Characterization of the surface interacting ability of carbon black by means of electron paramagnetic resonance analysis of adsorbed Cu2+, supported by surface analysis and atomic absorption.

Electron paramagnetic resonance (EPR) has been used to investigate the adsorption capability and the surface interacting ability towards Cu(II) solutions (CuCl2, Cu(NO3)2, CuSO4 in water or ethanol) of various carbon blacks, both graphitized and ungraphitized, selected on the basis of the surface area, namely, Carbograph1 (area = 100 m2/g), Carbograph4 (area = 210 m2/g), and Carbograph5 (area = 560 m2/g), which were indicated as C1g, C4g, C5g (g = graphitized), and C1ng, C4ng, C5ng (ng = ungraphitized). The EPR analysis was supported by surface analysis, for evaluating the surface area, the pore volume and the porosity, and by atomic absorption to obtain the adsorbed Cu(II) amounts. Graphitization provokes a decrease in surface area, but C1g, at low surface area, showed a unexpected increase of the adsorption ability ascribed to the formation of new surface porosity closed by graphite layers. The carbon samples showed a broad unresolved EPR signal due to mobile unpaired electrons in the carbon matrix. Graphitized samples presented a narrower signal than ungraphitized samples, which increases in width with the increase in surface area (with the exception of C5ng due to the high exposition of the wide surface to oxydizing external agents) and upon prolonged thermal treatment. The signal intensity of the carbon paramagnetic centers decreases upon Cu(II) adsorption. Computer aided analysis of the EPR spectra of the solids after Cu(II) adsorption allowed to extract structural information on the Cu-surface site complexes. The Cu2+ ions coordinated with surface polar sites, mainly oxygenated. Adsorption depends on the different Cu(II) salts, caused by the salt solubility and the interacting ability of the counter-ion. In several cases the solutions concentrated in the carbon porosity leading to precipitation of the salt. Ethanol solutions are more adsorbed at the carbon surface than water solutions; Cu(II) partially retains its solvation shell and partially presents electron transfer to the carbon surface. Adsorption is favored to ungraphitized carbons with respect to the graphitized ones due to both the higher surface area, and the higher hydrophilicity of the surface. In summary, these carbon powders, widely used for chromatographic applications, show an adsorption capability towards Cu(II) solutions higher than expected due to both a definite porosity, and the presence of polar groups which are not eliminated with chemical surface treatments.