## 2 Materials and Methods
### Modification of Commercial Activated Carbons  
Pristine activated carbon (DT0) underwent plasma treatment in the installation depicted in Figure 1 in the following two types of discharges: dielectric barrier or spark.  
Both discharges were powered by a pulse current with positive polarization. The repetition rate of voltage pulses (f) was 340 Hz and 2300 Hz in the dielectric barrier discharge and spark discharge, respectively. The course of current (i) and voltage (u) over time (t) is shown in Figures 2 and 3.  
Figure 1: Scheme of the installation for the active carbon plasma treatment. 1—mass flow controller, 2—plasma reactor, 3—power supply, 4—oscilloscope, 5—current probe, 6—voltage probe.  
Figure 3: Current and voltage waveforms of spark discharge.  
Figure 2: Current and voltage waveforms of dielectric barrier discharge.  
These waveforms were recorded with a two-channel Tektronix TDS 3032B oscilloscope with Tektronix TCP202 current and Tektronix P6015A voltage probes. The power (P) of the dielectric barrier discharge was 1 W, and the spark discharge was 5.9 W. The power was calculated using Equation (1), as follows:  
\[\mathrm{P}=\mathrm{f}\cdot\int\mathrm{i}(\mathrm{t})\cdot\mathrm{u}(\mathrm{t} )\mathrm{dt} \tag{1}\]  
In both discharges, activated carbon was modified in a flow system, and a mixture of nitrogen and helium was used. The gas flow rate was 10 L/h, and the volume ratio of nitrogen to helium was 1:1. The gas flow rate was regulated by Bronkhorst mass flow controllers. Before turning on the discharge, the reactors were flushed with a mixture of nitrogen and helium for 15 min. The time of the plasma treatment of activated carbon in both reactors was the same and amounted to 30 min.  
The dielectric barrier discharge was generated in a reactor, in which the high-voltage electrode was placed centrally in a quartz tube. The inner diameter of the quartz tube was 12 mm, and the wall thickness was 1.2 mm. The quartz tube was covered from the outside with a layer of electrically conductive paint, which was a grounded electrode. The length of the grounded electrode was 190 mm. The high-voltage electrode was a stainless-steel rod with a diameter of 3.2 mm. The reactor was mounted vertically. In the lower part, there was a 5 mm layer of quartz wool, which served as a grate, in which activated carbon was placed. A diagram and photo of the dielectric barrier discharge reactor are shown in Figure 4.  
The spark discharge was generated in a reactor, in which electrodes were placed in two arms of a quartz four-piece. The internal diameter of the quartz tubes from which the four-piece was made was 12 mm. The electrodes were made of graphite rods with a diameter of 5 mm. One electrode was connected to high voltage, and the other was grounded. The distance between the electrodes was 6 mm. Activated carbon was placed 15 mm below the electrodes. The activated carbon layer was 110 mm high and rested on a quartz wool grate. A diagram and photo of a barrier discharge reactor are shown in Figure 5.  
Figure 4: Dielectric barrier discharge reactor. 1—quartz case, 2—high-voltage electrode, 3—grounded electrode, 4—active carbon, 5—quartz wool, 6—Teflon shapes for electrode centering.