Removal of Cr(VI) from aqueous solution using amino-modified Fe3O4-SiO2-chitosan magnetic microspheres with high acid resistance and adsorption capacity

Polycarboxylate functionalized magnetic nanoparticles Fe3O4@SiO2@CS-COOH: Preparation, characterization, and immobilization of bovine serum albumin

Magnetic nanoparticles with increasing superparamagnetism and magnetic targeting have found widespread application in fields such as food and medicine. In this study, polycarboxylated magnetic nanoparticles (Fe3O4@SiO2@CS-COOH) were prepared by surface functionalizing iron tetraoxide (Fe3O4) nanoparticles with ethylenediaminetetraacetic acid (EDTA) as a modifier. The appropriate degree of functionalization modification was obtained by adjusting the EDTA concentration and the ratio of cross-linking agents. The prepared magnetic nanoparticles were analyzed with structural and property characterization. The results showed that the Fe3O4@SiO2@CS-COOH magnetic nanoparticles prepared with 4 % EDTA and cross-linking agents at a molar ratio of 3:4 were uniform in particle size, with an average size of roughly 7 nm, and possessed an abundant carboxylate content (310.8064 μmol/g) and a high magnetization intensity (35.05 emu/g). As a model protein, bovine serum albumin (BSA) was immobilized on the surface of magnetic particles. The largest amount of immobilized protein was 500.4376 mg BSA/g at pH 4.0 and no extra salt ions. According to molecular docking simulations, its immobilization was due to the interaction of amino and carboxyl groups at the Fe3O4@SiO2@CS-COOH/BSA interface. Fe3O4@SiO2@CS-COOH possesses a large number of carboxyl groups, strong protein immobilization, and magnetic responsiveness, which may have potential applications in biomedical and food fields.

Insights into the proton-enhanced mechanism of hexavalent chromium removal by amine polymers in strong acid wastewater: Reduction of hexavalent chromium and sequestration of trivalent chromium

Adsorption is a green technology of treating heavy metal-contaminated strong acid wastewaters for the recycling of heavy metal and reuse of strong acid. Herein, three amine polymers (APs) with different alkalinities and electron donating abilities were prepared to investigate the adsorption-reduction processes of Cr(VI). It was found that the removal of Cr(VI) was controlled by the concentration of -NRH+ on the surface of APs at pH > 2, which relies on the alkalinity of APs. However, the high concentration of NRH+ significantly facilitated the adsorption of Cr(VI) on the surface of APs and accelerated the mass transfer between Cr(VI) and APs at strong acid environment (pH ≤ 2). More importantly, the reduction of Cr(VI) was enhanced at pH ≤ 2, due to the high reduction potential of Cr(VI) (E ≥ 0.437). The ratio of reduction to adsorption (α) of Cr(VI) was above 0.70, and the proportion of Cr(III) bonding on Ph-AP excessed 67.6 %. Finally, a proton-enhanced mechanism of Cr(VI) removal was verified by analyzing FTIR and XPS spectra as well as constructing DFT model. This study provides a theoretical basis for the removal of Cr(VI) in the strong acid wastewater.

Preparation of porous silica materials using a eucalyptus template method and its efficient adsorption of methylene blue

Methylene blue (MB) is a prevalent pollutant in organic wastewater. For this research, eucalyptus wood was used as a template, into which quartz powder dissolved in NaOH was grown, resulting in a low-cost and efficient porous silica adsorbent material (PSAM). This PSAM successfully replaces expensive materials for MB removal from water. Through the application of Scanning Electron Microscopy (SEM) and Brunauer-Emmett-Teller (BET) analysis, it became evident that PSAM displays a porous slit pore structure characterized by numerous active sites, leading to an impressive maximum specific surface area of 88.05 m²/g. The central objective of this research was to investigate the impact of experimental temperature, initial dye concentration, and pH on the adsorption process. The adsorption kinetics were analyzed using the pseudo-first-order and pseudo-second-order models, as well as the Langmuir model. Remarkably, PSAM exhibited a substantial maximum adsorption capacity of 90.01 mg/g at 293 K, achieving an adsorption rate of over 85% within a mere 10-minute timeframe. The thermodynamic analysis revealed that the adsorption of MB onto PSAM was characterized by spontaneity and accompanied by heat absorption. Fourier Transform Infrared (FTIR) and SEM comparisons of PSAM before and after adsorption indicated that MB adsorption primarily occurred through electrostatic gravitational binding. In comparison to other adsorbents, PSAM exhibited exceptional efficacy in removing MB from water.

Facile synthesis of direct Z-scheme UiO-66-NH2/PhC2Cu heterojunction with ultrahigh redox potential for enhanced photocatalytic Cr(VI) reduction and NOR degradation

Z-scheme heterojunction-based photocatalysts typically have robust removal efficiencies for water contaminants. Herein, we employed p-type PhC₂Cu and n-type UiO-66-NH₂ to develop a direct Z-scheme UiO-66-NH₂/PhC₂Cu photocatalyst with an ultrahigh redox potential for Cr(VI) photoreduction and norfloxacin (NOR) photodegradation. Moreover, UV-vis diffuse reflectance, photoelectrochemical measurements, photoluminescence (PL) spectra and electron spin resonance (ESR) technique revealed that the UiO-66-NH₂/PhC₂Cu composite boosted light capturing capacities to promote photocatalytic efficiencies. Strikingly, the optimized UiO-66-NH₂/PhC₂Cu50 wt% rapidly reduced Cr(VI) (96.2%, 15 min) and degraded NOR (97.9%, 60 min) under low-power blue LED light. In addition, the UiO-66-NH₂/PhC₂Cu photocatalyst also exhibited favorable mineralization capacity (78.4%, 120 min). Benefitting from the enhanced interfacial electron transfer and ultrahigh redox potential of the Z-scheme heterojunction, the UiO-66-NH₂/PhC₂Cu photocatalyst greatly enhanced the separation efficacies of photogenerated carriers. This resulting abundance of active species (e.g., e^-, h⁺, O₂^•-, and •OH) were generated to photo-reduce Cr(VI) and photo-oxidize NOR. Base on the identified intermediates, four degradation pathways of NOR were proposed. Finally, the Z-scheme mechanism were systematically confirmed through X-ray photoelectron spectroscopy (XPS), ESR, cyclic voltammetry (CV) tests, and photodeposition techniques.

Selective removal of heavy metals by Zr-based MOFs in wastewater: New acid and amino functionalization strategy

Zr-based metal-organic frameworks (MOFs) have been developed in recent years to treat heavy metals, e.g. hexavalent chromium Cr⁶⁺ pollution, which damages the surrounding ecosystem and threaten human health. This kind of MOF is stable and convenient to prepare, but has the disadvantage of low adsorption capacity, limiting its wide application. To this end, a novel formic acid and amino modified MOFs were prepared, referred to as Form-UiO-66-NH₂. Due to the modification of formic acid, its specific surface area, pore size, and crystal size were effectively expanded, and the adsorption capacity of Cr⁶⁺ was significantly enhanced. Under optimal conditions, Form-UiO-66-NH₂ exhibited an excellent adsorption capacity (338.98 mg/g), ∼10 times higher than that reported for unmodified Zr-based MOFs and most other adsorbents. An in-depth study on the photoelectronic properties and pH confirmed that the adsorption mechanism of Form-UiO-66-NH₂ to Cr⁶⁺ was electrostatic adsorption. After modification, the improvement of Cr⁶⁺ adsorption capacity by Form-UiO-66-NH₂ was attributed to the expansion of its specific surface area and the increase in its surface charge. The present study revealed an important finding that Form-UiO-66-NH₂ elucidated selective adsorption to Cr⁶⁺ in mixed wastewater containing toxic heavy metal ions and common nonmetallic water quality factors. This research provided a new acid and amino functionalization perspective for improving the adsorption capacity of Zr-based MOF adsorbents while simultaneously demonstrating their pertinence to target contaminant adsorption.

Removal of Chromium Species by Adsorption: Fundamental Principles, Newly Developed Adsorbents and Future Perspectives

Emerging chromium (Cr) species have attracted increasing concern. A majority of Cr species, especially hexavalent chromium (Cr(VI)), could lead to lethal effects on human beings, animals, and aquatic lives even at low concentrations. One of the conventional water-treatment methodologies, adsorption, could remove these toxic Cr species efficiently. Additionally, adsorption possesses many advantages, such as being cost-saving, easy to implement, highly efficient and facile to design. Previous research has shown that the application of different adsorbents, such as carbon nanotubes (carbon nanotubes (CNTs) and graphene oxide (GO) and its derivatives), activated carbons (ACs), biochars (BCs), metal-based composites, polymers and others, is being used for Cr species removal from contaminated water and wastewater. The research progress and application of adsorption for Cr removal in recent years are reviewed, the mechanisms of adsorption are also discussed and the development trend of Cr treatment by adsorption is proposed.

Cationic surface-modified regenerated nanocellulose hydrogel for efficient Cr(VI) remediation

Because nanocellulose has a large specific surface area and abundant hydroxyl functional groups due to its unique nanomorphology, interest increases as an eco-friendly water treatment material. However, the distinctive properties of nanocellulose, which exists in a dispersion state, strongly hamper its usage in practical water treatment processes. Additionally, nanocellulose shows low performance in removing anionic pollutants because of its anionic characteristics. In an effort to address this challenge, regenerated cellulose (RC) hydrogel was fabricated through cellulose's dissolution and regeneration process using an eco-friendly aqueous solvent system. Subsequently, a crosslinking process was carried out to introduce the cationic functional groups to the RC surface PEI coating (P/RC). As a result, the PEI surface cationization process improved the mechanical rigidity of RC and showed an excellent Cr(VI) removal capacity of 578 mg/g. In addition, the prepared P/RC maintained more than 90% removal efficiency even after seven reuses.

High exposure effect of the adsorption site significantly enhanced the adsorption capacity and removal rate: A case of adsorption of hexavalent chromium by quaternary ammonium polymers (QAPs)

Enhancing the performance of adsorbents to the utmost extent is an objective but challenging in applying adsorption technology to wastewater treatment. In this work, novel quaternary ammonium polymers (QAPs) with high density adsorption site (i.e., quaternized N, confirmed by FT-IR results) were designed and prepared for rapid selective removal of Cr(VI) from water. The results of EDS analysis indicated the maximum exposure rate of N on the surface of QAPs was as high as 86.1%, which almost doubled comparing to that of Cr(VI) ions imprinted polymers (Cr(VI)-IIP) (46.2%). Interestingly, the maximum adsorption capacity (211.8 mg/g) and initial adsorption rate (h0, 66.6 mg/ (g·min)) of QAPs (i.e., 5:1(TRIM)) for Cr(VI) are about 3.6 times and 4.9 times those of Cr(VI)-IIP (63.0 mg/g and 13.5 mg/(g·min)), respectively. Impressively, flow-through adsorption experiments demonstrated 5:1(TRIM) can completely remove 5 mg/L of Cr(VI) within five seconds. Additionally, 5:1(TRIM) exhibited a remarkable selectivity for Cr(VI) adsorption, and high purity (100%) of chromium can be readily obtained. The proposed idea of high exposure effect of the adsorption site can provide a valuable guidance for designing rapid selective adsorbents to remove and reclaim Cr(VI) from wastewater.

Hexavalent Cr ion adsorption and desorption behaviour of expanded poly(tetrafluoro)ethylene films grafted with 2-(dimethylamino)ethyl methacrylate

A new polymeric adsorbent for Cr(VI) ions based on an expanded poly(tetrafluoroethylene) (ePTFE) film was prepared by the combined use of the pretreatment with oxygen plasma and photografting of 2-(dimethylamino)ethyl methacrylate (DMAEMA). The grafting of DMAEMA was characterized by XPS and FT-IR spectroscopic measurements. The adsorption behaviour of DMAEMA-grafted ePTFE (ePTFE-g-PDMAEMA) films was investigated as a function of the experimental parameters, such as the initial pH value, temperature, and grafted amount. The adsorption capacity and initial adsorption rate had the maximum values at the initial pH value of 3.0. On the other hand, the adsorption capacity became almost constant at temperatures higher than 30°C, although the adsorption rate increased over the temperature. The adsorption behaviour obeyed the pseudo-second-order kinetic model and well expressed by the Langmuir isotherm equation with higher correlation coefficients. These results indicate that the adsorption of Cr(VI) ions occurs through the electrostatic interaction between protonated dimethylamino groups on a grafted PDMAEMA chain and HCrO4- ions. Cr(VI) ions were successfully desorbed from Cr(VI)-loaded ePTFE-g-PDMAEMA films in the eluents, such as NaCl at 0.50 M, NH₄Cl at 0.50M, and NaOH at 1.0 mM, and ePTFE-g-PDMAEMA films were repeatedly used for adsorption of Cr(VI) ions without appreciable loss in the adsorption capacity. It should be noted that Cr(VI) ion adsorptivity with a high initial rate was conferred to the ePTFE films. The results obtained in this study emphasize that ePTFE-g-PDMAEMA films can be applied as an adsorbent for Cr(VI) ions.

Magnetically Recyclable Wool Keratin Modified Magnetite Powders for Efficient Removal of Cu2+ Ions from Aqueous Solutions

The treatment of wastewater containing heavy metals and the utilization of wool waste are very important for the sustainable development of textile mills. In this study, the wool keratin modified magnetite (Fe3O4) powders were fabricated by using wool waste via a co-precipitation technique for removal of Cu2+ ions from aqueous solutions. The morphology, chemical compositions, crystal structure, microstructure, magnetism properties, organic content, and specific surface area of as-fabricated powders were systematically characterized by various techniques including field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM), thermogravimetric (TG) analysis, and Brunauer-Emmett-Teller (BET) surface area analyzer. The effects of experimental parameters such as the volume of wool keratin hydrolysate, the dosage of powder, the initial Cu2+ ion concentration, and the pH value of solution on the adsorption capacity of Cu2+ ions by the powders were examined. The experimental results indicated that the Cu2+ ion adsorption performance of the wool keratin modified Fe3O4 powders exhibited much better than that of the chitosan modified ones with a maximum Cu2+ adsorption capacity of 27.4 mg/g under favorable conditions (0.05 g powders; 50 mL of 40 mg/L CuSO4; pH 5; temperature 293 K). The high adsorption capacity towards Cu2+ ions on the wool keratin modified Fe3O4 powders was primarily because of the strong surface complexation of -COOH and -NH2 functional groups of wool keratins with Cu2+ ions. The Cu2+ ion adsorption process on the wool keratin modified Fe3O4 powders followed the Temkin adsorption isotherm model and the intraparticle diffusion and pseudo-second-order adsorption kinetic models. After Cu2+ ion removal, the wool keratin modified Fe3O4 powders were easily separated using a magnet from aqueous solution and efficiently regenerated using 0.5 M ethylene diamine tetraacetic acid (EDTA)-H2SO4 eluting. The wool keratin modified Fe3O4 powders possessed good regenerative performance after five cycles. This study provided a feasible way to utilize waste wool textiles for preparing magnetic biomass-based adsorbents for the removal of heavy metal ions from aqueous solutions.

Surface-modified spherical lignin particles with superior Cr(VI) removal efficiency

Lignin, natural aromatic polymer derived from plant dry matter, is second abundant biopolymer. Recently, interest in applications of lignin, especially as an adsorbent material is increasing. However, the physicochemical complexity of lignin significantly reduces access to practical environmental remediation processes. Also, there is a limitation because the adsorption performance of the pristine lignin materials is not superior to that of commercial adsorbent and ion exchange resin material. In this study, spherical lignin particles with high physicochemical stability and excellent Cr(VI) adsorption capacity are prepared using a polyethylenimine (PEI) modification strategy. This modification process significantly improves the mechanical properties and water stability of lignin by complementing the instability of lignin particles. In addition, the PEI-lignin particles exhibit a superior Cr(VI) removal capability (657.9 mg/g, the highest value for a PEI-modified natural adsorbent), which is attributed to their structural stability and introduced amine functional groups. The Cr(VI) removal with PEI-lignin particles is performed via intra-particle diffusion and adsorption followed by covalent bonding combined with a reduction process. Moreover, the PEI-lignin particles exhibit excellent reusability, which sustains their high adsorption efficiency over a long and repeated adsorption period. The results herein strongly support the potential use of PEI-lignin particles as a high performance bio-sorption material for heavy metal removal and its detoxification in aqueous wastewater streams. Evidently, this lignin-based bio-sorbent manufacturing system can provide sustainable bio-resource recycling and cost efficiency.

Efficient novel amphiphilic double shells layer coupled with nanoscale zero-valent composite for the degradation of trichloroethylene

An efficient novel amphiphilic material composed of core-double shells nanocomposite (CDSN) with nanoscale zero-valent iron (NZVI) as the core and PS₁₀₀-b-PAA₁₆ as inner shell and chitosan as outmost shell has been synthesized successfully. Its application to remove the trichloroethylene (TCE) in stimulated TCE solution with 7.3 ± 0.3 mg/L dissolved oxygen was investigated. The results showed that CDSN after exposure to air for a month could still remove 92.6% of TCE as compared to 61.5% removal rate of NZVI in 360 min (the gram ratio of material: TCE equals to 10:1), exhibiting the great oxidation resistance performance. Specifically, dynamic research of the total removal divided into adsorption by shell layer and degradation by reducibility of NZVI at a predetermined interval was engaged to understand the complete mass transfer process of TCE. The results revealed that CDSN adsorbed 1.5 to 2 folds time TCE as compared to NZVI in the same initial pH = 8.5 aqueous solution. Importantly, CDSN could sustain fixed reactivity to remove about 94.8% of TCE from the start to end. NZVI exhibited greater removal capacity in first 180 min, but later it lost the reducibility and finial removal rate was 89%. The selective adsorption to protonated CDSN was strengthened to increase the removal of TCE at pH 3.5 while NZVI had a worse removal in pH 3.5 performance than pH 8.5.

Two-step grafting of 2-hydroxyethyl methacrylate (HEMA) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) onto a polyethylene plate for enhancement of Cr(VI) ion adsorption

Polyethylene (PE) plates grafted with a neutral monomer, 2-hydroxyethyl methacrylate (HEMA), and a cationic monomer, 2-(dimethylamino)ethyl methacrylate (DMAEMA), (PE-g-PHEMA)-g-PDMAEMA plates were prepared by the two-step photografting. The Cr(VI) ion adsorption behavior of the (PE-g-PHEMA)-g-PDMAEMA plates was investigated as a function of the amounts of grafted HEMA, amount of grafted DMAEMA, initial pH value, and temperature. The adsorption capacity of the DMAEMA-grafted PE (PE-g-PDMAEMA) and (PE-g-PHEMA)-g-PDMAEMA plates had the maximum value at the initial pH value of 3.0, independent of the temperature. The adsorption capacity of (PE-g-PHEMA)-g-PDMAEMA plates increased with the amount of grafted HEMA (G_HEMA) in the first-step grafting. The increase in the water absorptivity of the grafted layers and thereby the increase in the degree of protonation of dimethylamino groups on grafted PDMAEMA chains were found to lead to the increase in the adsorption capacity. This adsorption capacity was higher than or comparable to those of other polymeric adsorbents for Cr(VI) ions. The Cr(VI) ion adsorption behavior on both PE-g-DMAEMA and (PE-g-PHEMA)-g-PDMAEMA plates obeyed the mechanism of the pseudo-second-order kinetic model and was well expressed by Langmuir isotherm. The high values of the Langmuir constant suggest that the adsorption of Cr(VI) ions occurs through an electrostatic interaction between protonated dimethylamino groups on grafted PDMAEMA chains and HCrO4- ions. Cr(VI) ions were successfully desorbed from PE-g-PDMAEMA and (PE-g-PHEMA)-g-PDMAEMA plates in eluents such as NaCl, NaCl containing NaOH, NH₄Cl, NH₄Cl containing NaOH, and NaOH.

Distinguished Cr(VI) capture with rapid and superior capability using polydopamine microsphere: Behavior and mechanism

Toxic heavy metal containing Cr(VI) species is a serious threat for ecological environment and human beings. In this work, a new mussel-inspired polydopamine microsphere (PDA-sphere) is prepared through in situ oxidative polymerization at air condition with controllable sizes. The adsorption of Cr(VI) ions onto PDA-sphere is highly pH dependent with the optimal pH ranging from 2.5 to 3.8. A rapid Cr(VI) removal can approach in 8min for equilibrium. More importantly, the prepared materials exhibit a remarkable sorption selectivity, coexisting SO₄^2-, NO₃^- and Cl^- ions at high levels; The applicability model further proves its effective performances with treated capacity of 42,000kg/kg sorbent, and the effluent can be reduced from 2000ppb to below 50ppb, which meets the drinking water criterions recommended by WHO. 1kg sorbent can also purify approximately 100t Cr(VI) contaminated wastewaters basing on the wastewater discharges of China. Such capacity for application ranks the top level for Cr(VI) removal. Additionally, the exhausted materials can be well regenerated by binary alkaline and salts mixtures. Such efficient adsorption can be ascribed to the well-dispersed morphology as well as the strong affinity between Cr(VI) and catechol or amine groups by XPS investigation. All the results suggest that polydopamine microspheres may be ideal materials for Cr(VI) treatment in waters.

Nitrogen rich core-shell magnetic mesoporous silica as an effective adsorbent for removal of silver nanoparticles from water

The production and increasing use of silver nanoparticles (AgNPs) obviously results in their release into the environment, leading to a risk to the environment due to their toxic effects. Thus, the removal of AgNPs from water is highly needed. Here, we demonstrate that nitrogen rich (∼10% nitrogen content) core-shell magnetic mesoporous silica is a promising adsorbent for the removal of AgNPs. For this, the poly(ethylenimine) functionalized core-shell magnetic mesoporous silica composites (Fe₃O₄@SiO₂-PEI) were prepared, and characterized by TEM, FT-IR, XRD, TG and N₂ adsorption-desorption. The removal of AgNPs by Fe₃O₄@SiO₂-PEI as a function of contact time, concentration of AgNPs, solution pH and ionic strength were studied. The adsorption kinetic data could be described by the pseudo-second-order rate model. Both Langmuir and Freundlich models fitted the adsorption data well. The adsorption capacity for AgNPs is 909.1mg/g, which is 5-181 times higher than that of the previously reported adsorbents for AgNPs. Interestingly, the silver adsorbed onto Fe₃O₄@SiO₂-PEI exhibits highly catalytic activity for 4-nitropheol (4-NP) reduction with a rate constant of 0.072min^-1, which is much higher than those by other AgNPs reported before. The silver-loaded Fe₃O₄@SiO₂-PEI promises good recyclability for at least five cycles, showing great potential in practical applications.

Removal of chromium(vi) from wastewater using weakly and strongly basic magnetic adsorbents: adsorption/desorption property and mechanism comparative studies

Two novel strongly basic magnetic adsorbents were prepared, and the adsorption/desorption property and mechanism of weakly and strongly basic magnetic adsorbents were compared.