The impact of cerium (Ce) and neodymium (Nd) rare-earth metal doping of TiO₂ prepared by the hydrothermal method was investigated to tailor effective photocatalytic degradation of coloured wastewater under UV or visible illumination. The hydrothermal treatment of TiO₂ decreased the pH_pzc from 6.3 to 3.1–3.8 favouring the affinity for cationic water contaminants. Doping with Ce and Nd modified the crystallinity and the morphology of the photocatalysts and significantly increased the BET surface area and the adsorption capacity of cationic dyes. The photocatalytic activity under UV light irradiation decreased due to shielding of the catalyst active area by excessive amount of dye adsorbed. Conversely, the photocatalytic activity of the Ce and Nd doped TiO₂ increased under visible light irradiation by 1.2 times as a result of the dye photosensitization effect. It was demonstrated that two-steps dark adsorption and photocatalytic reaction or one-step simultaneous adsorption and reaction can produce significantly different results for the photocatalytic degradation of dyes in coloured waters, the rate being controlled by the competitive adsorption of the reacting organics and the H₂O/OH⁻ species. The reaction is driven by the radical oxygen species (ROS) formed on the catalyst surface the nature of which, differs under UV or visible light irradiation. The Ce-doped TiO₂ and Nd-doped TiO₂ photocatalysts with 0.5% rare-earth content were found to be efficient in the degradation of MB in aqueous solution, removing the colour and reducing the toxicity of wastewaters. GRAPHICAL ABSTRACT

A laccase from Oudemansiella canarii was immobilized using the crosslinked enzyme aggregate (CLEA) methodology and applied in the degradation of the anthraquinonic dye Remazol Brilliant Blue R (RBBR). The immobilized laccase was superior to the free laccase in both thermal and storage stabilities. Both immobilized and free laccase decolourized 100 mg/L RBBR within 24 h at 30 °C and pH 5.0, but the former was still efficient in degrading the dye after at least 6 cycles. The relationship between the decolorization rate and the RBBR concentration obeyed Michaelis–Menten kinetics, with K_M of 0.126 ± 0.044 mmol/L and V_max of 1.412 ± 0.295 µmol/min for free laccase and K_M of 0.159 ± 0.050 mmol/L and V_max of 1.214 ± 0.242 µmol/min for immobilized laccase. Fourier transform infra-red spectroscopy (FTIR) and mass spectrometry allowed to conclude that the O. canarii laccase acts not only on the chromophore group of the dye, but that it also cleaves other covalent bonds, causing an effective fragmentation of the molecule. The Microtox assay detected a significant diminution in toxicity, a finding corroborated by the phytotoxicity test performed with lettuce seeds. Our results indicate that the immobilized laccase from O. canarii could be useful in biological strategies aiming at degrading unwanted dyes in the environment.

In this study, treatment for the removal of 1,4-dioxane by ozone and by catalytic ozonation using CuO as the catalyst was investigated. While the removal of 1,4-dioxane was small (20%) and mineralization negligible after 6 h of ozonation treatment, the removals of 1,4-dioxane and total organic carbon increased by factors of 10.35 and 81.25, respectively, after catalytic ozonation in the presence of CuO. The mineralization during catalytic ozonation was favoured at pH 10 (94.91 min⁻¹), although it proceeded even at pH 3 (54.41 min⁻¹). The CuO catalyst decreased the equilibrium concentration of soluble ozone and favoured its decomposition to reactive oxidative species. Radical scavenging experiments demonstrated that superoxide radicals were the main species responsible for the degradation of 1,4-dioxane. Further scavenging experiments with phosphate confirmed the presence of Lewis active sites on the surface of CuO, which were responsible for the adsorption and decomposition of ozone. The reaction mechanism proceeded through the formation of ethylene glycol diformate, which quickly hydrolyzed to ethylene glycol and formic acid as intermediate products. The stability of CuO indicated weak copper leaching and high catalytic activity for five recycling cycles. The toxicity of the water, assessed by Vibrio fischeri bioluminescence assays, remained the same (low toxicity) after catalytic ozonation while it increased after treatment with ozonation alone. GRAPHICAL ABSTRACT

Iron oxide with a high degree of purity was recovered from waste and used as an environmentally friendly, low-cost catalyst in the application of the photo-Fenton process to simulated petrochemical wastewater (SPW). Iron oxide nanoparticles were characterized by X-ray powder diffraction, transmission electron microscopy, N₂ adsorption/desorption isotherms, zeta potential, toxicity and atomic absorption spectrometry. The experiments were performed in a batch photochemical reactor, at 20 ± 2.0°C and pH 3.0. The SPW was efficiently mineralized and oxidized using a low catalyst dosage. The results showed that the organic compounds present in the wastewater were not adsorbed onto the solid surface. The solid was found to be stable with negligible leaching and low toxicity. The k_TOC/k_COD ratios were calculated and varied according to the process: for a homogeneous reaction, the ratio obtained was 0.31 and for the heterogenous photo-Fenton process, it was closer to 1. The chemical oxygen demand and total organic carbon removal values were very close, indicating that the SPW is immediately mineralized, without producing partially oxidized compounds. The residue-based goethite studied represents a good alternative to commercially available catalysts in terms of sources and availability.