Selective adsorption and determination of hexavalent chromium ions using graphene oxide modified with amino silanes

SERS-based microdevices for use as in vitro diagnostic biosensors

Advances in surface-enhanced Raman scattering (SERS) detection have helped to overcome the limitations of traditional in vitro diagnostic methods, such as fluorescence and chemiluminescence, owing to its high sensitivity and multiplex detection capability. However, for the implementation of SERS detection technology in disease diagnosis, a SERS-based assay platform capable of analyzing clinical samples is essential. Moreover, infectious diseases like COVID-19 require the development of point-of-care (POC) diagnostic technologies that can rapidly and accurately determine infection status. As an effective assay platform, SERS-based bioassays utilize SERS nanotags labeled with protein or DNA receptors on Au or Ag nanoparticles, serving as highly sensitive optical probes. Additionally, a microdevice is necessary as an interface between the target biomolecules and SERS nanotags. This review aims to introduce various microdevices developed for SERS detection, available for POC diagnostics, including LFA strips, microfluidic chips, and microarray chips. Furthermore, the article presents research findings reported in the last 20 years for the SERS-based bioassay of various diseases, such as cancer, cardiovascular diseases, and infectious diseases. Finally, the prospects of SERS bioassays are discussed concerning the integration of SERS-based microdevices and portable Raman readers into POC systems, along with the utilization of artificial intelligence technology.

Comprehensive assessment of carbon-, biomaterial- and inorganic-based adsorbents for the removal of the most hazardous heavy metal ions from wastewater

Owing to the high cost of recycling waste, underdeveloped countries discharge industrial, agricultural, and anthropogenic effluents without pretreatment. As a result, pollutant-loaded waste enters water bodies. Among the diverse toxic contaminants, heavy metal ions are the most detrimental because of their chronic toxicity, non-degradability, prevalence, and bioaccumulation. The growing shortage of water resources demands the removal of heavy metal ions from wastewater. Three SDGs of the sustainability agenda of the United Nations appeal for clean water to protect life beneath water and on land depending on the water sources. Therefore, efficient environmentally friendly approaches for wastewater treatment are urgently required. In this regard, several methods have been developed for the removal of heavy metal ions from wastewater, including adsorption as the most widely used method owing to its eco-friendly, cost-effective, and sustainable nature. The present review discusses the progress in the preparation and application of various adsorbents based on carbon, micro-organisms, agricultural waste and inorganic materials for the extraction of toxic metal ions such as Pb2+, Cr6+, As3+, As5+, Hg2+ and Cd2+. Herein, we provide information on the role of the homogeneity and heterogeneity of adsorbents, kinetics of the adsorption of an adsorbate on the surface of an adsorbent, insights into adsorption reaction pathways, the mechanism of the sorption process, and the uptake of solutes from solution. The present review will be useful for researchers working on environmental protection and clean environment.

Employ a Clay@TMSPDETA hybrid material as an adsorbent to remove textile dyes from wastewater effluents

A grafting of N1-(3-trimethoxysilylpropyl)diethylenetriamine (TMSPDETA) on natural clay was carried out to obtain an organic-inorganic hybrid clay material that was applied as an adsorbent to the uptake of Reactive Blue 19 (RB-19) and Reactive Green 19 (RG-19) dyes from aqueous wastewaters. This research demonstrates the effect of TMSPDETA contents on amino-functionalized clay materials' hydrophobic/hydrophilic behavior. The resultant material was utilized to uptake reactive dyes in aqueous solutions. The clay@TMSPDETA hybrid material was characterized by isotherm of adsorption and desorption of nitrogen, FTIR, elemental analysis, TGA, pHpzc, total acidity, total basicity groups, and hydrophilic balance. The hybrid samples were more hydrophilic than the pristine clay for ratios from 0.1 up to 0.5 due to adding amino groups to the pristine clay. FTIR spectra suggest that TMSPDETA was grafted onto the clay. The hybrid material presents a surface area 2.17-fold (42.7 m2/g) lower than pristine clay (92.7 m2/g). The total volume of pores of hybrid material was 0.0822 cm3/g, and the pristine clay material was 0.127 cm3/g, corresponding to a diminution of the total pore volume (Vtot) of 1.54 times. The kinetic data followed the pseudo-second-order (PSO) model for RB-19 and RG-19 reactive dyes. The equilibrium data were better fitted to the Liu isotherm model, displaying a Qmax as 178.8 and 361.1 mg g-1 for RB-19 and RG-19, respectively, at 20.0 °C. The main mechanism of interactions of the reactive dyes with the hybrid clay is electrostatic interaction. The clay@TMSPDETA has a very good effect on treating synthetic dye-textile wastewater. The removal percentage of simulated wastewater was up to 97.67% and 88.34% using distilled water and plastic industry wastewater as the solvents, respectively. The clay@TMSPDETA-0.1 could be recycled up to 5 cycles of adsorption and desorption of both dyes, attaining recoveries of 98.42% (RB-19) and 98.32% (RG-19) using 0.1 M HCl + 10% ethanol.

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).

Contemporary updates on bioremediation applications of graphene and its composites

Graphene, a 2D single-layered carbon sp² hybrid substance set in a honeycomb network, is widespread in many carbon-based materials. Due to its extraordinary optical, electrical, thermal, mechanical, and magnetic competences as well as its significant specific surface area, it has attracted a lot of interest recently. Synthesizing graphene refers to any process for creating or extracting the material, depending on the desired purity, size, and efflorescence of the finished good. Numerous methods have been employed for graphene synthesis categorized as top-down procedures and bottom-up procedures. Graphene finds its implementations in various industries such as electronics, energy, chemical, transport, defence, and biomedical areas such as accurate biosensing. It has been widely used in water treatment as a binder for organic contaminants and heavy metals. Many researches have fixated on creating various modified graphene, graphene oxide composites, graphene nanoparticle composites and semiconductor hybrids of graphene for contaminant removal from water. In this review, we have tried to address various production methods for graphene and its composites along with their advantages and disadvantages. Furthermore, we have presented a summary on graphene's outstanding immobilization of a variety of contaminants like toxic heavy metals, organic dyes, inorganic pollutants and pharmaceutical wastes. Additionally, a development of graphene-based microbial fuel cell (MFC) has been evaluated in an effort to produce ecological wastewater treatment and bioelectricity.

Surface functionalization of bamboo leave mediated synthesized SiO2 nanoparticles: Study of adsorption mechanism, isotherms and enhanced adsorption capacity for removal of Cr (VI) from aqueous solution

Green synthesis of nanoparticles (NPs) provides economic and environmental benefits as an alternative to chemical or physical methods. Furthermore, the surface properties of such NPs can be modulated by means of the functionalization with different groups making them suitable for various advanced functional applications including water pollutants removal using adsorption technique. In the present work, an eco-friendly synthesis route for nano-adsorbent SiO₂ NPs and subsequent surface modifications for enhanced adsorption capacity in removal of Cr(VI) ions from aqueous solution are reported. The green synthesis of SiO₂ NPs was carried out using simple bamboo leaves followed by surface modification with amine (A-SiO₂) and carboxylic (C-SiO₂) functional groups with aim to study the effect of functionalization on adsorption capacity. These nano-adsorbents were characterized by FTIR, SEM, XPS, BET, and zeta potential. and adsorption of Cr(VI) was studied at varying parameters i.e. NPs mass, contact time, and solution pH. The investigation shows interesting results revealing the importance of interactions between the surface functional groups on SiO₂ NPs and Cr(VI) species as well as experimental conditions for the choice of surface modifier to achieve a maximum adsorption capacity. The adsorption mechanism has been studied using Langmuir, Freundlich and Temkin adsorption isotherms. The maximum adsorption capacity has been achieved in the case of A-SiO₂ NPs which was found to 174 mg/g and much higher than that of SiO₂ and C-SiO₂ NPs attributed to the selective adsorption and pH conditions. Additionally, A-SiO₂ NPs exhibit excellent recyclability indicating their suitability for promising and long term potential applications. This study provides a novel, simple and cost-effective synthesis/surface engineering technology for producing high performance recyclable nano-adsorbents for adsorptive removal of Cr(VI).

Flame atomic absorption spectrometry combined with surface-modified magnetic mesoporous silica microspheres by polyethyleneimine for enrichment, isolation and determination of Cu2+ in preserved eggs after high-temperature digestion

A new efficient magnetic solid-phase extractant based on a surface-modified magnetic mesoporous silica microsphere referred as MMSM-PEI was synthesised and used for the enrichment and isolation of copper ions (Cu²⁺) in preserved eggs. The physicochemical properties and morphology of MMSM-PEI were characterized by X-ray diffraction (XRD) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), vibration sample magnetometry (VSM), scanning electron microscopy (SEM) and thermos-gravimetric analyses (TGA). The concentrations of trace Cu²⁺ in the preserved egg were determined by flame atomic absorption spectroscopy (FAAS). The effects of important parameters were examined. The most suitable pH values and temperature for adsorbing Cu²⁺ were 6.5 and 25 °C, respectively. According to the determination of Cu²⁺ in egg white, egg yolk and the outer coating mixture (TOCM) of preserved eggs, the spiked recovery and RSD were 94.1-103.8% and 0.96-4.35%, respectively. The limit of detection (LOD) and the limit of quantitation (LOQ) were 0.14 mg/kg and 0.46 mg/kg, respectively. The developed method improved the sensitivity and accuracy of FAAS for the determination of Cu²⁺ and it could be applied to the determination of trace Cu²⁺ in real samples.

Bioengineered magnetic graphene oxide microcomposites for bioremediation of chromium in ex situ - A novel strategy for aggrandized recovery by electromagnetic gadgetry

Novel magnetic microcomposites consisting of graphene oxide and iron oxide was synthesized to immobilize metabolically versatile Paracoccus sp. MKU1 and Leucobacter sp. AA7 and tested for the simultaneous adsorption and enhanced biological detoxification of hexavalent chromium (Cr(VI)) from tannery wastewater. This study reports highest chromium adsorption of 272.6 mg/g and 179.3 mg/g with complete reduction of Cr(VI) to Cr(III) by the microcomposites of AA7 and MKU1 from wastewater in a bioreactor (10 L) at large-scale for first time in ex situ. Furthermore, both the microcomposites displayed an enhanced detoxification of tannery wastewater by reducing various physicochemical conditions such as ammonia, nitrate, TDS, fluoride, CaCO_3, Ca, Mg, NO₃ and SO₂ under the permissible limits. Use of electromagnetic device for magnetic microcomposites recovery from bioreactor yielded a maximum of 88% and 80.6% recovery for AA7 and MKU1, respectively. The rate of chromium recuperation achieved following desorption from the microcomposites of AA7 and MKU1 was 90.71% and 93.97%, respectively. Thus, the multifarious benefits including adsorption, metabolic detoxification, recovery, and recuperation by single functional microcomposites seems to be an intriguing and profitable approach for practicing in real-time operations to effectively remove heavy metals from the contaminated wastewater for environmental protection.

Modeling of Hexavalent Chromium Removal with Hydrophobically Modified Cellulose Nanofibers

Cellulose nanofibers (CNF) are sustainable nanomaterials, obtained by the mechanical disintegration of cellulose, whose properties make them an interesting adsorbent material due to their high specific area and active groups. CNF are easily functionalized to optimize the performance for different uses. The hypothesis of this work is that hydrophobization can be used to improve their ability as adsorbents. Therefore, hydrophobic CNF was applied to adsorb hexavalent chromium from wastewater. CNF was synthetized by TEMPO-mediated oxidation, followed by mechanical disintegration. Hydrophobization was performed using methyl trimetoxysilane (MTMS) as a hydrophobic coating agent. The adsorption treatment of hexavalent chromium with hydrophobic CNF was optimized by studying the influence of contact time, MTMS dosage (0-3 mmol·g^-1 CNF), initial pH of the wastewater (3-9), initial chromium concentration (0.10-50 mg·L^-1), and adsorbent dosage (250-1000 mg CNF·L^-1). Furthermore, the corresponding adsorption mechanism was identified. Complete adsorption of hexavalent chromium was achieved with CNF hydrophobized with 1.5 mmol MTMS·g^-1 CNF with the faster adsorption kinetic, which proved the initial hypothesis that hydrophobic CNF improves the adsorption capacity of hydrophilic CNF. The optimal adsorption conditions were pH 3 and the adsorbent dosage was over 500 mg·L^-1. The maximum removal was found for the initial concentrations of hexavalent chromium below 1 mg·L^-1 and a maximum adsorption capacity of 70.38 mg·g^-1 was achieved. The kinetic study revealed that pseudo-second order kinetics was the best fitting model at a low concentration while the intraparticle diffusion model fit better for higher concentrations, describing a multi-step mechanism of hexavalent chromium onto the adsorbent surface. The Freundlich isotherm was the best adjustment model.

Selective adsorption and ultrafast fluorescent detection of Cr(VI) in wastewater using neodymium doped polyaniline supported layered double hydroxide nanocomposite

Neodymium-doped polyaniline supported Zn-Al layered double hydroxide (PANI@Nd-LDH) nanocomposite has been prepared via an ex-situ oxidative polymerization process. The as-prepared nanocomposite shows selective fluorescence detection and adsorption of hexavalent chromium Cr(VI) within a short period. The fluorescence intensity of PANI@Nd-LDH decreases linearly with Cr(VI) concentrations ranging from 200 ppb to 1000 ppb with a limit of detection (LOD) of 1.5 nM and a limit of quantification (LOQ) of 96 nM. The sensing mechanism can be ascribed by the inner filter effect of Cr(VI), the intercalation of Cr(VI) within the intergallery region of LDH, and the synergistic affinity of metal ions along with the polymer chain for Cr(VI). The adsorption performance of PANI@Nd-LDH nanocomposite is evaluated for Cr(VI) from wastewaters, which displayed high removal capacity towards Cr(VI) (219 mg/g) as compared on bare Nd-LDH (123 mg/g) and LDH (88 mg/g) respectively. The adsorption of Cr(VI) on PANI@Nd-LDH depends on the pH of the aqueous solution. The adsorption isotherm and kinetics are supported by the Langmuir model and pseudo-second-order model, respectively. Owing to the highly sensitive detection and adsorption of Cr(VI) from aqueous water samples demonstrated the potential application of PANI@Nd-LDH as an excellent environmental probe can be exploited.

Graphene-Based Materials Immobilized within Chitosan: Applications as Adsorbents for the Removal of Aquatic Pollutants

Graphene and its derivatives, especially graphene oxide (GO), are attracting considerable interest in the fabrication of new adsorbents that have the potential to remove various pollutants that have escaped into the aquatic environment. Herein, the development of GO/chitosan (GO/CS) composites as adsorbent materials is described and reviewed. This combination is interesting as the addition of graphene to chitosan enhances its mechanical properties, while the chitosan hydrogel serves as an immobilization matrix for graphene. Following a brief description of both graphene and chitosan as independent adsorbent materials, the emerging GO/CS composites are introduced. The additional materials that have been added to the GO/CS composites, including magnetic iron oxides, chelating agents, cyclodextrins, additional adsorbents and polymeric blends, are then described and discussed. The performance of these materials in the removal of heavy metal ions, dyes and other organic molecules are discussed followed by the introduction of strategies employed in the regeneration of the GO/CS adsorbents. It is clear that, while some challenges exist, including cost, regeneration and selectivity in the adsorption process, the GO/CS composites are emerging as promising adsorbent materials.

Preparation of environmental samples for chemical speciation of metal/metalloids: A review of extraction techniques

Chemical speciation is a relevant topic in environmental chemistry since the (eco)toxicity, bio (geo)chemical cycles, and mobility of a given element depend on its chemical forms (oxidation state, organic ligands, etc.). Maintaining the chemical stability of the species and avoiding equilibrium disruptions during the sample treatment is one of the biggest challenges in chemical speciation, especially in environmental matrices where the level of concomitants/interferents is normally high. To achieve this task, strategies based on chemical properties of the species can be carried out and pre-concentration techniques are often needed due to the low concentration ranges of many species (μg L^-1 - ng L^-1). Due to the significance of the topic and the lack of reviews dealing with sample preparation of metal (loid)s (usually, sample preparation reviews focus on the total metal content), this work is presented. This review gives an up-to-date overview of the most common sample preparation techniques for environmental samples (water, soil, and sediments), with a focus on speciation of metal/metalloids and determination by spectrometric techniques. Description of the methods is given, and the most recent applications (last 10 years) are presented.

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.

Ion-imprinted modified molecular sieves show the efficient selective adsorption of chromium(vi) from aqueous solutions

Molecular sieve 5A was modified with (3-aminopropyl) triethoxysilane (APTES) as the support matrix, on which 4-VP was used as the ionic imprinting group. The as-prepared IIZMS-APTES was applied as the adsorbent for the recovery of chromium(vi) from aqueous solutions. The adsorbent was characterized via Fourier transform infrared spectroscopy (FT-IR), scanning electronic microscopy (SEM), and X-ray diffraction (XRD). The influences of adsorption time, concentration of the ions, initial pH, and temperature on the adsorption performance to Cr(vi) were investigated. The selectivity and reusability of IIZMS-APTES are also evaluated. The results showed that the maximum adsorption capacity reached 56.46 mg g-1 when the initial concentration of metal ions was at 100 mg L-1 at pH 2 and 30 °C. The adsorption process followed the pseudo-second-order kinetic model and Langmuir adsorption isotherm model. The IIZMS-APTES exhibits an efficient selective adsorption of Cr(vi) from aqueous solutions.

Ultrasound-assisted dispersive micro-solid phase extraction using molybdenum disulfide supported on reduced graphene oxide for energy dispersive X-ray fluorescence spectrometric determination of chromium species in water

Molybdenum disulfide (MoS2) was supported on graphene oxide (GO) by hydrothermal method. The resulting nanocomposite (MoS2-rGO) was characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The experiments show that at pH 2, MoS2-rGO has a great affinity for adsorption of hexavalent chromium ions while Cr(III) ions remain in aqueous sample. In the adsorption process, the dominant role plays chemisorption. The determined adsorption capacity is 583.5 mg g-1. Parameters affecting the extraction process, namely sample pH, sample volume, contact time, and matrix ions, were investigated by sequential batch tests. Under optimal conditions (pH 2, sample volume 50 mL, sonication time 10 min, adsorbent mass 1 mg), the calibration curve covers the 1-200 ng mL-1 range with a correlation coefficient (R2) of 0.998. The recovery of the method is 97 ± 3%. Other data of merit include a relative standard deviation of < 3.5%, enrichment factor of 3350, and detection limit of 0.050 ng mL-1. The accuracy of the method was confirmed by analysis of the reference materials QC1453 (chromium VI in drinking water) and QC3015 (chromium VI in seawater). The method was successfully applied to chromium speciation in water samples, including high salinity ones. The concentration of Cr(III) was calculated as the difference between the total concentration of chromium (after oxidation of Cr(III) to Cr(VI) with potassium permanganate) and the initial Cr(VI) content.Graphical abstract Schematic presentation of a method for determination of chromium species by energy dispersive X-ray fluorescence spectrometry after preconcentration on molybdenum disulfide supported on reduced graphene oxide.

Chemically modified Quercus dilatata plant leaves for Pb (II), Cd (II), and Cr (VI) ions remediation from aqueous solution

The current investigation deals with the removal of Pb (II), Cd (II), and Cr (VI) ions by using chemically modified Quercus dilatata leaves (CMQDL) treated with nitric acid (HNO3), and calcium chloride (CaCl2). Batch biosorption experiments were performed to determine the optimal conditions of pH, biomass dose, temperature, contact time, and initial metal concentration for the utmost removal of heavy metals from water. The structural morphology and functionalities were explained by SEM and FTIR analysis. The maximum biosorption capacities for remediation of Pb (II), Cd (II), and Cr (VI) ions via CMQDL were 17.54, 20.408, 20.83 mg g−1, respectively at the optimal conditions. The Langmuir and Freundlich isotherm were applied to explore the equilibrium data however Freundlich isotherm model best evaluate the equilibrium data with high regression correlation coefficient (R2) values of 0.985, 0.826, and 0.919 for the elimination of Pb (II) Cd (II), and Cr (VI) ions, respectively. The kinetic study proposed that the remediation operation best obeyed the kinetic pseudo 2nd order model. The calculated thermodynamics functions like change in entropy (ΔS°), change in enthalpy (ΔH°) and Gibbs free energy (ΔG°) revealed that the removal of Pb (II) ions via the CMQDL was viable, exothermic and spontaneous, Cd (II) was endothermic and spontaneous and Cr (VI) was endothermic and non-spontaneous. The current study explored that CMQDL can be used for the remediation of Pb (II), Cd (II), and Cr (VI) ions, respectively.

Preparation of a new magnetic ion-imprinted polymer and optimization using Box-Behnken design for selective removal and determination of Cu(II) in food and wastewater samples

A new magnetic Cu(II) IIP (Fe₃O₄@IIP-IDC) is synthesized by polymerization of Imidazole-4,5-dicarboxylic acid functionalized Allyl chloride, and significant improvement of its performance has been compared. SPE parameters were optimized using Box-Behnken design to achieve the twin objectives of quantitative determination and removal of Cu(II). FLPSO kinetic model and BS isotherm model fits well with the capacity of 175 mg g^-1. Analytical figures of merit includes a linearity range of 10-5,000 µg L^-1 (R² = 0.9986), preconcentration factor of 50 after eluting with 5 mL of 1 M HNO₃, LOD of 1.03 µg L^-1 and LOQ of 4.5 µg L^-1. Accuracy was assessed by analysis of SRM (Standard Reference Material) and recovery experiments after spiking in food samples (Tea, coffee, chocolate, spinach, infant milk substitute) and battery wastewater. Ease of use, reusability (15 cycles), rapid adsorption and high selectivity makes it a promising candidate for efficient and selective removal and trace determination.

Most suitable amino silane molecules for surface functionalization of graphene oxide toward hexavalent chromium adsorption

Adsorption is a simple and effective method for the removal of hexavalent chromium (Cr(VI)) from contaminated water. Several amino silane-graphene oxide (GO) composites with different species of amino groups (pN-GO, psN-GO, and pssN-GO; p: primary, s: secondary, N: amine) were evaluated to investigate their adsorption capacity and the effects of primary and secondary amines on Cr(VI) adsorption. We conducted a quantitative analysis to reveal the difference between primary and secondary amines in terms of Cr(VI) removal efficiency. A synergic effect was observed between the neighboring secondary amines in pssN-GO. From the Langmuir model prediction, we found that the composite with pssN-GO exhibited the highest maximum adsorption capacity (260.74 mg/g), followed by those with psN-GO (208.22 mg/g) and pN-GO (189.47 mg/g). Monolayer adsorption was more dominant when using pssN-GO, with the pseudo-second-order model best fitting the kinetic experiment results, whereas multilayer adsorption was dominant when using psN-GO and pN-GO.

Performance Differences of Hexavalent Chromium Adsorbents Caused by Graphene Oxide Drying Process

In this study, the influence of drying conditions on amine (−NH₃) functionalization of graphene oxide (GO) was evaluated, and the hexavalent chromium (Cr(VI)) adsorption efficiency of the prepared materials was compared. 3-[2-(2-aminoehtylamino) ethylamino]propyl-trimethoxysilane (3N) was used for amine functionalization. The synthesized materials were analyzed by SEM, BET, TGA, XPS, and EA. TGA results showed that the solution-GO (SGO) was functionalized by more 3N molecules than freeze-dried GO (FDGO) and oven-dried GO (ODGO). Additionally, XPS analysis also showed that the ratio of N/C and Si/C was relatively high in SGO than FDGO and ODGO. The maximum adsorption capacity of SGO, FDGO, and ODGO for Cr(VI) was 258.48, 212.46, and 173.45 mg g⁻¹, respectively. These results indicate that it is better to use SGO without drying processes for efficient amine functionalization and Cr(VI) removal. However, when the drying process is required, freeze-drying is better than oven-drying.

Laponites® for the Recovery of 133Cs, 59Co, and 88Sr from Aqueous Solutions and Subsequent Storage: Impact of Grafted Silane Loads

In this study, silylated Laponites^® (LAP) were synthetized with various loads of 3-aminopropyltriethoxysilane (APTES) to evaluate their adsorption properties of ¹³³Cs, ⁵⁹Co, and ⁸⁸Sr during single-solute and competitive experiments. The increase in the initial load of APTES increased the adsorbed amount of APTES in the resulted grafted clay. The characterization of LAP-APTES exhibited a covalent binding between APTES and LAP and emphasized the adsorption sites of APTES for each tested load. In comparison with raw LAP, LAP-APTES displayed significantly higher adsorption properties of Co²⁺, Cs⁺, and Sr²⁺. The competitive adsorption of these three contaminants provides a deeper understanding of the affinity between adsorbate and adsorbent. Therefore, Co²⁺ displayed a strong and specific adsorption onto LAP-APTES. Except for Cs⁺, the adsorption capacity was improved with increasing the load of APTES. Finally, the desorption behavior of the three contaminants was tested in saline solutions. Cs⁺ and Sr²⁺ were significantly released especially by inorganic cations displaying the same valence. Conversely, desorption of Co²⁺ was very low whatever the saline solution. LAP-APTES, therefore, presented suitable adsorption properties for the removal of radionuclides especially for Co²⁺, making this material suitable to improve the decontamination of radioactive wastewaters.

Nanomaterials-based treatment options for chromium in aqueous environments

Sustainable development and the restoration of ecosystems are the important goals for civilization. Currently, heavy metal contamination of aquatic environments has become a serious issue. Chromium (Cr) is simultaneously an essential metallic element and one of 20 chemicals posing a maximum threat to living beings. To mitigate that threat, various treatment methods have been developed, including adsorption, electrocoagulation, photoelectrocatalysis, fuel cells, bioremediation, chemical precipitation, ultrafiltration, ion exchange, and co-precipitation. However, selection of the most energy- and cost-efficient wastewater treatment option has proven challenging, as each approach is subject to shortcomings involving energy consumption, treatment capacity, and efficiency. This review describes the potential role of diverse functional nanomaterials (e.g., iron/iron oxide nanoparticles, carbon nanostructures, metal organic frameworks, and their commercial counterparts) in treatment of Cr in aqueous environments with respect to key figure of merits, such as, adsorption capacity, removal efficiency, and partition coefficient. In addition, their performance was compared with the most common treatment options. The results of this study will help determine the most effective and economical options for control of Cr in aquatic environments.

Recent advances in hexavalent chromium removal from aqueous solutions by adsorptive methods

Chromium exists mainly in two forms in environmental matrices, namely, the hexavalent (Cr(vi)) and trivalent (Cr(iii)) chromium. While Cr(iii) is a micronutrient, Cr(vi) is a known carcinogen, and that warrants removal from environmental samples. Amongst the removal techniques reported in the literature, adsorption methods are viewed as superior to other methods because they use less chemicals; consequently, they are less toxic and easy to handle. Mitigation of chromium using adsorption methods has been achieved by exploiting the physical, chemical, and biological properties of Cr(vi) due to its dissolution tendencies in aqueous solutions. Many adsorbents, including synthetic polymers, activated carbons, biomass, graphene oxide, and nanoparticles as well as bioremediation, have been successfully applied in Cr(vi) remediation. Initially, adsorbents were used singly in their natural form, but recent literature shows that more composite materials are generated and applied. This review focused on the recent advances, insights, and project future directions for these adsorbents as well as compare and contrast the performances achieved by the mentioned adsorbents and their variants.

Graphene-based adsorbents for the removal of toxic organic pollutants: A review

The synthesis and application of efficient materials for remediation of environmental contaminants from water is an emerging area of research. Graphene has received tremendous attention in various fields due to its exceptional properties. Graphene and its derivatives have also been extensively explored for the adsorptive removal of pollutants from water. The recent trends are inclined toward functionalization of graphene-based materials to get the advantage of their improved properties. The functionalized graphene materials are efficient due to their enhanced properties resulting from synergistic effects. This article reviews the synthesis and application of graphene-based adsorbents for the removal of organic pollutants from water. A critical account is provided on synthesis methods, applications, adsorption mechanisms, the figure of merits, and removal performances. The accomplishments, limitations, challenges, and future research directions are also highlighted.

Polystyrene-divinyl benzene microspheres with amino methyl phosphonic acid functional hairy brushes for the sorption and speciation of chromium prior to inductively coupled plasma mass spectrometric determination

This article describes the synthesis and application of a novel sorbent for Cr(III) and Cr(VI) speciation prior to their quantitation by inductively coupled plasma mass spectrometry. The sorbent consists of polystyrene-divinyl benzene microbeads that were graft-coated with poly(oligo (ethylene glycol) methacrylate)-block-poly(glycidyl methacrylate). The particles were finally modified with phosphomethylated triethylene tetramine. The resulting microbeads are shown to be a viable sorbent for Cr(VI). The total concentration of chromium was determined after oxidation of Cr(III) to Cr(VI) with KMnO₄ using the novel sorbent. The Cr(III) amount was then calculated by subtracting the concentration of Cr(VI) from that of total chromium. The optimum conditions for batch type sorption were established. Under optimal conditions, the limit of detection and quantification are 0.015 μg L^-1 and 0.050 μg L^-1, respectively. The kinetics and isotherms of the sorption of Cr(VI) were investigated. Following desorption with 0.1 M hydroxylamine hydrochloride, the method was successfully applied to spiked real water samples and a certified reference material. Graphical abstract Schematic presentation of a method for the sorption and speciation of chromium using amino methyl phosphonic acid functional brushes on polystyrene-divinyl benzene microspheres.

Photophysical deactivation behaviour of Rhodamine B using different graphite materials

In the present work, an attempt has been made to elucidate the structural features of synthesized graphite materials, i.e., expanded graphite (EG) and an expanded graphite/silver nanoparticles (EG/AgNPs) nanocomposite. In order to obtain knowledge about the functional groups present, the interlayer spacing between the carbon layers, topographical features, and the characterization of the materials were carried out using Fourier-transformer infrared spectroscopy, X-ray diffraction, Raman spectroscopy, field emission scanning electron microscopy-energy dispersive X-ray spectroscopy and atomic force microscope. Furthermore, the quenching efficiency of the synthesized graphite materials was also compared using Rhodamine B (Rhd B) as a fluorescent probe. The non-linear behaviour of the Stern–Volmer plots suggested that the complex quenching mechanism (a combination of static and dynamic quenching) was responsible for the decrease in photoluminescence intensity. At a lower concentration of the quencher, the static quenching mechanism was dominant whereas at a higher concentration dynamic processes seemed to be more likely. The binding strength of the complexation between the fluorophore and the quencher at lower concentrations was studied in detail for both of the synthesized materials. The analysis showed that the EG/AgNPs exhibited better quenching efficiency and possessed a strong binding strength in comparison to EG. The thermodynamic parameters of this association suggested that the interaction process was spontaneous and exothermic in nature. Thus, this work offers helpful insights into the fluorescence quenching mechanisms of the Rhd B/EG and its composite system.

Graphical representation of varying quenching mechanism of RhD B dye using different graphite materials i.e. EG and EG/AgNPs.

A label-free nano-probe for sequential and quantitative determination of Cr(VI) and ascorbic acid in real samples based on S and N dual-doped carbon dots

A fluorescent sulfur and nitrogen dual-doped carbon dots (S,N-CDs) was prepared by a simple and one-step acid-base neutralization and exothermic carbonization method. Hexavalent chromium (Cr(VI)) could effectively quench the fluorescence of S,N-CDs based on inner filter effect (IFE) and dynamic quenching, whereas ascorbic acid (AA) could recover the fluorescence of S,N-CDs/Cr(VI) because of IFE weakening. So an "on-off-on" and label-free nano-probe consecutive determination of Cr(VI) and AA was constructed. This nano-probe system demonstrated excellent selectivity and sensitivity to Cr(VI) and AA with linear range of 0.065-198 μmol/L (3.38-10,296 μg/L) and 6.6-892 μmol/L (1.16-157 mg/L), respectively. Meanwhile, the as-prepared S,N-CDs possess low toxicity and could be used for multi-color cell imaging in SMMC 7721 cells. More importantly, this nano-probe was successfully employed for detection of Cr(VI) in tap water and AA in food samples. In view of its simple detection condition, rapid response, wide linear range, low detection limit and inexpensive instrument, the as-constructed nano-probe system could have a wide range of potential application, including water quality monitoring and evaluation, food inspection and testing and biomedical analysis.

The development of a ternary nanocomposite for the removal of Cr(vi) ions from aqueous solutions

The aim of this study is to develop a ternary nanocomposite (NC) of polyaniline (PANI)/2-acrylamido-2-methylpropanesulfonic acid (AMPSA)-capped silver nanoparticles (NPs)/graphene oxide quantum dots (PANI/Ag (AMPSA)/GO QDs) as an efficient adsorbent for the removal of the highly toxic hexavalent chromium (Cr(vi)) from polluted water. PANI/Ag (AMPSA)/GO QDs NC was synthesized via in situ oxidative polymerization. The effects of pH, adsorbent dose, initial concentration, temperature, contact time, ionic strength and co-existing ions on the removal of Cr(vi) by PANI/Ag (AMPSA)/GO QDs were investigated. The PANI/Ag (AMPSA)/GO QDs NC (25.0 mg) removed 99.9% of Cr(vi) from an aqueous solution containing 60 mg L⁻¹ Cr(vi) ions at pH 2. Energy dispersive X-ray (EDX) and inductively coupled plasma spectrometry (ICP) studies confirmed the adsorption of Cr(vi) and that some of the adsorbed Cr(vi) was reduced to Cr(iii). Cr(vi) removal by the PANI/Ag (AMPSA)/GO QDs NC followed the pseudo-second order kinetic model, and the removal was highly selective for Cr(vi) in the presence of other co-existing ions. In summary, the PANI/Ag (AMPSA)/GO QDs NC has potential as a novel adsorbent for Cr(vi).

The aim is to develop a ternary nanocomposite of polyaniline/2-acrylamido-2-methylpropanesulfonic acid-capped silver nanoparticles/graphene oxide quantum dots as an efficient adsorbent for the removal of the highly toxic hexavalent chromium (Cr(vi)) from polluted water.

Determination and speciation of ultratrace arsenic and chromium species using aluminium oxide supported on graphene oxide

Alumina supported on graphene oxide (Al₂O₃/GO) nanocomposite as new nanosorbent in dispersive micro-solid phase extraction (DMSPE) for As(V) and Cr(III) preconcentration is described. The crucial issue of the study is synthesis of novel nanocomposite suitable for sorption of selected species of arsenic and chromium. Al₂O₃/GO demonstrates selectivity toward arsenates in the presence of arsenites at pH 5 and chromium(III) ions in the presence of chromate anions at pH 6. The Al₂O₃/GO nanocomposite was characterized by scanning electron microscopy (SEM) transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and the Raman spectroscopy. The maximum adsorption capacity calculated based on the Langmuir adsorption model were 43.9 mg g^-1 and 53.9 mg g^-1 for As(V) and Cr(III), respectively. The nanocomposite was used as solid sorbent in preconcentration of As(V) and Cr(III)_ions from water samples and their determination using energy dispersive X-ray fluorescence spectrometry (EDXRF). The As(V) and Cr(III) ions can be quantitatively preconcentrated from 25 to 100 mL aqueous samples within 5 min using DMSPE procedure and 1 mg of Al₂O₃/GO. The nanocomposite was also used for preparation of Al₂O₃/GO membrane. Then, As(V) and Cr(III)_ions can be retained under flow condition by passing analyzed solution through Al₂O₃/GO membrane. Under the optimized conditions, As(V) and Cr(III) ions can be determined with very good recovery (92-108%), precision (RSD 2.7-4.0%) and excellent limit of detection (0.02 ng mL^-1 As and 0.11 ng mL^-1 Cr). The accuracy of the method was studied by analyzing certified reference materials (NIST 1640a) and spiked real water samples.