Pirarucu hydroxyapatite applied to ternary competitive adsorption of synthetic basic dyes as contaminants of emerging concern: kinetic, equilibrium, and ANN studies

This study investigated the single and multicomponent adsorption of three emerging pollutants, the basic dyes Rhodamine 6G (R6G), Auramine-O (AO), and Brilliant Green (BG) by using hydroxyapatite synthesized from Pirarucu scales as adsorbent (HAP). The adsorption process was studied using seven different systems: AO-single, R6G-single, BG-single, R6G + AO, BG + AO, BG + R6G, and R6G + AO + BG. For kinetics, the initial concentration of each adsorbate per system was 50 mg/L, the results showed that the singular adsorption of these dyes was best-represented by the pseudo-second-order model (qAO = 62.54 mg/g, qR6G = 7.91 mg/g, qBG = 62.40 mg/g), however, the multicomponent adsorption was well-fitted by a pseudo-first-order model (ternary system: qAO = 56.21 mg/g, qR6G = 14.95 mg/g, qBG = 60.62 mg/g). For equilibrium, the initial concentration of each adsorbate per system was 10-300 mg/L, and the single adsorption systems were best represented by the Langmuir model. Nonetheless, the results displayed in the multicomponent mixture showed the presence of inflection points of AO and R6G whenever BG was present in solution with C0 > 150 mg/L, thus indicating that BG has greater affinity with HAP. The presence of inflection points in the curves represented a limitation for applying traditional equilibrium models, thus, an artificial neural network (ANN) was applied to non-linear curve fit this process and satisfactorily predicted the kinetics and equilibrium data. Finally, the analysis of thermodynamics for the ternary mixture revealed that the adsorption process is spontaneous (ΔG < 0), endothermic (ΔH > 0), and increases to a disorganized state as the temperature rises (ΔS > 0).

Cost effective porous areca nut carbon nanospheres for adsorptive removal of dyes and their binary mixtures

Novel porous nanospheres from areca nuts (ACNPs) were synthesized via one-step pyrolysis without the use of any chemical treatment and the materials were used as adsorbents for the removal of cationic methylene blue (MB) and anionic methyl orange (MO) as well as their binary mixtures. Around, 6-7 tonnes of areca nut biowaste is generated every year which are then burnt due to their slow rate of decomposition resulting in higher carbon footprints. Biosorbents are generally a preferable alternative for dye adsorption but involve chemical modification for surface enhancement and complex sample treatment. In this work, ACNPs, were investigated for their efficiency in the raw form and were characterized by SEM, EDS, FTIR, XRD, and BET techniques before and after subjecting to the dye adsorption studies. The BET analysis of the adsorbents showed a high specific surface area of 693.8 m²/g when prepared at 1000 °C, while the N₂ adsorption-desorption plot showed type-IV isotherm, suggesting the microporous nature of the carbon matrix. Batch equilibrium studies showed the removal efficiency of >95% for both the dyes and their binary mixtures under the optimum conditions of 0.15 g/L dosage, 10 μM concentration and contact time of 70 min. Due to the synergistic effects of the binary dyes, higher removal efficiency of MB compared to MO was observed in the binary mixture. Adsorption results were tested using Langmuir, Freundlich, Temkin, Redlich-Peterson, and Elovich isotherms to assess the best fit of the models. The q_m value of MB was found to be 97.37 mg/g, while that of MO was 71.22 mg/g which is higher compared to individual dye components having lower values of 86.12 mg/g and 50.35 mg/g, respectively. Extended Langmuir and Jain and Snoeyink isotherms were used for binary data interpretation. The kinetic results showed good agreement with the Pseudo-second order equation, indicating internal diffusion. The possible mechanism involved electrostatic and ᴨ-ᴨ interactions between the dye molecules and ACNPs. This approach is comprehensible and cost effective and can be utilized for dye removal in textile industries.

Zeolites as Adsorbents and Photocatalysts for Removal of Dyes from the Aqueous Environment

This study investigated the potential of zeolites (NH₄BETA, NH₄ZSM-5, and NaY) to remove two frequently used dyes, methylene blue (MB) and rhodamine B (RB), from an aqueous environment. The removal of dyes with zeolites was performed via two mechanisms: adsorption and photocatalysis. Removal of dyes through adsorption was achieved by studying the Freundlich adsorption isotherms, while photocatalytic removal of dyes was performed under UV irradiation. In both cases, the removal experiments were conducted for 180 min at two temperatures (283 K and 293 K), and dye concentrations were determined spectrophotometrically. Additionally, after photodegradation, mineralization was analyzed as chemical oxygen demand. A computational analysis of the structures of MB and RB was performed to gain a deeper understanding of the obtained results. The computational analysis encompassed density functional theory (DFT) calculations and analysis of two quantum-molecular descriptors addressing the local reactivity of molecules. Experimental results have indicated that the considered zeolites effectively remove both dyes through both mechanisms, especially NH₄BETA and NH₄ZSM-5, due to the presence of active acidic centers on the outer and inner surfaces of the zeolite. The lowest efficiency of dye removal was achieved in the presence of NaY zeolite, which has a lower SiO₂/Al₂O₃ ratio. A more efficient reduction was completed for RB dye, which agrees with the computationally obtained information about reactivity.

Natural kaolinite-based hierarchical porous microspheres as effective and highly recyclable adsorbent for removal of cationic dyes

The development of efficient, recyclable, and environment-friendly adsorbent for wastewater remediation is considered a challenge. In this study, a hierarchical porous kaolinite microsphere (HPKS) with three-dimensional (3D) structure was fabricated based on natural-layered kaolinite mineral via an environmentally friendly direct hydrothermal strategy. Characterization results revealed that HPKS microsphere with 3D hierarchical porous structure was constructed with numerous nanospheres which are assembled by ultrafine aluminosilicate flakes. HPKS exhibited negative charge feature ranging from strong acid to high alkaline solution. The influence of contact time, solution pH, initial dye concentration, adsorbent dosage, and foreign ions on methylene blue (MB) adsorption capability was systematically investigated. The synthesized HPKS with higher specific surface area (250.6 m²/g) shows an outstanding adsorption capacity towards MB (411.8 mg/g) and excellent selectivity for cationic MB dyes over anionic methyl orange and competitive metal ions. The adsorption kinetic experiment results fit very well with the pseudo-second-order model and reflect the fast adsorption rate of MB on HPKS. The sorption isotherm study reveals the chemisorption of electrostatic attraction between the cationic MB molecules and the negative charged surfaces of HPKS. More importantly, the MB removal efficiency is more than 99% in a broad range of solution pH value. The adsorption capacities of HPKS can be easily recovered by calcination at 600 °C to remove the adsorbed dyes and without obvious diminishment even after six successive cycles. Therefore, the HPKS is a cost-effective and environmentally friendly adsorbent which has is promising to use in practical applications.

Dynamics and thermodynamics for competitive adsorptive removal of methylene blue and rhodamine B from binary aqueous solution onto durian rind

Concurrent adsorptive removal of methylene blue (MB) and rhodamine B (RhB) onto durian rind (DR) agricultural waste, from an aqueous binary solution as a model of wastewater containing multiple synthetic dyes, was investigated. The concurrent adsorption of the dyes followed pseudo-second-order kinetics. The adsorption isotherm was well simulated by the Langmuir model, implying a monolayer adsorption to the surface with a homogeneous binding energy. The adsorption process was governed by external mass transfer through two-step intraparticle diffusion of the dyes onto the adsorbent surface. The adsorption efficiency of MB (96.4%) is much higher than that of RhB (56.3%). This is attributed to the higher rate constant for the adsorption of MB (0.348 g mg^-1 min^-1) as compared to that of RhB (0.151 g mg^-1 min^-1). The adsorption behavior suggested that the two cationic dyes in the binary solution diffused and adsorbed independently and randomly onto the DR surface. The adsorption capacity of MB and RhB in the binary solution (47.4 mg g^-1 and 32.9 mg g^-1, respectively) is lower than those of their single solute solutions (93.3 mg g^-1 and 62.8 mg g^-1, respectively), suggesting a competitive effect in their concurrent adsorption. This was confirmed based on the adsorption characteristics of the binary solution with different molar ratios. The competitive effect was attributed to either non-interactive or repulsive electrostatic interactions between the positively charged dyes in the binary system. The domination of MB is attributed to its smaller molecular size, higher planarity, and faster adsorption kinetics compared with RhB.

Trimeric Cationic Surfactant Coacervation as a Versatile Approach for Removing Organic Pollutants

A versatile method to remove a broad spectrum of dye pollutants from wastewater rapidly and efficiently is highly desirable. Here, we report that the complex coacervation of cationic trimeric imine-based surfactants (TISn) with negatively charged hydrolyzed polyacrylamide (HPAM) can be used for this purpose. The coacervation occurs in a wide concentration and composition range and requires the HPAM and TISn concentrations as low as 0.1 g/L and 0.1 mM, respectively. Dye effluents treated by trimeric surfactants and HPAM complete phase separation within 30 s under turbulent conditions, which generates an exceedingly small volume fraction (0.4%) of viscoelastic coacervate and a clear supernatant with a dye removal efficiency of up to 99.9% for anionic and neutral dyes in dosages of up to 120 mg/L. Crowded molecular arrangement and thick framework in coacervate are responsible for the rapid phase separation rate and low volume fraction. The trimeric imine surfactant/polymer coacervation provides a simple, effective, and sustainable approach for the rapid removal of dyes and other organic pollutants.

Graphitic carbon nitride nanosheets (g-C3N4 NS) as dual responsive template for fluorescent sensing as well as degradation of food colorants

In this work, fluorescent g-C₃N₄ NS with laminar morphology and ultrathin thickness were fabricated. The as synthesized NS were well characterized by UV-Visible and Fluorescence spectroscopy, FT-IR, XRD and HR-TEM. The bright blue fluorescent suspension of g-C₃N₄ NS was utilized for efficient detection of food colorant; tartrazine (Tz) and sunset yellow (SY). Both food colorant were able to quench fluorescence of NS efficiently were able to detect them selectively over other interfering analytes. The chemosensor showed linear range response for low concentration of Tz and SY with limit of detection for Tz and SY as 0.0325 μM (32.5 nM) and 0.221 μM (221 nM), respectively. They served as non-toxic and low cost photocatalyst. The catalytic degradation process was confirmed by mass and UV-Visible spectra analysis. The g-C₃N₄ NS served dual role of detection as well as photocatalytic degradation of food colorant.

Utilizing waste corn-cob in biosynthesis of noble metallic nanoparticles for antibacterial effect and catalytic degradation of contaminants

In the present study, cost-effective, and environmentally friendly fabrication of silver and gold nanoparticles was performed by using aqueous extract of waste corn-cob. The formation of the metallic nanoparticles (MNPs) was optimized by UV-Vis method. The phytoconstituents were responsible for reduction of silver and gold ions to silver nanoparticles (CC-AgNPs) and gold nanoparticles (CC-AuNPs) which were demonstrated by Fourier-transform infrared (FTIR) spectroscopy while formation of AgCl was attributed to the presence of chloride ions in the aqueous extract. The crystalline nature of the AgNPs, AgCl, and AuNPs was confirmed using the X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns. Morphological studies showed that the synthesized CC-AgNPs existed in spherical shape with the size ranging from 2 to 28 nm possessing an average value of 11 nm while CC-AuNPs were present in the multiple shapes with size ranging from 5 to 50 nm possessing an average value of 35 nm. For studies on bioactive application, the CC-AgNPs exhibited a high antibacterial activity against three bacterial strains including Salmonella typhimurium, Bacillus cereus, and Staphylococcus aureus. In addition, the catalytic efficiency of MNPs was investigated for reduction of o-, m-, p-nitrophenols, and degradation of organic dyes including Eosin Y and Rhodamine 6G. The rate constants calculated from the kinetical data revealed that the biosynthesized nanoparticles are excellent catalysts in potential applications for treatment of wastewater. Graphical abstract .

Reactive adsorption of Safranin O: surface - pore diffusion modeling and degradation study

Granular activated carbon was doped with iron (Fe-AC) and was used to study the removal of Safranin O (SO) using the Fe-AC/H₂O₂ system for reactive adsorption and Fe-AC for adsorption. Fe-AC and H₂O₂ doses were optimized to obtain maximum removal of SO. Maximum removal was found to be 96.1% after 5 h using 1.0 g/L Fe-AC and 5.0 mM hydrogen peroxide doses for 10 mg/L initial SO concentration. Kinetic study suggested the suitability of the pseudo-first-order model for reactive adsorption. The Langmuir isotherm explained well the sorption of SO onto Fe-AC. Parallel-pore-reactive adsorption model was applied and validated. By fitting the experimental data to the model, it is observed that the surface reaction rate coefficient, k_r, was found to be five times that of the apparent rate constant, k_app. Parameters such as the external liquid film mass transfer coefficient, macro-pore and micro-pore diffusivities were estimated by regression analysis. Pore diffusion and surface reaction were found to be rate controlling for adsorption and reactive adsorption, respectively. An oxidative degradation of SO took place via hydroxylation and ring cleavage processes.