Colorants, Volume 5, Issue 1 (March 2026) – 9 articles

Advanced oxidation processes (AOPs) have been widely applied to treat textile wastewater, in which synthetic dyes are among the main pollutants. Some of these processes, such as the Fenton reaction, exhibit enhanced efficiency when coupled with radiation sources, particularly when combined with a compound parabolic concentrator (CPC). In this study, a UV-A photo-Fenton process assisted by CPC, constructed using reused fluorescent lamps as reaction tubes and operating with recirculation was applied to treat real textile wastewater. A preliminary factorial design was employed to optimize reagent concentrations, identifying optimal conditions of 2647.8 g·L⁻¹ of H₂O₂ and 15 mg·L⁻¹ of Fe²⁺. Overall, the use of the CPC led to an increase in photon availability, resulting in COD degradation efficiencies of 83%, corresponding to an ~19% relative increase in treatment efficiency, compared to the system without the CPC, as well as 79% removal efficiency for apparent color and 57% for turbidity. Results demonstrate that the CPC-assisted UV-A photo-Fenton process is an efficient and robust approach for treating real textile wastewater. Meanwhile, the reuse of fluorescent lamps represents a low-cost, environmentally sustainable alternative that contributes to waste valorization and process intensification. Full article

Dye-sensitized solar cells (DSSCs) represent a promising low-cost photovoltaic technology with relatively high conversion efficiency and a simple fabrication process. Natural dyes have drawn growing interest compared to ruthenium-based dyes since they are greener. However, the power conversion efficiency (PCE) of natural dyes is generally low. In this study, we investigated novel approaches to improve the PCE of DSSCs using Delonix regia extracts by polarity-based separation using preparative thin-layer chromatography (PTLC). Our study indicated that polarity-based separation can significantly enhance the PCE, with one fraction achieving a PCE of 1.13%, which is high compared to most natural dye-based DSSCs, and is also 1.85 times that of the crude methanol extract. The major compounds in the highest-efficiency layer were flavanol-based dyes. Our study demonstrates the potential antagonistic effects within Delonix regia extracts in DSSC applications, which play a critical role in improving PCE. The study is expected to support future efforts to enhance the PCE of natural compound-based DSSCs, especially those using flavanol-based natural dyes. Full article

Water pollution from industrial dyes is a critical challenge due to the resistance of these types of compounds to degradation and potentially harmful effects on living organisms and human health. In this study, the electrochemical degradation of methylene blue (MB) was investigated using ink-based copper foam electrodes with reduced graphene oxide (rGO), antimony trioxide (Sb₂O₃), and rGO/Sb₂O₃ composites. The materials used to synthesize the electrodes were characterized by X-ray diffraction (XRD), which showed the successful synthesis of GO, rGO, and the Sb₂O₃-rGO composite. Additionally, the synthesized electrodes were examined using SEM. The MB degradation was studied using kinetic behavior and removal efficiency at pH levels from 3 through 6, monitored using UV-Vis spectroscopy. The electrocatalytic degradation was studied using sodium sulfate as the electrolyte across a pH range of 3 to 8. All electrodes investigated were determined to follow first-order kinetics. The Sb₂O₃-rGO composite showed the highest rate constants of MB degradation at pH 7 and 8, with rate constants of 0.0160 and 0.0159 min⁻¹, respectively. At the same time, the rGO ink-based electrode worked fastest at pH 3 and pH 4 with rate constants of 0.0178 and 0.0158 min⁻¹, respectively. The Sb₂O₃ also works best at pH 3 and 4 with rate constants of 0.0151 and 0.0152 min⁻¹. SEM analysis shows the composite electrode was more resilient to degradation than other materials. Full article

Heritage materials science techniques, including pXRF, FTIR-ATR, XRD, microphotography, and microsampling, have peeled back concealed layers of polychromy on a Roman Mithraic altar near Hadrian’s Wall. The results break new ground by exposing the interplay between light and dark and the transformative impact of colour cast onto cultic carved stone. A powerful pigment palette is revealed, including vibrant cinnabar/vermilion letters overlying an unprecedented purple inscription panel created from a compound of Egyptian blue, ultramarine, cinnabar/vermilion, red ochre, red lead, and realgar framed with purple, Egyptian blue, ultramarine, and orpiment. The panel was covered in a golden surface crafted from a previously unknown recipe of pyrite (fool’s gold) mixed with beeswax. Tantalising traces are also detected on some iconographic features, but conservator intervention and degradation processes combined to leach colour from the sculpted relief. These are paradigm-shifting results. They force a reinterpretation of the symbolism and performance of altars as personifications of dedicators, and we identify, for the first time, high-ranking Mithraic initiates by name and grade. Critically, we present a digital reconstruction of the altar with original polychromy that revolutionises our understanding of relief-sculpted Roman inscriptions—a category that has, until now, remained underexplored by the burgeoning polychromy research community. Full article

The trend toward developing sustainable nanotechnology has driven researchers to explore environmentally friendly techniques for nanomaterial fabrication. This review examines the utilisation of Commelina benghalensis plant extracts as an effective biological tool for the green synthesis of various nanomaterials. The procedures involve reducing metal salt precursors with aqueous or polar solvent extracts rich in phytochemicals such as flavonoids and polyphenols, followed by a calcination step that yields crystalline products. The findings show that the properties of ZnO, TiO₂, Ag, NiO, and their composites are directly influenced by synthesis factors, including solvent, plant component, and extract concentration. This directly influenced their specific sizes, morphologies, and phases. Furthermore, these C. benghalensis-mediated nanomaterials showed high efficiency in the photocatalytic degradation of textile dyes and pharmaceuticals, as well as potential antibacterial and antioxidant properties. The Commelina benghalensis plant is flexible and renewable for efficient nanomaterial synthesis; nevertheless, issues with standardisation and scalability must be overcome to fully realise its promise for commercial and industrial uses. Full article

Hair dyes are widely used across all socioeconomic groups and regions worldwide. However, some studies indicate that these products contain substances known to be toxic to a wide variety of organisms. Moreover, dyeing practices generate effluents that may carry the toxicity of hair dyes into the environment. Due to these facts, there is great concern about the impacts these products may have on the environment, as well as on the health of their users and professionals in the field of cosmetology. This scoping review analyzed 184 publications from major databases (PubMed, SciELO, Scopus, Google Scholar, and MEDLINE). Ultimately, 126 scientific studies published between 1981 and 2024 were included based on methodological rigor and their relevance to the One Health framework. According to the literature, the components of hair dyes can induce adverse responses in biological systems, ranging from reversible topical irritations to severe systemic effects. Among the studies evaluated, more than half reported significant toxicological or genotoxic associations related to oxidative dye components such as p-phenylenediamine and its derivatives. These compounds are frequently associated with various types of human cancers, including breast, prostate, bladder, skin, ocular cancers, and brain tumors. In addition to their effects on humans, hair dyes exhibit ecotoxicity, which may threaten the maintenance of ecosystems exposed to their residues. The reported environmental impacts result from effluent emissions after successive hair washes that release unreacted dye residues. Due to the low biodegradability of these compounds, conventional wastewater treatment methods are often ineffective, leading to environmental accumulation and changes in aquatic ecosystems, soil fertility, and trophic balance. Data on the toxicity of hair dye effluents remain scarce and sometimes contradictory, particularly regarding the effects of their transformation products and metabolites. Overall, the evidence underscores the need for continuous monitoring, updated risk assessments, and the adoption of advanced treatment technologies specific to beauty salon effluents. The information presented in this work may support further studies and guide public management agencies in developing policies for mitigating the impacts of hair dye pollutants within the One Health perspective. Full article

Water polluted by dye colorants has been on the rise in the last decade. This is due to the over reliance on the textile industry, and it is holding a high economic value in most countries. This industry is the highest consumer of fresh water whilst also discharging several natural and synthetic pollutants to the environment. Several methods have been used for the removal of these pollutants and one of the most efficient technologies to be developed includes the photocatalysis method, via advanced oxidation processes. This review highlights the developments of green iron oxide nanoparticles as photocatalysts in the last decade. It was noted that tuning and controlling the phytochemical concentration and synthesis conditions, can assist with forming uniform and non-agglomerated materials, as this has limited the vast usage of these materials in major applications. Also, upon controlling the synthesis conditions, improved surface area and charge separation efficiency was noted. Their limitations and need for modification through forming composites are highlighted. Moreover, future perspectives are given on the use of green IONPs as photocatalysts. Full article

The correct identification of historical artists’ earth pigments is mandatory for cultural, scholarly, and historical applications. This paper focuses on the definition of the distinctive mineralogical, geological, and geochemical properties and the discussion of the geological genesis and place of origin of the natural Fe-Mn-based earth pigment named terra d’ombra (umber). It one of the dark-brown earth pigment most widely used by Italian and European painters from the Renaissance to the nineteenth century. The terra d’ombra earth pigment is a primary chemical sediment mainly composed of Fe (oxy)hydroxide and Mn oxide, produced by the authigenic precipitation from oceanic or lacustrine waters rich in metal solutes of volcanic hydrothermal origin. The principal areas of provenance are the island of Cyprus and the Monte Amiata volcano (southern Tuscany, Italy). Its peculiar properties in painting derive from this specific mineralogical composition and genetic process, which also exclude its definition as a particular type of ochre and as a clay pigment. Further misinterpretations include confusion with pigments composed of organic materials and the erroneous attribution of the name and area of origin to the Italian region of Umbria. Full article

This Perspective sets out to raise awareness about the chemical and photophysical properties of an assortment of blue colorants; it is generally regarded that blue is the most popular color worldwide and is recognized for its serenity and calming effect. In fact, blue colorants have a long and rich history, perhaps starting with Egyptian Blue, and have found colossal usage in the dyeing of uniforms and popular clothing. Other blue colorants have made major contributions to our understanding of the fields of molecular spectroscopy and photophysics and continue to underpin contemporary opto-electronic devices. This is in addition to their socio-cultural, economic, and ecological benefits to society. Originally, blue colorants were extracted from minerals by tedious and ineffectual grinding to give a product carrying an exorbitant price. Later, these materials were supplemented by synthetic analogues, such as copper phthalocyanine, more affordable to the general public. It is stressed that the journal Colorants would welcome submissions that describe the chemistry and/or spectroscopy of other archetypal colorants. Full article