A comparative study of free chlorine and peroxymonosulfate activated by Fe(II) in the degradation of iopamidol: Mechanisms, density functional theory (DFT) calculatitons and formation of iodinated disinfection by-products

Highlights

• Fe(II)/PMS had a better removal performance than Fe(II)/FC at a similar formation rate of reactive species.

• Lower formation of I-THMs during Fe(II)/PMS process.

• Radicals were the main contributor for IPM degradation, not Fe(IV).

• The main reaction sites of IPM were on chains B, B’ and the iodine group in between.

Abstract

Both Fe(II)/free chlorine (Fe(II)/FC) and Fe(II)/peroxymonosulfate (Fe(II)/PMS) have a faster reactive species formation rate than the classic Fenton process, so their efficiencies of removing pollutants are better than that of the Fenton process. In this study, we compared the differences in the mechanisms and kinetics of iopamidol (IPM) degradation by Fe(II)/FC and Fe(II)/PMS. Although the production rate of reactive species of the two processes is at the same level (10⁴ M⁻¹ s⁻¹), the Fe(II)/PMS process performed better regarding IPM removal. Reactive chlorine species (RCS) were the major contributor to the degradation of IPM (73.8%) by Fe(II)/FC, and hydroxyl radicals (HO^•) and sulfate radicals (SO₄^•–) contributed 50.7% and 49.3% to the degradation of IPM by Fe(II)/PMS, respectively. Fe(IV) showed low contributions (<0.1%) to the degradation of IPM during the Fe(II)/FC and Fe(II)/PMS processes because of the low values of k_{Fe(IV), IPM} ((25 ± 5) M⁻¹ s⁻¹) and the low steady-state concentration of Fe(IV) in the Fe(II)/FC ((5.1 ± 0.7) × 10⁻⁸ M) and Fe(II)/PMS processes ((2.2 ± 0.09) × 10⁻⁸ M) at pH 3. Based on density functional theory (DFT) calculations and the MS results, the transformation pathway of IPM has been confirmed to mainly occur on chains B and B’ and the iodine group in between. The formation of disinfection by-products (DBPs) indicates that the Fe(II)/PMS process has lower environmental hazards. In general, we should pay more attention to the type of active species to evaluate the Fenton-like process. This study provides a basis for choosing a suitable alternative to the Fenton process and understanding the process in the Fenton-like procedure.

Introduction

The Fenton process is a classic advanced oxidation technology (AOP) that has been widely used in the field of water treatment. Its principle is to use the highly active hydroxyl radicals (HO^•) formed by the reaction of Fe²⁺ and H₂O₂ to remove pollutants that are difficult to biodegrade. However, due to the high cost of H₂O₂, Fenton-like reactions have also been proposed in recent years. As a common disinfectant, free chlorine (FC) is widely used in the field of drinking water disinfection. The O-Cl structure in HO-Cl can be regarded as similar to the O-OH structure in HO-OH. Previous studies have shown that HO-Cl can replace H₂O₂ in the Fenton process to produce a stronger oxidant to remove pollutants. Due to its lower cost, FC is potentially more practical than H₂O₂ [1]. A type of AOP based on persulfates (peroxodisulfate (PDS) and peroxymonosulfate (PMS)) has become part of an important method to address water environmental problems [2]. Sulfate radicals (SO₄^•−) are the main highly reactive substance after persulfate activation [3], [4]. Compared with PDS, PMS has a peroxy radical, which makes PMS more easily activated (Eqs. 5–7). The methods of activating PMS include heat, ultraviolet light, ultrasounds, transition metals and ozone. Among these methods, transition metal activation is the simplest and cheapest activation method, so it has received widespread attention. Fe(II) has a strong activation ability, and Fe(III) is the main by-product after the reaction, which has a lower impact on the environment compared to that of other transition metals; therefore, Fe(II) is usually selected as a persulfate activator. Previous studies have shown that Fe(II)/FC and Fe(II)/PMS are two effective processes for reducing organic pollutants in water. However, the Fe(II)/FC and Fe(II)/PMS processes have different main active substances. Except for HO^•, reactive chlorine radicals (RCS) and SO₄^•− are thought to exist as particular active species in the Fe(II)/FC and Fe(II)/PMS processes, respectively. In addition, the formation of Fe(IV) in the Fe(II)/FC and Fe(II)/PMS processes was reported in recent studies (Eqs. (1), (4)). Unlike the non-selective oxidation of radicals, Fe(IV) has also attracted widespread attention because of its selective oxidation for reducing pollutants [5], [6], [7].

$${\text{Fe}}^{2+}\text{+ HOCl}\to {\text{Fe}}^{3+}\text{+ reactive species}k\text{= 1.7}\times {\text{10}}^{\text{4}}{\text{M}}^{-\text{1}}{\text{s}}^{-\text{1}}$$

$${\text{Fe}}^{2+}{\text{+ H}}_{\text{2}}{\text{O}}_{\text{2}}\to {\text{reactive species + H}}_{\text{2}}\text{O}k{\text{= 70 M}}^{-\text{1}}{\text{s}}^{-\text{1}}$$

$${\text{2Fe}}^{2+}{\text{+ H}}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_8\to {\text{reactive species + Fe}}^{3+}{\text{+ H}}_{\text{2}}{\text{SO}}_{\text{4}}k{\text{= 27 M}}^{-\text{1}}{\text{s}}^{-\text{1}}$$

$${\text{Fe}}^{2+}{\text{+ HSO}}_5^{-}\to {\text{Fe}}^{3+}\text{+ reactive species}k\text{= 3}\times {\text{10}}^4{\text{M}}^{-\text{1}}{\text{s}}^{-\text{1}}$$

Previous studies have shown that the Fe(II)/FC and the Fenton processes have a similar removal rate of COD, but the Fe(II)/FC process has a higher rate in terms of degradation kinetics [1]. Because of the faster generation rate of reactive oxygen species in the Fe(II)/FC process, the pollutants can be degraded faster in the Fe(II)/FC process. The reactive oxygen species produced in the Fe(II)/FC process is chlorine radicals. However, the generation rate of chlorine radicals in the Fe(II)/FC process is too fast and the chlorine radicals are prone to self-consumption reactions, which lead to insufficient oxidant in the subsequent degradation process. Xu et al., [8]{Xu, 2019 #1414} also found that the two processes with different reactive oxygen species generation rates exhibited different degradation pseudo first-order reaction rates, but both processes effectively removed pollutants. In actual application, the process with faster processing capacity will have more advantages. Anipsitakis and Dionysiou [9] and Ling et al., [10] showed that the Fe(II)/PMS process, which has a faster formation rate of reactive oxygen species than Fe(II)/PDS (Eqs. (3), (4)), has a higher free radical generation rate after further increase of UV. Therefore, it is important to rationally select Fenton's alternative process applications by comparing the efficiency and mechanistic differences of these two processes, which have similar formation rates of reactive species. Both the Fe(II)/FC and Fe(II)/PMS systems can accelerate the degradation of pollutants. Interestingly, k_{Fe(II), PMS} and k_{Fe(II), FC} are close in value (∼10⁴ M⁻¹ s⁻¹) (Eqs. 1–4) [11], [12], [13], [14], [15], so the Fe(II)/FC and Fe(II)/PMS processes theoretically generate reactive species at similar rates and have more advantages than the Fenton process in terms of kinetics. Therefore, the one with higher treating capacity and lower by-products forming is selected by comparing the two types of Fenton-like processes, this is meaningful for the subsequent development of derived processes based on this advantageous Fenton-like process.

In this study, iopamidol (IPM) was selected as the target pollutant. IPM is an iodinated X-ray contrast medium that is commonly used to enhance the visualization of soft tissues, internal organs and blood vessels in medical imaging. IPM have been frequently detected in source waters at concentrations of up to 2.7 μg L⁻¹ [16]. IPM not only contributes substantially to the total organic iodine in water but also acts as a potential iodine source for the formation of iodine disinfection by-products (I-DBPs), which are more cytotoxic and genotoxic than their chlorinated or brominated analogues [17]. This study aims to compare the mechanism of degradation of IPM via Fe(II)/FC and Fe(II)/PMS processes, determine the contribution of reactive substances, measure the reaction rate constant of Fe(IV) and IPM, investigate the effect of the water matrix on degradation, identify the transformation products of IPM by the Fe(II)/FC and Fe(II)/PMS processes, evaluate the toxicity of the oxidation process by observing the formation of highly toxic I-DBPs.