Extraction, characterisation and metal biosorption of extracellular polysaccharides from activated sludge

Abstract

Waste activated sludge is a biological adsorbent whose potential to remove metals from solution and effluent has been demonstrated. Extracellular polysaccharides (EPS) as components of activated sludge are thought to contribute to activated sludge metal biosorption. During the present study characterisation and determination of the metal biosorptive capabilities of domestic and industrial extracellular polysaccharides (EPS) revealed similarities both in terms of chemical composition and metal adsorption potential. Extracellular polysaccharides were extracted from activated sludge, obtained from domestic and industrial sludge treatment plants, using chemical techniques which involved sodium hydroxide extraction and solvent precipitation. A purification technique, which involved precipitation of protein with chloroform and removal of nucleic acids was developed. To assess the efficiency of the purification method, the ratio of extracted polysaccharide to the amount of protein present was determined. This provided an indication of the magnitude of EPS extracted in relation to the degree of cellular disruption. The type of activated sludge being treated was shown to be of particular importance. The quantity of EPS present in the original sample was found to be higher in domestic sludge than in industrial sludge. Purified EPS was fractionated in a column of DEAE-Sepharose CL-6B using stepwise pH gradient elution. Molecular weight distribution was conducted on a column of Sepharose CL-4B. Component monosaccharides were identified by paper chromatography. Monomers identified were glucose, fructose, glucuronic acid and galactosamine. Ion-exchange chromatography results demonstrated the presence of a number of different polysaccharide fractions while gel filtration results indicated a wide molecular weight distribution range of EPS from both domestic and industrial activated sludge. This indicated potential for variety in the EPS content of the activated sludge. Metal adsorption studies were conducted to determine the capabilities of EPS to adsorb metals