Complex wastewater matrices present a major environmental concern. Besides the biodegradable organics, they may contain a great variety of toxic chemicals, heavy metals, and other xenobiotics. The electrochemically activated persulfate process, an efficient way to generate sulfate radicals, has been widely applied to the degradation of such complex effluents with very good results. This review presents the fundamentals of the electro-persulfate processes, highlighting the advantages and limitations, followed by an exhaustive evaluation on the application of this process for the treatment of complex industrial effluents. An overview of the main relevant experimental parameters/details and their influence on the organic load removal is presented and discussed, having in mind the application of these technologies at an industrial scale. Finally, the future perspectives for the application of the electro-persulfate processes in the treatment of complex wastewater matrices is outlined.

In this study, Bi-doped SrTiO₃ perovskites (Sr_1−xBi_xTiO₃, x = 0, 0.03, 0.05, 0.07 and 0.1) were synthesized using the solid-state method, characterized, and tested as photocatalysts in the degradation of the azo dye acid orange 7 (AO7) under visible light. The perovskites were successfully synthesized, and XRD data showed a predominant, well-crystallized phase, belonging to the cubic perovskite symmetry. For the doped samples, a minority phase, identified as bismuth titanate, was detected. All doped samples exhibited improved photocatalytic activity under visible light, on the degradation of AO7 (10 mg L⁻¹), when compared with the undoped SrTiO₃, with an increase in relative Abs_{484 nm} decay from 3.7% to ≥67.8% after 1 h, for a powder suspension of 0.2 g L⁻¹. The best photocatalytic activity was exhibited by the Sr_0.95Bi_0.05TiO₃ perovskite. Reusability studies showed no significant loss in photocatalytic activity under visible light. The final solutions showed no toxicity towards D. magna, proving the efficiency of Sr_0.95Bi_0.05TiO₃ as a visible-light-driven photocatalyst to degrade both the AO7 dye as well as its toxic by-products. A degradation mechanism is proposed.

Wastewater reuse has been addressed to promote the sustainable water utilization in textile industry. However, conventional technologies are unable to deliver treated wastewater with the quality required for reuse, mainly due to the presence of dyes and high salinity. In this work, the feasibility of electrooxidation, using a boron-doped diamond anode, to provide treated textile dyeing wastewater (TDW) with the quality required for reuse, and with complete recovery of salts, was evaluated. The influence of the applied current density on the quality of treated TDW and on the consecutive reuse in new dyeing baths was studied. The ecotoxicological evaluation of the process towards Daphnia magna was performed. After 10 h of electrooxidation at 60 and 100 mA cm⁻², discolorized treated TDW, with chemical oxygen demand below 200 (moderate-quality) and 50 mg L⁻¹ (high-quality), respectively, was obtained. Salt content was unchanged in both treatment conditions, enabling the consecutive reuse without any salt addition. For the two reuse cycles performed, both treated samples led to dyed fabrics in compliance with the most restrictive controls, showing that an effective consecutive reuse can be achieved with a moderate-quality water. Besides the water reuse and complete salts saving, electrooxidation accomplished an ecotoxicity reduction up to 18.6-fold, allowing TDW reuse without severe ecotoxicity accumulation.

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.

The behaviors of the electrodes Ti/PbO₂ and Ti/Pt/PbO₂ as anodes in the electro-oxidation of two antibiotics—tetracycline and oxytetracycline—were evaluated at different applied current densities, to evaluate the influence of the Pt interlayer. In the preparation of the electrodes, the electrodeposited β-PbO₂ phase was homogeneous; no Ti or Pt peaks were detected in the diffractograms. The β-PbO₂ surface presented significant roughness when deposited over the Pt interlayer, which also conferred significant conductivity to the material. In the electro-oxidation assays, the COD, TOC and absorbance removals increased with the current density due to an increase in the concentration of hydroxyl radicals, for both electrode materials and antibiotics tested. Slightly better results were obtained with Ti/PbO₂. The primary differences observed in the antibiotics concentration decay consisted of zero-order kinetics at the Ti/Pt/PbO₂ anode and first-order kinetics at the Ti/PbO₂ anode with a higher oxytetracycline concentration decay than the tetracycline concentration decay. A greater amount of total nitrogen was eliminated with the Ti/PbO₂ electrode. At the Ti/Pt/PbO₂ anode, the organic nitrogen primarily transformed into NH₄⁺ and the total nitrogen remained unchanged. The specific energy consumption with the Ti/Pt/PbO₂ anode was significantly lower than the specific energy consumption with the Ti/PbO₂ anode due to the higher electrical conductivity of the Ti/Pt/PbO₂ anode. Both anode materials were also utilized in the electro-oxidation of a leachate sample collected at sanitary landfill and spiked with tetracycline, and the complete elimination of the antibiotic molecule was observed.