The effect of pre-heating conditions of active carbons on the adsorption of organic compounds from aqueous solutions
Abstract
The effect of preliminary heating of active carbon in an oxidizing atmosphere under the conditions when the accumulation of surface compounds of oxygen prevails over the process of destruction of the organic part of the carbon adsorbent in adsorption of organic compounds was investigated. It was determined that during the heat treatment in the chosen conditions a redistribution of the volume of micro and mesopores of the adsorbent occurs, as well as the change in the amount of titratable surface acid and basic groups on the surface of active carbon. The equilibrium adsorption of phenol, aniline and pyridine from aqueous solutions with heated samples of active carbon was studied. It was established that the form of adsorption isotherms of studied species suggests a strong specific interaction of sorbent - sorbate. It was revealed that the adsorption of organic components is influenced not only by the characteristics of the porous structure and the chemical state of the surface of adsorbent, but also the molecule structure of the adsorbtion. Heat treatment of the adsorbent at the chosen conditions has little effect on the adsorption of phenol, but significantly increases the adsorption of pyridine. In the case of aniline a discernible increase in adsorption is observed only for the sample of active carbon, heated at 250° C. The resulting adsorption isotherms were analyzed in the linearized coordinates of the equations of BET and TMVF (Dubinin – Radushkevich). It was determined that the monomolecular adsorption of all studied organic substances is realized by specific interaction of adsorbent – adsorbate, the type of which (hydrogen bond and/or donor-acceptor interaction) depends on the prevalence of acidic or basic groups on the surface of the adsorbent. The formation of the second layer of the component occurs due to the dispersion interactions. The calculated value of the maximum adsorption volume occupied by aniline and phenol, assumes volume filling of micro - and mesopores of activated carbon. Upon the adsorption of pyridine a mixed adsorption layer is formed on all the samples of activated carbon. The ratio of organic material – solvent in this layer depends on the surface characteristics of the adsorbents. The proportion of pyridine, even for the sample heated at 250° C, does not exceed 35% in the formation of the first layer on the carbon surface and 70% in the volumetric filling of micropores. In the case of phenol, for which dispersive interactions with the carbon surface are more typical, increased adsorption due to the rise in the number of carbonyl/quinoid groups is compensated by the decrease in the total pore volume of the adsorbent and increased adsorption of the solvent (water).
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