J M Hartmann

Very low temperature growth of GeSi alloys with digermane, disilane and dichlorosilane

Keywords: A3, Chemical vapor deposition processes; B1, Germanium silicon alloys

2

-Introduction

We have studied a few years ago the low temperature epitaxy of pure Ge and of Ge-rich SiGe alloys in a 200 mm industrial Reduced Pressure -Chemical Vapor Deposition (RP-CVD) tool [1]. We first of all benchmarked germane (GeH4) and digermane (Ge2H6) for the growth of pure Ge. Using Ge2H6 instead of GeH4 enabled us to dramatically increase the Ge growth rate at temperatures 425 °C and lower. We also evaluated, at 400 °C, 100 Torr, the impact of the GeH4 or Ge2H6 mass-flow on the Ge growth rate. For the same Ge flow, the higher surface reactivity of digermane yielded roughly five times higher growth rates than with germane. We then combined digermane with disilane (Si2H6) or dichlorosilane (SiH2Cl2) in order to study the GeSi growth kinetics at 475 °C, 100 Torr. While the SiH2Cl2 mass-flow did not have any clear influence on the GeSi growth rate (with a 14 nm min -1 mean value, then, i.e. half that of pure Ge with the same digermane flow), a Si2H6 mass-flow increase resulted in a slight GeSi growth rate increase (from 11 nm min -1 up to 14 nm min -1 ). Using dichlorosilane instead of disilane resulted in definitely lower Si concentrations, in the 18% -23% range compared to the 47%-61% range, respectively.

Being able to grow GeSi alloys with Si contents in the few % -30% range at temperatures significantly lower than 475°C would be interesting for a lot of applications. Ge/GeSi Multi Quantum Wells (MQWs) grown on top of Ge-rich SiGe virtual substrates can for instance be used for the fabrication of optical modulators, photodetectors and light emitters [2]. Ge/GeSi MQWs could be the core of terahertz quantum cascade lasers [3]. GeSi layers grown at temperatures lower than or equal to 350°C could be used, instead of SiGeSn ternaries, as the confinement barriers in GeSi/GeSn MQWs and Double Hetero-Structures for Mid-Infra-Red lasers [4][5]. Provided that they could be grown selectively in cavities on each side of Ge p-type Metal Oxyde Semiconductor Field Effect Transistors, GeSi alloys would inject uniaxial tensile strain in short gate length devices, resulting in higher electron mobilities [6][7].

While the interest of such alloys is obvious, there is, apart from Ref. [1], a lack of growth kinetics data for high Ge content GeSi alloys, at least in RP-CVD, the workhorse of the microelectronics industry. The main reason for that is, to my opinion, the scarcity of large diameter wafers with the right lattice parameter. The lattice parameter mismatch between Si(001) substrates and GeSi layers with Si contents in the few % -30% range is indeed so large that only a few nm of the latter can be grown pseudomorphically, preventing their easy characterization with for instance X-Ray Diffraction or X-Ray Reflectivity. Bulk Ge wafers would be ideal for such investigations, but they are very expensive in 200mm and do not exist in 300mm. One thus has to master the growth of high crystalline quality, rather flat Ge Strain-Relaxed Buffers on Si(001) to have the right templates for such studies. Bauer et al. explored in 2013 the 20 Torr growth kinetics of GeSi alloys in the 350°C -500°C range with a GeH4 + SiH2Cl2 chemistry [START_REF]Handbook of Silicon Photonics[END_REF]. The following trends were identified: at a given temperature and for a fixed GeH4 flow, the GeSi growth rate decreased sub-linearly and the Si content increased sub-linearly with the SiH2Cl2 mass-flow. As the temperature changed, there was, besides the expected growth rate decrease, a decrease of the Si content in the film (Fig. 1.15 of Ref. [START_REF]Handbook of Silicon Photonics[END_REF]). Growth rates were low, however. At 450°C, the GeSi growth rate indeed decreased from 7 down to barely 0.4 nm min -1 and the Si content increased from 21% up to 43% as the F(SiH2Cl2)/F(GeH4) mass-flow ratio increased from 1 up to 25. Along the same lines, the GeSi growth rate decreased from 60 down to 0.6 nm min -1 and the Si content from 27% down to 19% as the temperature decreased from 550°C down to 350°C (F(SiH2Cl2)/F(GeH4) = 1).

As the growth rate of pure Ge from Ge2H6 is definitely higher than that from GeH4, I will, in the coming sections, explore the 100 Torr growth kinetics of GeSi at temperatures less than 500°C, the aim being to quantify the impact of the addition of SiH2Cl2 and/or Si2H6 on it and obtain reasonable growth rates at temperatures 350°C and less. GeSi / GeSn hetero-epitaxial growth would be feasible, then. Sn precipitation and surface segregation indeed prevent the use of temperatures significantly higher than 350°C for the deposition of flat, high crystalline quality GeSn layers. Temperatures closer to 300°C have to be adopted when Sn contents above 10% are targeted [START_REF] Aubin | [END_REF].

-Experimental details

A 200mm Epi Centura 5200 cluster tool from Applied Materials was used for the growths of the Ge Strain Relaxed Buffers (SRBs) and the GeSi layers on top. The 2.5 µm thick Ge SRBs were grown with GeH4 on Si(001) in the High Temperature epitaxy chamber. A Ge "seed" layer ~ 120 nm thick was first of all grown at 400°C, 100 Torr. After a temperature ramping-up under GeH4 (another 80 nm of Ge grown), the rest of the Ge SRB was grown at 750°C, 20 Torr. Afterwards, a short duration Thermal Cycling under H2 was used to lower the Threading Dislocations Density down to 10 7 cm -2 values and have flat surfaces (the surface root mean square roughness and Z range were around 0.8 nm and 6 nm, respectively) [10]. Ge SRBs were then stored in one of the N2-purge load locks before the re-epitaxy of tens of nm thick, nominally pseudomorphic GeSi layers in the Low Temperature epitaxy chamber, which is equipped with pyrometers having a linear response in the 200°C -500°C range. The growth with Ge2H6 (diluted at 2% in H2), pure SiH2Cl2 and/or pure Si2H6 of those GeSi layers was conducted at 100 Torr. The flow of purified H2, several tens of standard litres per minute, was not changed throughout the study. H2 was injected to (i) lower the partial pressures of unwanted impurities (such as O2, H2O or CHx compounds) that might enter the chamber which is not as tight as molecular beam epitaxy chambers, (ii) ensure that a laminar flow of gases is flowing over the wafer surface and (iii) passivate dangling bonds, notably at low temperatures, inhibiting unwanted adsorption of impurities on the surface.

X-Ray Reflectivity (XRR) experiments were performed in a fully automated Jordan-Valley tool with a convergent incident X-Ray beam and a 1024 pixel Charge-Coupled Display detector that sampled the reflected beam over an ~ 3.5° range. A X'Pert Panalytical tool with a copper anticathode as the X-Ray source, a 4 bounces symmetric Ge(220) Bartels' monochromator for incident beam conditioning and another 2 bounces Ge(220) analyser crystal in front of the detector was otherwise used for the high resolution X-Ray Diffraction (XRD) measurements around the (004) order.

-The metrology used on the GeSi/Ge samples

As stated in Section 2, I have used XRR and XRD to gain access, at the wafer center, to the thickness and Si composition of the tens of nm thick, pseudomorphic GeSi layers grown on the 2.5 µm thick Ge SRBs (grown beforehand on Si(001) substrates). XRR and XRD curves acquired on the GeSi layers grown with Ge2H6 and the lowest of the two SiH2Cl2 mass-flows probed can be found in Figure 1. Omega-2Theta scans around the (004) XRD order associated with 18 to 58 nm thick GeSi layers grown at 100 Torr with a Ge2H6 + SiH2Cl2 chemistry on Ge SRBs, themselves on Si(001). F(Ge2H6)/F(H2) = 1.98x10 -4 , F(SiH2Cl2)/F(H2) = 1.25x10 -3 , growth temperature in the 350°C -500°C range.

As the Si content and thus the optical index difference between GeSi and Ge were high (Si has a much lower density than Ge, 2.33 g cm -3 compared to 5.33 g cm -3 ), thickness fringes up to high incidence angles were present on the XRR curves of layers grown in the 425°C -500°C range. This meant that the GeSi surfaces were flat. An AFM image typical of such surfaces can be found in Figure 10a. By contrast, such XRR thickness fringes were of low intensity or not distinguishable for some of the samples grown at 400°C and below. This was likely due to Si contents not high enough to yield meaningful density and thus optical index differences, as we will see in the following. The presence of small islands on the surface for temperatures less than 400°C might also have reduced the amplitude of the thickness fringes (see Figures 10b-d). A Fast Fourier Transform (FFT) of the XRR curves with fringes yielded GeSi layer thicknesses. XRD, by contrast, was not hampered by small Si contents in the GeSi layers. Besides the Si(001) substrate and Ge SRB peaks located at 34.56° and 33.04°, there was always a well-defined, intense GeSi XRD peak in-between, with a thickness fringe at high incidence angles for the thickest layers, which was another sign that layers were of good crystalline quality. This peak moved towards the Ge SRB peak as the growth temperature decreased. This was due to a Si concentration decrease, as shown in Fig. 2. Fits based on the Takagi-Taupin dynamical diffraction theory of the experimental profiles yielded, with the assumption that GeSi layers were fully tensile strained on the larger lattice parameter Ge SRBs underneath, the Si content and another estimation of the thickness. Those fits took into account the fact the Ge SRBs were also slightly tensile-strained because of differences in thermal dilation coefficients between Ge and Si coming into play during the cooling-down to room temperature after growth (macroscopic degree of strain relaxation of 104%, typically [10]).

-Impact of SiH2Cl2 on the 100 Torr growth kinetics of GeSi in the 350°C -500°C range

I have started this study by quantifying the impact of "Low" or "High" amounts of SiH2Cl2 (DCS) on the 100 Torr growth kinetics of GeSi in the 350°C -500°C range. The digermane flow was half that used for GeSi growth at 475°C in Ref. [1], in order to have reasonably high Si contents at low temperatures: F(Ge2H6)/F(H2) = 1.98x10 -4 3.96x10 -4 in Ref. [1]. The SiH2Cl2 mass-flows probed, F(SiH2Cl2)/F(H2) = 1.25x10 -3 or 1.125x10 -2 , resulted in F(Si)/F(Ge) atomic flow ratios of 6.3 or 56.7 (see Table 1).

The pure Ge growth rate (from Ref. [1]) and the GeSi growth rates with those mass-flows are provided as functions of the growth temperature in Figure 2a. The Si contents in the resulting GeSi layers are plotted in Figure 2b.

I will first discuss the peculiar behaviour of the pure Ge Growth Rate (GR) from Ge2H6. It definitely increases with the temperature in the 325°C -400°C range, with a 36 kcal. mol. -1 activation energy which is very close to that of the Ge-H bond strength (37 kcal. mol. -1 [11]). Such GR, which are definitely higher than those with GeH4, are likely due to the ability digermane molecules have of creating their own adsorption sites on Hterminated Ge surfaces. The following mechanisms would be at play, then: the Ge-Ge bond of a digermane molecule and a surface Ge-H bond nearby break. One of the two GeH3 fragments bonds to the surface site vacated by the H atom, which simultaneously bonds to the other GeH3 fragment, forming gaseous GeH4 (i.e. Ge2H6(g) + H(a) → GeH3(a) + GeH4(g); (g) stands for gaseous while (a) means adsorbed) [1], [12]. At 400°C and above, the Ge growth rate reaches a plateau then decreases. Such a behaviour would be explained by a Ge2H6(g) + 2GeH(a) → 2GeH4(g) + 2Ge(s) alternative reaction pathway becoming predominant at the expense of the Ge2H6(g) + H(a) → GeH3(a) + GeH4(g) reaction. A gradual shift to a "GeH4-like" growth behavior would then occur as the temperature increases. The same reactions, with Si instead of Ge, were used in Refs. [13][14][15] to explain, for Si2H6, the sharp Si growth rate increase with temperature up to 575°C (with a 53 kcal. mol. -1 activation energy close to the 47 kcal. mol. -1 Si-H bond strength [11]), the plateau then the Si GR decrease (in the 575°C -675°C range).

Adding SiH2Cl2 to Ge2H6 results, whatever the amount and the growth temperature, in GeSi GR which are lower than that of pure Ge, in line with Ref. [1] findings at 475°C for the same chemistry and also in line with Ref. [START_REF]Handbook of Silicon Photonics[END_REF] findings for GeH4 + SiH2Cl2. This is likely due to higher Cl coverages and lower Ge coverages on GeSi surfaces with such a chlorinated chemistry. Si-Cl, Ge-Cl and Si-H bonds, which are harder to break that Ge-H bonds, would hamper growth, then (respective bond strengths: 90 kcal. mol. -1 , 51 kcal. mol. -1 [16] and 47 kcal. mol. -1 , to be compared with 37 kcal. mol. -1 only). HCl vapors, a by-product of DCS decomposition, might also etch Si and Ge surface atoms. Indeed, Si growth from dichlorosilane occurs as follows [17][18][19]: (i) gaseous SiH2Cl2 decomposes in the gaseous phase into SiCl2 and H2 (SiH2Cl2(g) ↔ SiCl2(g) + H2(g));

(ii) gaseous SiCl2 adsorbs on the surface (SiCl2(g) + _ ↔ SiCl2(a)); (iii) adsorbed SiCl2 is reduced to Si and HCl vapors by H2 (SiCl2(a) + 2H2(g) ↔ Si(s) + 2HCl(g)). Ge2H6 + SiH2Cl2 chemistry on Ge SRBs, themselves on Si(001). F(Ge2H6)/F(H2) = 1.98x10 -4 , F(SiH2Cl2)/F(H2) = 1.25x10 -3 or 1.125x10 -2 ("Low" and "High" DCS flows), growth temperature in the 325°C -500°C range.

As for pure Ge, the GeSi GR increases exponentially with the temperature in the 350°C -400°C range, with activation energies which are lower than for pure Ge (Ea(GeSi, "Low" DCS flows) = 26 kcal. mol. -1 and Ea(GeSi, "High" DCS flows) = 35 kcal. mol. -1 , to be compared with Ea (pure Ge) = 36 kcal. mol. -1 ). There is otherwise, in that temperature range, a monotonic decrease of the growth rate as the SiH2Cl2 mass-flow increases, with, on average, values 85% and 53% that of pure Ge for "Low" and "High" DCS flows. The situation is more complex above 400°C, with a sharper decrease of the GeSi growth rate as the temperature increases for "Low" than for "High" DCS flows, resulting in GR which are counterintuitively higher for the later than for the former for temperatures above 425°C.The Si content in the resulting GeSi alloys increases slowly then more rapidly with the temperature, from 7% up to 31% for "Low" DCS flows and from 12% up to 29% for "High" DCS flows. The slope of that increase was faster in the former than in the latter case, which was unexpected. It resulted in Si contents which were, for "Low" DCS flows, close to or higher than for "High" DCS flows in the 475°C-500°C range. Below 475°C, Si contents were, as expected, lower for DCS flows 9 times lower.

This monotonous increase of the Si content with the temperature was not a surprise. It was indeed in line with (i) Ref. [START_REF]Handbook of Silicon Photonics[END_REF] findings for a GeH4 + SiH2Cl2 chemistry and the same temperature range and (ii) Ref. [20] findings for a Ge2H6 + Si2H6 chemistry and a 450°C -550°C range (although Si contents were much higher in the latter study).

In order to gain more insight about the growth mechanisms with such a chemistry, I have plotted in Figure 3 the Ge and the Si growth rate components functions of temperature for pure Ge and GeSi with the two DCS flows (Si GR component = x * GeSi GR and Ge GR component = (1-x) * GeSi GR, x being the Si content in the GeSi alloys). Growth rate (nm min.

-1

)

Growth temperature (°C)

Figure 3: Ge and Si growth rate components at 100 Torr functions of the growth temperature in the 325°C -500°C range for Ge2H6 and Ge2H6 + SiH2Cl2 chemistries. F(Ge2H6)/F(H2) = 1.98x10 -4 , F(SiH2Cl2)/F(H2) = 1.25x10 -3 or 1.125x10 -2 ("low" and "high" DCS flows).

Adding SiH2Cl2 resulted in a reduction of the Ge GR component compared to that of pure Ge whatever the temperature (same Ge2H6 flow). The trend is the same in all cases: the Ge GR component increases exponentially with the temperature from 350°C up to 400°C, with activation energies very close to that of GeSi (see above). It decreases for temperatures above 400°C, at a faster pace for pure Ge and "Low" DCS flows than for "High" DCS flows. The situation is somewhat different for the Si GR components. They first of all increase exponentially with the temperature in the 350°C -400°C range (Ea(Si GR component, "Low" DCS flows) = 27 kcal. mol. -1 and Ea(Si GR component, "High" DCS flows) = 30 kcal. mol. -1 ). They reach a plateau in the 425°C -500°C range, with growth rates which are barely higher for a DCS flow 9 times higher (on average, 2.4 compared to 1.9 nm min -1 ). Such values are surprisingly high compared to the homoepitaxial growth rate of Si from DCS, which is around 0.5 nm min -1 at 650°C, 20 Torr and virtually nil below [12]. Based on that, it is surprising to reach 7-12% Si contents in GeSi alloys grown at 350°C, i.e. a temperature 300°C below that threshold.

-Impact of Si2H6 and Si2H6 + SiH2Cl2 mixtures on the 100 Torr growth kinetics of GeSi in the 350°C -425°C range

I have continued this study by quantifying the impact of moderate amounts of SiH2Cl2, Si2H6 or mixtures of Si2H6 + SiH2Cl2 on the 100 Torr growth kinetics of GeSi in the 350°C -425°C range. Mixtures were evaluated in order to have an additional degree of freedom on the Si content in GeSi films. Using SiH2Cl2 only indeed yields low Si contents, while the use of Si2H6 results in high Si contents, as shown in Figures 4b and6b. Mixing the two enables to access medium Si contents, as seen for instance in Figure 7b. The digermane flow was the same as in Section 4, i.e. F(Ge2H6)/F(H2) = 1.98x10 -4 . The disilane and dichlorosilane mass-flows probed, F(Si2H6)/F(H2) = 6.25x10 -4 and F(SiH2Cl2)/F(H2) = 1.25x10 -3 , resulted in F(Si)/F(Ge) atomic flow ratios of 6.3 (single Si precursors) or 12.6 (mixture of Si precursors): see Table 1. There are indeed two Si atoms in a disilane molecule, compared with one only in a dichlorosilane molecule.

The pure Ge growth rate (from Ref. [1]) and the GeSi growth rates with those chemistries and mass-flows are plotted as functions of the growth temperature in Figure 4a. The Si contents in the resulting GeSi layers are plotted in Figure 4b.

Adding Si2H6 or Si2H6 + SiH2Cl2 to Ge2H6 results, whatever the temperature, in GeSi GR which are, on average, (i) 2.3 times lower than that of pure Ge and (ii) 1.9 times lower that of GeSi with a Ge2H6 + SiH2Cl2 chemistry. Activation energies associated with the exponential increase of the GeSi GR with the temperature in the 350°C -400°C range were of 42 kcal. mol. -1 for Ge2H6 + Si2H6 and Ge2H6 + Si2H6 + SiH2Cl2, to be compared with 25 kcal. mol. -1 only for Ge2H6 + SiH2Cl2 and this DCS flow. A doubling of the Si flow by switching from Ge2H6 + Si2H6 to Ge2H6 + Si2H6 + SiH2Cl2 has almost no impact of the GeSi GR, which is barely lower with the mixture of Si precursors than with disilane only.

The Si content in the resulting GeSi alloys increases super-linearly with the temperature, from 24% up to 38% -39.5% for Ge2H6 + Si2H6 and Ge2H6 + Si2H6 + SiH2Cl2. By contrast, there was an increase from 7% up to 15.5% for Ge2H6 + SiH2Cl2, a "Low" DCS flow and that temperature range. This would be due to the relative ease Si atoms have of incorporating with Si2H6, which has the ability of creating its own adsorption site by breaking a surface Ge-H or Si-H bond (Si2H6(g) + H(a) → SiH3(a) + SiH4(g)). This results in Si contents which are, over the 350°C -425°C range and with the same F(Si)/F(Ge) MFR of 6.3, roughly 3 times higher with Si2H6 than with SiH2Cl2. A doubling of the Si atomic flow by adding SiH2Cl2 to Si2H6 has otherwise no real impact of the Si concentration, which is almost that with Si2H6. The same lack of impact on GeSi GR was highlighted just above. For those GeSi layers grown at very low temperatures, the predominance Si2H6 has over SiH2Cl2 is definitely more important than for lower Ge content SiGe layers grown at 525°C, 20 Torr with a Si2H6 + SiH2Cl2 + GeH4 chemistry [20]. Ge2H6 + SiH2Cl2, Ge2H6 + Si2H6 or Ge2H6 + Si2H6 + SiH2Cl2 on Ge SRBs, themselves on Si(001). F(Ge2H6)/F(H2) = 1.98x10 -4 , F(SiH2Cl2)/F(H2) = 1.25x10 -3 and/or F(Si2H6)/F(H2) = 6.25x10 -4 , growth temperature in the 350°C -425°C range.

The evolution with the temperature of the Si and Ge GR components can be found in Figure 5. It is obvious that a switch from SiH2Cl2 to Si2H6 or a mixture of Si2H6 + SiH2Cl2 results in (i) a drastic decrease of the Ge GR component and (ii) a definite increase of the Si GR component. The activation energies over the 350°C -400°C range, 41 kcal. mol. -1 for the Ge GR components and 44 kcal. mol. -1 for the Si GR components, are higher for Si2H6 or Si2H6 + SiH2Cl2 than for SiH2Cl2 (26-27 kcal. mol. -1 ). For temperatures 400°C and above, the Si GR component increases at a faster pace and the Ge GR decreases less with the formers than with the latter type of chemistry. The other interesting feature is that, maybe because of the generation of HCl during the decomposition of SiH2Cl2, Si and GR components are slightly lower with Si2H6 + SiH2Cl2 than with Si2H6.

-Impact of high flows of Si and Ge precursors on the very low temperature growth kinetics of GeSi

I have then quantified the impact of using higher Si and Ge precursor flows to boost, at temperatures less than 400°C, the GeSi growth rate, which was low indeed for the moderate flows used in Section 4. To that end, I have kept the F(Si)/F(Ge) atomic mass-flow ratio constant at 6.3 and increased, at 350°C, 100 Torr, the Ge2H6 mass-flow (and thus also the Si flow) by 2, 3 and 4 (see Table 1). Two kinds of chemistries were investigated: Ge2H6 + SiH2Cl2 or Ge2H6 + SiH2Cl2 + Si2H6. In the latter case, the atomic Si flows from SiH2Cl2 and Si2H6 molecules were the same and half the overall Si flow (i.e. F(Si2H6) = 0.5xF(SiH2Cl2) = 0.25xF(Si), as there are two Si atoms in a disilane molecule, versus one in a dichlorosilane molecule).

GeSi growth rates and Si contents in the resulting GeSi alloys can be found in Figure 6. They are given as functions of the F(Si)/F(H2) mass-flow ratio, F(Si) being equal to F(SiH2Cl2) +2* F(Si2H6) (F(Ge) = 2* F(Ge2H6)). As expected, the GeSi growth rate increases linearly with the mass-flows, from 4.6 up to 10.5 nm min -1 for a Ge2H6 + SiH2Cl2 chemistry and from 1.6 up to 3.55 nm min -1 for a Ge2H6 + SiH2Cl2 + Si2H6 chemistry. For the same Si atomic flow, growth rates are 2.9 times higher with SiH2Cl2 than with SiH2Cl2 + Si2H6 (as in Section 4). A quadrupling of the flows did not yield a quadrupling of the GeSi growth rate, however, but a 2.2 times increase for both chemistries, instead.

Given that the F(Si)/F(Ge) atomic mass-flow ratio was kept constant at 6.3, I would otherwise have expected the Si content in those GeSi alloys to be steady. This is to some extent the case, with a slight linear decrease of the Si content as mass-flows increase, however, from 24.2% down to 22.2% and from 8.4% down to 7.4%, respectively. Si contents from Ge2H6 + SiH2Cl2 + Si2H6 are once again ~ 3 times higher than Si contents from Ge2H6 + SiH2Cl2, although the Si atomic flow was the same (in line with Section 4 findings).

For F(Si)/F(Ge) = 6.3, Si contents are, on average, equal to 0.232 and 0.079. Incoming Ge atoms are thus 6.3 / 0.232 = 27 times more likely to incorporate in the lattice than Si atoms with a Ge2H6 + SiH2Cl2 + Si2H6 chemistry (13.5 times when discounting SiH2Cl2 molecules). Meanwhile, the Ge incorporation efficiency is 6.3 / 0.079 = 80 times that of Si with a Ge2H6 + SiH2Cl2 chemistry. GeSi growth rate (nm min. F(Ge2H6)/F(H2) = 1.98x10 -4 , 3.96x10 -4 , 5.94x10 -4 and 7.92x10 -4 . Pure SiH2Cl2: F(SiH2Cl2)/F(H2) = 2.5x10 -3 , 5.0x10 -3 , 7.5x10 -3 and 10 -2 . SiH2Cl2 and Si2H6: F(SiH2Cl2)/F(H2) = 2* F(Si2H6)/F(H2) = 1.25x10 -3 , 2.5x10 -3 , 3.75x10 -3 and 5.0x10 -3 .

I have adopted the highest Ge2H6 mass-flow which could reproducibly be delivered in they epitaxy chamber and probed the impact of (i) varying flows of Si2H6 (for a fixed SiH2Cl2 flow) and (ii) varying flows of SiH2Cl2 (for a fixed Si2H6 flow) on the 350°C, 100 Torr growth kinetics of GeSi (see Table 1).

Growth rates and Si contents in the resulting GeSi alloys can be found in Figure 7. They are given as functions of the F(Si)/F(H2) mass-flow ratio. Increasing by up to 4 the dichlorosilane flow has, for a fixed disilane flow, almost no impact on the Si content, which stays constant at 22.3%. The GeSi growth rate decreases, however, from 4.0 down to 2.7 nm min -1 . By contrast, an increase by up to 4 of the disilane flow results, for a fixed dichlorosilane flow, in (i) a definite increase of the Si content, from 16.7% up to 29.7%, and (ii) a three folds decrease of the GeSi growth rate, from 5.45 down to 1.8 nm min -1 only. I thus confirm, for higher precursor flows, the trends identified in Figures 4 and5: (i) the flow of Si2H6, and only that, governs Si incorporation in GeSi with such a mixture; (ii) the only impact SiH2Cl2 has is on growth rate, which is reduced for high flows (as if dichorosilane was a source of gaseous HCl, an etchant, and/or if it generated, on the growing surface, a significant amount of Ge-Cl and Si-Cl bonds, which are harder to break than Ge-H bonds); (iii) the growth rate goes down as the Si content goes up, which is logical as Si-H, Ge-Cl and Si-Cl bonds are stronger than Ge-H bonds.

I have concluded this study by quantifying, for the highest digermane flow achievable in our tool, the impact of really low temperatures on GeSi growth kinetics. The F(Si)/F(Ge) atomic mass-flow ratio was, for the Ge2H6 + SiH2Cl2 and Ge2H6 + SiH2Cl2 + Si2H6 chemistries probed, constant and equal to 6.3 (as in Fig. 4; see Table 1). The atomic Si flows from SiH2Cl2 and Si2H6 molecules were, for the latter kind of chemistry, the same and half the overall Si flow (i.e. F(Si2H6) = 0.5xF(SiH2Cl2) = 0.25xF(Si)).

Growth rates and Si contents in the resulting GeSi alloys can be found in Figure 8. As expected, the GeSi growth rate increases exponentially with the temperature, faster with Ge2H6 + SiH2Cl2 + Si2H6 than with Ge2H6 + SiH2Cl2 (Ea = 53 kcal. mol. -1 , to be compared with Ea = 43 kcal. mol. -1 ). Those values are higher than those in Fig. 4 for four times lower flows (Ea = 42 kcal. mol. -1 , to be compared with Ea = 25 kcal. mol. -1 ). This might partly be due to a temperature range over which activation energies were extracted which was closer, in Fig. 4, to a high temperature regime. This likely resulted in values slightly lower than in reality, as 400°C data points started to bow downwards from pure exponentials. Fig. 8 GeSi growth rates are otherwise 2.6 times higher, on average, with Ge2H6 + SiH2Cl2 than with Ge2H6 + SiH2Cl2 + Si2H6, once again in line with Figs. 4 to 7 findings.

Ge2H6 + SiH2Cl2 + Si2H6 Ge2H6 + SiH2Cl2
GeSi growth rate (nm min. Meanwhile, the Si contents in the resulting GeSi layers are 3 times higher, over the 325°C -375°C range, with Ge2H6 + SiH2Cl2 + Si2H6 than with Ge2H6 + SiH2Cl2 and the same F(Si)/F(Ge) ratio of 6.3. This is fully in line with Fig. 4 findings for flows 4 times lower and higher temperatures. Those higher Si contents partly explain why activation energies associated with the exponential increase of the GeSi GR with the temperature were higher for Ge2H6 + SiH2Cl2 + Si2H6 than for Ge2H6 + SiH2Cl2: there was a higher density of more energetic Si-Cl and Si-H bonds on the surface. Si contents otherwise increase linearly with the temperature in that range, from 7.2 (325°C) up to 7.7% (375°C) for Ge2H6 + SiH2 and from 22.2 (337°C) up to 24.0% (375°C) for Ge2H6 + SiH2Cl2 + Si2H6. The increase is 5 times slower with the former than the latter type of chemistry, however (0.09% 0.45% gain per 10°C increase).

The Ge and Si GR components with high precursor flows are plotted in Figure 9 as functions of the growth temperatures. They increase exponentially with the temperature, with activation energies which are similar for both components and higher for a Ge2H6 + SiH2Cl2 + Si2H6 than for a Ge2H6 + SiH2Cl2 chemistry (Ea = 51-53 kcal. mol. -1 , to be compared with Ea = 43-44 kcal. mol. -1 ). Using SiH2Cl2 + Si2H6 otherwise results in Ge GR components which are, on average, 0.37 times those with SiH2Cl2 and Si GR components which are, on average, 1.36 times those with SiH2Cl2. This reflects, once again, the relative ease Si2H6 molecules have of injecting Si atoms in the lattice by creating their own adsorption sites (higher Si GR components), with a Ge GR component which is then lower as, amongst other reasons, there are more Si atoms on the surface with Si-H bonds which are harder to break than Ge-H ones.

-GeSi surface morphology

Before ending this paper, I will say a few words about the surface morphology of such GeSi layers. AFM images typical of the surface of GeSi layers grown with various chemistries and flows at different temperatures can be found in Figure 10. GeSi layers grown at temperatures 400°C and above were, irrespectively of the growth chemistry used, smooth and islands-free, with a replication of the cross-hatched morphology of the underlying Ge strain-relaxed buffers, with terraces bordered by bi-atomic steps on the slopes of small hills (Fig. 10a). GeSi layers grown with a Ge2H6 + SiH2Cl2 chemistry, moderate flows and temperatures below 400°C, had a few islands on the cross-hatched surface (Fig. 10b). Meanwhile, GeSi layers grown at temperatures below 400°C with (i) a Ge2H6 + SiH2Cl2 chemistry and high precursor flows, (ii) a Ge2H6 + Si2H6 chemistry and moderate precursor flows or (iii) a Ge2H6 + Si2H6 + SiH2Cl2 chemistry and moderate flows, had a larger number of islands on the surface (Fig. 10c). The situation was the worse on GeSi layers grown with a Ge2H6 + Si2H6 + SiH2Cl2 chemistry and high flows, with a significant amount of islands on the surface, then (Fig. 10d). inclusions in the films because of gas phase reactions during growth (for high precursor flows). Having HCl, an etchant generated during the decomposition of SiH2Cl2, present in the gaseous mixture most likely helped in having smooth surfaces over the vast majority of the layers (preferential etching of growth defects after their formation). Meanwhile, the presence of vast amounts of Si2H6 molecules, which are more reactive on the surface and prone to gas phase reactions than SiH2Cl2 molecules, resulted in the formation of a larger amount of islands at temperatures less than 400°C.

A bearing analysis of Fig. 10 pictures showed that the fraction of the surface covered by islands was small, however: 0.1% only for Fig. 10(b), 0.8% for Fig. 10(c) and 7.8% for Fig. 10(d) image, one of the worse in terms of roughness. Those values are actually higher than reality, as islands are smaller than "seen" in AFM (convolution with the AFM tip, which has a 10 nm radius, typically). Those small surface coverages explain why XRR and XRD profiles were typical of smooth, pseudomorphic films: the flat portions of the films diffracted coherently (XRD) and yielded well-defined thickness fringes in XRR (monitoring of the specular signal only). Trends in Figs. 2 to 9 are logical and thus meaningful even for the highest amounts of islands (exponential growth rate increases, steady or linear fluctuations of the Si content as the temperature and/or the flows were changed and so on).

Similar islands were seen on the surface of Si (and SiGe) layers grown at low temperatures with Si2H6 [21], Si3H8 [22] and other high order Si precursors [23]. Reducing the Si precursor flow (and thus the growth rate) and switching to higher growth temperatures then helped in minimizing the island density or suppressing them, in line with Fig. 10 findings. Using Cyclic Deposition / Etch schemes enabled, thanks to the deposition of a reduced thickness and the selective etch of defects at the end of each cycle, to recover flat Si surfaces even at low temperatures [23][24]. The same strategy, with HCl (+ H2) [16] or Cl2 (+ N2 or He) [25][26] as etchant gases, could be used here to have flat GeSi films at temperatures less than 400°C with the current growth data points. Adding HCl to Ge2H6, SiH2Cl2 and/or Si2H6 might also help in recovering flat surfaces. It would however have an impact on growth kinetics, which would have to be quantified, then (growth rate and Si content reductions expected; adding HCl to Si2H6 + Ge2H6 indeed resulted, at 500°C, 20 Torr in lower growth rates and higher Ge contents SiGe layers [20]). Another path might be to reduce the precursor partial pressures by lowering the chamber pressure (20 Torr only in Ref. [START_REF]Handbook of Silicon Photonics[END_REF]). This would potentially result in a lesser number of harmful gas phase reactions, a lower density of limited mobility adatoms on the surface and higher crystalline quality layers. GeSi growth rates would be lower, however, hindering epitaxy at temperatures 350°C and less.

-Conclusions

The aim being to grow GeSi layers (with Si contents in the few % -30% range) at temperatures significantly less than 500°C for use in various types of devices (passivation of IR imagers, GeSi/GeSn multi-quantum wells, embedded stressors in Ge pMOS devices and so on), I have investigated the epitaxy of tens of nm thick, pseudomorphic layers on top of Ge Strain Relaxed Buffers, themselves grown on top of Si(001) substrates. The growth temperature T was in the 325°C -500°C range and the pressure was always 100 Torr. Ge2H6 + SiH2Cl2, Ge2H6 + Si2H6 and Ge2H6 + SiH2Cl2 + Si2H6 chemistries with moderate to high flows were used for the deposition of those layers.

Adding "Low" or "High" amounts of SiH2Cl2 (DCS) to Ge2H6 resulted in GeSi growth rates, in the 350°C to 500°C range, which were lower than the growth of pure Ge with the same Ge2H6 flow. A switch from an exponential increase of the growth rate with the temperature (for T ≤ 400°C) to a growth rate which decreased at T increased above 400°C was evidenced. The Si content in the resulting GeSi alloys increases slowly then more rapidly with the temperature, from 7% up to 31% for "Low" DCS flows and from 12% up to 29% for "High" DCS flows.

Switching over to a Ge2H6 + Si2H6 chemistry resulted, in the 350°C -425°C range, in GeSi growth rates which were 2 times lower and Si contents which were 3 times higher. The Si content increased monotonously with the temperature, from 24% up to 39%. Doubling the Si atomic flow by adding SiH2Cl2 to Si2H6 and Ge2H6 had, quite surprisingly, almost no impact on the GeSi growth kinetics, with almost the same Si contents and GeSi growth rates barely lower than those with Si2H6 only.

I have then quantified, for Ge2H6 + SiH2Cl2 and Ge2H6 + SiH2Cl2 + Si2H6 chemistries, the impact of higher precursor flows on the 100 Torr GeSi growth kinetics at 350°C. The GeSi growth rate increased linearly with the mass-flows, from 4.6 up to 10.5 nm min -1 for a Ge2H6 + SiH2Cl2 chemistry and from 1.6 up to 3.55 nm min -1 for a Ge2H6 + SiH2Cl2 + Si2H6 chemistry. Although the F(Si)F(Ge) atomic flow ratio was constant at 6.3, slight Si content decreases, from 24.2% down to 22.2% (Ge2H6 + SiH2Cl2 + Si2H6) and from 8.4% down to 7.4% (Ge2H6 + SiH2Cl2), were evidenced as the individual massflows were quadrupled. The GeSi growth rate increased exponentially with the temperature and those high flows, faster with Ge2H6 + SiH2Cl2 + Si2H6 than with Ge2H6 + SiH2Cl2 (Ea = 53 kcal. mol. -1 , to be compared with Ea = 43 kcal. mol. -1 over the 325°C -375°C range). Meanwhile, Si contents increased linearly, from 7.2 (325°C) up to 7.7% (375°C) for Ge2H6 + SiH2 and from 22.2 (337°C) up to 24.0% (375°C) for Ge2H6 + SiH2Cl2 + Si2H6.

Finally, GeSi layers grown at temperatures 400°C and above were, irrespectively of the growth chemistry used, smooth, with a replication of the cross-hatched morphology of the underlying Ge strain-relaxed buffers. GeSi layers grown with a Ge2H6 + SiH2Cl2 chemistry, moderate flows and temperatures below 400°C, had a few islands on the cross-hatched surface. Meanwhile, GeSi layers grown at temperatures below 400°C with (i) a Ge2H6 + SiH2Cl2 chemistry and higher precursor flows, (ii) a Ge2H6 + Si2H6 chemistry and moderate precursor flows or (iii) a Ge2H6 + Si2H6 + SiH2Cl2 chemistry and moderate flows, had a larger number of islands on the surface. The situation was the worse on GeSi layers grown with a Ge2H6 + Si2H6 + SiH2Cl2 chemistry and high flows, with a significant amount of islands on the surface, then. Cyclic Deposition / Etch schemes, with HCl or Cl2 periodically injected to selectively remove islands, which are most likely the surface signature of crystalline defects or amorphous inclusions in the bulk of the layers, will likely have to be called upon to make full use of data points below 400°C. Adding HCl to the gaseous mixture might also solve this problem. Its impact on growth kinetics would have to be quantified, however. 1 : F(Ge2H6)/F(H2), F(SiH2Cl2)/F(H2) and F(Si2H6)/F(H2) mass-flow ratios, F(Si)/F(Ge) atomic mass-flow ratios and temperatures probed during this study (100 Torr epitaxial growth of GeSi alloys on Ge SRBs, themselves on Si(001)).

Figure 1 :

 1 Figure1: (a) XRR curves and (b) Omega-2Theta scans around the (004) XRD order associated with 18 to 58 nm thick GeSi layers grown at 100 Torr with a Ge2H6 + SiH2Cl2 chemistry on Ge SRBs, themselves on Si(001). F(Ge2H6)/F(H2) = 1.98x10 -4 , F(SiH2Cl2)/F(H2) = 1.25x10 -3 , growth temperature in the 350°C -500°C range.

Figure 2 :

 2 Figure2 :(a) Growth rates and (b) Si content in GeSi layers grown at 100 Torr with a Ge2H6 + SiH2Cl2 chemistry on Ge SRBs, themselves on Si(001). F(Ge2H6)/F(H2) = 1.98x10 -4 , F(SiH2Cl2)/F(H2) = 1.25x10 -3 or 1.125x10 -2 ("Low" and "High" DCS flows), growth temperature in the 325°C -500°C range.

  . -Low DCS flow Ge GR comp.-High DCS flow Si GR comp. -Low DCS flow Si GR comp.-High DCS flow

Figure 4 :

 4 Figure 4 :(a) Growth rates and (b) Si contents in GeSi layers grown at 100 Torr with Ge2H6 + SiH2Cl2, Ge2H6 + Si2H6 or Ge2H6 + Si2H6 + SiH2Cl2 on Ge SRBs, themselves on Si(001). F(Ge2H6)/F(H2) = 1.98x10 -4 , F(SiH2Cl2)/F(H2) = 1.25x10 -3 and/or F(Si2H6)/F(H2) = 6.25x10-4 , growth temperature in the 350°C -425°C range.

Figure 5 :

 5 Figure 5: Ge and Si growth rate components at 100 Torr functions of the growth temperature in the 350°C -425°C range for Ge2H6, Ge2H6 + SiH2Cl2, Ge2H6 + Si2H6 and Ge2H6 + Si2H6 + SiH2Cl2 chemistries. F(Ge2H6)/F(H2) = 1.98x10 -4 , F(SiH2Cl2)/F(H2) = 2* F(Si2H6)/F(H2) = 1.25x10 -3 .

Figure 6 :

 6 Figure6: (a) Growth rates and (b) Si contents in GeSi layers grown at 350°C, 100 Torr with Ge2H6 + SiH2Cl2 or Ge2H6 + Si2H6 + SiH2Cl2 on Ge SRBs, themselves on Si(001). F(Ge2H6)/F(H2) = 1.98x10 -4 , 3.96x10 -4 , 5.94x10 -4 and 7.92x10 -4 . Pure SiH2Cl2: F(SiH2Cl2)/F(H2) = 2.5x10 -3 , 5.0x10 -3 , 7.5x10 -3 and 10 -2 . SiH2Cl2 and Si2H6: F(SiH2Cl2)/F(H2) = 2* F(Si2H6)/F(H2) = 1.25x10 -3 , 2.5x10 -3 , 3.75x10 -3 and 5.0x10 -3 .

Figure 7 :

 7 Figure 7 : (a) Growth rates and (b) Si contents in GeSi layers grown at 350°C, 100 Torr with Ge2H6 + Si2H6 + SiH2Cl2 on Ge SRBs, themselves on Si(001). F(Ge2H6)/F(H2) = 7.92x10 -4 . Blue circles: fixed dichlorosilane flow (F(SiH2Cl2)/F(H2) = 5.0x10 -3 ), varying Si2H6 flows. Brown squares: fixed disilane flow (F(Si2H6)/F(H2) = 2.5x10 -3 ), varying SiH2Cl2 flows.

Figure 8 :

 8 Figure 8 : (a) Growth rates and (b) Si content in GeSi layers grown at 100 Torr with Ge2H6 + SiH2Cl2 or Ge2H6 + SiH2Cl2 + Si2H6 + on Ge SRBs, themselves on Si(°001). F(Ge2H6)/F(H2) = 7.92x10 -4 . SiH2Cl2 only: F(SiH2Cl2)/F(H2) = 10 -2 . SiH2Cl2 + Si2H6 chemistry: F(SiH2Cl2)/F(H2) = 2*F(Si2H6)/F(H2) = 5x10 -3 . Growth temperature in the 325°C -375°C range.

Figure 9 :

 9 Figure 9: Ge and Si growth rate components at 100 Torr functions of the growth temperature in the 325°C -375°C range for Ge2H6 + SiH2Cl2 and Ge2H6 + SiH2Cl2 + Si2H6 chemistries. SiH2Cl2 only: F(SiH2Cl2)/F(H2) = 10 -2 . SiH2Cl2 + Si2H6: F(SiH2Cl2)/F(H2) = 2*F(Si2H6)/F(H2) = 5x10 -3 .

Figure 10 :

 10 Figure 10 : 5 µm * 5 µm AFM images of the surface of (a) a 29 nm thick GeSi layer with 26% of Si grown at 475°C with moderate amounts of Ge2H6 and SiH2Cl2, (b) a 41 nm thick GeSi layer with 7% of Si grown at 350°C with moderate amounts of Ge2H6 and SiH2Cl2, (c) a 24 nm thick GeSi layer with 24% of Si grown at 350°C with moderate amounts of Ge2H6, Si2H6 and SiH2Cl2 and, finally, (d) a 23 nm thick GeSi layer with 23% of Si grown at 350°C with high amounts of Ge2H6, Si2H6 and SiH2Cl2. Image sides along the <100> directions.

Table

	Growth conditions	F(Ge2H6)/	F(SiH2Cl2)/	F(Si2H6)/	F(Si)/F(Ge)	Growth
	(Figures)	F(H2)	F(H2)	F(H2)		temperature
						(°C)
	Ge2H6 + SiH2Cl2,	1.98x10 -4	1.25x10 -3	0	6.3	350°C
	moderate Ge2H6		or		or	to
	flow		1.125x10 -2		56.7	500°C
	(Figs. 2 and 3)
	Ge2H6 + SiH2Cl2 +				6.3 (single Si	350°C
	Si2H6, moderate	1.98x10 -4	1.25x10 -3	6.25x10 -4	precursors) or	to
	Ge2H6 flow				12.6 (mixture	425°C
	(Figs. 4 and 5)				of precursors)
	Ge2H6 + SiH2Cl2 or	1.98x10 -4	2.5x10 -3 up to	0
	Ge2H6 + SiH2Cl2 +	up to	10 -2 or	or	6.3	350°C
	Si2H6, increasing	7.92x10 -4	1.25x10 -3 up to	6.25x10 -4 up
	Ge2H6 flow		5x10 -3	to 2.5x10 -3
	(Fig. 6)
	Ge2H6 + SiH2Cl2 +	7.92x10 -4	5x10 -3	1.25x10 -3 to
	Si2H6, high Ge2H6		or	5x10 -3 or	9.5 up to 19	350°C
	flow, varying Si		2.5x10 -3 to 10 -2	2.5x10 -3
	flows
	(Fig. 7)
	Ge2H6 + SiH2Cl2 or
	Ge2H6 + SiH2Cl2 +	7.92x10 -4	10 -2 or	0 or	6.3	325°C
	Si2H6, high Ge2H6		5x10 -3	2.5x10 -3		to
	flow					375°C
	(Figs. 8 and 9)

Acknowledgements: L. Saidi, S. Rousseau, P. Roche, B. Assie and P. Nal are gratefully acknowledged for their help in operating and maintaining the Epi Centura 5200 tool.


Yassine Ouarda

Clément Trellu

Geoffroy Lesage

Matthieu Rivallin

Patrick Drogui

Marc Cretin
email: marc.cretin@umontpellier.fr

Prof Marc Cretin

Electro-oxidation of secondary effluents from various wastewater plants for the removal of acetaminophen and dissolved organic matter

Keywords: Organic pollutants, Electro-Fenton, Anodic oxidation, Magneli phases, 3D EEM, Mineralization

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Over the past decade, the effectiveness of wastewater treatment systems has been questioned by many researchers, particularly in terms of removing emerging contaminants (pesticides, pharmaceuticals, endocrinal disrupting compounds, …) often detected in treated effluents. Conventional physical, chemical or biological treatment systems are unable to efficiently eliminate these contaminants and there is often a transfer of pollution from one phase to another (liquid to solid phase or vice-versa) or an incomplete degradation of these contaminants. Several researchers have reported that the exposure, especially in long term, to emerging contaminants such as personal care products, pharmaceuticals and hormones causes disruption of aquatic fauna and flora and risks to human health. As reported by Tiwari et al. [START_REF] Tiwari | Review on fate and mechanism of removal of pharmaceutical pollutants from wastewater using biological approach[END_REF][START_REF] Solano | Application of electrochemical advanced oxidation processes with a boron-doped diamond anode to degrade acidic solutions of Reactive Blue 15 (Turqueoise Blue) dye[END_REF], prolonged exposure to pharmaceutical residues results in a change in the species character and behavior of aquatic organisms. The feminization of male fish due to the presence of estrogen in the aquatic environment is a well-known example of trait change in species [START_REF] Gross-Sorokin | Assessment of feminization of male fish in English rivers by the Environment Agency of England and Wales[END_REF]. The development of bacterial resistance to antibiotics is also a major risk of exposure to pharmaceutical residues in the environment. To overcome this problem, additional, efficient and sustainable treatments of wastewater treatment plant (WWTP) secondary effluents are necessary to improve the quality of the effluents before their release into the environment.

Electrochemical advanced oxidation processes (EAOPs) represent an attractive solution for tertiary treatment of treated effluents [START_REF] Solano | Application of electrochemical advanced oxidation processes with a boron-doped diamond anode to degrade acidic solutions of Reactive Blue 15 (Turqueoise Blue) dye[END_REF][START_REF] Brillas | Incineration of acidic aqueous solutions of dopamine by electrochemical advanced oxidation processes with Pt and BDD anodes[END_REF][START_REF] Sires | Electrochemical advanced oxidation processes: today and tomorrow. A review[END_REF][START_REF] Ganzenko | Removal of psychoactive pharmaceutical caffeine from water by electro-Fenton process using BDD anode: Effects of operating parameters on removal efficiency[END_REF]. These processes are considered environmentally friendly as they require little or no added chemicals like H2O2 for the Fenton reaction or O3 for ozonation. The advantages of such processes are their nonpolluting appearance, ease of automation and reduced processing times. The principle of EAOPs consists into the in situ electrochemical generation of reagents, particularly hydroxyl radicals • OH, which are considered as the second most powerful oxidizing species after fluorine (E o ( • OH/H2O) = 2.80 V / SHE). [START_REF] Yahya | Mineralization of the antibiotic levofloxacin in aqueous medium by electro-Fenton process: kinetics and intermediate products analysis[END_REF][START_REF] Brillas | Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry[END_REF]. They are non-selective oxidants that can oxidize pollutants until complete mineralization to water, carbon dioxide and inorganic ions.

Among the EOAPs, electro-Fenton (EF) is widely used in wastewater treatment. Several studies have reported its efficiency on eliminating different types of pharmaceutical pollutants such as levofloxacin, enoxacin, amoxicillin, tetracycline, etc. [START_REF] Oturan | Electrocatalytic destruction of the antibiotic tetracycline in aqueous medium by electrochemical advanced oxidation processes: Effect of electrode materials[END_REF][START_REF] Annabi | Degradation of enoxacin antibiotic by the electro-Fenton process: Optimization, biodegradability improvement and degradation mechanism[END_REF][START_REF] Panizza | Complete mineralization of the antibiotic amoxicillin by electro-Fenton with a BDD anode[END_REF]. EF process is based on the continuous production of hydroxyl radicals by the means of electrochemically generated Fenton's reagent (Eq. 1). Depending on cathode material, H2O2 can be electro-generated by reduction of dissolved oxygen (Eq. 2), while the Fe 2+ initially added is also continuously regenerated (Eq. 3).

Fe 2+ + H2O2 + H + → Fe 3+ + H2O + • OH (Eq. 1)

O2 + 2H + + 2e -→ H2O2 (Eq. 2)

Fe 3+ + e -→ Fe 2+ (Eq.

3)

The chemical nature of the anode is also a key parameter for the efficiency of the electrochemical process [START_REF] García-Gómez | Experimental design methodology applied to electrochemical oxidation of carbamazepine using Ti/PbO2 and Ti/BDD electrodes[END_REF]. Several types of anodes can be used during electrolysis, which are divided into two categories, active and non-active [START_REF] Komtchou | Removal of carbamazepine from spiked municipal wastewater using electro-Fenton process[END_REF][START_REF] Jardak | Electrochemical degradation of ethylene glycol in antifreeze liquids using boron doped diamond anode[END_REF][START_REF] Ouarda | Synthetic hospital wastewater treatment by coupling submerged membrane bioreactor and electrochemical advanced oxidation process: Kinetic study and toxicity assessment[END_REF]. Non-active anodes are characterized by their inert surface and their low adsorption properties, resulting in a weak interaction between the produced hydroxyl radicals and the surface of the anode.

Thus, the physisorbed hydroxyl radicals react easily with the organic pollutants, resulting in a complete mineralization of pollutants [START_REF] Salazar | Electrochemical degradation of the antihypertensive losartan in aqueous medium by electro-oxidation with boron-doped diamond electrode[END_REF]. On the other hand, hydroxyl radicals are not efficiently formed at active anodes such as Ti/IrO2 and Ti/RuO2. However, they promote efficient conversion of chloride present in wastewater, to hypochlorous acid or hypochlorite ions that can oxidize organic pollutants [START_REF] Boudreau | Competition between electrochemical advanced oxidation and electrochemical hypochlorination of sulfamethoxazole at a boron-doped diamond anode[END_REF].

In this study, we have mainly investigated the treatment of secondary effluents by the electro-Fenton process combined with anodic oxidation (AO) by using substoichiometric titanium oxide anode (Ti4O7). Ti4O7 has been reported to be a suitable non-active anode (Eq. 4) in EF process for the removal of pharmaceuticals such as propranolol and sulfamethoxazole [START_REF] Ganiyu | Use of Sub-stoichiometric Titanium Oxide as a Ceramic Electrode in Anodic Oxidation and Electro-Fenton Degradation of the Beta-blocker Propranolol: Degradation Kinetics and Mineralization Pathway[END_REF][START_REF] Ganiyu | Electrochemical mineralization of sulfamethoxazole over wide pH range using Fe II Fe III LDH modified carbon felt cathode: Degradation pathway, toxicity and reusability of the modified cathode[END_REF][START_REF] Trellu | Mineralization of organic pollutants by anodic oxidation using reactive electrochemical membrane synthesized from carbothermal reduction of TiO2[END_REF]. This material has the potential to be a low cost alternative to the well-known boron-doped diamond electrode [START_REF] Ganiyu | Use of Sub-stoichiometric Titanium Oxide as a Ceramic Electrode in Anodic Oxidation and Electro-Fenton Degradation of the Beta-blocker Propranolol: Degradation Kinetics and Mineralization Pathway[END_REF]. Moreover, it is possible to synthesize porous Reactive Electrochemical Membranes (REM), which allow a strong enhancement of mass transport conditions when operated in flow-through mode for the treatment of low concentrations of pollutants.. Unprecedented high removal rates of total organic carbon and mineralization current efficiency were achieved on REM (without coupling with EF process) after a single passage through the membrane for ACT, phenol and oxalic acid [START_REF] Trellu | Mineralization of organic pollutants by anodic oxidation using reactive electrochemical membrane synthesized from carbothermal reduction of TiO2[END_REF]. While further development of REM is a promising challenge, it is also important to continue to raise understanding on reaction and competition mechanisms thanks to batch experiments.

Here, the efficiency of this combined process (EF/AO) was also compared with the use of the EF process as well as with the use of the AO process (AO/H2O2). For the EF process, a Ti/Pt plate anode was used instead of Ti4O7. For the AO/H2O2, none Fe 2+ was added, thus avoiding the decomposition for H2O2 by the Fenton's reaction. Unfortunately, it is impossible to easily determine the degree of mineralization of ACT at a concentration of about 100 μg L -1 by TOC analysis in effluents containing from 7 to 75 mgC L -1 . Then, based on our previous study [START_REF] Le | Correlation between degradation pathway and toxicity of acetaminophen and its by-products by using the electro-Fenton process in aqueous media[END_REF], we used a concentration of ACT of 10 mg L -1 , i.e. 6.36 mgC L -1 to monitor the degree of mineralization of ACT in the real effluents and to prove the interest and the efficiency of the method coupling electro-Fenton process and anodic oxidation.

Three-dimensional fluorescence excitation-emission matrix (3DEEM) is increasingly employed to understand dissolved and colloidal organic matter (DCOM) evolution and distinguish the effects of different types of DCOM on wastewater treatment technologies (i.e., membrane fouling, AOP scavenging, and disinfection by-product formation) [START_REF] Henderson | Evaluation of effluent organic matter fouling in ultrafiltration treatment using advanced organic characterisation techniques[END_REF][START_REF] Henderson | Fluorescence as a potential monitoring tool for recycled water systems: a review[END_REF][START_REF] Hao | Use of three-dimensional excitation and emission matrix fluorescence spectroscopy for predicting the disinfection by-product formation potential of reclaimed water[END_REF].

A recent study also demonstrated that 3DEEM can be used to distinguish proteins from biopolymers and humic substances and to quantify building blocks, with potential use as an on-line indicator to describe DOM fate and behavior [START_REF] Jacquin | Three-dimensional excitation and emission matrix fluorescence (3DEEM) for quick and pseudo-quantitative determination of protein-and humic-like substances in full-scale membrane bioreactor (MBR)[END_REF].

The performance of the different EAOPs was first evaluated in the synthetic effluent. The effectiveness of the EF/AO process was then evaluated in terms of ACT degradation and TOC removal over time for all secondary effluents. The evolution of three dimensional excitation-emission matrix (3DEEM) was also assessed during the process in order to characterize dissolved and colloidal organic matter (DCOM) from wastewater treatment effluents, raise the understanding of reaction mechanisms in such complex effluent and more especially the interactions between DCOM and EAOP efficiencies. The evolution of the toxicity of the three effluents was evaluated using the Microtox ® method. Finally, energy consumption was calculated according to the objective to achieve (degradation, mineralization, toxicity removal).

Materials and Methods

Chemicals

All chemicals were reagent grade purchased from Sigma Aldrich (acetaminophen, Na2SO4, trifluoroacetic acid, acetonitrile) or Alfa Aesar (Fe2SO4,7H2O). Synthetic solutions were prepared with pure water (Milli-Q, Millipore Co. Ltd, resisitivity > 18.2 MΩ cm).

Secondary effluents from various wastewater plants

Three different effluents were used in this study. Two effluents were provided by two municipal wastewater treatment plants in the area of Montpellier, France: the first uses conventional activated sludge process (medium organic loading rate) and the second one uses MBR (continuous aeration, chlorinated PES UF membrane (Kubota, Japan)), they are referenced MWWTP and MWWTP MBR respectively. The third effluent was from a lab-scale MBR treating real source-separated urine and is named Urine MBR. A 4-L submerged MBR, equipped with a flat-sheet microfiltration membrane module made of PVDF (A3 Company, USA), was fed with fresh urine for a period of 120 days [START_REF] Jacquin | Link between dissolved organic matter transformation and process performance in a membrane bioreactor for urinary nitrogen stabilization[END_REF].

More details concerning this effluent could be found in a previous study [START_REF] Jacquin | Link between dissolved organic matter transformation and process performance in a membrane bioreactor for urinary nitrogen stabilization[END_REF]. All samples have been collected at stationary phase (stable operating parameters and stable removal efficiencies) of the systems and the main characteristics of the three effluents are presented in table 1.

Electrochemical setup

Experiments were performed using an undivided 500-mL single cylindrical glass with a two-electrode system. Ti4O7 plate (for EF/AO and AO/H2O2 processes) and platinised titanium mesh (Ti/Pt) (for EF process) anodes with a surface area of 24 cm 2 (4 cm x 6 cm), were provided by Saint Gobain Research Provence (France) and Goodfellow SARL (France) respectively. The Ti4O7 film was synthesized as follow: a mixture of TiOx (x<2) particles in the range 20-60 µm prepared as described in [START_REF] Ganiyu | Sub-stoichiometric titanium oxide (Ti4O7) as a suitable ceramic anode for electrooxidation of organic pollutants: a case study of kinetics, mineralization and toxicity assessment of amoxicillin[END_REF] was used to make a plasma coating on a Ti substrate (4 cm x 6 cm) pretreated by sandblasting to create a rough surface. In the conditions described in [START_REF] Ganiyu | Sub-stoichiometric titanium oxide (Ti4O7) as a suitable ceramic anode for electrooxidation of organic pollutants: a case study of kinetics, mineralization and toxicity assessment of amoxicillin[END_REF], a continuous thick film of 100 µm and 95% density was obtained. X-ray diffraction of the prepared electrode shows that the main phase was Ti4O7 with the presence of TiO2 rutile and Ti3O5. The typical electronic conductivity of the plasma-sprayed coating, determined by the four-probes method, is 10 -3 Ω cm. Carbon-felt cathodes with a surface area of 126 cm 2 (21 cm x 6 cm) and thickness of 0.5 cm, were provided by Alfa Aesar. In all experiments, the cathode covered the inner wall of the cell while the anode was in the center, similarly to the configuration reported in several articles [START_REF] Brillas | Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry[END_REF][START_REF] Oturan | Electrocatalytic destruction of the antibiotic tetracycline in aqueous medium by electrochemical advanced oxidation processes: Effect of electrode materials[END_REF][START_REF] Ganiyu | Electrochemical mineralization of sulfamethoxazole over wide pH range using Fe II Fe III LDH modified carbon felt cathode: Degradation pathway, toxicity and reusability of the modified cathode[END_REF][START_REF] Sirés | Electro-Fenton degradation of antimicrobials triclosan and triclocarban[END_REF][START_REF] Oturan | Reaction sequence for the mineralization of the short-chain carboxylic acids usually formed upon cleavage of aromatics during electrochemical Fenton treatment[END_REF][START_REF] Olvera-Vargas | Electro-Fenton and solar photoelectro-Fenton treatments of the pharmaceutical ranitidine in pre-pilot flow plant scale[END_REF]. The agitation and homogenization of the solution was carried out using a magnetic PTFE stirrer during electrolysis. Experiments were carried out at room temperature with 250 mL of secondary effluents from various wastewater plants doped with 10 mg L -1 of ACT as well as with the same volume of a synthetic solution containing 10 mg L -1 of ACT in 50 mM of Na2SO4 as supporting electrolyte. The solutions were continuously saturated with O2 by bubbling compressed air at atmospheric pressure at about 1 L min -1 . Iron (II) (0.2 mM) was added as catalyst to perform EF and EF/AO processes. . Additions of aliquots of a commercial concentrated solution of sulfuric acid (Sigma Aldrich 96% pure) were done to adjust the pH at 3. The ionic strength (I) was different for the four media. For the synthetic effluent, I was fixed by the 50 mM of Na2SO4 used as electrolyte (I = 0.15). For Urine MBR, MWWWTP and MWWTP MBR effluents, I was determined from the ionic concentration of salts listed in

ACT quantification

Concentrations of ACT and its by-products were analyzed by high-performance liquid chromatography (Waters 717 Autosampler and Waters 616 Pump) with a reverse phase C18 column (Thermo Scientific), thermostated at 30 °C, coupled with a UV detector (photodiode array detector Waters 2996). The injection volumes were 20 µL and the mobile phase was a mixture of water and acetonitrile, with 0.1% (v/v) of trifluoroacetic acid. The flow rate was 1 mL min -1 . The system was operated under gradient elution mode in order to ensure the proper separation of acetaminophen and p-benzoquinone (degradation by-product).

TOC analysis

The total organic carbon (TOC) analysis was performed using TOC-L Shimadzu 1.05.00 analyzer. Total carbon and inorganic carbon were analyzed and TOC was obtained from the difference between total carbon and inorganic carbon.

Characterization of the natural organic matter by 3DEEM

Three-dimensional excitation and emission matrix fluorescence (3DEEM) using a Perkin-Elmer LS-55 spectrometer (USA) was used for characterizing DCOM. For 3DEEM analysis, samples were filtered through 1.2 µm glass microfiber filters (Whatman GF/C) and diluted by a factor 60. The procedure reported by [START_REF] Jacquin | Three-dimensional excitation and emission matrix fluorescence (3DEEM) for quick and pseudo-quantitative determination of protein-and humic-like substances in full-scale membrane bioreactor (MBR)[END_REF]

Acute toxicity assessments

The Microtox ® bacterial toxicity test was used as an indicator of the acute toxicity of the effluents during the electrochemical treatment. Bacterial luminescence of Vibrio fisheri is directly related to cellular respiration and cell activity. Thus, this method is based on the measure of the inhibition of Vibrio fisheri luminescence. Samples were diluted at 45% by adding a 22% NaCl solution. The pH of all samples was adjusted between 6.5 and 7.5

with sodium hydroxide (NaOH, 1N). Measurements and data extraction were performed as reported in the study of Le et al. [START_REF] García-Gómez | Experimental design methodology applied to electrochemical oxidation of carbamazepine using Ti/PbO2 and Ti/BDD electrodes[END_REF].

Energy consumption

An important concern in EAOPs is to know the energy consumption (EC), expressed as kWh per m -3 and calculated from Eq. 5.

EC kWh. m

-3 = × × ( 5
)
Where Ecell is the average cell voltage (V), I the applied current (A), t the duration of electrolysis (h), V the volume of treated solution (m -3 ).

Results and Discussion

Degradation and mineralization of ACT

The degradation of ACT was first studied for the synthetic solution using the three different EAOPs: EF, AO/H2O2 and EF/AO. The electrolyses were carried out at 250 mA for 8 hours using the electrochemical cell previously described. The performance of the treatment was evaluated by following TOC removal (Table 2). Results showed that the highest TOC removal was obtained using AO/EF.

The higher TOC removal rate achieved by the EF/AO process can be explained by the action of hydroxyl radicals produced both at the surface of the Ti4O7 anode and in the bulk solution from the Fenton's reaction. The use of non-active anodes, such as Ti4O7 or boron-doped diamond (BDD), instead of active anodes such as mixed metal oxides (MMO) and dimensionally stable anodes (DSA), allows for a better degradation of organic substances and improves the mineralization of solutions. In fact, several studies have reported a better removal of pharmaceutical residues like propanol, ibuprofen, carbamazepine, venlafaxine and estradiol when using non-active anodes. The non-active anodes have a great potential for generating hydroxyl radicals which are powerful oxidants that are able to oxidize pollutants present in wastewater until their complete mineralization (transformation into CO2) [START_REF] Sires | Electrochemical advanced oxidation processes: today and tomorrow. A review[END_REF][START_REF] Ouarda | Synthetic hospital wastewater treatment by coupling submerged membrane bioreactor and electrochemical advanced oxidation process: Kinetic study and toxicity assessment[END_REF][START_REF] Ganiyu | Use of Sub-stoichiometric Titanium Oxide as a Ceramic Electrode in Anodic Oxidation and Electro-Fenton Degradation of the Beta-blocker Propranolol: Degradation Kinetics and Mineralization Pathway[END_REF][START_REF] Bejan | Mechanistic investigation of the conductive ceramic Ebonex® as an anode material[END_REF].

In this study, several current intensities were also tested (100, 250 and 300 mA) for treatment of the synthetic solution (data not shown). The best results, in terms of TOC removal, were obtained using a current intensity of 250 mA. Similar results were reported by Komtchou et al [START_REF] Komtchou | Removal of carbamazepine from spiked municipal wastewater using electro-Fenton process[END_REF] during the electrolysis of carbamazepine where the electrodes surface was 113 cm 2 . The highest removal rates were observed with 200 mA compared to 500, 1000 and 2000 mA. Generally, increasing the current intensity has a positive effect on electrochemical degradation of pollutants because a greater amount of hydroxyl radicals is produced when applying higher current intensities. However, researchers have reported that increasing current intensity usually decreases the energy-efficiency of the process because side reactions (mainly oxygen and hydrogen evolution reactions) and parasitic reactions (hydroxyl radical dimerization, reaction of hydroxyl radicals with excess H2O2) are also promoted [START_REF] Quand-Même | Electrochemical oxidation of amoxicillin in its pharmaceutical formulation at boron doped diamond (BDD) electrode[END_REF]. Therefore, for the next experiments, a current intensity of 250 mA was chosen as the optimal current intensity for simultaneous regeneration of Fe 2+ , production of H2O2 at the cathode and generation of hydroxyl radicals at the anode. The EF/AO process, identified as the most effective process, was then applied for tertiary treatment of ACT-doped secondary effluents from three different wastewater treatments plants.

As shown in figure 1, a rapid decrease of TOC was observed within the first hour of electrolysis for all the different wastewaters. In fact, a removal rate of 64 ± 0.5%, 56 ± 5.3 % and 30 ± 0.5 % was obtained for MWWTP MBR (curve B), MWWTP (curve C) and Urine MBR (curve A), respectively. At longer times, mineralization rate slowly increased, which is usually ascribed to the formation of degradation by-products such as shortchain carboxylic presenting lower reaction kinetics with hydroxyl radicals. Similar behaviors were reported in previous studies regarding the removal of propranolol and acetaminophen by EAOPs [START_REF] Ganiyu | Use of Sub-stoichiometric Titanium Oxide as a Ceramic Electrode in Anodic Oxidation and Electro-Fenton Degradation of the Beta-blocker Propranolol: Degradation Kinetics and Mineralization Pathway[END_REF][START_REF] Sirés | Paracetamol Mineralization by Advanced Electrochemical Oxidation Processes for Wastewater Treatment[END_REF]. The authors reported a similar phenomenon with an initial rapid drop of the TOC concentration during the first hour, which was then followed by a continuous slow decrease over time. The lower mineralization rate was achieved with Urine MBR (curve A), which reaches only 46 ± 1 % of TOC removal after 8h of electrolysis, most probably because of the presence of DCOM recalcitrant to oxidation. Under the same experimental conditions, higher mineralization (curve B and C) was obtained for the effluents from MWWTP MBR and MWWTP with a TOC removal of about 76 ± 1 % and 90 ± 3 %, respectively. It is important to note that the initial concentration of TOC in Urine MBR effluent doped with ACT was 80 mg C L -1 while for the other two effluents this concentration was 22 mg C L -1 and 12 mg C L -1 for MWWTP and MWWTP MBR, respectively. Thus, 20 to 40% of initial TOC was ascribed to ACT for these latter, while for Urine MBR, 94% of initial TOC was ascribed to the residual DCOM that was not eliminated during the treatment of urine by the MBR.

Figure 2 shows the degradation of 10 mg L -1 of ACT over electrolysis time, for the three secondary effluents and the synthetic solution. A rapid drop of ACT concentration is observed for the synthetic solution and for the three doped secondary effluents. In fact, a complete removal of ACT was achieved after 8, 8, 15 and 20 min of treatment for synthetic solution, MWWTP MBR, MWWTP and Urine MBR respectively with then a similar kinetic for the synthetic solution and MWWTP MBR (Apparent first order kinetic rate: 0.640 min -1 and 0.629 min -1 respectively) and lower kinetics for MWWTP and Urine MBR (0.374 min -1 and 0.314 min -1 respectively). Brillas et al. have studied the anodic oxidation of ACT (with concentrations up to 1 g.L -1 ) in a cell with a BDD or Pt anode and a graphite cathode [START_REF] Brillas | Mineralization of paracetamol in aqueous medium by anodic oxidation with a boron-doped diamond electrode[END_REF]. Results showed a complete removal of acetaminophen after 120 to 240 min of electrolysis with the BDD anode and after 240 to 360 min with the Pt anode (pseudo-first order rate constant was 0.013 +/-0.002 min -1 .

In both case, the time required for ACT elimination was much longer than in our study.

The faster degradation of ACT can be explained by (i) the lower concentration of ACT in our study and (ii) the production of large amount of both heterogeneous and homogeneous hydroxyl radicals from anodic oxidation at the Ti4O7 anode [START_REF] Ganiyu | Use of Sub-stoichiometric Titanium Oxide as a Ceramic Electrode in Anodic Oxidation and Electro-Fenton Degradation of the Beta-blocker Propranolol: Degradation Kinetics and Mineralization Pathway[END_REF] and electrochemically induced Fenton's reaction in the bulk solution (Eq. ( 1)). Besides, Sirès et al. [START_REF] Sirés | Paracetamol Mineralization by Advanced Electrochemical Oxidation Processes for Wastewater Treatment[END_REF] have studied the degradation of ACT solution (157 mg.L -1 ) by electro-Fenton and photo-electro-Fenton processes using a cell with a Pt anode and an O2 diffusion cathode for H2O2 electro-generation. A complete removal was observed after 20-25 min of process. The authors confirmed that ACT was mainly eliminated by the hydroxyl radicals during electrolysis and that UVA lighting did not contributed significantly. The electrolysis time required for the total removal of acetaminophen in the aqueous solution reported was also longer than that obtained in this study (8 min vs 20-25 min).

It might be ascribed to (i) the lower concentration level of ACT in our study and (ii) the use of a non-active anode such as Ti4O7, which has a much greater potential than Pt for generating large amounts of hydroxyl radicals at the anode surface [START_REF] Ganiyu | Use of Sub-stoichiometric Titanium Oxide as a Ceramic Electrode in Anodic Oxidation and Electro-Fenton Degradation of the Beta-blocker Propranolol: Degradation Kinetics and Mineralization Pathway[END_REF][START_REF] Bejan | Mechanistic investigation of the conductive ceramic Ebonex® as an anode material[END_REF].

The variation in the required time for the complete removal of ACT in the different effluents is mainly due to the composition of the treated water as well as to the process applied for the secondary treatment (activated sludge for WWTP effluent, MBR for WWTP MBR effluent and for Urine MBR effluent). Hydroxyl radicals are known to be non-selective and may react with DCOM (natural organic matter, microbial residues), inorganic species, suspended solids, which constitutes radical scavengers and can increase the time required for ACT degradation. During the treatment of MWWTP MBR effluent, it was not observed any significant effect of the real effluent matrix on the degradation rate of ACT. It might be ascribed to the high effectiveness of the MBR used as secondary treatment, which strongly reduces the presence of particulate and colloidal radical scavengers in the solution. Besides, a significant reduction of the degradation kinetic of ACT was observed in MWWTP and Urine MBR effluents, most probably because of the higher TOC and MLSS (for MWWTP) content that promotes hydroxyl radical scavenging. However, from the much higher TOC content of Urine MBR effluent, a stronger adverse effect could have been expected. The similar adverse effect observed from both MWWTP and Urine MBR might be ascribed to the lower reactivity of the organic compounds in the Urine MBR effluent, as it was observed from the lower TOC decrease. Thus, both the amount and the nature/reactivity of the organic compounds must be considered for understanding hydroxyl radical scavenging. Moreover, these results also highlight that the effectiveness of the secondary treatment might be crucial for the removal of organic micropollutants during tertiary treatment by EAOP.

Evolution of the toxicity of treated effluents

The evolution of the toxicity during the treatment of ACT-doped real effluents by EF/AO was evaluated by the Microtox ® method (Figure 3). The three raw effluents doped with ACT showed relatively low luminescence inhibition to V. fischeri bacteria (< 10%) which is in good accordance with EC50 observed in the literature for paracetamol on V. fischeri with values around 100 mg/L [START_REF] Nunes | Toxic potential of paracetamol to freshwater organisms: a headache to environmental regulators?[END_REF]. However, luminescence inhibition increased rapidly after 1 hour of treatment to reach 85% (+-1%) and 65% (+-5%), for MWWTP MBR and Urine MBR effluents respectively. This strong increase in bacterial inhibition can be attributed to the formation of degradation by-products that may be more toxic than parent compounds [START_REF] Dirany | Study of the toxicity of sulfamethoxazole and its degradation products in water by a bioluminescence method during application of the electro-Fenton treatment[END_REF][START_REF] Dirany | Electrochemical treatment of the antibiotic sulfachloropyridazine: kinetics, reaction pathways, and toxicity evolution[END_REF]. The stronger increase of toxicity in Urine MBR effluent might be ascribed to the formation of further toxic N-containing and halogenated organic by-products because of the higher N and Cl content of this effluent (Table 1).

Luminescence inhibition may also come from the presence of residual oxidants generated during electrolysis such as HClO resulting from the oxidation of Cl -ions present in the effluents. Subsequently, luminescence inhibition decreased during electrolysis to reach around 20% (+-5%) for Urine MBR after 8 hours of treatment, while a quasi-zero (+-12%) luminescence inhibition was observed for MWWTP MBR after 4 hours of process. The slower decrease of the toxicity in the Urine MBR effluent might be explained by the formation of toxic degradation by-products that are more recalcitrant to oxidation. This phenomenon is consistent with the slow decrease of the TOC between 1 and 8 h of treatment of this effluent (Figure 1). Such increase in the toxicity of a ACT solution was also reported by Hamdi El Najjar et al. [START_REF] El Najjar | Kinetics of paracetamol oxidation by ozone and hydroxyl radicals, formation of transformation products and toxicity[END_REF] during the oxidation by ozone and hydroxyl radicals. The authors observed a gradual increase in toxicity up to nearly 64% inhibition of luminescence bacteria after less than 20 minutes because of the formation of degradation by-products more toxic than ACT [START_REF] Roberts | The occurrence of selected pharmaceuticals in wastewater effluent and surface waters of the lower Tyne catchment[END_REF]. Recently, Le et al. identified the aromatic compounds 2-hydroxy-4-(N-acetyl) aminophenol, 1,4benzoquinone, benzaldehyde and benzoic acid as the toxic sub-products formed from ACT degradation at the first stage of the electrochemical treatment and responsible of toxicity enhancement [START_REF] García-Gómez | Experimental design methodology applied to electrochemical oxidation of carbamazepine using Ti/PbO2 and Ti/BDD electrodes[END_REF].

Luminescence inhibition of the MWWTP effluent during EF/AO process shows a singular behavior. The high concentration of MLSS of this effluent may explain why the toxicity was very low during the first six hours of treatment (Table 1). Indeed, particulate organic matter might be preferentially oxidised without production of toxic by-products.

After 6 hours of treatment of MWWTP effluent, oxidation of soluble organic matter (like ACT) and Cl-ions might promote a slight increase of luminescence inhibition.

Overall, these results highlight that the nature and composition of the effluent might be also crucial parameters for the evolution of the toxicity during a tertiary treatment based on EAOPs.

Evolution of dissolved and colloidal organic matter by 3DEEM

3DEEM has been reported to be a useful tool for characterization of dissolved and colloidal organic matter (DCOM) in real WWTP effluents [START_REF] Jacquin | Three-dimensional excitation and emission matrix fluorescence (3DEEM) for quick and pseudo-quantitative determination of protein-and humic-like substances in full-scale membrane bioreactor (MBR)[END_REF] and is used in this study to follow the scavenging effect of DCOM during EAOPS (Figures 4 and5).

The initial MWWTP and MWWTP MBR effluents show similar 3DDEM spectrums and percentages of volume of fluorescence of the three different zones. Colloidal proteins fluorophores from zone I ' , which are known to exert quenching effect on AOPs [START_REF] Maghsoodi | Emerging investigator series: photocatalysis for MBR effluent post-treatment: assessing the effects of effluent organic matter characteristics[END_REF],

represent around 54% of fluorescent DCOM of those effluents. Urine MBR has the higher content of DCOM fluorophores (higher 3DEEM intensities) because of its higher TOC concentrations (Table 1) and its DCOM composition is relatively well distributed between the three different fluorescence zones.

As shown on figure 5

Energy consumption

Energy consumption (in term of kWh m -3 ) was calculated for each different effluent according to different objectives. For example, the energy required for >99% degradation of ACT was 1.1, 1.9 and 2.0 kWh m -3 for MWWTP MBR, Urine MBR and MWWTP effluents, respectively. By comparison, the average energy consumption of a conventional MWWTP is about 0.6-1 kWh m -3 [START_REF] Sid | Cost minimization in a full-scale conventional wastewater treatment plant: associated costs of biological energy consumption versus sludge production[END_REF]. The energy consumption can be increased to 2 kWh m -3 for a MBR. Overall, the higher energy consumption related to the use of MBR could be almost offset by the cost reduction during the tertiary treatment related to the better quality of the secondary effluent. However, >99% of degradation should not be the only objective because of the potential toxicity of by-products. For an objective related to the acute toxicity (<5%) or the percent of mineralization (>80%), the energy consumption calculated for the MWWTP MBR effluent was strongly increased to 33 and 49 kWh m -3 . Therefore, it is required to develop more cost-effective electrochemical processes for such application. In fact, the low concentration of pollutants in secondary effluents strongly reduces the efficiency of electro-oxidation processes because of mass transport limitations [START_REF] Trellu | Electro-oxidation of organic pollutants by reactive electrochemical membranes[END_REF]. Several studies currently focus on different ways to overcome this drawback, including pre-concentration of effluents by combination with the adsorption process [START_REF] Trellu | Regeneration of activated carbon fiber by the electro-Fenton process[END_REF] or using reactive electrochemical membranes for convection-enhanced mass transport of pollutants during AO. Another approach related to energy-consumption optimization is to develop milli-or microfluidic reactors in order to reduce the interelectrode distance [START_REF] Mousset | Advanced Electro-Oxidation with Boron-Doped Diamond for Acetaminophen Removal from Real Wastewater in a Microfluidic Reactor: Kinetics and Mass-Transfer Studies[END_REF].

Conclusions

The electrochemical oxidation of ACT in different secondary effluents was successfully realized using EF/AO process with carbon felt cathode and Ti4O7 anode. The complete elimination of acetaminophen was achieved after 20 minutes of electrolysis for the three types of effluents (MWWTP, MWWTP MBR and Urine MBR). However, lower adverse effect from the organic matrix was observed for the MWWTP MBR effluent because of the higher effectiveness of the secondary treatment (MBR). A relatively high removal of TOC was observed for MWWTP and MWWTP MBR effluents (90 ± 15 %, 76 ± 3.8 % respectively) after 8h treatment. However, only 46 ± 1.3 % of TOC removal was achieved for Urine MBR effluent under the same conditions. An increase in acute toxicity during ACT oxidation was also observed. A higher increase was observed in Urine MBR effluent.

This increase could be attributed to the formation of ACT degradation by-products during electrolysis. Then, further oxidation of these by-products reduced the acute toxicity of the solution and Vibrio fisheri luminescence inhibition reached quasi-zero (+-12%) for MWWTP MBR and MWWTP. However, toxicity reduction was lower for Urine MBR and luminescence inhibition reached 20% (+-5%) at the end of the treatment.

Lower TOC removal and higher toxicity during the treatment of Urine MBR might be ascribed to the formation of N-containing and halogenated organic by-products with higher toxicity and lower reactivity for further oxidation by oxidant species generated during the treatment. This hypothesis was confirmed by 3DEEM results, which highlight the formation of recalcitrant degradation by-products during the treatment of Urine MBR. Such phenomenon was not observed during the treatment of MWWTP and MWWTP MBR effluents. Overall, EF/AO process seems to be an effective technology for the treatment of ACT in municipal sewage effluents. Next scientific and technological challenges are related to the treatment of effluents without pre-treatment for pH adjustment. This approach is possible by coupling anodic oxidation with heterogeneous electro-Fenton process on carbon felt modified with iron based compounds like Layer Double Hydroxyde (LDH) as we recently proposed in [START_REF] Ganiyu | Electrochemical mineralization of sulfamethoxazole over wide pH range using Fe II Fe III LDH modified carbon felt cathode: Degradation pathway, toxicity and reusability of the modified cathode[END_REF] and [START_REF] Yang | Highly efficient and stable FeIIFeIII LDH carbon felt cathode for removal of pharmaceutical ofloxacin at neutral pH[END_REF]. It will be also necessary to improve the cost-efficiency of these processes, particularly for the treatment of effluents containing a cocktail of pollutants with concentration in the order of 100 μg L -1 . For such low concentration of pollutants, REM operated in flow-through mode might be one of the most promising solutions.

  Ti4O7 + H2O → Ti4O7 ( • OH) + H + + e - (Eq. 4) WWTP secondary effluents were sampled from three different wastewater treatment systems, including an effluent from a full-scale WWTP using activated sludge ([TOC°]= ([TOC°]= 7 mgC L -1 ) and a third from a lab-scale membrane bioreactor (MBR) treating real source-separated urine ([TOC°]= 75 mgC L -1 ). The three effluents were then doped with acetaminophen (ACT) as indicator of pharmaceutical pollutant. Results were also compared with a synthetic effluent containing only ACT and Na2SO4 as electrolyte. ACT (also known under the name of paracetamol) was selected as pharmaceutical contaminant because it is usually detected in wastewater treatment plants, natural water resources and rivers with concentrations varying from 6 to 65 μg L -1 [21-23].

  was used for fluorescence spectra acquisition and data extraction based on the division of the 3D fluorescence spectra representation into three different areas corresponding to different groups of fluorophores [33]. Jacquin et al. have recently developed a semi-quantitative method for the assessment of the amount of fluorophores associated with DCOM in real effluents [27]. This method is based on the calculation of the volume of fluorescence in zones I', II' and III' and was applied in the present work. Zone III' volume of fluorescence (HA+FAlike fluorophores) is correlated to humic substances concentration (MW ≈ 1000 Da) and degradation by-products from humic substances (hydrophobic compounds, MW ≈ 300-500 Da). Similarly, a correlation exists between the volume of fluorescence of zone II' (SMP-like fluorophores) and the concentration of proteins from biopolymers (MW ≈ 20,000 -7.5 x 10 11 Da). Finally, volume of fluorescence of zone I' is associated with colloidal proteins, which are the bigger and more hydrophilic compounds present in DCOM.

  , molecules with fluorophores of zone I' are less degraded during EAOPs than fluorophores associated with zones II' and III'. In previous studies, we have already observed that EAOPs lead to a slow decrease of colloidal proteins from zone I' due to lower availability of colloids for reaction with OH in the liquid phase. Besides, a fast decrease of fluorophores of zone II' and III' was observed because of the degradation and mineralization of these compounds with an aromatic structure that reacts quickly with OH and active chlorine[20; 42]. Humic substances found in WWTP effluent are mainly biorefractory (because they have not been degraded during secondary treatment) with high stability and aromaticity. Those substances, known as potentially cytotoxic, are transformed into more biodegradable and small-molecule DCOM (fulvic-like substances and protein-like substances) by oxidants generated during EAOPs[START_REF] Sid | Cost minimization in a full-scale conventional wastewater treatment plant: associated costs of biological energy consumption versus sludge production[END_REF]. Fluorophores from zone I' and III' present in Urine MBR effluent shows an increase during the first two hours of treatment. High Cl and N concentration in Urine MBR led to the quick formation of by-products, which have some fluorophores in those zones. Then, between two to eight hours of treatment, fluorophores from zone I' decreased, while fluorophores from zone III' were stable. It means that by-products from these two zones have different oxidation rates during EAOPs. The stability of zone III' is consistent with the evolution of TOC removal (Figure 1), which is fast during the first hour of treatment and shows a slight decrease after this first hour.

Figure 1 :

 1 Figure 1: Mineralization of acetaminophen-doped ([ACT]0 = 10 mg L -1) real effluents by

Figure 2 :

 2 Figure 2: Degradation of acetaminophen ([ACT]0 = 10 mgC L -1 ) in synthetic solution

Figure 3 :

 3 Figure 3: Mircrotox® analysis. Evolution of Vibrio fisheri luminescence inhibition (15 min

Figure 4 -

 4 Figure 4 -Initial 3D fluorescence map of acetaminophen-doped real effluents and

Figure 5 -

 5 Figure 5 -Volume of fluorescence evolution (zones I', II', III') during the treatment of

Tableau 2 :

 2 Comparison of TOC removal of the synthetic solution of ACT after 8 h of treatment by electro-Fenton (EF), anodic oxidation/H2O2 (AO/H2O2) and electro-Fenton/anodic oxidation (EF/AO) processes. ACT0 = 10 mg L -1 ; I = 250 mA ; [Na2SO4] = 50 mM ; [Fe 2+ ]0 = 0.2 mM or 0 mM (for AO/H2O2) ; pH = 3.

Table 1

 1 (taking the hypothesis of monovalent cations as counter-ions). Then I=0.21 for Urine MBR; I=0.0075 for MWWWTP and I=0.016 for MWWTP MBR. Because

of the difference in ionic strength, the solution resistance varied from one effluent to the other. Resistance of the different systems are 19.6, 19.5, 27.2, 26.6 Ω for the synthetic solution, Urine MBR, MWWWTP and MWWTP MBR effluents respectively.

Table 1 -

 1 MWWTP, MWWTP MBR and Urine MBR effluents main characteristics σ: Conductivity ; LOQ: Limit of Quantification ; MLSS: Mixed Liquor Suspended Solid

Acknowledgements

Sincere thanks are extended to Mitacs. The work herein was financially supported Mitacs globalink programme. The authors also thank Saint Gobain CREE company for supplying the anodes.