Elimination of radiocontrast agent Diatrizoic acid from water by electrochemical advanced oxidation: Kinetics study, mechanism and mineralization pathway

Toward decentralized wastewater treatment: A flow-through module using microtubular gas diffusion electrodes for micropollutants removal

Electro-Fenton (EF) represents an eco-friendly and cost-effective advanced oxidation process that can remove highly persistent and hazardous pharmaceuticals, e.g., contrast media agents, from water bodies. However, up to date, EF modules incorporate a planar carbonaceous gas diffusion electrode (GDE) cathode containing fluorinated compounds as polymeric binders. Here, we introduce a novel flow-through module that deploys freestanding carbon microtubes (CMT) as microtubular GDEs, omitting any risks of secondary pollution by highly-persistent fluorinated compounds (e.g., Nafion). The flow-through module was characterized for electrochemical hydrogen peroxide (H₂O₂) generation and micropollutant removal via EF. H₂O₂ electro-generation experiments illustrated high production rates (1.1 ± 0.1-2.7 ± 0.1 mg cm^-2 h^-1) at an applied cathodic potential of - 0.6 V vs. SHE, depending on the porosity of CMTs. Diatrizoate (DTZ), as the model pollutant, with a high initial concentration of 100 mg L^-1 was successfully oxidized (95-100 %), reaching mineralization (TOC-total organic carbon removal) efficiencies up to 69 %. Additionally, Electro-adsorption experiments demonstrated the capability of positively charged CMTs to remove negatively charged DTZ with a capacity of 11 mg g^-1 from a 10 mg L^-1 DTZ solution. These results reveal the potential of the as-designed module to serve as an oxidation unit coupled with other separation techniques, e.g., electro-adsorption or membrane 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.

Application and performance evaluation of a cost-effective vis- LED based fluidized bed reactor for the treatment of emerging contaminants

Visible light induced photocatalysis is considered as one of the most potential technologies which can achieve new levels of sustainability in water treatment. The current study explores the performance of immobilized visible light active catalyst on inert media for light driven catalysis of pharmaceuticals. These coated media is used in a continuous flow fluidized column reactor equipped with spirally arranged visible Light Emitting Diodes (LEDs) as irradiation source. The treatment efficiency of the system is evaluated for the removal of pharmaceutical drugs such as carbamazepine, diclofenac and ibuprofen. For the present study, system parameters such as light intensity and flow rate are optimized for maximum removal rate. The system shows complete elimination of the pharmaceuticals under the given experimental conditions. Complete mineralization of the target compounds are confirmed by TOC analysis. Recyclability is an important attribute for full scale commercialization of a treatment technology. An investigation on the reusability study of the photocatalyst displayed no significant reduction in the removal efficiency for a run of six cycles, hence rendering the photocatalyst reusable. The results acquired indicate an immense potential for scaling up the photoreactor as a sustainable tertiary treatment technology in water treatment plants.

Removal of organochlorine pesticides from lindane production wastes by electrochemical oxidation

This study is focused on the effective removal of recalcitrant pollutants hexaclorocyclohexanes (HCHs, isomers α, β, γ, and δ) and chlorobenzenes (CBs) present in a real groundwater coming from a landfill of an old lindane factory. Groundwater is characterized by a total organic carbon (TOC) content of 9 mg L^-1, pH₀ = 7, conductivity = 3.7 mS cm^-1, high salt concentration (SO₄^2-, HCO₃^-, Cl^-), and ferrous iron in solution. The experiments were performed using a BDD anode and a carbon felt (CF) cathode at the natural groundwater pH and without addition of supporting electrolyte. The complete depletion of the four HCH isomers and a mineralization degree of 90% were reached at 4-h electrolysis with a current intensity of 400 mA, the residual TOC (0.8 mg L^-1) corresponding mainly to formic acid. A parallel series reaction pathway was proposed: HCHs and CBs are transformed into chlorinated and hydroxylated intermediates that are rapidly oxidized to non-toxic carboxylic acids and/or mineralized, leading to a rapid decrease in solution pH.

Removal of lindane wastes by advanced electrochemical oxidation

The effective removal of recalcitrant organochlorine pesticides including hexachlorocyclohexane (HCH) present in a real groundwater coming from a landfill of an old lindane (γ-HCH) factory was performed by electrochemical oxidation using a BDD anode and a carbon felt cathode. Groundwater (ΣHCHs = 0.42 mg L^-1, TOC₀ = 9 mg L^-1, pH₀ = 7, conductivity = 3.7 mS cm^-1) was treated as received, achieving the complete depletion of the HCH isomers and a mineralization degree of 90% at 4 h electrolysis at constant current of 400 mA. Initial groundwater contains high chloride concentration (Cl₀^- = 630 mg L^-1) that is progressively decreased due to its oxidation to different oxychlorine species: Cl₂, HClO, ClO^-, ClO₂^- ClO₃^- and ClO₄^- some of them (Cl₂, HClO, ClO^-) playing an important role in the oxidation of organic pollutants. The oxidation rate of chloride (and its oxidized intermediates) depends on the applied current value. Although some of the species generated from them are active oxidants, the presence of inorganic salts is detrimental to the efficiency of the electrochemical process when working at current densities above 100 mA due to the high consumption of hydroxyl radicals in wasting reactions. The initial organic carbon content is not crucial for the extension of the process but high organic loads are more profitable for cost effectiveness. The addition of a supporting electrolyte to the solution could be interesting since it increases the conductivity, reducing the cell potential and therefore, decreasing the energy consumption.

Removal of artificial sweetener aspartame from aqueous media by electrochemical advanced oxidation processes

The degradation and mineralization of aspartame (ASP) in aqueous solution were investigated, for the first time, by electrochemical advanced oxidation processes (EAOPs) in which hydroxyl radicals were formed concomitantly in the bulk from Fenton reaction via in situ electrogenerated Fenton's reagent and at the anode surface from the water oxidation. Experiments were performed in an undivided cylindrical glass cell with a carbon-felt cathode and a Pt or boron-doped diamond (BDD) anode. The effect of Fe²⁺ concentration and applied current on the degradation and mineralization kinetics of ASP was evaluated. The absolute rate constant for the reaction between ASP and OH was determined as (5.23 ± 0.02) × 10⁹ M^-1 s^-1 by using the competition kinetic method. Almost complete mineralization of ASP was achieved with BDD anode at 200 mA constant current electrolysis. The formation and generation of the formed carboxylic acids (as ultimate end products before complete mineralization) and released inorganic ion were monitored by ion-exclusion high performance liquid chromatography (HPLC) and ion chromatography techniques, respectively. The global toxicity of the treated ASP solution during treatment was assessed by the Microtox^® method using V. fischeri bacteria luminescence inhibition.

Elimination of radiocontrast agent diatrizoic acid by photo-Fenton process and enhanced treatment by coupling with electro-Fenton process

The removal of radiocontrast agent diatrizoic acid (DIA) from water was performed using photo-Fenton (PF) process. First, the effect of H2O2 dosage on mineralization efficiency was determined using ultraviolet (UV) irradiation. The system reached a maximum mineralization degree of 60 % total organic carbon (TOC) removal at 4 h with 20 mM initial H2O2 concentration while further concentration values led to a decrease in TOC abatement efficiency. Then, the effect of different concentrations of Fenton's reagents was studied for homogeneous Fenton process. Obtained results revealed that 0.25 mM Fe(3+) and 20 mM H2O2 were the best conditions, achieving 80 % TOC removal efficiency at 4 h treatment. Furthermore, heterogeneous PF treatment was developed using iron-activated carbon as catalyst. It was demonstrated that this catalyst is a promising option, reaching 67 % of TOC removal within 4 h treatment without formation of iron leachate in the medium. In addition, two strategies of enhancement for process efficiency are proposed: coupling of PF with electro-Fenton (EF) process in two ways: photoelectro-Fenton (PEF) or PF followed by EF (PF-EF) treatments, achieving in both cases the complete mineralization of DIA solution within only 2 h. Finally, the Microtox tests revealed the formation of more toxic compounds than the initial DIA during PF process, while, it was possible to reach total mineralization by both proposed alternatives (PEF or PF-EF) and thus to remove the toxicity of DIA solution.