Photochemical and electrochemical reduction of graphene oxide thin films: tuning the nature of surface defects.

Individual and combined photo(electro)chemical reduction treatments of graphene oxide thin films have been performed to modulate the type of defects introduced into the graphene sheets during the reduction. These were characterized by X-ray photoelectron and Raman spectroscopies, nuclear reaction analysis and electrochemical methods. Illumination of the graphene oxide thin film electrodes with low irradiance simulated solar light provoked the photoassisted reduction of the material with negligible photothermal effects. The photoreduced graphene oxide displayed a fragmented sp2 network due to the formation of a high density of defects (carbon vacancies) and the selective removal of epoxides and hydroxyl groups. In contrast, the electrochemical reduction under mild polarization conditions favored the formation of sp3 defects over vacancies, with a preferential removal of carbonyl and carboxyl groups over hydroxyl/epoxides. Used in conjunction, mild photochemical and electrochemical treatments allowed the obtainment of reduced graphene oxides with varied reduction degrees (ca. C/O ratio ranging from 4.9 to 2.2), and surface defects. Furthermore, the electrochemical reduction prevented the formation of vacancies during the subsequent illumination step. In contrast, both types of defects were accumulated when the GO electrode was first exposed to illumination and then polarized.

Synergistic effect of zeolite/chitosan in the removal of fluoride from aqueous solution.

Today, fluoride represents one of the most often found, and resilient, pollutants threatening the health of millions of people around the globe. The use of biosorbents is an interesting alternative technique for the removal of fluorine-ions. Chitosan is a natural biopolymer with surface groups capable of removing fluorine; however, their lack of mechanical stability restricts its application. In the present work, we proposed that such limitations can be overcame by forming a composite with zeolite (ZCC). A proper zeolite-to-chitosan ration must be kept to prevent a collapse of the material's capacity. Two ZCCs at ratios of 1:1 and 1:3 were formed and tested for the removal of fluoride from aqueous solution. The composites were characterized by Electron Microscopy, FT-IR, N2 physisorption, and potentiometric titration techniques. During fluoride adsorption studies, the effects of pH and temperature were analysed and thermodynamic parameters for adsorption were calculated. The results demonstrated that there is a chemical interaction between the zeolite and chitosan components leading to a superior adsorption performance than if there was a simple physical mixture of the precursors. Maximum adsorption capacities were reached using the composite material with the lowest chitosan content due to reduced constriction of the zeolite pores and a better dispersion of overall the adsorption sites. Both pH and temperature had a significant, and negative, impact on the adsorption; these effects were discussed. The present work represents an advance in the development of functional biocomposites for the removal of pollutants from aqueous solutions.

The adsorption kinetics of cadmium by three different types of carbon nanotubes.

Oxidized nitrogen-doped multiwall carbon nanotubes (ox-N-MWCNTs), oxidized multiwall carbon nanotubes (ox-MWCNTs), and oxidized single-wall carbon nanotubes (ox-SWCNTs) were evaluated via batch adsorption kinetic experiments to determine the effect of nanotube morphology on the adsorption rate of cadmium. The nanotubes were characterized by HRTEM, XRD and Raman spectroscopy. Cadmium adsorption isotherms were determined at pH 6. Analyses of the kinetic data with an external mass transport model and an intraparticle diffusion model considered two cases: (1) single nanotubes suspended in aqueous solution and (2) agglomerates of nanotubes suspended in aqueous solution. The intraparticle diffusion model produced the best fit to the experimental data. However, only the diffusivity coefficients for single nanotubes suspended in solution were similar to literature values: about 4×10(-9), 1×10(-9) and 2.4×10(-11) cm(2)/s for ox-N-MWCNTs, ox-MWCNTs and ox-SWCNTs, respectively. The morphology of the various carbon nanotubes might determine cadmium diffusivity. The high amount of sidewall pores observed in the single-walled carbon nanotubes could limit cadmium diffusion and account for the slow diffusion rate of 180 min. Conversely, the short length, small surface area and bamboo-type morphology observed with nitrogen-doped multiwall carbon nanotubes may account for the relatively fast adsorption rate of 15 min as this morphology prevents cadmium diffusion through the internal tubular space of these nanotubes.