Sustainable Wastewater Treatment via Dye–Surfactant Interaction: A Critical Review

Recent progress in magnetic polydopamine composites for pollutant removal in wastewater treatment

Various water pollution issues pose a significant threat to human water safety. Magnetic polydopamine composites (MPCs), which can be separated by magnetic fields after the adsorption process, exhibit outstanding adsorption capacity and heterogeneous catalytic properties, making them promising materials for water treatment applications. In particular, by modifying the polydopamine (PDA) coating, MPCs can acquire enhanced high reactivity, antibacterial properties, and biocompatibility. This also provides an attractive platform for further fabrication of hybrid materials with specific adsorption, catalytic, antibacterial, and water-oil separation capabilities. To systematically provide the background knowledge and recent research advances in MPCs, this paper presents a critical review of MPCs for water treatment in terms of both structure and mechanisms of effect in applications. Firstly, the impact of different PDA positions within the composite structure is investigated to summarize the optimization of properties contributed by PDA when acting as the shell, core, or bridge. The roles of various secondary modifications of magnetic materials by PDA in addressing water pollution problems are explored. It is anticipated that this work will be a stimulus for further research and development of magnetic composite materials with real-world application potential.

Role of Alkyl Chain Length in Surfactant-Induced Precipitation of Reactive Brilliant Blue KN-R

To develop a cost-effective method for the effective removal of reactive brilliant blue KN-R (RBB KN-R) from wastewater, we investigated the interactions between RBB KN-R and three cationic surfactants with different alkyl chain lengths, namely dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), and cetyltrimethylammonium bromide (CTAB). Employing a conductivity analysis, surface tension analysis, ultraviolet-visible spectrophotometry, and molecular dynamics simulation, we ascertained that RBB KN-R formed a 1:1 molar ratio dye-surfactant complex with each surfactant through electrostatic attraction. Notably, an augmentation in alkyl chain length correlated with increased binding strength between RBB KN-R and the surfactant. The resulting dye-surfactant complex exhibited heightened surface activity, enabling interactions through hydrophobic forces to generate dye-surfactant aggregates when the molar ratio was below 1:1. Within these mixed aggregates, self-assembly of RBB KN-R molecules occurred, leading to the formation of dye aggregates. Due to the improved hydrophobicity with increased alkyl chain length, TTAB and CTAB could encapsulate dye aggregates within the mixed aggregates, but DTAB could not. The RBB KN-R aggregates tended to distribute on the surface of the RBB KN-R-DTAB mixed aggregates, resulting in low stability. Thus, at a DTAB concentration lower than CMC, insoluble particles readily formed and separated from surfactant aggregates at an RBB KN-R and DTAB molar ratio of 1:4. Analyzing the RBB KN-R precipitate through scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) and measuring the DTAB concentration in the supernate revealed that, at this molar ratio, all RBB KN-R precipitated from the dye-surfactant mixed solution, with only 7.5 ± 0.5% of DTAB present in the precipitate. Furthermore, the removal ratio of RBB KN-R reached nearly 100% within a pH range of 1.0 to 9.0 and standing time of 6 h. The salt type and concentration did not significantly affect the precipitation process. Therefore, this simultaneous achievement of successful RBB KN-R removal and effective separation from DTAB underscores the efficacy of the proposed approach.

Intermolecular interactions in mixed dye systems and the effects on dye wastewater treatment processes

Dye wastewater discharge is a critical concern across textiles, paper, cosmetics, and other industries. This study explores the impact of dye-dye interactions on chemical coagulation and ultrafiltration process. Using basic and reactive dyes, representing cationic and anionic compounds, the intricate interplay between these dyes was examined through spectroscopic analysis. Remarkably, interactions between dyes of opposite charges exhibited significant effects on both techniques. Electrostatic attractions played a key role. Positive coagulant hydrolysates selectively attracted negative dyes, while negatively charged membranes effectively captured positive dyes. Combining dyes with opposite charges resulted in enhanced removal efficiency, addressing challenging dyes collectively. This discovery offers a novel approach to improving dye removal, utilizing opposite-charged dye mixtures can tackle stubborn dyes unmanageable by conventional methods.

Banana Peel Derived Chitosan-Grafted Biocomposite for Recovery of NH4+ and PO43

Biomass-derived adsorbents afford accessible and inexpensive harvesting of nitrogen and phosphorus from wastewater sources. Human urine is widely accepted as a rich source of nitrogen and phosphorus. However, direct use of urine in agriculture is untenable because of its unpleasant smell, pathogen contamination, and pharmaceutical residues. In this work, we have grafted chitosan onto dried and crushed banana peel (DCBP) to generate the biocomposite DCBP/Ch. A combination of FTIR, TGA, XRD, FESEM, EDX, and NMR analyses were used to characterize DCBP/Ch and reveal condensation-aided covalent conjugation between O-H functionalities of DCBP and chitosan. The adsorption performance of DCBP/Ch toward NH4+ and PO43- is in sync with its attractive surface porosity, elevated crystallinity, and thermostability. The maximum adsorption capacity of DCBP/Ch toward NH4+/PO43- was estimated as 42.16/15.91 mg g-1 at an operating pH of 7/4, respectively, and ranks highly when compared to previously reported bioadsorbents. DCBP/Ch performs admirably when tested on artificial urine. While nitrogen and phosphorus harvesting from human urine using single techniques has been reported previously, this is the first report of a single adsorbent for recovery of NH4+ and PO43-. The environmental compatibility, ease of preparation, and economic viability of DCBP/Ch present it as an attractive candidate for deployment in waste channels.

Deep dewatering of refinery oily sludge by Fenton oxidation and its potential influence on the upgrading of oil phase

Highly efficient dewatering is essential to the reduction and reclamation disposal of oily sludge, which is a waste from the extraction, transportation, and refining of crude oil. How to effectively break the water/oil emulsion is a paramount challenge for dewatering of oily sludge. In this work, a Fenton oxidation approach was adopted for the dewatering of oily sludge. The results show that the oxidizing free radicals originated from Fenton agent effectively tailored the native petroleum hydrocarbon compounds into smaller organic molecules, hence destructing the colloidal structure of oily sludge and decreasing the viscosity as well. Meanwhile, the zeta potential of oily sludge was increased, implying the decrease of repulsive electrostatic force to realize easy coalescence of water droplets. Thus, the steric and electrostatic barriers which restrained the coalescence of dispersed water droplets in water/oil emulsion were removed. With these advantages, the Fenton oxidation approach derived the significant decrease of water content, in which 0.294 kg water was removed from per kilogram oily sludge under the optimal operation condition (i.e., pH value of 3, solid-liquid ratio of 1:10, Fe²⁺ concentration of 0.4 g/L and H₂O₂/Fe²⁺ ratio of 10:1, and reaction temperature of 50 °C). In addition, the quality of oil phase was upgraded after Fenton oxidation treatment accompanying with the degradation of native organic substances in oily sludge, and the heating value of oily sludge was increased from 8680 to 9260 kJ·kg^-1, which would facilitate to the subsequent thermal conversion like pyrolysis or incineration. Such results demonstrate that the Fenton oxidation approach is efficient for the dewatering as well as the upgrading of oily sludge.

Interaction of Direct Blue 86 with cationic surfactant micelles: spectroscopic, conductometric and thermodynamic aspects

Due to the amphiphilic structure of surfactants, aqueous surfactant solutions can behave like very good solvents and dissolve both polar and non-polar solutes. This study reports on the solubilisation of a direct dye (Direct Blue 86) in a micellar medium using the cationic surfactant cetyltrimethylammonium bromide (CTAB). Solubilisation of dyes is important for their subsequent removal from aqueous media. UV spectroscopy and conductometry, among others, were used to quantitatively evaluate this process. The extent of solubilisation, the interaction between the molecules and the stability of the processes were checked using the partition coefficient (K x), the binding constant (K b) and the corresponding thermodynamic parameters. From the results, it could be concluded that the solubilisation of Direct Blue 86 is a spontaneous process supported by an increase in entropy. It was also found that the micellar medium CTAB is efficient for solubilisation and binding of the dye and can be used economically.

Effect of ultrasound on the physical properties and processing of major biopolymers-a review

Designing and developing modern techniques to facilitate the extraction and modification of functional properties of biopolymers are key motivations among researchers. As a low-cost, sustainable, non-toxic, and fast process, ultrasound has been considered a method to improve the processing of carbohydrate and protein-based biopolymers such as cellulose, chitin, starch, alginate, carrageenan, gelatine, and guar gum. A better understanding of the complex physicochemical behavior of biopolymers under ultrasonication may fortify the eminence of this technology in advanced-level applications. This review summarizes the recent advances in biopolymer processing and the effect of ultrasound on the physical properties of the selected biopolymers. A major focus will be given to the mechanisms of action and their impact on the properties and extraction. At the end, some possible suggestions are highlighted which need future investigation for amending the physical properties of biopolymers using ultrasonication.

Electrochemical Sensor for Detection and Degradation Studies of Ethyl Violet Dye

In this work, a simple and sensitive electrochemical method was developed to determine ethyl violet (EV) dye in aqueous systems by using square wave anodic stripping voltammetry (SWASV) employing a glassy carbon electrode modified with acidic-functionalized carbon nanotubes (COOH-fCNTs). In square wave anodic stripping voltammetry, EV exhibited a well-defined oxidation peak at 0.86 V at the modified GCE. Impedance spectroscopy and cyclic voltammetry were used to examine the charge transduction and sensing capabilities of the modified electrode. The influence of pH, deposition potential, and accumulation time on the electro-oxidation of EV was optimized. Under the optimum experimental conditions, the limit of detection with a value of 0.36 nM demonstrates high sensitivity of COOH-fCNTs/GCE for EV. After detection, it was envisioned to devise a method for the efficient removal of EV from an aqueous system. In this regard a photocatalytic degradation method of EV using Ho/TiO₂ nanoparticles was developed. The Ho/TiO₂ nanoparticles synthesized by the sol-gel method were characterized by UV-vis, XRD, FTIR, SEM, and EDX. The photocatalytic degradation studies revealed that basic medium is more suitable for a higher degradation rate of EV than acidic and neutral media. The photodegradation kinetic parameters were evaluated using UV-vis spectroscopic and electrochemical methods. The results revealed that the degradation process of EV follows first-order kinetics.

A comprehensive review on recent advances toward sequestration of levofloxacin antibiotic from wastewater

Among various classes of antibiotics, fluoroquinolones, especially Levofloxacin, are being administered on a large scale for numerous purposes. Being highly stable to be completely metabolized, residual quantities of Levofloxacin get accumulated into the food chain proving a great global threat for aquatic as well as terrestrial ecosystems. Various removal techniques including both conventional and advanced methods have been reported for this purpose. This review is a novel attempt to make a critical analysis of the recent advances made exclusively toward the sequestration of Levofloxacin from wastewater through an extensive literature survey (2015-2021). Adsorption and advanced oxidation processes especially photocatalytic degradation are the most tested techniques in which assorted nanomaterials play a significant role. Several photocatalysts exhibited up to 100% degradation of LEV which makes photocatalytic degradation the best method among other tested methods. However, the degraded products need to be further monitored in terms of their toxicity. Biological degradation may prove to be the most environment-friendly with the least toxicity, unfortunately, not much research is reported in the field. With these key findings and knowledge gaps, authors suggest the scope of hybrid techniques, which have been experimented on other antibiotics. These can potentially minimize the disadvantages of the individual techniques concurrently improving the efficiency of LEV removal. Besides, techniques like column adsorption, membrane treatment, and ozonation, being least reported, reserve good perspectives for future research. With these implications, the review will certainly serve as a breakthrough for researchers working in this field to aid their future findings.

A Biocompatible, pH-Sensitive, and Magnetically Separable Superparamagnetic Hydrogel Nanocomposite as an Efficient Platform for the Removal of Cationic Dyes in Wastewater Treatment

A series of environment-friendly cationic dye adsorbents, namely, pH-sensitive superparamagnetic hydrogel nanocomposite AA-VSA-P/SPIONs systems with different concentrations of superparamagnetic iron oxide nanoparticles (SPIONs; 1.2, 3.2, and 5.2 wt %), was synthesized by free-radical polymerization reaction using two pH-sensitive monomers, acrylic acid (AA) and vinylsulfonic acid (VSA), in an optimum ratio, in the presence of presynthesized SPIONs. The structural properties, thermal stability, and chemical configuration of AA-VSA-P/SPIONs systems with different weight percentages of SPIONs were characterized by XRD, TGA, Raman spectroscopy, and FTIR spectroscopy. The systems show substantial efficiency as dye adsorbents for removing cationic dyes (MB dye) from aqueous solution in neutral to alkaline medium. Further, these systems exhibit easy magnetic separation capabilities from aqueous solutions after dye adsorption, even for a very low weight percentage of SPIONs. The adsorption kinetics, mechanism, and isotherms of these systems were evaluated. The study suggests consistency with the pseudo-second-order kinetic model, following an intraparticle diffusion mechanism, where the heterogeneous surface of the system having different activation energies for adsorption plays the crucial role in dye adsorption via chemisorption for higher pH medium, which was further substantiated by excellent data fit with the Freundlich isotherm model. Biocompatibility and regeneration-ability studies establish the environment-friendliness and cost effectivity of the system.

Cellulose-Based Hydrogels for Wastewater Treatment: A Concise Review

Finding affordable and environment-friendly options to decontaminate wastewater generated with heavy metals and dyes to prevent the depletion of accessible freshwater resources is one of the indispensable challenges of the 21st century. Adsorption is yet to be the most effective and low-cost wastewater treatment method used for the removal of pollutants from wastewater, while naturally derived adsorbent materials have garnered tremendous attention. One promising example of such adsorbents is hydrogels (HGs), which constitute a three-dimensional polymeric network of hydrophilic groups that is highly capable of adsorbing a large quantity of metal ions and dyes from wastewater. Although HGs can also be prepared from synthetic polymers, natural polymers have improved environmental benignity. Recently, cellulose-based hydrogels (CBHs) have been extensively studied owing to their high abundance, biodegradability, non-toxicity, and excellent adsorption capacity. This review emphasizes different CBH adsorbents in the context of dyes and heavy metals removal from wastewater following diverse synthesis techniques and adsorption mechanisms. This study also summarizes various process parameters necessary to optimize adsorption capacity followed by future research directions.

Influence of the AOT Counterion Chemical Structure on the Generation of Organized Systems

The impact of the imidazolium counterion structure on the organized systems formed by the surfactant 1,4-bis-2-ethylhexylsulfosuccinate, AOT, both in aqueous solutions and in nonpolar solvents is investigated. With this in mind, we investigated if the ionic liquid-like (IL-like) surfactant 1-ethyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate, emim-AOT, forms direct micelles or vesicles in water. Dynamic light scattering, zeta potential, conductivity, fluorescence spectroscopy, and UV-visible spectroscopy measurements were performed to characterize the organized systems in aqueous solutions. We also studied the self-aggregation of emim-AOT, 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate, bmim-AOT, and of 1-hexyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate, hmim-AOT, in nonpolar solvents. The results obtained showed that the IL-like surfactant emim-AOT forms direct micelles in water, as sodium 1,4-bis-2-ethylhexylsulfosuccinate (Na-AOT) does. However, emim-AOT aggregates are larger, have a lower surface charge, are more stable, and have a more polar and less fluid micellar interface than Na-AOT micelles. It was also observed that emim-AOT and hmim-AOT form reverse micelles in nonpolar solvents. The size of the imidazolium cations dramatically influences the size of the reverse micelles and their ability to solubilize water.