## 3 Applications and Limits of ATPs in Water and Wastewater Treatment
In addition to the E0 potential, reaction rates are paramount for the efficiency of process development: Literature-reported reaction rate constants determined for different organic compounds reacting with ozone and hydroxyl radicals show that the latter originates rates eight to nine orders of magnitude (10\({}^{8}\)-10\({}^{9}\)) faster than the former. Hence, a process generating large amounts of OH radicals will be much faster than one based on ozone, or on lower concentrations of hydroxyl radicals, although the mere oxidative potential of the two basic species is not that different, as seen in Table 4.  
In radiolytic processes, the kinetics is determined by the reaction order and the values of rate constants for specific reactions. The irradiation process yield is commonly expressed by G values, i.e., the number of molecules of reactant consumed per 100 eV of energy absorbed, the value of the dose constant, \(d\), and the dose magnitude required for 50% (\(D_{0.5}\)) or 90% (\(D_{0.9}\)) decomposition of chemical solutes [57]. _G-values_, commonly reported in the applied radiolysis literature, however, are not useful in practice to predict the required decomposition dose for a compound, since this depends on its actual concentration in solution, while G values are often calculated according to a linear kinetics approximation, not fully representative of the actual reaction. The dose constant, \(d\), represents decomposition rates as a function of the absorbed radiation dose, calculated as the slope of the line obtained from plotting \(ln(C_{D}/C_{0})\) against the absorbed dose. This is considered a more reliable parameter than the former, because it represents the entire irradiation process. Large values of \(G_{0}\) and \(d\) indicate high pollutant decomposition yields; their increase corresponds to a lower required dose for pollutant decomposition. Experimental dose-constants serve as a first approximation of the irradiation required to remove a major component compound from a multi-component solution; however, for these and other considerations exposed below, the actual required dose depends on the overall matrix composition, and on compounds' decomposition pathway(s). Dose estimates can also be used for a preliminary evaluation of treatment costs (equipment size and energy requirements). An important factor affecting the efficiency of all ATPs is the composition of the original water matrix, which may contain natural compounds acting as radical scavengers. Reaction rates involving scavengers may be of the same magnitude as those involving target pollutants; as a result, the pollutant decomposition yield in real conditions may significantly differ from that assessed in a pure solution. Some examples of scavenger species, typically occurring in natural waters and wastewater, are summarized in Table 5, together with the rate constants of their reaction with the main radical species. In addition to those reported in the table, the presence of other scavengers in wastewater \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Reactive Species** & \multicolumn{3}{c}{**Generated by**} & \multicolumn{2}{c}{**Standard Redox**} \\  & **AOP** & **ARP** & **AORP** & **Potential, E0 (V)** & \\ \hline Hydroxy radical (-OH) & X & & X & 2.86 \\ Oxygen atom (-O) & (X) & & (X) & 2.42 \\ Ozone molecule (O\({}_{3}\)) & (X) & & (X) & 2.07 \\ Peracetic Acid & = & = & = & 1.81 \\ Hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) & X & & X & 1.78 \\ Hypochlorite (HClO) & = & = & = & 1.63 \\ Chlorine (Cl2) & = & = & = & 1.36 \\ Oxygen molecule (O2) & = & = & = & 1.23 \\ Ferric Iron (Fe3+) & = & = & = & 0.77 \\
**Hydrogen (H)** & = & = & = & **0** \\ Ferrous Iron (Fe2+) & = & = & = & \(-\)0.44 \\ Sulfur dioxide radical (SO2\(-\)) & & X & X & \(-\)0.66 \\ Hydrogen radical (H) & = & X & X & \(-\)2.3 \\ Hydrated electron (e\({}^{-}\)aq) & & X & X & \(-\)2.87 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Oxidation and reduction potential of different radical and commonly used reagents.