Polyaniline-based adsorbents for removal of hexavalent chromium from aqueous solution: a mini review

Elimination of mutagenic contaminants from water using cellulose bearing ferrous-phthalocyanine

BACKGROUND

We previously investigated methods for separating mutagenic contaminants from aqueous solutions using cellulose-bearing covalently bound trisulfo-Cu-phthalocyanine (blue cotton and blue rayon). Mutagenic contaminants with three or more fused aromatic rings in their structures were adsorbed onto blue cotton and rayon. Since Cu-phthalocyanine is considered an unsuitable absorption ligand for byproducts of water chlorination, such as 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (Mutagen X or MX), we investigated the development of a new material for the elimination of MX from aqueous solvents.

RESULTS

We selected green cellulose powder bearing ferrous phthalocyanine (FePh), hereafter referred to as green cellulose or GP, as the candidate material. GP is composed of cationized cellulose (white cellulose, WP) and FePh tetracarboxylic acid. The mutagenicity of MX dissolved in buffer or dimethyl sulfoxide (DMSO) solution significantly decreased after treatment with GP. The effects of GP on the elimination of MX from the solvent were very close to being expired after 70 cycles of repeated adsorption of the same GP, and the capacity of GP for MX removal was estimated to be exhausted after 120 cycles of repeated adsorption based on the extrapolation of the obtained result; thus, the interacting ligands on GP may be saturated after complete MX adsorption. The mutagenicity of MX dissolved in aqueous buffer significantly decreased after treatment at pH7.4 but not at pH 4.0. Since MX is dissociated to be the anionic form at pH 6 or higher, the negative charge of MX in the buffer at pH 7.4 may interact with the positive charge of ferrous ions in GP to create a linkage between MX and GP. After GP adsorbed MX, mutagenicity was extracted with water or acetonitrile and recovered in the eluent. Thus, the reversible interaction between MX and FePh may have caused adsorption of MX onto GP.

CONCLUSION

GP could be used as a new eliminator and recovery agent for MX in chlorinated drinking water. Developing new materials for the removal and recovery of agents for the detection of mutagenic contaminant-related chlorination in water is beneficial for environmental health.

Revealing the interface chemistry of polyaniline grafted biomass via statistical modeling of multi-component dye systems: optimization, kinetics, thermodynamics, and adsorption mechanism

The growing need to examine the adsorption capabilities of innovative materials in real-world water samples has encouraged a shift from single to multicomponent adsorption systems. In this study, a novel composite, PANI-g-SM was synthesized by covalently grafting a lignocellulosic biomass, Saccharum munja (SM) with polyaniline (PANI). The as-synthesized composite was investigated for the simultaneous adsorption of cationic (Methylene Blue (MB); Crystal Violet (CV)) and anionic dyes (Reactive Red 35 (RR); Fast Green FCF (FG)) from four single components and two binary systems, MB + RR and CV + FG. Further, the effect and interaction of pH (2-11), dosage (0.01-0.04 g/10 mL), and initial concentration (0.0313 to 0.1563 mmol/L) on the elimination of dyes by PANI-g-SM were studied through a novel design of Box-Behnken of Response Surface Methodology (RSM) technique which was found to be highly useful for revealing the chemistry of interfaces in multi-component systems. The extended Langmuir model for the binary system indicated the presence of synergism, as result the maximum monolayer adsorption capacity increased by 44.44%, 645.83%, 67.88%, and 441.07% for MB, RR, CV, and FG dye, respectively. Further, the adsorption process mainly followed a pseudo-second-order kinetic model, and the thermodynamic studies revealed the exothermic nature of adsorption for RR and FG dye while endothermic for MB and CV dye, respectively with Δ G varying from - 1.68 to - 6.12 kJ/mol indicating the spontaneity of the process. Importantly, the efficacy of the composite was evaluated for the treatment of textile industry effluent highlighting its potential as an adsorbent for wastewater treatment.

Fruit waste-derived cellulose-polyaniline composite for adsorption-coupled reduction of chromium oxyanions

Hexavalent chromium oxyanions, known as potentially toxic micropollutants, exist in the effluents and discharges of metallurgical, electroplating, refractory, chemical, and tanning industries. The exposure of chromium-contaminated water causes severe health hazards. The present work outlines a facile approach to grow polyaniline (PANI) on fruit-waste-derived cellulose (CEL) via oxidative polymerization of aniline; followed by chemical processing with NH4OH to obtain CEL-PANI-EB composites for adsorptive separation-coupled reduction of highly toxic hexavalent chromium oxyanions. The spectroscopic analyses of the CEL-PANI-EB composite before and after adsorption of Cr(VI) oxyanions revealed hydrogen bonding, electrostatic, and complexation as major interactive pathways. The adsorbed hexavalent chromium oxyanions are reduced into Cr(III) species by oxidation of PANI-based benzenoid amine into quinoid imine in the CEL-PANI-EB composite. The adsorption of Cr(VI) oxyanions by the CEL-PANI-EB composite showed negligible effects of other anionic co-pollutants, like NO3- and SO42-. The CEL-PANI-EB composite adsorbed Cr(VI) oxyanions with a removal capacity of 469 mg g-1, based on the Langmuir adsorption isotherm model. The hydroxyl functionalities in cellulose and amine/imine functionalities in PANI facilitate the electrostatic attraction between the CEL-PANI-EB and Cr(VI) oxyanions in an acidic environment beside the hydrogen linkages. The adsorbed Cr(VI) oxyanions are reduced to Cr(III)-based species by the benzenoid amines of PANI, as revealed from the XPS analyses. The CEL-PANI-EB composite showed excellent recyclability and maintained 83.4% adsorption efficiency after seven runs of chromium adsorption-desorption. The current findings reveal the potential of CEL-PANI-EB composites for the adsorptive removal of Cr(VI) oxyanions and their conversion into a lesser toxic form, making them promising materials for wastewater treatment applications.

Efficiency of Ppy-PA-pani and Ppy-PA composite adsorbents in Chromium(VI) removal from aqueous solution

In this study, first time the combination of composites with Phytic acid (PA) as the organic binder cross-linker is reported. The novel use of PA with single and double conducting polymers (polypyrrole (Ppy) and polyaniline (Pani)) were tested against removal of Cr(VI) from wastewater. Characterizations (FE-SEM, EDX, FTIR, XRD, XPS) were performed to study the morphology and removal mechanism. The adsorption removal capability of Polypyrrole - Phytic Acid - Polyaniline (Ppy-PA-Pani) was deemed to be higher than Polypyrrole - Phytic Acid (Ppy-PA) due to the mere existence of Polyaniline as the extra polymer. The kinetics followed 2nd order with equilibration at 480 min, but Elovich model confirmed that chemisorption is followed. Langmuir isotherm model exhibited maximum adsorption capacity of 222.7-321.49 mg/g for Ppy-PA-Pani and 207.66-271.96 mg/g for Ppy-PA at 298K-318K with R2 values of 0.9934 and 0.9938 respectively. The adsorbents were reusable for 5 cycles of adsorption-desorption. The thermodynamic parameter, ΔH shows positive values confirmed the adsorption process was endothermic. From overall results, the removal mechanism is believed to be chemisorption through Cr(VI) reduction to Cr(III). The use of phytic acid (PA) as organic binder with combination of dual conducting polymer (Ppy-PA-Pani) was invigorating the adsorption efficiency than just single conducting polymer (Ppy-PA).

Fabrication of Polyamide6/Polyaniline as an Effective Nano-web Membrane for Removal of Cr (VI) from Water and a Black Box Approach in Modeling of Adsorption Process

Chromium (Cr), as a highly toxic heavy metal ion, is still a severe environmental issue, although many research efforts have been put into its removal from water. Polyaniline (PANI), as a conductive polymer, demonstrated great capability in heavy metal adsorption due to its low cost, ease of synthesis, reversible redox behavior, and chemical stability. However, using PANI powder alone in heavy metal removal causes secondary pollution and aggregation in water. The PANI coating on a substrate could tackle this problem. In this study, polyaniline-coated polyamide6 (PA6/PANI) nano-web membrane was used for the removal of Cr(VI) in both adsorption and filtration-adsorption modes. The PA6/PANI nano-web membrane was fabricated via PA6 electrospinning followed by in-situ polymerization of the aniline monomer. The electrospinning condition of PA6 was optimized by the Taguchi method. The PA6/PANI nano-web membrane was characterized by FESEM, N2-adsorption/desorption, FT-IR, contact angle measurement, and tensile test. FT-IR and FESEM results demonstrated the successful synthesis of PA6/PANI nano-web and PANI homogeneous coating on PA6 nanofibers, respectively. The N2 adsorption/desorption results indicated that the pore volume of the PA6/PANI nano-web decreased by 39% compared to PA6 nanofibers. The tensile test and water contact angle studies showed that the coating of PANI on PA6 nanofibers improves the mechanical properties and hydrophilicity of PA6 by 10% and 25%, respectively. The application of PA6/PANI nano-web in the removal of Cr(VI) in batch and filtration modes exhibits excellent removal of 98.4 and 86.7%, respectively. A pseudo first order model well described the adsorption kinetics, and the adsorption isotherm was best fitted by the Langmuir model. A black box modeling approach based on artificial neural networks (ANN) was developed to predict the removal efficiency of the membrane. The superior performance of PA6/PANI in both adsorption and filtration-adsorption systems makes it a potential candidate for the removal of heavy metals from water on an industrial scale.

Sorption enhancement of Cr(VI) from aqueous solution by polyaniline confined in three-dimensional network of composite porous hydrogel

Hexavalent chromium Cr(VI) is a typical harmful pollutant, which is carcinogenic or mutagenic to aquatic animals and humans. In this study, sepiolite/humic acid/polyvinyl alcohol@ polyaniline (SC/HA/PVA@PANI) composite porous hydrogel adsorbent was synthesized by Pickering emulsion template in situ chemical oxidative polymerization for adsorption of Cr(VI) from aqueous solution. The in situ polymerization of aniline at the Pickering emulsion interface and the unique three-dimensional network structure of the hydrogel act as an effective "confinement" for the growth of the polymer. The porous structure of the material acts as a water channel, which effectively accelerates the binding of the adsorbate to the adsorption sites, and significantly improves the adsorption rate and adsorption capacity. The adsorption capacity of PANI for Cr(VI) confined in three-dimensional network of composite porous SC/HA/PVA@PANI hydrogel reached 1180.97 mg/g-PANI, which increased about 27-fold compared the adsorption capacity of pure PANI (43.48 mg/g). It is shown that the experimental design effectively avoids the agglomeration of PANI and improves its potential adsorption performance. In addition, the analysis of FESEM-EDX, FT-IR, and XPS spectra before and after adsorption confirmed that the main adsorption mechanisms of Cr(VI) on SC/HA/PVA@PANI included ion exchange, electrostatic attraction, and redox reaction. In conclusion, SC/HA/PVA@PANI has good stability and excellent adsorption performance, which is a new type of Cr(VI) ion adsorbent with great potential.

Ion-Selective Electrodes with Solid Contact Based on Composite Materials: A Review

Potentiometric sensors are the largest and most commonly used group of electrochemical sensors. Among them, ion-selective electrodes hold a prominent place. Since the end of the last century, their re-development has been observed, which is a consequence of the introduction of solid contact constructions, i.e., electrodes without an internal electrolyte solution. Research carried out in the field of potentiometric sensors primarily focuses on developing new variants of solid contact in order to obtain devices with better analytical parameters, and at the same time cheaper and easier to use, which has been made possible thanks to the achievements of material engineering. This paper presents an overview of new materials used as a solid contact in ion-selective electrodes over the past several years. These are primarily composite and hybrid materials that are a combination of carbon nanomaterials and polymers, as well as those obtained from carbon and polymer nanomaterials in combination with others, such as metal nanoparticles, metal oxides, ionic liquids and many others. Composite materials often have better mechanical, thermal, electrical, optical and chemical properties than the original components. With regard to their use in the construction of ion-selective electrodes, it is particularly important to increase the capacitance and surface area of the material, which makes them more effective in the process of charge transfer between the polymer membrane and the substrate material. This allows to obtain sensors with better analytical and operational parameters. Brief characteristics of electrodes with solid contact, their advantages and disadvantages, as well as research methods used to assess their parameters and analytical usefulness were presented. The work was divided into chapters according to the type of composite material, while the data in the table were arranged according to the type of ion. Selected basic analytical parameters of the obtained electrodes have been collected and summarized in order to better illustrate and compare the achievements that have been described till now in this field of analytical chemistry, which is potentiometry. This comprehensive review is a compendium of knowledge in the research area of functional composite materials and state-of-the-art SC-ISE construction technologies.

One-Step Accelerated Synthesis of Conducting Polymer/Silver Composites and Their Catalytic Reduction of Cr(VI) Ions and p-Nitrophenol

In this paper, silver nitrate was used as an oxidant to prepare polyaniline, polypyrrole, and poly(3,4-ethylene dioxythiophene)/silver composites through a simultaneous oxidation/reduction process. In addition, p-phenylenediamine was added with 1 mole% relative to the concentrations of the monomers to accelerate the polymerization reaction. The prepared conducting polymer/silver composites were characterized by scanning and transmission electron microscopies to study their morphologies; Fourier-transform infrared and Raman spectroscopies to confirm their molecular structures; and thermogravimetric analysis (TGA) to study their thermal stabilities. The silver content in the composites was estimated by energy-dispersive X-ray spectroscopy, ash analysis, and TGA. The conducting polymer/silver composites were utilized for the remediation of water pollutants through catalytic reduction. Hexavalent chromium ions (Cr(VI)) were photocatalytically reduced to trivalent chromium ions, and p-nitrophenol was catalytically reduced to p-aminophenol. The catalytic reduction reactions were found to follow the first-order kinetic model. Among the prepared composites, polyaniline/silver composite has shown the highest activity for the photocatalytic reduction of Cr(VI) ions with an apparent rate constant of 0.226 min^-1 and efficiency of 100% within 20 min. Additionally, poly(3,4-ethylene dioxythiophene)/silver composite showed the highest catalytic activity towards the reduction of p-nitrophenol with an apparent rate constant of 0.445 min^-1 and efficiency of 99.8% within 12 min.

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.

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.

Polyaniline coated Pt/CNT as highly stable and active catalyst for catalytic hydrogenation reduction of Cr(VI)

Liquid phase catalytic hydrogenation reduction is a feasible method to eliminate Cr(VI) in water, while supported noble metal catalysts are liable to deactivation. In this study, carbon nanotube supported Pt catalyst (Pt/CNT) coated by polyaniline (Pt/CNT@PANI) was prepared and applied in the liquid phase catalytic hydrogenation of Cr(VI). Characterization results disclose that after coating Pt/CNT is completely wrapped by PANI layers and active Pt particles are no longer accessible. Despite complete embedment of Pt particles by PANI layers, Pt/CNT@PANI remains highly active for Cr(VI) reduction in liquid phase catalytic hydrogenation. The catalytic Cr(VI) reduction on Pt/CNT@PANI can be described by a PANI oxidation-reduction mechanism, by which PANI is first oxidized by Cr(VI) to form Cr(III), and oxidized PANI is reduced by catalytic hydrogenation. The Cr(VI) reduction on Pt/CNT@PANI complies with the Langmuir-Hinshelwood model, reflecting the pivotal role of Cr(VI) adsorption. Furthermore, the catalytic activity of Pt/CNT@PANI differs with PANI layer thickness and Cr(VI) reduction is positively correlated with reaction temperature. Catalyst recycling results show that after 4 cycles Pt/CNT loses 92.4% of catalytic activity, while the initial activity of Pt/CNT@PANI slightly decreases by 11.6%, demonstrating its high catalyst stability.

Recent Advances in the Synthesis, Characterization, and Application of Carbon Nanomaterials for the Removal of Endocrine-Disrupting Chemicals: A Review

The large-scale production and frequent use of endocrine-disrupting chemicals (EDCs) have led to the continuous release and wide distribution of these pollutions in the natural environment. At low levels, EDC exposure may cause metabolic disorders, sexual development, and reproductive disorders in aquatic animals and humans. Adsorption treatment, particularly using nanocomposites, may represent a promising and sustainable method for EDC removal from wastewater. EDCs could be effectively removed from wastewater using various carbon-based nanomaterials, such as carbon nanofiber, carbon nanotubes, graphene, magnetic carbon nanomaterials, carbon membranes, carbon dots, carbon sponges, etc. Important applications of carbon nanocomposites for the removal of different kinds of EDCs and the theory of adsorption are discussed, as well as recent advances in carbon nanocomposite synthesis technology and characterization technology. Furthermore, the factors affecting the use of carbon nanocomposites and comparisons with other adsorbents for EDC removal are reviewed. This review is significant because it helps to promote the development of nanocomposites for the decontamination of wastewater.

Study on the Preparation of Nano-FeS Loaded on Fly Ash and Its Cr Removal Performance

Chromium has been considered as one of the most hazardous heavy metals because of its strong and persistent toxicity to the ecosystem and human beings. In this study, fly ash-loaded nano-FeS (nFeS-F) composites were constructed with fly ash as the carrier, and the performance and mechanism of the composites for the removal of Cr(VI) and total chromium from water were investigated. The composite was characterized by X-ray diffraction and transmission electron microscopy. The effects of fly ash size, molarity of FeSO₄, and flow rate of FeSO₄ solution on the removal of Cr(VI) and total chromium were investigated by a single factor experiment. The interaction of various factors was studied by the Box-Behnken response surface methodology. The optimum conditions of removal of Cr(VI)and total chromium by nFeS-F were determined. The results show that ① the optimal preparation conditions for nFeS-F were an FeSO₄ concentration of 0.45 mol/L, a fly ash particle size of 120-150 mesh, and a flow rate of 0.43 mL/s.② The response surface model provides reliable predictions for the removal efficiencies of Cr(VI) and total chromium.③ The removal efficiencies of Cr(VI) and total chromium were 92.87 and 83.53%, respectively, under the optimal preparation conditions by the experimental test. This study provides an effective method for the removal of Cr(VI) and total chromium.

Disinfection and Photocatalytic Degradation of Organic Contaminants Using Visible Light-Activated GCN/Ag2CrO4 Nanocomposites

Visible-light-driven photocatalysts have gained increasing attention in the past few decades in treating emerging contaminants in water and wastewater. In this work, the photocatalytic activity of the coupled graphitic carbon nitride (GCN) and silver chromate (Ag2CrO4), herein denoted as GCN/Ag2CrO4, nanocomposites was evaluated for degrading organic pollutants and inactivating microorganisms under visible light irradiation using a royal blue light-emitting diode (LED). The organic pollutants studied were 2,4-dichlorophenoxyacetic acid (2,4-D) and methyl chlorophenoxy propionic acid (MCPP or Mecoprop-P) present in KillexR, a commercially available herbicide, bovine serum albumin (BSA) protein, and SARS-CoV-2 spike protein. The disinfection experiments were conducted on wastewater secondary effluent. The results showed that over 85% degradation was achieved for both 2,4-D and Mecoprop-P in 120 min while 100% of BSA protein and 77.5% of SARS-CoV-2 protein were degraded in 20 min and 30 min, respectively. Additionally, GCN/Ag2CrO4 nanocomposites led to over one log reduction of cellular ATP (cATP), total coliforms, and E. coli in wastewater treatment plant (WWTP) secondary effluent after 60 min of royal blue LED irradiation. It was observed that the degradation performance of a photocatalyst under light irradiation is contaminant-specific. The binding affinity of the released metal ions from GCN/Ag2CrO4 with protein and ATP functional groups was responsible for the degradation of proteins and the reduction of cATP, while the generated ROS was responsible for the disinfection of total coliforms and E. coli. Overall, the results indicate that GCN/Ag2CrO4 nanocomposite is a promising photocatalyst in degrading organic pollutants and disinfecting microorganisms under visible light irradiation within a reasonable time.

Removal of Cr(VI) from Wastewater Using Graphene Oxide Chitosan Microspheres Modified with α-FeO(OH)

Graphene oxide and chitosan microspheres modified with α−FeO(OH) (α−FeO(OH)/GOCS) are prepared and utilized to investigate the performance and mechanism for Cr(VI) removal from aqueous solutions and the possibility of Fe secondary pollution. Batch experiments were carried out to identify the effects of pH, mass, and volume ratio (m/v), coexisting ions, time (t), temperature (T), and Cr(VI) initial concentration (C0) on Cr(VI) removal, and to evaluate adsorption kinetics, equilibrium isotherm, and thermodynamics, as well as the possibility of Fe secondary pollution. The results showed that Cr(VI) adsorption increased with C0, t, and T but decreased with increasing pH and m/v. Coexisting ions inhibited Cr(VI) adsorption, and this inhibition increased with increasing concentration. The influence degrees of anions and cations on the Cr(VI) adsorption in descending order were SO42− > PO42− > NO3− > Cl− and Ca2+ > Mg2+ > Mn2+, respectively. The equilibrium adsorption capacity of Cr(VI) was the highest at 24.16 mg/g, and the removal rate was 97.69% under pH = 3, m/v = 1.0 g/L, T = 298.15 K, and C0 = 25 mg/L. Cr(VI) adsorption was well fitted to a pseudo-second-order kinetic model and was spontaneous and endothermic. The best fit of Cr(VI) adsorption with the Langmuir and Sips models indicated that it was a monolayer and heterogeneous adsorption. The fitted maximum adsorption capacity was 63.19 mg/g using the Sips model under 308.15 K. Cr(VI) removal mainly included electrostatic attraction between Cr(VI) oxyanions with surface Fe−OH2+, and the adsorbed Cr(VI) was partially reduced to Cr(III) and then precipitated on the surface. In addition, there was no Fe secondary pollution during Cr(VI) adsorption.

Synthesis and characterization of lanthanum-based metal organic framework decorated polyaniline for effective adsorption of lead ions from aqueous solutions

The novel La-MOF@x%PANI composite was synthesized via a two-step procedure with ultra-sonication, and the adsorption mechanism of Pb²⁺ ions from synthetic aqueous solutions was systematically studied. The Pb²⁺ adsorption on the La-MOF@x%PANI was evaluated by the Fourier transform infrared spectroscopy, powder X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray analysis, Brunauer-Emmett-Teller analysis, X-ray photoelectron spectroscopy, and elemental mapping analyses. The effects of the adsorption-influencing parameters, including contact time, solution pH, and co-existing cations on the maximum adsorption capacity of Pb²⁺ onto the prepared composite material were investigated. Moreover, the adsorption of Pb²⁺ ions could be eliminated with rapid adsorption kinetics using the water-stable La-MOF@x%PANI composite. The as-synthesized La-MOF@50%PANI exhibited excellent adsorption performance toward Pb²⁺ ions with an extraordinary adsorption capacity of 185.19 mg/g at pH 6. The Pb²⁺ adsorption onto the La-MOF@x%PANI composite follows the pseudo-second-order kinetics and fits well with the Langmuir isotherm model, indicating the Pb²⁺ adsorption depended on the solution pH as the adsorption mechanism was mainly governed by the electrostatic attraction. Notably, La-MOF@x%PANI composite possesses outstanding regeneration ability and stability after up to four successive cycles. The satisfactory findings reflect that the La-MOF@50%PANI hybrid composite holds a great promise for remediating Pb²⁺ ions from aqueous environments.

Removal of Cr(VI) by polyaniline embedded polyvinyl alcohol/sodium alginate beads - Extension from water treatment to soil remediation

Efficient nano-scale chromium (Cr) remediating agents used in the water industry may find their application in soil difficult because of the strong aggregation effect. In this study, a millimeter-sized PANI/PVA/SA composite (PPS) was synthesized by embedding polyaniline (PANI) into polyvinyl alcohol (PVA)/sodium alginate (SA) gel beads. Additionally, the PPS was used to recover hexavalent chromium (Cr(VI)) contaminated water and soil to study the remediation impacts and mechanism. Results showed that the PPS was an irregular sphere with a pore size of 24.24 nm and exhibited strong adsorption capacity (83.1 mg/g) for removing Cr(VI) in water. The Cr(VI) adsorption by PPS could be well described with the pseudo-second-order kinetics and the Redlich-Peterson isotherm model, indicating that the chemical reactions were the controlling step in the Cr(VI) adsorption process. PPS also exhibited excellent physicochemical properties (< 13 mg/L TOC release) and reusability (efficiency of 95.25% after four runs) for Cr(VI) removal. Soil incubation results showed that the 5% PPS (5PPS) treatment could efficiently remove 24.17% of total Cr and 52.47% of Cr(VI) in the contaminated soil after 30 days. Meanwhile, the water-soluble and the leaching Cr contents were decreased by 43.37% and 61.78% in the 5PPS group, respectively. Elemental speciation by XPS revealed that Cr(VI) removal from solution and soil proceeded mainly by electrostatic attraction, reduction, and complexation/chelation. The study implied that PPS could be a useful amendment to remediate both the Cr(VI)-contaminated water and soil.

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.

Progress in surface-modified silicas for Cr(VI) adsorption: A review

Various toxic chemicals are discharging to the environment due to rapid industrialization and polluting soil, water, and air causing numerous diseases including life-threatening cancer. Among these pollutants, Cr(VI) or hexavalent chromium is one of the most carcinogenic and toxic contaminants hostile to human health and other living things. Therefore, along with other contaminants, the removal of Cr(VI) efficiently is very crucial to keep our environment neat and clean. On the other hand, silica has a lot of room to modify its surfaces as it is available with various sizes, shapes, pore sizes, surface areas etc. and the surface silanol groups are susceptible to design and prepare adsorbents for Cr(VI). This review emphases on the progress in the development of different types of silica-based adsorbents by modifying the surfaces of silica and their application for the removal of Cr(VI) from wastewater. Toxicity of Cr(VI), different silica surface modification processes, and removal techniques are also highlighted. The adsorption capacities of the surface-modified silica materials with other parameters are discussed extensively to understand how to select the best condition, silica and modifiers to achieve optimum removal performance. The adsorption mechanisms of various adsorbents are also discussed. Finally, future prospects are summarized and some suggestions are given to enhance the adsorption capacities of the surface-modified silica materials.

Use of experimental design to evaluate the adsorption of chromium (VI) by alginate/polyaniline beads

Low-cost decorated sodium alginate beads with polyaniline (Alg@PANI beads) were easily prepared using a cross-linking method, and employed for the adsorption of Cr(VI) from aqueous solutions. The effect of several influencing parameters, including temperature, contact time, Cr(VI) concentration, and adsorbent dosage, was investigated and optimized using central composite design (CCD) under response surface methodology (RSM). The analysis of variance (ANOVA) of the quadratic model and the analyzed model revealed that the models were statistically significant, with a low P-value (<0.0001) and a high correlation coefficient value (R2 = 0.93). The optimum parameters for total adsorption were as follows: adsorbent dose 0.027 g, pH 2, contact time 45 min, temperature 38?C, and Cr(VI) concentration 29.24 ppm. The findings of this study indicate that the prepared Alg@PANI beads could be effectively used to remove Cr(VI) ions from aqueous solutions.

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.

Application of polyaniline nanofibers for the construction of nitrate all-solid-state ion-selective electrodes

The application of polyaniline nanofibers doped with chloride and nitrate ions (PANINFs-Cl and PANINFs-NO3) in potentiometry was described. Both kinds of nanofibers were used as an ion-to-electron transducer in ion-selective electrodes with solid contact (SCISEs). Extensive research on the properties of the nanofibers themselves (SEM, UV–Vis spectroscopy, FTIR) and the constructed electrodes (potentiometric methods, electrochemical impedance spectroscopy) has been carried out. Basic analytical parameters of electrodes containing various nanofibers contents in the ion-selective membrane and with nanofibers as an intermediate layer were determined. It was found that application of PANI nanofibers resulted in improvement of electrode performance (among others, better stability and reversibility of the electrode potential). The obtained sensors were characterized by a high slope of the calibration curve, a wide measuring range and a fast response time. Moreover, they were insensitive to change of redox potential, as well as light and the presence of oxygen in the solution, what is important from a practical point of view. They were also successfully used for nitrate determination in real environmental samples.

Conductive Polymers and Their Nanocomposites as Adsorbents in Environmental Applications

Proper treatment and disposal of industrial pollutants of all kinds are a global issue that presents significant techno-economical challenges. The presence of pollutants such as heavy metal ions (HMIs) and organic dyes (ODs) in wastewater is considered a significant problem owing to their carcinogenic and toxic nature. Additionally, industrial gaseous pollutants (GPs) are considered to be harmful to human health and may cause various environmental issues such as global warming, acid rain, smog and air pollution, etc. Conductive polymer-based nanomaterials have gained significant interest in recent years, compared with ceramics and metal-based nanomaterials. The objective of this review is to provide detailed insights into different conductive polymers (CPs) and their nanocomposites that are used as adsorbents for environmental remediation applications. The dominant types of CPs that are being used as adsorbent materials include polyaniline (PANI), polypyrrole (Ppy), and polythiophene (PTh). The various adsorption mechanisms proposed for the removal of ODs, HMIs, and other GPs by the different CPs are presented, together with their maximum adsorption capacities, experimental conditions, adsorption, and kinetic models reported.

Nano-adsorbents an effective candidate for removal of toxic pharmaceutical compounds from aqueous environment: A critical review on emerging trends

Advancements in medical research has resulted in the modernization of healthcare facilities, subsequently leading to a higher level of production and usage of pharmaceuticals to sustain better quality of life. Pharmaceutical active compounds (PhACs) possess high genotoxicity and eco-toxicity thus presenting numerous side effects to living beings on long-term exposure. The fate and toxicity of PhACs were explored in detail, aiming to elucidate their occurrence and transmission in wastewater treatment systems (WWTPs). Adsorption of pharmaceutical compounds using Nano-adsorbents has gained momentum in recent years owing to their low-cost, high surface area and effectiveness. This review has been conducted in order to widen the utilization of Nano adsorbents in the adsorption of pharmaceutical compounds with a focus on the aqueous environment. The synthesis routes and properties of Nano-adsorbents for removal of PhACs were assessed in a comprehensive way. The recovery and reuse ability of nano-adsorbents also forms an integral part of its application in the removal of PhACs and has hence been delineated.

Synthesis of Polyaniline Coating on the Modified Fiber Ball and Application for Cr(VI) Removal

In this study, polyaniline (PANI) is prepared by means of chemical oxidization polymerization and directly loaded on the modified fiber ball (m-FB) to obtain macroscale polyaniline/modified fiber ball (PANI/m-FB) composite, and then its removal ability of Cr(VI) is investigated. The effects of different parameters such as contact time, pH value and initial concentration on Cr(VI) removal efficiency are discussed. The experimental results illustrate that the favorable pH value is 5.0 and the maximum removal capacity is measured to be 293.13 mg g^-1. Besides, PANI/m-FB composites can be regenerated and reused after being treated with strong acid. The kinetic study indicates that the adsorption procedure is mainly controlled by chemical adsorption. More importantly, the macroscale of composites can avoid secondary pollution efficiently. Benefiting from the low cost, easy preparation in large scale, environmentally friendly, excellent recycling performance as well as high removal ability, PANI/m-FB composites exhibit a potential possibility to remove Cr(VI) from industrial waste water. The polyaniline (PANI) was coated on modified fiber ball (m-FB) to remove Cr(VI) in waste water, and this kind of PANI/m-FB composites can avoid secondary pollution efficiently due to its macrostructure. Furthermore, the removal capacity can reach to 291.13 mg/g and can be multiple reused.

In-situ self-assembly construction of hollow tubular g-C3N4 isotype heterojunction for enhanced visible-light photocatalysis: Experiments and theories

A highly reactive hollow tubular g-C₃N₄ isotype heterojunction (SCN-CN) was designed to enhance visible light absorption and manipulate the directed transfer of electrons and holes. The results of UV-vis DRS, XPS valence band and DFT theoretical calculations indicated S doping increases the visible-light absorption capacity and changed the ba nd gap structure of g-C₃N₄ (CN), resulting in the transfer of electrons from the CN to the SCN and holes from the SCN to the CN under visible light. In addition, the tubular structure of the SCN-CN facilitated the transfer of electrons in the longitudinal direction, which reduced charge carrier recombination. Furthermore, the optical properties, electronic structure, and electron transfer of SCN-CN were also studied by experiments and theoretical calculations. The antibiotic tetracycline hydrochloride (TCH) and dye Rhodamine B (RHB) were subjected to evaluate the photocatalytic performance of SCN-CN. The scavenger tests and ESR data showed that the h⁺, ·O₂^- and ·OH worked together in the photocatalytic process. Moreover, the photocatalytic degradation pathway was analyzed by LC-MS. This study synthesized a hollow tubular CN isotype heterojunction with high visible-light photocatalytic performance and provided a theoretical basis for CN isotype heterojunction.

Mechanisms of enhanced hexavalent chromium removal from groundwater by sodium carboxymethyl cellulose stabilized zerovalent iron nanoparticles

Chromium (Cr) contamination poses serious threats to the environment and human health. Thus, batch and column experiments were performed to investigate hexavalent chromium [Cr (VI)] removal from solution and porous media using nanoscale zerovalent iron nanoparticles (NZVI) stabilized by sodium carboxymethyl cellulose (CMC). Batch experiments indicated that the mass ratio of Fe/CMC = 1, the presence of 150-200 mg L^-1 CMC and lower ionic strength led to optimum Cr (VI) removal in aqueous solution. Column experiments demonstrated that Cr (VI) removal was enhanced with decreasing solution pH and increasing CMC-NZVI concentration. The presence of CMC can increase Cr (VI) removal by NZVI in both aqueous solution and porous media by complexation precipitation of Cr (VI) compounds and better dispersion of NZVI. X-ray photoelectron spectroscopy (XPS) analysis revealed that an appropriate amount of CMC supported the redox reaction of Cr (VI) and NZVI. The removal of Cr (VI) through columns was 20.8% and 88.5% under no additional CMC and optimized CMC content, respectively. However, Cr (VI) removal decreased to 64.6% under excessive CMC content. The CMC modified NZVI nanoparticles were characterized by XRD, XPS and TEM techniques. These findings imply that CMC can be used as an effective stabilizer on NZVI which can in turn be applied for the efficient removal of Cr (VI) from industrial wastewater and groundwater.

Nanoadsorbents for water and wastewater remediation

Water has a wide-ranging effect on all aspects of human life, such as health and food. However, the water has often become polluted by the waste of our industrial, agricultural, and day-to-day activities due to the impact of humans. Therefore, there is an urgent need for new technologies to remove the contaminants from water and wastewater. Thence, many ways and techniques have been developed for water and wastewater remediation. Among all the methods of water and wastewater remediation techniques, the adsorption process has gained tremendous importance as a suitable water and wastewater remediation. The application of nanoadsorbent materials is a growing solution to solving this environmental problem. The unique physical and chemical properties of nanoadsorbents enhance their application due to its higher in ranking, status, and quality and beneficial in different fields compared to traditional adsorbents. Recently, numerous studies reported that the nanosorbent materials have a great and quite promising effect on water and wastewater treatment such as carbon tube, polymeric, zeolites, metal and metal oxides nanosorbents. Thus, the aim of this review article is to provide new data on the study and the improvement in this specific field, and to provide a version of the uses, benefits and restrictions of nanosorbents in water and wastewater remediation.

Semicontinuous enhanced electroreduction of Cr(VI) in wastewater by cathode constructed of copper rods coated with palladium nanoparticles followed by adsorption

Conventional techniques used for reduction of Cr(VI) in wastewater product great amounts of metal sludge due to the use of reducing chemicals. Since in electrochemical process, the reducing agent is the electron, so the main advantage of this method is its adaptability to the environment. The aim of the current study is to reduce Cr(VI) from electroplating wastewater by the electrochemical method and to adsorb Cr(III) by cellulose sulfate adsorbent. Furthermore, to enhance the reduction efficiency of Cr(VI), the cathode was modified with Pd nanoparticles. In the present study, recovery in the electrochemical column was conducted continuously and semi-continuously. In addition, the effect of pH, amperage, flow rate, and initial concentration of Cr(VI) was investigated. To remove Cr(III) from the wastewater, the cellulose sulfate adsorbent was provided from modification of cotton health wastes. The highest recovery rate (99.63%) was witnessed at pH = 1.5, 1 A amperage, flow rate of 4.24 mL min^-1, and initial concentration of 50 mg L^-1. The sewage was removed from the system after several consecutive cycles and during 20-55 min reached recovery efficiency of 99.99%. Based on the results, pH had the highest effect on the process. The optimum removal percentage was 85.74% occurred at a pH of 5.6, chromium concentration of 150 mg L^-1, and adsorbent concentration of 400 mg L^-1. The removal rate of the pollutant was 97.32%, done by cellulose sulfate adsorbent.

Elaboration of novel polyaniline@Almond shell biocomposite for effective removal of hexavalent chromium ions and Orange G dye from aqueous solutions

A novel polyaniline@Almond shell (PANI@AS) biocomposite was synthesized via facile in situ chemical polymerization method. The as-synthesized adsorbent was characterized using various analytical techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and potentiometric titration. A batch adsorption system was applied with the aim of investigating as-synthesized adsorbent ability to remove Cr(VI) ions and Orange G (OG) textile dye from aqueous solutions. Obtained results revealed that adsorption process was strongly depended upon the physicochemical parameters. The adsorption of Cr(VI) and OG dye onto PANI@AS was better described by the pseudo second-order-kinetic model and followed the Freundlich isotherm model. The maximum uptakes were 335.25 for Cr(VI) and 190.98 mg g^-1 for OG dye. We further evaluated that PANI@AS biocomposite could be regenerated easily with NaOH solution and efficiently reused for Cr(VI) and OG dye removal from aqueous media. Thus, these results indicated the potential practical application of PANI@AS biocomposite for wastewater treatment.