Removal of direct dyes by coagulation: the performance of preformed polymeric aluminum species

Trypan blue removal from water with zein sorbents and laccase

Abstract

Zein-based materials were used to remove Trypan blue from water under flow conditions and in batch tests. In flow tests, zein dissolved at pH = 13 was injected in sand columns and subsequently coagulated with CaCl₂, to create an adsorbent filter which removed over 99% of Trypan blue. Batch tests were conducted using zein powder, zein dissolved at pH = 13 and coagulated with CaCl₂, Fe₂Cl₃ or citric acid, and zein dissolved in ethanol and then coagulated with water. The highest Trypan blue removal was achieved with zein powder (4000 mg Trypan blue/kg sorbent, as determined through spectrophotometry), followed by zein coagulated with Fe₂Cl₃ (500 mg Trypan blue/kg sorbent) and with other salts (140 mg Trypan blue/kg sorbent). Differences in the sorption efficiency are attributed to differences in the surface area. The sorption isotherm of Trypan blue onto zein-based sorbents was a Type II isotherm, suggesting physisorption. Desorption of Trypan blue was limited when zein-based coagulated sorbents were immersed in pure water. Trypan blue could be degraded by free laccase in water, as determined through spectrophotometry and electrospray ionization mass spectroscopy (ESI-MS). Trypan blue could also be degraded by laccase when zein-based laccase-containing sorbents were prepared at pH = 10, using Fe₂Cl₃ as coagulant.

Graphic abstract

Supplementary information

The online version contains supplementary material available at 10.1007/s42452-020-04107-w.

Effective and viable photocatalytic degradation of rhodamine B dye in aqueous media using CuO/PVA nanocomposites

Graphical representation of CuO/PVA nanocomposite synthesis to degrade rhodamine B dye in aqueous medium

Adsorption, kinetics and thermodynamics studies of methyl orange dye sequestration through chitosan composites films

Different films comprising pure chitosan (CS) and chitosan coated sodium zeolites composites films designated as CSZ1, CSZ2, CSZ3 and CSZ4 respectively are presented here for the sequestration of MO dye. The as-synthesized films were characterized by using FSESM, XPS XRD, and TGA analysis. The sequestration of methyl orange dye (MO) was studied under various adsorption parameters i.e. pH effect, reaction temperature, catalytic dosage, interaction period, and original dye concentration in batch experiments. The adsorption power of MO dye sequestration in the presence of CSZ3 was 287 mg g^-1 higher than the fine CS (201 mg g^-1), and lowest for CSZ4 (173 mg g^-1). The experimental data is fitted in the pseudo-second order of chemical kinetics. The Freundlich and Langmuir adsorption models were used on behalf of the analysis of experimental data that revealed multilayered adsorption of MO dye. Kinetics, equilibrium and thermodynamic process were discussed in detailed, suggesting the endothermic and spontaneous process of the adsorption of MO dye on the exterior of films. The present work is general for the MO adsorption, however, it can be applied on large scale applications and can be easily adjustable in the water purification assemblies.

Evaluation of polyvinyl alcohol (PVA) loading in the PVA/titanium dioxide (TiO2) thin film coating on polyvinylidene fluoride (PVDF) membrane for the removal of textile dyes

Polyvinylidene fluoride (PVDF) membrane was coated with polyvinyl alcohol (PVA)/titanium dioxide (TiO₂) solution using dip-coating method. The effect of PVA loading (0-12 wt%) at a fixed TiO₂ concentration (1 wt%) was investigated through physical and morphological characterization of the membranes. The experimental results showed that increasing the PVA content from 0 to 12 wt% in the coating solution increased the hydrophilicity and tensile strength as well as the young's modulus of the coated membranes. It also increased the thickness of the thin film coating which enhanced the removal of dyes but affected the permeate flux adversely. Larger the molecular weight of the dye, higher the rejection and lower the permeate flux obtained. The antifouling performance of the membrane was studied using Bovine Serum Albumin (BSA) solution and the fouling was measured using the irreversible fouling factor. The results indicated that the PVA/TiO₂ coated PVDF membrane had lower irreversible fouling factors compared to plain PVDF membrane. The PVDF membrane with a thin film coating containing 3 wt% of PVA and 1 wt% of TiO₂ showed high photocatalytic degradation for the three dyes studied under UV irradiation due to uniform dispersion of TiO₂ nanoparticles over the membrane surface. Further, smaller the molecular weight of the dye, larger the photodegradation that was observed.

A novel chitosan-vanadium-titanium-magnetite composite as a superior adsorbent for organic dyes in wastewater

In this research, a novel chitosan (CS)-vanadium-titanium-magnetite (VTM) composite was designed and synthesized. The interaction between CS-VTM and Congo red (CR) dye conformed to a pseudo-second-order model to support the potent involvement of chemisorption. The effects of adsorbent dosage, reaction temperature, and initial solution pH on adsorption of CR were investigated. Approximately 99.1% of CR (100 mg/L) was adsorbed at a CS-VTM dose of 2.0 g/L or above, while such a reaction was favored at temperatures of 65 °C and pH of 6.0. Thermodynamically, the adsorption of CR proceeded spontaneously (ΔG < 0) above 35 °C. According to scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectrometer, and zeta potential analysis, its adsorption on CS-VTM can be attributed to electrostatic attraction and hydrogen bonds. The prepared CS-VTM exhibited superior adsorption performance on removal of CR as evidenced by significantly large partition coefficient of 108.3 mg g^-1 μM^-1 (equilibrium adsorption capacity of 62.2 mg/g at CR dose of 100 mg/L). Overall, the CS-VTM proved to be a promising and environmentally friendly adsorbent for the highly efficient and effective removal of organic dyes among the comparable sorbents studied to date.

A nanocomposite paper comprising calcium silicate hydrate nanosheets and cellulose nanofibers for high-performance water purification

Removal of soluble organic and inorganic contaminants from wastewater to produce clean water has received much attention recently. However, the simultaneous enhancement of water permeability and removal efficiency is still a challenge for filtration membranes. Here, we present a new kind of nanocomposite paper (CSH/CNF) consisting of calcium silicate hydrate (CSH) nanosheets and cellulose nanofibers (CNFs), and demonstrate the rapid water filtration and highly efficient contaminant (e.g., dyes, proteins, and metal ions) adsorption properties. The CNFs can serve as the bridging material to connect the CSH nanosheets to form a porous network structure and vital channels in the CSH/CNF paper for rapid water transportation. The weight ratio of CSH nanosheets in the paper is up to 75–85%. The weight ratio of CSH nanosheets has a significant effect on the water permeability and removal efficiency. The water permeability of the CSH/CNF paper with 82.5 wt% CSH nanosheets reaches as high as 312.7 L m⁻² h⁻¹ bar⁻¹, which is about 14.7 times that of the CSH/CNF paper with 75 wt% CSH nanosheets. Because of the high specific surface area and abundant adsorption sites of CSH nanosheets, the CSH/CNF paper with 82.5 wt% CSH nanosheets exhibits high adsorption capacities and removal efficiencies for methyl blue (242.6 mg g⁻¹, 97.3%), bovine serum albumin (289.2 mg g⁻¹, 98.5%) and Pb²⁺ ions (366.2 mg g⁻¹, 98.2%). The CSH/CNF nanocomposite paper holds great potential for application in environmental wastewater purification.

A nanocomposite paper with high water permeability and removal efficiency was prepared for the removal of organic and inorganic contaminants by filtration.

A water resistance magnetic graphene-anchored zeolitic imidazolate framework for efficient adsorption and removal of residual tetracyclines in wastewater

Water-resistant magnetic graphene-anchored zeolite imidazolate (Fe₃O₄/ZIF-8-G) composite materials with the largest surface area are formed by directly growing a hydrophobic ZIF-8 skeleton onto a graphene support through self-assembly in methanol. Fe₃O₄/ZIF-8-G hybrid composite has water resistance and super strong adsorption capacity, and is used as an effective adsorbent for adsorption and removal of residual tetracycline in wastewater. The morphologies and structure, as well as water resistance of Fe₃O₄/ZIF-8-G, were characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetry analysis (TGA), N₂ adsorption and pH_PZC. The adsorption for tetracycline (TC), oxytetracycline (OTC) and chlortetracycline (CTC) followed pseudo-second-order kinetics and fitted the Freundlich adsorption model with the simultaneous adsorption capacity for TC (382.58 mg g^-1), OTC (565.94 mg g^-1) and CTC (608.06 mg g^-1) at pH 5-6 for 10 h. These were much higher than previously reported results for the removal of tetracycline from aqueous solutions. The used Fe₃O₄/ZIF-8-G could be effectively reused and recycled at least five times without significant loss of adsorption capacity. The hydrophobic and π-π interaction between the aromatic rings of TCs and the aromatic imidazole rings of the ZIF-8-G framework were the main adsorption mechanism on the surface of Fe₃O₄/ZIF-8-G. Constructing a hydrophobic surface of ZIF-8/G framework resulted in a reduction of the hydrophilic sites of the surface. This can improve stability and selective adsorption of ZIF-8-G framework. In addition, the results show no significant difference in the adsorption kinetics and adsorption capacity of Fe₃O₄/ZIF-8-G for TC, OTC and CTC in pure water and wastewater.

In-situ characterization of dissolved organic matter removal by coagulation using differential UV-Visible absorbance spectroscopy

Removing dissolved organic matter (DOM) is of great concern due to its adverse effects on water supplies. Great effort has been given to studying DOM removal by coagulation, while the mechanism of DOM removal and the changes in its properties during coagulation have not been clearly illustrated due to the limitations of detection methods under practical environmental conditions. In this paper, the changes in DOM during coagulation were quantified using differential UV-Visible absorbance spectroscopy, and the differential spectra of DOM in the wavelength range of 200-600 nm could be deconvoluted into six Gaussian bands with maxima at approximately 200, 240, 276, 316, 385, and 457 nm after coagulation, respectively. The intensity of these maxima decreased with the type and dosage of coagulants. These observations should reflect the difference in the removability of DOM by coagulation, and this perspective was further confirmed by examining the deprotonation-protonation properties of DOM before and after coagulation. The affinity sites of DOM in coagulated waters, quantified by spectra parameter DlnA₄₀₀ (differential log-transformed spectra at wavelength 400 nm) in combination with the revised NICA model, increased as the coagulant dosage, which indicates that coagulation is inclined to remove the DOM fraction with fewer functional groups. Polyaluminum chloride (PAC) and Al-aggregate (Al₁₃) were more efficient than Alum for removing DOM due to their high efficiency for removing DOM fractions with fewer functional groups. The residual dissolved Al concentration depended on the total amount of reactive binding sites in DOM, and there was a strong linear correlation between residual dissolved Al and the total amount of reactive binding sites in DOM for Alum, while a weaker correlation was presented for PAC and Al₁₃. This indicates that Al_a was the dominant species to bind with the affinity sites in DOM to form residual dissolved Al.

Brief bibliometric analysis of "ionic liquid" applications and its review as a substitute for common adsorbent modifier for the adsorption of organic pollutants

The importance of improving adsorbent's adsorption efficiency in organic pollutants has been reported by many researchers. Surfactant-based modified adsorbents were a tasteful choice. As a result, the use of surfactants as a modifier for removing organic pollutants has shown to play a very big role in enhancing the adsorption efficiency of different materials. Ionic liquids are receiving extensive interest as green multipurpose compounds, primarily as a replacement for traditional chemicals that are used in many chemical processes. This work gives a brief bibliometric analysis of application of ionic liquid from 1930 to 2017, documents were collected from Scopus database and keywords from the abstracts and titles were analyzed using VOSviewer software. Furthermore, the work presents a review of conventionally known surfactants and the recent likelihood of ionic liquids for modifying adsorbents for adsorption of organic pollutants. Over the period of years between 1930 and 2017, 13,144 documents were published on the application of ionic liquids. VOSviewer software further confirms that adsorption is one of the leading areas in applications of ionic liquids. Review also showed that ionic liquid is a good modifier of adsorbents.

Preparation, Characterization, and Application of Metakaolin-Based Geopolymer for Removal of Methylene Blue from Aqueous Solution

Metakaolin-based geopolymers are aluminosilicate materials that can be used as cationic dye adsorbents in aqueous system treatment. Our aim in this paper is to study the ability of geopolymer powder produced from metakaolin and alkaline activators to act as an adsorbent to remove methylene blue (MB). The solid materials were systematically analyzed by X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier-transform infrared spectrometery (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and the point of zero charge. XRF, FTIR, XRD, SEM, and EDX analyses confirmed the formation of a geopolymer composite by geopolymerization reaction. The influence of various experimental factors such as geopolymer dosage, pH, initial dye concentration, contact time, and temperature was assessed. Adsorption isotherms were evaluated by Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherms. Kinetics data were studied using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. The thermodynamic parameters, namely, Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°), were determined. The results indicated that the maximum decolorization was found in high pH values. The collected isotherm data were best fitted by the Langmuir isotherm, and the maximum adsorption capacity of dye onto the geopolymer was 43.48 mg/g. The experiment kinetics followed the pseudo-second-order kinetic models. The thermodynamic results demonstrated that the adsorption of the obtained material occurs spontaneously as an endothermic process. The results confirmed that the prepared adsorbent can be used for remediation of water contaminated by MB dye.

Mechanistic Insights into Photodegradation of Organic Dyes Using Heterostructure Photocatalysts

Due to its low cost, environmentally friendly process, and lack of secondary contamination, the photodegradation of dyes is regarded as a promising technology for industrial wastewater treatment. This technology demonstrates the light-enhanced generation of charge carriers and reactive radicals that non-selectively degrade various organic dyes into water, CO2, and other organic compounds via direct photodegradation or a sensitization-mediated degradation process. The overall efficiency of the photocatalysis system is closely dependent upon operational parameters that govern the adsorption and photodegradation of dye molecules, including the initial dye concentration, pH of the solution, temperature of the reaction medium, and light intensity. Additionally, the charge-carrier properties of the photocatalyst strongly affect the generation of reactive species in the heterogeneous photodegradation and thereby dictate the photodegradation efficiency. Herein, this comprehensive review discusses the pseudo kinetics and mechanisms of the photodegradation reactions. The operational factors affecting the photodegradation of either cationic or anionic dye molecules, as well as the charge-carrier properties of the photocatalyst, are also fully explored. By further analyzing past works to clarify key active species for photodegradation reactions and optimal conditions, this review provides helpful guidelines that can be applied to foster the development of efficient photodegradation systems.

Preparation, characterization and kinetics study of chitosan/PVA electrospun nanofiber membranes for the adsorption of dye from water

In this study, chitosan (CS) nanofibers with two different degrees of deacetylation (DDA) were first successfully fabricated from its solution in 1% aqueous acetic acid solution by mixing with poly(vinyl alcohol) (PVA) solution at a weight ratio of 50/50 via the electrospinning method. Then, the CS/PVA membranes were further modified by glutaraldehyde vapor. The prepared nanofibers were characterized by field electron scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), the tensile test, the contact angle test, the weight loss test and the adsorption test for Congo red (CR). SEM analysis showed defect-free nanofibers and a uniform diameter distribution, with an average diameter of 100–125 nm. Subsequently, FTIR spectroscopy, XRD and TGA indicated that the modified CS/PVA membranes had a relatively higher thermal stability, because the thermal decomposition temperature of the unmodified CS/PVA membranes (~250°C) increased to a higher temperature (~ 300°C) for the modified CS/PVA. The nanofiber membranes after modification possessed better mechanical tensile properties. The membranes with lower DDA had a relatively higher tensile strength, which can withstand the maximum tensile strength of up to 6.36 MPa. Furthermore, the resulting membranes showed excellent hydrophilicity and kept their stability in distilled water, acidic, and basic media for 20 days. In the adsorption study, the maximum adsorption capacity of the membrane for CR was 358 mg/l in the optimum operating conditions of 25°C, pH = 6, 0.3 g membrane and 50 ml of 100 mg/l CR solutions. The resulting nanofibers membranes showed a better fitting to the Langmuir isotherm model and pseudo-second-order kinetic model.

Optimizing Chemically Enhanced Primary Treatment Processes for Simultaneous Carbon Redirection and Phosphorus Removal

There has been increased interest towards maximizing wastewater energy recovery by enhancing the carbon captured through the primary treatment process such as chemically enhanced primary treatment (CEPT). This research was conducted to optimize the CEPT performance in terms of redirection of carbon and nutrients in both bench- and full-scale operations. In order to improve the CEPT process, the performance of ferric chloride and seven types of polymers were evaluated through jar testing. The optimal coagulant (15 mg/L ferric chloride) and flocculant (0.5 mg/L poly aluminum chloride (PACl)) combination achieved total COD, soluble COD, total suspended solids (TSS), and total phosphorus (TP) removal efficiencies of 76, 58, 89, and 84, respectively, in a full-scale primary clarifier operation. In doing so the organic matter and phosphorus were concentrated in CEPT sludge, making them available for recovery. Furthermore, the relationship between influent characteristics and removal rates under varying operating conditions was investigated. It was found that soluble COD removal appeared to be season-dependent, and TSS removals were independent of influent TSS concentrations in all scenarios. The removal of tCOD, sCOD, and TP had a positive relationship with their corresponding concentrations when the polymer Alcomer® 120L was used, whereas no correlation between removal and concentration was observed with PACl.

Biosynthesis of Au nanoparticles supported on Fe3O4@polyaniline as a heterogeneous and reusable magnetic nanocatalyst for reduction of the azo dyes at ambient temperature

In this research Au nanoparticles, supported on Fe₃O₄@polyaniline as a magnetic nanocatalyst, was synthesized and its performance has been evaluated to reduce Methylene Blue (MB) and Methyl Orange (MO) from aqueous solutions. Gold nanoparticles, as a nanocatalyst with excellent activity, were prepared through the reduction of Au³⁺ using a wild herbal extract (Allium Sp) without any toxic chemical compounds and harmful materials. The synthesized Fe₃O₄@PANI-Au magnetic nanocatalyst was characterized by different instruments. To investigate the effect of nanocatalyst concentration on the degradation rate of azo dyes, two different catalyst concentrations were used at ambient temperature. According to the carried out calculations, degradation reaction of azo dyes with NaBH₄ in presence of the nanocatalyst is 10³ to 10⁴ times faster than degradation without using the catalyst.

Enhanced photocatalytic New Coccine degradation and Pb(II) reduction over graphene oxide-TiO2 composite in the presence of aspartic acid-β-cyclodextrin

In this work, aspartic acid-β-cyclodextrin (ACD) was synthesized by the reaction of β-cyclodextrin with aspartic acid and epichlorohydrin, and graphene oxide-TiO₂ (GO-TiO₂) composite catalyst was prepared by a hydrothermal method. The complexation of ACD with New Coccine (NC) and Pb²⁺ was characterized with FT-IR and XPS, respectively, the results show that ACD can simultaneously complex NC and Pb²⁺. XRD analysis and SEM images of GO-TiO₂ show that TiO₂ platelets are well distributed on both sides of the graphene oxide sheets, and display a similar XRD pattern to the pure TiO₂ nanoparticles with the typical diffraction peak of anatase phase. The effects of ACD on the photocatalytic degradation of NC and photocatalytic reduction of Pb²⁺ were investigated in the single pollution system, and the synergistic effects on the simultaneous photocatalytic NC degradation and Pb²⁺ reduction in the presence of ACD were also evaluated. The results showed that the presence of ACD was favorable to the acceleration of photocatalytic oxidation of NC and photocatalytic reduction of Pb²⁺ in the single pollution system, and the photocatalytic reaction rate constants of NC and Pb²⁺ in the presence of ACD increased 58% and 42%, respectively. For the combined pollution system, the synergistic effects on the simultaneous conversion of NC and Pb²⁺ in aqueous solutions were also further enhanced. ACD enhanced photocatalytic activity was attributed to the improvement of the electron transfer and mass transfer at the GO-TiO₂ interface.

Modelling dye removal by adsorption onto water treatment residuals using combined response surface methodology-artificial neural network approach

In this study, response surface methodology (RSM)-artificial neural network (ANN) approach was used to optimise/model disperse dye removal by adsorption using water treatment residuals (WTR). RSM was first applied to evaluate the process using three controllable operating parameters, namely WTR dose, initial pH (pH_initial) and dye concentration, and optimal conditions for colour removal were determined. In the second step, the experimental results of the design data of RSM were used to train the neural network along with a non-controllable parameter, the final pH (pH_final). The trained neural networks were used for predicting the colour removal. A colour removal of 52.6 ± 2.0% obtained experimentally at optimised conditions (pH_initial 3.0, adsorbent dose 30 g/L and dye concentration 75 mg/L) was comparable to 52.0% and 52.2% predicted by RSM and RSM-ANN, respectively. This study thus shows that optimising/predicting the colour removal process using the RSM-ANN approach is possible, and it also indicates that adsorption onto WTR could be used as a primary treatment for removal of colour from dye wastewater.

Characterization and optimization of spectrophotometric colour removal from dye containing wastewater by Coagulation-Flocculation

The performance of Vigna unguiculata coagulant (VUC) for colour removal from acid dye was investigated in this study. The proximate, structure and morphology of the coagulant were investigated using standard official methods, Fourier-Transform Infrared (FTIR) spectrometer and scanning electron microscopy (SEM), respectively. Response surface methodology (RSM) using face-centred central composite design (FCCD) optimized four process variables including pH, coagulant dosage, dye concentration and time. The colour removal efficiency obtained from the optimization analysis was 99.26% at process conditions of pH 2, coagulant dosage 256.09 mg/l, dye concentration 16.7 mg/l and time 540 min. The verification experiments agreed with the predicted values having a standard error value of 1.96%. Overlay contour plot established optimum areas where the predicted response variable is in an acceptable range (≥ 70%) with respect to optimum conditions. The FCCD approach was appropriate for optimizing the process giving higher removal efficiency when compared to the main effect plots.

Zwitterionic superabsorbent polymer hydrogels for efficient and selective removal of organic dyes

A novel zwitterionic superabsorbent polymer hydrogel [ZI-SAH] was synthesized by free radical polymerization and used for the removal of crystal violet (CV) and congo red (CR) from an aqueous medium.

Operating Cost Reduction of In-line Coagulation/Ultrafiltration Membrane Process Attributed to Coagulation Condition Optimization for Irreversible Fouling Control

This study examined the optimum coagulation conditions for reducing irreversible fouling during the in-line coagulation/ultrafiltration (UF) membrane process and assessed the decrease in operating cost. The coagulation conditions that generated charge-neutralization, sweep-flocculation, and under-dosing mechanisms were obtained by a jar-test, and a pilot-scale in-line coagulation/UF membrane process was operated under the coagulation conditions. Charge-neutralization and sweep-flocculation mechanisms reduced irreversible fouling effectively, and the under-dosing mechanism was able to reduce irreversible fouling only when flocs of a certain size or larger were formed. This revealed that floc size was a more important factor in reducing irreversible fouling than floc structure, and once initial cake layers were created by flocs of a fixed size, the structure of formed cake layers had only a minor effect on irreversible fouling. Regarding reduction in operating cost, 0.5 mg/L and 3 h, which were necessary to produce an under-dosing mechanism, were deemed the optimum coagulant dosage and coagulant injection time, respectively, to reduce irreversible fouling. In order to analyze the operating cost reduction effect, a pilot plant was operated under optimum operating conditions, and the total operating cost was approximately 11.2% lower than without in-line coagulation.

Coagulation performance of cucurbit[8]uril for the removal of azo dyes: effect of solution chemistry and coagulant dose

Dye wastewater has attracted significant attention because of its wide pH range and high content of color. In this work, the coagulation performances of cucurbit[8]uril for the removal of color from acid red 1 (AR1), orange II (OII), and Congo red (CR) dye wastewaters were investigated. Experimental results showed that color removal rates of greater than 95% for AR1, OII and CR were achieved at pH 6.0, when the dosage of cucurbit[8]uril was 1.51, 3.01 and 0.38 mmol·L^-1, respectively. Under identical conditions, the color removal efficiencies of AR1 and CR were higher than OII, due to the larger molecular weights and more active hydroxyl and amino groups. Moreover, steady increases in AR1, OII and CR removal rates were recorded with increasing ionic strength. Such increases may be related to the reduction in thickness of the surface solvent membrane surrounding the dye colloids at high ionic strengths. Furthermore, Fourier transform infrared spectra demonstrated that no new bonds or functional groups were formed during coagulation, which indicates that the removal of AR1, OII and CR was primarily a physical process. The hydrogen bonds and inclusion complexes formed between cucurbit[8]uril and AR1, OII and CR contributed to the removal of color in coagulation predominantly.

One step synthesis of Co/Cr-codoped ZnO nanoparticle with superb adsorption properties for various anionic organic pollutants and its regeneration

Adsorption is an effective means to remove organic pollutant. However, it is challenging to prepare the adsorbents with high adsorption capacities and their regeneration. Herein, Co/Cr-codoped ZnO nanoparticles (NPs) with superb adsorption for dyes and antibiotics have been successfully synthesized by a mild solvothermal method. At the optimal Co:Cr:Zn doping moral ratio of 4:6:100, the maximum adsorption capacities of methyl orange (MO) and tetracycline hydrochloride (TC-HCl) on Co/Cr-codoped ZnO NPs is 1057.90 mg g^-1 and 874.46 mg g^-1, respectively. The adsorption process of the sample over MO and TC-HCl both agreed well with the pseudo-second-order kinetic model and Langmuir isotherm model. Adsorption thermodynamics proved that the adsorption of MO and TC-HCl on Co/Cr-codoped ZnO NPs was a spontaneous and endothermic process. The mechanism shows that the surface of Co/Cr-codoped ZnO NPs have more positive charges, larger specific surface area and more crystal defects due to Co³⁺ and Cr³⁺ substitutes Zn²⁺ in ZnO lattice, improving their adsorption property. In addition, Co/Cr-codoped ZnO NPs have also excellent adsorption capacity for Direct Red, Congo Red, Evans Blue and Methyl Blue. More importantly, the regeneration of adsorbents was studied to achieve the reuse of materials, and avoid secondary pollution. Co/Cr-codoped ZnO NPs will be a promising choice for wastewater treatment owing to its excellent adsorption capacity and relatively low cost.