In the framework of wastewater treatment plants, sewage sludge can be directed to biochar production, which when coupled with an external iron source has the potential to be used as a carbon–iron composite material for treating various organic pollutants in advanced oxidation processes. In this research, “green” synthesized nano zero-valent iron (nZVI) supported on sewage sludge-based biochar (BC)–nZVI-BC was used in the Fenton process for the degradation of the recalcitrant organic molecule. In this way, the circular economy principles were supported within wastewater treatment with immediate loop closing; unlike previous papers, where only the water treatment was assessed, the authors proposed a new approach to wastewater treatment, combining solutions for both water and sludge. The following phases were implemented: synthesis and characterization of nano zero-valent iron supported on sewage sludge-based biochar (nZVI-BC); optimization of organic pollutant removal (Reactive Blue 4 as the model pollutant) by nZVI-BC in the Fenton process, using a Definitive Screening Design (DSD) model; reuse of the obtained Fenton sludge, as an additional catalytic material, under previously optimized conditions; and assessment of the exhausted Fenton sludge’s ability to be used as a source of nutrients. nZVI-BC was used in the Fenton treatment for the degradation of Reactive Blue 4—a model substance containing a complex and stable anthraquinone structure. The DSD model proposes a high dye-removal efficiency of 95.02% under the following optimal conditions: [RB4] = 50 mg/L, [nZVI] = 200 mg/L, [H₂O₂] = 10 mM. pH correction was not performed (pH = 3.2). Afterwards, the remaining Fenton sludge, which was thermally treated (named FS_treated), was applied as a heterogeneous catalyst under the same optimal conditions with a near-complete organic molecule degradation (99.56% ± 0.15). It could be clearly noticed that the cumulative amount of released nutrients significantly increased with the number of leaching experiments. The highest cumulative amounts of released K, Ca, Mg, Na, and P were therefore observed at the fifth leaching cycle (6.40, 1.66, 1.12, 0.62, 0.48 and 58.2 mg/g, respectively). According to the nutrient release and toxic metal content, FS_treated proved to be viable for agricultural applications; these findings illustrated that the “green” synthesis of nZVI-BC not only provides innovative and efficient Fenton catalysts, but also constitutes a novel approach for the utilization of sewage sludge, supporting overall process sustainability.

The sorption behavior of the herbicides atrazine, alachlor and trifluralin on four sorbents (two modified organoclays, one model sediment, and one natural sediment) in three water matrices (synthetic water, natural groundwater and surface water) was investigated. The influence of the sorbent and water matrix characteristics was elucidated. All adsorption isotherms fitted well with the Freundlich model. Based on the K_d values, the organically modified bentonite and model sediment proved to be the most effective sorbents for the investigated herbicides. A linear correlation of K_d with the total organic carbon (TOC) concentration was found for trifluralin in the synthetic water matrix (c₀ = 0.01 mg L⁻¹). In the case of higher trifluralin concentration and for other tested herbicides at all concentrations the influence of both the sorbent organic matter quality and possible interactions with the mineral phase was indicated. Furthermore, the results showed different influence of the type and the content of dissolved organic carbon (DOC) in the water phase on the sorption of all the herbicides, depending on the herbicide concentration and the sorbent applied. It was confirmed that the comparison of sorbent efficiency should be done in native matrix since sorption coefficients vary depending on pollutant concentration, DOC concentration and DOC fraction. This finding is considered to be important for remediation of contaminated sites.

This research investigates the potential use of pyrite cinder (PC) as an efficient Fenton-like catalyst for the removal of the reactive azo dye Reactive Red 120 (RR120) from aqueous solutions. The characterization of its PC structure and composition confirmed its great potential to act as catalytic iron source in a heterogeneous Fenton system. Dye removal optimization was performed in terms of PC dosage (0.4–8 g/L), H₂O₂ concentration (2–25 mM), pH value (2–4.6), initial dye concentration (50–200 mg/L), and mixing time. The highest decolorization efficiency (92%) was achieved after a reaction time of 480 min under following conditions: RR120 = 50 mg/L, PC = 4 g/L, H₂O₂ = 10 mM, and pH = 3. After decolorization, an extensive analysis of the generated effluent was performed regarding metal leaching, mineralization, toxicity, and degradation product formation. The metal leaching indicated the necessity for a pH increase in order to remove the settled metal hydroxides. The mineralization efficiency was satisfactory, reaching 85% and 62% of the COD and TOC removal, respectively. The respirometry measurements and bioluminescence tests indicated the detoxification of the treated solution. The absorption spectra and GC/MS analysis confirmed the changes in the molecular structure in the form of the destruction of the azo bond, with a simpler aromatic and aliphatic intermediates formation. This study provides an effective method for removing azo dye in polluted water by employing waste tailings as alternative Fenton-like catalysts, while also using waste tailings as the secondary resource.

Diclofenac (DCF) and metformin (MET) are pharmaceuticals often detected in influents and effluents of municipal sewage treatment plants and surface waters which may cause adverse effects to human health and the environment. In recent years, advanced oxidation processes (AOPs) have been demonstrated to be effective technology for the removal of many organic pollutants. The objective of this study was to evaluate the removal and toxicity of investigated pharmaceuticals by UV and UV/H₂O₂ processes. UV irradiation was provided by a Pen Ray lamp emission at 254 nm which was covered with a quartz tube and placed in the middle of the reactor. Experimental conditions of the process were: [DIC, MET] = 10 mg/L, [H₂O₂] = 2.5 mM, reaction time 3 h. Results obtained by only UV exposition of solution showed that diclofenac and metformin were degradated 30% and 50% during 3 h test. Addition of 2.5 mM H₂O₂ to photo reactor contributed to 90% and 100% removal of DCF and MET, respectively. Inhibition toxicity test of MET increased in the following range: 13% (C0) < 38% (UV) < 77% (UV/ H₂O₂), while toxicity measured for DCF solutions followed the range: 33% (UV) < 75% (C0) < 78% (UV/H₂O₂). The results confirmed that addition of hydrogen-peroxide accelerated the removal of investigated pharmaceuticals, but at the same time, lead to formation of the more toxic intermediates. The possible reason for better removal efficiency of metformin can be related to its simpler aliphatic structure in comparison to more aromatic diclofenac.

The production of essential oils generates substantial quantities of solid post-distillation residues, a largely unutilized waste stream rich in bioactive compounds (e.g., phenolics, flavonoids) as well as polysaccharides. Managing this organic waste presents both environmental and economic challenges. This review critically examines environmentally friendly green innovations and resource-efficient technologies within circular bio-economy strategies for valorizing these residues, focusing on four primary conversion pathways: physico-mechanical, thermochemical, biological, and chemical methods. We highlight their potential for practical applications, including the extraction of active compounds for food, cosmetic, and pharmaceutical industries, utilization in agriculture, incorporation into construction materials and wastewater treatment. Despite these opportunities, wider industrial adoption remains limited by high processing costs and the lack of scalable, cost-effective technologies. Key research gaps included the need for methods applicable at the farm level, optimization of the residue-specific conversion process, and life-cycle assessments to evaluate environmental and economic impacts. Addressing these gaps is crucial to fully exploit the economic and ecological potential of post-distillation solid residues and integrate them into sustainable circular bio-economy practices through various processes.