Search Results (57)

The secondary effluent from printing and dyeing wastewater contains recalcitrant organic pollutants, such as azo dye derivatives. Their persistence in aquatic environments not only creates ecological risks but also hampers the high-value reuse of reclaimed water. This study investigated the influence of typical binary anions on the degradation performance of low-concentration azo dye wastewater using a Ti/RuO₂-IrO₂ anode electrochemical oxidation system. The results demonstrated that maximum COD removal efficiency could reach 50.22%, and the controlling factors synergistically regulated the contribution and competition between Reactive Chlorine Species and free radicals. This led to a characteristic “rapid rise–decline–slow rebound” phenomenon in the COD removal rate, with the inflection points co-influenced by the current density, conductivity, and binary anion ratio of the electrochemical process. Furthermore, it alters the degradation pathway of the azo dye to “azo bond cleavage → demethylation/desulfonation → dehydroxylation/deamination oxidation → benzene ring opening”. Within a fixed duration of 60 min, the Response Surface Methodology model identified the optimal COD degradation conditions as follows: current density of 19.72 mA/cm², Cl⁻/SO₄²⁻ ratio of 5.40, and conductivity of 8.30 mS/cm. This research elucidates the differences between the electrochemical oxidation degradation pathway of low-concentration azo dye wastewater under the regulation of typical binary anions and the conventional pathway. It also reveals the regulatory effects of current density, conductivity, and binary anion ratio on the degradation patterns. Full article

The main objective of the work was to prepare a series of new activated biocarbons by chemical activation of black chokeberry seed and to assess their suitability for removing cationic and anionic dyes from an aqueous medium. Activation of the precursor was performed at 550 °C with orthophosphoric acid, using conventional or microwave-assisted heating. The activated biocarbons were characterized in terms of elemental composition, textural parameters, surface morphology, acid-base character of the surface, as well as electrokinetic properties. Adsorption tests were carried out against two organic compounds: methylene blue (thiazine dye of cationic character) and Congo red (azo dye of anionic character). The influence of the initial dye concentration (5–120 mg/L), temperature (20–40 °C), and solution pH (2–10) on dye removal efficiency from the liquid phase was investigated. Additionally, kinetic adsorption tests were carried out to determine the rate and mechanism of the dyes removal process. Microwave-assisted chemical activation with H₃PO₄ proved to be a very effective approach for generating a high specific surface area (884 m²/g) and a micro/mesoporous structure, which directly increases the adsorption capacity of activated biocarbons towards cationic and anionic synthetic dyes. The maximum adsorption capacities for methylene blue and Congo red were 194.5 and 68.6 mg/g, respectively. It was also confirmed that the choice of heating method at the activation stage plays a key role in determining the physicochemical properties and adsorption performance of the activated biocarbons prepared from waste biomass. In general, carbonaceous adsorbents derived from black chokeberry seeds exhibit high potential for the treatment of dye-contaminated wastewater. Full article

The ability of ultrasound/peracetic acid (US/PAA) to degrade the azo dye Sunset Yellow FCF (SSY) was evaluated considering the impacts of power, pH, inorganic carbon, common salts, radical scavengers, and real water matrices. Pseudo-first-order rate constants revealed synergy indices of 2.90, 3.28, 2.22, and 2.03 at electrical powers of 40, 60, 80, and 100 W, respectively. Selective scavenger assays revealed a mixed radical regime. ^•OH radical involvement was confirmed by inhibition with alcohols (tert-butanol, 2-propanol), benzoic acid, nitrobenzene, sodium azide, and phenol, while suppression by TEMPO highlighted the key role of PAA-derived acyl and peroxyl radicals. Nitrobenzene caused pronounced inhibition at elevated doses, while nitrite acted as a decisive quencher by converting ^•OH and other oxidants into less reactive species. Carbonate alkalinity exerted dual effects: at acidic pH (3.7–4.4) it diverted ^•OH radicals to carbonate radicals and reduced cavitation through dissolved CO₂, whereas at near-neutral pH it buffered conditions toward the optimum (pH 9) and enhanced degradation. Common anions (chloride, sulfate, nitrate) at ≤10 mM produced minor effects. Tests in environmental waters revealed the following reactivity order: seawater > ultrapure water > tap water ≈ Zamzam water > tertiary effluent. Enhanced performance in seawater was attributed to halide-mediated formation of reactive chlorine and bromine species, while inhibition in effluent was linked to organic matter scavenging. Overall, US/PAA emerges as a robust and adaptable advanced oxidation process for azo dye abatement across diverse water matrices. Full article

Multilayer and free-standing films self-assembled from water-soluble anionic azo dye acid yellow 9 (AY9) and both poly(allylamine hydrochloride) (PAH) and chitosan (CS) cationic polyelectrolytes were fabricated from water solution using a layer-by-layer (LbL) technique and characterized by UV–Vis and Raman spectroscopy. Observations were made of a strong, unexpected, and highly unusual colour change from deep red to a distinct dark blue upon exposure of the multilayer films to an acidic environment. The colour change was attributed to the multilayer films only and was not observed either for the polymer or the dye alone, or their mixture in water solution, nor when cast as free-standing films. The significant shift to blue colour of the absorption peaks was quantified with UV–Vis spectroscopy, and a proposed explanation is presented based on density functional theory (DFT) calculations exploring possible and most likely acid-base equilibria configurations of the azo dye that result from being self-assembled. Full article

Efficient water decolorization techniques are vital for ensuring fresh water for future generations. Azo dyes are used heavily in the textile industry and are a challenge to remove from industrial wastewater. This research expands on recent innovative work where anionic azo dyes themselves were used to functionalize track-etched porous polycarbonate filtration membranes with decolorized water obtained as a byproduct. The objective of this research is to determine whether the observed dye rejection is dependent on the magnitude of the intrinsic charge of the dye molecule or on its structure, using two selectively chosen anionic azo dye series during functionalization. The first group is a negative two intrinsic charge series with six dyes, each differing in structure, and the second group is a five-dye series that increases from −1 to −6 in intrinsic charge. Rejection measurements as a function of both time and concentration during functionalization are made using ultraviolet-visible light spectroscopy. For 100 µM aqueous dyes, comparing pre- and post-functionalization, a systematically increasing trend in the ability to functionalize porous polycarbonate based on the number of double 6-carbon ring structures in the dyes is illustrated and found to be independent of intrinsic charge. Full article

Nowadays, nitrate ions and azo dyes are a significant source of water pollution due to their high toxicity, persistence, and potential to be carcinogenic. Both contaminants are the result of anthropogenic sources, such as sewage or industrial wastewater discharge; the first one results also as a consequence of the intensive use of fertilizers. In this work we report the use of a new quaternized pentablock copolymer (PTBr) for the removal of nitrate ions and methyl orange (MO) dye from water by adsorption processes. Morphological, chemical, and thermal properties of the pentablock copolymer were investigated, respectively, by scanning electron microscopy (SEM), Attenuated Total Reflectance Infrared Spectroscopy (ATR-FTIR) (FT-IR), and X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) analyses. Anionic removal ability and adsorption rate in water solutions containing either a single contaminant species or a mix of the two contaminants were studied by UV–VIS absorbance spectroscopy as a function of time and initial concentration. The presence of imidazole groups confers on PTBr a positive charge and a hydrophilic character that are responsible for an effective removal of anions from water. PTBr film reports an adsorption efficiency of 10.15 mg/g for nitrate removal and this value is in line with others reported in the literature. In the case of the simultaneous presence of nitrate and MO, it is found that nitrate ions removal is slightly affected by the presence of the dye, since both contaminants compete for electrostatic interaction with imidazole groups. On the contrary, the dye removal does not show significant change with or without the presence of nitrate ions, probably due to other kinds of interaction that it can establish with the polymer surface (π-π interaction). The adsorption process and the related mechanisms are described using kinetic and isothermal models. Despite a certain reduction in the adsorption efficiency for one of the investigated contaminants, the results confirm the possibility of using the quaternized pentablock copolymer for the co-adsorption of both inorganic and organic anions. Full article

UiO-66-NH₂ is a metal–organic framework (MOF) with open metal sites, making it a promising candidate for adsorption and catalysis. However, the powdery texture of MOFs and the use of toxic solvents during synthesis limit their application. A novel solution to this issue is to create a layered porous composite by encasing the MOF within a flexible and structurally robust aerogel substrate using safe, eco-friendly, and green solvents such as ethanol. The fibrous MOF aerogels, characterized by a desirable macroscopic shape of cylindrical block and hierarchical porosity, were synthesized by two approaches: in situ growth of amine-functionalized UiO-66-NH₂ crystals on a TEMPO-oxidized cellulose nanofiber (TOCNF) and ex situ crosslinking of UiO-66-NH₂ crystals onto a TOCNF network to form UiO-66-NH₂/TOCNF. The incorporation of MOF into the cellulose nanofibrils via the in situ method reduces their aggregation potential, alters the nucleation/growth balance to produce smaller MOF crystals, and enhances mechanical flexibility, as evidenced by SEM images. The three adsorbents, including UiO-66-NH₂, ex situ UiO-66-NH₂/TOCNF, and in situ UiO-66-NH₂/TOCNF, were synthesized and used in this study. The effects of pH, time, temperature, and initial concentration were studied. A maximum adsorption capacity (Qmax) of 549.45 mg/g for Congo Red (CR) and 171.23 mg/g for Orange II (ORII) was observed at pH 6, using 10 mg of in situ UiO-66-NH₂/TOCNF at 40 °C with a contact time of 75 min for CR and 2 h for ORII. The adsorption of both dyes primarily occurs through monolayer chemisorption on the in situ UiO-66-NH₂/TOCNF. The main removal mechanisms were hydrogen bonding and surface complexation. The noteworthy adsorption capacity of in situ UiO-66-NH₂/TOCNF coupled with environment-friendly fabrication techniques indicates its potential applications on a large scale in real wastewater systems. Full article

This study presents the synthesis and utilization of a conductive polymer/clay nanocomposite for the adsorptive removal of an azo dye, methyl orange (MO), from artificial wastewater. The PANI-CLAY nanocomposites were synthesized by means of the oxidative polymerization route and characterized using the Brunauer, Emmett and Teller thermogravimetric analysis, Fourier-Transform Infrared spectra and Scanning Electron Microscopy. The surface area of the clay mineral decreased from 37.38 to 13.44 m²/g for 10 g of PANI/CLAY when made into a composite with PANI. Such behavior is most likely due to the possible coverage of the clay surface by a layer of PANI. Further, TGA revealed that incorporating CLAY significantly improved the thermal stability of PANI. The effects of adsorption process parameters such as adsorbent dosage (0.006–0.4 g), solution pH (1, 3, 5, 7, 9, 11 and 13), initial dye concentration (50–300 ppm), contact time (1–80 min) and temperature (25 °C, 30 °C, 35 °C and 40 °C) on the % removal efficiency were investigated. The experimental data were well fitted by the pseudo-second-order kinetic model. The maximum uptake capacity (q_max) values increased from 42.017 mg/g (PANI/CLAY 10 g) to 55.87 mg/g for PANI alone. The uptake capacity implies that the prepared adsorbents possess excellent adsorption characteristics with high affinity towards organic dye removal. Full article

Bentonite-supported TiO₂ (Montmorillonite (MMT)-TiO₂) and Cu₃TiO₅ oxides (MMT-Cu₃TiO₅) nanomaterials were synthesized via a facile and sustainable sol–gel synthesis approach. The XRD results indicate the presence of mixed phases, namely, TiO₂ anatase and a new semiconductor, Cu₃TiO₅, in the material. The specific surface area (S_BET) exhibits a notable increase with the incorporation of TiO₂ and Cu₃TiO₅, rising from 85 m²/g for pure montmorillonite to 245 m²/g for MMT-TiO₂ and 279 m²/g for MMT-Cu₃TiO₅. The lower gap energy of MMT-Cu₃TiO₅ (2.15 eV) in comparison to MMT-TiO₂ (2.7 eV) indicates that MMT-Cu₃TiO₅ is capable of more efficient absorption of visible light with longer wavelengths. The immobilization of TiO₂ and Cu₃TiO₅ on bentonite not only enhances the textural properties of the samples but also augments their visible light absorption capabilities, rendering them potentially more efficacious for adsorption and photocatalytic applications. The photocatalytic efficacy of both MMT-TiO₂ and MMT-Cu₃TiO₅ was evaluated through the monitoring of the degradation of Orange G, an anionic azo dye. The MMT-Cu₃TiO₅ photocatalyst was observed to induce complete degradation (100%) of the Orange G dye in 120 min when tested in an optimized reaction medium with a pH of 3 and a catalyst concentration of 2 g/L. MMT-Cu₃TiO₅ was demonstrated to be an exceptionally effective catalyst for the degradation of Orange G. Following the synthesis of the catalyst, it can be simply washed with the same recovered solution and reused multiple times for the photocatalytic process without the need for any chemical additives. Full article

pH-responsive polyamidoamine (PAMAM) dendrimers are used as well-defined building blocks to design light-switchable nano-assemblies in solution. The complex interplay between the photoresponsive di-anionic azo dye Acid Yellow 38 (AY38) and the cationic PAMAM dendrimers of different generations is presented in this study. Electrostatic self-assembly involving secondary dipole–dipole interactions provides well-defined assemblies within a broad size range (10 nm–1 μm) with various shapes. The size and shape of these assemblies were determined using dynamic and static light scattering (DLS/SLS) and small-angle neutron scattering (SANS); ζ-potential measurements were performed to elucidate the charge characteristics, revealing the effective surface charge density of the nano-objects as an important parameter in the size and shape control. UV–vis spectroscopy and isothermal titration calorimetry (ITC) were employed to investigate the interaction on a molecular level and from a thermodynamic point of view. The results show that the amount of isomerized cis dye depends on the dendrimer generation because of a photoprotective effect through electrostatics for lower generations and through dipole–dipole interactions for higher generations; as the cis dye and trans dye bind with different strength, the amount of cis dye then again encodes the charge density and thereby the particle size and shape. Full article

The single-atom catalyst (SAC) activated persulfate process has emerged as a highly efficient technology for eliminating refractory organic compounds in aqueous environments. This review delves into the intricacies of utilizing SACs for the effective removal of various contaminants in water. The common supports and the preparation procedures of SACs are summarized at first. The synthesis methods of SACs (i.e., wet chemical method, one-pot hydrothermal method, and high-temperature pyrolysis method) are also described. Then, a comprehensive overview of the diverse reaction mechanisms in SAC-activated persulfate systems is presented, including a radical oxidation process via sulfate or hydroxyl radicals and superoxide radicals, or a nonradical process via single oxygen, surface active complex, and high-valent metal-oxo species oxidation. The impact of key factors such as peroxides concentration, SAC dosage, reaction pH, inorganic anions, organic matter, operando stability, and real water is also delved. The removal of various pollutants (i.e., azo dyes, phenolic compounds, pharmaceuticals, and bacteria) by this process is further summarized. Finally, the challenges and perspectives in the field of water treatment utilizing SACs are discussed. Full article

Advanced oxidation process based on heterogeneous activation of peroxymonosulfate (PMS) has received significant attention in wastewater remediation. Herein, a facile and effective electrochemical method was introduced in a tungsten sulfide (WS₂)-activated PMS process for the removal of a typical azo dye Acid Orange 7 (AO7) in aqueous solution. It was found that the electrochemical activation could remarkably promote the removal of organic pollutants by coupling with WS₂/PMS system. The elimination of AO7 in the electro-assisted WS₂-activated PMS (E/WS₂/PMS) system achieved 95.8% of AO7 removal in 30 min, with the optimal conditions of 1.0 g/L WS₂, 1.0 mM PMS, current density of 1.0 mA/cm² and initial pH of 6.5. Based on quenching experiments and EPR techniques, mechanistic studies confirmed that hydroxyl radical (^•OH) and singlet oxygen (¹O₂) are the primary reactive oxygen species for the oxidation of pollutants. In addition, the influences of pH, WS₂ dosage, PMS concentration, current density, common anions and humic acid on the AO7 removal are also investigated in detail. Furthermore, the system exhibited resistance to aqueous matrices, verifying the accepted applicability in real water (i.e., Yangtze River water and Shahu Lake water). In summary, this study demonstrates a green system for the effective removal of contaminants in water, holding significant implications for practical application. Full article

Trace concentrations of dyes are often present in textile wastewater streams and present a serious environmental problem. Thus, these dyes must be removed from wastewater either by degradation or sequestration prior to discharge of the wastewater into the environment. Existing processes to remove these wastewater contaminants include the use of solid sorbents to sequester dyes or the use of biochemical or chemical methods of dye degradation. However, these processes typically generate their own waste products, are not necessarily rapid because of the low dye concentration, and often use expensive or non-recyclable sequestrants or reagents. This paper describes a simple, recyclable, liquid–liquid extraction scheme where ionic dyes can be sequestered into poly(α-olefin) (PAO) solvent systems. The partitioning of anionic and cationic dyes from water into PAOs is facilitated by ionic PAO-phase anchored sequestering agents that are readily prepared from commercially available vinyl-terminated polyisobutylene (PIB). This is accomplished by a sequence of reactions involving hydroboration/oxidation, conversion of an alcohol into an iodide, and conversion of the resulting primary alkyl iodide into a cationic nitrogen derivative. The products of this synthetic sequence are cationic nitrogen iodide salts which serve as anionic sequestrants that are soluble in PAO. These studies showed that the resulting series of cationic PIB-bound cationic sequestering agents facilitated efficient extraction of anionic, azo, phthalein, and sulfonephthalein dyes from water into a hydrocarbon PAO phase. Since the hydrocarbon PAO phase is completely immiscible with water and the PIB derivatives are also insoluble in water, neither the sequestration solvent nor the sequestrants contaminate wastewater. The effectiveness and efficiency of these sequestrations were assayed by UV–visible spectroscopy. These spectroscopic studies showed that extraction efficiencies were in most cases >99%. These studies also involved procedures that allowed for the regeneration and recycling of these PAO sequestration systems. This allowed us to recycle the PAO solvent system for at least 10 sequential batch extractions where we sequestered sodium salts of methyl red and 4′,5′-dichlorofluorescein dyes from water with extraction efficiencies of >99%. These studies also showed that a PIB-bound derivative of the sodium salt of 1,1,1-trifluoromethylpentane-2,4-dione could be prepared from a PIB-bound carboxylic acid ester by a Claisen-like reaction and that the sodium salt of this β-diketone could be used to sequester cationic dyes from water. This PIB-bound anion rapidly and efficiently extracted >99% of methylene blue, malachite green, and safranine O from water based on UV–visible and ¹H NMR spectroscopic assays. Full article

In this investigation, Zn/Al carbonate layered double hydroxide (ZAC-LDH) and its derived material on calcination were synthesized for removing the anionic azo dye Congo red (CR) from wastewater. Numerous factors were methodically investigated, including temperature, adsorbent dosage, pH, starting Dye Concentration (DC), and contact time. The CR elimination percentage dropped as the initial DC increased from 25 mg/L to 100 mg/L at 30 °C for uncalcined LDH, and from 97.96% to 89.25% for calcined LDH. The pH analysis indicates that the highest level of dye removal was recorded within the acidic pH range through the electrostatic attraction mechanism. The sorption kinetics analysis results demonstrated that the pseudo-second-order kinetic model exhibited a stronger fit to both uncalcined LDH and CZA-LDH, with the maximum correlation coefficient value. The Van’t Hoff plots indicate the spontaneous nature of the physisorption process with a negative ΔG° (<−20 kJ/mol), while the endothermic adsorption process exhibited a positive ΔH°. The X-ray diffraction of calcined LDH reveals a significant intercalation of CR dye molecules, both prior to and following adsorption, showcasing a distinctive memory effect. The Brunauer–Emmett–Teller (BET) gas sorption measurements were performed to support the mesoporous nature of ZAC-LDH and CZA-LDH. The FTIR spectrum confirms the interaction of dye molecules on the surface of uncalcined and calcined LDH. These findings emphasize the efficacy of both the synthesized LDHs in removing CR dye, with CZA-LDH demonstrating superior efficiency compared to uncalcined LDH in the context of CR removal from wastewater. Full article

This paper presents the preparation and characterization of bentonite coated with hydroxide double lamellar Mg/Al-bentonite and Zn/Al-bentonite as a potential adsorbent material. The coating process involved co-precipitation of mixed metal nitrate solution (Mg-Al) or (Zn-Al), followed by immersion of bentonite (B-Na⁺) dispersion. The structures and morphologies of the coated bentonites were characterized using XRD, FTIR, BET, and SEM analysis. The results of the BET analysis indicate that Mg/Al-bentonite and Zn/Al-bentonite have larger surface areas and pore volumes compared to bentonite alone. Specifically, the surface area of Mg/Al-bentonite is 209.25 m²/g with a pore volume of 0.423 cm³/g, while Zn/Al-bentonite has a surface area of 175.95 m²/g and a pore volume of 0.313 cm³/g. In contrast, the surface area and pore volume of bentonite alone are 110.43 m²/g and 0.132 cm³/g, respectively. The Mg/Al-bentonite reaches 85% uptake within 3 h (equivalent to 724.20 mg/g at 25 °C and pH 7), achieving rapid equilibrium. In contrast, the Zn/Al-bentonite achieves a maximum adsorption of 74% within 5 h under identical pH and temperature conditions, corresponding to 650.34 mg/g. The error function values, including the correlation coefficient R², chi-square test χ², and residual sum of squares RSS, were calculated to evaluate both kinetic and isotherm models. The kinetic adsorption data agreed well with a pseudo-second-order model. The adsorption process followed the Sips isotherm model, and the monolayer adsorption capacity of Mg/Al-bent and Zn/Al-bent composites was 872.41 (R² = 0.974) and 678.45 mg/g (R² = 0.983), respectively. The thermodynamic analysis of the adsorption process revealed that it occurred spontaneously with an endothermic characteristic. The parameters ΔS, ΔH, and ΔG were used to determine this. Full article