Metal-organic frameworks/alginate composite beads as effective adsorbents for the removal of hexavalent chromium from aqueous solution

Covalent organic framework-sodium alginate-Ca2+-polyacrylic acid composite beads for convenient dispersive solid-phase extraction of neonicotinoid insecticides in fruit and vegetables

Neonicotinoids, the fastest-growing class of insecticides, have posed a multi-media residue problem with adverse effects on environment, biodiversity and human health. Herein, covalent organic framework-sodium alginate-Ca2+-polyacrylic acid composite beads (CACPs), facilely prepared at room temperature, were used in convenient dispersive solid-phase extraction (dSPE) and combined with high-performance liquid chromatography (HPLC) for the detection of five neonicotinoid insecticides (thiamethoxam, acetamiprid, dinotefuran, clothianidin, imidacloprid). CACPs can be completely separated within 1 min without centrifugation. After seven adsorption/desorption cycles, it maintained high extraction efficiencies (>90%). The developed method exhibited a wide linear range (0.01 ∼ 10 μg mL-1), low limits of detection (LODs, 0.0028 ∼ 0.0031 mg kg-1), and good repeatability (RSD ≤ 8.11%, n = 3). Moreover, it was applied to the determination of five neonicotinoids in fruit and vegetables (peach, pear, lettuce, cucumber, tomato), and recoveries ranged from 73.6% to 116.2%.

Synthesis of fenugreek gum-based metal-organic framework (FG/Zr-AIPA MOF) composite beads for sequestration of heavy metal ions from aqueous solution

Metal-organic frameworks (MOFs) are a prominent class of materials due to their large surface area and customized structures. This gives them specificity and high adsorption capacity while they lack mechanical strength and reusability. Integrating MOFs with polysaccharide matrix may retain MOF characteristics along with imparting structural integrity. In the present study, zirconium MOF-based fenugreek composite (FG/Zr-AIPA) beads were synthesised by a single droplet method and utilised for removal of Cr(VI), Pb(II) and Fe(III) from aqueous solution. The structure, morphology and composition of beads were evaluated by FTIR, XRD, TGA, BET, FESEM, EDX, XPS and zeta potential analysis. Adsorption isotherm, kinetics and thermodynamics were studied for Cr(VI), Pb(II) and Fe(III) adsorption. Adsorption kinetics and isotherm study revealed that all the metal ions were adsorbed through a monolayer chemisorption process. The maximum adsorption capacity was 344.43, 270.02 and 223.21 mg g-1 for Cr(VI), Pb(II) and Fe(III), respectively, based on the Langmuir isotherm study. The thermodynamics study revealed that the interaction between the metal ions and the composite beads was spontaneous and endothermic. The FG/Zr-AIPA composite beads exhibited good reusability for the removal of Cr(VI), Pb(II) and Fe(III). The results open new possibilities for the preparation of polysaccharide MOF-based composite beads which exhibit substantial potential for water treatment applications.

Recent advancements in modifications of metal-organic frameworks-based materials for enhanced water purification and contaminant detection

Metal-organic frameworks (MOFs) have emerged as a key focus in water treatment and monitoring due to their unique structural features, including extensive surface area, customizable porosity, reversible adsorption, and high catalytic efficiency. While numerous reviews have discussed MOFs in environmental remediation, this review specifically addresses recent advancements in modifying MOFs to enhance their effectiveness in water purification and monitoring. It underscores their roles as adsorbents, photocatalysts, and in luminescent and electrochemical sensing. Advancements such as pore modification, defect engineering, and functionalization, combined synergistically with advanced materials, have led to the development of recyclable MOF-based nano-adsorbents, Z-scheme photocatalytic systems, nanocomposites, and hybrid materials. These innovations have broadened the spectrum of removable contaminants and improved material recyclability. Additionally, this review delves into the creation of multifunctional MOF materials, the development of robust MOF variants, and the simplification of synthesis methods, marking significant progress in MOF sensor technology. Furthermore, the review addresses current challenges in this field and proposes potential future research directions and practical applications. The growing research interest in MOFs underscores the need for an updated synthesis of knowledge in this area, focusing on both current challenges and future opportunities in water remediation.

Heavy metal sequestration from wastewater by metal-organic frameworks: a state-of-the-art review of recent progress

Metal-organic frameworks (MOFs) have emerged as highly promising adsorbents for removing heavy metals from wastewater due to their tunable structures, high surface areas, and exceptional adsorption capacities. This review meticulously examines and summarizes recent advancements in producing and utilizing MOF-based adsorbents for sequestering heavy metal ions from water. It begins by outlining and contrasting commonly employed methods for synthesizing MOFs, such as solvothermal, microwave, electrochemical, ultrasonic, and mechanochemical. Rather than delving into the specifics of adsorption process parameters, the focus shifts to analyzing the adsorption capabilities and underlying mechanisms against critical metal(loid) ions like chromium, arsenic, lead, cadmium, and mercury under various environmental conditions. Additionally, this article discusses strategies to optimize MOF performance, scale-up production, and address environmental implications. The comprehensive review aims to enhance the understanding of MOF-based adsorption for heavy metal remediation and stimulate further research in this critical field. In brief, this review article presents a comprehensive overview of the contemporary information on MOFs as an effective adsorbent and the challenges being faced by these adsorbents for heavy metal mitigation (including stability, cost, environmental issues, and optimization), targeting to develop a vital reference for future MOF research.

Effective chromium removal of metal anchored alginate-chitosan binary bio-composites

Water is the most essential resource for the biotic and abiotic components of an ecosystem. Any change in the quality of this water may cause adverse impact on the ecosystem. Hexavalent chromium is one such important pollutant that gets exposed in the water mainly through anthropogenic processes. Adsorption is considered to be an effective, economic and easiest method for remediation of such pollutants. Amongst the innumerable adsorbents available, biopolymers fetch the interest due to its cost effectiveness, efficiency and biocompatibility. But, the mechanical strength and workability of such biopolymers makes it unfit to use as an adsorbent. To improve these drawbacks, synthesis of biopolymeric composites become the need of the hour. So, an attempt was made here to synthesize metal cross-linked binary bio-composites using Alginate and Chitosan polymer matrix. Synthesized bio-composites were characterised with the aid of FTIR, XPS, Thermal analysis, SEM with EDAX and subjected for hexavalent chromium removal from water. Analysis of variance (ANOVA) with 95 % confidence intervals was used to assess the significance of independent variables and their interactions. Adsorption studies were done using batch process and to achieve greater sorption, various influencing parameters were optimized one by one. While investigating one parameter, other parameters were kept unaltered. Optimization was done for the parameters like contact time, dosage of the adsorbent, pH of the medium and presence of co-ions. Contact time and dosage for all the composites was 30 mins and 0.1 g respectively. Amongst the composites, Zirconium loaded binary composite possess high sorption capacity of around 14.8 mg/g. While Calcium and Iron loaded composites exhibit sorption capacity of around 9.8 mg/g and 10.4 mg/g respectively. Presence of other co-ions in the medium doesn't affect the sorption process. Isothermal studies infer the adsorption follows Langmuir model and thermodynamic parameters concludes the endothermic and randomness of the adsorption. The bio-composites can be recycled and used upto three cycles. Field trial was conducted and the composites work well in such conditions.

Modification of cellulosic adsorbent via iron-based metal phenolic networks coating for efficient removal of chromium ion

Surface modification of durian rind cellulose (DCell) was done by utilizing the strong coordination effect of polyphenol-based metal phenolic networks (MPNs). MPNs from Fe(III)-tannic acid (FTN) and Fe(III)-gallic acid (FGN) were coated on DCell via a self-assembly reaction at pH 8, resulting in adsorbent composites of FTN@DCell and FGN@DCell for removal of Cr(VI). Batch adsorption experiments revealed that FTN coating resulted in an adsorbent composite with higher adsorption capacity than FGN coating, owing to the greater number of additional adsorption sites from phenolic hydroxyl groups of tannic acid. FTN@DCell exhibits an equilibrium adsorption capacity at 30°C of 110.9 mg/g for Cr(VI), significantly higher than FGN@DCell (73.63 mg/g); the adsorption capacity was increased at higher temperature (i.e., 155.8 and 116.8 mg/g at 50°C for FTN@DCell and FGN@DCell, respectively). Effects of pH, adsorbent dose, initial concentration, and coexisting ions on Cr(VI) removal were investigated. The kinetics fractal-based model Brouers-Sotolongo indicates the 1st and 2nd order reaction for Cr(VI) adsorption on FTN@DCell and FGN@DCell, respectively. The isotherm data can be described with a fractal-based model, which implies the heterogeneous nature of the adsorbent surface sites. The Cr(VI) adsorption via surface complexation with phenolic hydroxyl groups was confirmed by evaluating the functional groups shifting. FGN@DCell and FTN@DCell were found to have good reusability, maintaining over 50 % of their adsorption efficiency after four adsorption-desorption cycles. Environmental assessment with Arabidopsis thaliana demonstrated their potential in eliminating the Cr(VI) phytotoxic effect. Thus, this study has shown the efficient and economical conversion of durian waste into environmentally benign adsorbent for heavy metal treatment.

Adsorption of metal ions by oceanic manganese nodule and deep-sea sediment: Behaviour, mechanism and evaluation

Deep-sea mining disturbs the sediment on the seabed to form plumose flows, carrying metal ions that are transmitted through the food chain, posing a serious threat to marine ecosystems and human health. In this study, two types of marine raw materials were screened: Oceanic Manganese Nodules (OMN) and Deep-sea Sediments (DSS), and prepared the spherical regenerative adsorption materials OMN@SA, DSS@SA and OMN&DSS@SA using sodium alginate (SA) by sol-gel method. Preliminary investigations on the adsorption effect of metal ions were carried out. OMN@SA exhibited the best adsorption capacity, with the adsorption quantities for Cu2+, Co2+ and Ni2+ reaching 31.12, 21.11 and 16.66 mg/g, respectively. The adsorption behaviour is consistent with the Langmuir, pseudo-second-order kinetics and particle diffusion model, indicating that the adsorption process is mainly spontaneous, monolayer chemical adsorption, and the adsorption rate is mainly controlled by internal particle diffusion. SEM-EDS, XRD, FTIR and XPS analyses suggest that the adsorption mechanism includes surface physical adsorption, ion exchange, functional group complexation, electrostatic attraction and precipitation. The fixed bed column experiment shows that OMN@SA can effectively remove metal ions Cu2+, demonstrating excellent stability, safety and good regenerability. This study paves a new direction for the design of efficient and sustainable materials for heavy metal adsorption. More importantly, as marine primordial materials, OMN and DSS have strong technical and economic feasibility for future use in in-situ fixation of metal ions in seafloor sediments and restoration of the original seabed environment.

Efficient structure tuning over the defective modulated zirconium metal organic framework with active coordinate surface for photocatalyst CO2 reduction

Structure engineering of zirconium-based metal organic frameworks (MOFs) aims to develop efficient catalysts for transforming intermittent renewable energy into value-added chemical fuels. In order to have a deeper understanding of industrial scaling, it is vital to ascertain the favourable operational parameters that are necessary for projecting at the atomic level. The proposed paradigm provides a robust basis for the efficient design of MOFs based heterogeneous photocatalysts. In this study, set of defective MOF (D-NUiO66) was effectively produced using a modular acidic method. Afterwards, the D-NUiO66 was combined with CeO2 to form the D-CeNUiO66 heterojunction for the purpose of carbon dioxide reduction. The morphological aspect of the composite investigation suggested that D-CeNUiO66 had a mesoporous structure with favourable adsorption properties. The optimized D-CeNUiO66 photocatalyst showed the high activity for the reduction of CO2 to CO, with a rate of 38.6 µmolg-1h-1 and demonstrated remarkable repeatability in terms of CO production. The incorporation of defect sites in the D-NUiO66 enhanced the light response to visible light, resulting in reduced band gap of 2.9 eV. The photoelectrochemical tests indicated that the introduction of defects in the UiO66 and coupling CeO2 in the D-CeNUiO66 composite induced fast charge transfer, therefore suppressing the charge recombination rate. This study provides valuable insights into the use of defective engineering and heterojunction approaches to metal-organic frameworks for photocatalytic applications.

Batch Experimental Studies and Statistical Modeling for the Effective Removal of Tetracycline from Wastewater Using Bimetallic Zn-Cu-Metal-Organic Framework@Hydrogel Composite Beads

Antimicrobial resistance (AMR) is on an upsurge as more and more broad-spectrum antibiotics are being used haphazardly, resulting in imbalances in the ecosystem and disrupting common/systematic clinical protocols. To combat this issue, metal-organic framework embedded zinc-copper-benzenedicarboxylate@calcium alginate composite beads (Zn-Cu-BDC@CA CBs) were synthesized and utilized for the adsorption of tetracycline (TC) from water. The surface morphology, presence of functional groups, surface area, and thermal stability of Zn-Cu-BDC@CA CBs were evaluated by field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET), and thermal gravimetric analysis (TGA), respectively. Batch adsorption experiments were also carried out to optimize the adsorption performance of Zn-Cu-BDC@CA CBs for TC by adjusting the key parameters, including pH of the solution, contact time, adsorbent dosage, temperature, and initial concentration of TC. From the RSM model, 96.8% removal of TC takes place under the optimum conditions (pH = 7.3, mass = 17.2 mg, concentration = 21.3 ppm, time = 3.4 h, and temperature = 31.8 °C), which aligns closely with the experimental batch study, where the addition of 20 mg of adsorbent to a 20 mL TC solution (20 mg/L) at a pH of 7 and a temperature of 27 °C yielded an impressive TC removal efficiency of 96.55% within 180 min. Zn-Cu-BDC@CA CBs possess homogeneous adsorption surfaces, and TC is adsorbed via monolayer chemisorption, according to the results derived from the Langmuir isotherm model and pseudo-second-order kinetic model. The thermodynamic analysis indicated that the adsorption process is both endothermic and spontaneous. In their entirety, the synthesized Zn-Cu-BDC@CA CBs exhibit certain operational advantages, such as simple separation, satisfactory adsorption performance, and decent recyclability, indicating their viability for industrial application of elimination of TC residues from aquatic environments.

Fabrication and Morphological Control of Nonspherical Alginate Hydrogel Particles

We report a simple platform for the fabrication of nonspherical alginate hydrogel particles using a dripping method. Hydrogel particles with novel morphologies, such as vortex ring, teardrop, disk, sphere, and mushroom, are fabricated by controlling various parameters. We monitored the deformation process of the hydrogel particles after they penetrated the crosslinking solution using a high-speed camera. Then, we proposed a mechanism showing a unique morphological transformation from a spherical to a disk shape. We demonstrated how controlling the collecting height that causes the drop impact force against the crosslinking solution surface was critical to producing hydrogel particles with these intriguing shapes. In particular, disk-shaped alginate particles show their ability as potential platforms for culturing mouse adrenocortical tumor cells (Y1) and a hippocampal neuronal cell (HT-22). To modify alginate particles, cell-adhesive gelatin is incorporated into the alginate matrix and then alginate particles are coated with poly(allylamine hydrochloride). Two modified alginate particles show good adhesion and proliferation rates on their surfaces. In particular, the hybrid hydrogel particles provide great potential to be developed into promising materials for cell culture, drug delivery, and tissue engineering.

Hydrothermally magnetic particles fabricated hydrocalumite based biopolymeric composites for toxic chromium removal

Hexavalent chromium is an emerging environmental pollutant that leads to various effects on living organisms. The developed clay material, hydrocalumite (HC) possesses promising chromium adsorption capacity but because of its powder form it cannot be used in column studies. Hence, it is aimed to prepared HC in an usable hybrid bio-composite form by dispersing HC in biopolymeric matrixes like chitosan (CS) and cellulose (Cel) as HCCS and HCCel bio-composites for Cr(VI) removal from water. For quick separation after adsorption, the magnetic particles sprayed HCCS (Fe3O4@HCCS) and HCCel (Fe3O4@HCCel) bio-composites were prepared which possess high adsorption capacity. Different instrumental techniques like FTIR, SEM, and EDAX studies were used to examine the synthesized magnetic bio-composites in order to determine their physicochemical properties. The promising adsorbents namely Fe3O4@HCCS and Fe3O4@HCCel bio-composites were examined for Cr(VI) removal in batch mode. The maximum chromium adsorption capacity of Fe3O4@HCCS and Fe3O4@HCCel bio-composites were found at 43.4 mg/L and 31.8 mg/L, respectively within 45 min. The Freundlich, Langmuir, and Dubinin-Radushkevich (D-R) isotherms were used to reinterpret the equilibrium data of the synthetic magnetic bio-composites. According to the thermodynamic findings, chromium adsorption onto magnetic bio-composites is an endothermic and spontaneous reaction. The NaOH solution makes it simple to regenerate the chromium adsorbed magnetic bio-composites, which can be successfully employed upto four times. The synthesized Fe3O4@HCCS and Fe3O4@HCCel bio-composites act as efficient adsorbents for chromium removal.

Research Progress of Treatment Technology and Adsorption Materials for Removing Chromate in the Environment

Cr is used extensively in industry, so the number of Cr (VI) hazards is increasing. The effective control and removal of Cr (VI) from the environment are becoming an increasing research priority. In order to provide a more comprehensive description of the research progress of chromate adsorption materials, this paper summarizes the articles describing chromate adsorption in the past five years. It summarizes the adsorption principles, adsorbent types, and adsorption effects to provide methods and ideas to solve the chromate pollution problem further. After research, it is found that many adsorbents reduce adsorption when there is too much charge in the water. Besides, to ensure adsorption efficiency, there are problems with the formability of some materials, which impact recycling.

Comparative study for removal of cationic and anionic dyes using alginate-based hydrogels filled with citric acid-sawdust/UiO-66-NH2 hybrid

Nowadays, a big challenge is developing a sustainable and effective method for removing contaminants like dyes from aqueous solutions. In this regard, Zr-based metal-organic framework (UiO-66-NH₂) and sawdust as the ideal adsorbents were used. Due to their low separation in adsorption processes, embedding into alginate and obtaining composite beads are suggested as a suitable strategy. The achieved Ca-alginate/citric acid (CA)-sawdust/UiO-66-NH₂ hydrogel beads were used to compare cationic and anionic dyes removal. This sorbent indicated an excellent selectivity for removing methylene blue versus methyl orange in a binary system. pH = 6, adsorbent amount = 80 mg, methylene blue concentration = 10 mg/L, and contact time = 420 min were achieved as optimal parameters on methylene blue adsorption with an adsorption capacity of about 26 mg/g. The removal process of methylene blue followed linear Freundlich isotherm and nonlinear pseudo-2nd-order kinetic models. The regeneration test demonstrated methylene blue removal efficiency higher than about 89 % after 9 cycles. According to the outcomes, methylene blue could be attached to the adsorbent surface through the electrostatic, hydrogen bonding, and π-π interactions of the aromatic rings. These results confirm the potential of Ca-alginate/CA-sawdust/UiO-66-NH₂ hydrogel beads as a selective bio-sorbent for cationic dye removal.

Turning waste into valuables: In situ deposition of polypyrrole on the obsolete mask for Cr(VI) removal and desalination

The global mask consumption has been exacerbated because of the coronavirus disease 2019 (COVID-19) pandemic. Simultaneously, the traditional mask disposal methods (incineration and landfill) have caused serious environmental pollution and waste of resources. Herein, a simple and green mass-production method has been proposed to recycle carbon protective mask (CPM) into the carbon protective mask/polydopamine/polypyrrole (CPM/PDA/PPy) composite by in situ polymerization of PPy. The CPM/PDA/PPy composite was used for the removal of Cr(VI) and salt ions to produce clean water. The synergistic effect of PPy and the CPM improved the removal capability of Cr(VI). The CPM/PDA/PPy composite provided high adsorption capacity (358.68 mg g^-1) and economic value (811.42 mg $^-1). Consequently, the CPM/PDA/PPy (cathode) was combined with MnO₂ (anode) for desalination in CDI cells, demonstrated excellent desalination capacity (26.65 mg g^-1) and ultrafast salt adsorption rate (6.96 mg g^-1 min^-1), which was higher than conventional CDI cells. Our work proposes a new low-carbon strategy to recycle discarded masks and demonstrates their utilization in Cr(VI) removal and seawater desalination.

Inclusion of bimetallic Fe0.75Cu0.25-BDC MOFs into Alginate-MoO3/GO as a novel nanohybrid for adsorptive removal of hexavalent chromium from water

Metal-organic frameworks (MOFs) as porous materials have recently attracted research works in removal of toxic pollutants from water. Cr(VI) is well-known as one of the most toxic forms of chromium and the selection of efficient and effective Cr(VI)-remediation technology must be focused on a number of important parameters. Therefore, the objective of this work is to fabricate a novel nanohybrid adsorbent for removal of Cr(VI) by using assembled bimetallic MOFs (Fe_0.75Cu_0.25-BDC)-bound- Alginate-MoO₃/Graphene oxide (Alg-MoO₃/GO) via simple solvothermal process. The aimed Fe_0.75Cu_0.25-BDC@Alg-MoO₃/GO nanohybrid was confirmed by FTIR, SEM, TEM, XRD and TGA. Adsorptive extraction of Cr(VI) from aqueous solution was aimed by various optimized experimental parameters providing optimum pH = 3, dosage = 5-10 mg, starting concentration of Cr(VI) = 5-15 mg L^-1, shaking time = 5-10 min. The point of zero charge (pH_Pzc) was 3.8. For Cr(VI) removal by Fe_0.75Cu_0.25-BDC@Alg-MoO₃/GO, four isotherm models were estimated: Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R) with calculated correlation coefficient (R² = 0.9934) for Langmuir model which was higher than others. The collected results from the kinetic study clarified that pseudo-second order model is the most convenient one for describing the adsorption behavior of Cr(VI) and therefore, the adsorption process was suggested to rely on a chemisorption mechanism. Thermodynamic parameters referred that the adsorption mechanism is based on a spontaneous and exothermic process. Finally, the emerged Fe_0.75Cu_0.25-BDC@Alg-MoO₃/GO nanohybrid was confirmed as an effective adsorbent for extraction of hexavalent chromium from real water specimens (tap, sea water and wastewater) with percentage recovery values > 98%.

Ni-alginate hydrogel beads for establishing breakthrough curves of lead ions removal from aqueous solutions

The scientific impact of this work is the protection of the environment from hazardous pollutants using a column technique. Besides its higher stability at working pH and its time persisting, Ni-alginate has a higher ability to remove lead ions compared to the other prepared beads (Sr-alginate, Co-alginate, and Ca-alginate). Also, Ni-alginate possessed a higher removal percent, 93.3%, toward Pb2+ than the other ions, taking the sorption order of Pb2+ > Sr2+ > Co2+ > Cd2+ > Zn2+. Therefore, this study focused on using Ni-alginate as a selective sorbent for lead ions. Fixed-bed column was employed for the sorption process. The results for that efficiency are presented as breakthrough curves in view of the impact of various parameters; influent flow rate (1.5, 3.0, and 5.0 mL/min), lead concentration (100, 150, and 200 mg/L), and bed depth of sorbent (3.0, 5.0, and 7.0 cm). Breakthrough modeling including Thomas and Yan models was employed. The outcomes indicated that Thomas theory is more applicable. The overall outcomes indicated that Ni-alginate is recommended for selective removal of Pb2+ from waste solutions.

Reusable composite membranes for highly efficient chromium removal from real water matrixes

Natural or industrial hexavalent chromium water pollution continues to be a worldwide unresolved threat. Today, there is intense research on new active and cost-effective sorbents for Cr(VI), but most still exhibit a critical limitation: their powdered nature makes their recovery from water cost and energy consuming. In this work, Al(OH)₃, MIL-88-B(Fe), and UiO-66-NH₂ Cr(VI) sorbents were immobilized into a poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymeric substrate to develop an easily reactivable and reusable water filtering technology. The immobilization of the sorbents into the PVDF-HFP porous matrix modified the macro and meso-porous structure of the polymeric matrix, tuning in parallel its wettability. Although a partial blocking of the Cr(VI) adsorptive capacity was observed for of Al(OH)₃ and MIL-88-B(Fe) when immobilized into composite membranes, PVDF-HFP/UiO-66-NH₂ filter (i) exceeded the full capacity of the non-immobilized sorbent to trap Cr(VI), (ii) could be reactivated and reusable, and (iii) it was fully functional when applied in real water effluents.

Kinetic and Isothermal Investigations on the Use of Low Cost Coconut Fiber-Polyaniline Composites for the Removal of Chromium from Wastewater

Pollution due to various heavy metals is increasing at an alarming rate. Removal of hexavalent chromium from the environment is a significant and challenging issue due to its toxic effects on the ecosystem. Development of a low-cost adsorbent with better adsorption efficiency is presently required. In this study, waste coconut fibers (CF) were used to prepare its composite with polyaniline (PANI) via in-situ oxidation. The obtained composites with varying loading of PANI (15, 25, 50, and 75% w/w) were characterized by FE-SEM, TGA, and FTIR spectroscopy. The prepared composites were evaluated for their adsorption performance for removal of Cr(VI). It was concluded that the composite with 50% w/w polyaniline loading on coconut fiber exhibited a maximum adsorption efficiency of 93.11% in 30 min. The effect of pH, dosage, and concentration of the aqueous solution of chromium on the Cr(VI) adsorption efficiency of the composite was also studied. From the optimization studies it was observed that the absorbents exhibited the best adsorption response for Cr(VI) removal with 0.25 mg/mL adsorbent at pH 4, in 30 min. The effect of pH, dosage, and concentration of the aqueous solution of chromium on the Cr(VI) adsorption efficiency of the composite was also studied. This study highlights the application of low-cost adsorbent as a potential candidate for the removal of hexavalent chromium. A detailed study on the adsorption kinetics and isothermal analysis was conducted for the removal of Cr(VI) from aqueous solution using coconut fiber-polyaniline composite. From the kinetic investigation, the adsorption was found to follow the pseudo second order model. The data obtained were best fitted to the Elovich model confirming the chemisorption of the Cr(VI) on coconut polymer composites. The analysis of the isothermal models indicated monolayer adsorption based on the Langmuir adsorption model.

Iron-sulphur transformation control for enhancing Cr(VI) removal in flake and nanoscale porous pyrrhotite (Fe7S8) added wastewater

Hexavalent chromium (Cr(VI)) contaminated wastewater should be addressed efficiently in the environmental field. In previous applications, nano iron sulfides amendment has not been well controlled for iron-sulfur transformation. In this study, the novel flake and nanoscale porous pyrrhotite (Fe₇S₈) (FNPP) amendment was synthesized. The iron-sulphur transformation of FNPP was controlled and optimized for enhancing Cr(VI) removal. The specific surface area and average pore diameter of the FNPP amendment reached 115.7 m²/g and 2.1 nm. The maximum adsorption capacity of total chromium reached 66.3 mg/g. The optimized iron-sulphur transformation condition was an initial FNPP and Cr(VI) molar ratio of 8, pH at 5.6, in which the Cr(VI) removal reached 96.5% and all producing S^2- was utterly consumed. It is confirmed that S^2- fast induced Fe³⁺/Fe²⁺ circulation and FNPP has a speedier adsorption rate for Cr(III) than Cr(VI). Fe²⁺ and S^2- mediated the Cr(VI) reduction to Cr(III), thus, much faster Cr(VI) removal was achieved. High efficiency removal mechanism of Cr(VI) was combined with surface adsorption/reduction and solution reduction/precipitation. The research demonstrated that controlling and optimizing the iron-sulphur transformation of Fe₇S₈ amendment can significantly enhance Cr(VI) removal.

Amine-bilayer-functionalized cellulose-chitosan composite hydrogel for the efficient uptake of hazardous metal cations and catalysis in polluted water

Herein, we represent a novel ecofriendly bilayer-amine group incorporated microcrystalline cellulose (MCC)/chitosan (CS) hydrogel, fabricated via integrating polydopamine (PDA) and polyethyleneimine (PEI) for reliable and effective extraction of copper (Cu²⁺), zinc (Zn²⁺), and nickel (Ni²⁺) ions from effluents. Owing to abundant adsorptive sites, the MCC-PDA-PEI/CS-PDA-PEI hydrogel showed excellent Cu²⁺, Zn²⁺, and Ni²⁺ adsorbabilities of ~434.8, ~277.7, and ~261.8 mg/g, respectively, in a single-ion adsorption system with the adsorption kinetics and isotherm complied with pseudo-second-order and Langmuir models, respectively. In a multi-ion adsorption system, hydrogel removes mixed metal cations with slightly higher selectivity for Cu²⁺. In accordance with X-ray photoelectron and Fourier-transform-infrared spectrometric analyses, a plausible binding mechanism of metal cations on the as-prepared hydrogel was proposed by chelation between hydrogel functional groups and metal ions. In the repetitive adsorption/desorption experiments, the hydrogel retained >40% metal ion adsorption and desorption capacities after four cycles. Furthermore, the Cu²⁺-adsorbing hydrogel could serve as a support for the in situ development of Cu nanoparticles, which showed excellent catalytic performance as demonstrated by the transformation of 4-nitrophenol (4-NP) to 4-aminophenol. This work provides a novel ecofriendly, reusable, and highly-efficient adsorbent, as well as a biocatalyst for remediation of heavy metal cations and 4-NP polluted effluents.

Chitosan-based nanocomposites for removal of Cr(VI) and synthetic food colorants from wastewater

Current study aims to synthesize chitosan/polyvinyl alcohol (CS/PVA), poly(ethyleneimine), and Fe₃O₄ impregnated beads for co-removal of Cr(VI) and toxic azo-dyes from wastewater. The mesoporous PEI@AC@Fe₃O₄ exhibits magnetism and enhanced physisorption by higher specific-porosity (2.1 nm) from Cr(VI) radii (0.044 nm). Moreover, surface functional groups (-OH, -NH, -NH₂, -COOH etc.), especially amines enhance ionic bonding due to positive zeta potential. Hence, it is unique for anionic dyes removal under a wide pH range. It showed maximum adsorption capacity 98, 85.5, 85.8, and 91%, or 199.8, 148, 167, 176.5 mg g^-1 respectively for Cr(VI), tartrazine, sunset yellow, and erythrosine. Surface adsorption of Cr(VI) and its transition into Cr(III) was confirmed by EDX. Langmuir isotherm and pseudo-first-order kinetics best fit the adsorption of Cr(VI) and azo-dyes confirming their monolayer physisorption on adsorbent surface. Synthesized adsorbent examined in wastewater purification prototype for efficient removal of different simulated wastewaters confirms its potential for real-world applications.

Production of polyacrylonitrile/ionic covalent organic framework hybrid nanofibers for effective removal of chromium(VI) from water

Hexavalent Cr(VI) found in industrial wastewater is a proven carcinogen which causes serious health issues in humans around the world. This study presents a novel method to enhance the Cr(VI) oxyanion removal from wastewater by polyacrylonitrile (PAN) nanofibers through incorporation of a guanidinium-based ionic covalent organic framework (BT-DG) in the nanofibers structure. Simple electrospinning technique was employed to produce PAN nanofibers and BT-DG was synthesized through condensation between benzene-1,3,5-tricarbaldehyde and N,N'-diaminoguanidine monohydrochloride. In-situ polymerization of BT-DG onto PAN nanofibers resulted in generation of hybrid PAN-BT-DG nanofibers. This modified PAN-BT-DG was characterized by obtaining its point of zero charge (PZC), differential scanning calorimeter (DSC), scanning electron microscopy (SEM) morphology and surface elements and oxidation states by X-ray photoelectron spectroscopy (XPS). PAN-BT-DG exhibited positive surface charge below pH 4, making it an outstanding adsorbent, for Cr(VI) removal. Cr(VI) adsorption onto PAN-BT-DG followed pseudo second order kinetics and adsorption data fitted well to Freundlich isotherm model. Highest Cr(VI) removal was obtained at 55 ℃ with a maximum Langmuir adsorption capacity of 173 mg/g at pH 3. Kinetic studies revealed that Cr(VI) adsorption onto PAN-BT-DG is endothermic and thermodynamically feasible. Desorption studies were conducted on PAN-BT-DG using 1 M NaOH as the stripping solvent and PAN-BT-DG exhibited excellent regeneration after five consecutive cycles.

Removal and recycling of hexavalent chromium from alkaline wastewater via a new ferrite process to produce the valuable chromium ferrite

Current technologies for removal of Cr(VI) are generally fit for acidic wastewater. In this study, a new ferrite process for removal and recycling of Cr(VI) from alkaline wastewater to produce the valuable chromium ferrite has been developed. The results show that this new ferrite method is a one-step process which can be divided into two successive reactions including Cr(VI) reduction to form coprecipitation (Cr_0.25Fe_0.75(OH)₃) and subsequently magnetic conversion of Cr_0.25Fe_0.75(OH)₃ induced by Fe²⁺ under the same alkaline condition. The total Fe/Cr mole ratio of 5:1 is at least required for the chromium ferrite transformation. Increasing temperature and pH can enhance the interaction of Fe²⁺ with Cr_0.25Fe_0.75(OH)₃ and further promote the formation of chromium ferrite, while suppressing the generation of nonmagnetic by-product goethite. Almost pure chromium ferrite is formed under proposed optimum conditions (Fe/Cr = 7:1, 65 °C and pH of 9) with Cr(VI) removal ratio around 100%. The Cr(VI) remained in the filtrate can be reduced to 0.01 mg/L which is much lower than the limits concentration for surface water (≤0.05 mg/L). The chromium ferrite product whose molecular formula can be expressed as Cr_0.5-xFe_2.5+xO₄ (where 0 ≤ x < 0.5) presents good magnetic properties and has the potential to be recycled as a useful material.

Alginate-based nano-adsorbent materials - Bioinspired solution to mitigate hazardous environmental pollutants

Population growth and industrialization is associated with the elevation of hazardous pollutants, including heavy metals, biomedical wastes, personal-care products, endocrine-disrupters, pharmaceutically active compounds, and colorants in the environment. The scientific focus has been devoted to developing novel adsorbents to mitigate hazardous pollutants by constructing hybrids of different polymers and nano-structured materials for improved workability and physicochemical attributes. Recently, much attention has been devoted to nanomaterials in environmental remediation, owning to their exceptional characteristics including novel electrical/chemical features, quantum size effects, tunable functionalization, high scalability, and surface-area-to-volume ratio. Target-specific designing of nanocomposites impart high functionality. The cost-effective and eco-friendly synthesis of bioadsorbent materials is increasing for the removal of hazardous pollutants. Due to biocompatible, biodegradable, and eco-friendly nature, sodium alginate has been widely reported for the preparation of bioadsorbent materials to remove different inorganic/organic pollutants. In this review, the potentialities of alginate-based nanocomposites have been described for environmental remediation purposes. Different nanomaterials, including silica, metallic oxide, graphene oxide, hybrid inorganic-organic, non-magnetic-magnetic, carbon nanorods, nanotubes, polymeric nanocarriers, and several other materials have been described in combination with alginate biopolymer for environmental remediation.

Efficient Removal of Cr(VI) by TiO2 Based Micro-Nano Reactor via the Synergy of Adsorption and Photocatalysis

The low-toxicity treatment of chromium-containing wastewater represents an important way of addressing key environmental problems. In this study, a core-shell structural ZIF-8@TiO2 photocatalyst was synthesized by a simple one-step hydrothermal method. The obtained composite photocatalyst possessed improved photocatalytic activity compared with TiO2. The results indicated that the optimized ZIF-8@TiO2 composite exhibited the highest removal efficiency with 93.1% of Cr(VI) after 120 min under UV-vis irradiation. The removal curves and XPS results indicated that the adsorbed Cr(VI) on the ZIF-8 during the dark process was preferentially reduced. The superior removal efficiency of ZIF-8@TiO2 is attributed to the combination of both high adsorption of ZIF-8, which attracted Cr(VI) on the composite surface, and the high separation efficiency of photo-induced electron-hole pairs. For the mixture of wastewater that contained methyl orange and Cr(VI), 97.1% of MO and 99.7% of Cr(VI) were removed after 5 min and 60 min light irradiation, respectively. The high removal efficiency of multiple pollutants provides promising applications in the field of Cr(VI) contaminated industrial wastewater treatment.

Biogenic Synthesis of Iron Oxide Nanoparticles Using Enterococcus faecalis: Adsorption of Hexavalent Chromium from Aqueous Solution and In Vitro Cytotoxicity Analysis

The biological synthesis of nanoparticles is emerging as a potential method for nanoparticle synthesis due to its non-toxicity and simplicity. In the present study, a bacterium resistant to heavy metals was isolated from a metal-contaminated site and we aimed to report the synthesis of Fe3O4 nanoparticles via co-precipitation using bacterial exopolysaccharides (EPS) derived from Enterococcus faecalis_RMSN6 strains. A three-variable Box-Behnken design was used for determining the optimal conditions of the Fe3O4 NPs synthesis process. The synthesized Fe3O4 NPs were thoroughly characterized through multiple analytical techniques such as XRD, UV-Visible spectroscopy, FTIR spectroscopy and finally SEM analysis to understand the surface morphology. Fe3O4 NPs were then probed for the Cr(VI) ion adsorption studies. The important parameters such as optimization of initial concentration of Cr(VI) ions, effects of contact time, pH of the solution and contact time on quantity of Cr(VI) adsorbed were studied in detail. The maximum adsorption capacity of the nanoparticles was found to be 98.03 mg/g. The nanoparticles could retain up to 73% of their efficiency of chromium removal for up to 5 cycles. Additionally, prepared Fe3O4 NPs in the concentration were subjected to cytotoxicity studies using an MTT assay. The investigations using Fe3O4 NPs displayed a substantial dose-dependent effect on the A594 cells. The research elucidates that the Fe3O4 NPs synthesized from EPS of E. faecalis_RMSN6 can be used for the removal of heavy metal contaminants from wastewater.

Removal of organic pollutants from aqueous solution using metal organic frameworks (MOFs)-based adsorbents: A review

The development of metal organic frameworks (MOFs) has recently drawn a lot of scientific interest in water treatment due to the unique properties such as tunable porosities, large pore volumes, hierarchical structures, excellent adsorption and regeneration performances. MOFs represent an eco-friendly alternative to conventional adsorbents especially for the adsorptive removal of noxious organic pollutants from aqueous solution. Advanced MOFs' performances are justified by the introduction of functional groups, magnetic moieties, and specific foreign materials onto MOFs. This however leads to increase in the manufacturing costs of MOFs and consequently possess a huge challenge in large-scale applications. This review hence critically discusses the recent progresses in the development of MOFs-based adsorbents for the removal of selected organic pollutants (e.g., dyes, antibiotics and pesticides) from aqueous solution. Furthermore, major interaction mechanisms between MOFs and organic pollutants in response to numerous experimental conditions, such as pH, temperature, coexisting ions are put forward. Finally, some recommendations in support for designing MOFs with improved adsorption performances are also highlighted.

Insight into the synergistic adsorption-reduction character of chromium(VI) onto poly(pyrogallol-tetraethylene pentamine) microsphere in synthetic wastewater

Facile fabrication of the ultra-high-performance adsorbent can effectively ameliorate the Cr(VI)-pollution elimination in sewage control. Herein, a simple synthesis strategy is proposed to tap a versatile chelating resin poly(pyrogallol-tetraethylene pentamine) (PPTA) with respect to Cr(VI) removal from solution. Multiple changing factors which affect the adsorption behavior of PPTA are explored sequentially, such as initial pH, adsorbate concentration, adsorbent dosage, temperature, foreign ions, etc. The microstructure and functional mechanism of synthetic adsorbent are investigated systematically by means of various characterizations including TEM, EDS, FT-IR, XPS, etc. Consequently, the as-prepared PPTA-3 microsphere by reactant ratio of 1: 1 represents a brilliant synergistic adsorption and reduction result for Cr(VI) by the drastic electrostatic interaction of -NH₃⁺ and -OH₂⁺ groups, including satisfactory removal efficiency which closes to 100 % in low concentration, favorable specificity for the influence from coexistent ions (Mo(VI), Mn(VII), Cl^-, Cr(III), etc), and passable recyclability. Following the surpassingly fitting with Langmuir isotherm model, its maximum capacity reaches 714.29 mg g^-1 at 30 °C. The removal performance is essentially in agreement with the pseudo-second-order kinetics, simultaneously, suffers the rate-limiting impact depending on intra-particle diffusion process. In brief, this newly developed chelating resin presents an effective means with regard to the Cr(VI)-wastewater treatment or other uses in the future.

The Z-scheme NH2-UiO-66/PTCDA composite for enhanced photocatalytic Cr(VI) reduction under low-power LED visible light

Series Z-scheme NH₂-UiO-66/PTCDA (NU100PX) composites constructed from NH₂-UiO-66 and PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) were obtained by simple ball-milling method. The photocatalytic Cr(VI) reduction activities of the NU100PX composites were conducted upon the irradiation of low power LED visible light. The results revealed that the introduction of a small amount of PTCDA on the surface of NH₂-UiO-66 could broaden the light absorption range and boost the separation of photo-induced charge carriers to promote the photocatalysis efficiency. The influence factors toward photocatalytic Cr(VI) cleanup performances of NU100P10 like pH, initial Cr(VI) concentrations, the impacts of small organic acids as hole capture agents along with various co-existing foreign matters were clarified. After 5 runs' adsorption-photoreduction towards Cr(VI), the NU100P10 still exhibited superior reduction activity and reusability. The Z-scheme mechanism of photocatalytic Cr(VI) removal over NU100P10 was put forward and certificated by electrochemical experiment, ESR (electron spin resonance) test, XPS determination, photo-deposition and DFT (density functional theory) calculation.

Polydopamine-coated UiO-66 nanoparticles loaded with perfluorotributylamine/tirapazamine for hypoxia-activated osteosarcoma therapy

Background

Hypoxia is a characteristic of solid tumors that can lead to tumor angiogenesis and early metastasis, and addressing hypoxia presents tremendous challenges. In this work, a nanomedicine based on oxygen-absorbing perfluorotributylamine (PFA) and the bioreductive prodrug tirapazamine (TPZ) was prepared by using a polydopamine (PDA)-coated UiO-66 metal organic framework (MOF) as the drug carrier.

Results

The results showed that TPZ/PFA@UiO-66@PDA nanoparticles significantly enhanced hypoxia, induced cell apoptosis in vitro through the oxygen-dependent HIF-1α pathway and decreased oxygen levels in vivo after intratumoral injection. In addition, our study demonstrated that TPZ/PFA@UiO-66@PDA nanoparticles can accumulate in the tumor region after tail vein injection and effectively inhibit tumor growth when combined with photothermal therapy (PTT). TPZ/PFA@UiO-66@PDA nanoparticles increased HIF-1α expression while did not promote the expression of CD31 in vivo during the experiment.

Conclusions

By using TPZ and PFA and the enhanced permeability and retention effect of nanoparticles, TPZ/PFA@UiO-66@PDA can target tumor tissues, enhance hypoxia in the tumor microenvironment, and activate TPZ. Combined with PTT, the growth of osteosarcoma xenografts can be effectively inhibited.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-01013-0.

Polysaccharide-based (nano)materials for Cr(VI) removal

Chromium is a potentially poisonous and carcinogenic species, which originates from human activities and various industries such as leather, steel, iron, and electroplating industries. Chromium is present in various oxidation states, among which hexavalent chromium (Cr(VI)) is highly toxic as a natural contaminant. Therefore, chromium, particularly Cr(VI), must be eliminated from the environment, soil, and water to overcome significant problems due to its accumulation in the environment. There are different approaches such as adsorption, ion exchange, photocatalytic reduction, etc. for removing Cr(VI) from the environment. By converting Cr(VI) to Cr(III), its toxicity is reduced. Cr(III) is essential for the human diet, even in small amounts. Today, biopolymers such as alginate, cellulose, gum, pectin, starch, chitin, and chitosan have received much attention for the removal of environmental pollutants. Biopolymers, particularly polysaccharides, are very useful compounds due to their OH and NH₂ functional groups and some advantages such as biodegradability, biocompatibility, and accessibility. Therefore, they can be widely applied in catalytic applications and as efficient adsorbents for the removal of toxic compounds from the environment. This review briefly investigates the application of polysaccharide-based (nano)materials for efficient Cr(VI) removal from the environment using adsorption/reduction, photocatalytic, and chemical reduction mechanisms.

Batch and Continuous Chromate and Zinc Sorption from Electroplating Effluents Using Biogenic Iron Precipitates

Nanoparticles of iron precipitates produced by a microbial consortium are a suitable adsorbent for metal removal from electroplating industry wastewaters. Biogenic iron precipitates were utilized as adsorbents for chromate and zinc in batch conditions. Furthermore, the iron precipitates were embedded in alginate beads for metal removal in fixed-bed columns, and their performance was evaluated in a continuous system by varying different operational parameters such as flow rate, bed height, and feeding system (down- and up-flows). The influence of different adsorption variables in the saturation time, the amount of adsorbed potentially toxic metals, and the column performance was investigated, and the shape of the breakthrough curves was analyzed. The optimal column performance was achieved by increasing bed height and by decreasing feed flow rate and inlet metal concentration. The up-flow system significantly improved the metal uptake, avoiding the preferential flow channels.