Adsorption of Aqueous Cr(VI) by Novel Fibrous Adsorbent with Amino and Quaternary Ammonium Groups

One-pot synthesis of magnetic cellulose nanocrystal and its post-functionalization for doxycycline adsorption

The composite of magnetite (Fe₃O₄) and cellulose nanocrystal (CNC) is considered a potential adsorbent for water treatment and environmental remediation. In the current study, a one-pot hydrothermal procedure was utilized for magnetic cellulose nanocrystal (MCNC) development from microcrystalline cellulose (MCC) in the presence of ferric chloride, ferrous chloride, urea, and hydrochloric acid. The x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and Fourier-transform infrared spectroscopy analysis confirmed the presence of CNC and Fe₃O₄, while transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis verified their respective sizes (< 400 nm and ≤ 20 nm) in the generated composite. To have an efficient adsorption activity for doxycycline hyclate (DOX), the produced MCNC was post-treated using chloroacetic acid (CAA), chlorosulfonic acid (CSA), or iodobenzene (IB). The introduction of carboxylate, sulfonate, and phenyl groups in the post-treatment was confirmed by FTIR and XPS analysis. Such post treatments decreased the crystallinity index and thermal stability of the samples but improved their DOX adsorption capacity. The adsorption analysis at different pHs revealed the increase in the adsorption capacity by reducing the basicity of the medium due to decreasing electrostatic repulsions and inducing strong attractions.

Polyethylenimine-Functionalized Nanofiber Nonwovens Electrospun from Cotton Cellulose for Wound Dressing with High Drug Loading and Sustained Release Properties

Electrospun cellulose nanofiber nonwovens have shown promise in wound dressing owing to the highly interconnected pore structure, high hydrophilicity coupled with other coveted characteristics of biodegradability, biocompatibility and renewability. However, electrospun cellulose wound dressings with loaded drugs for better wound healing have been rarely reported. In this study, a novel wound dressing with a high drug loading capacity and sustained drug release properties was successfully fabricated via electropinning of cellulose followed by polyethylenimine (PEI)-functionalization. Remarkably, the grafted PEI chains on the surface of electrospun cellulose nanofibers provided numerous active amino groups, while the highly porous structure of nonwovens could be well retained after modification, which resulted in enhanced adsorption performance against the anionic drug of sodium salicylate (NaSA). More specifically, when immersed in 100 mg/L NaSA solution for 24 h, the as-prepared cellulose-PEI nonwoven displayed a multilayer adsorption behavior. And at the optimal pH of 3, a high drug loading capacity of 78 mg/g could be achieved, which was 20 times higher than that of pristine electrospun cellulose nonwoven. Furthermore, it was discovered that the NaSA-loaded cellulose-PEI could continuously release the drug for 12 h in simulated body fluid (SBF), indicating the versatility of cellulose-PEI as an advanced wound dressing with drug carrier functionalities.

Chromium Adsorption from Aqueous Solution onto Dowex Retardion 11A8 and Amberlite IRA 743 Free Base: An Insight into the Mechanism

Background:

The presence of heavy metal contaminants such as chromium, lead, mercury, cadmium, arsenic, nickel, and copper have become a major issue towards human health. Chromium is extremely toxic to living organisms as it acts as carcinogen and mutagen. High concentration of chromium may cause detrimental effects to human health in the long term. The mutagenic and carcinogenic properties, included Cr(VI) in the group “A” of human carcinogens. Cr(VI) can easily penetrate into the cell wall and exert its noxious effect due to its mobility in the environment. Cr(VI) is nearly 100 times more toxic than Cr(III). Cr(VI) causes skin and stomach irritation or ulceration, damage to liver, kidney ulceration, damage to nerve tissue, and long-term exposure above the maximum contaminated level even led to death. Therefore, it is essential to remove chromium from wastewater prior to its final discharge into the environment. This study attempts to explore the mechanism by which chromium ions had been adsorbed by these two ion exchange resins and will be extended further to investigate the uptake mechanism of other metal ions within future research.

Methods:

Equilibrium isotherms were obtained by contacting 20 mL of aqueous metal ion solution with different amounts of adsorbents in a shaker bath controlled at 25±0.5oC. The initial concentration of metal ion in the aqueous solution was varied between 40-100 mg L -1 . Equilibrium isotherms for the above metal ion were generated at pH 3, 4 and 5. The pH of the solution was varied between pH 3 to 5 using appropriate doses of buffer. Preliminary runs exhibited that the adsorption equilibrium was achieved after 1–1.30 h of contact time for both the tested resins. The adsorbents used were DOWEX and AMB resins. For estimation of adsorption enthalpy, adsorption equilibrium experiments were performed at temperatures 30, 40 and 55oC. The amount of metal ion adsorbed per unit mass of the adsorbent (mg g-1) was calculated as q= V∆C/W, where ∆C is the change in solute concentration (mg L -1 ), V is the solution volume (L) and W is the weight of the adsorbent (g). Experiments on adsorption kinetics were performed in a stirred constant volume vessel. The liquid volume was 100 cm3 with 10g of adsorbent sample. The initial concentration of metal ion was 80 mg L -1 at 25±0.5oC. The aqueous phase concentration was examined at equal time intervals till equilibration.

Results:

The electrostatic interaction of Cr(VI) with the positively charged nitrogen atom of the functional groups and chelation of Cr(III) with the electron donor groups were the possible mechanistic pathways through which the adsorption had occurred onto both the ion-exchange resins. Though electrostatic interaction was the predominant interaction in both the resins for the adsorption of anionic Cr(VI) species, but it had been observed that the mechanism of Cr(VI) adsorption was not only “anionic adsorption” but also the complexation of the reduced Cr(III) with the ammonium group of the resins. Thus, “adsorption- coupled reduction” was the main mechanism for the uptake of chromium ions.

Conclusion:

The present work demonstrated that both resins could effectively adsorb Cr(VI) ions from aqueous solution. More adsorption had taken place onto DOWEX compared to AMB. The adsorption characteristics of both the resins were studied under various equilibrium and thermodynamic conditions which proposed the spontaneous nature of the process. The adsorption capacities of both resins were influenced by the pH of the medium and exhibited high adsorption performances at pH 3. The mechanism of adsorption onto the two resins studied here was anionic adsorption of Cr (VI) and chelation of Cr (III) ion. The Cr(III) ions might have formed because of the reduction of Cr(VI) by the electron donor atoms present in the resins and interacted with the adsorbent surface. FTIR spectra also supported the interaction of chromium ions with functional groups present in the resin structures. Thus chromium uptake by DOWEX and AMB resins was mainly governed by “adsorption- coupled reduction”. Desorption studies revealed that regeneration of both the ionexchange resins are possible at basic pH and can be reused. However, the application of these two ion-exchange resins using real effluent is under consideration.

A Review on Polymer Nanocomposites and Their Effective Applications in Membranes and Adsorbents for Water Treatment and Gas Separation

Globally, environmental challenges have been recognised as a matter of concern. Among these challenges are the reduced availability and quality of drinking water, and greenhouse gases that give rise to change in climate by entrapping heat, which result in respirational illness from smog and air pollution. Globally, the rate of demand for the use of freshwater has outgrown the rate of population increase; as the rapid growth in town and cities place a huge pressure on neighbouring water resources. Besides, the rapid growth in anthropogenic activities, such as the generation of energy and its conveyance, release carbon dioxide and other greenhouse gases, warming the planet. Polymer nanocomposite has played a significant role in finding solutions to current environmental problems. It has found interest due to its high potential for the reduction of gas emission, and elimination of pollutants, heavy metals, dyes, and oil in wastewater. The revolution of integrating developed novel nanomaterials such as nanoparticles, carbon nanotubes, nanofibers and activated carbon, in polymers, have instigated revitalizing and favourable inventive nanotechnologies for the treatment of wastewater and gas separation. This review discusses the effective employment of polymer nanocomposites for environmental utilizations. Polymer nanocomposite membranes for wastewater treatment and gas separation were reviewed together with their mechanisms. The use of polymer nanocomposites as an adsorbent for toxic metals ions removal and an adsorbent for dye removal were also discussed, together with the mechanism of the adsorption process. Patents in the utilization of innovative polymeric nanocomposite membranes for environmental utilizations were discussed.

Polyethylenimine-functionalized polyacrylonitrile anion exchange fiber as a novel adsorbent for rapid removal of nitrate from wastewater

The development of an adsorbent with high adsorption ability and favorable cyclic regeneration performance for the removal of nitrate residues from wastewater is a task of vital importance. To this end, polyacrylonitrile fiber (PANF) was modified with polyethyleneimine (PEI), and alkyl groups were then introduced around the active amine groups to prepare three polymer-based anion exchange fibers (PAN-PEI-3C, PAN-PEI-5C, and PAN-PEI-8C). The novel fibers were characterized using techniques such as scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The adsorption isotherms of the fibers were best fitted by the Langmuir model, and PAN-PEI-5C exhibited a higher adsorption amount of nitrate (31.32 mg/g) than the other adsorbents. The equilibrium was reached expeditiously (within 10 min), and both pseudo-first-order and pseudo-second-order models could well describe the adsorption kinetics. More attractively, the saturated PAN-PEI-5C could be eluted using a low-concentration (0.3 M) NaCl solution, without any sharp loss of adsorption amount for five consecutive cycles in the presence of dissolved organic matter (DOM). Furthermore, PAN-PEI-5C could effectively adsorb low-concentration nitrate from real secondary effluents in a fixed-bed column experiment. Our work provides a promising and low-cost material for the removal of nitrate residues in practical applications.

Three dimensional flower-like magnetic polyethyleneimine@MoS2 composites for highly efficient removal of Cr(VI) and Pb(II) ions

Magnetic Fe₃O₄ nanoparticles were coated by polyethyleneimine (PEI), and then Fe₃O₄@PEI was further modified with MoS₂ by the hydrothermal method to fabricate 3D flower-like structured magnetic polyethyleneimine@MoS₂ (MP@MoS₂) composites, and the composites were served as efficient adsorbents to capture Cr(VI) and Pb(II) from aqueous solution. The effects of temperature, pH, shaking time and environmental conditions on adsorption performance of MP@MoS₂ towards Cr(VI) and Pb(II) have been conducted by batch adsorption experiments. The prepared MP@MoS₂ exhibited high adsorption capacities (192.30 mg/g for Cr(VI) at pH 3.0 and 256.41 mg/g for Pb(II) at pH 6.0) and the adsorption equilibrium could be achieved in a short time. Moreover, MP@MoS₂ composites with high saturation magnetization could be simply separated under an external magnet. Combined experiments and spectral analysis, the underlying adsorption mechanism for Cr(VI) on MP@MoS₂ was mainly attributed to the reduction of Cr(VI) to Cr(III), and the removal of Pb(II) was due to the complexation with sulfur groups and amino-groups. Consequently, the prepared 3D flower-like structured MP@MoS₂ has a great potential for the practical application in removing Cr(VI) and Pb(II) from the aquatic environment.

Synthesis and characterization of novel N-methylimidazolium-functionalized KCC-1: A highly efficient anion exchanger of hexavalent chromium

A key challenge in adsorption process of toxic organic and inorganic species is the design and development of adsorbent materials bearing an abundance of accessible adsorption sites with high affinity to achieve both fast adsorption kinetics and elevated adsorption capacity for toxic contaminants. Herein, a novel anion-exchange adsorbent based on fibrous silica nanospheres KCC-1 was synthesized by a facile hydrothermal-assisted post-grafting modification of KCC-1 with 1-methyl-3- (triethoxysilylpropyl)imidazolium chloride for the first time. Silica fibers with micro-mesoporous structure display the proper combination of features to serve as a potential scaffold for decorating adsorption sites to create desired ion-exchange adsorbent. The obtained N-methylimidazolium-functionalized KCC-1 (MI-Cl-KCC-1) with fibrous nanosphere morphology showed a high surface area (∼241 m² g^-1) and high pore volume (0.81 m² g^-1). The adsorption behaviors of toxic hexavalent chromium from aqueous media by the MI-Cl-KCC-1 were systematically studied using the batch method. The adsorption rate was relatively fast, and MI-Cl-KCC-1 possesses a high capacity for the adsorption of Cr(VI). The maximum Cr(VI) adsorption was obtained at pH 3.0-4.0. Different non-linear isotherm equations were tested for choosing an appropriate adorption isotherm behavior, and the adsorption data for MI-Cl-KCC-1 were consistent with the Langmuir model with a maximum adsorption capacity of 428 ± 8 mg g^-1.

Monolithic cellulose supported metal nanoparticles as green flow reactor with high catalytic efficiency

A highly effective, stable and reusable flow microreactor was developed by utilizing the environmentally sustainable porous monolithic cellulose based on a facile temperature induced phase separation (TIPS) method. The obtained microreator could be applied to efficiently and continuously catalysing the reduction reaction of 4-nitrophenol (an important reaction in water treatment) without any post-treatment or regeneration of catalysts. Moreover, the monolith overcame the brittleness of the crystalline cellulose and showed a good mechanical resilience, suggesting a great potential for the practical application in severe environment. Compared with previous reported Pd supported catalytic systems, this microreactor exhibited extremely high catalytic efficiency (turnover frequency, TOF = 4660 h^-1, almost 4 times higher than that of cellulose nanocrystals supported catalyst) and long-term stability. This work provided a new strategy to construct highly effective and reusable metal NPs involved catalytic system by utilizing biodegradable cellulose materials.

Design of a preorganized adsorbent for the removal of chromate from wastewater under neutral conditions

A preorganized structure with two positive nitrogen ions linked by a carbon atom was designed. This structure acts like crab pincers to tightly entrap chromate.

An efficient and reusable quaternary ammonium fabric adsorbent prepared by radiation grafting for removal of Cr(VI) from wastewater

A novel quaternary ammonium polyethylene nonwoven fabric for removing chromium ions from water was prepared via radiation-induced grafting of glycidyl methacrylate and further modification with N,N'-dimethylethylenediamine. The structural and morphological characteristics of the adsorbent were analyzed using Fourier transform infrared spectroscopy (FTIR), thermogravimetry and differential thermogravimetry (TG/DTG), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The influences of several principal factors, including pH value, initial Cr(VI) concentration, contact time, and coexisting anions (including SO₄^2-, CO₃^2-, NO₃^-, PO₄^3-, and Cl^-), on adsorption performance were investigated via batch tests. The results showed that the optimum removal efficiency was 99.2% at pH 3 and the maximum adsorption quantity for Cr(VI) at 25 °C was 336 mg/g. The adsorption kinetic parameters were better fitted with the pseudo-second-order kinetic model, and the equilibrium data were described very well by the Freundlich isotherm model. Furthermore, the as-synthesized adsorbent exhibited excellent regeneration and recyclability while maintaining high adsorption performance after five adsorption/desorption cycles.

Recycling Spent Cr Adsorbents as Catalyst for Eliminating Methylmercaptan

Waste adsorbents generated from treating Cr(VI)-containing wastewater are hazardous materials and generally landfilled or treated by acid or base desorption, with concomitant high cost and toxic effects. The present work shows that these Cr adsorbents can be reused as highly efficient catalysts for treating sulfur-containing VOCs (CH₃SH), not only avoiding the economic and environmental impact from the conventional approaches, but also achieving the efficient treatment of sulfur-containing waste gas. Importantly, these reused Cr adsorbents exhibit enhanced activity and stability compared with the catalysts reported elsewhere, indicating a new avenue of green chemistry. The highly toxic adsorbed Cr(VI) species are reduced to a Cr₂O₃ crystalline phase by calcination and finally immobilized as a Cr₂S₃ solid phase while converting and eliminating CH₃SH. Still, the presence of Cr(VI) species on the reused Cr adsorbent provides enough reactive sites for reaction, but high concentration of Cr(VI) species causes serious accumulation of coke deposit on the catalyst, leading to fast deactivation of the catalyst.

Efficient removal of Cr(VI) from water by quaternized chitin/branched polyethylenimine biosorbent with hierarchical pore structure

A novel chitin-based biosorbent (QCP) was synthesized by cross-linking quaternized chitin and branched polyethylenimine with the aid of epichlorohydrin for efficient removal of Cr(VI) from water. Because it possessed both quaternary ammonium groups and amino groups as well as the hierarchical pore structure, QCP presented a maximum adsorption capacity of 387.7 mg/g according to the Langmuir isotherm at 25 °C. The biosorption of QCP achieved the equilibrium within 40 min and followed the pseudo-second-order kinetic model. QCP worked well even in the solution with high pH and high content of competing anions and, it exhibited an excellent reusability. The main Cr(VI) uptake mechanism was confirmed to be electrostatic attractions between Cr(VI) anions and quaternary ammonium groups as well as the protonated amino groups, and followed by partial reduction of Cr(VI) to Cr(III) by amines and hydroxyls. This work may provide a potential for Cr(VI) removal by chitin-based biosorbents.

One-pot preparation of MnOximpregnated cotton fibers for methylene blue dye removal

Mn-X@BCF, prepared by one-pot sono-assisted KMnO₄reduction, could remove methylene blue efficiently by a proposed adsorption-partial oxidation mechanism.

Electrospun nanofibrous polyethylenimine mat: a potential adsorbent for the removal of chromate and arsenate from drinking water

A nanofibrous adsorbent for sub-ppm level chromate and arsenate removal from drinking water has been fabricated via an electrospinning technique.

Facile preparation of amidoxime-functionalized fiber by microwave-assisted method for the enhanced adsorption of chromium(vi) from aqueous solution

In this study, a facile and highly efficient approach, the microwave-assisted (MW-aid) method, was applied for the synthesis of amidoxime-functionalized fibrous adsorbent, which exhibited enhanced adsorption capacities for Cr(vi) in aqueous solution.

Structure regulation of silica nanotubes and their adsorption behaviors for heavy metal ions: pH effect, kinetics, isotherms and mechanism

Silica nanotubes (SNTs) with controlled nanotubular structure were synthesized via an electrospinning and calcination process. In this regard, SNTs were found to be ideal adsorbents for Pb(II) removal with a higher adsorption capacity, and surface modification of the SNTs by sym-diphenylcarbazide (SD-SNTs) markedly enhanced the adsorption ability due to the chelating interaction between imino groups and Pb(II). The pH effect, kinetics, isotherms and adsorption mechanism of SNTs and SD-SNTs on Pb(II) adsorption were investigated and discussed detailedly. The adsorption capacity for Pb(II) removal was found to be significantly improved with the decrease of pH value. The Langmuir adsorption model agreed well with the experimental data. As for kinetic study, the adsorption onto SNTs and SD-SNTs could be fitted to pseudo-first-order and pseudo-second-order model, respectively. In addition, the as-prepared SNTs and SD-SNTs also exhibit high adsorption ability for Cd(II) and Co(II). The experimental results demonstrate that the SNTs and SD-SNTs are potential adsorbents and can be used effectively for the treatment of heavy-metal-ions-containing wastewater.

Reduction of Cr (VI) by organic acids in the presence of Al (III)

The effects of Al (III) on the reduction of Cr (VI) by three α-hydroxy acids, tartaric, malic and citric acids, were investigated through batch experiments at pH from 2.5 to 4.0 and temperatures from 25 °C to 35 °C. These reactions could be described as pseudo-zero-order with respect to Cr (VI) when the concentrations of α-hydroxy acids were greatly in excess. The transformation rates of Cr (VI) to Cr (III) in the presence of Al (III) without light were in the decreasing order of tartaric acid>malic acid>citric acid. This order suggested that the two α-hydroxyl groups in tartaric acid could play an important role in the reduction of Cr (VI) by organic acids. The possible mechanism was that the formed complex between organic acids and Al (III) significantly enhanced the reductivity of α-hydroxy acids and further led to the more complicated Cr (VI)-tartaric acid-Al (III) cyclic ester which greatly accelerated the reduction rate. The Cr (VI) reduction reaction rate increased with the decrease of pH but with the increase of Al (III) concentration, tartaric acid concentration, and temperature. As the pH decreased, the increase of [H(+)] led to an increase in {Al(III)H₂Tar₂}(+), the most active species, and thus the enhanced reduction rate.