Functional rGO aerogel as a potential adsorbent for removing hazardous hexavalent chromium: adsorption performance and mechanism

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.

Free-standing N, S co-doped graphene aerogels coupled with Eucalyptus wood tar-based activated carbon and cellulose nanofibers for high-performance supercapacitor and removal of Cr(VI)

N, S-dual doping graphene aerogels with three dimensional interconnected network and large specific surface area have been fabricated by cellulose nanofibers (CNF), Eucalyptus wood tar-based activated carbon (AC), and reduced graphene oxide (rGO) for the energy storage applications as well as the removal of Cr(VI). Benefiting from the particular pore structural characteristics, the optimized activated carbon aerogel electrode (GDAC) exhibited prominent capacitances of 813.8 F/g at 1 A/g, and prominent cycling stability. The Ragone plot for the GDAC supercapacitor depicted that the energy density reached maximum (50 Wh/kg) when the power density was 370 W/kg. As far as the adsorption capacity of GDAC for Cr(VI), GDAC achieved a removal rate of 97 % for Cr(VI) and a maximum adsorption capacity of 939.20 mg/g. The fabrication method and excellent performance of GDAC proposed in this study provided new perspective into the potential application of Eucalyptus wood tar-based materials in the supercapacitor applications. Additionally, the comprehensive analysis of the structure-function relationship also provided important theoretical foundations for the removal of Cr(VI).

Magnetic Polypyrrole-Gelatin-Barium Ferrite Cryogel as an Adsorbent for Chromium (VI) Removal

Polypyrrole-gelatin aerogels, containing magnetic barium ferrite (BaFe) particles, (PPy-G-BaFe) were synthesized by oxidative cryopolymerization and used as adsorbents for the removal of Cr(VI) from aqueous media. The removal was performed at pH 4, which was shown to be the optimal value, due to HCrO4- being the dominant species in these conditions and its more favorable adsorption and reduction compared to CrO42-, present at pH > 4. It was found that the presence of magnetic BaFe particles had no effect on the adsorption performance of PPy aerogels in terms of capacity and kinetics, which was attributed to its relatively low content in the composite. After the adsorption, the presence of chromium in the composites was confirmed by EDX and its electrostatic interaction with the adsorbent was pointed at by vibrational spectroscopy, corresponding to the accepted adsorption mechanism. The adsorption kinetics followed the pseudo-second-order model pointing at chemisorption being the rate-limiting step. The adsorption isotherm data was best fitting with the Temkin model. The maximum adsorption capacity, calculated using the Langmuir model, was 255.8 mg g-1 (the maximum experimental value was 161.6 mg g-1). Additionally, the possibility of Cr(VI) adsorption in the presence of Cl-, Br-, NO3- and SO42- as interfering ions was shown.

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.

One-step synthesis of nitrogen-functionalized graphene aerogel for efficient removal of hexavalent chromium in water

The efficient removal of hexavalent chromium (Cr(VI)) with high toxicity has attracted widespread concern since it causes serious harm to ecological environment and public health. Herein, we report a novel nitrogen-functionalized graphene aerogel with stereoscopic structure through a simple hydrothermal method and freeze drying for Cr(VI) removal from water. Graphene oxide (GO) and nitrogenous organics polyethyleneimine (PEI) and pyrrole are used as raw materials for preparing PEI/polypyrrole/GO aerogel (PPGA) adsorbent. PEI and pyrrole act as nitrogen sources to introduce nitrogenous functional groups, and also take on the role of cross-linkers for helping GO sheets to form stereoscopic structure. The obtained PPGA has a fast adsorption rate, excellent reusability, and shows a remarkable adsorption capacity for Cr(VI) up to 458.24 mg/g under the optimal conditions (pH 2.0, 298 K, Cr(VI): 600 mg/L, dosage: 0.4 g/L). The adsorption process of Cr(VI) on PPGA can fit the pseudo-second-order kinetic model well. Analysis of intraparticle diffusion shows that the Cr(VI) removal process is a multi-step process. Adsorption thermodynamic and isotherm results demonstrate Cr(VI) adsorption on PPGA is a spontaneous endothermic process. The adsorption mechanism involves electrostatic attraction, redox, and chelation. In general, this work provides a simple and eco-friendly way to prepare PPGA, which has a great potential for actual application of Cr(VI) removal in effluent.

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.

Reduced graphene oxide (rGO) aerogel: Efficient adsorbent for the elimination of antimony (III) and (V) from wastewater

3D porous, thin sheet-like rGO aerogel was fabricated to explore its antimony (Sb) removal potential from wastewater. Langmuir isothermal and pseudo-second-order kinetic model best-suited the adsorption process. The maximum adsorption capacities were 168.59 and 206.72 mg/g for Sb (III and V) at pH 6.0 respectively. The thermodynamic parameters designated the process to be thermodynamically spontaneous, endothermic reaction, a result of dissociative chemisorption. The rGO aerogel bestowed good selectively among competing ions and reusability with 95% efficiency. rGO posed excellent practicability with Sb-spiked tap water and fixed-bed column experiments showing 97.6% of Sb (III) (3.6 μg/L) and 96.8% of Sb (V) (4.7 μg/L) removal from tap water and from fixed column bed experiments breakthrough volumes (BV) for the Sb (III) and Sb (V) ions were noted to be 540 BV and 925 BV respectively, until 5 ppb, which are below the requirement of MCL for Sb in drinking water (6 μg/L). XPS and DFT analyses explained adsorption mechanism and depicted a higher affinity of Sb (V) towards rGO surface than Sb (III).