Highly efficient heavy-metal extraction from water with carboxylated graphene nanoflakes

Rearrangement of GO nanosheets with inner and outer forces under high-speed spin for supercapacitor

The self-standing graphene membranes are considered as ideal electrode materials for supercapacitors. However, maintaining highly regularized and uniform graphene membranes with satisfied electrochemical performance is still a challenge. Herein, with the chelation of metal cation and the radial shear force introduced by high-speed spinning, the uniform interlayer channels and shrunken cracks between adjacent nanosheets can be achieved in the metal-intercalated graphene oxide (GO) membranes, thus realizing regularization both in normal and radial direction. With the promotion in electron transfer and electrolyte penetration, the iron cross-linked GO membrane with spin coating for 40 cycles exhibits a high specific capacitance (427 F g^-1 at 1 A g^-1) and rate capability (42.6% capacitance retention from 1 to 40 A g^-1), as well as excellent cyclic capability (90.5% capacitance retention after 20,000 cycles). Particularly, a 21% increasement in capacitance can be achieved after high-speed spinning treatment. Moreover, the spin regularization strategy can be extended to GO membranes cross-linked by various multi-valence metal cations, the electrochemical performance of metal-cation cross-linked GO membrane electrodes after high-speed spinning treatment can also be improved obviously. Therefore, this paper provides a novel method to fabricate highly ordered GO membranes with promising electrochemical performance, which presents an immense potential application in membrane materials applied in energy storage, separation and catalysis.

A comprehensive review of capacitive deionization technology with biochar-based electrodes: Biochar-based electrode preparation, deionization mechanism and applications

The recently developed techniques for desalination and wastewater treatment are costly and unsustainable. Therefore, a cost-effective and sustainable approach is essential to achieve desalination through wastewater treatment. Capacitive deionization (CDI), an electrochemical desalination technology, has been developed as a novel water treatment technology with great potential. The electrode material is one of the key factors that promotes the development of CDI technology and broadens the scope of CDI applications. Biochar-based electrode materials have attracted increasing attention from researchers because of their advantages, such as environmentally friendly, economical, and renewable properties. This paper reviews the methods for preparing biochar-based electrode materials and elaborates on the mechanism of CDI ion storage. We then summarize the applications of CDI technology in water treatment, analyze the mechanism of pollutant removal and resource recovery, and discuss the applicability of different CDI configurations, including hybrid CDI systems. In addition, the paper notes that environmentally friendly green activators that facilitate the development of pore structure should be developed more often to avoid the adverse environmental impact. The development of ion-selective electrode materials should be enhanced and it is necessary to comprehensively assess the impact of heteroatoms on selective ion removal and CDI performance. Electrooxidation of organic pollutants should be further promoted to achieve organic degradation by extending to redox reactions.

Synthesis of 3D graphene-based materials and their applications for removing dyes and heavy metals

Contamination of water streams by dyes and heavy metals has become a major problem due to their persistence, accumulation, and toxicity. Therefore, it is essential to eliminate and/or reduce these contaminants before discharge into the natural environment. In recent years, 3D graphene has drawn intense research interests owing to its large surface area, superior charge conductivity, and thermal conductivity properties. Due to their unique surface and structural properties, 3D graphene-based materials (3D GBMs) are regarded as ideal adsorbents for decontamination and show great potential in wastewater or exhaust gas treatment. Here, this minireview summarizes the recent progress on 3D GBMs synthesis and their applications for adsorbing dyes and heavy metals from wastewater based on the structures and properties of 3D GBMs, which provides valuable insights into 3D GBMs' application in the environmental field.

Graphene-based materials for adsorptive removal of pollutants from water and underlying interaction mechanism

Graphene-based materials have received much attention as attractive candidates for the adsorptive removal of pollutants from water due to their large surface area and diverse active sites for adsorption. The design of graphene-based adsorbents for target pollutants is based on the underlying adsorption mechanisms. Understanding the adsorption performance of graphene-based materials and its correlation to the interaction mechanisms between the pollutants and adsorbents is crucial to the further development of graphene-based functional materials and their practical applications. This review summarizes recent advances on the development of graphene-based materials for the adsorption of heavy metal ions, dyes, and oils, and the co-adsorption of their mixture from water. The material design, performance, regeneration and reuse of adsorbents, and the associated adsorption mechanisms are discussed. Various techniques for mechanistic studies of the adsorption of heavy metal ions, dyes, and oils on graphene-based materials are highlighted. The remaining challenges and perspectives for future development and investigation of graphene-based materials as adsorbents are also presented.

Toward green flotation: Interaction of a sophorolipid biosurfactant with a copper sulfide

The United Nations' Sustainable Development Goals have sparked growing interest in biosurfactants from many surfactant-loaded industries including those utilizing froth flotation for mineral separation. However, the interaction of biosurfactants with mineral surfaces is currently poorly understood. We bridge this gap by studying adsorption of a yeast-derived bola acidic sophorolipid (ASL) biosurfactant on djurleite (Cu_1.94S). The methods used include Hallimond flotation, contact angle, adsorption isotherm, zeta potential, leaching measurements, and X-ray photoelectron spectroscopy (XPS). To facilitate the interpretation of the adsorption results, we characterize the activity of ASL at the air-water interface and measure its critical micelle concentration (CMC) at different pH using static surface tension. We find ASL to be a multifunctional surfactant with an unusual, pH-sensitive interfacial behavior. At the air-water interface, ASL is most active at pH 8, while its CMC goes through minimum as low as 40 μM at pH 7. The surfactant adsorption at the djurleite-water interface makes the sulfide surface hydrophilic at acidic pH and hydrophobic at neutral and basic pH. In addition, ASL has strong affinity to copper sulfide and demonstrates metal leaching properties. Finally, ASL demonstrates detergency properties. We offer a mechanistic interpretation of these findings. Our results provide a basis for the application of acidic glycolipids in froth flotation and have implications for their application in ion separation using hydrometallurgical routes, as well as for the chemical stability of metal sulfides in environmental systems.

State of the art: synthesis and characterization of functionalized graphene nanomaterials

Nanomaterials play nowadays a preponderant role in the field of materials science due to the wide range of applications and synergy with other fields of knowledge. Recently, carbonaceous nanomaterials, most notably bi-dimensional graphene (2D graphene), have been highlighted by their application in several areas: electronics, chemistry, medicine, energy and the environment. The search for new materials has led many researchers to develop new routes of synthesis and the expansion of the current means of production, by the anchoring of other nanomaterials on graphene surface, or by modifications of its hexagon sp2 structure, through the doping of heteroatoms. By adding functional groups to the graphene surface, it is possible to increase its affinity with other materials, such as polymers, magnetic nanoparticles and clays, leading to the formation of new nanocomposites. Several covalent and non-covalent functionalization processes, their advantages and disadvantages with respect to their interactions with other chemical species, are discussed in this review. The characterization of these materials is a sensitive topic, since the insertion of functional groups over the graphene basal plane causes changes in its morphology and the so-called chemistry of surface. In this sense, beyond the classical techniques, such as x-ray Diffraction (XRD), Infrared Spectroscopy (FTIR), Raman Spectroscopy and Transmission Electron Microscopy (TEM), modern characterization techniques of graphene-based nanomaterials are discussed, focusing on those more indicated according to the proposed modifications. A significant attention was driven to environmental applications of functionalized graphenes, specifically in the removal of pollutants from wastewaters.

Carbon Transition-metal Oxide Electrodes: Understanding the Role of Surface Engineering for High Energy Density Supercapacitors

Supercapacitors store electrical energy by ion adsorption at the interface of the electrode-electrolyte (electric double layer capacitance, EDLC) or through faradaic process involving direct transfer of electrons via oxidation/reduction reactions at one electrode to the other (pseudocapacitance). The present minireview describes the recent developments and progress of carbon-transition metal oxides (C-TMO) hybrid materials that show great promise as an efficient electrode towards supercapacitors among various material types. The review describes the synthetic methods and electrode preparation techniques along with the changes in the physical and chemical properties of each component in the hybrid materials. The critical factors in deriving both EDLC and pseudocapacitance storage mechanisms are also identified in the hope of pointing to the successful hybrid design principles. For example, a robust carbon-metal oxide interaction was identified as most important in facilitating the charge transfer process and activating high energy storage mechanism, and thus methodologies to establish a strong carbon-metal oxide contact are discussed. Finally, this article concludes with suggestions for the future development of the fabrication of high-performance C-TMO hybrid supercapacitor electrodes.

Sample Preparation Using Graphene-Oxide-Derived Nanomaterials for the Extraction of Metals

Graphene oxide is a compound with a form similar to graphene, composed of carbon atoms in a sp2 single-atom layer of a hybrid connection. Due to its significant surface area and its good mechanical and thermal stability, graphene oxide has a plethora of applications in various scientific fields including heterogenous catalysis, gas storage, environmental remediation, etc. In analytical chemistry, graphene oxide has been successfully employed for the extraction and preconcentration of organic compounds, metal ions, and proteins. Since graphene oxide sheets are negatively charged in aqueous solutions, the material and its derivatives are ideal sorbents to bind with metal ions. To date, various graphene oxide nanocomposites have been successfully synthesized and evaluated for the extraction and preconcentration of metal ions from biological, environmental, agricultural, and food samples. In this review article, we aim to discuss the application of graphene oxide and functionalized graphene oxide nanocomposites for the extraction of metal ions prior to their determination via an instrumental analytical technique. Applications of ionic liquids and deep eutectic solvents for the modification of graphene oxide and its functionalized derivatives are also discussed.

Effect of Composition Ratios on the Performance of Graphene/Carbon Nanotube/Manganese Oxide Composites toward Supercapacitor Applications

Herein, we describe the preparation and characterization of graphene/carbon nanotube (CNT)/MnO v composites and the effects of chemical composition and phase transformation on the properties of the corresponding electrode film. In general, the effect of graphene-to-CNT ratio (G/C ratio) and the manganese (Mn) content on the morphology, chemical state, crystallization properties, and microstructure of the composite material was examined by scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and selected area electron diffraction. The bonding mechanism between MnO v and graphite-based materials, that is, graphene and CNTs, is discussed. The influence of the composition of the composites on the performance of the electrode was investigated using charge-discharge curves. The faradically active MnO v also functioned as a considerable cobinder and allowed for a reduced amount of polymeric binder, which enhanced the conductivity and capacitance of the electrode. The optimized electrode composition was obtained based on our present graphene and CNT specifications. In summary, the results discussed in this article provide significant background information for future applications of graphene/CNT/MnO v composite electrodes.

Solvent-free functionalisation of graphene oxide with amide and amine groups at room temperature

A new solvent free protocol is presented to introduce amide and amine functionalities (N-aliphatic groups) onto graphene oxide in an energy efficient manner. Nitrogen contents of 3.6 wt% are obtained in only 5 minutes at room temperature by using ammonia gas as the nitrogen source for the ammonolysis of graphene oxide.

Graphene Oxide/Carbon Nanotube Membranes for Highly Efficient Removal of Metal Ions from Water

Graphene oxide (GO) has an excellent adsorption capacity toward metal ions. Therefore, it is widely recognized as an auspicious material for fabrication of membranes applied in metal ion separation. However, GO membranes are not stable in aqueous solution because of electrostatic repulsion between GO nanosheets which are negatively charged. This paper shows that stable GO membranes can be easily obtained by the noncovalent interaction of GO with oxidized carbon nanotubes (CNTs). The experiment also shows that the GO/CNTs membranes can be used for the effective adsorption of metal ions. The kinetic data, adsorption isotherms, competitive adsorption experiment, and X-ray photoelectron spectroscopy indicate that the adsorption of metal ions is based on chemisorption. The membranes are remarkably durable in acidic, neutral, and basic solutions. Although the significant stabilization of the membranes by CNTs is observed, they strongly influence the adsorption process. Our study reveals that even a small amount of CNTs (GO/CNTs in the ratio 8:1) significantly reduces adsorption capacities of the membranes which were as follows: 37, 40, 50, 42, 48, and 98 mg g^-1 for Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Pb(II), respectively. The reduction of the membrane adsorption capacities results from the creation of micro- and nanochannels formed by entangled CNTs. Durability and adsorptive properties of studied membranes indicate their potential use for the removal of metals from water.

Graphene Composites for Lead Ions Removal from Aqueous Solutions

The indiscriminate disposal of non-biodegradable, heavy metal ionic pollutants from various sources, such as refineries, pulp industries, lead batteries, dyes, and other industrial effluents, into the aquatic environment is highly dangerous to the human health as well as to the environment. Among other heavy metals, lead (Pb(II)) ions are some of the most toxic pollutants generated from both anthropogenic and natural sources in very large amounts. Adsorption is the simplest, efficient and economic water decontamination technology. Hence, nanoadsorbents are a major focus of current research for the effective and selective removal of Pb(II) metal ions from aqueous solution. Nanoadsorbents based on graphene and its derivatives play a major role in the effective removal of toxic Pb(II) metal ions. This paper summarizes the applicability of graphene and functionalized graphene-based composite materials as Pb(II) ions adsorbent from aqueous solutions. In addition, the synthetic routes, adsorption process, conditions, as well as kinetic studies have been reviewed.