Electrochemical reductive dehalogenation of iodine-containing contrast agent pharmaceuticals: Examination of reactions of diatrizoate and iopamidol using the method of rotating ring-disc electrode (RRDE)

Occurrence of iodinated contrast media (ICM) in water environments and their control strategies with a particular focus on iodinated by-products formation: A comprehensive review

Iodinated contrast media (ICM), one of the pharmaceutical and personal care products (PPCPs), are frequently detected in various water bodies due to the strong biochemical stability and recalcitrance to conventional water treatment. Additionally, ICM pose a risk of forming iodinated by-products that can be detrimental to the aquatic ecosystem. Consequently, effectively removing ICM from aqueous environments is a significant concern for environmental researchers. This article provides a comprehensive review of the structural characteristics of ICM, their primary source (e.g., domestic and hospital wastewater), detected concentrations in water environments, and ecological health hazards associated with them. The current wastewater treatment technologies for ICM control are also reviewed in detail with the aim of providing a reference for future research. Prior researches have demonstrated that traditional treatment processes (such as physical adsorption, biochemical method and chemical oxidation method) have inadequate efficiencies in the removal of ICM. Currently, the application of advanced oxidation processes to remove ICM has become extensive, but there are some issues like poor deiodination efficiency and the risk of forming toxic intermediates or iodinated by-products. Conversely, reduction technologies have a high deiodination rate, enabling the targeted removal of ICM. But the subsequent treatment issues related to iodine (such as I- and OI-) are often underestimated, potentially generating iodinated by-products during the subsequent treatment processes. Hence, we proposed using combined reduction-oxidation technologies to remove ICM and achieved synchronous control of iodinated by-products. In the future, it is recommended to study the degradation efficiency of ICM and the control efficiency of iodinated by-products by combining different reduction and oxidation processes.

Removal of Iodine-Containing X-ray Contrast Media from Environment: The Challenge of a Total Mineralization

Iodinated X-ray contrast media (ICM) as emerging micropollutants have attracted considerable attention in recent years due to their high detected concentration in water systems. It results in environmental issues partly due to the formation of toxic by-products during the disinfection process in water treatment. Consequently, various approaches have been investigated by researchers in order to achieve ICM total mineralization. This review discusses the different methods that have been used to degrade them, with special attention to the mineralization yield and to the nature of formed by-products. The problem of pollution by ICM is discussed in the first part dedicated to the presence of ICM in the environment and its consequences. In the second part, the processes for ICM treatment including biological treatment, advanced oxidation/reductive processes, and coupled processes are reviewed in detail. The main results and mechanisms involved in each approach are described, and by-products identified during the different treatments are listed. Moreover, based on their efficiency and their cost-effectiveness, the prospects and process developments of ICM treatment are discussed.

Comprehensive review on iodinated X-ray contrast media: Complete fate, occurrence, and formation of disinfection byproducts

Iodinated contrast media (ICM) are drugs which are used in medical examinations for organ imaging purposes. Wastewater treatment plants (WWTPs) have shown incapability to remove ICM, and as a consequence, ICM and their transformation products (TPs) have been detected in environmental waters. ICM show limited biotransformation and low sorption potential. ICM can act as iodine source and can react with commonly used disinfectants such as chlorine in presence of organic matter to yield iodinated disinfection byproducts (IDBPs) which are more cytotoxic and genotoxic than conventionally known disinfection byproducts (DBPs). Even highly efficient advanced treatment systems have failed to completely mineralize ICM, and TPs that are more toxic than parent ICM are produced. This raises issues regarding the efficacy of existing treatment technologies and serious concern over disinfection of ICM containing waters. Realizing this, the current review aims to capture the attention of scientific community on areas of less focus. The review features in depth knowledge regarding complete environmental fate of ICM along with their existing treatment options.

Effects of fulvic acids on the electrochemical reactions and mass transfer properties of organic cation toluidine blue: Results of measurements by the method of rotating ring-disc electrode

This study examined effects of aquatic and soil natural organic matter (NOM) exemplified by standard Suwannee River fulvic acid (SRFA) and Pahokee Peat fulvic acid (PPFA), respectively, on the electrochemical (EC) reactivity and mass transfer properties of the cationic organic probe toluidine blue (TB) that forms complexes with NOM. EC measurements that were carried out using the method of rotating ring-disc electrode (RRDE) showed that for disc potentials below -0.4 V vs. the standard Ag/AgCl reference electrode, TB molecules undergo EC reduction accompanied by the formation of EC-active products that undergo oxidation at the ring electrode. EC reactions of TB in the range of potentials -0.2 to -0.4 V were determined to involve free TB⁺ cations and TB species adsorbed on the electrode surface. The EC reduction of TB species at the disc potentials < -0.4 V was controlled by the mass transfer of the free TB⁺ cations and TB/NOM complexes to the electrode surface. Formation of TB/NOM complexes caused the mass transfer-controlled TB currents to undergo a consistent decrease. The observed changes were correlated with the extent of TB/NOM complexation and decreases of the diffusion coefficients of TB/NOM complexes that have higher molecular weights (MW) than the free cations. Properties of the intermediates formed upon the reduction of TB⁺ cations were also affected by NOM. These results demonstrate that RRDE measurements of EC reactions of TB or possibly other EC active probes allow probing the complexation of EC-active organic species with NOM and mass transfer properties of NOM complexes and ultimately NOM itself.

Electrochemical Processes Coupled to a Biological Treatment for the Removal of Iodinated X-ray Contrast Media Compounds

Iodinated X-ray contrast media (ICM) compounds are a form of intravenous radiocontrast containing iodine, which are rapidly eliminated via urine or feces. The issue with the accumulation of ICM has received considerable critical attention since they are ubiquitously distributed in municipal wastewater effluents and in the aquatic environment and are not significantly eliminated by most biological sewage treatment processes. Among the methods that have been tested to eliminate ICM, electrochemical methods have significant advantages, since they can selectively cut the carbon-iodine bonds that are suspected to decrease their biodegradability. On the production sites, the recovery of iodine ions due to the carbon-iodine cleavage can be envisaged, which is particularly interesting to reduce the cost of the ICM production process. The coupling of an electrochemical process and a biological treatment can be carried out to mineralize the organic part of the formed by-products, allowing the recovery of the iodide ions. Therefore, the degradation of diatrizoate, a typical ionic ICM compound, by an electrochemical process was the purpose of this study. The electrochemical reduction of diatrizoate was performed using a flow cell with a graphite felt electrode at different potentials. The removal yield of diatrizoate reached ~100% in 2 h and the main product, 3,5-diacetamidobenzoic acid, was quantitatively formed, showing that diatrizoate was almost completely deiodinated. According to the BOD₅/COD ratio, the biodegradability of diatrizoate after electrolysis was considerably improved. Cyclic voltammetry analysis of the electroreduced solution showed several oxidation peaks. The electrochemical oxidation of the by-products formed after the first treatment by electroreduction was then performed at three different potentials to study the influence of electrochemical oxidation on biodegradability. Results showed that the degradation yield of the deiodinated by-products increased with the potential and reached 100% at 1.3 V/SCE. Four different biological treatments were implemented during 21 days in stirred flasks with fresh activated sludge. The evolution of the mineralization during the biological treatment highlighted the biorecalcitrance of diatrizoate as previously estimated by the BOD₅/COD ratio. Interestingly, the mineralization yield increased from 41 to 60% when electrochemical oxidation at 1.3 V/SCE was implemented after electroreduction.

Amorphous nickel phosphide as a noble metal-free cathode for electrochemical dechlorination

Nickel phosphide (Ni₂P) is an emerging efficient catalyst for the hydrogen evolution and water splitting. Herein, we report that Ni₂P is also a promising catalyst for enhancing electrochemical dechlorination of chlorinated disinfection byproducts (DBPs). Amorphous Ni₂P (ANP) mini-nanorod arrays were in-situ fabricated on nickel foam (NF) via a facile phosphidation process, and then used as a binder-free cathode for electrochemical dechlorination of trichloroacetic acid (TCAA). Results showed that ANP exhibited superior performance on electrochemical dechlorination of TCAA than other metal cathodes (e.g., NF and Pd/C). Scavenging experiments and electron spin resonance (ESR) technique indicated that atomic H* was generated from water reduction through ANP catalysis, and primarily contributed to TCAA dechlorination. Indeed, the superhydrophilic surface of ANP favored electrocatalyst/electrolyte contact, and its low impedance further afforded rapid electron transport from the electrode to water or protons for atomic H* generation. The kinetic modelling and mass balance evaluation revealed the transformation mechanism of TCAA dechlorination. This study is among the first to develop ANP as a binder-free cathode for electrochemical dechlorination, and have important implications for eliminating chlorinated DBPs in water.

Electrochemical filtration process for simultaneous removal of refractory organic and particulate contaminants from wastewater effluents

Versatile electrochemical reactions are effective for removing a wide range of water contaminants. This study focuses on the development and testing of bifunctional electrocatalytic filter anodes as reactive and separating media for the simultaneous removal of refractory dissolved organic and particulate contaminants from real wastewater effluents. The results show that the TiO₂ particle interlayers formed between the Ti fiber support and the top composite metal oxide catalyst layers assist in reducing filter pores to an effective size range that enables removal of most particulates within the wastewater. The double-sheet design, which is a sandwich-structured module with an internal void space for permeate, prevents filter fouling, and transmembrane pressure can be maintained at a very low level of <5 kPa during batch and continuous flow reactor operations. Substantive and simultaneous removal of dissolved organics (e.g., chromophores, fluorophores, 1,4-dioxane, chemical oxygen demand, and total organic carbon) and particulate matter (i.e., turbidity) are achieved, although removal rates and efficacies differ depending on the electric current density applied. Decolorization and particulate rejection occur swiftly and immediately, but 1,4-dioxane degradation is relatively slow and quite time-dependent. Possible 1,4-dioxane degradation pathways during electrocatalysis are also proposed. Electrochemical filtration technology shows considerable promise for use in the next generation of advanced wastewater treatment solutions.