Application of different equations to adsorption isotherms of phenolic compounds on activated carbons prepared from cork

Activated carbons were prepared from solid cork wastes by physical activation with carbon dioxide or steam, and chemical activation by impregnation with phosphoric acid. In this work we show the possibility of using these activated carbons for the adsorption of phenolic compounds from the aqueous phase. The materials present a different response to the adsorptives used (p-nitrophenol, p-chlorophenol, p-cresol and phenol), depending on the type of activation and the parameters (burn-off, absolute concentration) used in each case. All the samples were capable of retaining the contaminants, with the best result being reached by the sample with higher burnoff and the worst with the carbonised, while intermediate values were reached with the remaining samples. The experimental isotherms were analysed with two and three parameters equations (Freundlich, Langmuir, Dubinin–Radushkevich–Kaganer and Redlich–Peterson). The results obtained from the application of the equations are similar in some aspects, but the degree of confidence is quite different. The best fit was achieved with the Redlich–Peterson equation, which can be explained by the fact that this has three adjustable parameters. However, overall the Freundlich and DRK equations appear to be more useful and provide parameters which can be correlated with the structural characteristics of the solids obtained from N2 adsorption measurements

Controlling the micropore size of activated carbons for the treatment of fuels and combustion gases

Exclusively microporous activated carbons have been prepared from cork by physical and chemical activation under different conditions. The results show that it is possible to control the pore size of the activated carbons and to obtain materials with narrow micropore size ( 0.69 nm) and high micropore volume ( 0.64 cm3 g 1) equal to or better than the best activated carbon fibres. Higher micropore volumes are generally obtained by chemical activation at higher temperature using dry or potassium hydroxide impregnation. On the other hand, wet or carbonate impregnation, as well as high temperature, or physical activation with CO2 or H2O under appropriate conditions, favours low mean pore widths

Pore size control in activated carbons obtained by pyrolysis under different conditions of chemically impregnated cork

Activated carbons were prepared by the pyrolysis of cork impregnated with potassium and sodium hydroxides and carbonates as well as phosphoric acid and the effect of five experimental parameters, namely method of impregnation, impregnant concentration, mass ratio, precursor particle size and pyrolysis temperature, were studied. It is shown that cork is a versatile precursor and allows us to prepare a wide variety of materials with quite different pore structural characteristics by precise control of the impregnation and pyrolysis conditions. Even under relatively mild conditions, it was possible to produce cork based carbons with high pore volumes, in the range 0.5–0.7 cm3 g 1, and to simultaneously control the mean pore width over a three-fold range from a value as low as 0.7 nm up to a value as high as 2.2 nm. The best materials produced present pore structural characteristics which are significantly different to the vast majority of commercial activated carbons. In particular, the possibility of obtaining such high pore volumes in essentially microporous materials, containing virtually no mesoporosity in most cases, is noteworthy. Furthermore, the fact that it was possible with some samples to combine high pore volume and very narrow micropore size is a particularly notable achievement

Adsorption of Bovine Serum Albumin onto Mesocellular Silica Foams with Differently Sized Cells and Windows

As a model protein with quite large dimensions, Bovine Serum Albumin (BSA) has been used to evaluate the influence of the distinct pore structural characteristics of three mesocellular foam (MCF) materials prepared with or without the addition of ammonium fluoride and with varying 1,3,5-trimethylbenzene/Pluronic P123 (TMB/P123) ratios. SBA-15 was also studied for comparative purposes. Characterisation by X-ray diffraction and electron microscopy confirmed the characteristic spheroid cell structure of the MCF pores. Nitrogen adsorption/desorption isotherms at 77 K revealed the different pore structural parameters of the MCF, viz. pore volume (1.8–2.4 cm3/g), cell size (24.6–28.5 nm) and window size (11.3–17.3 nm), as obtained by the NLDFT method. The equilibrium adsorption isotherms and the kinetic adsorption data for BSA at 298 K and pH 5 were well fitted by the Langmuir model and pseudo-second-order kinetic model, respectively. The results showed that adsorption onto the material possessing a window size of 11.3 nm was mostly restricted to the external surface, while a considerable increase in the maximum adsorption capacity from 120 mg/g to 500–600 mg/g was observed when the window size was above the critical dimension of 13.9 nm. In the latter cases, the maximum adsorption capacities could be related to the pore volume rather than to the total surface area. The confined BSA molecules were strongly immobilised in the cells, since only a small proportion was desorbed from the material with windows of 17.3 nm dimensions on contact with a buffer solution of pH 7

Separating Surface and Solvent Effects and the Notion of Critical Adsorption Energy in the Adsorption of Phenolic Compounds by Activated Carbons

A modified form of the Freundlich equation in which the solute equilibrium concentration is normalized with respect to the solute solubility is analyzed and applied to adsorption isotherms of phenol, 4-nitrophenol, 4-chlorophenol, and 2-chlorophenol at different values of pH on commercial activated carbon before and after oxidation. The analysis confirms the importance of normalizing the solute equilibrium concentration when analyzing the adsorption isotherms, and it is suggested that a parameter, KF10, obtained by taking 10%solubility as the reference point when applying the Freundlich equation, is probably the best comparative estimate of the relative adsorption capacity of the carbon for different phenolic compounds. In combination with the Freundlich exponent, nF, estimates of the adsorption capacity at any other reference point can then be obtained. Analysis of the experimental results also indicates a need to distinguish between two regimes of adsorption, characterized by an adsorption energy, Eads, greater than or less than a critical value, Eca. When Eads>Eca, the shape of the adsorption isotherm is determined by solute-solid interactions. On the other hand, when Eads < Eca, solute-solution interactions become more important

Competitive naphthalene adsorption on activated carbons. Effect of porosity and hydrophobicity

The goal of this work was to investigate the mechanism of competitive adsorption of naphthalene on activated carbons from diluted solutions. The polarity of the adsorption media was varied, by selecting solvents ranging from high to low dielectric constants (ethanol, cyclohexane and heptane). By altering the hydrophobicity of the adsorption media, the preferential adsorption of the organic probe is changed; therefore the selectivity and efficiency of naphthalene adsorption may be tuned by optimising the features of the adsorbent upon the adsorption media. Liquid phase adsorption using solvent molecules of different sizes and hydrophobicity can be an effective means of characterising the accessibility and mechanism of competitive adsorption. The results show that the surface polarity of the carbons can be modulated to favour the retention of a non-polar adsorbate (i.e., naphthalene), by controlling the competitive adsorption of the solvent. Oxidation of an activated carbon might result in a better adsorbent even of non-polar compounds, when the competitive adsorption of the solvent becomes important. However, one must also take into account the dimensions of the molecules, and therefore the accessibility to the porosity of the adsorbents. These factors would compensate the slight loss in pore structure and gain in polarity of oxidised carbons, when adsorption takes place from organic media