Adsorption of Polycyclic Aromatic Hydrocarbons on Graphene Oxides and Reduced Graphene Oxides

Graphene-based nanomaterials for the removal of emerging contaminants of concern from water and their potential adaptation for point-of-use applications

Considering the plethora of work on the exceptional environmental performance of 2D nanomaterials, there is still a missing link in addressing their practical application in point-of-use (POU) water treatment. By reviewing the exceptional environmental performance of 2D nanomaterials with specific emphasis on graphene and its derivatives, this review aims at inspiring further discussions and research in graphene-based POU water treatment with particular focus on the removal of emerging contaminants of concern (ECCs), which is largely missing in the literature. We outlined the prevalence of ECCs in the environment, their health effects both on humans and marine life, and the potential of efficiently removing them from water using three-dimensional graphene-based macrostructures to ensure ease of adsorbent recovery and reuse compared to nanostructures. Given various successful studies showing superior adsorption capacity of graphene nanosheets, we give an account of the recent developments in graphene-based adsorbents. Moreover, several cost-effective materials which can be easily self-assembled with nanosheets to improve their environmental performance and safety for POU water treatment purposes were highlighted. We highlighted the strategy to overcome challenges of adsorbent regeneration and contaminant degradation; and concluded by noting the need for policy makers to act decisively considering the conservative nature of the water treatment industry, and the potential health risks from ingesting ECCs through drinking water. We further justified the need for the development of advanced POU water treatment devices in the face of the growing challenges regarding ECCs in surface water, and the rising cases of drinking water advisories across the world.

Ultrasonic-assisted synthesis of CaCu3Ti4O12/reduced graphene oxide composites for enhanced photocatalytic degradation of pharmaceutical products: Ibuprofen and Ciprofloxacin

The objective of this research is to create a highly effective approach for eliminating pollutants from the environment through the process of photocatalytic degradation. The study centers around the production of composites consisting of CaCu3Ti4O12 (CCTO) and reduced graphene oxide (rGO) using an ultrasonic-assisted method, with a focus on their capacity to degrade ibuprofen (IBF) and ciprofloxacin (CIP) via photodegradation. The impact of rGO on the structure, morphology, and optical properties of CCTO was inspected using XRD, FTIR, Raman, FESEM, XPS, BET, and UV-Vis. Morphology characterization showed that rGO particles were dispersed within the CCTO matrix without any specific chemical interaction between CCTO and C in the rGO. The BET analysis revealed that with increasing the amount of rGO in the composite, the specific surface area significantly increased compared to the CCTO standalone. Besides, increasing rGO resulted in a reduction in the optical bandgap energy to around 2.09 eV, makes it highly promising photocatalyst for environmental applications. The photodegradation of IBF and CIP was monitored using visible light irradiation. The results revealed that both components were degraded above 97% after 60 min. The photocatalyst showed an excellent reusability performance with a slight decrease after five runs to 93% photodegradation efficiency.

Adsorption of Pyrene and Arsenite by Micro/Nano Carbon Black and Iron Oxide

Polycyclic aromatic hydrocarbons (PAHs) and arsenic (As) are common pollutants co-existing in the environment, causing potential hazards to the ecosystem and human health. How their behaviors are affected by micro/nano particles in the environment are still not very clear. Through a series of static adsorption experiments, this study investigated the adsorption of pyrene and arsenite (As (III)) using micro/nano carbon black and iron oxide under different conditions. The objectives were to determine the kinetics and isotherms of the adsorption of pyrene and As (III) using micro/nano carbon black and iron oxide and evaluate the impact of co-existing conditions on the adsorption. The microstructure of micro/nano carbon black (C 94.03%) is spherical-like, with a diameter of 100-200 nm. The micro/nano iron oxide (hematite) has irregular rod-shaped structures, mostly about 1 µm long and 100-200 nm wide. The results show that the micro/nano black carbon easily adsorbed the pyrene, with a pseudo-second-order rate constant of 0.016 mg/(g·h) and an adsorption capacity of 283.23 μg/g at 24 h. The micro/nano iron oxide easily adsorbed As (III), with a pseudo-second-order rate constant of 0.814 mg/(g·h) and an adsorption capacity of 3.45 mg/g at 24 h. The mechanisms of adsorption were mainly chemical reactions. Micro/nano carbon black hardly adsorbed As (III), but its adsorption capability for pyrene was reduced by the presence of As (III), and this effect increased with an increase in the As (III) concentration. The adsorbed pyrene on the micro/nano black carbon could hardly be desorbed. On the other hand, the micro/nano iron oxide could hardly adsorb the pyrene, but its adsorption capability for As (III) was increased by the presence of pyrene, and this effect increased with an increase in the pyrene concentration. The results of this study provide guidance for the risk management and remediation of the environment when there is combined pollution of PAHs and As.

Montmorillonite modified Ni/Mg/Al ternary layered double hydroxide nanoflowers with enhanced adsorption features

A hydrothermal technique was employed to synthesize Ni/Mg/Al ternary L.D.H.s modified with montmorillonite (NMA-MMT-LDHs). Many characterization methods, including X-ray diffraction (XRD), scanning electron microscopy (S.E.M.), Fourier transform infrared (FTIR), and Brunauer, Emmett, and Teller (B.E.T.), were used to assess the physiochemical properties of the produced analytes. Congo red and methylene blue were utilized as model dyes to treat textile waste with the synthesized analytes. The batch adsorption model was utilized to conduct the adsorption experiments under varying contact time, adsorbent dosage, and solution pH conditions. A pseudo-second-order kinetics and the Langmuir adsorption model control the adsorption process. The maximum monolayer adsorption capacities of C.R. and M.B. were determined to be 344 and 200 mg/g, respectively. As the quantity of dosage increased from the 0.01-0.04 g, the percent removal efficiency (%) increased from 75 to 87 % for S2-LDH, 84-88 % for S2-MMT, 86-93 % for S3-MMT, and 95-97% for S4-MMT for C.R. dye and 82-85 % for S2-LDH, 83-89 % for S2-MMT, 83-91 % for S3-MMT, and 84-92 % for S4-MMT for M.B. dye. The removal percentage of C.R. dye for adsorbents S2-LDH, S2-MMT, S3-MMT, and S4-MMT were 75 %, 84 %, 86 %, and 95 %, respectively and 82 %, 83 %, 83 %, and 85 %, respectively for the M.B. dye removal. The presence of MMT significantly increases the affinity of Ni/Mg/Al-LDHs (NMA-LDHs), and the designed production technique can be used to produce a variety of compositionally distinct adsorbent materials.

Porous Hierarchical Ni/Mg/Al Layered Double Hydroxide for Adsorption of Methyl Orange from Aqueous Solution

A basic urea technique was successfully used to synthesize Mg/Al-Layered double hydroxides (Mg/Al LDHs), which were then calcined at 400 °C to form Mg/Al-Layered double oxides (Mg/Al LDOs). To reconstruct LDHs, Mg/Al LDOs were fabricated with different feeding ratios of Ni by the co-precipitation method. After synthesis, the Ni/Mg/Al-layered double hydroxides (NMA-LDHs) with 20% and 30% Ni (S1 and S2) were roasted at 400 °C and transformed into corresponding Ni/Mg/Al-layered double oxides (NMA-LDOs) (S1a and S2b, respectively). The physiochemical properties of synthesized samples were also evaluated by various characterization techniques, such as X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared (FTIR), and Brunauer, Emmett, and Teller (BET). The adsorption behavior of methyl orange (MO) onto the synthesized samples was evaluated in batch adsorption mode under varying conditions of contact time, adsorbent quantity, and solution pH. As the dosage amount increased from 0.01-0.04 g, the removal percentage of MO dye also increased from 83% to 90% for S1, 84% to 92% for S1a, 77% to 87% for S2, and 93% to 98% for S2b, respectively. For all of the samples, the adsorption kinetics were well described by the pseudo-second-order kinetic model. The equilibrium adsorption data were well fitted to both Langmuir and Freundlich models for methyl orange (MO). Finally, three adsorption-desorption cycles show that NMA-LDHs and NMA-LDOs have greater adsorption and reusability performance for MO dye, signifying that the design and fabrication strategy can facilitate the application of the natural hydrotalcite material in water remediation.

Adsorption of naphthalene polycyclic aromatic hydrocarbon from wastewater by a green magnetic composite based on chitosan and graphene oxide

A green magnetic composite mCS/GO was synthesized using water hyacinth extract, as a reducing agent, and proanthocyanidin, as a crosslinking agent, for the adsorption of naphthalene from effluents. The green composite was evaluated using different characterization techniques to determine its thermal (TG/DTG), structural (BET, XPS and FTIR), crystallographic (XRD), and textural (SEM) properties in natura and post-adsorption. The results obtained through a central composite design (CCD) experiment indicated that the initial concentration of NAP and the adsorbent dosage are significant for the adsorption capacity. The adsorption assays indicated that physisorption, through π-π and hydrophobic interactions, were the main mechanism involved in the NAP adsorption. However, the adjustment to the PSO and Freundlich models, obtained through kinetic and equilibrium studies, indicated that chemisorption also influences the adsorptive process. The thermodynamic study indicated physisorption as the mechanism responsible for the NAP adsorption. Also, the adsorbent has high affinity for the adsorbate and the process is spontaneous and endothermic. The maximum adsorption capacity (q_max) of the green mCS/GO was 334.37 mg g^-1 at 20 °C. Furthermore, the green mCS/GO was effectively regenerated with methanol and reused for five consecutive cycles, the percentage of NAP recovery went from approximately 91 to 75% after the fifth cycle. The green composite was also applied in the adsorption of NAP from river water samples, aiming to evaluate the feasibility of the method in real applications. The adsorption efficiency was approximately 70%. From what we know, this it is the first time that a green adsorbent was recycled after the polycyclic aromatic hydrocarbon (PAHs) adsorption process.

A Sustainable Approach for Graphene Oxide-supported Metal N-Heterocyclic Carbenes Catalysts

Sustainable noble metal-N-heterocyclic carbenes (NHC's) are a topic of arising concern in both the chemical industry and the academic community due to a growing consciousness of environmental pollution and scarcity. Recovering and reusing homogeneous catalysts from the reaction mixture requires a tremendous amount of capital investment in the chemical manufacturing industry. Heterogeneous catalysts are proved to have better functional groups tolerance; however, catalysts support largely influences the active catalyst sites to affect catalyst efficiency and selectivity. Thus the, choice of catalyst supports plays an almost decisive role in this emerging area of catalysis research. Graphene oxide (GO)/reduced graphene oxide (rGO) support has a potential growth in heterogeneous catalysis owing to their commercial availability, considerably larger surface area, inert towards chemical transformations, and easy surface functionalization to attached metal complexes via covalent and non-covalent aromatic π-conjugates. To take advantage of two independently well-established research areas of noble metal-N-heterocyclic carbenes and GO/rGO support via covalent or non-covalent interactions approach would offer novel heterogeneous complexes with improved catalytic efficiency without sacrificing product selectivity. This unique concept of marrying metal-N-heterocyclic carbenes with GO/rGO support has potential growth in the chemical and pharmaceutical industry, however, limited examples are reported in the literature. In this perspective, a comprehensive summary of metal-NHC synthesis on GO/rGO support and synthetic strategies to graft M-NHC onto GO/rGO surface, catalytic efficiency, for the catalytic transformation are critically reviewed. Furthermore, a plausible mechanism for non-covalent grafting methodology is summarized to direct readers to give a better understanding of M-NHC@rGO complexes. This would also allow the designing of engineered catalysts for unexplored catalytic applications.

Biochar application strategies for polycyclic aromatic hydrocarbons removal from soils

Polycyclic aromatic hydrocarbons (PAHs) are known as a hazardous group of pollutants in the soil which causes many challenges to the environment. In this study, the potential of biochar (BC), as a carbonaceous material, is evaluated for the immobilization of PAHs in soils. For this purpose, various bonding mechanisms of BC and PAHs, and the strength of bonds are firstly described. Also, the effect of impressive criteria including BC physicochemical properties (such as surface area, porosity, particle size, polarity, aromaticity, functional group, etc., which are mostly the function of pyrolysis temperature), number of rings in PAHs, incubation time, and soil properties, on the extent and rate of PAHs immobilization by BC are explained. Then, the utilization of BC in collaboration with biological tools which simplifies further dissipation of PAHs in the soil is described considering detailed interactions among BC, microbes, and plants in the soil matrix. The co-effect of BC and biological remediation has been authenticated by previous studies. Moreover, recent technologies and challenges related to the application of BC in soil remediation are explained. The implementation of a combined BC-biological remediation method would provide excellent prospects for PAHs-contaminated soils.

A Comprehensive DFT Investigation of the Adsorption of Polycyclic Aromatic Hydrocarbons onto Graphene

To better understand graphene and its interactions with polycyclic aromatic hydrocarbons (PAHs), density-functional-theory (DFT) computations were used. Adsorption energy is likely to rise with the number of aromatic rings in the adsorbates. The DFT results revealed that the distance between the PAH molecules adsorbed onto the G ranged between 2.47 and 3.98 Å depending on the structure of PAH molecule. The Non-Covalent Interactions (NCI) plot supports the concept that van der Waals interactions were involved in PAH adsorption onto the Graphene (G) structure. Based on the DFT-calculated adsorption energy data, a rapid and reliable method employing an empirical model of a quantitative structure–activity relationship (QSAR) was created and validated for estimating the adsorption energies of PAH molecules onto graphene.

Graphene Based Nanohybrid Aptasensors in Environmental Monitoring: Concepts, Design and Future Outlook

In view of ever-increasing environmental pollution, there is an immediate requirement to promote cheap, multiplexed, sensitive and fast biosensing systems to monitor these pollutants or contaminants. Aptamers have shown numerous advantages in being used as molecular recognition elements in various biosensing devices. Graphene and graphene-based materials/nanohybrids combined with several detection methods exhibit great potential owing to their exceptional optical, electronic and physicochemical properties which can be employed extensively to monitor environmental contaminants. For environmental monitoring applications, aptamers have been successfully combined with graphene-based nanohybrids to produce a wide range of innovative methodologies. Aptamers are immobilized at the surface of graphene based nanohybrids via covalent and non-covalent strategies. This review highlights the design, working principle, recent developmental advances and applications of graphene based nanohybrid aptasensors (GNH-Apts) (since January 2014 to September 2021) with a special emphasis on two major signal-transduction methods, i.e., optical and electrochemical for the monitoring of pesticides, heavy metals, bacteria, antibiotics, and organic compounds from different environmental samples (e.g., water, soil and related). Lastly, the challenges confronted by scientists and the possible future outlook have also been addressed. It is expected that high-performance graphene-based nanohybrid aptasensors would find broad applications in the field of environmental monitoring.

Adsorption of polycyclic aromatic hydrocarbons from wastewater using graphene-based nanomaterials synthesized by conventional chemistry and green synthesis: A critical review

Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants formed mainly by the incomplete combustion of organic matter, such as oil, gas and coal. The presence of PAHs can cause irreparable damage to the environment and living beings, which has generated a global concern with the short and long term risks that the emission of these pollutants can cause. Many technologies have been developed in the last decades aiming at the identification and treatment of these compounds, mainly, the PAHs from wastewater. This review features an overview of studies on the main methods of PAHs remediation from wastewater, highlighting the adsorption processes, through the application of different adsorbent nanomaterials, with a main focus on graphene-based nanomaterials, synthesized by conventional and green routes. Batch and fixed-bed adsorptive processes were evaluated, as well as, the mechanisms associated with such processes, based on kinetic, equilibrium and thermodynamic studies. Based on the studies analyzed in this review, green nanomaterials showed higher efficiency in removing PAHs than the conventional nanomaterials. As perspectives for future research, the use of green nanomaterials has shown to be sustainable and promising for PAHs remediation, so that further studies are needed to overcome the possible challenges and limitations of green synthesis methodologies.

Sorption of benzo[a]pyrene by Chernozem and carbonaceous sorbents: comparison of kinetics and interaction mechanisms

Benzo[a]pyrene (BaP) is a polycyclic aromatic hydrocarbon, highly persistent and toxic and a widespread environmental pollutant. Although various technologies have been developed to remove BaP from the environment, its sorption through solid matrixes has received increasing attention due to cost-effectiveness. The present research compares the adsorption capacity of Haplic Chernozem, granular activated carbon and biochar in relation to BaP from water solution. Laboratory experiments with different initial BaP concentrations in the liquid phase and different ratios of the solid and liquid phases show that Freundlich model describes well the adsorption isotherms of BaP by the soil and both sorbents. Moreover, the BaP isotherm sorption by the Haplic Chernozem is better illustrated by the Freundlich model than the Langmuir equation. The results reveal that the sorption capacity of the carbonaceous adsorbents at a ratio 1:20 (solid to liquid phases) is orders of magnitude higher (13 368 ng mL^-1 of activated carbon and 3 578 ng mL^-1 of biochar) compared to the soil (57.8 ng mL^-1). At the ratio of 0.5:20, the adsorption capacity of the carbonaceous sorbents was 17-45 times higher than that of the soil. This is due to the higher pore volume and specific surface area of the carbonaceous sorbents than soil particles, assessed through scanning electron microscopy. The sorption kinetic of BaP by Chernozem was compared with the adsorption kinetics by the carbonaceous sorbents. Results indicate that the adsorption dynamic involves two steps. The first one is associated with a fast BaP adsorption on the large available surface and inside macro- and meso-pores of the sorbent particles of the granular activated carbon and biochar. Then, the adsorption is followed by a slower process of BaP penetration into the microporous space and/or redistribution into a hydrophobic fraction. The effectiveness of the sorption process depends on both the sorbent properties and the solvent competition. Overall, the granular activated carbon and biochar are highly effective adsorbents for BaP, whereas the Haplic Chernozem has a rather limited capacity to remove BaP from contaminated solutions.

Removal Performance and Mechanism of Benzo(b)Fluorathene Using MnO2 Nanoflower/Graphene Oxide Composites

High-ring polycyclic aromatic hydrocarbons (PAHs, Benzo[b]fluorathene (BbFA), etc.) are difficult to biodegrade in the water environment. To address this issue, an innovative method for the preparation of MnO₂ nanoflower/graphene oxide composite (MnO₂ NF/GO) was proposed for adsorption removal of BbFA. The physicochemical properties of MnO₂ NF/GO were characterized by SEM, TEM, XRD, and N₂ adsorption/desorption and XPS techniques. Results show that the MnO₂ NF/GO had well-developed specific surface area and functional groups. Batch adsorption experiment results showed that adsorption capacity for BbFA was 74.07 mg/g. The pseudo-second-order kinetic model and Freundlich isotherm model are fitted well to the adsorption data. These show electron-donor-acceptor interaction; especially π-π interaction and π complexation played vital roles in BbFA removal onto MnO₂ NF/GO. The study highlights the promising potential adsorbent for removal of PAHs.

Strategies to improve the adsorption properties of graphene-based adsorbent towards heavy metal ions and their compound pollutants: A review

Heavy metal-containing wastewater can be treated by adsorption technology to obtain ultra-low concentration or high-quality treated effluent. Due to the constraints of the specific surface area, surface electrical structure and spatial effect of conventional adsorbents, it is often difficult to obtain adsorbents within high adsorption capacity. Graphene has characteristics of large specific surface area, small particle size, and high adsorption efficiency. It is considered as one of the research hotspots in recent years. However, despite graphene's unique properties, graphene-based adsorbents still have some drawbacks, i.e. graphene nanosheets are easier to be stacked with each other via π-π stacking and van der Waals interactions, which affect the site exposure, impede the rapid mass transport and limit its adsorption performance. Special strategy is needed to overcome its drawbacks. This work summarizes recent literatures on utilization of three strategies-surface functionalization regulation, morphology and structure control and material composite, to improve the adsorption properties of graphene-based adsorbent towards heavy metal removal. A brief summary, perspective on strategies to improving adsorption properties of graphene-based materials for heavy metal adsorption are also presented. Certainly, this review will be useful for designing and manufacturing of graphene-based nanomaterials for water treatment.

Micro-droplet Trapping and Manipulation: Understanding Aerosol Better for a Healthier Environment

Understanding the physicochemical properties and heterogeneous processes of aerosols is key not only to elucidate the impacts of aerosols on the atmosphere and humans but also to exploit their further applications, especially for a healthier environment. Experiments that allow for spatially control of single aerosol particles and investigations on the fundamental properties and heterogeneous chemistry at the single-particle level have flourished during the last few decades, and significant breakthroughs in recent years promise better control and novel applications aimed at resolving key issues in aerosol science. Here we propose graphene oxide (GO) aerosols as prototype aerosols containing polycyclic aromatic hydrocarbons, and GO can behave as two-dimensional surfactants which could modify the interfacial properties of aerosols. We describe the techniques of trapping single particles and furthermore the current status of the optical spectroscopy and chemistry of GO. The current applications of these single-particle trapping techniques are summarized and interesting future applications of GO aerosols are discussed.

Uptake and effects of graphene oxide nanomaterials alone and in combination with polycyclic aromatic hydrocarbons in zebrafish

Because of its surface characteristics, once in the aquatic environment, graphene could act as a carrier of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), to aquatic organisms. In this study we aimed to (1) assess the capacity of graphene oxide (GO) to sorb PAHs and (2) to evaluate the toxicity of GO alone and in combination with PAHs on zebrafish embryos and adults. GO showed a high sorption capacity for benzo(a)pyrene (B(a)P) (98% of B(a)P sorbed from a nominal concentration of 100 μg/L) and for other PAHs of the water accommodated fraction (WAF) of a naphthenic North Sea crude oil, depending on their log K_ow (95.7% of phenanthrene, 84.4% of fluorene and 51.5% of acenaphthene). In embryos exposed to different GO nanomaterials alone and with PAHs, no significant mortality was recorded for any treatment. Nevertheless, malformation rate increased significantly in embryos exposed to the highest concentrations (5 or 10 mg/L) of GO and reduced GO (rGO) alone and with sorbed B(a)P (GO-B(a)P). On the other hand, adults were exposed for 21 days to 2 mg/L of GO, GO-B(a)P and GO co-exposed with WAF (GO + WAF) and to 100 μg/L B(a)P. Fish exposed to GO presented GO in the intestine lumen and liver vacuolisation. Transcription level of genes related to cell cycle regulation and oxidative stress was not altered, but the slight up-regulation of cyp1a measured in fish exposed to B(a)P for 3 days resulted in a significantly increased EROD activity. Fish exposed to GO-B(a)P and to B(a)P for 3 days and to GO + WAF for 21 days showed significantly higher catalase activity in the gills than control fish. Significantly lower acetylcholinesterase activity, indicating neurotoxic effects, was also observed in all fish treated for 21 days. Results demonstrated the capacity of GO to carry PAHs and to exert sublethal effects in zebrafish.

Advances in water treatment technologies for removal of polycyclic aromatic hydrocarbons: Existing concepts, emerging trends, and future prospects

In the last two decades, environmental experts have focused on the development of several biological, chemical, physical, and thermal methods/technologies for remediation of PAH-polluted water. Some of the findings have been applied to field-scale treatment, while others have remained as prototypes and semi-pilot studies. Existing treatment options include extraction, chemical oxidation, bioremediation, photocatalytic degradation, and adsorption (employing adsorbents such as biomass derivatives, geosorbents, zeolites, mesoporous silica, polymers, nanocomposites, and graphene-based materials). Electrokinetic remediation, advanced phytoremediation, green nanoremediation, enhanced remediation using biocatalysts, and integrated approaches are still at the developmental stage and hold great potential. Water is an essential component of the ecosystem and highly susceptible to PAH contamination due to crude oil exploration and spillage, and improper municipal and industrial waste management, yet comprehensive reviews on PAH remediation are only available for contaminated soils, despite the several treatment methods developed for the remediation of PAH-polluted water. This review seeks to provide a comprehensive overview of existing and emerging methods/technologies, in order to bridge information gaps toward ensuring a green and sustainable remedial approach for PAH-contaminated aqueous systems. PRACTITIONER POINTS: Comprehensive review of existing and emerging technologies for remediation of PAH-polluted water. Factors influencing efficiency of various methods, challenges and merits were discussed. Green nano-adsorbents, nano-oxidants and bio/phytoremediation are desirous for ecofriendly and economical PAH remediation. Adoption of an integrated approach for the efficient and sustainable remediation of PAH-contaminated water is recommended.

Characterization of graphene/pine wood biochar hybrids: Potential to remove aqueous Cu2

Biochar-based hybrid composites containing added nano-sized phases are emerging adsorbents. Biochar, when functionalized with nanomaterials, can enhance pollutant removal when both the nanophase and the biochar surface act as adsorbents. Three different pine wood wastes (particle size < 0.5 mm, 10 g) were preblended with 1 wt% of three different graphenes in aqueous suspensions, designated as G1, G2, and G3. G1 (S_BET, 8.1 m²/g) was prepared by sonicating graphite made from commercial synthetic graphite powder (particle size 7-11 μm). G2 (312.0 m²/g) and G3 (712.0 m²/g) were purchased commercial graphene nanoplatelets (100 mg in 100 mL deionized water). These three pine wood-graphene mixtures were pyrolyzed at 600 °C for 1 h to generate three graphene-biochar adsorbents, GPBC-1, GPBC-2, and GPBC-3 containing 4.4, 4.9, and 5.0 wt% of G1, G2, and G3 respectively. Aqueous Cu²⁺ adsorption capacities were 10.6 mg/g (GPBC-1), 4.7 mg/g (GPBC-2), and 5.5 mg/g (GPBC-3) versus 7.2 mg/g for raw pine wood biochar (PBC) (0.05 g adsorbent dose, Cu²⁺ 75 mg/L, 25 mL, pH 6, 24 h, 25 ± 0.5 °C). Increased graphene surface areas did not result in adsorption increases. GPBC-1, containing the lowest nanophase surface area with the highest Cu²⁺ capacity, was chosen to evaluate its Cu²⁺ adsorption characteristics further. Results from XPS showed that the surface concentration of oxygenated functional groups on the GPBC-1 is greater than that on the PBC, possibly contributing to its greater Cu²⁺ removal versus PBC. GPBC-1 and PBC uptake of Cu²⁺ followed the pseudo-second-order kinetic model. Langmuir maximum adsorption capacities and BET surface areas were 18.4 mg/g, 484.0 m²/g (GPBC-1) and 9.2 mg/g, 378.0 m²/g (PBC). This corresponds to 3.8 × 10^-2 versus 2.4 × 10^-2 mg/m² of Cu²⁺ removed on GPBC-1 (58% more Cu²⁺ per m²) versus PBC. Cu²⁺ adsorption on both these adsorbents was spontaneous and endothermic.

An Overview and Evaluation of Highly Porous Adsorbent Materials for Polycyclic Aromatic Hydrocarbons and Phenols Removal from Wastewater

Polycyclic aromatic hydrocarbons (PAHs) and phenolic compounds had been widely recognized as priority organic pollutants in wastewater with toxic effects on both plants and animals. Thus, the remediation of these pollutants has been an active area of research in the field of environmental science and engineering. This review highlighted the advantage of adsorption technology in the removal of PAHs and phenols in wastewater. The literature presented on the applications of various porous carbon materials such as biochar, activated carbon (AC), carbon nanotubes (CNTs), and graphene as potential adsorbents for these pollutants has been critically reviewed and analyzed. Under similar conditions, the use of porous polymers such as Chitosan and molecularly imprinted polymers (MIPs) have been well presented. The high adsorption capacities of advanced porous materials such as mesoporous silica and metal-organic frameworks have been considered and evaluated. The preference of these materials, higher adsorption efficiencies, mechanism of adsorptions, and possible challenges have been discussed. Recommendations have been proposed for commercialization, pilot, and industrial-scale applications of the studied adsorbents towards persistent organic pollutants (POPs) removal from wastewater.

Unravelling the Potential of Graphene Quantum Dots in Biomedicine and Neuroscience

Quantum dots (QDs) are semiconducting nanoparticles that have been gaining ground in various applications, including the biomedical field, thanks to their unique optical properties. Recently, graphene quantum dots (GQDs) have earned attention in biomedicine and nanomedicine, thanks to their higher biocompatibility and low cytotoxicity compared to other QDs. GQDs share the optical properties of QD and have proven ability to cross the blood-brain barrier (BBB). For this reason, GQDs are now being employed to deepen our knowledge in neuroscience diagnostics and therapeutics. Their size and surface chemistry that ease the loading of chemotherapeutic drugs, makes them ideal drug delivery systems through the bloodstream, across the BBB, up to the brain. GQDs-based neuroimaging techniques and theranostic applications, such as photothermal and photodynamic therapy alone or in combination with chemotherapy, have been designed. In this review, optical properties and biocompatibility of GQDs will be described. Then, the ability of GQDs to overtake the BBB and reach the brain will be discussed. At last, applications of GQDs in bioimaging, photophysical therapies and drug delivery to the central nervous system will be considered, unraveling their potential in the neuroscientific field.

Enhanced adsorption of hydrophobic organic contaminants by high surface area porous graphene

The relatively low surface area and micropore volume of graphene nanosheets (GNS) limit their potential application as effective adsorbents for hydrophobic organic contaminants (HOCs). In this study, KOH etching was used to develop activated GNS (K-GNS) for adsorption of model HOCs such as naphthalene, phenol, nitrobenzene, and bisphenol A. After activation, the specific surface area (SSA) of K-GNS increased to 885 m²/g, which was three times larger than that of GNS. The micropore volume of K-GNS substantially increased and the C/O ratio was doubled. Accordingly, the adsorption capacity of these HOCs on K-GNS was larger than that of pristine GNS (P-GNS) by 2-8 times. The kinetic data was fitted by the pseudo-second-order model, and the adsorption isotherms of HOCs on P-GNS and K-GNS were fitted by the Freundlich model. The desorption studies showed the K-GNS had a lower rate of release than P-GNS. The high adsorption of naphthalene, phenol, nitrobenzene, and bisphenol A on P-GNS and K-GNS is dominated by hydrophobic and π-π interactions. Additionally, the π-π EDA interaction and hydrogen bond between K-GNS and substituents cannot be ignored.

Nanotechnology-based sorption and membrane technologies for the treatment of petroleum-based pollutants in natural ecosystems and wastewater streams

Petroleum processing wastewater (PPW) is a complex mixture of free, soluble, and emulsive hydrocarbons that often contain heavy metals and/or solid particles. As these hazardous constituents can accumulate in human beings and the environment, exposure to the PPW can have harmful effects in various respects. The use of environmental nanotechnologies (E-Nano) is considered an attractive option to resolve the problems associated with PPW. Among different treatment technologies, E-Nano-based sorption (adsorption/absorption) and membrane filtration approaches have been proven to have outstanding efficacy in remediation of PPW pollutants. It is, however, crucial to determine the appropriate technological option (e.g., low-cost operational conditions) for the practical application of such technologies. In this review, the potential of E-Nano-based sorption and membrane technologies in the treatment of various PPW pollutants is discussed based on their performances in comparison to traditional technologies. Their suitability is evaluated further in relation to their merits/disadvantages and economic feasibility with the goal of constructing a perspective map to efficiently implement the E-Nano technologies.

Application of multivariate analysis on naphthalene adsorption in aqueous solutions

Naphthalene (NAP) is found as a pollutant in water, soil, and air, and adsorption is the most prominent removal process of this compound, among the methods studied. A study concerning the types of adsorbents and the parameters with the greatest influence on the adsorption process is interesting to direct future works on new adsorbents. The use of multivariate data analysis tools becomes an appealing way to compile data obtained from bibliographic reviews and to establish a behavior in NAP adsorption. This work aims to evaluate the parameters with greater influence on NAP adsorption process regarding adsorption capacity (qe_exp) with the principal component analysis (PCA), and to group common NAP adsorbents by chemical characteristics through hierarchical cluster analysis (HCA). The variables qe_exp, S, [NAP]₀, T, CT, and [Ads] were used to perform PCA with correlation matrix. For the HCA, the variables S, [NAP]₀, T, CT, and [Ads] with average linkage method (UPGMA) and Euclidean distance were used. Through PCA, it is possible to infer that S and [NAP]₀ are the factors with greater influence in qe_exp of NAP, while T, CT, and [Ads] have little correlation. PCA also shows that activated charcoal is the adsorbent with higher qe_exp. HCA grouped the adsorbents into four groups by their chemical classes, except group A. Both PCA and HCA methods show themselves as potential tools to evaluate a data set of NAP adsorption processes.

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.

Integrating Biolayer Interferometry, Atomic Force Microscopy, and Density Functional Theory Calculation Studies on the Affinity between Humic Acid Fractions and Graphene Oxide

The interactions between nanoparticles and humic acid (HA) are critical to understanding the environmental risks and applications of nanoparticles. However, the interactions between HA fractions and graphene oxide (GO, a popular carbon nanosheet) at the molecular level remain largely unclear. Four HA fractions with molecular weights ranging from 4.6 to 23.8 kDa were separated, and the large HA fractions presented low oxygen contents and many aromatic structures. The binding constants of the large HA fractions on GO were 2.6- to 3551-fold higher than those of the small HA fractions, while the maximum adsorption capacities of the larger HA fractions onto GO were much higher. Atomic force microscopy (AFM) found that the small and large HA fractions were spread over the center and the edge of the GO nanosheets, respectively. Density functional theory (DFT) simulation and nuclear magnetic resonance spectroscopy confirmed the above phenomena (three adsorption patterns, "vs", "ps", and "pea") and revealed that HA bonded to the GO nanosheets mainly through van der Waals force and π-π interactions. The integrating analysis of binding affinity, AFM, and DFT provides new insights into the environmental behavior of GO and the applications of GO in pollutant removal under exposure from HA.

Tailored Design of Differently Modified Mesoporous Materials To Deeply Understand the Adsorption Mechanism for Polycyclic Aromatic Hydrocarbons

A series of SBA-15 with different modifications have been successfully prepared and applied as adsorbents to remove polycyclic aromatic hydrocarbons (PAHs) from aqueous solutions. The morphology and structural properties of the chemically modified materials are all similar to those of pure SBA-15, and thus the difference of PAHs adsorption capacity can be directly attributed to the different functional groups, which is favorable to deeply explore the adsorption mechanism. Adsorption kinetics and isotherm experiments for naphthalene (Nap), anthracene (Ant), and pyrene (Pyr) were carried out, and the results reveal that the adsorption processes follow a pseudo-second-order rate equation and the equilibrium can be achieved within 120 min for Nap and Ant, whereas only 90 min for Pyr, indicating that the more hydrophobic molecules, the easier and faster adsorption can be obtained. All of the adsorption isotherms fit well with the Freundlich model, suggesting the unevenly distributed active sites on adsorbents. The phenyl-functionalized materials possess the highest adsorption capacity, implying that the π-π interaction is the most primary interaction and plays the predominant role in the studied PAHs adsorption, superior to the acidic and hydrophobic interaction. Our research sheds light on the design and synthesis of advanced and highly efficient adsorbents to remove PAHs from aqueous solutions.

Adsorption mechanism of emerging and conventional phenolic compounds on graphene oxide nanoflakes in water

Emerging contaminants (ECs) such as bisphenol A (BPA), 4-nonylphenol (4-NP) and tetrabromobisphenol A (TBBPA) have gained immense attention worldwide due to their potential threat to humans and environment. Graphene oxide (GO) nanomaterial is considered as an important sorbent due to its exceptional range of environmental application owing to its unique properties. GO was also considered as one of ECs because of its potential hazard. The adsorption of organic contaminants such as phenolic ECs on GO affects the stability of GO nanoflakes in water and the fate of organic contaminants, which would cause further environmental risk. Therefore, the adsorption behaviors of emerging and common phenolic compounds (PCs) including phenol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 4-NP, BPA and TBBPA on GO nanoflakes and their stability in water were studied. The adsorption equilibrium for all the compounds was reached <10h and was fitted with Langmuir and Freundlich isotherms. In addition to hydrophobic effect, adsorption mechanisms included π-π bonding and hydrogen bonding interactions between the adsorbate and GO, especially the electrostatic interactions were observed. Phenol has the highest adsorption affinity due to the formation of hydrogen bond. GO has a good stability in water even after the adsorption of PCs in the presence of a common electrolyte, which could affect its transport with organic contaminants in the environment. These better understandings illustrate the mechanism of emerging and common PC interaction with GO nanoflakes and facilitate the prediction of the contaminant fate in the aquatic environment.

Impact of emerging, high-production-volume graphene-based materials on the bioavailability of benzo(a)pyrene to brine shrimp and fish liver cells

With increasing commercialization of high volume, two-dimensional carbon nanomaterials comes a greater likelihood of environmental release. In aquatic environments, black carbon binds contaminants like aromatic hydrocarbons, leading to changes in their uptake, bioavailability, and toxicity. Engineered carbon nanomaterials can also adsorb pollutants onto their carbon surfaces, and nanomaterial physicochemical properties can influence this contaminant interaction. We used 2D graphene nanoplatelets and isometric carbon black nanoparticles to evaluate the influence of particle morphology and surface properties on adsorption and bioavailability of benzo(a)pyrene, a model aromatic hydrocarbon, to brine shrimp (Artemia franciscana) and a fish liver cell line (PLHC-1). Acellular adsorption studies show that while high surface area carbon black (P90) was most effective at a given concentration, 2D graphene nanoplatelets (G550) adsorbed more benzo(a)pyrene than carbon black with comparable surface area (M120). In both biological models, co-exposure to nanomaterials lead to reduced bioavailability, with G550 graphene nanoplatelets cause a greater reduction in bioavailability or response than the M120 carbon black nanoparticles. However, on a mass basis the high surface area P90 carbon black was most effective. The trends in bioavailability and adsorption were consistent across all biological and acellular studies, demonstrating the biological relevance of these results in different models of aquatic organisms. While adsorption is limited by surface area, 2D graphene nanoplatelets adsorb more benzo(a)pyrene than carbon black nanoparticles of similar surface area and charge, demonstrating that both surface area and shape play important roles in the adsorption and bioavailability of benzo(a)pyrene to carbon nanomaterials.

Polycyclic Aromatic Hydrocarbons Adsorption onto Graphene: A DFT and AIMD Study

Density functional theory (DFT) calculations and ab-initio molecular dynamics (AIMD) simulations were performed to understand graphene and its interaction with polycyclic aromatic hydrocarbons (PAHs) molecules. The adsorption energy was predicted to increase with the number of aromatic rings in the adsorbates, and linearly correlate with the hydrophobicity of PAHs. Additionally, the analysis of the electronic properties showed that PAHs behave as mild n-dopants and introduce electrons into graphene; but do not remarkably modify the band gap of graphene, indicating that the interaction between PAHs and graphene is physisorption. We have also discovered highly sensitive strain dependence on the adsorption strength of PAHs onto graphene surface. The AIMD simulation indicated that a sensitive and fast adsorption process of PAHs can be achieved by choosing graphene as the adsorbent. These findings are anticipated to shed light on the future development of graphene-based materials with potential applications in the capture and removal of persistent aromatic pollutants.

Reduced Graphene Oxide-Hybridized Polymeric High-Internal Phase Emulsions for Highly Efficient Removal of Polycyclic Aromatic Hydrocarbons from Water Matrix

Reduced graphene oxide (RGO)-hybridized polymeric high-internal phase emulsions (RGO/polyHIPEs) with an open-cell structure and hydrophobicity have been successfully prepared using 2-ethylhexyl acrylate and ethylene glycol dimethacrylate as the monomer and the cross-linker, respectively. The adsorption mechanism and performance of this RGO/polyHIPEs to polycyclic aromatic hydrocarbons (PAHs) were investigated. Adsorption isotherms of PAHs on RGO/polyHIPEs show that the saturated adsorption capacity is 47.5 mg/g and the equilibrium time is 8 h. Cycling tests show that the adsorption capacity of RGO/polyHIPEs remains stable in 10 adsorption-desorption cycles without observable structure change in RGO/polyHIPEs. Moreover, the PAH residues in water samples after being purified by RGO/polyHIPEs are lower than the limit values in drinking water set by the European Food Safety Authority. These results demonstrate that the RGO/polyHIPEs have great potentiality in PAH removal and water purification.

Water treatment by new-generation graphene materials: hope for bright future

Water is the most important and essential component of earth's ecosystem playing a vital role in the proper functioning of flora and fauna. But, our water resources are contaminating continuously. The whole world may be in great water scarcity after few decades. Graphene, a single-atom thick carbon nanosheet, and graphene nanomaterials have bright future in water treatment technologies due to their extraordinary properties. Only few papers describe the use of these materials in water treatment by adsorption, filtration, and photodegradation methods. This article presents a critical evaluation of the contribution of graphene nanomaterials in water treatment. Attempts have been made to discuss the future perspectives of these materials in water treatment. Besides, the efforts are made to discuss the nanotoxicity and hazards of graphene-based materials. The suggestions are given to explore the full potential of these materials along with precautions of nanotoxicity and its hazards. It was concluded that the future of graphene-based materials is quite bright.

Adsorptive removal of aromatic hydrocarbons from water over metal azolate framework-6-derived carbons

Metal azolate framework-6 (MAF-6) was pyrolyzed at 1000°C to yield MOF-derived carbons (MCs). The obtained MCs were used to eliminate aromatic hydrocarbons, including polyaromatic hydrocarbons (PAHs; e.g., naphthalene (NAP), anthracene (ATC), and pyrene (PRN)) and benzene (BZ) from water via adsorption. The adsorption results over the MCs were compared with that of pristine MAF-6 and commercial activated carbon (AC). MC obtained after 24h (MC-24) exhibited a remarkable adsorption efficiency compared to that of the other MCs (obtained after different durations), MAF-6, and AC. For example, MC-24 led to adsorptions of NAP around 17 and 2.5 times those of pristine MAF-6 and AC, respectively. Or, the maximum adsorption capacities (Q₀) of MAF-6, AC and MC-24 for NAP were 14, 104 and 237mg/g, respectively. Moreover, Q₀ values of MC-24 for ATC and PRN were also very high of 284 and 307mg/g, respectively. Based on the properties of PAHs and the hydrophobicity of MC-24, hydrophobic interaction was suggested as the main mechanism for the adsorption of PAHs and BZ. In addition, MC-24 can be recycled by washing with acetone with little loss in performance. Therefore, MC-24 is recommended as a competitive adsorbent for aromatic hydrocarbon removal from water.