# Sewage Sludge Derived Materials for CO2 Adsorption
## 2 Materials and Methods  
The sewage sludge was obtained from a local sewage treatment plant (WWTP Ramnicu Valcea, Romania) after the anaerobic digestion. From the textural point of view, the sewage sludge was similar to plasticine, having high humidity, more than 80 wt % (gravimetrically determined up to constant weight) [38]. Therefore, prior to pyrolysis process, in order to obtain the char, there were two necessary stages (steps) for the material preparation. First, the material was dried using a moisture extractor to achieve a humidity less than 5%, and further it was grounded in a ball mill (Fritsch, Idar-Oberstein, Germany), to obtain a particle size less than 250 \(\upmu\)m [38]. A previously developed experimental setup was used for the pyrolysis process [39]. The pyrolysis temperature interval was 23-450 \({}^{\circ}\)C, with a gradient of 5 \({}^{\circ}\)C/min. The temperature increased after 25 min, resulting in three products: oil, gas and char. Further, an oxidation process was applied to 20 g of solid waste (char) using a sandwich support. This setup facilitated the continuous flow of oxidizing gas (10 vol % O\({}_{2}\) in N\({}_{2}\)). The oxidation process was achieved in 4 h with a flow rate of 100 mL/min and a gradient of 10 \({}^{\circ}\)C/min up to 280 \({}^{\circ}\)C. Prior to the oxidation process, an inert atmosphere was insured in the reactor using the same flow rate of the binary gas for 30 min. After the oxidation, a chemical activation process was performed using KOH (Sigma Aldrich, Darmstadt, Germany). The oxidized \(\mathrm{SSPYCHAR}\) and alkaline hydroxide were manually grinded until the mixture became homogenous. In order to assess the influence of KOH, three mixtures were obtained: (i) \(\mathrm{SSPYCHAR}\):KOH (1:1) (further noted I_1); (ii) \(\mathrm{SSPYCHAR}\):KOH (1:2) (further noted I_2); and (iii) \(\mathrm{SSPYCHAR}\):KOH (1:3) (further noted I_3). The first type of mixture, \(\mathrm{SSPYCHAR}\):KOH (1:1), was also treated with 5M HCl (Sigma Aldrich, Darmstadt, Germany) (1 g mixture to 20 mL HCl) by using a known method [40] in order to observe the influence on the material properties (further noted I_4). The obtained solution was filtered (cellulose filter, with 25 \(\mathrm{\SIUnitSymbolMicro m}\) pore size and 580 mm \(\times\) 580 mm size-Merck KGaA, Darmstadt, Germany), and its pH was adjusted to 6. The filtrate, together with the filter paper, was placed on the watch glass and dried at 105 \({}^{\circ}\)C for 6 h in an oven (Nahita 631, Stuttgart, Germany) without ventilation. The obtained materials, due to poor content of volatiles, were pelletized by using starch powder as a binder (9:1). The pellets were obtained at 1 g weight by using a pelletizer (IKA, Konigswinter, Germany) and dried at 105 \({}^{\circ}\)C for 4 h.  
After the pelletizing process, a thermal activation was performed by introducing the pellets into the reactor [17] placed into a calcination oven equipped with PID panel for temperature control. The process was carried up to 750 \({}^{\circ}\)C, with an increment of 10 \({}^{\circ}\)C/min, under inert atmosphere, by using a flow rate of 100 mL/min N\({}_{2}\) (99.999 vol.%, Messer, Bucharest, Romania). The obtained pellets were weighted prior to and after these processes, and a mass loss of 30% was noted. The pellets aspect and their physical properties are presented in Figure 1 and Table 1, respectively. For CO\({}_{2}\) adsorption tests, two gas mixtures were used: (i) 0.1 vol.% CO\({}_{2}\) in N\({}_{2}\) (Messer, Bucharest, Romania) and (ii) 74 vol.% CO\({}_{2}\) in N\({}_{2}\) (Messer, Bucharest, Romania). The tests were performed three times in order to observe the repeatability and reproducibility. The morphology of the materials was determined using a Variable Pressure Field Emission Scanning Electron Microscope-FESEM VP (Carl Zeiss, Oberkochen, Germany) with a resolution of 0.8 nm at 30 kV or 2.5 nm at 30 kV in VP mode.