Hexavalent chromium (Cr(VI)) is a heavy metal that is highly soluble and exhibits toxic effects on biological systems. Nevertheless, it is used in many industrial applications. The adsorption process of Cr(VI), using activated carbon (AC), is under investigation globally. On the other hand, around six million tons of spent coffee is sent to landfill annually. In the spirit of cyclic economy, this research investigated the production of AC from spent coffee for the removal of Cr(VI) from wastewater. The AC was produced via pyrolysis process under a nitrogen atmosphere. Chemical activation using potassium hydroxide (KOH) occurred simultaneously with the pyrolysis process. The produced AC was tested as an absorber of Cr(VI). The best fitted kinetic model was the diffusion–chemisorption model. A 24-h adsorption experiment was carried out using a solution with a pH of 3 and an initial Cr(VI) concentration of 54.14 ppm. This resulted in an experimental maximum capacity of 109 mg/g, while the theoretical prediction was 137 mg/g. It also resulted in an initial adsorption rate (ri) of 110 (mg/(g h)). The Brunauer–Emmett–Teller surface area (SgBET) was 1372 m2/g, the Langmuir surface area (SgLang.) was 1875 m2/g, and the corrugated pore structure model surface area (SgCPSM) was 1869 m2/g. The micropore volume was 84.6%, exhibiting micropores at Dmicro1 = 1.28 and Dmicro2 = 1.6 nm. The tortuosity factor (τ) was 4.65.

Magnetic carbon nanocages (Mag@CNCs) were synthesized via a green one-step process using pine resin and iron nitrate salt as a carbon and iron source, respectively. To produce Mag@CNCs, pristine materials have been carbonized at high temperature under inert atmosphere. The structural, textural, and surface properties of as-synthesized Mag@CNCs were studied employing microscopic, spectroscopic, and surface physicochemical methods. The obtained results showed that the new Mag@CNCs have significant surface area (177 m2 g–1) with both microporosity and mesoporosity. Moreover, the material exhibits a homogeneous distribution of core–shell-type magnetic nanoparticles within the carbon matrix, formed by iron carbide (Fe3C) and metallic iron (α-Fe), with sizes of 20–100 nm, surrounded by a few graphitic layers-walls. Most importantly, Mag@CNCs were tested as absorbents for As­(III) removal from aqueous solutions, showing a total of 263.9 mg As­(III)-uptake capacity per gram of material at pH = 7, a record sorption capacity value among all previously tested iron-based materials and one of highest values among all reported sorbents so far. The adsorbed As­(III) species are anchored at the surface of Mag@CNCs, as demonstrated by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy measurements. The pH-edge As­(III)-adsorption experiments combined with theoretical surface complexation modeling allowed a detailed understanding of the interfacial properties of Mag@CNCs, and hence the As­(III) uptake mechanism. The analysis revealed that As­(III) binds on two types of surface sites of Mag@CNCs, i.e., on carbon-surface species (C x OH2) and on Fe-oxide layer (FeOH2) of nanoparticles. This exemplifies that the advanced morphology- and surface-driven synergistic properties of the Mag@CNCs material are crucial for its As­(III)-uptake performance.

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Carbon into polymer nanocomposite is so far a common additive for the enhancement of the polymer properties. The properties of the polymer, such as thermal, and especially its mechanical properties, are improved by the homogeneously dispersed carbon nanoparticles on the polymer matrix. In this study, carbon wires in nano dimensions are, for the very first time, synthesized via the hard templating method from the silicate matrix MCM-41, and used as nano additives of polystyrene. The carbon nanowires were chemically oxidized, and further modified by attaching octadecylamine molecules, for the development of organic functionalities onto carbon nanowires surface. The nanocomposite materials of polystyrene with the modified carbon nanowires were prepared by a solution-precipitation method at three nano additive to polymer loadings (1, 3 and 5 wt%). The as-derived nanocomposites were studied with a combination of characterization and analytical techniques. The results showed that the thermal and mechanical properties of the polystyrene nanocomposites gradually improved while increasing nano-additive loading until 3 wt%. More specifically, the 3 wt% loading sample showed the best mechanical properties, while the 5 wt% sample was difficult to achieve satisfactory dispersion of carbon nanowires and consequently has a wide range of values.