STXM results

Figure 6.3 shows X-ray absorption spectra at the C K-edge region (278−320 eV) of particles containing citric acid (black), iron citrate (red) and FeCit/CA (1:1) after 3 hours of UV irradiation in Helium (50 mbar) and oxygen (100 mbar) (blue) and in absence of oxygen (green) at 60 %RH. The background of the averaged pre-edge absorption at 278−282 eV was subtracted and all the spectra were normalized by the

averaged post-edge absorption at 305−320 eV. The main features are at 286.8 eV and at 288.6 eV, which indicate the absorption of carbonylic and carboxylic groups, respectively. Figure 6.4 shows X-ray absorption spectra at the C K-edge region at only 6 energies (278, 286.8, 288.6, 290.8, 298.4 and 320 eV) of particles containing FeCit/CA (1:1), freshly prepared and after different times of UV irradiation.

Figures 6.3a shows the absorption spectra of CA and Fe-Cit in comparison. The CA spectrum is domi-nated by the carboxyl peak at 288.6 eV, which appears broader in the Fe-Cit spectrum. Figure 6.3b shows the appearance of new features upon UV irradiation such as the feature at 286.8 eV, corresponding to car-bonyl compounds, or the feature at 285.0 eV, corresponding to double bounds. The appearance of a feature at 290.8 eV is a consequence of the exposure of the particles to X-rays, which damages them and generates

280 290 300 310 320

Figure 6.3: C-NEXAFS spectra from 278 eV to 320 eV of particles containing citric acid (black), iron citrate (red) and FeCit/CA (1:1) after 3 hours of UV irradiation in presence (blue) and absence (green) of oxygen. The shady zones correspond with the absorption of double bond, carbonyl and carboxyl compounds at 285.0 eV, 286.8 eV and at 288.6 eV respectively.

Figure 6.4: 6-Energy spectra of particles containing FeCit/CA (1:1) before (black) and after 0.5, 1 and 3 hours of irradiation (red, blue and green respectively) over the same particles. b) Optical density of the carbonyl (black) and carboxyl (red) peak to the optical density in the post edge (320 eV).

carbonate. Even so, the damage is very low so we do not consider it. We can first conclude that Fe-Cit photochemistry induces the production of new compounds by photolysis and further reaction. We observed that the concentration of carbonyl compounds is higher when the irradiation is made in absence of oxygen while the double bound compounds are only detectable in absence of oxygen. This can be understood by assuming that in presence of oxygen Fe-Cit cycling occurs several times, leading to further CA degradation and also leading to higher generation oxidation products, including OVOC, which can leave the particles.

In absence of oxygen, the cycle (Fig. 6.1) presumably is occurring once, but cannot be maintained due to lack of oxidants for Fe(II), and thus later oxidation generations are not formed. Consequently, the organic radicals produced in the first step of photolysis of Fe-Cit can drive radical recombination or intramolecular reactions. The latter can be elimination reactions which may lead to the formation of double bounds³³¹.

Figure 6.4a shows that Fe-Cit photochemistry drives the degradation of citric acid over several hours.

The decarboxylation produced by the initial photolysis or Fe^IIICit becomes apparent with the continuous depletion of the absorption at 288.6 eV, indicating lower concentration of carboxylate groups. The increase in the absorption at 286.8 eV indicates the production of carbonyl groups upon UV irradiation. The optical density in the post-edge decreases substantially after 3 hours of irradiation due to most likely the release of OVOCs and thus the decrease of mass and of total carbon of the particles. Figure 6.4b shows the optical density of the carbonyl (black) and carboxyl (red) peak normalized to the optical density at the post edge (320 eV). In the first hour of irradiation the main loss in terms of functionalities is driven by the loss of carboxylic compounds, presumably CO2 and acetic acid, while the carbonyl content increases. From 1 hour to 3 hours of irradiation the carboxyl content increases due to the mass loss of the particles (dominated by release of OVOCs without carboxylic groups) while the carbonyl content decreases most likely due to the higher concentration of C4compounds which are precursors of OVOCs (acetone). This is in agreement with HPLC-MS results and OVOCs results shown later below. The decrease of the carbonyl signal after 3 hours of irradiation can be due to the decrease of the total carbon rather than an effective decrease of the carbonyl concentration.

Figure 6.5 shows a map of Fe(III) fraction (φ) of particles containing FeCit/CA (1:1 molar ratio) after illumination with UV light (375 nm) over 15 minutes in an atmosphere of 110 mbar of oxygen and 40 % RH. Images of particles reveal that Fe(III) is highly depleted while it is still present near the surface as seen as the colour contrast from blue (more Fe²⁺) to red (more Fe³⁺). O2 supply into the bulk can have been restricted only to surface layers with a small depth. We calculated the lower limit of the reacto-diffusive length by assuming that the rate coefficient of the reaction between oxygen and the organic radical is∼10⁶ M⁻¹ s^{−1 155}, and that the diffusion coefficient of oxygen in citric acid at 40 % RH is 10⁻¹³ cm² s^{−1 11}. If every iron atom leads to a radical (potential concentration of∼1 M at the beginning) the reacto-diffusive length (l =p

D/k²¹⁶) , which is the distance over which O2 is lost due to the reaction with radicals, would be 0.01 nm .

Figure 6.5: Map of FeCit/CA particles (1:1 molar ratio) after illumination with UV light over 15 minutes in an atmosphere of 110 mbar of oxygen and 40 %RH. The color scale indicates (Fe(III)/(Fe(II)+Fe(III)) and is generated by means of the parameterization described in Moffet et al.¹²