Nanomaterials-based treatment options for chromium in aqueous environments

Simultaneous Cr(VI) reduction and Cr(III) sequestration in a wide pH range by using magnetic chitosan-based biopolymer

The simultaneous reduction of Cr(VI) and sequestration of the resulting Cr(III) in one process is highly desirable as a cost-effective and environmental-friendly approach for the decontamination of Cr(VI)-polluted wastewater. However, most of the existing adsorptive materials are only effective in low pH environments (pH = 1-3), severely restricting the adsorption efficiency and cost effectiveness. Herein, we proposed a chitosan-based magnetic porous microsphere (PPy@PMCS) for simultaneous Cr(VI) reduction and Cr(III) sequestration in a wide pH range. Benefiting from its abundant interaction sites, Cr(VI) was effectively adsorbed on the surface and then immediately reduced to Cr(III) with much lower toxicity. Most importantly, the resulting Cr(III) was in-situ sequestrated by the complexation of chitosan matrix. As a result, PPy@PMCS exhibited a maximum Cr(VI) adsorption capacity of 330.42 mg/g at pH 2.0 and an adsorption capacity of 167.82 mg/g even at near neutral pH (6.0), which is superior to most reported adsorbents. Furthermore, the exhausted PPy@PMCS can be rapidly separated from solutions under an external magnetic field and facilely regenerated. The proposed novel biopolymer-based material shows great application potentials in wastewater treatment.

Enhanced removal of Cr(VI) by polyethyleneimine-modified bamboo hydrochar

Hydrochar is an environmentally friendly and cheap adsorbent, but its adsorption amounts for anions is very limited. The functionalized hydrochar can overcome this shortcoming. Herein, polyethyleneimine-modified hydrochar (PEI-HC) was synthesized from hydrothermal carbonization (HTC) of methyl acrylate and bamboo after addition of initiator ammonium persulfate, and then modified by polyethyleneimine (PEI), which was used to treat Cr(VI). PEI-HC was tested by XANES, EXAFS, SEM-EDS, XPS, FTIR, N2 sorption isotherms, zeta potential, and elemental analyses. The characterizations showed that PEI was successfully grafted onto hydrochar, and the PEI-HC was rich in N and O functional groups, which presented high Cr(VI) sorption ability (528.41 mg·g-1 at pH 2). The bath experiments found the pseudo-second-order kinetic and Freundlich equations can well describe the adsorption kinetics and isotherm of the Cr(VI) adsorption onto PEI-HC, respectively. Electrostatic interaction, reduction, complexation, and H-bonding are the main removal mechanisms as supported by XANES, EXAFS, XPS, and FTIR. This study provides a strategy of combining HTC and free radical graft polymerization to convert agricultural and forestry wastes into functionalized hydrochar, showing highly efficient removal of Cr(VI).

Development of fluorescent carbon nanoparticles from Madhuca longifolia flower for the sensitive and selective detection of Cr6+: a collective experimental-computational approach

Herein, blue-emitting carbon nanoparticles (CNPs) were synthesized using the Madhuca longifolia flower for the highly selective and sensitive detection of Cr6+ ions in aqueous media using a simple, green, and cost-effective approach, and computational experiments were also performed. The prepared CNPs were well-dispersed in water with an average diameter of 12 nm and functionalized with carbonyl, hydroxyl and carboxylic acid groups. The decrease in the fluorescence intensity of the CNPs with an increase in the content of Cr6+ provided an important signal for the sensitive and selective detection of Cr6+ in aqueous media. The limit of detection for Cr6+ in an aqueous medium was found to be 103 ppb, which is more sensitive in comparison with the previously reported study. Furthermore, the validation of the proposed higher sensing feature and more selective nature of the CNPs towards Cr6+ was also explained using an in silico approach. The results from the theoretical calculations based on the DFT approach demonstrated that the binding energy (BE) of the CNPs with three transition metal (TM) cations (Cr6+, Fe3+, and Hg2+) follows the order of Cr6+ > Fe3+ > Hg2+, where the Cr6+ TM cation associated with the CNPs possesses the highest valence state, showing the highest sensing feature and highest selectivity among the investigated ions, as expected. The metal ions associated with the CNPs having a higher charge and a smaller radius indicated a higher BE and larger degree of deformation of the CNPs. Moreover, to achieve new insights into the structural, stability/energetics, and electronic features, some useful tools, such as NCI-plot, HOMO-LUMO gap, MESP, and QTAIM analysis were employed, which facilitated noteworthy outcomes.

Effective aqueous chromate treatment using triethanolamine anacardate coated magnetic nanoparticles

Low-cost adsorbents derived from agricultural by-products incorporated magnetic nanoparticles (NPs) are promising for wastewater treatment. They are always preferred due to their great performance and easy separation. This study reports cobalt superparamagnetic (CoFe₂O₄) nanoparticles (NPs) incorporated with triethanolamine (TEA) based surfactants from cashew nut shell liquid, namely TEA-CoFe₂O₄, for the removal of chromium (VI) ions from aqueous solutions. To have detailed characteristics of the morphology and structural properties, scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and vibrating sample magnetometry (VSM) were employed. The fabricated TEA-CoFe₂O₄ particles exhibit soft and superparamagnetic properties, which make the nanoparticles easily recycled by using a magnet. Chromate adsorption on the TEA-CoFe₂O₄ nanomaterials reached an optimal efficiency of 84.3% at pH = 3 with the initial adsorbent dose of 10 g/L and chromium (VI) concentration of 40 mg/L. The TEA-CoFe₂O₄ nanoparticles can maintain the effective adsorption of chromium (VI) ion (by 29% of efficiency loss) and retain the magnetic separation using a magnet up to three cycles of the regeneration, which promise a high potential of this low-cost adsorbent for long-term treatment of heavy metal ions from polluted waters.

The Cytotoxicity, DNA Fragmentation, and Decreasing Velocity Induced By Chromium(III) Oxide on Rainbow Trout Spermatozoa

The present study aimed to determine the cytotoxicity of chromium(III) oxide micro particles (Cr₂O₃-Ps) in rainbow trout (Oncorhynchus mykiss) spermatozoa. Firstly, Cr₂O₃-Ps were synthesized and structurally characterized the surface, morphological for particle size and thermal properties. In addition, its structural and elemental purity was determined using energy-dispersive X-ray (EDX) spectrum and elemental maps. Structural purity, thermal properties, and stability of Cr₂O₃-Ps were also examined in detail by performing thermal analysis techniques. The cytotoxicity of Cr₂O₃-Ps was measured by the observation of velocities, antioxidant activities, and DNA damages in rainbow trout spermatozoa after exposure during 3 h in vitro incubation. The straight line velocity (VSL), the curvilinear velocity (VCL), and the angular path velocity (VAP) of spermatozoa decreased after exposure to Cr₂O₃-Ps. While the superoxide dismutase (SOD) and the catalase (CAT) decreased, the lipid peroxidation increased in a dose-dependent manner. However, the total glutathione (tGSH) was not affected in this period. DNA damages were also determined in spermatozoa using Comet assay. According to DNA in tail (%) data, DNA damages have been detected with gradually increasing concentrations of Cr₂O₃-Ps. Furthermore, all of class types which are categorized as the intensity of DNA fragmentation has been observed between 50 and 500 µg/L concentrations of Cr₂O₃-Ps exposed to rainbow trout spermatozoa. At the end of this study, we determined that the effective concentrations (EC50) were 76.67 µg/L for VSL and 87.77 µg/L for VCL. Finally, these results about Cr₂O₃-Ps may say to be major risk concentrations over 70 µg/L for fish reproduction in aquatic environments.

Potential of magnetic quinoa biosorbent composite and HNO3 treated biosorbent for effective sequestration of chromium (VI) from contaminated water

The present study aims to prepare novel quinoa biosorbent (QB), acid activated QB (QB/Acid) and its nanocomposite with magnetic nanoparticles (QB/MNPs) for batch scale Cr removal from contaminated water. The Cr adsorption was systematically studied at different pH (2-9), adsorbent dosage (1-3 g/L), initial concentration (25-200 mg/L), contact time (180 min) and competing ions in water. Maximum Cr adsorption was observed onto QB/MNPs (57.4 mg/L), followed by QB/Acid (46.35 mg/g) and QB (39.9 mg/g). The Cr removal by QB/MNPs was higher than QB/Acid and QB. Results revealed that the highest Cr removal was obtained at optimum pH 4, 25 mg/L, and 2 g/L dosage. The FTIR spectra displayed various functional groups on adsorbents surface serving as a potential scaffold to remove Cr from contaminated water. The equilibrium and kinetic Cr adsorption data best fitted with Freundlich and pseudo-second order models, respectively (R² ≥ 0.96). The QB/MNPs showed excellent reusability in five adsorption/desorption cycles (4.7% decline) with minor leaching of Fe (below threshold level). The coexisting ions in groundwater showed an inhibitory effect on Cr sequestration (5%) from water. The comparison of Cr adsorption by QB/MNPs and QB/Acid showed better potential for Cr sequestration than various previously explored adsorbents in the literature.

Removal of Pb2+, CrT, and Hg2+ Ions from Aqueous Solutions Using Amino-Functionalized Magnetic Nanoparticles

In this paper, a circular economy approach with the adsorption and desorption of heavy metal (HM) ions—i.e., lead (Pb2+), chromium (CrT), and mercury (Hg2+)—from aqueous solutions was studied. Specific and selective binding of HM ions was performed on stabilized and amino-functionalized iron oxide magnetic nanoparticles (γ-Fe2O3@NH2 NPs) from an aqueous solution at pH 4 and 7. For this purpose, γ-Fe2O3@NH2 NPs were characterized by thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), specific surface area (BET), transmission electron microscopy (TEM), EDXS, and zeta potential measurements (ζ). The effects of different adsorbent amounts (mads = 20/45/90 mg) and the type of anions (NO3−, Cl−, SO42−) on adsorption efficiency were also tested. The desorption was performed with 0.1 M HNO3. The results showed improvement of adsorption efficiency for CrT, Pb2+, and Hg2+ ions at pH 7 by 45 mg of g-Fe2O3@NH2 NPs, and the sequence was as follows: CrT > Hg2+ > Pb2+, with adsorption capacities of 90.4 mg/g, 85.6 mg/g, and 83.6 mg/g, respectively. The desorption results showed the possibility for the reuse of γ-Fe2O3@NH2 NPs with HNO3, as the desorption efficiency was 100% for Hg2+ ions, 96.7% for CrT, and 91.3% for Pb2+.

TiO2-NPs and cadmium co-exposure: in vitro assessment of genetic and genomic DNA damage on Dicentrarchus labrax embryonic cells

The increased titanium dioxide nanoparticles (TiO₂-NPs) spread and their interaction with organic and inorganic pollutants arouses concern for the potential hazards for organisms and environment. This study tested in vitro the genotoxic effects of TiO₂-NPs (1 μg/mL) and cadmium (Cd) (0.1 μg/mL) co-exposure using Dicentrarchus labrax embryonic cells (DLEC) as experimental model. The genotoxicity tests (Comet assay, Diffusion Assay and Random Amplification of Polymorphic DNA (RAPD-PCR) were conducted after 3, 24 and 48 hours of exposure to TiO₂-NPs and Cd alone and in combination. The results showed that the percentage of DNA damage and apoptotic cells increases following 48 hours TiO₂-NPs exposure, while DNA instability was detected for all the times tested. Cd induced genotoxic effects starting from 3 hour-exposure and for all the treatment times. Cd + TiO₂-NPs co-exposure did not cause any genomic damage or apoptosis for all the exposure times. The possibility that Cd and TiO₂-NPs form aggregates no longer able of penetrating the nucleus and damaging the genetic material is discussed.

Efficient removal of Cr(VI) at alkaline pHs by sulfite/iodide/UV: Mechanism and modeling

Efficient removal of toxic hexavalent chromium (Cr(VI)) under alkaline conditions is still a challenge due to the relatively low reactivity of CrO₄^2-. This study proposed a new sulfite/iodide/UV process to remove Cr(VI). The removal of Cr(VI) followed pseudo-zero-order kinetics at alkaline pHs, and was enhanced by sulfite and iodide with synergy. Compared with sulfite/UV, iodide in sulfite/iodide/UV showed about 40 times higher concentration-normalized enhancement for Cr(VI) removal, and reduced the requirement of sulfite ([S(IV)]₀/[Cr(VI)]₀ of about 2.1:1) by more than 90%. The Cr(VI) removal was accelerated by decreasing pH and by increasing temperature, and was slightly influenced by dissolved oxygen, carbonate, and humic acid. The process was still effective in real surface water and industrial wastewater. Mechanism and pathways of Cr(VI) removal were revealed by quenching experiments, competition kinetic analysis, product identification and quantification, and mass and electron balance. Both e_aq^- and SO₃^•- were responsible for Cr(VI) removal, making contributions of about 75% and 25%, respectively. When e_aq^- mainly reacted with Cr(VI), SO₃^•- participated in reduction of Cr(V) and Cr(IV) intermediates, with Cr(III), S₂O₆^2-, and SO₄^2- as the final products. A model was developed to predict removal kinetics of Cr(VI), and well interpreted the roles of S(IV) and iodide in the process. This study sheds light on mechanism of Cr(VI) removal at alkaline pHs by kinetic modeling, and thus advances the applicability of this promising process for water decontamination.

Metallic nanoparticles for catalytic reduction of toxic hexavalent chromium from aqueous medium: A state-of-the-art review

The ever increasing concentration of toxic and carcinogenic hexavalent chromium (Cr (VI)) in various environmental mediums including water-bodies due to anthropogenic activities with rapid civilization and industrialization have become the major issue throughout the globe during last few decades. Therefore, developing new strategies for the treatment of Cr(VI) contaminated wastewaters are in great demand and have become a topical issue in academia and industry. To date, various techniques have been used for the remediation of Cr(VI) contaminated wastewaters including solvent extraction, adsorption, catalytic reduction, membrane filtration, biological treatment, coagulation, ion exchange and photo-catalytic reduction. Among these methods, the transformation of highly toxic Cr(VI) to benign Cr(III) catalyzed by metallic nanoparticles (M-NPs) with reductant has gained increasing attention in the past few years, and is considered to be an effective approach due to the superior catalytic performance of M-NPs. Thus, it is a timely topic to review this emerging technique for Cr(VI) reduction. Herein, recent development in synthesis of M-NPs based non-supported, supported, mono-, bi- and ternary M-NPs catalysts, their characterization and performance for the reduction of Cr(VI) to Cr(III) are reviewed. The role of supporting host to stabilize the M-NPs and leading to enhance the reduction of Cr(VI) are discussed. The Cr(VI) reduction mechanism, kinetics, and factors affecting the kinetics are overviewed to collect the wealthy kinetics data. Finally, the challenges and perspective in Cr(VI) reduction catalyzed by M-NPs are proposed. We believe that this review will assist the researchers who are working to develop novel M-NPs catalysts for the reduction of Cr(VI).

Pyrolyzed biomass-derived nanoparticles: a review of surface chemistry, contaminant mobility, and future research avenues to fill the gaps

Nanoparticles are abundant in the subsurface, soil, streams, and water bodies, and are often a critical control on elemental speciation, transport and cycling in the natural environment. This review provides an overview of pyrolyzed biomass-derived nanoparticles (PBNPs), their surface properties and reactivity towards aqueous species. We focus specifically on biochar-derived nanoparticles and activated carbon-derived nanoparticles which fall under our classification of PBNPs. Activated carbon-iron (nano)composites are included in some instances where there are significant gaps in literature because of their environmental relevance. Increased use of activated carbon, along with a resurgence in the manufacture and application of biochar for water treatment and soil amendment, has generated significant concerns about the mobility and toxicity of PBNPs derived from the bulk material in environmental applications. Recent examples are discussed to highlight current progress in understanding the influence of PBNPs on contaminant transport, followed by a critical discussion of gaps and future research directions.

A figure of merits-based performance comparison of various advanced functional nanomaterials for adsorptive removal of gaseous ammonia

The implementation of sustainable industrial development based on energy/cost-effective techniques with zero/low rate of pollutant emission is an ideal strategy for the proper management of a natural environment. Gaseous ammonia released from a variety of anthropogenic sources (e.g., agriculture, pharmaceuticals, commercial cleaning products, and refrigerant) is estimated to be as high as 150 million tons∙year^-1. To reduce the negative effects of atmospheric ammonia, the great utility of advanced functional nanomaterials (e.g., metal organic frameworks, covalent organic polymers, metal/metal oxide nanoparticles, and carbon nanostructures) has been recognized. To gain a better understanding of the sorptive removal potential of diverse materials, their performance has been evaluated based on the key performance merits (e.g., initial concentration, sorption capacity, and partition coefficient). Generally, the PC values can be applied to significantly estimate the contaminant adsorption potential of NMs via balancing the biased influences of operating parameters (e.g., initial concentration of pollutants) as perceived for the partitioning of compounds between aqueous phases at equilibrium (e.g., Henry's Law). Therefore, in this work, we have proposed the PC as a prosperous performance merit (in terms of heterogeneity of surface and strength of adsorption process) for the selection of high performance nano-adsorbents for gaseous ammonia. Moreover, the water stability, recyclability, economic aspects, and future perspectives have also been discussed for real-world applications of advanced nanomaterial against gaseous ammonia adsorption. The outcome of this evaluation will be expedient for the classification/selection of the most effectual and cost-effective options for mitigation of environmental pollutants like gaseous ammonia.

Enhanced removal of Cr(VI) by cation functionalized bamboo hydrochar

Chemical modification on hydrochars can significantly improve their ability of removing heavy metal ions from wastewater, but so far no research has focused on the chemical modification through free radical reaction. In this work, a cation functionalized hydrochar (CFHC) bearing - N⁺H₂R was synthesized by grafting-polymerization of glycidyl methacrylate (GMA) onto bamboo hydrochar under initiation by benzoyl peroxide, followed by the amination with the introduced epoxy group and diethylenetriamine and a subsequent hydrochloric acid treatment. The resulted CFHC exhibited a superior removal capacity of 424.09 mg·g^-1 for Cr(VI), and the highest sorption occurred at pH of 2. Combining a series of characterizations and tests, it was concluded that the sorption conformed to the pseudo-second-order and Freundlich equations, indicating a multilayer chemisorption process that mainly driven by electrostatic reaction, reduction, and surface complexation. This research proved that a free radical polymerization treatment could effectively transform hydrochars into super adsorbents for wastewater treatment.

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.

Impact of the choice of buffer on the electrochemical reduction of Cr(vi) in water on carbon electrodes

The nature of the buffer influences the PCET step gating Cr(vi) reduction in water at pH 4.75, as well as the extent of deposition on carbon electrodes. Electrode activity is recovered without polishing, through a simple acid wash step.

Fabrication and characterization of electrospun zein/nylon-6 (ZN6) nanofiber membrane for hexavalent chromium removal

Zein has drawn attention for its great potential for biodegradability and adsorption of hexavalent chromium Cr(VI) that is a carcinogenic industrial pollutant. Zein is a biopolymer extracted from corn and is used for many purposes, but because of its poor stability in aqueous solution, a novel composite of zein and nylon-6 was used to synthesize a nanofibrous membrane using electrospinning to improve its stability and tensile strength. The scanning electron microscope (SEM) image of the zein/nylon-6 (ZN6) nanofiber membrane showed a smooth, beadless, and continuous structure of the nanofibers, but the Fourier transform infrared (FTIR) spectrum of pristine and Cr(VI) saturated ZN6 showed that peaks of secondary amide, carbonyl, and hydroxyl functional groups were involved in adsorption. Optimized experimental parameters were obtained with pH 2.0, contact time 60 min, adsorbent dosage 25 mg, and adsorbate concentration 5.0 mg Cr-VI/mL. Experimental results show that the ZN6 nanofibers removed 87% Cr(VI) with an adsorption capacity of 4.73 mg/g at ambient temperature. Also, the Langmuir isotherm fits well, and the adsorption process followed a pseudo-2^nd-order kinetics with r² of 0.90 and 0.99 respectively.

Carbon dots derived from kanamycin sulfate with antibacterial activity and selectivity for Cr6+ detection

Among antibacterial nanomaterials, carbon dots (CDs) have attracted much attention because of their unique physical and chemical properties and good biosafety. In this study, kanamycin sulfate (Kan), a broad-spectrum antibiotic, was used to synthesize novel carbon dots (CDs-Kan) by a one-step hydrothermal method. CDs-Kan showed good inhibitory effects on Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. Further, scanning electron microscopy revealed that treatment with CDs-Kan and Kan resulted in the same phenomena. In particular, the morphologies of S. aureus cells treated with CDs-Kan and Kan became smaller and irregular, whereas the surfaces of E. coli cells protruded and formed vesicles. These results indicated that CDs-Kan was shown to retain the good antibacterial activity of Kan as well as its main bactericidal functional groups, namely, the amino sugar and amino cyclic alcohol, We refer to this phenomenon as the "preservation property". We also found that CDs-Kan has good biocompatibility and nontoxic properties. Moreover, CDs-Kan was successfully applied to the biological imaging of fungi and plant cells. In addition, CDs-Kan could be used as a fluorescent probe for the quick, sensitive, and selective detection of Cr⁶⁺. Therefore, CDs-Kan not only retained the good bacteriostatic properties of Kan but also expanded its application in bioimaging and biosensors.

Electrochemical reduction of Cr(VI) in water: lessons learned from fundamental studies and applications

Converting toxic Cr(vi) to benign Cr(iii) would offer a solution to decontaminate drinking water. Electrochemical methods are ideally suited to carry out this reduction without added external reductants. Achieving this transformation at low overpotentials requires mediating the transfer of protons and electrons to Cr(vi). In this review thermodynamic parameters will be discussed to understand Cr(vi) speciation in water and identify reduction pathways. The electrochemical reduction of Cr(vi) at bare electrodes is reviewed and mechanistic considerations are discussed. Works on modified electrodes are compared to identify key parameters influencing the reduction. An overview of current applications to Cr(vi) reduction is briefly discussed to link fundamental studies to applications.

Sequential Abatement of FeII and CrVI Water Pollution by Use of Walnut Shell-Based Adsorbents

In this study walnut shells, an inexpensive and readily available waste, were used as carbonaceous precursor for preparation of an innovative adsorbent (walnut-shell powder (WSP)) which was successfully tested for the removal of FeII from synthetic acid mine drainage (AMD). Then, the exhausted iron-contaminated adsorbent (WSP-FeII) was recovered and treated with sodium borohydride for the reduction of adsorbed FeII to Fe0. The resulting material (WSP-Fe0) was subsequently tested for the removal of CrVI from aqueous solutions. Treatability batch experiments were employed for both FeII and CrVI-contaminated solutions, and the influence of some important experimental parameters was studied. In addition, the experimental data was interpreted by applying three kinetic models and the mechanism of heavy metal removal was discussed. The overall data presented in this study indicated that fresh WSP and WSP-Fe0 can be considered as promising materials for the removal of FeII and CrVI, respectively. Furthermore, the present work clearly showed that water treatment residuals may be converted in upgraded materials, which can be successfully applied in subsequent water treatment processes. This is an example of sustainable and environmentally-friendly solution that may reduce the adverse effects associated with wastes and delay expensive disposal methods such as landfilling or incineration.

Inorganic nanoparticles for reduction of hexavalent chromium: Physicochemical aspects

Hexavalent Chromium [Cr(VI)] is a highly carcinogenic and toxic material. It is one of the major environmental contaminants in aquatic system. Its removal from aqueous medium is a subject of current research. Various technologies like adsorption, membrane filtration, solvent extraction, coagulation, biological treatment, ion exchange and chemical reduction for removal of Cr(VI) from waste water have been developed. But chemical reduction of Cr(VI) to Cr(III) has attracted a lot of interest in the past few years because, the reduction product [Cr(III)] is one of the essential nutrients for organisms. Various nanoparticles based systems have been designed for conversion of Cr(VI) into Cr(III) which have not been critically reviewed in literature. This review present recent research progress of classification, designing and characterization of various inorganic nanoparticles reported as catalysts/reductants for rapid conversion of Cr(VI) into Cr(III) in aqueous medium. Kinetics and mechanism of nanoparticles enhanced/catalyzed reduction of Cr(VI) and factors affecting the reduction process have been discussed critically. Personal future insights have been also predicted for further development in this area.

Biosorption of methylene blue and malachite green on biodegradable magnetic Cortaderia selloana flower spikes: modeling and equilibrium study

This study involves the production of a novel biosorbent obtained from Cortaderia selloana flower spikes (CSFs). Magnetic C. selloana flower spikes (nM∞CSFs) was applied as an ideal biosorbent for the elimination of dyes from water. They were utilized for the removal from aqueous solutions of malachite green (MG) and methylene blue (MB) dyes. The analyses of the equilibrium were done under certain experimental parameters such as contact time, initial dye concentration, pH, and quantity of biosorbent. The rapid intake of dyes to reach the equilibrium in a short period time showed the effectiveness of nM∞CSFs to adsorb MG and MB. The experimental information of MB and MG was obtained from the Langmuir model and it confirmed the magnificent dye biosorption ability; 72.99 mg/g for CSFs/MB, 119.05 mg/g for nM∞CSFs/MB, 31.06 mg/g for CSFs/MG, and 56.50 mg/g for nM∞CSFs/MG. Langmuir's model affirmed the excellent dye biosorption ability. The pseudo-second-order kinetic model displayed a great fit to the experimental result for the removal of MG and MB. The nM∞CSFs compared with raw biosorbent affirmed that the magnetic form of the biosorbent has a greater removal ability for MB and MG. nM∞CSFs is a noteworthy biosorbent for MB and MG removal from wastewater. [Figure: see text] HIGHLIGHTS Magnetic Cortaderia selloana flower spikes (nM∞CSFs) was synthesized for the biosorption of dyes FT-IR and SEM analysis were used for characterization. The Langmuir isotherm model fitted the data of the adsorption for nM∞CSFs nM∞CSFs is a noteworthy biosorbent for MB and MG removal from wastewater. A NOVELTY STATEMENT   This novel biodegradable biosorbent (magnetic-C. selloana flower spikes-(nM∞CSFs)) has many different functional groups to bind MG and MB from aqueous medium. The method to bring the magnetic form was well described and gives an astronomically immense capacity for the abstraction of the dyes. It resists in acidic or basic medium and has a vigorous structure. It has an immense capacity for the dyes compared to other biosorbents. It can be cited by a sizably voluminous number of investigators or researchers when it is published because it is incipient biosorbent in the literature and can be utilized as a novel biosorbent for the removal of dyes.

Genetical Surface Display of Silicatein on Yarrowia lipolytica Confers Living and Renewable Biosilica-Yeast Hybrid Materials

In this work, a biological engineering-based biosilica-yeast hybrid material was developed. It was obtained by the aggregation of Yarrowia lipolytica through biosilicification catalyzed using genetically displayed silicatein on its cell surface. With orthosilicate or seawater as the substrate, the silicatein-displayed yeast could aggregate into flocs with a flocculation efficiency of nearly 100%. The resulting floc was found to be a sheetlike biosilica-yeast hybrid material formed by the biosilica-mediated immobilization of yeast cells via cross-linking and embedding, turning the original hydrophilicity of yeast cells into hydrophobicity. In addition, this material was characterized to be porous with an average pore diameter of approximately 10 μm and porosity of over 70%. Because of these properties, this hybrid material could achieve enhanced removal efficiencies for chromium ions and n-hexadecane, which were both above 99%, as compared to the free cells of Y. lipolytica in aqueous environments. Importantly, this hybrid material could be recultivated to generate new batches of yeast cells that maintain parallel properties to the first generation so that the same hybrid material could be reproduced with unchanged highly efficient removal of chromium and n-hexadecane to those of the first generation, demonstrating that this biosilica-yeast hybrid material was living and renewable. This work presented a novel way of harnessing silicatein and Y. lipolytica to achieve biological synthesis of a living inorganic-organic hybrid material that has potential to be applied in water treatment.

Magnetic magnesium ferrite-doped multi-walled carbon nanotubes: an advanced treatment of chromium-containing wastewater

Magnetic magnesium ferrite (MgFe₂O₄) nanoparticles (MMFNPs) were synthesized by employing the sol-gel method. These nanoparticles were ultrasonically decorated onto the multi-walled carbon nanotubes (MWCNTs) to produce magnetic magnesium ferrite nanocomposites (MMFNCs). The as-prepared materials were investigated for their capability to treat wastewater loaded with heavy metals. The synthesized nanocomposites were characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transmission infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and zeta analyzer. Besides, the effect of the environmental chemistry of the solution was determined by varying the critical parameters. The adsorption isotherm of Cr(VI) adsorption onto the as-synthesized MMFNC best fitted the Langmuir adsorption isotherm model. The high adsorption capacity of 175.43 mg/g was achieved at a temperature of 40 °C under optimized conditions. Due to the magnetic nature of MMFNC, they are easily recoverable from the aqueous solution making them cost-friendly. Even after seven consecutive adsorption-desorption cycles, the MMFNC presented an efficiency loss of less than 20% for the removal of Cr(VI) ions. The presented development method offers prospects in developing a highly effective magnetic adsorbent for heavy metal removal from wastewater.

A review of chromite mining in Sukinda Valley of India: impact and potential remediation measures

Sukinda Valley, one of the highly polluted areas of the world is generating tons of mining waste and causing serious health and environmental issues in its surroundings. Several reports are available reporting the severity of hexavalent chromium, yet little efforts have been made to address the pollution and its remediation due to a lack of proper remedial measures. The review highlights the pros and cons of various physical, chemical and biological techniques used worldwide for the treatment of chromium waste and also suggests better and reliable bioremediation measures. Microbes such as Acidophilium and Acidithiobacillus caldus (Bioleaching), Pseudomonas, Micrococcus and Bacillus (Bioreduction), Aereobacterium and Saccharomyces (Biosorption), are widely used for bioremediation of hexavalent chromium owing to their unique metabolic activities, ionic movement through an extracellular membrane, and other cellular adsorptions and reduction properties. The use of native and hybrid combinations of microbes supported by organic supplements is projected as a fast and efficient technique that not only reduces chromium quantity but also maintains the integrity of the microbial sources. Innovation and emphasis on nano-based products like nanocomposite, nano adsorbent, nanoscale zerovalent iron (nZVI) particles and multifunctional plant-growth-promoting bacteria (PGPB) will serve as the next generation environmental remediation technologies in the near future.

Advances in Template Prepared Nano-Oxides and their Applications: Polluted Water Treatment, Energy, Sensing and Biomedical Drug Delivery

The nano-oxide materials with special structures prepared by template methods have a good dispersion, regular structures and high specific surface areas. Therefore, in some areas, improved properties are observed than conventional bulk oxide materials. For example, in the treatment of dye wastewater, the treatment efficiency of adsorbents and catalytic materials prepared by template method was about 30 % or even higher than that of conventional samples. This review mainly focuses on the progress of inorganic, organic and biological templates in the preparation of micro- and nano- oxide materials with special morphologies, and the roles of the prepared materials as adsorbents and photocatalysts in dye wastewater treatment. The characteristics and advantages of inorganic, organic and biological template are also summarized. In addition, the applications of template method prepared oxides in the field of sensors, drug carrier, energy materials and other fields are briefly discussed with detailed examples.

Advances in the application, toxicity and degradation of carbon nanomaterials in environment: A review

Carbon nanomaterials (CNMs) are novel nanomaterials with excellent physicochemical properties, which are widely used in biomedicine, energy and sensing. Besides, CNMs also play an important role in environmental pollution control, which can absorb heavy metals, antibiotics and harmful gases. However, CNMs are inevitably entering the environment while they are rapidly developing. They are harmful to living organisms in the environment and are difficult to degrade under natural conditions. Here, we systematically describe the toxicity of carbon nanotubes (CNTs), graphene (GRA) and C₆₀ to cells, animals, humans, and microorganisms. According to the current research results, the toxicity mechanism is summarized, including oxidative stress response, mechanical damage and effects on biological enzymes. In addition, according to the latest research progress, we focus on the two major degradation methods of chemical degradation and biodegradation of CNTs, GRA and C₆₀. Meanwhile, the reaction conditions and degradation mechanisms of degradation are respectively stated. Moreover, we have prospects for the limitations of CNM degradation under non-experimental conditions and their potential application.