Recent advances for dyes removal using novel adsorbents: A review

Novel green strategy for CuO-ZnO-C nanocomposites fabrication using marigold (Tagetes spp.) flower petals extract with and without CTAB treatment for adsorption of Cr(VI) and Congo red dye

The CuO-ZnO-Carbon (CZC) nanocomposites (NCs) were synthesized via a green method at 300 and 400 °C calcinated temperatures, using waste marigold (Tagetes spp.) flower petal extract as a reducing agent and carbon source. A novel green strategy for the synthesis of highly effective CZC NCs was developed which showed better adsorption of toxic Cr(VI) and Congo red (CR) dye compared to unsupported carbon NCs. In this strategy, fine powder of petals as carbon source were passed with the flower liquid extract during the filtration process, which supported the metal oxides nanorods(NRs)/nanoparticles(NPs) on the surface. Furthermore, the surface of the synthesized NCs was modified by Cetyl Trimethyl Ammonium Bromide (CTAB) cationic surfactant to increase surface functionality, surface area, and positive charge density of NCs. Additionally, the adsorption performance of Cr(VI) and CR dye improved from acidic pH to neutral pH after surfactant modification of NCs compared to unmodified NCs. The characterization techniques such as Powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) surface area analysis, Point of zero charge (pHpzc), Field Emission Scanning Electron Microscopy (FE-SEM), Transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were performed to examine physio-chemical properties of NCs and CTAB modified NCs. The FTIR and BET analysis confirmed that CTAB modified NCs showed excellent functionality and more than 49% and ~67% greater surface area than CZC-300 and CZC-400, respectively, which prepared at 300 and 400 °C temperature. XRD analysis confirmed that NCs were highly crystalline and no phase change after surfactant modification. The FE-SEM and TEM analysis confirmed the pentagonal NRs and spherical NPs of ZnO and CuO, respectively, were formed on the carbon surface. After CTAB modification, no change in the surface morphology of NCs was observed. Thus, comparative study of NCs and CTAB modified NCs was done for Cr(VI) and CR dye adsorption by varying batch conditions, such as initial pH, contact time, temperature, and initial concentration of Cr(VI)/CR dye. The equilibrium time and concentration data were fitted with non-linear forms of kinetic and isotherm models, respectively. CTAB modified CZC-300 NCs showed excellent adsorption capacity for both pollutants up to pH 6 compared to CZC-300 and CZC-400 NCs. Additionally, the maximum adsorption capacity of CTAB modified NCs for Cr(VI) and CR dye were 201.56 and 331.36 mg/g, respectively, at pH 2 and 30 °C and increased with increasing temperature. The effect of co-existing anions on adsorption capacity of both NCs for Cr(VI) and CR dye adsorption was investigated. The regeneration and reusability experiments of both NCs were also performed.

Activated biochar derived from spent Auricularia auricula substrate for the efficient adsorption of cationic azo dyes from single and binary adsorptive systems

In this study, spent Auricularia auricula substrate (AS)-derived biochar (ASBCs) and activated biochar with NaOH (A-ASBC) were evaluated for the adsorption of cationic azo dyes, including methylene blue (MB), rhodamine B (RB), and crystal violet (CV), from single and binary adsorptive systems. A-ASBC showed a higher maximum adsorption capacity for these dyes (MB: 53.62 mg·g^-1, RB: 32.33 mg·g^-1, CV: 735.73 mg·g^-1) than ASBCs in a single system because it had a greater specific surface area and more oxygen containing-functional groups on the surface. The adsorption process of the three dyes onto the adsorbents was in good agreement with the Freundlich adsorption isotherm and fit the pseudo-second-order kinetic model, which revealed sorbate polymolecular layer formation over the adsorbent surface and the involvement of chemisorption. The adsorption mechanism showed that the adsorption of three dyes on adsorbents could be postulated as a multistep process with extraordinary affinity-induced adsorption in terms of both physisorption and chemisorption. In the binary adsorptive system, the results showed that all MB, RB, and CV had antagonistic/competitive effects on each other's adsorption (Q_Binary/Q_Single < 1). Furthermore, a phytotoxic assay affirmed the effectiveness of the adsorbent in adsorbing dye species from aqueous solutions using Brassica pekinensis L. seeds as the model. Therefore, activated biochar prepared from AS can be used as a potentially economical and effective adsorbent for treating printing and dyeing wastewater.

Biodegradation of Indanthrene Blue RS dye in immobilized continuous upflow packed bed bioreactor using corncob biochar

The current study describes the aerobic biodegradation of Indanthrene Blue RS dye by a microbial consortium immobilized on corn-cob biochar in a continuous up-flow packed bed bioreactor. The adsorption experiments were performed without microbes to monitor the adsorption effects on initial dye decolorization efficiency. The batch experiments were carried out to estimate the process parameters, and the optimal values of pH, temperature, and inoculum volume were identified as 10.0, 30 °C, and 3.0 × 10⁶ CFU mL⁻¹, respectively. During the continuous operation, the effect of flow rate, initial substrate concentration, inlet loading rate of Indanthrene Blue RS on the elimination capacity, and its removal efficiency in the bioreactor was studied. The continuous up-flow packed bed bioreactor was performed at different flow rates (0.25 to 1.25 L h⁻¹) under the optimal parameters. The maximum removal efficiency of 90% was observed, with the loading rate varying between 100 and 300 mg L⁻¹ day⁻¹. The up-flow packed bed bioreactor used for this study was extremely useful in eliminating Indanthrene Blue RS dye using both the biosorption and biodegradation process. Therefore, it is a potential treatment strategy for detoxifying textile wastewater containing anthraquinone-based dyes.

PEG/Sodium Tripolyphosphate-Modified Chitosan/Activated Carbon Membrane for Rhodamine B Removal

Textile dyes from wastewater effluent are highly toxic to both living species and aqueous environments. An environmentally friendly method to remove hazardous dyes from wastewater in the textile industry has been a challenge. Chitosan (CS) and activated carbon (AC) are widely used as adsorbents for dye removal. However, the poor porosity and unsatisfactory stability of CS and the unfriendly cost of AC limited their applications to be used alone as a single adsorbent. Here, we report a novel method to prepare a CS/AC membrane using PEG10000 as a porogen and sodium tripolyphosphate (TPP) as a cross-linking agent. The adsorption efficiency and reusability of the PEG/TPP-modified CS/AC membrane to remove RhB were investigated based on dynamic and static adsorption models. The results reveal that the adsorption performance of CS/AC membranes was significantly improved after the PEG/TPP modification based on the abundance macroporous structure. The modified CS/AC membrane with a 30% AC doping ratio exhibited an excellent adsorption efficiency of 91.29 and 73.91% in the dynamic and static adsorption processes, respectively. These results provide new insights into designing membranes to remove dyes from wastewater efficiently.

A novel strategy for enhancing heterogeneous Fenton degradation of dye wastewater using natural pyrite: Kinetics and mechanism

Hydrogen peroxide activation by pyrite for degradation of recalcitrant contaminants receives increasing attention. The improvement of catalytic efficiency of natural pyrite is still a challenging issue. This work provides a novel strategy of enhancing catalytic efficiency via pre-reaction between pyrite and hydrogen peroxide. Effects of process factors including pre-reaction time, hydrogen peroxide, solution pH and initial dye concentration were examined. Natural pyrite with low purity was characterized by Raman, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Reaction kinetic verifies tremendous improvement of the reaction rate through pre-reaction. Enhanced dye degradation is ascribed to hydroxyl radical production promoted by self-regulation of pH, Fe²⁺ releasing and Fe²⁺/Fe³⁺ cycle. The plausible mechanism was proposed based on multiple determinations. Dye degradation in different water matrix was efficiently obtained, as well as multicomponent dyes. Additionally, broad operation pH and good reusing performance make the developed process highly attractive for application. This work provides a solid step-forward of pyrite/hydrogen peroxide Fenton process for treatment of recalcitrant contaminants in wastewater.

Efficient adsorption removal of anionic dyes by an imidazolium-based mesoporous poly(ionic liquid) including the continuous column adsorption-desorption process

The mesoporous poly(N,N'-methylene-bis(1-(3-vinylimidazolium)) chloride), labeled as PDVIm-Cl, with double anions (Cl^-) and low monomer molecular weight was synthesized and applied in the adsorption of anionic dyes (acid orange 7 (AO7), sunset yellow (SY), reactive blue 19 (RB19), congo red (CR)). Due to the mesoporous structure, abundant Cl^- and positively charged imidazole rings, the poly(ionic liquid) (PIL) exhibited superior adsorption ability towards anionic dyes. What is more, the RB19 adsorption by PDVIm-Cl could achieve the highest capacity (2605 ± 254 mg g^-1) which was nearly twice higher than the maximum adsorption capacity of the previously reported materials. All the adsorption kinetic data and isotherms fitted well with the pseudo second-order model and Langmuir-Freundlich model. To better explore the practical potential of the PIL for dye adsorption, the adsorption under different pH values and column adsorption performances were also evaluated. Results showed that PDVIm-Cl exhibited high removal efficiencies for anionic dyes over a wide pH range (2-10). Also, the great reusability could be well demonstrated by the achievable continuous column adsorption-desorption process. It is worth mentioning that the regeneration could be realized with very little desorbent which was far less than the adsorption volume flowing through the column and the desorption efficiency was well maintained after three consecutive cycles. At last, the adsorption mechanism was explored by experiments combined with quantum chemical calculations and showed anionic dyes adsorption by PDVIm-Cl was a joint process dominated by the ion exchange, electrostatic interaction, hydrogen bond and π-π stacking.

Modification of porous polyetherimide hollow fiber membrane by dip-coating of Zonyl® BA for membrane distillation of dyeing wastewater

Wetting and fouling have significantly affected the application of membrane distillation (MD). In this work, a dip-coating method was used for improving surface hydrophobicity of the polyetherimide (PEI) hollow fiber membrane. An air gap membrane distillation (AGMD) process was applied for treatment of the methylene blue (MB) solution. The porous PEI membrane was fabricated by a dry-wet spinning process and the hydrophobic 2-(Perfluoroalkyl) ethanol (Zonyl^® BA) was used as the coating material. From FESEM, the modified PEI-Zonyl membrane showed an open structure with large finger-like cavities. The modified membrane displayed a narrow pore size distribution with mean pore size of 0.028 μm. The outer surface contact angle of the PEI-Zonyl membrane increased from 81.3° to 100.4° due to the formation of an ultra-thin coated layer. The pure water flux of the PEI-Zonyl membrane was slightly reduced compared to the pristine PEI membrane. A permeate flux of 6.5 kg/m² h and MB rejection of 98% were found for the PEI-Zonyl membrane during 76 h of the AGMD operation. Adsorption of MB on the membrane surface was confirmed based on the Langmuir isotherm evaluation, AFM and FESEM analysis. The modified PEI-Zonyl membrane can be a favorable alternative for AGMD of dyeing wastewaters.

Transformation of Glass Fiber Waste into Mesoporous Zeolite-Like Nanomaterials with Efficient Adsorption of Methylene Blue

Recycling and reusing glass fiber waste (GFW) has become an environmental concern, as the means of disposal are becoming limited as GFW production increases. Therefore, this study developed a novel, cost-effective method to turn GFW into a mesoporous zeolite-like nanomaterial (MZN) that could serve as an environmentally benign adsorbent and efficient remover of methylene blue (MB) from solutions. Using the Taguchi optimizing approach to hydrothermal alkaline activation, we produced analcime with interconnected nanopores of about 11.7 nm. This MZN had a surface area of 166 m2 g−1 and was negatively charged with functional groups that could adsorb MB ranging from pH 2 to 10 and all with excellent capacity at pH 6.0 of the maximum Langmuir adsorption capacity of 132 mg g−1. Moreover, the MZN adsorbed MB exothermically, and the reaction is reversible according to its thermodynamic parameters. In sum, this study indicated that MZN recycled from glass fiber waste is a novel, environmentally friendly means to adsorb cation methylene blue (MB), thus opening a gateway to the design and fabrication of ceramic-zeolite and tourmaline-ceramic balls and ceramic ring-filter media products. In addition, it has environmental applications such as removing cation dyes and trace metal ions from aqueous solutions and recycling water.

A Stochastic Model for Adsorption Kinetics

A novel stochastic model is proposed to characterize the adsorption kinetics of pollutants including dyes (direct red 80 and direct blue 1), fluoride ions, and cadmium ions removed by calcium pectinate (Pec-Ca), aluminum xanthanate (Xant-Al), and reed leaves, respectively. The model is based on a transformation over time following the Ornstein–Uhlenbeck stochastic process, which explicitly includes the uncertainty involved in the adsorption process. The model includes stochastic versions of the pseudo-first-order (PFO), pseudo-second-order (PSO), and pseudo- $n$ -order (PNO) models. It also allows the estimation of the adsorption parameters, including the maximum removal capacity ( $q_e$ ), the adsorption rate constant ( $k_n$ ), the reaction pseudoorder ( $n$ ), and the variability $\left({\sigma }^2\right)$ . The model fitted produced $R^2$ values similar to those of the nonstochastic versions of the PFO, PSO, and PNO models; however, the obtained values for each parameter indicate that the stochastic model better reproduces the experimental data. The $q_e$ values of the Pec-Ca-dye, Xant-Al-fluoride, and reed leaf-Cd+2 systems ranged from 2.0 to 9.7, 0.41 to 1.9, and 0.04 and 0.29 mg/g, respectively, whereas the values of $k_n$ ranged from 0.051 to 0.286, 0.743 to 75.73, and 0.756 to 8.861 (mg/g)1-n/min, respectively. These results suggest a variability in the parameters $q_e$ and $k_n$ inherent to the natures of the adsorbate and adsorbent. The obtained $n$ values ranged from 1.13 to 2.02 for the Pec-Ca-dye system, 1.0–3.5 for the Xant-Al-fluoride system, and 1.8–3.8 for the reed leaf-Cd+2 system. These ranges indicate the flexibility of the stochastic model to obtain fractional $n$ values, resulting in high $R^2$ values. The variability in each system was evaluated based on ${\sigma }^2$ . The developed model is the first to describe pollutant removal kinetics based on a stochastic differential equation.

Enhanced Photocatalytic Activity of Cu2O Cabbage/RGO Nanocomposites under Visible Light Irradiation

Towards the utilization of Cu₂O nanomaterial for the degradation of industrial dye pollutants such as methylene blue and methyl orange, the graphene-incorporated Cu₂O nanocomposites (GCC) were developed via a precipitation method. Using Hummers method, the grapheme oxide (GO) was initially synthesized. The varying weight percentages (1-4 wt %) of GO was incorporated along with the precipitation of Cu₂O catalyst. Various characterization techniques such as Fourier-transform infra-red (FT-IR), X-ray diffraction (XRD), UV-visible diffused reflectance (UV-DRS), Raman spectroscopy, thermo gravimetric analysis (TGA), energy-dispersive X-ray analysis (EDX), and electro chemical impedance (EIS) were followed for characterization. The cabbage-like morphology of the developed Cu₂O and its composites were ascertained from field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HR-TEM). In addition, the growth mechanism was also proposed. The results infer that 2 wt % GO-incorporated Cu₂O composites shows the highest value of degradation efficiency (97.9% and 96.1%) for MB and MO at 160 and 220 min, respectively. Further, its catalytic performance over visible region (red shift) was also enhanced to an appreciable extent, when compared with that of other samples.

Tailoring Chitosan/LTA Zeolite Hybrid Aerogels for Anionic and Cationic Dye Adsorption

Chitosan (CS) is largely employed in environmental applications as an adsorbent of anionic dyes, due to the presence in its chemical structure of amine groups that, if protonated, act as adsorbing sites for negatively charged molecules. Efficient adsorption of both cationic and anionic dyes is thus not achievable with a pristine chitosan adsorbent, but it requires the combination of two or more components. Here, we show that simultaneous adsorption of cationic and anionic dyes can be obtained by embedding Linde Type A (LTA) zeolite particles in a crosslinked CS-based aerogel. In order to optimize dye removal ability of the hybrid aerogel, we target the crosslinker concentration so that crosslinking is mainly activated during the thermal treatment after the fast freezing of the CS/LTA mixture. The adsorption of isotherms is obtained for different CS/LTA weight ratios and for different types of anionic and cationic dyes. Irrespective of the formulation, the Langmuir model was found to accurately describe the adsorption isotherms. The optimal tradeoff in the adsorption behavior was obtained with the CS/LTA aerogel (1:1 weight ratio), for which the maximum uptake of indigo carmine (anionic dye) and rhodamine 6G (cationic dye) is 103 and 43 mg g⁻¹, respectively. The behavior observed for the adsorption capacity and energy cannot be rationalized as a pure superposition of the two components, but suggests that reciprocal steric effects, chemical heterogeneity, and molecular interactions between CS and LTA zeolite particles play an important role.

Key structural features that determine the selectivity of UV/acetylacetone for the degradation of aromatic pollutants when compared to UV/H2O2

Acetylacetone (AA) has proven to be a potent photo-activator for the decolorization of dyes. However, there is very limited information on the quantitative structure-activity relationship (QSAR) and the mechanisms of dye degradation by UV/AA. Herein, the photolysis of 65 aromatic compounds (dyes and dye precursors) was investigated at three pH values (4.0, 6.0, 9.0) by UV/AA and UV/H₂O₂. The obtained pseudo-first-order photodegradation rate constants (k₁) were processed using statistical analysis. The correlation between the k₁ values and the number of photons absorbed by AA, together with the observed pH effect, suggested that the protonated enol structure of AA plays a crucial role in the photodecolorization of dyes. According to quantum chemical computation, photo-induced direct electron transfer between the excited state of AA and the dye was the main mechanism in the UV/AA process. QSAR models demonstrated that the molecular size and stability were the key factors that determined the efficiency of UV/H₂O₂ for dye degradation. Statistically, the UV/AA process was target-selective and suffered less from the inner filter effect, which made it more effective than the UV/H₂O₂ process for dye degradation. The selectivity of the UV/AA process was mainly embodied in the substituent effects: dyes with hydroxyl groups in conjugated systems decomposed faster than those with nitro-substitution or ortho-substituted sulfonate groups. The results can be used for the selection of appropriate photochemical approaches for the treatment of dye-contaminated water.

Biodegradation of azo dye-containing wastewater by activated sludge: a critical review

The effluent from the textile industry is a complex mixture of recalcitrant molecules that can harm the environment and human health. Biological treatments are usually applied for this wastewater, particularly activated sludge, due to its high efficiency, and low implementation and operation costs. However, the activated sludge microbiome is rarely well-known. In general, activated sludges are composed of Acidobacteria, Bacillus, Clostridium, Pseudomonas, Proteobacteria, and Streptococcus, in which Bacillus and Pseudomonas are highlighted for bacterial dye degradation. Consequently, the process is not carried out under optimum conditions (treatment yield). Therefore, this review aims to contextualize the potential environmental impacts of azo dye-containing wastewater from the textile industry, including toxicity, activated sludge microbiome identification, in particular using the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) as a novel, rapid and accurate strategy for the identification of activated sludge microbiome (potential to enhance treatment yield).

Al and Zr Porous Clay Heterostructures as Removal Agents of Basic Blue-41 Dye from an Artificially Polluted Solution: Regeneration Properties and Batch Design

The removal of Basic Blue-41 dye molecules was carried out by using two doped porous clay heterostructures by aluminum (Al) or zirconium (Zr) species. The proposed method of synthesis showed its efficiency, starting from Al or Zr intercalated hydrolyzed species, prior to its reaction with dodecylamine (C12 amine) and tetraethyl orthosilicate (TEOS) as a silica source. The intercalated precursors and their porous clay heterostructures (PCH) derivatives were characterized by different techniques. Solid NMR technique proved the presence of Al species into the intercalated silica between the clay sheets, and in addition to Si in different environments within the PCH materials. The Zr-PCH material exhibited a higher surface area and pore volume compared to its Al-PCH counterpart, with a mesoporous character for both materials. A maximum removed amount of 279 and 332 mg/g was achieved and deduced from the Langmuir equation. The regeneration tests revealed that the removal efficiency of Zr-PCH was retained after five regeneration runs, with a loss of 15% of the original value; meanwhile, the Al-PCH lost 45% of its efficiency after only three cycles. A single-stage batch design was proposed based on the Langmuir isotherm parameters. The increase of the removal capacity of Zr-PCH led to the reduction of the required amounts for the target removal of BB-41 dye compared to Al-PCH.

Ion-Exchange Resins for Efficient Removal of Colorants in Bis(hydroxyethyl) Terephthalate

Bis(hydroxyethyl) terephthalate (BHET) obtained from waste poly(ethylene terephthalate) (PET) glycolysis often have undesirable colors, leading to an increased cost in the decoloration of the product and limiting the industrialization of chemical recycling. In this work, eight types of ion-exchange resins were used for BHET decoloration, and resin D201 showed an outstanding performance not only in the decoloration efficiency but also in the retention rate of the product. Under the optimal conditions, the removal rate of the colorant and the retention efficiency of BHET were over 99% and 95%, respectively. D201 showed outstanding reusability with five successive cycles, and the decolored BHET and its r-PET showed good chromaticity. Furthermore, the investigations of adsorption isotherms, kinetics, and thermodynamics have been conducted, which indicated that the decoloration process was a natural endothermic reaction. Adsorption interactions between the colorant and resin were extensively examined by various characterizations, revealing that electrostatic force, π-π interactions, and hydrogen bonding were the dominant adsorption mechanisms.

Simultaneous removal of Cu(II) and reactive green 6 dye from wastewater using immobilized mixed fungal biomass and its recovery

Immobilized fungal biomass (Aspergillus niger and Aspergillus flavus) was prepared and analysed for the simultaneous removal of Cu(II) ion and Reactive Green 6 dye from aqueous phase. Different characterization analysis was utilized to exploit the adsorption characteristics of fungal biomass. Batch biosorption tests, performed to investigate the factors influencing biosorption process inferred optimal values of 25 mg/L of adsorbate with equilibrium time of 60 min, 2.5 g of immobilized fungal biomass, temperature of 303 K and pH of 5.0 for the maximal removal of pollutants. The obtained experimental data was utilized to evaluate the kinetic, thermodynamic and equilibrium models. Langmuir isotherm model has higher correlation coefficient [Cu(II) ion = 0.8625 and RG 6 dye = 0.8575] with small values of errors (RMSE = 3.746 and SSE = 56.12 for Cu(II) ion; RMSE = 4.872 and SSE = 11.87 for RG 6 dye). Kinetic studies performed to evaluate the adsorption rate mechanism of this present study indicated that pseudo-first order and pseudo-second order kinetics to be most fitting model for removal of Cu(II) ions and Reactive green dye respectively. Thermodynamic analysis inferred the spontaneous, random, and exothermic nature of the biosorption process based on ΔG^o, ΔH^o, and ΔS^o values respectively. The prepared biomass can be an alternative for the elimination of toxic pollutants from wastewater.

Formation and mechanisms of nano-metal oxide-biochar composites for pollutants removal: A review

Biochar, a carbon-rich material, has been widely used to adsorb a range of pollutants because of its low cost, large specific surface area (SSA), and high ion exchange capacity. The adsorption capacity of biochar, however, is limited by its small porosity and low content of surface functional groups. Nano-metal oxides have a large SSA and high surface energy but tend to aggregate and passivate because of their fine-grained nature. In combining the positive qualities of both biochar and nano-metal oxides, nano-metal oxide-biochar composites (NMOBCs) have emerged as a group of effective and novel adsorbents. NMOBCs improve the dispersity and stability of nano-metal oxides, rich in adsorption sites and surface functional groups, maximize the adsorption capacity of biochar and nano-metal oxides respectively. Since the adsorption capacity and mechanisms of NMOBCs vary greatly amongst different preparations and application conditions, there is a need for a review of NMOBCs. Herein we firstly summarize the recent methods of preparing NMOBCs, the factors influencing their efficacy in the removal of several pollutants, mechanisms underlying the adsorption of different pollutants, and their potential applications for pollution control. Recommendations and suggestions for future studies on NMOBCs are also proposed.

Locally available agroresidues as potential sorbents: modelling, column studies and scale-up

Sawdust, cotton stalk and groundnut shell were used for removal of methylene blue from aqueous solution using batch sorption. Effect of initial dye concentration, temperature, and particle size of sorbents on methylene blue removal was investigated. Sorption capacity increases with rise in initial dye concentration and temperature. Impact of particle size on sorption of methylene blue was investigated and indicated that removal of dye increases with decrease in particle size of sorbents. Maximum sorption for sawdust, cotton stalks and groundnut shell were 9.22 mg g-1, 8.37 mg g-1 and 8.20 mg g-1 respectively; at 60 °C and 100 ppm initial dye concentration. Sorption isotherms were analyzed using fundamental Freundlich isotherm. Subsequently, sips isotherm model was employed for better fitting. Kinetic study shows that, biosorption process is pseudo-second-order in nature. During the course of this study, adsorption dynamics revealed that film diffusion was key step for biosorption. In addition, thermodynamics of sorption was studied; and it was found that Gibbs free energy (∆G°) decreases with increase in temperature. Sawdust was found to be best among all the sorbents. Therefore, column studies and breakthrough curve modelling were performed using sawdust. Furthermore, it was estimated that a scaled-up column using sawdust can treat 6672 L of wastewater in 24 h with 80% efficiency.

Treatment of Wastewaters with Zirconium Phosphate Based Materials: A Review on Efficient Systems for the Removal of Heavy Metal and Dye Water Pollutants

Layered zirconium phosphate (ZrP) is a versatile material with phosphate (POH ) groups able to exchange inorganic and organic cations or to intercalate basic molecules. The present review deals with the use of this material as a sorbent for heavy metal cations or dye molecules in wastewater treatments. The possibility to combine ZrP with polymers or other inorganic materials, in order to have suitable systems for real and large scale applications, was investigated, as well as the combination with photocatalytic materials to obtain hetrogeneous photocatalysts for the capture and photodegradation of organic dye molecules.

Engineering Iron Oxide Nanocatalysts by a Microwave-Assisted Polyol Method for the Magnetically Induced Degradation of Organic Pollutants

Advanced oxidation processes constitute a promising alternative for the treatment of wastewater containing organic pollutants. Still, the lack of cost-effective processes has hampered the widespread use of these methodologies. Iron oxide magnetic nanoparticles stand as a great alternative since they can be engineered by different reproducible and scalable methods. The present study consists of the synthesis of single-core and multicore magnetic iron oxide nanoparticles by the microwave-assisted polyol method and their use as self-heating catalysts for the degradation of an anionic (acid orange 8) and a cationic dye (methylene blue). Decolorization of these dyes was successfully improved by subjecting the catalyst to an alternating magnetic field (AMF, 16 kA/m, 200 kHz). The sudden temperature increase at the surface of the catalyst led to an intensification of 10% in the decolorization yields using 1 g/L of catalyst, 0.3 M H₂O₂ and 500 ppm of dye. Full decolorization was achieved at 90 °C, but iron leaching (40 ppm) was detected at this temperature leading to a homogeneous Fenton process. Multicore nanoparticles showed higher degradation rates and 100% efficiencies in four reusability cycles under the AMF. The improvement of this process with AMF is a step forward into more sustainable remediation techniques.

Enhanced removal of Congo red dye from aqueous solution by surface modified activated carbon with bacteria

AIMS

The adsorption behaviour and mechanisms of the surface modified activated carbon with bacteria was evaluated.

METHODS AND RESULTS

16S rRNA was employed to identify the hydrocarbon-degrading bacteria. The bacteria was characterized by TEM and electron microscope. The surface modified activated carbon with bacteria was characterized by SEM. The adsorption behaviour was tested by static adsorption and dynamic adsorption.

CONCLUSION

The adsorption efficiency of the modified activated carbon was high when pH was weak acidic, and the adsorption capacity increased with the increase of temperature ranging from 20 to 35°C. The adsorption capacity peaked at 234·6 mg g^-1 at 25°C, which was sixfold higher than that of activated carbon. The pseudo-first-order kinetic can more accurately assess Congo red adsorption on the two adsorbents. The adsorption of Congo red by bacteria surface modified activated carbon fitted well with the Langmuir's model. The adsorption process was endothermic, and the biological floccules were formed during the adsorption. The physical adsorption is the main driving force.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results indicate that the bacteria surface-modified activated carbon can be used effectively as an adsorbent to eliminate Congo red from aqueous solutions.

Risk assessment and the adsorptive removal of some pesticides from synthetic wastewater: a review

Background

The need for environmental protection and remediation processes has been an increasing global concern. Pesticides are used as biological agents, disinfectants, antimicrobials, and also in a mixture of some chemical substances. Their modes of application are through selective dispensing and attenuation processes which act upon any pest that compete with the production, processing, and storage of foods and also in agricultural commodes. The pests might comprise weeds, insects, birds, fish, and microbes.

Main body

Pesticides are commonly found in water surface, landfill leachate, ground water, and wastewater as pollutant. An overview of recently studied adsorption processes for the pesticide elimination from polluted water has been reported in this study utilizing activated carbon, clay materials, biomass materials, metal organic frame work, graphene, and carbon-based materials as well as agricultural wastes as adsorbents. The risk assessment and cost analysis of adsorbents were also provided.

Conclusion

Evidences from literature recommend modified adsorbent and composite materials to have a prospective use in pesticide removal from wastewater. The adsorption data obtained fitted into different isotherm and kinetic models and also the thermodynamic aspect have been discussed.

Graphical abstract

Performance of UV/acetylacetone process for saline dye wastewater treatment: Kinetics and mechanism

Futility of traditional advanced oxidation processes (AOPs) in saline wastewater treatment has stimulated the quest for novel "halotolerant" chemical oxidation technology. Acetylacetone (AA) has proven to be a potent photo-activator in the degradation of dyes, but the applicability of UV/AA for saline wastewater treatment needs to be verified. In this study, degradation of crystal violet (CV) was investigated in the UV/AA system in the presence of various concentrations of exogenic Cl^- or Br^-. The results reveal that degradation, mineralization and even accumulation of adsorbable organic halides (AOX) were not significantly affected by the addition of Cl^- or Br^-. Rates of CV degradation were enhanced by elevating either AA dosage or solution acidity. An apparent kinetic rate equation was developed as r = -d[CV]/dt = k[CV]^a[AA]^b = (7.34 × 10^-4 mM^1-(a+b) min^-1) × [CV]^a=0.16 [AA]^b=0.97. In terms of results of radical quenching experiments, direct electron/energy transfer is considered as the major reaction mechanism, while either singlet oxygen or triplet state (³(AA)*) might be involved. Based on identification of degradation byproducts, a possible degradation pathway of CV in the UV/AA system is proposed.

Comparative Study of CoFe2O4 Nanoparticles and CoFe2O4-Chitosan Composite for Congo Red and Methyl Orange Removal by Adsorption

(1) Background: A comparative research study to remove Congo Red (CR) and Methyl Orange (MO) from single and binary solutions by adsorption onto cobalt ferrite (CoFe2O4) and cobalt ferrite-chitosan composite (CoFe2O4-Chit) prepared by a simple coprecipitation method has been performed. (2) Methods: Structural, textural, morphology, and magnetic properties of the obtained magnetic materials were examined by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, N2 adsorption-desorption analysis, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and magnetic measurements. The optimal operating conditions of the CR and MO removal processes were established in batch experiments. The mathematical models used to describe the processes at equilibrium were Freundlich and Langmuir adsorption isotherms. (3) Results: Cobalt ferrite-chitosan composite has a lower specific surface area (SBET) and consequently a lower adsorption capacity than cobalt ferrite. CoFe2O4 and CoFe2O4-Chit particles exhibited a superparamagnetic behavior which enabled their efficient magnetic separation after the adsorption process. The research indicates that CR and MO adsorption onto prepared magnetic materials takes place as monolayer onto a homogeneous surface. According to Langmuir isotherm model that best fits the experimental data, the maximum CR/MO adsorption capacity is 162.68/94.46 mg/g for CoFe2O4 and 15.60/66.18 mg/g for CoFe2O4-Chit in single solutions. The results of the kinetics study revealed that in single-component solutions, both pseudo-first-order and pseudo-second-order kinetics models represent well the adsorption process of CR/MO on both magnetic adsorbents. In binary solutions, adsorption of CR/MO on CoFe2O4 better follows the pseudo-second-order kinetics model, while the kinetic of CR/MO adsorption on CoFe2O4-Chit is similar to that of the dyes in single-component solutions. Acetone and ethanol were successfully used as desorbing agents. (4) Conclusions: Our study revealed that CoFe2O4 and CoFe2O4-Chit particles are good candidates for dye-contaminated wastewater remediation.

Adsorption and Desorption Properties of Polyethylenimine/Polyvinyl Chloride Cross-Linked Fiber for the Treatment of Azo Dye Reactive Yellow 2

In this study, the optimal conditions for the fabrication of polyethylenimine/polyvinyl chloride cross-linked fiber (PEI/PVC-CF) were determined by comparing the adsorption capacity of synthesized PEI/PVC-CFs for Reactive Yellow 2 (RY2). The PEI/PVC-CF prepared through the optimal conditions was characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) analyses. Several batch adsorption and desorption experiments were carried out to evaluate the sorption performance and reusability of PEI/PVC-CF for RY2. As a result, the adsorption of RY2 by PEI/PVC-CF was most effective at pH 2.0. A pseudo-second-order model fit better with the kinetics adsorption data. The adsorption isotherm process was described well by the Langmuir model, and the maximum dye uptake was predicted to be 820.6 mg/g at pH 2.0 and 25 °C. Thermodynamic analysis showed that the adsorption process was spontaneous and endothermic. In addition, 1.0 M NaHCO₃ was an efficient eluent for the regeneration of RY2-loaded PEI/PVC-CF. Finally, the repeated adsorption–desorption experiments showed that the PEI/PVC-CF remained at high adsorption and desorption efficiencies for RY2, even in 17 cycles.

Film-like bacterial cellulose/cyclodextrin oligomer composites with controllable structure for the removal of various persistent organic pollutants from water

The adsorptive removal of persistent organic pollutants (POPs) is reckoned as a simple, convenient, and practical technology, especially in decentralized systems and remote areas. For this purpose, it is important to design new adsorbents, with controllable structure and convenient shape, for the highly efficient removal of POPs. In this study, we describe a strategy for film-like water purifier, prepared by loading cyclodextrin (CD) oligomer onto the ultrafine nanofibers of 3D bacterial cellulose. The optimum product exhibits remarkable removal ability toward various target pollutants such as phenol, bisphenol A (BPA), glyphosate and 2,4-Dichlorophenol (2,4-DCP), with capacities higher than most adsorbents including porous carbon based materials reported previously. Moreover, our sample demonstrated stable adsorption ability over a broad pH range and under more complex water conditions, and more importantly excellent reusability. A rough cost analysis highlights the commercial potential of our sample. We reckon our study provides new insight for the design of adsorbent with high yet stable adsorption ability and controllable structure. Furthermore, the product can be used to treat actual sewage with its convenient film-like shape and excellent performance, which improves its potential in complex systems and large-scale applications.

Adsorptive removal of cationic methylene blue and anionic Congo red dyes using wet-torrefied microalgal biochar: Equilibrium, kinetic and mechanism modeling

This study aims to investigate the adsorption behavior of cationic and anionic dyes of methylene blue (MB) and Congo red (CR) onto wet-torrefied Chlorella sp. microalgal biochar respectively, as an approach to generate a waste-derived and low-cost adsorbent. The wet-torrefied microalgal biochar possessed microporous properties with pore diameter less than 2 nm. The optimum adsorbent dosage of wet-torrefied microalgal biochar for MB and CR dyes removal were determined at 1 g/L and 2 g/L, respectively, with their natural pHs as the optimum adsorption pHs. The determined equilibrium contact times for MB and CR were 120 h and 4 h, respectively. Based on the equilibrium modeling, the results revealed that Langmuir isotherm showed the best model fit, based on the highest R² coefficient, for both the adsorption processes of MB and CR using the wet-torrefied microalgal biochar, indicating that the monolayer adsorption was the dominant process. From the modeling, the maximum adsorption capacities for MB and CR were 113.00 mg/g and 164.35 mg/g, respectively. The kinetic modeling indicated the adsorption rate and mechanism of the dyes adsorption processes, which could be crucial for future modeling and application of wet-torrefied microalgal biochar. From the results, it suggests that the valorization of microalgae by utilizing wet-torrefied microalgal biochar as the effective adsorbent for the removal of toxic dyes with an approach of microalgal biorefinery and value-added application to the environment is feasible.

Adsorption of Methylene blue and Congo red from aqueous solution using synthesized alumina-zirconia composite

Alumina-zirconia (Al₂O₃-ZrO₂) composite was prepared by combustion method and used to remove Congo red and Methylene blue from aqueous solutions. It was characterized using SEM-EDS, XRD and gas adsorption techniques. The results obtained from gas adsorption and SEM agree with each other, showing meso- and macro-porosity of inter-agglomerate pores. The removal of the two dyes was pH dependent, acidic pH favoured Congo red removal, while basic pH favoured Methylene blue. The, mechanism of adsorption was not limited to electrostatic attraction between the adsorbent and the dye molecules. Adsorption kinetic of both dyes was consistent with Pseudo-second-order model. The data obtained fitted to Langmuir and Liu isotherm models, with the maximum adsorption capacity of 57. 50 and 53.44 mg g^-1 for Congo red and Methylene blue, respectively. The thermodynamic parameters indicated that the adsorption is spontaneous and exothermic. The mechanism of adsorption was elucidated using XRD and FTIR techniques.

Couple of graphene oxide and functionalized carbon nanotubes for dye degradation enhancement of anatase under visible light and solar irradiation

Graphene oxide sheets (GO) were coupled with carbon nanotubes (CNTs) to enhance the photoactivity of anatase under visible and solar irradiation. The carbon nanotube surface was functionalized in the acidic reflux condition before coupling with GO and decoration of anatase by the sol-gel method. A modified kinetic model was appropriately applied to predict the breakthrough in the methylene blue degradation yield and determine the constant rate which was clearly affected by coupling architecture. The nanocomposite fabricated by the same proportions of GO and CNTs, 3.33%, exhibited the maximal degradation yield, 96.5%, in the dye solution with the initial concentration of 3.0 mg l-1. The characterizations based on X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and field emission scanning electron microscopy (FESEM) revealed that the functionalized CNTs could create the appropriate space between the graphene sheets for uniformly interconnection of anatase via oxygen-containing groups onto the material surfaces. This enhancement in the degradation efficiency could be ascribed to the unique architecture, leading to a decrease in bandgap energy, 2.2 eV, which facilitated the electron-hole separation. Besides of breakthrough in the photoreaction rate, the adequate architecture led to an efficient reduction in the content of carbon-based materials. Also, the performance of mentioned nanocomposite under sunlight photons was effectively higher than that under UV irradiation. The hybrid nanocomposite provided a large number of active sites for photoreactions to facilitate the treatment of wastewater under solar irradiation.

Design of PAMAM grafted chitosan dendrimers biosorbent for removal of anionic dyes: Adsorption isotherms, kinetics and thermodynamics studies

PAMAM grafted chitosan as biocompatible adsorbent was synthesized through Michael addition of methyl acrylate followed by amidation of ethylenediamine on the chitosan backbone. Then, the adsorption capacity of bioadsorbents were assessed by employing two anionic dyes. The adsorption experiments were carried out using a batch adsorption system. The influence of various operational variables such as different PAMAM generations, pH, adsorbent dosage, contact time, initial dye concentration and temperature on the maximum adsorption capacity (q_m) were investigated. The adsorbent consists of second generation of PAMAM (CS-PAMAM G2) demonstrated high removal efficiency for both dyes. The maximum adsorption capacity of CS-PAMAM G2 for Congo Red at certain operational conditions was 559.3 mg/g; while the maximum adsorption capacity for Amido Black 10B at certain operational conditions was 489.8 mg/g; which revealed endothermic and exothermic nature of adsorption process for Congo Red and Amido Black 10B, respectively. These results were then well confirmed by thermodynamics studies. Also, kinetic studies showed that the dye adsorption process followed a pseudo-second-order kinetic model. Moreover, among various applied isotherms, the experimental data were well-fitted by Sips model. Consequently, CS-PAMAM G2 showed superior potential for the removal of dyes from aqueous phase.

Toward the Limitation of Dealloying: Full Spectrum Responsive Ultralow Density Nanoporous Gold for Plasmonic Photocatalytic SERS

Plasmon-mediated chemical reaction has a great potential to create self-cleaning surface-enhanced Raman scattering (SERS) substrates. However, few works have been reported to promote this goal. Here, we report ultralow density nanoporous gold (ULDNPG) that possesses an impressive full spectrum responsive characteristic with a reflectivity lower than 5% in the waveband of 300-900 nm. ULDNPG was fabricated by a sandwich dealloying strategy from ultradilute Au-Ag solid solutions with the Au content as low as 1-5 at.%. The prepared ULDNPG presents excellent SERS properties, including high sensitivity, high uniformity, and reproducibility. The full spectrum responsive characteristic of ULDNPG leads to an obvious plasmonic photocatalytic activity. The short lifetime of the SP-excited hot carriers causes a restricted self-cleaning SERS property and a strong photothermal effect for ULDNPG structures.

Enhanced Water Purification Performance of Ionic Liquid Impregnated Metal-Organic Framework: Dye Removal by [BMIM][PF6]/MIL-53(Al) Composite

We incorporated a water-stable ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF₆], into a water-stable metal-organic framework (MOF), MIL-53(Al), to generate the [BMIM][PF₆]/MIL-53(Al) composite. This composite was examined for water purification by studying its capacity for methylene blue (MB) and methyl orange (MO) removal from aqueous solutions having either single dye or a mixture of both. Data illustrated that the removal efficiency and the maximum adsorption capacity of MIL-53(Al) were increased several times upon [BMIM][PF₆] incorporation. For instance, within 1 min, 10 mg of pristine MIL-53(Al) adsorbed 23.3% MB from 10 mg/L of MB solution, while [BMIM][PF₆]/MIL-53(Al) composite was adsorbed 82.3% MB in an identical solution. In the case of MO, 10 mg of pristine MIL-53(Al) achieved 27.8 and 53.6% MO removal from 10 mg/L of MO solution, while [BMIM][PF₆]/MIL-53(Al) composite removed 61.4 and 99.2% within 5 min and 3 h, respectively. Moreover, upon [BMIM][PF₆] incorporation, the maximum MB and MO adsorption capacities of the pristine MOF were increased from 84.5 to 44 mg/g to 204.9 to 60 mg/g, respectively. The adsorption of dyes in pristine MIL-53(Al) and [BMIM][PF₆]/MIL-53(Al) followed a pseudo-second-order kinetic model and a Langmuir isotherm model. In a mixture of both dyes, the IL/MOF composite showed a doubled MB selectivity after the IL incorporation. The composite was successfully regenerated at least two times after its use in water purification to remove MB, MO, and their mixtures. Infrared (IR) spectra indicated that the MB/MO adsorption occurs on [BMIM][PF₆]/MIL-53(Al) by electrostatic interactions, hydrogen bonding, and π-π interactions. These results showed that [BMIM][PF₆]/MIL-53(Al) composite is a highly promising material for efficient water purification.

A Carboxyl-Functionalized Covalent Organic Framework Synthesized in a Deep Eutectic Solvent for Dye Adsorption

Instead of using organic solvents, a deep eutectic solvent (DES) composed of tetrabutylammonium bromide and imidazole (Bu₄ NBr/Im) was employed as a solvent for the first time to synthesize covalent organic frameworks (COFs). Due to the low vapor pressure of the Bu₄ NBr/Im-based DES, a new carboxyl-functionalized COF (TpPa-COOH) was synthesized under environmental pressure. The as-synthesized TpPa-COOH has open channels, and the DES can be removed completely from the pores. The dye adsorption performance of TpPa-COOH was examined for three organic dyes with similar molecular sizes: one anionic dye (eosin B, EB) and two cationic dyes (methylene blue, MB and safranine T, ST). TpPa-COOH showed an excellent selective adsorption effect on MB and ST. The electronegative keto form in TpPa-COOH might help to form electrostatic and π-π interactions between the π-stacking frameworks of TpPa-COOH and the positive plane MB and ST molecules. The adsorption isotherms of MB and ST on TpPa-COOH were further investigated in detail, and the equilibrium adsorption was well modeled by using a Langmuir isotherm model. Together with hydrogen bonding, TpPa-COOH showed higher adsorption capacity for ST than for MB (1135 vs. 410 mg g^-1 ). These results could provide a guidance for the green synthesis of adsorbents in removing organic dyes from wastewater.

Physicochemical Interactions in Systems C.I. Direct Yellow 50—Weakly Basic Resins: Kinetic, Equilibrium, and Auxiliaries Addition Aspects

Intensive development of many industries, including textile, paper or plastic, which consume large amounts of water and generate huge amounts of wastewater-containing toxic dyes, contribute to pollution of the aquatic environment. Among many known methods of wastewater treatment, adsorption techniques are considered the most effective. In the present study, the weakly basic anion exchangers such as Amberlyst A21, Amberlyst A23 and Amberlyst A24 of the polystyrene, phenol-formaldehyde and polyacrylic matrices were used for C.I. Direct Yellow 50 removal from aqueous solutions. The equilibrium adsorption data were well fitted to the Langmuir adsorption isotherm. Kinetic studies were described by the pseudo-second order model. The pseudo-second order rate constants were in the range of 0.0609–0.0128 g/mg·min for Amberlyst A24, 0.0038–0.0015 g/mg·min for Amberlyst A21 and 1.1945–0.0032 g/mg·min for Amberlyst A23, and decreased with the increasing initial concentration of dye from 100–500 mg/L, respectively. There were observed auxiliaries (Na2CO3, Na2SO4, anionic and non-ionic surfactants) impact on the dye uptake. The polyacrylic resin Amberlyst A24 can be promising sorbent for C.I. Direct Yellow 50 removal as it is able to uptake 666.5 mg/g of the dye compared to the phenol-formaldehyde Amberlyst A23 which has a 284.3 mg/g capacity.