## 3 Results  
### Influence of Dose on DCF Degradation under EB Irradiation  
The effect of the adsorbed dose on DCF degradation under EB irradiation was studied using initial concentrations of DCF varying from 10 mg/L to 250 mg/L (Figure 2). The removal efficiency of DCF increased with increasing adsorbed dose, and it depended on initial concentration of DCF in the solution, being higher for the lowest concentration. It was 99%, 45% and 33% at the 0.5 kGy absorbed dose for initial concentrations of DCF of 10, 125 and 250 mg/L, respectively. As far as the authors know, this is the first study evaluating the influence of the initial concentration on the degradation of diclofenac by EB irradiation; however, the same tendency was also reported in studies on the decomposition of diclofenac using EB irradiation [21] and gamma radiation [33; 35] and other pharmaceutical compounds using gamma radiation [36; 37; 38].  
The degradation of DCF in aqueous solution by EB irradiation was previously studied by other authors with or without the use of additives [21; 22; 39]. [PERSON] et al. [22] reported the complete decomposition of DCF (50 mg/L) in aqueous solutions at 5 kGy, while [PERSON] et al. [21] indicated that the removal efficiency of DCF (10-40 mg/L) was almost 100% for all the samples at 0.5 kGy. [PERSON] and [PERSON] [20] observed that 86% of DCF (30 mg/L) was degraded after gamma radiation at 1 kGy.  
DCF decomposition was initiated by the primary products from water radiolysis, mainly H, e\({}_{\text{aq}}^{-}\) and OH radicals, which formed according to the following reaction (2) [40]:  
\[\text{H}_{2}\text{O}\xrightarrow[\tau]{\text{radiolysis}}[0.28]^{\bullet}\text {OH}+[0.27]^{\bullet}\text{H}+[0.07]\text{H}_{2}\text{O}_{2}+[0.05]\text{H} _{2}+[0.27]\text{H}^{+} \tag{2}\]  
The numbers in brackets represent the radiation chemical yield (G-value, in \(\upmu\)M/J).  
The main species generated from water radiolysis are \({}^{\bullet}\)OH, \({}^{\bullet}\)H and e\({}_{\text{aq}}^{-}\).  
DCF degradation can be written as  
\[\frac{-\text{d}[\text{DCF}]}{\text{dt}}=\text{k}_{\text{OH}}[{}^{\bullet}\text {OH}][\text{DCF}]+\text{k}_{\text{H}}[{}^{\bullet}\text{H}][\text{DCF}]+\text {k}_{\text{aq}^{-}}[\text{aq}^{-}][\text{DCF}] \tag{3}\]The initial pH of the DCF solution was 6, meaning that the \({}^{\bullet}\)H concentration was low and its contribution to DCF degradation can be neglected under this condition. The hydroxyl radical is a strong oxidant species with a standard reduction potential compared to a normal hydrogen electrode (NHE) of E\({}^{0}\) (\({}^{\bullet}\)OH/OH\({}^{-}\)) = 1.9 V, whereas a hydrated electron is a reductant, with a standard reduction potential (vs. NHE) of E\({}^{0}\) (\({\rm e}_{\rm aq}^{-}\)) = \(-\)2.9.  
The rate constants of DCF with \({}^{\bullet}\)OH and hydrated electrons are given in Equations (4) and (5), respectively [19; 39]:  
\[{}^{\bullet}\mathrm{OH}+\mathrm{DCF}=\mathrm{products}(0.93\,\sim\,1.24)\times 1 0^{10}\;\mathrm{M}^{-1}\mathrm{s}^{-1} \tag{4}\]  
\[{\rm e}_{\rm aq}^{-}+\mathrm{DCF}=\mathrm{Cl}^{-}+\mathrm{product}(1.53\,\sim \,3.1)\times 10^{9}\;\mathrm{M}^{-1}\mathrm{s}^{-1} \tag{5}\]  
The reaction rate of \({}^{\bullet}\)OH with DCF is faster than that with \({\rm e}_{\rm aq}^{-}\). The reaction of the OH radical with DCF involves an addition reaction to a benzene ring (4) and \({\rm e}_{\rm aq}^{-}\) reacts with DCF by a dissociative electron capture reaction and releases Cl\({}^{-}\) ions (5).