## 3 Applications and Limits of ATPs in Water and Wastewater Treatment  
While oxidation is considered a key removal mechanism for many compounds, especially when achieved by strong oxidants, such as chlorine, ozone, or AOPs, as discussed in the previous section, not all CECs are susceptible to destruction by it. Table 3 summarizes selected organic compounds' removal rates in solution by different radicals, and the relative importance of each reactive species in the process. It can be seen that several compounds (e.g., halogenated substances) respond poorly to oxidation but are well degraded by reduction. Others, such as, for example, trihalomethanes, are not degraded at all by oxidation, and seem to be poorly degraded also by reduction, although recent reports claimed an 82% total THMs removal by means of an AORP VUV/UV-C process [55] and complete degradation by a UV-C/free chlorine process [56]. The effectiveness of an ATP depends on the reactivity of the radical species generated. This is indicated by the redox potential, a measure of a species' tendency to reduce or oxidize, measured in volts (V). The standard oxidation-reduction potential, E\({}_{0}\), is defined relative to a standard reference hydrogen electrode (SHE), which is arbitrarily given a potential of 0 V. A high positive value of E\({}_{0}\) \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Radiation Source Rated Strength** & **Energy [kW]** & **Calculated Dose-Rate** & **Process Contact Time** \\  & & **[MGy h\({}^{-1}\)]** & **for 1 kGy Dose [s]** \\ \hline \hline \(X\)-\(ray\) & & & \\ \hline
250 kV & \(0.5\times 10^{-8}\) & \(1.8\times 10^{-5}\) & \(2.0\times 10^{5}\) (=55.6 h) \\ \hline \({}^{60}Co\)\(\gamma\)-source (Ci = Curie) & & & \\ \hline \(0.5\times 10^{6}\) Ci\({}^{*}\) & 0.18 & 0.65 & 5.5 \\ \(1.0\times 10^{6}\) Ci\({}^{*}\) & 0.36 & 1.30 & 2.8 \\ \hline \hline _Electron Beam (MeV = 10\({}^{6}\) electron-Volt)_ & & & \\ \hline
1 mA, 1 MeV & 2 & \(7.2\times 10^{3}\) & 0.2 \\
10 mA, 10 MeV & 100 & \(3.6\times 10^{5}\) & \(10^{-5}\) \\
50 mA, 10 MeV & 500 & \(1.8\times 10^{6}\) & \(2\times 10^{-6}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Achievable dose-rates and process contact times of different radiation sources. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & \multicolumn{3}{c}{**Bimolecular Reaction Rate Constants**} & \multicolumn{3}{c}{**Relative Importance of Radical**} \\
**Compound** & \multicolumn{3}{c}{**[10\({}^{9}\) M\({}^{-1}\)s\({}^{-1}\)]**} & \multicolumn{3}{c}{**Species in Compound Removal (\%)**} \\ \cline{2-6}  & \(\cdot\)**OH** & \(\mathbf{e^{-}}_{\mathbf{aq}}\) & \(\cdot\)**H** & \(\cdot\)**OH** & \(\mathbf{e^{-}}_{\mathbf{aq}}\) & \(\cdot\)**H** \\ \hline \hline _Compounds highly susceptible to oxidation_ & & & & & \\ \hline Ethlybenzene & 7.5 & - & - & 100 & 0 & 0 \\ DSEP & 0.73 & - & - & 100 & 0 & 0 \\ NDMA & 0.33 & - & - & 100 & 0 & 0 \\ Atrazine & 2.6 & - & - & 100 & 0 & 0 \\ Simazine & 208 & - & - & 100 & 0 & 0 \\ \(\alpha\)-Xylene & 6.7 & - & 2 & 94 & 0 & 6 \\ Toluene & 5.1 & 0.011 & 2.6 & 90 & \(\sim\)0.1 & \(\sim\)10 \\ \hline \hline _Compounds highly susceptible to reduction_ & & & & & \\ \hline Chloroform & 0.054 & 11 & 0.073 & 0.4 & 99 & 0.1 \\ Ethlyenedibromide & 0.26 & 14 & NF & 2 & 98 & 0 \\ Bromoform & 0.11 & 26 & 1.9 & 0.5 & 97.5 & 2 \\ \hline \hline _Other compounds_ & & & & & \\ \hline CHBrCl\({}_{2}\) & - & 21 & - & 0 & 10 & 0 \\ CHBr\({}_{2}\)Cl & - & 20 & - & 0 & 10 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Organic compounds’ removal rates by different radicals, and the relative importance of each species in the process (modified from [50]). therefore, indicates a high capacity to oxidate; a high negative value, on the contrary, a high capacity to reduce. Table 4 compares the E0 values of different radical and commonly used chemical reagents.