## 2 Materials and Methods
### Anode Consumption and Sludge Characterisation
\begin{table}
\begin{tabular}{c c c c c} \hline RUN 1 (pH = 7.0; \(i\) = 0.008 A cm\({}^{-2}\)) & \multicolumn{4}{c}{Spectrum} \\ \hline  & Element & 1 & 2 & 3 & 4 \\ \hline C & 5.14 & 9.35 & 5.64 & 7.77 \\ O & 57.31 & 55.53 & 56.81 & 56.00 \\ Na & 0.13 & 0.19 & 0.10 & 0.15 \\ Mg & & & 0.27 & 0.09 \\ Al & 36.37 & 33.88 & 35.98 & 34.61 \\ Si & 0.10 & 0.15 & 0.13 & 0.17 \\ S & 0.10 & 0.12 & 0.09 & 0.11 \\ Cl & 0.85 & 0.78 & 0.98 & 1.10 \\ Total & 100.00 & 100.00 & 100.00 & 100.00 \\ \hline  & & & & \\ \end{tabular}
\end{table} 
Table 4: SEM images and elemental composition (on marked spots) of sludge under different process conditions. The FTIR spectrum of Tartrazine (E102), a synthetic azo dye, was recorded in the range of 4000-450 cm\({}^{-1}\) (Figure 7a). The spectrum reveals characteristic absorption bands corresponding to the principal functional groups of the molecule, including sulfonate groups, azo linkages, aromatic rings, and a heterocyclic pyrazolone moiety. The analysis of the major peaks is presented below. A broad absorption band centred at 3443.7 cm\({}^{-1}\) is attributed to the stretching vibration of O-H bonds, most likely due to adsorbed moisture or hydrogen bonding interactions involving sulfonate groups. A series of intense peaks at 1633.8-1557.0 cm\({}^{-1}\) corresponds to the C=C stretching vibrations of aromatic rings (1621.9 and 1599.1 cm\({}^{-1}\)) and N=N azo stretching modes (1557.0 cm\({}^{-1}\)), reflecting the extensive conjugation between azo linkage and aromatic systems. The peak at 1478.9 cm\({}^{-1}\) is also associated with C-N stretching or symmetric azo bond vibrations. Sulfonate functional groups (-SO\({}_{3}\)-), which are important features of Tartrazine structure, are evidenced by the 1344.8 cm\({}^{-1}\) peak (S=O asymmetric stretching). An additional peak at 1123.1 cm\({}^{-1}\) further supports the presence of S-O stretching modes, commonly observed in sulfonated aromatic compounds. Bands in the region of 1174.2-1106.1 cm\({}^{-1}\) are primarily attributed to para-substituted aromatic compounds by sulphoxyl groups, while adsorption lines from 869.2 to 564.5 cm\({}^{-1}\) are related to di-substituted aromatic compounds [5]. FTIR spectra of sludge after TY removal through electrocoagulation (Figure 7b) show spectroscopic changes. Electrocoagulation causes destabilisation of tartrazine due to the formation of Al\({}^{3+}\) ions, which hydrolyze to Al(OH)\({}_{3}\). This hydroxide adsorbs dye molecules, resulting in shifts and/or the disappearance of characteristic vibrations (particularly those of azo and sulfonate groups). FTIR spectra indicate that interactions occur between the functional groups of tartrazine and aluminium hydroxide, including the possibility of complexation, adsorption, and hydrophobic interactions. The disappearance of the azo bond (-N=N-) suggests that, in addition to adsorption, partial degradation of the dye molecule also takes place. Similar results for azo dye sludge were found in the literature [8; 12]. \begin{table}
\begin{tabular}{c c c c c} \hline RUN 7 (pH = 7.0; \(i\) = 0.024 A cm\({}^{-2}\)) & & Spectrum & & \\ \cline{2-5}  & Element & 1 & 2 & 3 & 4 \\ \cline{2-5}  & C & 17.83 & 50.46 & 51.15 & 29.97 \\  & O & 48.65 & 38.00 & 35.61 & 36.66 \\  & Na & 0.76 & 0.26 & 0.11 & 0.05 \\  & Al & 30.78 & 10.78 & 12.65 & 32.61 \\  & S & 0.22 & 0.31 & 0.37 & 0.25 \\  & Cl & 1.76 & 0.19 & 0.11 & 0.46 \\  & Total & 100.00 & 100.00 & 100.00 & 100.00 \\ \hline RUN 12 (pH = 3.0; \(i\) = 0.024 A cm\({}^{-2}\)) & & Spectrum & & \\ \cline{2-5}  & Element & 1 & 2 & 3 & \\ \cline{2-5}  & C & 60.84 & 56.14 & 54.86 & \\  & O & 28.49 & 31.23 & 30.01 & \\  & Al & 10.14 & 12.21 & 14.73 & \\  & S & 0.53 & 0.42 & 0.40 & \\  & Cl & & & 0.10 \\  & Total & 100.00 & 100.00 & 100.00 & \\ \hline \end{tabular}
\end{table} 
Table 4: _Cont._Despite the organic content of the resulting sludge, its classification as hazardous waste cannot be definitively determined without further ecotoxicological assessment in accordance with EU Directive 2008/98/EC and Regulation No.