Engineered Carbon Nanotubes: Review on the Role of Surface Chemistry, Mechanistic Features, and Toxicology in the Adsorptive Removal of Aquatic Pollutants

Carbon nanotubes buckypapers: A new frontier in wastewater treatment technology

Occurrence of contaminants in water is one of the major global concerns humanity is still facing today: most of them are extremely toxic and dangerous for human health, obliging their removal for a proper and correct process of sanitation. Among wastewater treatment technologies, in the view of development of sustainable and environmentally friendly processes, membrane adsorption has proved to be a fast and simple method in the removal of pollutants, offering great contaminants recovery percentages, fast adsorbent regeneration and recycle, and easy scale-up. Due to their large surface area and tunable chemistry, carbon nanotubes (CNTs)-based materials revealed to be extraordinary adsorbents, exceeding by far performances of ordinary organic and inorganic membranes such as polyethersulfone, polyvinylidene fluoride, polytetrafluoroethylene, ceramics, currently employed in membrane technologies for wastewater treatment. In consideration of this, the review aims to summarize recent developments in the field of carbon nanotubes-based materials for pollutants recovery from water through adsorption processes. After a brief introduction concerning what adsorption phenomenon is and how it is performed and governed by using carbon nanotubes-based materials, the review discusses into detail the employment of three common typologies of CNTs-based materials (CNTs powders, CNTs-doped polymeric membranes and CNTs membranes) in adsorption process for the removal of water pollutants. Particularly focus will be devoted on the emergent category of self-standing CNTs membranes (buckypapers), made entirely of carbon nanotubes, exhibiting superior performances than CNTs and CNTs-doped polymeric membranes in terms of preparation strategy, recovery percentages of pollutants and regeneration possibilities. The extremely encouraging results presented in this review aim to support and pave the way to the introduction of alternative and more efficient pathways in wastewater treatment technologies to contrast the problem of water pollution.

Electrochemical Reaction Assisted 2D π-Stacking of Benzene on a MWCNT Surface and its Unique Redox and Electrocatalytic Properties

Turning the π-structure and electronic properties of carbon nanotubes (CNTs) is a cutting-edge research topic in interdisciplinary areas of material chemistry. In general, chemical functionalization of CNT has been adopted for this purpose, which has resulted in a few monolayer thickness increment of CNT diameter size. Herein, we report an interesting observation of >10-fold increment in the apparent diameter of multiwalled carbon nanotubes (MWCNTs) brought about by a process of self-assembly of the BZ moiety on MWCNT, which is formed by electrochemical oxidation of a surface-adsorbed benzene-water cluster, {BZ-nH₂O}. From physicochemical characterizations by transmission electron microscopy (TEM) and Raman and IR spectroscopic techniques and electrochemical characterizations by several radical scavenger species, it has been revealed that benzene radical moieties as a series of π-stacked layers ([BZ]-π-stack) were self-assembled on the MWCNT surface. A possible mechanism for their formation was proposed to be electrochemical oxidation of H₂O from the MWCNT@{BZ-nH₂O}_ads layer to oxygen gas via hydroxyl radical formation and benzene cationic radical species at 1.2 V vs Ag/AgCl followed by its self-assembly into a unique MWCNT@[BZ]-π-stack network. The scanning electrochemical microscopic (SECM) technique was used to identify the in situ ^•OH radical formation. The electrochemical studies of a glassy-carbon-modified MWCNT@[BZ]-π-stack system showed a well-defined and highly symmetrical redox peak at an equilibrium potential E_1/2 = 0.2 V vs Ag/AgCl (pH 2 HCl/KCl), with a peak-to-peak potential separation of 0 V, highlighting the ideal-surface-confined electron-transfer nature of the redox couple. Furthermore, enhanced electrical conductivity over the unmodified MWCNT was observed when testing the surface-sensitive redox couple Fe³⁺/Fe²⁺ with the modified electrode. This new redox material showed a specific electrocatalytic reduction of hydrogen peroxide at neutral pH (pH 7 phosphate buffer solution) unlike the quinone and other organic redox mediators, which show the reduction signal only in the presence of horseradish peroxidase enzyme.

Effects of ionic strength on removal of toxic pollutants from aqueous media with multifarious adsorbents: A review

Adsorption is one of the most widely used and effective wastewater treatment methods. The role of ionic strength (IS) in shaping the adsorption performances is much necessary due to the ubiquity of electrolyte ions in water body and industrial effluents. The influences of IS on adsorption are rather complex, because electrolyte ions affect both adsorption kinetics and thermodynamics by changing the basic characteristics of adsorbents and adsorbates. For a given adsorption system, multiple or even contradictory effects of IS may coexist under identical experimental conditions, rendering the dominant mechanism recognition and net effect prediction complicated. We herein reviewed the key advancement on the interaction and mechanisms of IS, including change in number of active sites for adsorbents, ion pair for metal ions, molecular aggregation and salting-out effect for organic compounds, site competition for both inorganic and organic adsorbates, and charge compensation for adsorbent-adsorbate reciprocal interactions. The corresponding fundamental theory was thoroughly described, and the efforts made by various researchers were explicated. The structural optimization of adsorbents affected by IS was detailed, also highlighting polyamine materials with exciting "salt-promotion" effects on heavy metal removal from high salinity wastewater. In addition, the research trends and prospects were briefly discussed.