## 3 Results and Discussion
### Removal of Ammonium, MEA, and TMAH by Various Processes (Synthetic Wastewater)
Similar pH effect was observed in the findings reported by Lu et al. [26] and the authors investigated using activated carbon/silver catalytic combined with ozone and hydrogen peroxide for the removal of TMAH. On the other hand, under acidic or neutral conditions, the direct attack on organic pollutants by molecular ozone occurs. In general, hydroxyl radicals could destroy organic compounds more effectively than ozone [16; 27]. Figure 4: Removal of ammonium by various processes at various solution pH (C\({}_{\text{o}}\) = 100 mg/L, V = 0.5 L, solid 1 g, 0.25 mmol O3/min, 0.125 mmol H2O2/min, 30 min). Since significant amounts of salts might be present in the wastewater, the effect of these salts on the processes has to be investigated in order to be able to apply the processes effectively in practice. Three types of salts such as sulfates, nitrates, and carbonates were chosen in this study as model compounds. As shown in Figure 6, these salts had a negative effect (Figure 6) on the removal of these salts. Figure 5: Removal of TMAH by various processes at various solution pH (C\({}_{o}\) = 100 mg/L, V = 0.5 L, solid 1 g, 0.25 mmol O\({}_{3}\)/min, 0.125 mmol H\({}_{2}\)O\({}_{2}\)/min, 30 min). pollutants at the tested pH values (3, 7, and 11). For example, ammonium removal decreased from 61 to 33, 45, and 36% due to the presence of sulfates (100 mg/L), nitrates (500 mg/L), and carbonates (500 mg/L) in solution, respectively (Figure 6). TMAH removal decreased from 80 to 46, 41, and 46% in aqueous solutions containing sulfates (100 mg/L), nitrates (250 mg/L), and carbonates (250 mg/L), respectively (Figure 7). Salts can affect the ozone oxidation process in several ways. For example, the presence of a salt generally lowers the solubility of a gas (salting out). A change in ozone solubility can be observed in the salt-containing solutions [28; 29]. Rischbieter et al. [28] reported that sulfates and nitrates could reduce the solubility of ozone significantly with Henry constants about 15.3 and 12.1 kPa m\({}^{3}\) mol\({}^{-1}\), respectively (salt concentration 0.5 kmol m\({}^{-3}\)). Salts can also influence the size distribution of gas bubbles in a bubble column. They might inhibit bubble coalescence, resulting in a lower average bubble diameter. As a result, mass transfer rates might increase [30; 31]. Carbonate ions are well-known species for scavenging the radicals involved in ozone decomposition [28; 29; 32]. On the other hand, sulfate, phosphate, nitrate, and chloride ions are extremely slow in reacting with \(\bullet\)OH. Their scavenging effect can generally be neglected [33]. Therefore, the negative effect of sulfates and nitrates on pollutants removal might due to the salting-out effect. Figure 6: Effect of salts on the removal of ammonium by the MnCe-GAC/H\({}_{2}\)O\({}_{2}\) process (C\({}_{o}\) = 100 mg/L, V = 0.5 L, solid 1 g, 0.25 mmol O\({}_{3}\)/min, 0.125 mmol H\({}_{2}\)O\({}_{2}\)/min, 30 min). This study proposed that MnCe-GAC could enhance the formation of free radicals (such as hydroxyl radicals) in the MnCe-GAC/O\({}_{3}\)/H\({}_{2}\)O\({}_{2}\) process and subsequent oxidation of adsorbed pollutants [34, 35]. This assumption was tested by the addition of t-butanol. t-Butanol has a high rate constant with \(\bullet\)OH as shown in Equation (1) [36]. \[\mathrm{CH_{3}C(CH_{3})}_{2}\mathrm{OH+\bullet OH\to CH_{2}C(CH_{3})}_{2} \mathrm{OH+H_{2}O}\ (k\mathrm{=6.0^{\bullet}10^{8}\ M^{-1}s^{-1}}) \tag{1}\]  
t-Butanol could be used as a hydroxyl radical scavenger [37] and served as an \(\bullet\)OH-scavenger in this study. If the removal of pollutants were significantly hindered by the addition of t-butanol, it indicated that the hydroxyl radical reaction might play an important role in the removal processes. The effect of t-butanol on the removal of ammonium and TMAH by the MnCe-GAC/O\({}_{3}\)/H\({}_{2}\)O\({}_{2}\) process is shown in Figure 8a,b, respectively. As indicated in the figure, the removal of ammonium as well as TMAH was significantly reduced due to the presence of t-butanol.