfonate, Photochem. Photobiol. Sci. 9 (2010) V. Maurino, D. Borghesi, D. Vione, C. Minero, Transformation of phenolic compounds upon UVA irradiation of anthraquinone-2-sulfonate, Photochem. Photobiol. Sci. 7 (2008) A. Bedini, E. De Laurentiis, B. Sur, V. Maurino, C. Minero, M. Brigante, G. Mailhot, D. Vione, Phototransformation of anthraquinone-2-sulphonate in aqueous solution, Photochem. Photobiol. Sci. 11 (2012) S 19

Supplementary Material for

Electronic Supplementary Material (ESI) for Environmental Science: Processes & Impacts. This journal is The Royal Society of Chemistry 2015 Supplementary Material for Effects of ph and dissolved oxygen