In recent years, heterogeneous photocatalysis has emerged as a new effective, powerful, clean, and safe decontamination technology for the treatment of organic pollutants and the transformation of hazardous chemicals into different forms. This review focuses on the recent development of various conventional technologies of reactors designated for the photodegradation of hazardous organic pollutants with their limitations. This phenomenon is strongly influenced by reaction conditions such as temperature of reaction, pH, light intensity and wavelength, pollutant concentration, photocatalyst quantity, relative humidity (RH), and other parameters. The catalyst photoactivity depends on the structural characteristics of the semiconductor, its morphology, and its particles size. This paper presents the progression of photocatalytic reactors for synthetic dyes degradation with special consideration to the use of supported photocatalyst and nanostructured titanium supported over volcanic ashes, owing to the major advantage of an easy separation of the catalyst when compared to homogenous system, namely suspended catalyst. In addition, special attention was paid to the literature dealing with the promotion of light efficiency by testing various light sources.

In the present study, we propose a new photocatalytic interface prepared by high-power impulse magnetron sputtering (HiPIMS), and investigated for the degradation of Reactive Green 12 (RG12) as target contaminant under visible light light-emitting diodes (LEDs) illumination. The CuxO/TiO2 nanoparticulate photocatalyst was sequentially sputtered on polyester (PES). The photocatalyst formulation was optimized by investigating the effect of different parameters such as the sputtering time of CuxO, the applied current, and the deposition mode (direct current magnetron sputtering, DCMS or HiPIMS). The results showed that the fastest RG12 degradation was obtained on CuxO/TiO2 sample prepared at 40 A in HiPIMS mode. The better discoloration efficiency of 53.4% within 360 min was found in 4 mg/L of RG12 initial concentration and 0.05% Cuwt/PESwt as determined by X-ray fluorescence. All the prepared samples contained a TiO2 under-layer with 0.02% Tiwt/PESwt. By transmission electron microscopy (TEM), both layers were seen uniformly distributed on the PES fibers. The effect of the surface area to volume (dye volume) ratio (SA/V) on the photocatalytic efficiency was also investigated for the discoloration of 4 mg/L RG12. The effect of the presence of different chemicals (scavengers, oxidant or mineral pollution or salts) in the photocatalytic medium was studied. The optimization of the amount of added hydrogen peroxide (H2O2) and potassium persulfate (K2S2O8) was also investigated in detail. Both, H2O2 and K2S2O8 drastically affected the discoloration efficiency up to 7 and 6 times in reaction rate constants, respectively. Nevertheless, the presence of Cu (metallic nanoparticles) and NaCl salt inhibited the reaction rate of RG12 discoloration by about 4 and 2 times, respectively. Moreover, the systematic study of reactive oxygen species’ (ROS) contribution was also explored with the help of iso-propanol, methanol, and potassium dichromate as •OH radicals, holes (h+), and superoxide ion-scavengers, respectively. Scavenging results showed that O2− played a primary role in RG12 removal; however, •OH radicals’ and photo-generated holes’ (h+) contributions were minimal. The CuxO/TiO2 photocatalyst was found to have a good reusability and stability up to 21 cycles. Ions’ release was quantified by means of inductively coupled plasma mass spectrometry (ICP-MS) showing low Cu-ions’ release.

Flax shives and beech wood residues represent biomass streams that are abundant in Northwest Europe. These primary feedstocks were evaluated for their suitability to produce biochar as a low environmental-impact adsorbent. The efficacy of the produced biochars was tested by their adsorption capacity towards methylene blue (MB). A series of adsorption tests with carbamazepine is also presented, focusing on the better performing beech wood biochar. Post treatment of the biochars with citric acid (CA) and oxidation of the surface by heating at 250 °C in a muffle oven were carried out to enhance the adsorption capacities of both flax shives biochar (FSBC) and beech biochar (BBC). The resulting physicochemical characteristics are described. The thermally treated biochars have specific surface areas of 388 m²·g⁻¹ and 272 m²·g⁻¹ compared to the untreated biochars with 368 and 142 m²·g⁻¹ for BBC and FSBC, respectively. CA treatment leads to enhancement of the oxygenated surface functional groups and the adsorption capacities of both studied biochars. The non-linear Langmuir and Freundlich models show the best fit for both the isotherm data for MB and the CMZ adsorption with a good correlation between the experimental and calculated adsorption capacities. The effect of adsorbent dosages and initial concentrations of MB and CMZ on the adsorption efficiency is discussed. It can be concluded that beech biochar is a very promising pollutant adsorbent only requiring a mild, low-cost, and low-environmental impact activation treatment for best performance.

In this work, a comparative study between two processes was performed—biodegradation and photocatalysis, as an advanced oxidation process—to discover which one is more efficient to degrade Rhodamine B, a synthetic dye widely used in the textile and food industries. The advantage of this study is that it correlates treatment efficiency with the ecotoxicity of the by-products resulting from the treatments. Since the COVID-19 pandemic, it has been difficult to use activated sludge because of the risk factor of COVID-19 infection. Therefore, biodegradation tests were conducted with the yeast Saccharomyces cerevisiae in this study. For the photocatalysis assays, TiO₂ doped with 5 per cent Cerium was used as a catalyst under UV light irradiation. S. cerevisiae cannot reduce RhB by biodegradation. However, a 13 per cent biosorption was observed with an uptake capacity of 4.2 mg g⁻¹ dry matter of S. cerevisiae cultivated in the presence of 5 mg L⁻¹ of RhB after 150 min. At a 5 mg L⁻¹ of RhB concentration, the 6 h photocatalysis treatment led to 55% color removal and 8.6% COT reduction. The biodegradability of the photocatalyzed solution increased since the BOD₅/COD ratio raised from 0.10 to 0.42. In the presence of glucose as a source of carbon, yeast can grow on the by-products generated by photocatalysis. The phytotoxicity of RhB in solution was measured using the germination index (GI) of watercress seeds. The GI decreases by 75% for an RhB solution of 100 mg L⁻¹ compared to the control sample. The by-products of the photocatalytic treatment, using crustaceans Daphnia magna and conducted with solutions of Rhodamine B, induced a decrease of 24% in the GI. Lethality test. After 3 or 6 h of treatment, no increase in immobilization or mortality of D. magna was observed compared to the negative control.