Selective removal of toxic Cr(VI) from aqueous solution by adsorption combined with reduction at a magnetic nanocomposite surface

Remediation of Cr(VI)-contaminated soil by CS/PPy coupling with Microbacterium sp. YL3

In this research, a new material, chitosan/polypyrrole (CS/PPy), was synthesized and linked with the Cr(VI)-reducing bacterial strain YL3 to treat Cr(VI)-polluted soil. The findings demonstrated that the synergistic application of strain YL3 and CS/PPy achieved the greatest reduction (99.6 %). During the remediation process, CS/PPy served as a mass-storage and sustained release agent in the soil, which initially decreased the toxic effects of high concentrations of Cr(VI) on strain YL3, thereby enhancing the Cr(VI) reduction efficiency of strain YL3. These combined effects significantly mitigated Cr(VI) stress in the soil and restored enzyme activities. Furthermore, wheat growth in the treated soil also significantly improved. High-throughput sequencing of the microorganisms in the treated soil revealed that CS/PPy was not only effective at removing Cr(VI) but also at preserving the original microbial diversity of the soil. This suggests that the combined treatment using strain YL3 and CS/PPy could rehabilitate Cr(VI)-contaminated soil, positioning CS/PPy as a promising composite material for future bioremediation efforts in Cr(VI)-contaminated soils.

Selective Adsorption of Hazardous Substances from Wastewater by Hierarchical Oxide Composites: A Review

Large volumes of wastewater containing toxic contaminants (e.g., heavy metal ions, organic dyes, etc.) are produced from industrial processes including electroplating, mining, petroleum exploitation, metal smelting, etc., and proper treatment prior to their discharge is mandatory in order to alleviate the impacts on aquatic ecosystems. Adsorption is one of the most effective and practical methods for removing toxic substances from wastewater due to its simplicity, flexibility, and economics. Recently, hierarchical oxide composites with diverse morphologies at the micro/nanometer scale, and the combination advantages of oxides and composite components have been received wide concern in the field of adsorption due to their multi-level structures, easy functionalization characteristic resulting in their large transport passages, high surface areas, full exposure of active sites, and good stability. This review summarizes the recent progress on their typical preparation methods, mainly including the hydrothermal/solvothermal method, coprecipitation method, template method, polymerization method, etc., in the field of selective adsorption and competitive adsorption of hazardous substances from wastewater. Their formation processes and different selective adsorption mechanisms, mainly including molecular/ion imprinting technology, surface charge effect, hard-soft acid-base theory, synergistic effect, and special functionalization, were critically reviewed. The key to hierarchical oxide composites research in the future is the development of facile, repeatable, efficient, and scale preparation methods and their dynamic adsorption with excellent cyclic regeneration adsorption performance instead of static adsorption for actual wastewater. This review is beneficial to broaden a new horizon for rational design and preparation of hierarchical oxide materials with selective adsorption of hazardous substances for wastewater treatment.

Rooibos tea waste binary oxide composite: An adsorbent for the removal of nickel ions and an efficient photocatalyst for the degradation of ciprofloxacin

In this study, rooibos tea waste (RTW) incorporated with a binary oxide (BO; Fe2O3-SnO2) has been reported for the first time as a highly efficient adsorbent material for the elimination of Ni(II) ions. The as-synthesised rooibos tea waste-binary oxide (RWBO) composite adsorbent was characterised using miscellaneous techniques such as FTIR, XRD, SEM, EDX, TGA, BET, and XPS. The RWBO was then tested for the removal of Ni(II) in a batch adsorption experiment. The composite adsorbent showed a great removal efficiency of about 99.75% for Ni(II) ions at 45 °C, 180 min agitation time, pH 7, and dosage of 250 mg. The adsorption process was found to be endothermic and spontaneous. Also, the spent adsorbent [RWBO-Ni(II)] was found to be solar light active with a narrow band gap of 1.4 eV. It was further used as a photocatalyst for the photocatalytic abatement of 10 mg/L ciprofloxacin with an extent of degradation of 83% obtained after 150 min. In addition, the extent of mineralisation of the ciprofloxacin by the spent adsorbent as obtained from the TOC data was found to be 64%. Overall, the RWBO composite adsorbent lends itself as an efficient, eco-friendly and promising material for environmental remediation.

Hyperbranched-Polyethylenimine-Functionalized Coal Fly Ash as an Adsorbent for the Removal of Hexavalent Chromium and Reuse as a Dye Photocatalyst

Coal fly ash (CFA) has been extensively researched as an adsorbent for heavy metals, but its application is limited by its low adsorption capacity. The modification of CFA with hyperbranched polymers results in improved adsorption capacities. Hyperbranched polyethylenimine (HPEI) is a hyperbranched polymer containing NH2 groups that can bind with heavy metal ions through complexation or electrostatic interactions. In this study, CFA-HPEI adsorbents with various HPEI loadings (1-5%) were prepared and evaluated for the removal of Cr(VI). The successful incorporation of HPEI onto CFA was confirmed using Fourier transform infrared, elemental analysis, and X-ray photoelectron spectroscopy (XPS). The 3% CFA-HPEI loaded adsorbent resulted in optimum results when the effect of pH and adsorbent dosage was studied. The pseudo-second-order kinetics model best described the adsorption kinetics at an initial concentration of 20 mg/L. The Freundlich adsorption isotherm model best fitted the equilibrium adsorption data with a maximum adsorption capacity of 85.93 mg/g. The Cr-loaded adsorbent was reused as a photocatalyst to degrade methylene blue (MB) in the presence of visible light. The loaded adsorbent degraded 98.9% of MB (5 mg/L) within 180 min and was accompanied by compounds with m/z of 173 and 234, corresponding to the intermediate degradation of Azure A. The XPS analysis confirmed the coexistence of Cr(III) and Cr(VI) on the surface of the adsorbent. In addition, the loaded adsorbent exhibited good stability following MB degradation with no structural changes observed. Thus, CFA-HPEI adsorbents can be utilized as low-cost adsorbents for the remediation of toxic Cr(VI) from water and wastewater. The Cr-loaded CFA-HPEI adsorbent can be effectively reused as a photocatalyst, thus reducing environmental pollution.

Polypyrrole Aerogels: Efficient Adsorbents of Cr(VI) Ions from Aqueous Solutions

Three-dimensional and porous polypyrrole (PPy) aerogels were prepared using a facile two-step procedure in which cryogels were synthesized via the cryopolymerization of pyrrole with iron (III) chloride in the presence of supporting water-soluble polymers (poly(N-vinylpyrrolidone), poly(vinyl alcohol), gelatin, methylcellulose or hydroxypropylcellulose), followed by freeze-drying to obtain aerogels. The choice of supporting polymers was found to affect the morphology, porosity, electrical conductivity, and mechanical properties of PPy aerogels. PPy aerogels were successfully used as adsorbents to remove toxic Cr(VI) ions from aqueous solutions.

Fe-N-Doped Conjugated Organic Polymer Efficiently Enhanced the Removal Rate of Cr(VI) from Water

A Fe-N conjugated organic polymer (SMP-Fr-Py) was prepared from ferrocene and pyrrole using a Scholl coupling reaction, which significantly improved the performance of Cr(VI) removal compared to the polymer (HCP-Fr-Py) prepared by adding the cross-linker formaldehyde dimethyl acetal (FDA). The results showed that at a pH of 2 and at 25 °C, the removal of Cr(VI) reached 90% for SMP-Fr-Py and only 58% for HCP-Fr-Py after 20 min of reaction. Subsequently, 99% and 78% were achieved after 120 min of reaction, respectively. The test results showed that the removal reaction followed a pseudo-second-order kinetic model. The removal efficiency decreased with increasing solution pH and initial Cr(VI) concentration, but increased with increasing SMP-Fr-Py dosage, reaching three cycles. The characterization of the reaction complexes and measurements of Cr species conversion revealed the near absence of Cr(VI) species in the solution. Approximately 38% of Cr(VI) was found to be adsorbed on the material surface, with another fraction present in solution (24%) and on the material surface (38%) in the form of Cr(III). The overall study showed that the direct connection of ferrocene and pyrrole in SMP-Fr-Py through C-C bonding increased the conjugated structure of the polymer backbone, which facilitated electron transfer and transport. Furthermore, the Fe-N elements worked synergistically with each other more easily, which improved the removal performance of Cr(VI) and provided a reference for the subsequent work.

Characterization and synthesis of new adsorbents with some natural waste materials for the purification of aqueous solutions

In this study, hexavalent chromium Removal from aqueous environments was investigated by using polyaniline composites with some natural waste materials. Batch experiments were used, and some parameters such as contact time, pH and adsorption isotherms were determined for the best composite with the highest removal efficiency. Scanning Electron Microscope (SEM), Fourier Transform Infrared (FTIR) spectroscopy, and X-ray Diffraction (XRD) were used to characterize the composites. According to the results, the polyaniline/walnut shell charcoal/PEG composite outperformed other composites and showed the highest chromium removal efficiency of 79.22%. Polyaniline/walnut shell charcoal/PEG has a larger specific surface area of 9.291 (m²/gr) which leads to an increase in its removal efficiency. For this composite, the highest removal efficiency was obtained at the pH = 2 and 30 min contact time. The maximum calculated adsorption capacity was 500 mg/g.

Coal Fly Ash Decorated with Graphene and Polyaniline Nanocomposites for Effective Adsorption of Hexavalent Chromium and Its Reuse for Photocatalysis

Coal fly ash was functionalized and modified with graphene oxide and polyaniline (CFA/GO/PANI nanocomposite) through hydrothermal synthesis, which was used for remediation of hexavalent chromium (Cr(VI)) ions. Batch adsorption experiments were carried out to investigate the effects of adsorbent dosage, pH, and contact time on the removal of Cr(VI). The ideal pH for this work was 2, and it was used for all other studies. The Cr(VI)-loaded spent adsorbent CFA/GO/PANI + Cr(VI) was reused as a photocatalyst for the degradation of bisphenol A (BPA). The CFA/GO/PANI nanocomposite removed Cr(VI) ions rapidly. The adsorption process was best described by pseudo-second-order kinetics and the Freundlich isotherm model. The CFA/GO/PANI nanocomposite demonstrated a high adsorption capacity of 124.72 mg/g for Cr(VI) removal. In addition, the Cr(VI)-loaded spent adsorbent played a significant role in the photocatalytic degradation of BPA with 86% degradation. The reuse of the Cr(VI)-loaded spent adsorbent as a photocatalyst presents a new solution for the reduction of secondary waste from the adsorption process.

A review of novel green adsorbents as a sustainable alternative for the remediation of chromium (VI) from water environments

The presence of heavy metal, chromium (VI), in water environments leads to various diseases in humans, such as cancer, lung tumors, and allergies. This review comparatively examines the use of several adsorbents, such as biosorbents, activated carbon, nanocomposites, and polyaniline (PANI), in terms of the operational parameters (initial chromium (VI) concentration (C_o), temperature (T), pH, contact time (t), and adsorbent dosage) to achieve the Langmuir's maximum adsorption capacity (q_m) for chromium (VI) adsorption. The study finds that the use of biosorbents (fruit bio-composite, fungus, leave, and oak bark char), activated carbons (HCl-treated dry fruit waste, polyethyleneimine (PEI) and potassium hydroxide (KOH) PEI-KOH alkali-treated rice waste-derived biochar, and KOH/hydrochloric acid (HCl) acid/base-treated commercial), iron-based nanocomposites, magnetic manganese-multiwalled carbon nanotubes nanocomposites, copper-based nanocomposites, graphene oxide functionalized amino acid, and PANI functionalized transition metal are effective in achieving high Langmuir's maximum adsorption capacity (q_m) for chromium (VI) adsorption, and that operational parameters such as initial concentration, temperature, pH, contact time, and adsorbent dosage significantly affect the Langmuir's maximum adsorption capacity (q_m). Magnetic graphene oxide functionalized amino acid showed the highest experimental and pseudo-second-order kinetic model equilibrium adsorption capacities. The iron oxide functionalized calcium carbonate (IO@CaCO₃) nanocomposites showed the highest heterogeneous adsorption capacity. Additionally, Syzygium cumini bark biosorbent is highly effective in treating tannery industrial wastewater with high levels of chromium (VI).

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.

Multifunctional interfaces for multiple uses: Tin(II)-hydroxyapatite for reductive adsorption of Cr(VI) and its upcycling into catalyst for air protection reactions

Evidence collected to date by our group has demonstrated that tin(II)-functionalized hydroxyapatites (Sn/HAP) are a newly discovered class of ecofriendly reductive adsorbents for Cr(VI) removal from wastewaters. In this work an upgraded series of Sn/HAP materials assured a maximum removal capacity of ≈ 20 mgCr/g, doubling the previously reported value for Sn/HAP materials, thanks to higher Sn-dispersion as proved by X-ray photoelectron spectroscopy and electron microscopy. Insights on kinetics and thermodynamics of the reductive adsorption process are provided and the influence of pH, dosage, and nature of Cr(VI) precursors on chromium removal performances have been investigated. Pseudo-second-order kinetics described the interfacial reductive adsorption process on Sn/HAP, characterized by low activation energy (21 kJ mol-1), when measured in the 278-318 K range. Tests performed in the 2-6 pH interval showed similar efficiency in terms of Cr(VI) removal. Conventional procedures of recycling and regeneration resulted ineffective in restoring the pristine performances of the samples due to surface presence of both Sn(IV) and Cr(III). To overcome these weaknesses, the used samples (Sn + Cr/HAP) were upcycled into catalysts in a circular economy perspective. Used samples were tested as catalysts in gas-phase catalytic processes for air pollution remediation: selective catalytic reduction of NOx (NH3-SCR), NH3 selective catalytic Oxidation (NH3-SCO), and selective catalytic oxidation of methane to CO2. Catalytic tests enlightened the interesting activity of the upcycled Sn + Cr/HAP samples in catalytic oxidation processes, being able to selectively oxidize methane to CO2 at relatively low temperature.

Advances in Sorptive Removal of Hexavalent Chromium (Cr(VI)) in Aqueous Solutions Using Polymeric Materials

Sorptive removal of hexavalent chromium (Cr(VI)) bears the advantages of simple operation and easy construction. Customized polymeric materials are the attracting adsorbents due to their selectivity, chemical and mechanical stabilities. The mostly investigated polymeric materials for removing Cr(VI) were reviewed in this work. Assembling of robust functional groups, reduction of self-aggregation, and enhancement of stability and mechanical strength, were the general strategies to improve the performance of polymeric adsorbents. The maximum adsorption capacities of these polymers toward Cr(VI) fitted by Langmuir isotherm model ranged from 3.2 to 1185 mg/g. Mechanisms of complexation, chelation, reduction, electrostatic attraction, anion exchange, and hydrogen bonding were involved in the Cr(VI) removal. Influence factors on Cr(VI) removal were itemized. Polymeric adsorbents performed much better in the strong acidic pH range (e.g., pH 2.0) and at higher initial Cr(VI) concentrations. The adsorption of Cr(VI) was an endothermic reaction, and higher reaction temperature favored more robust adsorption. Anions inhibited the removal of Cr(VI) through competitive adsorption, while that was barely affected by cations. Factors that affected the regeneration of these adsorbents were summarized. To realize the goal of industrial application and environmental protection, removal of the Cr(VI) accompanied by its detoxication through reduction is highly encouraged. Moreover, development of adsorbents with strong regeneration ability and low cost, which are robust for removing Cr(VI) at trace levels and a wider pH range, should also be an eternally immutable subject in the future. Work done will be helpful for developing more robust polymeric adsorbents and for promoting the treatment of Cr(VI)-containing wastewater.

Flexible and easy-handling pristine polypyrrole membranes with bayberry-like vesicle structure for enhanced Cr(VI) removal from aqueous solution

Polypyrrole has been extensively explored for Cr(VI) removal from wastewater towing to the advantages of superior performance, low cost, facile synthesis, and high environmental stability. However, the unsatisfactory adsorption capacity and complicated process of adsorbent separation from aqueous solutions remain a huge challenge, limiting its practical application. Herein, a flexible PPy membrane with bayberry-like vesicle structures (PPy-B) was prepared via template-assisted interfacial polymerization. It was found that sodium sulfosalicylate not only improved the flexibility and strength of the PPy-B membrane for easy-handling but also participated in the polymerization of PPy as a dopant to improve the specific surface area and doping level for increasing adsorption sites. Benefiting from these, the easy-handling PPy-B membrane exhibited a high adsorption capacity (586.90-682.50 mg/g at 298-318 K), a high reusability (five adsorption-desorption cycles), and a high ultimate adsorption capacity after adsorption-desorption cycles until membrane failure (1174.86 mg/g at 298 K). The proposed mechanisms of the enhanced Cr(VI) removal involve electrostatic adsorption, reduction, and ion exchange. This flexible PPy membrane therefore shows attractive advantages in wastewater treatment.

Polypyrrole-coated Boron-doped Nickel-Cobalt sulfide on electrospinning carbon nanofibers for high performance asymmetric supercapacitors

Although nickel-cobalt bimetallic sulfides have been widely studied for supercapacitor electrodes, how to obtain high specific capacity and cycle stability is still an important challenge. Here, an efficient chemical redox method is used to adjust the crystal and electronic structure of cobalt-nickel sulfide (NCS) via B doping, combined with electrospinning technology and conductive polymer polypyrrole (PPy) coating to facilitate faraday redox reactions and obtain high energy density electrode materials. The resulting composite with boron-doped nickel-cobalt sulfide on electrospinned carbon nanofibers with polypyrrole-coating (PPy@B-NCS/CNF) has a high specific capacity (751.61C/g at 1 A/g) and good cycle stability (82.49 % retention after 4000 cycles at 5 A/g). With PPy@B-NCS/CNF as the positive electrode and activated carbon as the negative electrode, an asymmetric supercapacitor (ASC) is prepared. It has excellent electrochemical properties with a power density of 65.58 Wh kg^-1 and an energy density of 819.72 W kg^-1. The low-temperature performance test shows high reversibility, which provides the possibility for the development of low-temperature electrolytes. Finally, density functional theory (DFT) explains that B-doped NCS has better electrochemical properties from the energy band and state density.

Enhanced sorption and reduction of Cr(VI) by the flowerlike nanocomposites combined with molybdenum disulphide and polypyrrole

Developing high-performance adsorbent for hexavalent chromium (Cr(VI)) elimination presents an enticing prospect in environmental remediation. Herein, three-dimensional flowerlike nanospheres composed of molybdenum disulphide and polypyrrole (MoS₂@PPy) were successfully prepared via a one-pot hydrothermal and subsequent carbothermal reduction process for the removal of Cr(VI). The effects of pH, adsorbent dosage, co-existing ions, initial Cr(VI) concentration and temperature were investigated systematically by batch experiments. Benefiting from the incorporation of MoS₂, the obtained MoS₂@PPy composites showed a dramatic increase of specific surface area (149.82 m²·g^-1) and adsorption capacity (230.97 mg·g^-1) when compared with the pure PPy nanoparticles. Based on the thermodynamics study and X-ray photoelectron spectroscopy analyses, the removal process of Cr(VI) was proved to be exothermic and spontaneous, and accessible under-coordinated Mo(IV) and pyrrolic N groups coupled with redox reactions were conducive to the efficient removal of Cr(VI). Attractively, the MoS₂@PPy acted as the electron donor could also activate peroxymonosulphate for the efficient degradation of organic contaminants. These results suggested that the MoS₂@PPy was promising in Cr(VI) elimination and other kinds of organic pollutants removal in wastewater.

Adsorption and reduction from modified polypyrrole enhance electrokinetic remediation of hexavalent chromium-contaminated soil

Toxic metal pollutant Cr(VI) in the environment will pose a severe threat to animal and human health. In this work, Fe₃O₄@PPy, Arg@PPy, and Arg/Fe₃O₄@PPy were prepared to enhance adsorption of Cr(VI) by doping Fe₃O₄ nanoparticles and amino radicals into the original PPy structure. Their characteristics were investigated by FTIR, SEM, EDS, BET analysis, and batch adsorption experiments. And they were used as permeable reaction barriers (PRB) to combine with electrokinetic remediation (EKR) to remediate Cr-contaminated soil. Adsorption experiment results showed that the maximum adsorption capacities of PPy, Fe₃O₄@PPy, Arg@PPy, and Arg/Fe₃O₄@PPy for Cr(VI) were 60.43 mg/g, 67.12 mg/g, 159.86 mg/g, and 141.50 mg/g, respectively. They all followed the kinetic pseudo-second-order model and the Langmuir isothermal model with a monolayer adsorption behavior. In the EKR/PRB system, the presence of Fe₃O₄@PPy, Arg@PPy, and Arg/Fe₃O₄@PPy obtained the higher Cr(VI) removal efficiency near the anode than that of the PPy, increasing by 74.60%, 26.04%, and 68.64%, respectively. A strong electrostatic attraction between anion contaminants and protonated modified PPy and a reduction from Cr(VI) to Cr(III) appeared in the EKR remediation process under acid conditions. This study opened up a prospect for applying modified PPy composites to treat toxic metal-contaminated soil.

A facile modification of cation exchange resin by nano-sized goethite for enhanced Cr(VI) removal from water

A novel macroporous strong acidic cation exchange resin (D001) modified by nano-sized goethite (nFeOOH@D001) was fabricated by using a facile ethanol dispersion and impregnation method, and its efficiency for Cr(VI) removal was tested thereafter. Due to the dispersing effect of ethanol, FeOOH particles of 20-150 nm were coated on the D001 surfaces. The nFeOOH@D001 obtained a Cr(VI) removal efficiency and capacity of 80.2% and 7.4 mg/g respectively, 5 times and 8 times higher than that of the pristine D001. The Cr(VI) removal by nFeOOH@D001 followed the pseudo second-order kinetics and the Langmuir adsorption model. Column experiments also demonstrated that the nFeOOH@D001 exhibited a much better ability to remove Cr(VI) as compared to the D001. Additionally, the nFeOOH@D001 showed a potential for reusability and renewability. The adsorbed nFeOOH@D001 could be easily desorbed by 0.1 M acetic acid and a reuse efficiency of 92.7% could be maintained after 4 desorption-adsorption cycles. The used nFeOOH@D001 could be eluted by 0.1 M HCl to remove nFeOOH, and the renewed D001 could be recoated by nFeOOH and achieved a regeneration rate of 97.8% for Cr(VI) removal. The above results indicated that nano-sized goethite modification is a promising method to endow D001 with the ability to remove Cr(VI) from water.

Polypyrrole-based adsorbents for Cr(VI) ions remediation from aqueous solution: a review

Anthropogenic activities are principally responsible for the manifestation of toxic and carcinogenic hexavalent chromium (Cr(VI)) triggering water pollution that threatens the environment and human health. The World Health Organisation (WHO) restricts Cr(VI) ion concentration to 0.1 and 0.05 mg/L in inland surface water and drinking water, respectively. The available technologies for Cr(VI) ion removal from water were highlighted with an emphasis on the adsorption technology. Furthermore, the characteristics of several polypyrrole-based adsorbents were scrutinized including amino-containing compounds, biosorbents, graphene/graphene oxide, clay materials and many other additives with reported effective Cr(VI) ion uptake. This efficiency in Cr(VI) ions adsorption is attributed to enhanced redox properties, increased number of functional groups as well as the synergistic behaviour of the materials making up the composites. The Langmuir isotherm best described the adsorption processes with maximum adsorption capacities ranging from 3.40-961.50 mg/g. The regeneration of Cr(VI) ion-laden adsorbents was studied. Ion exchange, electrostatic attractions, complexation, chelation reactions with protonated sites and reduction were the mechanisms of adsorption. Nevertheless, there are limited details on comprehensive adsorbent regeneration studies to prolong robustness in adsorption-desorption cycles and utilization of the Cr(VI) ion-laden adsorbent in other areas of research to limit the threat of secondary pollution.

Morphology and structure of in situ FeS affect Cr(VI) removal by sulfidated microscale zero-valent iron with short-term ultrasonication

The properties of sulfidated zero-valent iron (S-ZVI) are considered to be determined by the entire structure of Fe⁰ and Fe_xS_y as a whole, but few studies focus on the influence of the morphology and structure of the external Fe_xS_y layer on the performance of S-ZVI. In this study, after the sulfidation of microscale ZVI in acetate (HAc-NaAc) and 2-(N-morpholino) ethanesulfonic acid (MES) buffer solution, the S-mZVI^HAc-NaAc surface presented the in situ growth of the FeS nanosheet, while the S-mZVI^MES surface was dominated by agglomerated FeS sub-micron particles. Under short-term ultrasonication, S-mZVI^HAc-NaAc was superior to removing Cr(VI) than S-mZVI^MES, and the clearance of the passivation layer by ultrasound maximized the conductivity of the FeS nanosheet to strengthen the sulfidation contribution. However, agglomerated FeS particles were easily separated from S-mZVI^MES by ultrasonication, resulting in the suppression of its sulfidation contribution. The removal of Cr(VI) by S-ZVI increased linearly with FeS content, and the chemical combination of FeS with ZVI had more significant synergy than their physical mixture. The FeS nanosheet with excellent conductivity and large vertical space benefited the generation of dissolved and surface-associated Fe(II) as electron donors and structural Fe(II) as the electron shuttle. Understanding the relationship between FeS structure and S-ZVI performance will pave a way for optimizing the synthesis of S-ZVI.

High-performance flexible supercapacitor enabled by Polypyrrole-coated NiCoP@CNT electrode for wearable devices

As a pseudocapacitive electrode material, nickel-cobalt bimetallic phosphide has attracted wide attention with its advantage in capacitance and chemical activity. While, like Ni-Co oxides or sulfides, the application of nickel-cobalt bimetallic phosphide is generally hampered by its confined conductivity, low chemical stability and unsatisfactory cycle durability. Herein, this work demonstrates a NiCoP@CNT@PPy (NCP@CNT@PPy) composite that is obtained by polymerizing pyrrole monomer on the surface of NiCoP@CNT complex. According to density functional theory (DFT), it is theoretically demonstrated that the bimetallic Ni-Co phosphide (NiCoP) can exhibit more electrons near the Fermi level than single Ni or Co phosphide. Under the combined effects of carboxylic carbon nanotubes (c-CNTs) and polypyrrole (PPy), the NCP@CNT@PPy electrode exhibits excellent electrochemical performance. In addition, a flexible asymmetric supercapacitor (ASC) is prepared, which demonstrated high energy density and admirable heat-resistance and flexibility performance, showing huge potential in the application of heat-resistant storage energy systems and portable wearable devices.

Preparation of MES@Fe3O4@SiO2-PPy magnetic microspheres for the highly efficient removal of Cr(vi)

A novel magnetic adsorbent, PPy-modified silica-coated magnetic MES organic–inorganic composite (MFSP), with high dispersibility, abundant adsorption sites, and magnetic separation was prepared successfully for the adsorption or reduction of Cr(vi).

Synthesis of Magnetic Short-Channel Mesoporous Silica SBA-15 Modified with a Polypyrrole/Polyaniline Copolymer for the Removal of Mercury Ions from Aqueous Solution

A novel magnetic short-channel mesoporous silica SBA-15 composite adsorbent was prepared by the copolymerization of pyrrole and aniline. The prepared novel nanoadsorbent polypyrrole-polyaniline/CoFe2O4-SBA-15 (PPy-PANI/M-SBA-15) has a significant adsorption effect on heavy metal mercury ions. The batch adsorption experiment was carried out to study the effects of various parameters including solution pH, initial concentration (C 0), adsorbent dose (dosage), temperature (T), and contact time on the adsorption effect. The analysis results of the response surface method (RSM) and central composite design (CCD) show that the importance for adsorption factors is pH > C 0 > T > dosage, and the maximum capacity of PPy-PANI/M-SBA-15 is 346.2 mg/g under the optimal conditions of pH = 6.7, T = 310 K, C 0 = 29.5 mg/L, and a dosage of 0.044 g/L. The pseudo-second-order kinetic model and the Langmuir isotherm model simulate the adsorption behavior of mercury ions. In addition, thermodynamic parameters indicate self-heating and reversible adsorption processes. A covalent bond is formed between the nitrogen-containing functional group and the mercury ions. Excellent magnetic properties and high reproducibility indicate that PPy-PANI/M-SBA-15 has excellent recyclability and environmentally friendly properties and can become a potential heavy metal ion adsorbent in practical applications.

Investigating the selectivity and interference behavior for detoxification of Cr(VI) using lanthanum phosphate polyaniline nanocomposite via adsorption-reduction mechanism

A novel Lanthanum phosphate polyaniline (LaPO₄-PANI) nanocomposite was synthesized by the simple sol-gel technique. The nanocomposite prepared at 1:1 ratio provided the highest ion exchange capacity and selective adsorption of Cr(VI). The phase composition and particle morphology of the as-prepared material was evaluated by XRD, FESEM and TEM analyses. The FTIR, Raman, and TGA data inferred the definite chemical interaction between the organic and inorganic counterparts in the formation of LaPO₄-PANI. The selective adsorption of Cr(VI) was estimated by evaluating the distribution coefficient, electrical double layer theory as well as valency and Pauling's ionic radii of interfering ions (phosphate, iodide, sulfate, chloride, sulfide). The high tolerance capability of LaPO₄-PANI against the interfering ions made it appropriate for selective and efficient removal of Cr(VI) ions from solutions. The nanocomposite showed the highest removal percentage of 98.6% towards Cr(VI) in a wide pH range of 2-6 at room temperature, as compared to sole lanthanum phosphate (56%) and polyaniline (75%). The XPS analysis revealed the adsorption mechanism due to the combined effect of both adsorption and reduction. Cr(VI) is adsorbed through electrostatic interactions while the = N-/-NH- group facilitated the in situ chemical reduction. The procured results make the LaPO₄-PANI nanocomposite a promising adsorbent for the removal of Cr(VI).

Adsorption characteristics of Cr(VI) on microalgae immobilized by different carriers

To solve the problem of harvesting microalgae during heavy metal adsorption, six different carriers were selected in this study to compare the adsorption behavior of microalgae after immobilization. The results of the scanning electron microscope (SEM) and adsorption showed chitosan as a carrier showed the best immobilization effect and adsorption advantages after immobilizing microalgae. The optimal immobilized carrier-chitosan was obtained under the following conditions of chitosan: acetic acid (2:40), microalgae concentration (10⁸ cells mL^-1), and immobilization time (18 h). The optimal adsorption conditions were as follows: temperature: 30 °C, pH: 7.0, adsorption dose: 1.5 g L^-1, initial ion concentration: 40 mg L^-1. The adsorption capacity of metal ions can reach 37.1 mg g^-1 Cr(VI), 25.98 mg g^-1 Cu(II), 25.06 mg g^-1 Pb(II), and 24.62 mg g^-1 Cd(II), respectively. The desorption efficiency in 0.5 mol L^-1 NaOH desorption solution reached 90.01%. After five adsorption-desorption cycles, excluding chitosan (∼70%), the adsorption efficiency of other adsorbents decreased with an increase in the recycling times. Chitosan was a suitable carrier for the immobilization of Synechocystis sp. PCC6803. Fourier transform infrared spectroscopy and Raman spectra analysis showed that groups belonging to the microalgae were detected after the microalgae in different carriers, indicating that the microalgae were immobilized with the carriers. At the same time, the energy spectrum changed before and after adsorption indicated the specific functional groups of microalgae played an important role in the adsorption process. The kinetic and isothermal model data showed that the adsorption process was mainly chemical adsorption and homogeneous monolayer adsorption. Moreover, X-ray diffraction showed the interlayer peak strength decreased significantly, indicating that the interlayer structure was stretched after Cr(VI) ion exchange. X-ray photoelectron spectroscopy analysis showed that the Cr adsorption process involves the reduction of Cr(VI) to Cr(III).

Removal of Cr (VI) by Biochar Derived from Six Kinds of Garden Wastes: Isotherms and Kinetics

Garden waste is one of the main components of urban solid waste which affects the urban environment. In this study, garden waste of Morus alba L. (SS), Ulmus pumila L. (BY), Salix matsudana Koidz (LS), Populus tomentosa (YS), Sophora japonica Linn (GH) and Platycladus orientalis (L.) Franco (CB) was pyrolyzed at 300 °C, 500 °C, 700 °C to obtain different types of biochar, coded as SSB300, SSB500, SSB700, BYB300, etc., which were tested for their Cr (VI) adsorption capacity. The results demonstrated that the removal efficiency of Cr by biochar pyrolyzed from multiple raw materials at different temperatures was variable, and the pH had a great influence on the adsorption capacity and removal efficiency. GHB700 had the best removal efficiency (89.44%) at a pH of 2 of the solution containing Cr (VI). The pseudo second-order kinetics model showed that Cr (VI) adsorption by biochar was chemisorption. The Langmuir model showed that the adsorption capacity of SSB300 was the largest (51.39 mg·g-1), BYB500 was 40.91 mg·g-1, GHB700, CBB700, LSB700, YSB700 were 36.85 mg·g-1, 36.54 mg·g-1, 34.53 mg·g-1 and 32.66 mg·g-1, respectively. This research, for the first time, used a variety of garden wastes to prepare biochar, and explored the corresponding raw material and pyrolysis temperature for the treatment of Cr (VI). It is hoped to provide a theoretical basis for the research and utilization of garden wastes and the production and application of biochar.

Magnetic graphene-based nanocomposites as highly efficient absorbents for Cr(VI) removal from wastewater

Due to the merits of their high adsorption and convenient separation, magnetic graphene-based composites have become a promising adsorbent in terms of wastewater treatment. However, recycling and regeneration properties of magnetic graphene-based composites are still a conundrum, which remains to be resolved. Here, Fe₃O₄/reduced graphene oxide (RGO) (Fe₃O₄/RGO) nanocomposites were synthesized by one-step solvent-thermal reduction route and used as adsorbents for water purification. It was encouraging to find that the nanocomposites possessed many intriguing properties in removing of Cr(VI) ions, including high adsorption efficiency and excellent recycling and regeneration property. The results indicated that the magnetic separation process of the Fe₃O₄/RGO nanocomposites only took less than 5 s and the maximum removal efficiency of Cr(VI) reached 99.9% under the optimum experimental conditions. Most significantly, the adsorption rate of Cr(VI) can still be as high as 98.13% after 10 cycles and the single recycle quality of the nanocomposites can maintain at more than 80%. As a result, the Fe₃O₄/RGO nanocomposites could be a potential adsorbent for removing heavy metal ions effectively, especially in environmental protection and restoration.

Effect of hexavalent chromium on the environment and removal techniques: A review

Despite the importance of chromium (Cr) in most anthropogenic activities, the subsequent environmental adulteration is now a source of major concern. Cr occurs in numerous oxidation states, with the furthermost stable and frequently occur states being Cr(0), Cr(III) and Cr(VI). Cr(0) and Cr(III) are vital trace elements while Cr(VI) is dispensable and noxious to living organisms. Predominantly in plants, Cr at low concentrations of about 0.05-1 mg/L assist to boost growth as well as increase productivity. However, accumulation of Cr could represent a potential threat to living organisms. Cr absorption, displacement and accretion depend on its speciation, which also determines its toxicity which is often diverse. Indications of its toxicity include; reduction of seed germination, retardation of growth, reduction of yield, inhibition of enzymatic activities, weakening of photosynthesis, nutrient, oxidative disparities and genetic mutation in plants as well as several injurious diseases in animals and humans. In this study, we have presented a comprehensive review as well as an informative account of the influence of Cr on the environment drawn from researches carried out over the years following an analytical approach. Uniquely, this work presents a review of the effects and remediation of Cr from soil and wastewater drawn from several evidence and meta-data-based articles and other publications. Accordingly, the write-up is intended to appeal to the consciousness of the general public that the significance of Cr notwithstanding, its environmental toxicity should not be taken for granted.

Self-assembly preparation of lignin-graphene oxide composite nanospheres for highly efficient Cr(vi) removal

Recently, research interest in the application of lignin is growing, especially as adsorbent material. However, single lignin shows unsatisfactory adsorption performance, and thus, construction of lignin-based nanocomposites is worth considering. Herein, we introduced graphene oxide (GO) into lignin to form lignin/GO (LGNs) composite nanospheres by a self-assembly method. FTIR and 1H NMR spectroscopy illustrated that lignin and GO are tightly connected by hydrogen bonds. The LGNs as an environmental friendly material, also exhibit excellent performance for Cr(vi) removal. The maximum sorption capacity of LGNs is 368.78 mg g-1, and the sorption efficiency is 1.5 times than that of lignin nanospheres (LNs). The removal process of Cr(vi) via LGNs mainly relies on electrostatic interaction, and it also involves the reduction of Cr(vi) to Cr(iii). Moreover, LGNs still have high adsorption performance after repeating five times with the sorption capacity of 150.4 mg g-1 in 200 mg g-1 Cr(vi) solution. Therefore, the prepared lignin-GO composite nanospheres have enormous potential as a low-cost, high-absorbent and recyclable adsorbent, and can be used in wastewater treatment.

Enhanced visible-light driven multi-photoredox Cr(VI) and p-cresol by Si and Zr interplay in fibrous silica-zirconia

Multiple contaminants including heavy metals and phenolic compounds are normally co-exist in wastewater, which caused the treatment process is rather complicated. Herein, the synergistic photoredox of Cr(VI) and p-cresol (pC) by innovative fibrous silica zirconia (FSZr) photocatalyst was reported. The high surface area of FSZr comprised of microspheres with a bicontinuous concentric lamella structure morphology consisted of silica, while its core consisted of ZrO₂ structure. The rearrangement of FSZr framework increased the crystallinity, formed Si-O-Zr bonds and narrowed the band gap of ZrO₂ for enhanced of photoredox of Cr(VI) and pC. Compared to the reaction, the photoredox efficiency of FSZr for removing Cr(VI) and pC in simultaneous system was found to be 96 % and 59 %, respectively which are higher than that in its single system owing to the efficient electron-hole charge separation. Phenolic compound with high degree of electron donating group gave beneficial effect to photoreduction of Cr(VI). Consequently, a proposed mechanism involving multi-photoredox pathway were proposed based on photoredox reaction and scavengers studies. FSZr sustained the simultaneous photoredox activities after five runs demonstrating its possibility to be use in the wastewater treatment of various pollutants.

Magnetic poly-o-vanillin-functionalized core–shell nanomaterials as a smart sorbent for scavenging mercury(ii) from aqueous solution

In the present work, a poly-o-vanillin-functionalized magnetic nanoparticle of PoVan/CoFe₂O₄@mSiO₂ with core–shell structure was synthesized through a facile, green and low cost method.

Efficient removal of hexavalent chromium from electroplating wastewater using polypyrrole coated on cellulose sulfate fibers

In this study, cellulose sulfate was synthesized through sulfonation of cotton, and polypyrrole was coated on the surface of fibers. Then, the optimum ratio of pyrrole to cellulose sulfate was evaluated, and the physical, chemical, and morphological properties of the composite were assessed by using FESEM, EDS, FTIR, BET, and TGA analysis. Furthermore, adsorption of hexavalent chromium using the composite adsorbent was studied by the results of designed experiments with the Box-Behnken technique to assess the effect of pH, contact time, adsorbent dose and the initial concentration of hexavalent chromium and optimize the adsorption process. The removal percentage was 99.9% under the optimum conditions (adsorbent dose, 4 g L-1; initial concentration of Cr(VI), 200 mg L-1; pH value, 2; contact time, 200 min). The results of adsorption isotherms illustrated that the adsorption process followed Redlich-Peterson, Freundlich, Radke-Prausnitz, and UT models, and the calculated maximum adsorption capacity by the Langmuir model was 198 mg g-1. Based on the kinetic and thermodynamic studies, the adsorption process followed the intraparticle diffusion model and showed the endothermic and spontaneous adsorption with an increase in entropy on the adsorbent surface. The presence of copper, nickel and zinc cations had no adverse effect on the removal percentage of hexavalent chromium significantly. The adsorbent was reused successfully in four sequential treatments. Consequently, the synthesized adsorbent is efficient due to the high efficiency of hexavalent chromium removal percentage from electroplating effluent (99.87%).

Pyrolyzed fabrication of N/P co-doped biochars from (NH4)3PO4-pretreated coffee shells and appraisement for remedying aqueous Cr(VI) contaminants

In this study, an agricultural residue-derived biochar was fabricated by pyrolyzing coffee shells using (NH₄)₃PO₄ pretreatment. The influence of pyrolysis temperature on the structure and properties of biochars was investigated. The elemental analysis, spectroscopic and textural studies showed that the biochars were endued sufficient N and P co-doping and large specific surface area by (NH₄)₃PO₄-pretreatment. The appraisement for remedying aqueous Cr(VI) contaminants demonstrated that the N/P co-doped biochars offered high efficiencies above 95% for aqueous Cr(VI) removal. The mechanism investigation displayed that the adsorption and reduction of Cr(VI) were boosted by the synergistic effect between the hierarchical pore structure and the groups related to oxygen, nitrogen and phosphorus. Moreover, the biochar can be readily regenerated by HCl solution soaking for reuses several times. This work should permit for providing a convenient utilization of coffee shell agricultural residues, and the coffee shell-derived biochars supplied potential for remedying Cr(VI) in effluents.

New Perspectives on Iron-Based Nanostructures

Among all minerals, iron is one of the elements identified early by human beings to take advantage of and be used. The role of iron in human life is so great that it made an era in the ages of humanity. Pure iron has a shiny grayish-silver color, but after combining with oxygen and water it can make a colorful set of materials with divergent properties. This diversity sometimes appears ambiguous but provides variety of applications. In fact, iron can come in different forms: zero-valent iron (pure iron), iron oxides, iron hydroxides, and iron oxide hydroxides. By taking these divergent materials into the nano realm, new properties are exhibited, providing us with even more applications. This review deals with iron as a magic element in the nano realm and provides comprehensive data about its structure, properties, synthesis techniques, and applications of various forms of iron-based nanostructures in the science, medicine, and technology sectors.