João Paulo de Mesquita

Characterization of copper adsorption on oxidized activated carbon

This paper discusses Cu(II) ion adsorption onto oxidized activated carbon with HNO3. The modifications introduced on the activated carbon (AC) surface were characterized by surface area measurement, FT-IR spectroscopy, titration by the traditional Boehm procedure, and potentiometric titration. A nonlinear method for fitting acid-base potentiometric titration data was applied to determine the concentration and pKa distributions of the ionizable acidic sites on AC, before and after HNO3 oxidation. Adsorption studies were conducted in batch mode for the concentration range 0.08 to 1.75 mmol L-1 and pH = 4.0. Results showed that the acid treatment of AC leads to a decrease in apparent surface areas and produces slight modifications in pore structure. However, oxidation strongly influenced the AC surface chemical structures by increasing the concentration of acidic groups with pKa < 6.0, which significantly improved Cu(II) ion adsorption. The uptake enhanced for oxidized carbon was related to the presence of acidic groups with pKa lower than 4, revealing the important role of acidic groups in adsorption by ion-exchange mechanisms.

Cellulose Nanocrystals Assembled on the Fe3O4Surface as Precursor to Prepare Interfaced C/Fe3O4Composites for the Oxidation of Aqueous Sulfide

In this work, composites based on carbon (1, 10 and 20 wt.%) interfaced with Fe3O4 (magnetite) have been studied as catalysts for the oxidation of aqueous sulfide. The composites were prepared by assembling cellulose nanocrystals surrounding Fe3O4 followed by a controlled thermal decomposition at 400, 600 and 800 oC. Mossbauer, X-ray diffractometry (XRD), Raman, thermogravimetry (TG), elemental analysis CHN, scanning electron microscopy (SEM/EDS), Fourier transform infrared spectroscopy (FTIR) and potentiometric titration indicated that at 400 and 600 oC the cellulose nanocrystals decompose to different carbon forms, i.e., films, filaments and particles attached to the Fe3O4 crystals. At higher temperature, i.e., 800 oC, this carbon on the magnetite surface further reacted to produce Fe0. UV-Vis, Raman and electrospray ionization mass spectrometry (ESI-MS) measurements showed that these composites catalyze the oxidation of aqueous sulfide to convert S2-aq to polysulfides Sn2- (where n = 2-9) and also oxygen containing polysulfides HOSn-. Simple kinetic experiments showed very low sulfide oxidation activities for pure Fe3O4 and pure carbon. On the other hand, the composites, especially with 10% C obtained at 600 oC, were remarkably active. These results are discussed in terms of a possible participation of oxygen based redox groups present on the carbon surface and an electron transfer from the carbon to the Fe3O4 phase.

Bionanocompósitos preparados por incorporação de nanocristais de celulose em polímeros biodegradáveis por meio de evaporação de solvente, automontagem ou eletrofiação

This review reports the preparation and characterization of bionanocomposites based on biodegradable polymers reinforced with cellulose nanocrystals (CNC) described in the literature. The outstanding potential of cellulose nanocrystals as reinforcement fillers of biodegradable polymers is presented with an emphasis on the solution casting process, which is an appropriate method to investigate the physico-chemical effects of the incorporation of CNC into the polymeric matrices. Besides solution casting, other small scale methods such as electrospinning and layer-by-layer are also covered.

Micromesoporous Activated Carbons as Catalysts for the Efficient Oxidation of Aqueous Sulfide

KOH activation of a mesophase pitch produces very efficient carbons for the removal of sulfide in aqueous solution, increasing the sulfur oxidation rate with the degree of activation of the carbon. These carbons are characterized by their graphitic structures, with domains of sizes of around 20 nm, and a moderate concentration of surface oxygen groups (0.2-0.5 mmol·g-1) dominating the basic groups. Because the activation leads first to a strong development of the micropores and later to a development of the mesopores, the surface area values are always high, reaching values of as high as 3250 m2·g-1 in the most activated carbon, with a volume of mesopores of as high as 44% of the total pore volume. In the presence of this carbon, the sulfide oxidation rate is 100 times higher than that found for a commercial activated carbon, the results indicating that the porosity of the carbon, especially mesoporosity, plays a role more important than the structure or the chemical nature of the carbon in the kinetics of sulfide oxidation to different polysulfides.

Purification, Selection, and Partition Coefficient of Highly Oxidized Carbon Dots in Aqueous Two-Phase Systems Based on Polymer–Salt Pairs

In general, the methodologies for the preparation of carbon dots (CDs) lead to the formation of nanostructures with size and surface chemistry heterogeneity. Because the electronic and optical properties of these nanoparticles are directly associated with these properties, the development of purification and selection strategies is essential. Herein, we report a systematic study of the spontaneous partition and separation of highly oxidized carbon dots (OCDs) prepared by the dehydration and oxidation reactions of cotton cellulose in aqueous two-phase systems (ATPSs) based on polymer-salt pairs. The partition of the CDs was investigated in different ATPSs in which the effects of the cations and anions of the salts, molecular mass and nature of the polymer, tie-line length, initial pH, and surface modification of the nanoparticles on the partition coefficient (K) were evaluated. The results showed that the best separation occurred with a system consisting of PEO1500 + lithium sulfate + water using reduced CDs with hydrazine. Alternatively, the lowest value of K, 0.79, was obtained for a poly(ethylene oxide) PEO1500 + sodium tartrate + water system with pH = 6 using OCDs. The detailed analyses of the top and bottom phases of the systems with fluorescence and ultraviolet-visible spectroscopy showed that ATPSs are capable, in addition to partitioning, of separating the nanoparticles with different optical properties, which are directly associated with the surface properties and particle sizes. We believe that the presented methodology is an alternative, practical, fast, and potentially scalable technique for the separation of carbon nanostructures with different optical properties.

Surface modified mesoporous nanocast carbon as a catalyst for aqueous sulfide oxidation and adsorption of the produced polysulfides

In this work, a mesoporous nanocast carbon prepared using SBA-15 as a template was modified by surface oxidation to produce unique catalysts for sulfide oxidation/elimination from an aqueous medium. Different characterization techniques (BET, SEM, Raman, FTIR, potentiometric titration, elemental analyses, cyclic voltammetry) showed that treatment with concentrated HNO3 at 80 °C for 5, 15 and 30 min attacks the carbon structure creating different oxygen surface functionalities in concentrations varying from 0.4–1.1 mmol g−1 with a small effect on the surface area (1080–1148 m2 g−1) and pore volumes (micropore 0.17–0.20 cm3 g−1 and mesopores 0.36–0.50 cm3 g−1). These materials showed high activities for the oxidation of sulfide in an aqueous medium forming polysulfides, e.g., S22−, S32− and S42−, which are rapidly eliminated from the aqueous medium. These results are discussed regarding efficient sulfide oxidation at redox surface oxygen sites leading to the formation of higher polysulfides followed by adsorption of these relatively large and more hydrophobic molecules into the mesopores.