## 2 Materials and Methods
### Chloride Ion Determination  
#### 2.4.1 Dechlorination Process  
Chemical analyses were conducted following DIN 51527 guidelines [21]. The PCB congeners were extracted in acetone/n-hexane (1:3, \(v/v\)), and then purification and separation were conducted by solid-phase extraction [22]. The polarographic method with a liquid mercury electrode was used for determination of chloride ions. The ECO-TRIBO Polarograf software from the Czech company PolaroSensor was used [23]. The main electrode was a mercury drop electrode, and the reference electrode was a silver Ag/AgCl electrode, which was separated from the test solution by a salt bridge filled with 1M KNO\({}_{3}\), so that the sample was not contaminated with chloride ions. The electrode parameters used in the determination of chlorides are compiled in Table 2. \begin{table}
\begin{tabular}{c c c} \hline
**PCB Congener** & **Retention Time [min]** & \multicolumn{2}{c}{**The Congener**} \\  & & **Concentration [ppm]** \\ \hline
2,6-dichlorobifenyl & 25.104 & 368.2 \\
4,4\({}^{\prime}\)-dichlorobifenyl & 26.595 & 11.2 \\
2,5\({}^{\prime}\)-dichlorobifenyl & 27.135 & 36.7 \\
2,5-dichlorobifenyl & 27.445 & 652.3 \\
2,3,4-trichlorobifenyl & 28.396 & 93.1 \\
2,2\({}^{\prime}\),5-trichlorobifenyl & 29.727 & 689.2 \\
2,2\({}^{\prime}\),5\({}^{\prime}\)-trichlorobifenyl & 29.827 & 573.4 \\
2,4,6-trichlorobifenyl & 30.237 & 42.3 \\
2,4,4\({}^{\prime}\)-trichlorobifenyl & 30.607 & 691.0 \\
2,3\({}^{\prime}\),4-trichlorobifenyl & 31.658 & 73.3 \\
2,4\({}^{\prime}\),5-trichlorobifenyl & 31.758 & 25.3 \\
2,4\({}^{\prime}\),4\({}^{\prime}\)-trichlorobifenyl & 32.128 & 691 \\
2,2\({}^{\prime}\),3,5\({}^{\prime}\)-tetrachlorobifenyl & 32.588 & 630.3 \\
2,3,4-trichlorobifenyl & 32.878 & 373.1 \\
2,2\({}^{\prime}\),4,6-tetrachlorobifenyl & 33.139 & 51.2 \\
2,2\({}^{\prime}\),5\({}^{\prime}\),6-tetrachlorobifenyl & 33.479 & 19.3 \\
2,3\({}^{\prime}\),4\({}^{\prime}\)-tetrachlorobifenyl & 33.879 & 163.2 \\
2,2\({}^{\prime}\),5\({}^{\prime}\),5\({}^{\prime}\)-tetrachlorobifenyl & 34.079 & 123.8 \\
2,3\({}^{\prime}\),5\({}^{\prime}\)-tetrachlorobifenyl & 34.209 & 47.7 \\
2,2\({}^{\prime}\),4\({}^{\prime}\),4\({}^{\prime}\)-tetrachlorobifenyl & 34.289 & 145.3 \\
2,2\({}^{\prime}\),5\({}^{\prime}\),6-tetrachlorobifenyl & 34.810 & 131.2 \\
2,2\({}^{\prime}\),3,4\({}^{\prime}\),5\({}^{\prime}\)-heptachlorobifenyl & 35.859 & 134.3 \\
2,3\({}^{\prime}\),4\({}^{\prime}\)-tetrachlorobifenyl & 36.910 & 41.3 \\
2,3\({}^{\prime}\),4\({}^{\prime}\),4\({}^{\prime}\)-tetrachlorobifenyl & 37.641 & 91.7 \\
2,2\({}^{\prime}\),3,4\({}^{\prime}\),5-tetrachlorobifenyl & 37.792 & 92.7 \\
2,3\({}^{\prime}\),4\({}^{\prime}\),5-pentachlorobifenyl & 37.891 & 62.3 \\
2,2\({}^{\prime}\),3,4\({}^{\prime}\),5-pentachlorobifenyl & 38.171 & 13.2 \\
2,2\({}^{\prime}\),4\({}^{\prime}\),5\({}^{\prime}\)-heksachlorobifenyl & 38.352 & 17.1 \\
2,2\({}^{\prime}\),3,3\({}^{\prime}\),5\({}^{\prime}\)-heksachlorobifenyl & 39.542 & 12.3 \\ \hline \end{tabular}
\end{table} 
Table 1: The PCB congeners concentrations in transformer oil. \begin{table}
\begin{tabular}{c c} \hline
**Preparation** & **Potential** \\ \hline  & At rest: 0 mV \\  & Initial: 300 mV \\  & Final: \(-\)200 mV \\  & Cleaning: 0 mV \\  & 20 mV Oxidation: 0 mV \\ \hline   Drop & Impulse \\ \hline   Tap: 0.15 s & Height: 50 mV \\   Valve: 0.20 s & Width: 100 ms \\  & Measurement: 20\% to 80\% \\ \hline   Stripping & Browsing \\ \hline   Cleaning: 0 s Accumulation: 20–120 s & Speed: 20 mV/s \\   Standstill: 30 s Oxidation: 0 s & Number of cycles: 1 Standstill: 0 s \\ \hline \end{tabular}
\end{table} 
Table 2: Parameters of chloride determination carried out using the polarographic method. Chloride ions were determined in a medium containing the mixture: water/1M HNO\({}_{3}\)/CH\({}_{3}\)OH in the ratio 10/1/10 vol%. The measurement was carried out in accordance with the parameters presented in Table 2. The test sample was taken from the aqueous phase of the culture medium.