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

Extracellular bioreduction is the main Cr(VI) detoxification strategy of Bacillus sp. HL1

Bacterium with high Cr(VI) detoxification capability belonged to the genus Bacillus have been largely explored, yet their reduction strategies are still in debate. Cr(VI) removal performance and mechanism of Bacillus sp. HL1 isolated from tailings a site was comprehensively investigated in this study. Approximately 88.31% of 100 mg/L Cr(VI) was continuously removed within 72 h, while it could resist up to 300 mg/L Cr(VI). Metal ions Mn2+ and Cu2+ could effectively improve the Cr(VI) removal performance to 14.41% and 3.41% under the optimal conditions, respectively. Cr(VI) removal performances by subcellular extracts showed that nearly 45.28% of 100 mg/L extracellular Cr(VI) was efficaciously reduced to Cr(III), while only 14.27%, 6.40%, and 2.73% of the cell-free extract, resting cells, and cell debris were reduced, respectively. This suggested that extracellular bioreduction was the primary Cr(VI) detoxification strategy despite a small part of Cr(VI) reduction took place intracellularly. In particular, the reduction products of the intracellular and extracellular compounds significantly differed, with organo-Cr(III) complex outside the cell and crystalline Cr(III) precipitate inside. Such observation was also evidenced by the intracellular black precipitate observed in the TEM image. XRD, XPS, and EPR analysis showed different Cr(III) compositions of intracellular and extracellular products. This study deepens our insights into the different fates of microorganisms that reduce Cr(VI) intracellularly and extracellularly.

Monodisperse Polyaspartic Acid Derivative Microspheres for Potential Tumor Embolization Therapy

Polyaspartic acid derivatives are a well-known kind of polypeptide with good biocompatibility and biodegradability, and thus have been widely used as biomedical materials, including drug-loaded nano-scale micelles or macroscopic hydrogels. In this work, for the first time, monodisperse polyaspartic acid derivative microspheres with diameter ranging from 120 to 350 µm for potential tumor embolization therapy are successfully prepared by single emulsion droplet microfluidic technique. The obtained microsphere shows fast cationic anticancer drug doxorubicin hydrochloride loading kinetics with high loading capacity, which is much better than those of the commercial ones. Additionally, drug release behaviors of the drug-loaded microspheres with different diameters in different media are also studied and discussed in detail. These results provide some new insights for the preparation and potential application of polyaspartic acid derivative-based monodisperse microspheres, especially for their potential application as embolic agent.

Recent Advances in Molecular Imprinting for Proteins on Magnetic Microspheres

The separation of proteins in biological samples plays an essential role in the development of disease detection, drug discovery, and biological analysis. Protein imprinted polymers (PIPs) serve as a tool to capture target proteins specifically and selectively from complex media for separation purposes. Whereas conventional molecularly imprinted polymer is time-consuming in terms of incubation studies and solvent removal, magnetic particles are introduced using their magnetic properties for sedimentation and separation, resulting in saving extraction and centrifugation steps. Magnetic protein imprinted polymers (MPIPs), which combine molecularly imprinting materials with magnetic properties, have emerged as a new area of research hotspot. This review provides an overview of MPIPs for proteins, including synthesis, preparation strategies, and applications. Moreover, it also looks forward to the future directions for research in this emerging field.

Well-designed protein amyloid nanofibrils composites as versatile and sustainable materials for aquatic environment remediation: A review

Amyloid nanofibrils (ANFs) are supramolecular polymers originally classified as pathological markers in various human degenerative diseases. However, in recent years, ANFs have garnered greater interest and are regarded as nature-based sustainable biomaterials in environmental science, material engineering, and nanotechnology. On a laboratory scale, ANFs can be produced from food proteins via protein unfolding, misfolding, and hydrolysis. Furthermore, ANFs have specific structural characteristics such as a high aspect ratio, good rigidity, chemical stability, and a controllable sequence. These properties make them a promising functional material in water decontamination research. As a result, the fabrication and application of ANFs and their composites in water purification have recently gained considerable attention. Despite the large amount of literature in this field, there is a lack of systematic review to assess the gap in using ANFs and their composites to remove contaminants from water. This review discusses significant advancements in design techniques as well as the physicochemical properties of ANFs-based composites. We also emphasize the current progress in using ANFs-based composites to remove inorganic, organic, and biological contaminants. The interaction mechanisms between ANFs-based composites and contaminants are also highlighted. Finally, we illustrate the challenges and opportunities associated with the future preparation and application of ANFs-based composites. We anticipate that this review will shed new light on the future design and use of ANFs-based composites.

Highly efficient removal of Cr(VI) from wastewater using electronegative SA/EGCG@Ti/SA/PVDF sandwich membrane

The use of green, non-toxic raw materials is of great significance to the sustainable development of the environment, among which epigallocatechin gallate (EGCG) is a renewable carbon source from plants. At present, there is a lack of research on the metal-polyphenol nanomaterials their use in water decontamination. In this study, a novel SA/EGCG@Ti/SA/PVDF (SESP) sandwich membrane was prepared to effectively solve the problems of difficult recovery of nanomaterials and the leaching of metal ions. The membrane was made by scraping SA on the surface of the PVDF substrate as the bottom protective layer, depositing EGCG@Ti NPs as the functional layer, then coating SA as the surface isolation layer, and finally cross-linking with anhydrous calcium chloride. Results showed that EGCG@Ti NPs dispersed well on the surface of the SA/PVDF basement membrane. SESP sandwich membrane had good hydrothermal and acid-base stability, and it can be applied to wastewater with multiple co-existing heavy metals (e.g., Cu, Pb, Cd, and Ni). The contact angle and pure water flux of the SESP sandwich membrane with a negatively charged surface were 14.0-15.6° and 171.40 L/m2 h, respectively. The pure water flux of the regenerated membrane after BSA pollution recovered to 98.68 L/m2 h, and the interception efficiency and the interception flux of Cr(VI) were 100 % and 72.92 L/m2 h at 40 min of interception, respectively. Additionally, the removal efficiency of Cr(VI) by SESP sandwich membrane was maintained above 83 % for simulated wastewater and 100 % for actual wastewater after five adsorption-desorption cycles. Cr(VI) and Cr(III) can be removed simultaneously with the negatively charged SESP sandwich membrane. EDS and XPS analysis showed that the removal of Cr(VI) was controlled by the Donnan effect, anion exchange, chelation/complexation, and reduction mechanism. In contrast, Cr(III) was mainly influenced by electrostatic attraction and chelation/complexation mechanisms. In conclusion, the newly prepared sandwich membrane has good application potential in treating Cr(VI) wastewater.

Efficient removal of Cr from aqueous solution by catechol/m-phenylenediamine nanospheres combined with Fe(II)

The discharge of chromium-containing wastewater in industrial production causes resource loss and damage to the ecological environment. Currently, various phenolamine materials have been used to remove chromium, but their harsh adsorption conditions bring many difficulties. For example, ideal chromium removal is only achieved at low pH. In this study, we synthesized catechol/m-phenylenediamine nanospheres (CMN) and combined CMN with Fe(II) for Cr removal from aqueous solutions, and Fe(II) comes from FeSO4·7H2O. CMN was characterized and analyzed by field-emission scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), Fourier transformed infrared (FTIR), X-ray diffraction (XRD), X-ray photoelectron (XPS). The adsorption performance was studied through a series of adsorption experiments. When C0 = 900 mg/L and pH = 6, the maximum adsorption capacity obtained in the experiment was 977.1 mg/g. It maintains excellent adsorption properties in acidic, neutral and alkaline environments. The results of the adsorption mechanism showed that the ultra-high adsorption capacity of CMN and Fe(II) for Cr was the result of the synergistic effect of adsorption and reduction, including electrostatic attraction, reduction and coprecipitation. CMN is expected to be an ideal adsorbent for Cr removal in aqueous solution due to its low cost, high biocompatibility and high efficiency in Cr removal.

Synchronously Enhanced Removal Ability and Stability of MXene through Biomimetic Modification

Increasing environmental problems intensify the demand for high-performance environmental purification materials. MXene is a typical transition-metal carbide/nitride material with a two-dimensional geometric feature and a good deal of functional groups, and it is considered as an efficient adsorbent for removing pollutants from wastewater. However, the easy oxidation and relatively low adsorption capacity greatly restrict its application. In this study, the MXene/polydopamine (PDA) composite particles were fabricated through the biomimetic modification method of inducing the self-polymerization of dopamine in an MXene aqueous solution. Microstructure characterizations demonstrate that PDA facilitates the exfoliation of MXene. Adsorption measurements show that MXene and PDA exhibit an apparent synergistic effect in removing chromium hexavalent Cr(VI) from aqueous solution, and more PDA content leads to a larger synergistic effect. Consequently, the composite particles exhibit an ultrahigh adsorption capacity (862.3 mg/g). Specifically, even if the composite particles were stored in aqueous solution for 2 months, they still exhibit high adsorption ability with only a 3.3% loss in adsorption capacity, indirectly confirming the enhanced stability of MXene induced by PDA. Furthermore, the composite particles also show reduction ability to Cr(VI) and about 54.3% Cr(VI) can be reduced to harmless chromium trivalent Cr(III). This study provides a new method for the preparation of MXene-based adsorbents with excellent adsorption capacity and high stability, which has broad application prospects in the field of wastewater treatment.

Application of Poly(caffeic acid) for the Extraction of Critical Rare Earth Elements

Poly(caffeic acid) was synthesized and utilized for the extraction and determination of rare earth elements (REEs), thorium, and uranium. Oxidative polymerization of caffeic acid, a low-cost plant-based material, in the presence of ethylenediamine produced a granular, air-stable, and cross-linked polymer. The polymer is highly oxygenated and together with the amino group from ethylenediamine efficiently coordinates and preconcentrates these critical elements from aqueous media. Extraction was dependent on solution pH, amount of sorbent, and extraction time, while the concentration and flow rate of the desorption solution governed the recovery efficiency. Removal and recovery efficiencies greater than 98 and 90%, respectively, and low levels of detection ranging from 0.1 to 2.9 ng/L were achieved. Determination of these strategic elements in the presence of potentially interfering ions as well as in complex matrices such as well water and produced water samples also was demonstrated. The capacity of poly(caffeic acid) was determined with lanthanum as a representative REE to be 161.7 mg/g, establishing the promise of poly(caffeic acid) for larger-scale extractions in addition to the ability to screen sources for the presence of REEs.

Room temperature fabrication of cobalt mullite for the snappy adsorption of cationic and anionic dyes

Cobalt mullite adsorbent for the robust adsorption performance toward Victoria Blue (VB) and Metanil Yellow (MY) is fabricated by the sol-gel method at room temperature using dipropylamine as a structure-directing agent. The synthesized adsorbent is characterized by XRD, FT-IR, and HRTEM. From these analyses, it is found that dipropylamine binds with the alumina and cobalt oxide, which makes it into tetrahedral to octahedral form. This interaction causes the formation of cobalt mullite. It is observed that trigonal alumina and orthorhombic cobalt mullite are interlinked to form a hybrid network. The special feature of adopting this adsorbent for the adsorption of VB and MY is that it has a large amount of Brønsted acid site because of the octahedral coordination of Al and Co. The large availability of acid sites in the framework and hybridization of two different network systems favors robust adsorption. The rate of adsorption (K₂ = 0.00402 g/mg.min for VB and K₂ = 0.004 g/mg.min for MY) and adsorption capacity (Q_e = 102.041 mg/g for VB and Q_e = 19.0406 mg/g for MY) are greater for VB than MY. This may be due to the more steric factor involved in MY than VB. Thermodynamic parameter indicated that the adsorption of VB and MY is spontaneous, endothermic, and increased randomness in the adsorbent-adsorbate interface. The results from the enthalpy value (ΔH° = 65.43 kJ/mol for VB and ΔH° = 44.729 kJ/mol for MY) revealed that the chemisorption is involved in the adsorption process.

In Situ Reconstruction of Active Catalysis Sites Triggered by Chromium Immobilization for Sulfite Oxidation

Hexavalent chromium (Cr(VI)) is a highly toxic substance in wastewater, triggering grievous detriment to aquatic life and human health. Magnesium sulfite is spawned along with the desulfurization process in coal-fired power plants, which is usually disposed of as solid waste. Here, a "waste control by waste" method was proposed upon the redox of Cr(VI)-sulfite, in which highly toxic Cr(VI) is detoxicated and sequent enriched on a novel biochar-induced cobalt-based silica composite (BISC) due to the forced electron transfer from chromium to surface hydroxyl. The immobilized Cr on BISC gave rise to the reconstruction of catalytic active sites "Cr-O-Co", which further enhance its performance in sulfite oxidation by elevating O₂ adsorption. As a result, the sulfite oxidation rate increased by 10 times compared with the non-catalysis benchmark together with the maximum chromium adsorption capacity being 120.3 mg/g. Therefore, this study provides a promising strategy to simultaneously control highly toxic Cr(VI) and sulfite, achieving high-grade sulfur resource recovery for wet magnesia desulfurization.

Feasible synthesis of bifunctional polysilsesquioxane microspheres for robust adsorption of Hg(II) and Ag(I): Behavior and mechanism

The pollution of Hg(II) and Ag(I) to water system exerts hazardous effect to aquatic ecosystem and public security. Simple strategy for constructing adsorbents to efficient remove them is greatly desired. Thus, a series of thiol and amino groups containing bifunctional polysilsesquioxanes (ASPSS) microspheres with adjustable porous structure and functional group content were synthesized by one-step feasible sol-gel process. The adsorption behavior and mechanism of ASPSS microspheres toward Hg(II) and Ag(I) was thoroughly determined. The maximum adsorption capacity of ASPSS for Hg(II) and Ag(I) are 4.32 and 3.86 mmol·g^-1 under 25 ℃. The as-prepared ASPSS microspheres can 100% selectively capture Hg(II) with the coexisting of Mn(II), Co(II), Pb(II), Cd(II), Cu(II), Fe(III). And they can 100% adsorb Ag(I) with the presence of Cd(II), Pb(II), Co(II), Ni(II), and Zn(II). Moreover, the ASPSS microspheres exhibit good removal efficiency for Hg(II) and Ag(I) from simulated industrial wastewater with the coexistence of multiple pollutants. Adsorption mechanism suggests the adsorption for Hg(II) and Ag(I) is the synergistic coordination effect of amino and thiol groups. The excellent adsorption selectivity for Hg(II) and Ag(I) is attributed to the super binding ability of these functional group. ASPSS microspheres also exhibit good regeneration ability and could be reused for removing Hg (II) and Ag(I) from aqueous solution with practical value.

High capacity and fast removal of Cr(vi) by alkali lignin-based poly(tetraethylene pentamine-pyrogallol) sorbent

In this work, a novel alkali lignin-based adsorption material, alkali lignin-based poly(tetraethylene pentamine-pyrogallol) (AL-PTAP), was prepared using a Mannich reaction and catechol-amine reaction for removal of Cr(vi). It was characterized by thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The effects of tetraethylene pentamine (TEPA) dosage, pyrogallol (PL) dosage, contact time, pH, temperature and other factors on the adsorption behavior of the adsorbent were systematically investigated. These experimental data show that the adsorption behavior conforms to the pseudo-second-order kinetic model and the Langmuir isotherm model. The maximum adsorption capacity is 769.2 mg g^-1 at 303 K, which is much higher than that of alkali lignin (AL). AL-PTAP can achieve a removal rate of almost 100% for Cr(vi) solutions with a concentration of less than 90 mg L^-1 at 1 min. Furthermore, the toxic Cr(vi) is partly reduced to nontoxic Cr(iii) during the adsorption process. Therefore, AL-PTAP is a fast and efficient alkali lignin-based adsorbent, which is expected to improve the utilization value of alkali lignin in Cr(vi) wastewater treatment.

A metal-organic framework surrounded with conjugate acid-base pairs for the efficient capture of Cr(VI) via hydrogen bonding over a wide pH range

Given the large amount of toxic Cr(VI) wastewater from various industries, it is urgent to take effective treatment measures. Adsorption has been regarded as highly desirable for Cr(VI) removal, but the effectiveness of most adsorbents is significantly dependent on pH value, in which precipitous performance drop and even structural collapse generally occur in strong acidic/alkaline aqueous. Thus, maintaining high adsorption performance and structural integrity over a wide pH range is challenging. To efficiently remove Cr(VI), we designed and prepared of an acid-base resistant metal-organic framework (MOF) Zr-BDPO, by introducing weak acid-base groups (-NH-, -N= and -OH) onto the ligand. Zr-BDPO achieved a maximum adsorption capacity of 555.6 mg·g^-1 and retained skeletal structure at pH= 1-11. Interestingly, all these groups can generate conjugate acid-base pairs by means of H⁺ and OH^- in the external solution and then form buffer layer. The removal of Cr(VI) at a broad range of pH values primarily via hydrogen bonds between -NH- and -OH, and the oxoanion species of Cr(VI) is unusual. This strategy that insulating high concentrations of acids and bases and relying on hydrogen bonds to capture Cr(VI) oxoanions provides a new perspective for actual Cr(VI) wastewater treatment.

Biosorption of Cr (VI) by acid-modified based-waste fungal biomass from Calocybe indica fruiting bodies production

The world is going through a colossal drinking water scarcity. Unchecked discharge (even at trace levels) of Cr (VI) from industries into water bodies is a serious environmental concern. Here, we report waste fungal biomass (WFB) for the detoxification and removal of chromium ions. Biomass understudy was collected from Calocybe indica fruiting bodies. WFB was used after drying and pretreatment with two distinctive chemical methods, which improved the remediation effectiveness of Cr (VI). Light microscope and Field emission Scanning microscope (FESEM) were employed to elucidate the surface morphology of waste fungal biomass. While Fourier-Transform Infrared-Spectroscopy (FTIR) and Energy Dispersive X-Ray analysis (EDAX) were deployed to explore the mechanism of interaction between Cr (VI) anion and waste fungal biomass. X-ray Photoelectron Spectroscopy (XPS) analyses demonstrated considerable conversion of Cr (VI) into nontoxic Cr (III) species. The most favorable condition for optimum Cr (VI) remediation of 99.66% by treated waste fungal biomass (TWFB) occurred at pH 3, contact time 10 min, adsorbent dosage 3 gL^-1, Cr (VI) concentration 4 mgL^-1, stirring speed 140 rpm, and temperature 320 K, where for untreated waste fungal biomass (UWFB) the optimum of 85% remediation occurred at a contact time 15 min, and adsorbent dosage 2 gL^-1 whereas other experimental conditions remained identical as TWFB biosorbent. Pseudo-second-order kinetics (R² > 0.99) model matched the adsorption rate. And, the Freundlich isotherm model (R² > 0.99) is shown to be a better match for the experimental data. The optimum amount of Cr (VI) adsorbed by the TWFB and UWFB were 205.8 ± 10.1 and 72.85 ± 2.36 mgg^-1, respectively. Thermodynamic parameters revealed that the adsorption was spontaneous (ΔG ˂ 0), endothermic (ΔH > 0), and entropy-driven (ΔS > 0). The generated WFB adsorbent also has significant recycling potential. After five cycles of regeneration and adsorption. It can still keep up good remediation effectiveness of Cr (VI) ions to 85.5.

Iodine/iodide-doped polymeric nanospheres for simultaneous voltammetric detection of p-aminophenol, phenol, and p-nitrophenol

A sensor was developed for the first time based on polydopamine nanospheres doped with I₂, I^-, and IO₃^- species (PDA-Iodine), to determine the concentration of p-aminophenol (p-AP), phenol (Ph), and p-nitrophenol (p-NP) simultaneously. These polymeric nanospheres were successfully characterized using a variety of techniques including field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, Raman spectroscopy, and X-ray diffraction analysis. A carbon paste electrode was modified with the PDA-Iodine (CPE/PDA-Iodine). Because of the electrocatalytic activity of DA/DQ, I₂ and I^- species (in the structure of PDA-Iodine), CPE/PDA-Iodine shows enhancement in the electrooxidation peak currents as well as slight negative shift in peak potentials of p-AP, Ph, and p-NP compared with a bare carbon paste electrode. Under optimal experimental conditions, the linear calibration plots are linear in the ranges 0.5-120 μM for p-AP, 0.7-120 μM for Ph, and 1.0-100 μM for p-NP with limits of detection of 30, 40, and 80 nM for p-AP, Ph, and p-NP, respectively (S/N = 3). To prove the performance of the method, the repeatability of the signals of CPE/PDA-Iodine was evaluated and the RSD values obtained were 2.9%, 3.2%, and 3.1% for p-AP (45 µM), Ph (40 µM), and p-NP (40 µM), respectively. The CPE/PDA-Iodine is a promising new sensor for sensing p-AP, Ph, and p-NP simultaneously in tap and river water sample and the values of recoveries for spiked samples were in the range 94.0-104.4%.

Wastewater Treatment by Polymeric Microspheres: A Review

This review addresses polymer microspheres used as adsorbent for wastewater treatment. The removal of various pollutants (including dyes, heavy metal ions, and organic pollutants) is a prominent issue, as they can cause severe health problems. Porous microspheres can provide large specific area and active sites for adsorption or photo degradation. Enhancement in performance is achieved by various modifications, such as the introduction of nanoparticles, magnetic particles, and ZIF-8. Some microspheres were synthesized from synthetic polymers such as vinylic polymer and polydopamine (PDA) through a facile fabrication process. Natural polymers (such as cellulose, alginate, and chitosan) that are biodegradable and eco-friendly are also used. The adsorbents used in industrial application require high adsorption capacity, thermal stability, and recyclability. Batch adsorption experiments were conducted to investigate the optimal conditions, influence of related factors, and adsorption capacities. Insights regarding the adsorption mechanisms were given from the kinetic model, isotherm model, and various characterization methods. The recyclability is investigated through regeneration ratio, or their maintenance of their capability through repeated adsorption-desorption cycles. The high potential of polymer microsphere for the removal of pollutants from wastewater is shown through the high adsorption capacities, environmentally friendliness, and high stability.

Proteinaceous porous nanofiber membrane-type adsorbent derived from amyloid lysozyme protofilaments for highly efficient lead(II) biologic scavenging

To overcome the technical bottleneck of fine amyloid lysozyme fibrils in environmental engineering, a novel co-operative strategy was identified to fabricate free-standing lysozyme complex nanofibers based membrane-type adsorbent (Lys-CNFs membrane) through a combination of vacuum filtration for lead remediation. The composition of the membrane integrated the linear amyloid protofilaments that were obtained by acid-heating fibrillation and polydopamine that adjusted the fibers' diameters and surface chemistry. As expected, the Lys-CNFs membrane not only showed nanofibrous morphology and layer stacking architecture but presented a hierarchical macro-mesoporous structure along with a high surface area of 220.4 m²/g. Besides, the thermal stability up to 200 ℃ and wetting nature of below 2 s endowed its further applicability. Adsorption experiments showed that Lys-CNFs membrane can effectively uptake Pb(II) ions with acceptable selectivity, high adsorption capacity of 270.3 mg/g, rapid equilibrium kinetic within only 10 mins, and good reusability that dropped by 14.9% efficiency even after five cycles, indicating that Lys-CNFs membrane can be as an affordable technology for alleviating the lead pollution issues.

A systematic review on adsorptive removal of hexavalent chromium from aqueous solutions: Recent advances

The contamination of natural resources by hexavalent chromium (Cr(VI)) originating from natural and anthropogenic activities is a serious environmental concern. Although many articles on chromium remediation have been published, a comprehensive understanding of the mechanisms involved in remediation with different sorbents is not yet available. In this systematic review, the performance and applicability of several adsorptive materials for Cr(VI) removal from aqueous media are discussed, along with a detailed analysis of the mechanisms involved. Statistical analysis is applied to compare the efficacies of different adsorbents, while a similar approach is used to determine the effects of sorbent properties and experimental conditions on the adsorption capacity. A detailed analysis of the factors involved in fixed-bed column studies is also presented. A suitable desorption approach to the regeneration of the spent adsorbent and its adsorption performance in reuse is also examined. Among the different sorbents, nanoparticles and mineral-doped biochar were found to be the most effective sorbents, while the adsorption was higher at low pH (~4.0) than that at intermediate pH (6-8). Contrary to our expectation, adsorption was high for sorbents with low specific surface areas, suggesting that the adsorption of Cr(VI) is largely influenced by the chemical properties of the sorbents. The optimum adsorption in fixed-bed column systems is obtained at a lower Cr(VI) ion concentration, a lower influent flow rate, and a higher bed height. Since most of the studies reviewed herein were merely experimental and utilized ideal conditions with the presence of a single contaminant, i.e. Cr(VI) in water, further studies on adsorption dynamics with the presence of other interfering ions are suggested. This review is promising for the further development of Cr(VI) removal strategies and closes the research gaps pertaining to their challenges.

Simultaneous removal of typical flotation reagent 8-hydroxyquinoline and Cr(VI) through heterogeneous Fenton-like processes mediated by polydopamine functionalized ATP supported nZVI

The heavy metal and organic pollution caused by mining activities keep attracting attention, thus an economic and efficient treatment for combined pollution is pressing. In this study, the simultaneous removal performance of typical organic flotation reagent 8-hydroxyquinoline (8-HQ) and Cr(VI) was investigated via heterogeneous Fenton process induced by a novel polydopamine (PDA) functionalized attapulgite supported nano sized zero-valent iron (nZVI) composite (PDA/ATP-nZVI). Batch experiments showed that PDA/ATP-nZVI had better catalytic reactivity and reduction ability than both ATP-nZVI and nZVI. Under acidic condition, 96.0% of 8-HQ was degraded accompanied with the 42.5% of total organic carbon (TOC) decrease, while 95.8% of Cr(VI) removal efficiency was accomplished by PDA/ATP-nZVI. PDA not only served as redox mediator in expediting electron transfer, but also acted as electron donor that accelerated transformation from Fe(III) to both dissolved Fe(II) and surface Fe(II), which resulted in the increased degradation of 8-HQ. The synergic removal behavior between 8-HQ and Cr(VI) was discussed and the reaction mechanism in the persulfate (PS)-PDA/ATP-nZVI system was also explored. This study developed a highly efficient heterogeneous catalyst, and demonstrated that the PS-PDA/ATP-nZVI system had a potential for remediation of mine environment polluted by both heavy metals and organic flotation reagents.

Highly efficient Cr(VI) removal from industrial electroplating wastewater over Bi2S3 nanostructures prepared by dual sulfur-precursors: Insights on the promotion effect of sulfate ions

In this work, different Bi₂S₃ nanostructures were prepared from various single and dual sulfide precursors via a solvothermal method. It was found that Bi₂S₃ nanostructures prepared from dual sulfur precursors of L-cysteine and ammonium sulfide exhibited highest Cr(VI) removal ability with maximum Cr(VI) removal capacity of 148.95 mg/g in Cr(VI) solution (pH = 2). More importantly, the removal capacity strikingly increased to 223.33 and 240.25 mg/g in two kinds of actual industrial electroplating wastewater. By analyzing the components of actual electroplating wastewater and the results of control experiments in the absence and presence of different ions in Cr(VI) solution, it was found that SO₄^2- played a critical role in the Cr(VI) removal over Bi₂S₃. The addition of SO₄^2- could promote the conversion of Cr(VI) to Cr(III) on the surface of Bi₂S₃, thus leading to the enhanced Cr(VI) removal ability in actual electroplating wastewater. The Bi₂S₃ maintained its original Cr(VI) removal ability after four cycles in the electroplating wastewater, indicating the moderate reuse ability of the sample. This work not only demonstrated an highly efficient nanomaterials for the Cr(VI) removal in industrial electroplating wastewater, but also provided an insight on the influence of the components in wastewater on Cr(VI) removal.

Efficient, rapid and simple adsorption method by polydopamine polystyrene nanofibers mat for removal of multi-class antibiotic residues in environmental water

Polydopamine polystyrene nanofibers mat (PDA-PS NFsM) was prepared as an adsorbent for the simultaneous removal of multiple antibiotic residues in environmental water for the first time. PDA-PS NFsM can directly be used to adsorb 18 antibiotic residues belonging to 8 classes without any pretreatment of water samples. The adsorption process was completed within 5 min. All antibiotics could be removed at the same time, and the removal rate of each target was above 85%. Moreover, the used PDA-PS NFsM can be easily separated from the environmental water by taking out directly, and can be reused for 10 times after simple regeneration. The thermodynamic and kinetic properties of PDA-PS NFsM adsorption of antibiotic residues were further investigated consequently. It was found that the adsorptions of PDA-PS NFsM to the targets were spontaneous and endothermic process (ΔG<0, ΔH>0, ΔS>0). The results of adsorption kinetic experiments illustrated that the adsorption process was rapid, the adsorption equilibrium of which can be reached in 5 min. Adsorption isotherm experiments proved that the adsorption process of PDA-PS NFsM was consistent with Langmuir adsorption (R² > 0.994), and the maximum adsorption capacity of PDA-PS NFsM towards all targets were 123.76 mg g^-1. The developed method is rapid and simple, and can efficiently adsorb and remove a variety of antibiotics in environmental water, which has good practical application prospect.

DNA nanosensing systems for tunable detection of metal ions and molecular crypto-steganography

Various sensing platforms based on molecular or nanosystems are widely exploited through molecular diversity and specific recognition. However, it is extremely challenging to develop systems with tunable sensing ability and utilize the systems as information carriers/covers for communication and safety. Herein, DNA nanosensing systems based on cobalt oxyhydroxide (CoOOH) nanosheets were constructed for tunable detection and valence distinction of metal ions, molecular crypto-steganography, and information coding. CoOOH nanosheets absorb fluorescence-labeled single-stranded DNA with different bases and lengths, resulting in fluorescence quenching. The binding priority of bases with CoOOH nanosheets was guanine (G) > cytosine (C) > adenine (A) ≈ thymine (T) and the short chain excelled long chain. Due to the differences in the interaction among CoOOH, DNA, metal ions and variability of DNA bases, various DNA-CoOOH nanosystems have significantly different selective response patterns (that is selectivity) to metal ions and tunable linear ranges to Fe³⁺, Hg²⁺, Cr³⁺. Interestingly, by utilizing their molecular diversity, recognition, selective patterns, DNA-CoOOH sensing systems can be served as doubly cryptographic and steganographic systems to implement information encoding, encryption, and hiding and to reversely improve the selectivity of metal ions. This study provides an idea and platform for adjustable detection and valence distinction of metal ions, and gives a set of "molecular programming languages" for designing intelligent programmable sensing and molecular information communication and safety systems.

Superior elimination of Cr(VI) using polydopamine functionalized attapulgite supported nZVI composite: Behavior and mechanism

In this study, a polydopamine (PDA) modified attapulgite (ATP) supported nano sized zero-valent iron (nZVI) composite (PDA/ATP-nZVI) was rapidly synthesized under acidic conditions, and employed to alleviate Cr(VI) toxicity from an aqueous solution. Kinetic studies revealed that Cr(VI) adsorption process followed the pseudo-second order model, suggesting chemisorption was the dominant adsorption mechanism. Liu isotherm adsorption model was able to better describe the Cr(VI) adsorption isotherm with the maximum adsorption capacity of 134.05 mg/g. The thermodynamic study demonstrated that the adsorption process occurred spontaneously, accompanied by the increase in entropy and endothermic reaction. Low concentrations of coexisting ions had negligible effects on the removal of Cr(VI), while high concentrations of interfering ions were able to facilitate the removal of Cr(VI). Reactive species test revealed that Fe²⁺ played a key role in Cr(VI) reduction by PDA/ATP-nZVI. PDA enhanced the elimination of Cr(VI) via donation of electrons to Cr(VI) and acceleration of Fe³⁺ transformation to Fe²⁺. Furthermore, PDA was able to effectively inhibit the leaching of iron species and generation of ferric hydroxide sludge. Mechanistic study revealed that 72% of Cr(VI) elimination was attributed to reduction/precipitation, while 28% of Cr(VI) elimination was due to the surface adsorption.

Investigations of Cr(VI) removal by millet bran biochar modified with inorganic compounds: Momentous role of additional lactate

In this study, millet bran biochars modified with inorganic compounds (H₃PO₄: P-BC, NaOH: Na-BC and K₂CO₃: K-BC) were prepared and applied for Cr(VI) removal to evaluate the effects of modification on biochars' physicochemical properties. The results showed that Cr(VI) reduction capacity complied with the order of Na-BC > BC > P-BC > K-BC, and reductive groups such as -OH and -NH₂ played considerable roles in electrons donating. Based on this, lactate was added for further investigation of electrons transferring. The results displayed that Cr(VI) removal of all biochars was enhanced tremendously and modified biochars exhibited better Cr(VI) reduction. This may be due to the bridging effect of lactate, which could not only chelate with Cr(VI) via -COOH (or -OH) but also form hydrogen bonds with oxygen or nitrogen containing groups on biochars through the other groups, thus facilitating electrons transferring between biochars and Cr(VI). This work provided an insight into evaluation of the influence of inorganic compounds modification on both electrons donating capability of biochars and electrons transferring potential of biochars combined with lactate in Cr(VI) removal.

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.

Interconnected hierarchical nickel-carbon hybrids assembled by porous nanosheets for Cr(VI) reduction with formic acid

Magnetic carbon materials promise distinct advantages in the decontamination of heavy metal ions. In this work, a novel interconnected hierarchical nickel-carbon (Ni@IHC) hybrid was synthesized by combining the solvothermal method with a one-step pyrolysis under argon atmosphere. Benefitting from 3D flower-like morphology, interconnected porous nanosheets, large surface area, and abundant Ni nanoparticles, Ni@IHC hybrids can remove Cr(VI) within 25 min by using formic acid (FA) as a reductant at 25 ℃. Furthermore, the experimental parameters that can affect the material catalytic performance such as initial Cr(VI) concentration, catalyst dosage, FA concentration, and temperature were also investigated in detail. It was found that highly dispersed Ni nanoparticles contributed significantly to the reduction process. More importantly, the embedded Ni nanoparticles favor fast separation by a magnet and were helpful for the recycles use. This Ni@IHC hybrid was obtained by a facile and easy scale-up method, resulting in the fast transformation of Cr(VI) into Cr(III).

Adsorption behavior of heavy metal ions on a polymer-immobilized amphoteric biosorbent: Surface interaction assessment

Here we unveiled a novel magnetically separable amphoteric biosorbent (PD-Fe₃O₄@CCS) and investigated its adsorption behavior toward two classes of heavy metals, hexavalent chromium (Cr(VI)) and copper (Cu(II)) ions from water. Results indicated that the adsorption behavior of PD-Fe₃O₄@CCS for Cr(VI) was well described by Langmuir model; while for Cu(II) adsorption, the Freundlich model was the better one. Based on the kinetic results, both Cr(VI) and Cu(II) adsorption on PD-Fe₃O₄@CCS fitted well with the pseudo-second-order kinetic model. To evaluate the reusability and stability of PD-Fe₃O₄@CCS, regeneration tests were carried out for five cycles. Furthermore, the applicable feasibility of PD-Fe₃O₄@CCS in the real water matrix (including the single and binary pollutant systems) was studied, and results suggested the promising potential of PD-Fe₃O₄@CCS for large-scale application. Apart from these, the surface interactions between PD-Fe₃O₄@CCS and heavy metal ions in single and binary systems were systematically investigated based on FTIR and XPS analyses, which provided an essential implication for comprehending the interactions between biosorbents and contaminants in wastewater.

Polydopamine assists the continuous growth of zeolitic imidazolate framework-8 on electrospun polyacrylonitrile fibers as efficient adsorbents for the improved removal of Cr(vi)

PDA coating assists the growth of ZIF-8 particles on PAN fibers to fabricate composite ZIF-8@PDA/PAN fibers as efficient adsorbents for Cr(vi) removal.

Polydopamine Microsphere-Incorporated Electrospun Fibers as Novel Adsorbents for Dual-Responsive Adsorption of Methylene Blue

The usually inconvenient detection and uneasy recycling of polydopamine (PDA) with sphere morphology as an adsorbent restrict its actual applications in wastewater purification. Thus, novel composite fibers were fabricated via the electrospinning technique by integrating polydopamine microspheres (PDA-MPs) with pH/temperature dual-responsive copolymers. The insoluble fraction of the fabricated composite fibers can be maintained to a value above 89% after being immersed in aqueous solutions with different pH values. Also, the regeneration efficiency of the composite fibers can also remain above 80% after undergoing five adsorption-desorption cycles. These results both indicated that the fabricated composite fibers can avoid secondary pollution during the adsorption process effectively. In addition, the presence of abundant N-isopropyl acrylamide (NIPAM) units within the fibers could make it have a relatively higher water swelling ability of 4643%, which could further offer relatively larger inner spaces to accommodate the dye molecules. Meanwhile, by incorporating β-cyclodextrin (β-CD), methacrylic acid (MAA), PDA, and NIPAM components, plentiful active adsorption sites could be supplied to interact with methylene blue (MB) dye. So, the adsorption experiments of the composite fibers showed a maximum adsorption capacity of 1722.1 mg/g at pH 9.0 and a temperature of 55 °C. Furthermore, the pseudo-second-order kinetic model of adsorption suggested that it is a chemisorption process. Moreover, the adsorption experimental data can be better described by Langmuir models, inferring its monolayer adsorption. The adsorption thermodynamic studies revealed that adsorption is a spontaneous and endothermic process. Also, the increase of temperature facilitated the adsorption processes, owing to the increase of adsorbent's hydrophobicity and molecules' reactivity. The present work suggested that the combination of smart-responsive polymers and PDA-MPs could form an unprecedented system to be a promising candidate adsorbent for wastewater treatment.

Applications of Polydopamine-Modified Scaffolds in the Peripheral Nerve Tissue Engineering

Peripheral nerve injury is a common and complicated traumatic disease in clinical neurosurgery. With the rapid advancement and development of medical technologies, novel tissue engineering provides alternative therapies such as nerve conduit transplantation. It has achieved significant outcomes. The scaffold surface modification is vital to the reconstruction of a pro-healing interface. Polydopamine has high chemical activity, adhesion, hydrophilicity, hygroscopicity, stability, biocompatibility, and other properties. It is often used in the surface modification of biomaterials, especially in the peripheral nerve regeneration. The present review discusses that polydopamine can promote the adhesion, proliferation, and differentiation of neural stem cells and the growth of neuronal processes. Polydopamine is widely used in the surface modification of nerve conduits and has a potential application prospect of repairing peripheral nerve injury. Polydopamine-modified scaffolds are promising in the peripheral nerve tissue engineering.

Uniformly coating ZnAl layered double oxide nanosheets with ultra-thin carbon by ligand and phase transformation for enhanced adsorption of anionic pollutants

The increasing severity of water pollution has strongly urged to develop green and efficient adsorbents for waste-water treatment. In this work, ZnAl layered double oxide nanosheets uniformly coated with ultra-thin amorphous carbon shells (ZnAl-LDO@C) were fabricated by modifying ZnAl layered double hydroxides (LDHs) with molecular ligands followed by calcination. Compared with their counterparts derived from the pristine ZnAl-LDH, ZnAl-LDO@C nanosheets exhibit higher specific surface area with abundant and highly accessible active sites. The adsorption performance of the ZnAl-LDO@C nanosheets for methyl orange (MO) and hexavalent chromium [Cr(VI)] ions was investigated in detail. It is found that the channel-like hydrophilic carbon shells facilitate the diffusion of water molecules and ions, leading to the fast adsorption rate. In addition, the rich oxygen-containing functional groups in the amorphous carbon shells can efficiently improve the adsorption capacity through multiple interactions. As a result, ZnAl-LDO@C nanosheets exhibit superior adsorption performance for MO and Cr(VI), outperforming most LDH- or LDO-based adsorbents reported previously. Meanwhile, a new oriented overlapping intercalation mechanism for MO adsorption was proposed for the first time to clarify how MO molecules arrange at the interlayer space.

Copper microsphere hybrid mesoporous carbon as matrix for preparation of shape-stabilized phase change materials with improved thermal properties

Copper microsphere hybrid mesoporous carbon (MPC-Cu) was synthesized by the pyrolysis of polydopamine microspheres doped with copper ions that were prepared using a novel, facile and simple one-step method of dopamine biomimetic polymerization and copper ion adsorption. The resulting MPC-Cu was then used as a supporter for polyethylene glycol (PEG) to synthesize shape-stabilized phase change materials (PEG/MPC-Cu) with enhanced thermal properties. PEG/MPC-Cu was studied by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, differential scanning calorimetry and thermal constant analysis. The results demonstrated that the thermal conductivity of PEG/MPC-Cu was 0.502 W/(m K), which increased by 100% compared to pure PEG [0.251 W/(m K)]. The melting enthalpy of PEG/MPC-Cu was 95.98 J/g, indicating that PEG/MPC-Cu is a promising candidate for future thermal energy storage applications. In addition, the characterization results suggested that PEG-MPC-Cu possessed high thermal stability. Therefore, the method developed in this paper for preparing shape-stabilized phase change materials with improved thermal properties has substantial engineering application prospects.

Facile preparation of hierarchical porous polydopamine microspheres for rapid removal of chromate from the wastewater

Abstract

Cr(VI) containing industrial wastewaters are highly toxic and carcinogenic, and severely threats living creatures and the environment. Therefore, it is highly desired yet challenging to develop an available and economical adsorbent for simultaneously detoxifying Cr(VI) anions to Cr(III) ions and removing them from the wastewater. Here we propose a facile method for rapid removal of Cr(VI) ions from the wastewater by using a synthetic polydopamine microsphere (PPM) adsorbent with hierarchical porosity. The as-prepared PPM exhibits high Cr(VI) removal capacity of 307.7 mg/g and an outstanding removal efficiency. They can effectively decrease the Cr(VI) concentration to lower than 0.05 mg/L well below the limits for drinking water standard of WHO regulations in 60 s at pH 2. More importantly, PPMs can reduce the lethal Cr(VI) anions to Cr(III) ions with low toxicity, and simultaneously immobilize them on the matrices of PPMs.

Graphical abstract

New Mussel Inspired Polydopamine-Like Silica-Based Material for Dye Adsorption

A straightforward and economic procedure has been developed for the synthesis of a new polydopamine-like silica-based material that has been obtained by oxidation of catechol with KIO4 followed by reaction with 3-aminopropyltrimethoxysilane. All techniques adopted for characterization showed that the obtained material is rich in different functional groups and the morphological analyses revealed dimensions in the nanometric range. The hybrid material has been characterized by several techniques showing its polydopamine-like nature, and preliminary observations for dye adsorption have been reported.