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The adsorption of chromium(III) ions from aqueous solutions onto activated carbons prepared from air and CO2 activation of Eucalyptus sawdust is studied. HNO3 oxidation of these carbons greatly enhances their ability to retain Cr(III) mainly because of a significant increase of the concentration of carbon−oxygen surface groups. To characterize these surface groups, X-ray photoelectron spectroscopy and temperature-programmed desorption (TPD) were used. The influence of these carbon−oxygen surface groups on chromium adsorption is examined. Upon addition of acidic groups, some other functions of nonacidic character evolving as CO upon TPD seem to contribute to Cr(III) uptake. Modifications of the porous structure upon oxidation and chromium adsorption are investigated. Adsorption into the micropores appears to take place to a significant extent although oxidation of the active carbons leads to a narrowing of the average micropore widths.

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Activated carbons obtained from rice husk exhibit low specific surface areas when physical activation is applied due to its high silica content. The purpose of this work was to improve textural parameters of powdered activated carbons obtained from rice husk. To avoid the negative influence of the raw material ash content, a leaching step was included in the preparation process. Hydrofluoric acid, in two concentrations (25 and 50 wt %), was used as leaching agent and applied in different stages of the process. Physical activation using water vapor as activating agent was applied. Specific surface area and porosity were evaluated from nitrogen adsorption data. When a leaching step was included, specific surface area values between 700 and 1200 m2/g were obtained. These values are higher than that corresponding to the activated carbon prepared from rice husk not treated with acid (290 m2/g). Pore size distribution depends on the step sequence of the preparation process and on the HF concentration.

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