Phenolic wastewaters depuration by electrochemical oxidation process using Ti/IrO2 anodes

Toxicological aspect of water treated by chlorine-based advanced oxidation processes: A review

As well established in the literature, residual toxicity is an important parameter for evaluating the sanitary and environmental safety of water treatment processes, and this parameter becomes even more crucial when chlorine-based processes are applied for water treatment. Eliminating initial toxicity or preventing its increase after water treatment remains a huge challenge mainly due to the formation of highly toxic disinfection by-products (DBPs) that stem from the degradation of organic contaminants or the interaction of the chlorine-based oxidants with different matrix components. In this review, we present a comprehensive discussion regarding the toxicological aspects of water treated using chlorine-based advanced oxidation processes (AOPs) and the recent findings related to the factors influencing toxicity, and provide directions for future research in the area. The review begins by shedding light on the advances made in the application of free chlorine AOPs and the findings from studies conducted using electrochemical technologies based on free chlorine generation. We then delve into the insights and contributions brought to the fore regarding the application of NH₂Cl- and ClO₂-based treatment processes. Finally, we broaden our discussion by evaluating the toxicological assays and predictive models employed in the study of residual toxicity and provide an overview of the findings reported to date on this subject matter, while giving useful insights and directions for future research on the topic.

Role of hypochlorite in the harmless treatment of cyanide tailings through slurry electrolysis

The efficient and harmless treatment of cyanide tailings is necessary for gold extraction processes. The present study reports the effects of ClO^- generation in a slurry electrolysis system containing NaCl on the removal rate of cyanide and heavy metal ions in cyanide tailings. The chemical dissolution of metallic minerals and the reaction mechanisms were investigated by Fourier-transform infrared (FT-IR) and X-ray diffraction (XRD) analyses. The obtained results evidenced the key role of ClO^- in the removal of cyanide and heavy metal ions through the slurry electrolysis system with NaCl addition. Furthermore, ClO^- formation increased with the NaCl concentration, resulting in a higher removal rate of cyanide and heavy metal ions and enhanced metallic mineral dissolution. The cyanide tailings showed the best harmless effect with a NaCl concentration of 15 g/L. With this condition, the removal rates of CN_T, CN^-, Cu, Zn and Fe were 96.15%, 98.34%, 98.62%, 99.32% and 79.31%, respectively; furthermore, Cu and Fe precipitated at the cathode. The relative hematite content decreased by 3.12%. Under the effect of an electric field, the cyanide and metal cyanide complexes in the cyanide tailings oxidised and decomposed to release metal cations by the strongly oxidising ClO^- generated at the anode. The metal cations and hematite were reduced at the cathode, while the metal oxide mineral hematite in the electrolyte underwent chemical dissolution. In the toxic degradation of cyanide tailings, the comprehensive recovery of metals and destruction of metallic minerals in tailings will provide favourable conditions for subsequent comprehensive utilisation.

Removal of Recalcitrant Compounds from Winery Wastewater by Electrochemical Oxidation

The electro-oxidation of recalcitrant compounds, phthalic acid, tyrosol, and catechin was studied in simulated and real winery wastewater samples using a boron-doped diamond (BDD) anode. In the simulated samples, catechin, although presenting a higher removal rate than that of phthalic acid and tyrosol, attained lower combustion efficiency, indicating that this compound is readily converted into other products rather than being completely oxidized. On the other hand, phthalic acid was easily mineralized. Regarding the electro-oxidation assays performed with the spiked winery wastewater, recalcitrant compounds and overall organic load removal rates increased with applied current density (j), but the removal efficiency of recalcitrant compounds decreased with the increase in j, and the specific energy consumption was significantly raised. The increase in treatment time showed to be a feasible solution for the WW treatment at lower j. After 14 h treatment at 300 A m−2, phthalic acid, tyrosol, and catechin removals above 99.9% were achieved, with a chemical oxygen demand removal of 98.3%. Moreover, the biodegradability index was increased to 0.99, and toxicity towards Daphnia magna was reduced 1.3-fold, showing that the electro-oxidation process using a BDD anode is a feasible solution for the treatment of winery wastewaters, including phthalic acid, tyrosol, and catechin degradation.

Electrochemical degradation of psychotropic pharmaceutical compounds from municipal wastewater and neurotoxicity evaluations

Contaminants of emerging concern (CECs) are released daily into surface water, and their recalcitrant properties often require tertiary treatment. Electrochemical oxidation (EO) is often used as an alternative way to eliminate these compounds from water, although the literature barely addresses the neurotoxic effects of residual by-products. Therefore, this study investigated the performance of EO in the removal of five CECs (alprazolam, clonazepam, diazepam, lorazepam, and carbamazepine) and performed neurotoxicity evaluations of residual EO by-products in Wistar rat brain hippocampal slices. Platinum-coated titanium (Ti/Pt) and boron-doped diamond (BDD) electrodes were studied as anodes. Different current densities (13-75 A m^-2), pH values (3-10), electrolyte dosages (NaCl), and matrix effects were assessed using municipal wastewater (MWW). The drugs were successfully degraded after 5 min of reaction for both the Ti/Pt and BDD electrodes when a current density of 75 A m^-2 was applied. For Ti/Pt and BDD, neutral and acidic pH demonstrated better CEC removal performance, respectively. Compound degradation using MWW achieved 40% removal after 120 min for Ti/Pt and ranged between 33 and 52% for the BDD anode. For Ti/Pt, neurotoxicity studies using MWW indicated a decrease in reactive oxygen species (ROS) signals. However, when an artificial cerebrospinal fluid (ACSF) medium was reapplied, the signal recovered and increased to a value above the baseline, indicating that cells recovered part of their normal activity but remained in a different condition. For the BDD anode, the treated MWW did not cause significant ROS production variations, suggesting that he EO was effective in eliminating the toxicity of the treated solution.

Emerging contaminants removal from effluents with complex matrices by electrooxidation

The electrooxidation of methiocarb and bisphenol A was studied in complex matrices, namely, simulated and real sanitary landfill leachate samples, using a boron-doped diamond anode. With simulated sanitary landfill leachate samples, the influence of the type and ratio of carbon source (glucose/humic acid) and electrolyte (NaCl or Na₂SO₄) on the emerging contaminants removal was assessed. Using real sanitary landfill leachate, the influence of current density was evaluated. The experimental results showed that electrooxidation, using a boron-doped diamond anode, can be successfully utilized to degrade methiocarb and bisphenol A when present in complex matrices, such as sanitary landfill leachate, and that methiocarb is more easily oxidized than bisphenol A. Furthermore, it was found that the presence of chloride and high humic acid content increases emerging contaminants removal rate, showing that electrooxidation at boron-doped diamond is particularly adequate to solve the problems raised by sanitary landfill leachate, even when contaminated with emerging contaminants.

Carbon nanotube-grafted chitosan and its adsorption capacity for phenol in aqueous solution

Chitosan was covalently grafted onto the surface of multi-walled carbon nanotubes to create a novel chitosan/multi-walled carbon nanotube. The structure of the new material was characterized using Fourier transform-infrared spectroscopy, cross polarization magic angle spinning ¹³C nuclear magnetic resonance, thermogravimetric analysis, XRD ray diffraction analysis, differential scanning calorimetry and scanning electron microscopy. The phenol adsorption capacity was determined and the Langmuir and Freundlich models were used to describe the adsorption isotherms. The adsorption capacity of the novel chitosan/multi-walled carbon nanotube material for phenol (86.96 mg/g) was improved compared to the original chitosan (61.69 mg/g). The kinetic studies showed rapid adsorption, exhibiting Lagergren second-order kinetics. Therefore, this study provides a reference for preparing functional materials from biological substrates that are able to remove toxic pollutants from an aqueous environment.