Investigations On The Porosity Structure
INVESTIGATIONS ON THE POROSITY STRUCTURE
OF MINERAL-CARBON SORBENTS
B. Pacewska, D. Szychowski
Warsaw University of Technology Faculty of Building Engineering, Mechanics, and Petrochemistry, Institute of Chemistry,
ul. Lukasiewicza 17, 09-400 Plock, Poland
Adsorption and catalytic processes play a substantial role in many branches of modern technology. Among many applications of adsorption in unit processes also its importance in the removal of chemical pollutants from liquid wastes is often appreciated.
Active carbon is an adsorbent, which has been the most frequently applied for elimination of residues of dyestuffs, detergents, hydrocarbons and other organic compounds, as well as of heavy metal ions. Because of the high price, which is a heavy barrier in application of active carbon in sewage treatment processes, there has been a world-wide tendency of replacing active carbon with mineral-carbon sorbents.
In the present work an attempt was made to obtain mineral carbon sorbents by thermal decompositon. The mineral matrix for the sorbents (aluminium hydroxide) was based on petrochemical waste stream containing considerable amounts of aluminium chloride. Reference tests were carried out with a model solution prepared with the use of analytical grade AlCl3. Atactic polypropylene and hydrocarbon mixtures obtained in the flotation of petrochemical waste waters were used as the carbon-containing raw materials.
The aim of this work was to determine the adsorption and structural characteristics of the complex sorbents and to check the possibility of evaluation of their hydrophobic-hydrophilic properties. The porous structure parameters were determined by the adsorption methods using nitrogen and benzene as adsorbates.
The carbon-aluminium sorbents were prepared by thermal decomposition of mixtures composed of a mineral matrix (aluminium hydroxide) and a carbon-containing substance. Sorbent samples were prepared
either by mixing the two components (the mineral matrix and the carbon material) or by depositing the carbonaceous substance in the mineral matrix (by including the carbon sorbent particles between the gel particles in the course of the precipitation process). The final stage of the process consisted in carbonization of the mineral-carbon mixture in an inert gas (nitrogen) atmosphere.
The experimental studies performed have shown that materials of good sorption properties can be obtained using either pure aluminium chloride or a liquid waste containing aluminium chloride for preparation of a mineral matrix of Al(OH)3 and organic substances such as atactic polypropylene or a mixture of hydrocarbons obtained from flotation of petrochemical waste waters.
Optimum preparation conditions and optimum composition were established for the carbon-aluminium sorbents. The sorption properties of the obtained materials depend on the contents of the carbon substance, degree of coverage of the mineral matrix with the carbon pyrolysate, and conditions of the carbonisation process. Active sorbents were obtained by carbonisation under the following conditions: temperature 450° C, nitrogen atmosphere, heating time 30 min. The optimum content of the carbon component in the initial mixture was:
– 20% by wt. and 30% by wt. for sorbents obtained with addition of APP,
– 30% by wt. and 50% by wt. for sorbents obtained with addition of the hydrocarbon waste skim.
A high sorption capacity was found for the sorbent obtained by co-precipitation of aluminium hydroxide from the petrochemical waste water in an aqueous emulsion of hydrocarbon waste skim (200cm3/1dm3 wastewater), SBET=270 m2/g.
Comparative analysis of the specific surface determined from the adsorption of nitrogen and benzene may be an efficient method for determining the homogeneity of mineral-carbon materials.
A possibility of various orientations of benzene molecules in the adsorption layer has been postulated. In the case of sorbents based on aluminium hydroxide precipitated from the petrochemical waste water mixed with APP, a mixed orientation (planar and vertical) of benzene molecules is probably a predominating form. The vertical orientation is characteristic for the polar surface of aluminium oxide, thus it might be accounted for not complete coverage of the carrier surface with the carbon pyrolysate. A different relationship was found in the case of samples based on aluminium hydroxide precipitated from the precursor solutions. The similar results of SBET obtained from the nitrogen and benzene adsorptions with assumption of planar orientation of benzene molecules was probably due to the hydrophobic nature of the matrix surface. A decisive factor of procedure in this case consisted in preliminary trituration of the components heated at 180° C.
A horizontal orientation of benzene molecules in the adsorption layer was found for the sorbents prepared with the use of the hydrocarbon waste skim, except the samples obtained by co-precipitation of Al(OH)3 from aluminium chloride solution in aqueous emulsion of the hydrocarbon waste skim (25 and 100 cm3 /1dm3 AlCl3 solution). A higher content of the organic substance may give a more uniform coverage of the mineral matrix.
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