Synergetic Effect of Potassium Persulfate and Iron Oxalate on Photodegradation of Para-Arsanilic Acid

The p-arsanilic acid (p-ASA) is widely used in agriculture as a food additive to control parasites. It leaves the body almost unchanged and is subsequently destroyed by environmental factors with the formation of toxic forms of inorganic arsenic. UVA irradiation of p-ASA with the addition of the Fe(III) oxalate complex leads to an effective degradation of the target compound. However, this method needs high concentrations of reagents and the keeping high [oxalate]:[Fe(III)] ratio to maintain proper eciency of Fe(III) oxalate system. In this work, to overcome these problems, potassium persulfate (PS) was used as an additional oxidizer to improve Fe(III) oxalate system. It was found that the sulfate radical produced upon PS activation reacts readily with both neutral and monoanionic forms of p-ASA yielding corresponding organic cation radical, bimolecular rate constants are (7.3 ± 0.6)×10 9 and (2.4 ± 0.4)×10 9 M − 1 s − 1 , accordingly. Addition of PS allows one to reduce the working concentration of oxalate and achieve the complete degradation of both p-ASA and organic byproducts to the less toxic inorganic As(V) up to 3 ppm of initial p-ASA concentration. Also the proposed approach demonstrates eciency even for low (< 0.5 ppm) concentrations of p-ASA.


Introduction
The widespread use of p-ASA in agriculture has led to the water contamination with inorganic arsenic (As(III) and As(V)), as well as p-ASA itself ( Regeneration of active Fe(III) form during the photolysis of Fe(III)-oxalate complexes allows some researches to call such systems as "photocatalytical" though in fact their functioning demands the constant consumption of oxalate ligands as a "sacri cial agent". This is rst, but not the last drawback of Fe(III) oxalate system (Pozdnyakov et  In our previous work (Tyutereva et al., 2020) the in uence of iron oxalate complexes on p-ASA photodegradation was investigated. The photosystem showed a good degradation quantum yield of · OH radical production (f 308nm = 0.06) and allows to achieve full degradation of both p-ASA and its aromatic by-products at concentration of pollutant less than 7 µM. It is also demonstrated that during photooxidation signi cant part (~30%) of inorganic arsenic formed was absorbed by photogenerated Fe(III) complexes.
However, in the application of only the Fe(III) oxalate as photoactive agent the concentrations of oxalate and iron could not be below 0.5 mM and 20 µM, accordingly, to maintain stable degradation rate of p-ASA. To increase the e ciency of this process and to reduce the concentration of reagents, we suggest the use of PS as additional oxidizer.
The activation of PS yields the sulfate radical, a very strong oxidizing agent, which can quickly and more selectively react with aromatic compounds than the hydroxyl radical (Real et al. 2016). PS also reduces the concentration of oxalate and increases its e ciency in photodegradation processes, signi cantly accelerating the degradation of target compounds. The use of PS allows reducing the amount of "sacri cial" oxalate, while maintaining stabilization and regeneration of the most photoactive form of Fe(III).
Thus, the aim of this work is to study the synergetic effect of the iron oxalate complex and PS on the photodegradation of p-ASA. To shed the light on mechanisms of reactions we directly detected the primary intermediates, determined the rate constants of their reactions with p-ASA and optimized the photolysis conditions to achieve the best degradation of both the target compound and organic byproducts.

Arsenic determination in solutions
Concentration of total arsenic was determined by atomic-emission spectrometry with inductively coupled plasma (ICP-AES). iCap 6000 Duo ICP-AES instrument (Thermo Scienti c, USA) with concentric nebulizer and CID detector was applied. The data acquisition and processing was performed by iTEVA (Thermo Scienti c, USA) software. The working parameters of ICP-AES were the following: power supply 1150 W, nebulizer ow rate 0.75 L•min -1 , cooling gas ow 12 L•min -1 . The interference of other elements was eliminated by adding scandium nitrate, Sc(NO 3 ) 2 with concentration of 0.5 ppm, as internal standard, to all tested solutions. ICP-AES in combination with hydride generation technique (HG-ICP-AES) was used to measure As(III) at the concentration level < 0.1 ppm according to the guide (Thermo Scienti c, USA).

Arsenic speciation in solutions
The concentration of inorganic arsenic (As(III) and arsenate As(V) ions), p-ASA and oxalate ions were determined by capillary zone electrophoresis (CZE). Capel 3R CZE system (Lumex Company, Russia) equipped with photometric UV detector was used . Upon the registration of the analysts the following retention order was observed: oxalate, arsenate, p-ASA and arsenite. The correctness of peaks assignment was con rmed by using corresponding individual standards.
Determination of the total arsenic content to assess the balance of the element was performed by ICP-AES.

Optical spectroscopy and photolysis setups
Agilent 8453 spectrophotometer (Agilent Technologies, USA) was used to record optical spectra. The stationary photolysis experiments were carried out in quartz cells with a total volume of 10 ml and an optical path length of 5 cm at normal oxygen content in solution. Excimer XeCl lamp (308 nm) manufactured by the Institute of High Current Electronics, SB RAS, Tomsk, Russia (Sosnin et al., 2006) was used as a source of a stationary radiation.
Time-resolved experiments were performed using a conventional laser ash photolysis setup (Pozdnyakov et al., 2006). Quartz cells with a total volume of 3 ml and an optical path length of 1 cm at normal oxygen content in solutions were applied. A tunable LS-2137U laser (Lotis, Belarus) with an excitation wavelength of 355 or 266 nm was used as excitation source with pulse duration of about 6 ns, and pulse energy from 1 to 15 mJ. The optical sensitivity of the setup is up to 5 × 10 -4 , the spectral range is 270-800 nm, and the temporal resolution is 50 ns. For the numerical calculations of kinetic curves, the differential equations were solved by means of thef fourth-order Runge-Kutta method using RUNGE (developer Yu.V. Ivanov) software (Pozdnyakov et al., 2008).

Photodegradation studies
The photodegradation of p-ASA was also studied by LC 1200 high performance liquid chromatography (HPLC) system (Agilent Technologies, USA) equipped with a diode array detector from the Center of Collective Use «Mass spectrometric investigations» SB RAS. Separations were performed using an Agilent Zorbax Eclipse RapidResolution XBD-C18 column (4.6 × 100 mm, 80 Å, 1.8 μm) in isocratic elution of mobile phase (5% of acetonitrile and 0.1% v/v of formic acid) with the ow rate 0.5 mL/min. The injection volume was 80 μL. Retention time of p-ASA was 2.75 min. Concentration of total aromatics (TAR) was estimated by integration of all peaks in a chromatogram at 250 nm corresponding to the initial compound and its photoproducts. Analysis of the obtained results was performed using Agilent ChemStation software. The decay of RNH 2 +• radical at the initial times can be described by a rst-order equation (3)  respectively. At pH 3 the reaction rate constant of the sulfate radical with the neutral form of p-ASA is k 2 = (7.3 ± 0.6) ×10 9 M -1 s -1 that is three times greater than that obtained for the anionic one (Fig. S2). This indicates the strong in uence of Coulomb repulsion during the interaction of two negatively charged species at pH 7. However both rate constants are close to diffusion-controlled limit, so one can conclude that SO 4 -• radical could readily oxidize p-ASA at a wide pH range even at low concentration of the target molecule.

Stationary photolysis of the Fe(Ox) 3 3complex with PS in the presence of p-ASA.
In our previous article (Tyutereva et all., 2020) we demonstrated that at neutral pH the Fe(III) oxalate complexes effectively generate • OH radical under UV irradiation, which reacts with p-ASA with high rate constant, (8.6 ± 0.5) × 10 9 M -1 s -1 . Subsequent oxidation of organic radical formed leads to complete degradation of both p-ASA and basic aromatic photoproducts with formation of inorganic As(V) mainly, under optimal conditions. It is also worth to note, that p-ASA has own photochemistry under 308 nm irradiation but it is negligible in our experimental conditions due to high absorption and photoactivity of Fe(III) complexes.
The presence of both Fe(III) oxalate and PS in the solution accelerates the photodegradation of p-ASA and gives an opportunity to reduce the working concentration of oxalate ions in the solution (from 0.5 to 0.12 mM) without changes in the high degradation e ciency of not only the target compound but the aromatic photoproducts as well (Fig. 3). This can be explained by the catalytic decomposition of PS by photogenerated Fe(II) ions with the regeneration of the initial Fe(III) oxalate complex and the formation of additional oxidative sulfate radical: Addition of PS also allows to oxidize p-ASA at high concentration of the pollutant that cannot be done in the presence of iron oxalate alone. Figure 4 shows the dependence of the yield of different arsenic species on the irradiation time of p-ASA -Fe(III) oxalate system in the presence and absence of PS at initial concentration of p-ASA about 3 ppm (4×10 -5 M). Without PS only a partial (about 50%) degradation of p-ASA was observed after 40 min of irradiation (Fig. 4A). The total content of arsenic including p-ASA and inorganic species at the end of irradiation (~1.7 ppm) was signi cantly less than the total arcenic concentration (~2.8 ppm) before irradiation that indicates an accumulation of some organic arcenic byproducts with questionable toxicity.
In the presence of PS (Fig. 4B) the complete degradation of p-ASA was observed with generation of As(V) mainly, which can be removed by standard water treatment procedures (Vircikova et all., 1996;Lawrence and Higgs, 1999). It worth to note that the concentration of As(V) measured by CZE method shows evident decrease after 20 minutes of irradiation comparatively with the experiment withot PS (Fig. 4B). To explain such puzzling effect the test experiments with model As(V) -Fe(III) oxalate -PS system without and with irradiation were carried out under the same conditions. Figure 5A demonstrates the difference between actual (by preparation) concentration of As(V) measured using methods of ICP-AES and CZE before and after irradiation. Prior to irradiation both methods give results consistent with each other and actual As(V) concentration (Fig. 5A). In the case of photolysed samples, ICP-AES shows no in uence of irradiation on the measured concentrations of As(V), while CZE demonstrates a pronounced decrease of As(V) by 0.7 -2 ppm, depending on the initial arsenic content. This discrepancy increases with exposure ( Fig. 5B) that gives a right to assume that a portion of As(V) transforms into another form or product, which is out of registration by CZE, e.g., conjugate or complex, on account of Fe(III)-oxalate degradation. Analogous loss of As(V) measured by CZE was reported in our previous work (Tyutereva et al., 2020) where oxidation of p-ASA was studied in Fe(III)-oxalate system in the absence of PS. According to (Wang et all., 2020), this unknown product is most likely an As(V) -ferric oxyhydroxides colloid.
Another argument for the assumption that part of arsenic is sorbed on photogenerated iron oxyhydroxides are the results of total As determination using ICP-AES analysis of undisturbed samples few days after photolysis (Fig. S3). The total arsenic concentration is restored by a shaking of samples prior the measurements. Figure 5B also illustrates this effect (curves 1 and 1'). This important observation indicates that both processes including p-ASA oxidation and As(V) sorption occurs during a single photochemical process. The signi cance of this effect for further application of this photochemical approach for p-ASA removal from natural waters is in the reduction of the number and the cost of water puri cation steps. We intend to continue research in this prospective direction in the nearest future.
Assuming that p-ASA is completely converted to inorganic forms of arsenic we can estimate the loss of As(V) in CZE measurements after 20 and 40 min of irradiation, accordingly. Result was lower that one predicted by photolysis of the model system with As(V) (³1 ppm, Fig. 5B). However, it should be taken into account that in the model system ROS, generated by excitation of Fe(III) oxalate, react immediately with the complex itself. In the presence of p-ASA these active species rstly react with target pollutant and start the degradation of complex only at the nal stage of photolysis. So, we can expect the stabilization of Fe(III)-oxalate system in presence of p-ASA as compared with As(V).
After correction of As(V) concentrations measured by CZE, we can conclude that in the presence of 1 mM PS the complete degradation of p-ASA to inorganic arsenic occurs at concentrations of p-ASA up to 3 ppm. The ratio of As(III)/As(V) formed during p-ASA oxidation depends on the initial concentration of the pollutant. Figure 6 demonstrates the arsenic species distribution in solution after 20 min of irradiation depending on the starting concentration of p-ASA. Almost complete conversion of p-ASA to inorganic arsenic could be seen (Fig. 6). Another important result is the decrease of As(III)/As(V) ratio from 0.21 to negligible value (Fig. 6, inset)  on the interaction of two negatively charged partners at neutral pH. It was shown that addition of PS to Fe(III)-oxalate system has several advantages for p-ASA photooxidation. First, the presence of PS signi cantly reduces the concentration of oxalate required for complete p-ASA degradation. Second, higher degradation e ciency of the target compound leads to lower exposure doses and total time for photolysis. Third, the predominant formation of As(V) reduces the number and cost of subsequent puri cation cycles. The proposed approach demonstrates e ciency even for low concentrations of p-ASA. Also the observed effect of partial sorption of As(V) on photogenerated Fe(III) species formed upon p-ASA photooxidation opens a new opportunities of Fe(III)-oxalate -PS system application in water puri cation processes though further studies of the adsorption prosesses are necessary for their effective use in the real applications.

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