Study on the Transformation of I− in Fe2+ Activated PMS System

In order to solve the problem of more and more iodinated by-products in water, the advanced oxidation system of peroxymonosulfate (PMS) activated by ferrous (Fe2+) was used to study the transformation process and principle of iodine ion (I−) as the initial target under different influencing factors in this system. The results showed that when the pH value was 3, the molar concentration ratio of Fe2+ and PMS was 0.8:1, the conversion effect was the best. When the concentration of I− was 20 μmol/L, the conversion rate of I− was 94 %. Under the action of sulfate radical (SO4 −), the final conversion product of I− is only iodate (IO3 −). With the increase of pH value, the conversion effect will also decrease. The concentration of I− itself also affects the reaction.


Experimental method
Add 100 mL ultrapure water into 250 mL iodine bottle, and then add the calculated volume or I − to the solution to prepare the required concentration. Adjust the pH value of the solution to the required value with dilute sulfuric acid or sodium hydroxide solution. This process needs to be carried out on a magnetic stirrer to ensure uniform diffusion of the solution. Then add the calculated concentration of Fe 2+ and the corresponding molar ratio concentration of PMS, and use the timer to start timing. At the set time point, 6 mL of the sample was taken with a syringe, and the NaOH solution with the calculated volume of the alkaline solution and 200 μL of 2,6-dichlorophenol were immediately added to the sample. The sample was shaken for 10~15 s，to capture the intermediate active substance hypoiodous acid (IO − ), and then 50 μL of sodium sulfite (Na 2 SO 3 ) and 100 μL of methanol were added to terminate the reaction. 1 mL sample was injected into the liquid bottle, and IO − was determined by liquid chromatography. The remaining 5 mL sample was injected into the ion chromatography tube to determine the corresponding ions.

Analysis method
In the experiment, the content of IO − in the experiment was detected by the reaction of IO − with 2,6dichlorophenol to generate 4-I-2,6-dichlorophenol. 4-I-2,6-dichlorophenol was determined by Waters HPLC. The mobile phase was V (acetonitrile): V (water) = 7:3, the UV detection wavelength was 225 nm, the flow rate was 1 mL/min, and the injection volume was 100 μL. I − and IO 3 − were determined by ion chromatography (ICS1500) with 9 mmol/L anhydrous sodium carbonate solution as the eluent at a flow rate of 1 mL/min. The injection volume was 1000 μg/L. The detection limit of the method was 0.3 μg/L.

Effect of pH on I − transformation
Fe 2+ /PMS system is Fenton-like system, so the system is greatly affected by pH value. The oxidation effect is the best under acidic conditions, and the effect is poor under neutral or alkaline conditions [6].   Figure.1 Effect of pH on Fe 2+ /PMS oxidation of I − It can be seen from Fig. 1 that when pH value was 3, the conversion effect of I − was the best, and 94 % of I − was oxidized. With the increase of pH value, when pH value was 7, the conversion efficiency of I − continued to drop to 39 %. The main reason may be that in neutral or alkaline environment, Fe 2+ is prone to hydrolysis and precipitation, so that the effective Fe 2+ concentration involved in Fe 2+ /PMS is reduced, and SO 4 ꞏ− is easy to react with OH − under alkaline conditions. So that the conversion efficiency of I − is reduced. Under acidic conditions, and when pH value is 3, most Fe 2+ can participate in activation, and the number of effective SO 4 ꞏ− can also be maximized. So in the actual water treatment system, we should always pay attention to the change of pH value, control the pH value of water, so that the treatment effect is the best.   Fig. 2, when Fe 2+ increased from 100 μmol/L to 300 μmol/L, the conversion rate of I − increased from 59 % to 94 %, and then decreased to 70 %. It can be seen that when the concentration of Fe 2+ was 160 μmol/L, namely the concentration ratio of Fe 2+ to PMS was 0.8:1, the conversion effect was the best. The main reason for this result is that when the concentration of Fe 2+ is low, it can't play a good activation role, but it is not the more the better. Excessive Fe 2+ will compete with the SO 4 ꞏ− which plays a major oxidation role in the system. The content of effective SO 4 ꞏ− in the reaction is reduced, and the content of Fe 3+ is increased, which reduces the reaction efficiency. Therefore, in the relevant experiments, it is necessary to control the dosage of Fe 2+ , taking into account the processing cost and environmental protection and other issues, so that the concentration ratio of Fe 2+ and PMS is kept at about 0.8:1, and the cost performance is higher, which can maintain the maximum activation effect and oxidation effect [7].   Fig. 3, when I − concentration increased from 10 μmol/L to 30 μmol/L, the conversion rate of I − decreased from 97 % to 67 %. I − will react with SO 4 ꞏ− in Fe 2+ /PMS system, and SO 4 ꞏ− will react with other active substances in the whole reaction process [8]. When the concentrations of Fe 2+ and PMS remain unchanged, the conversion efficiency of I − will not be improved by continuously increasing the concentration of I − . Therefore, in the water with known I − concentration, the concentration relationship between Fe 2+ /PMS and I − should be appropriately adjusted to achieve the best response.

Analysis of transformation products of I −
I − is already in the lowest valence state and has a certain degree of reduction, which can be oxidized by the system to produce compounds with high valence state. In this experiment, the optimal reaction conditions were as follows. Temperature was 25 °C, pH value was 3, the concentration of Fe 2+ was 160 μmol/L, the concentration of PMS was 200 μmol/L, and the concentration of I − was 20 μmol/L. The reaction results are shown in Fig. 4. As shown in Figure 4, because of the strong reducibility of I − , the reaction rate in the first 30 minutes was faster and the conversion rate was higher. Finally, 18.82 μmol/L I − was oxidized, and all the oxidized parts generated IO 3 − , and no IO − was detected. The reason for this phenomenon may be that the iodine active substances produced during the reaction are more active, difficult to capture, and the reaction rate is very fast, directly forming the final product IO 3 − .

Effect of PMS alone on I −
From the above results, it can be seen that Fe 2+ is very important for the activation of I − transformation. In order to more intuitively and accurately see the specific role of Fe 2+ in the system of I − transformation, this section did a set of blank test for comparison. Only PMS was used to oxidize I − , without adding Fe 2+ , under the optimal conditions, namely the temperature was 25 °C, pH value was 3, PMS was 200 μmol/L, I − concentration was 20 μmol/L. The transformation of I − was observed and analyzed, and compared with the Fe 2+ /PMS system under the optimal conditions. The reasons for the differences between the two conditions were analyzed, as shown in Figure 5. As shown in Figure 5, PMS itself has a certain oxidation effect, but the effect is relatively poor. Compared with the 94 % conversion of I − in the Fe 2+ / PMS system under the optimum conditions, only 86 % of I − is converted and only IO 3 − is generated, and IO − is still not detected. It can be seen from the results that, under the same optimal conditions, the direction of action and reaction trend of PMS alone and Fe 2+ /PMS system on I − are generally consistent. However, under the activation of Fe 2+ , the