# Photooxidation of \(p\)-Arsanilic Acid in Aqueous Solution by UV/Persulfate Process
## Results and Discussion  
As depicted in Figure 1, \(>\)90% of \(p\)-ASA is photooxidized directly by the irradiation of UV-C light within 30 min, while the addition of 5 mmol L\({}^{-1}\) PS caused a noticeable enhancement to the degradation that it took only \(\sim\)7 min to achieve a complete removal of \(p\)-ASA. Additionally, the enhancement in photooxidation efficiency strengthened with PS dosage increase. Control experiments showed no obvious change in \(p\)-ASA concentration under dark conditions, either with or without PS, suggesting the absence of thermal degradation of \(p\)-ASA by PS. Our previous work [15] showed an insignificant TOC removal of _p_-ASA after UV-C treatment only, while in this work a sharp decrease in TOC was obtained in UV/PS system. As depicted in Figure 2, \(\sim\)90% TOC elimination was achieved after 180 min irradiation. It was worth to note that only 23% TOC was removed on the stage of _p_-ASA degradation (<45 min of irradiation) with a relatively slow rate, while a faster removal of TOC was gained under UV irradiation for a longer time (>45 min). This fact indicated that TOC removal was mainly connected with the oxidation of by-products. Consequently, a good TOC conversion in UV/PS system implied its potential of industrial application in the treatment for _p_-ASA containing wastewater. SO4\(\bullet^{-}\) has been reported as the dominant radical generated in the process of PS activation in neutral and acid solution [28]. To elucidate more insight of the promotion effect in UV/PS system, ethanol (EtOH) was used as a radical scavenger [29]. The initial reaction rate (calculated in terms of the degradation of _p_-ASA from the first three points during the irradiation) decreased from 1.14 umol L\({}^{-1}\) min\({}^{-1}\) to 0.49 umol L\({}^{-1}\) min\({}^{-1}\) in the presence of 40 mmol L\({}^{-1}\) EtOH (Figure 3) in UV/PS system, confirming the contribution of SO4\(\bullet^{-}\). Moreover, the presence of EtOH led to a more retarded degradation of _p_-ASA in UV/PS system than that under UV irradiation only (Figure 3, insert). The stronger inhibiting effect caused by the joint presence of EtOH and PS is caused by competition of PS with _p_-ASA for the light quanta [30]. The remaining _p_-ASA underwent direct photolysis and/or oxidation by \({}^{1}\)O\({}_{2}\), which was generated via photosensitization [13]. Figure 1: Degradation of _p_-ASA in different systems. Conditions: (_p_-ASA) = 8 μmol L\({}^{-1}\), (PS) = 1 mmol L\({}^{-1}\), (PB) = 20 mmol L\({}^{-1}\), pH = 6. Figure 2: Changes of total organic carbon (TOC) with the degradation of _p_-ASA. Conditions: (_p_-ASA) = 80 μmol L\({}^{-1}\), (PS) = 50 mmol L\({}^{-1}\), (PB) = 20 mmol L\({}^{-1}\), pH = 6. The generation of SO\({}_{4}\bullet^{-}\) by activating PS (Equation (1)) [31] is responsible for the TOC decrease, since TOC remains almost invariable in the presence of UV light only [15]. Photoproduct analysis identified the generation of several aromatic compounds: hydroxybenzaldehyde, nitrobenzene, benzenediol sulfate, o-ASA (photo-Fries rearrangement product) [15], and biphenylarsinic acid with [M + H]\({}^{+}\)\(m/z\) at 122.9, 123.7, 190.7, 217.7, and 278.9 respectively (Table 1). Previous works [15] reported the cleavage of As-C bond during photolysis, generating aniline at circumneutral conditions, which could be sequentially oxidized to nitrobenzene [21; 32; 33]. The generation of benzenediol sulfate could result from sulfonation of dihydrobenzene formed both by direct \(p\)-ASA photolysis [13; 14] or its oxidation of by SO\({}_{4}\bullet^{-}\). Ring-open products--for example, aldehydes and carboxylic acids--with low molecular weight have been reported as the intermediates in AOPs treatment of aromatic compounds [28; 32; 34].