# Photooxidation of \(p\)-Arsanilic Acid in Aqueous Solution by UV/Persulfate Process
## Results and Discussion
These products may go through further oxidation to CO\({}_{2}\) or form C-C bond with benzene ring, leading to the generation of aromatic aldehydes/carboxylic acids (like hydroxybenzaldehyde observed in this study) [28; 35; 36], which explicated the moderate decrease in TOC at the very beginning of irradiation. [PERSON] et al. [37] also proposed aldehydes as one of the possible intermediate products in the oxidation process of benzene by PS in the presence of Fe(III)- and Mn(IV)-containing oxides. However, the obvious cleavage of CO from the primary aldehyde-like products was not observed in our work, followed by the generation of resulting products, probably owing to the more complicated oxidation process caused by the additional involvement of UV light. Phenylarsonic acid [13] was recognized as one of the photoreaction products in the photolysis of \(p\)-ASA, which could form adducts (for example, biphenylarsinic acid) with aromatic byproducts. Formation of aromatic organoarsenic compounds like biphenylarsinic acid and \(o\)-ASA could explain the deviation of total arsenics concentrations (As\({}^{\rm T}\), the total amount of \(p\)-ASA, inorganic As\({}^{\rm III}\) and As\({}^{\rm V}\)) from initial concentration of \(p\)-ASA at intermediate stage of the irradiation (Figure 4). \[{\rm S_{2}O_{8}}^{2-}+{\rm hv}\to 2{\rm SO_{4}}\bullet^{-} \tag{1}\]  
Figure 3: Effect of different quenchers on the degradation of \(p\)-ASA in UV/PS system. Insert: the initial rate constant of \(p\)-ASA degradation (\(r_{0}\)) on different conditions. Conditions: (EtOH) = 40 mmol L\({}^{-1}\), (\(p\)-ASA) = 8 \(\upmu\)mol L\({}^{-1}\), (PS) = 1 mmol L\({}^{-1}\), (PB) = 20 mmol L\({}^{-1}\), pH = 6. Figure 4 demonstrated the evolution of arsenic species during \(p\)-ASA photolysis in PS/UV system. As\({}^{\text{III}}\) was generated as a minor photoproduct at the beginning of the reaction, reaching a plateau at 7 min, at a concentration of 0.36 \(\upmu\)mol L\({}^{-1}\) (<5% of initial \(p\)-ASA concentration). Concentration of As\({}^{\text{III}}\) was even lower than that in UV system [15] (\(\sim\)10%) due to the presence of oxidative agents (PS and SO\({}_{4}\bullet^{-}\)). As\({}^{\text{T}}\) got a decrease in the first 15 min of irradiation, caused by the generation of newly formed organoarsenicals (biphenylarsinic acid, \(o\)-ASA). These organic arsenicals could be oxidized by SO\({}_{4}\bullet^{-}\) afterwards, ultimately releasing inorganic arsenic, and as a result, As\({}^{\text{T}}\) went upward reaching value of 7.7 \(\upmu\)mol L\({}^{-1}\) after the irradiation for 30 min. As\({}^{\text{V}}\) gained a continuous growth during the photolysis and reached to 7.6 \(\upmu\)mol L\({}^{-1}\) (95% of initial \(p\)-ASA concentration) at the end of the photoreaction. The equally matched amount of As\({}^{\text{V}}\) and As\({}^{\text{T}}\) at the end of the reaction demonstrated a nearly complete transformation from organic arsenic to inorganic As\({}^{\text{V}}\). \begin{table}
\begin{tabular}{c c} \hline
**[M + H]\({}^{*}\)** & **Structure** \\ \hline
122.9 & \\
123.7 & \\ \hline
190.7 & \\ \hline
278.9 & \\ \hline \end{tabular}
\end{table} 
Table 1: The photooxidation products during UV-C irradiation identified by LC-MS.