Abstract
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.
Highlights
Since the application of X-ray on medical diagnostic imaging examination, iodinated X-ray contrast media (ICM) have been extensively used as contrast agents due to their ability to enhance the visibility of image
To associate the benefit of a selective electrochemical processes and low expensive biological treatments (Assassi et al, 2011; Fontmorin et al, 2014b; Belkheiri et al, 2015; Saidi et al, 2017; Zaghdoudi et al, 2017; Geneste, 2018), we investigated the coupling of both methods for the removal of the highly recalcitrant contrast agent, diatrizoate (DTR) (Scheme 1)
An electrochemical pretreatment was performed to improve its biodegradability before a biological process to complete its mineralization
Summary
Since the application of X-ray on medical diagnostic imaging examination, iodinated X-ray contrast media (ICM) have been extensively used as contrast agents due to their ability to enhance the visibility of image. ICM belong to the derivatives of 2,4,6-triiodobenzoic acid, which usually contain polar carboxyl, hydroxyl or amide moieties on their side chains. Three iodine atoms were introduced on the benzene ring for X-ray adsorption; other polar groups were added to assure their high water solubility and resistance to human body metabolism (Yu and Watson, 1999; Estep et al, 2000). ICM have been frequently detected in hospital effluents, domestic sewage systems as well as drinking water at relatively high concentrations. For example in the downstream of Danube river in Germany, more than 500 ng L−1 of diatrizoate acid and iopamidol were found (Seitz et al, 2006), whereas in a Spain Sewage Treatment Plant, the concentration of iopromide was in the range 6.60–9.30 μg L−1 (Carballa et al, 2004) and organic iodine concentration in Texas in the United States, varied between 5 and 40 μg L−1 (Drewes et al, 2001). ICM ecotoxicology still needs to be further confirmed, many researchers have claimed that ICM are transformed into toxic by-products during drinking water disinfection process (Jeong et al, 2017)
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