An Appraisal of The Lignin/Activated Carbon Adsorption System

Abstract

<p>The pore structure of a wide array of activated carbons has been evaluated by nitrogen adsorption and mercury penetration techniques. Carbons were found to exhibit total surface areas and pore volumes in the range 93 - 1500 m²/g and 0.34 -1.8 cm³/g respectively. The Darco carbons contain significant pore structure in all pores up to 1000 Ǻ radius. Columbia carbon contains negligible pore structure in pores of greater than approximately 20 Ǻ radius. Special "A" carbon contains virtually all its limited pore structure in pores of 20 - 200 Ǻ radius.</p> <p>A kraft lignin, Indulin A.T., has been shown by Ultra-filtration and Gel Filtration techniques to exist in aqueous solution as a variety of different molecular weight fragments, with molecular weights ranging from approximately 4,000 to greater than 300,000. Each fragment is characterised by a unique molecular weight, hydrodynamic radius, characteristic diffusivity, solubility and color and TOC content. The concentration, and color and TOC content per molecule of each species in solution determines overall solutions color and TOC. The change in overall solution color and/or TOC with change in solution pH and solution age has been shown to be associated with an alteration in the molecular distribution, and hence the relative concentrations, of the various fragments in solution.</p> <p>Adsorption of lignin from aqueous solution onto activated carbon has been shown to proceed by a selective adsorption mechanism. Here the smallest adsorbable fragments are preferentially adsorbed initially, with progressive preferential adsorption of increasingly larger fragments as either contact time or carbon dosage is increased. Correlations between observed adsorption behaviour and pore structure for various carbons tend to support Chen's hypothesis that each fragment will adsorb preferentially into pores of approximately 3.3 to 6.1 times its hydrodynamic radius. It appears that carbon pore structure in pores of greater than 16 Ǻ radius is primarily responsible for adsorption of color and TOC from aqueous lignin solution. Adsorption of each species appears to occur by an ion exchange or concentration-precipitation mechanism.</p> <p>Most activated carbons contain sufficient pore structure in pores of greater than 16 Ǻ radius to bring about good color and TOC removals at resonably low carbon dosages. Columbia carbon, however, is incapable of removing the bulk of lignin fragments, and hence the major portion of color and TOC, from solution due to lack of adequate pore structure in pores of greater than 15 Ǻ radius. Similarly, Special "A" cannot effectively adsorb the low molecular weight fragments from solution due to a deficiency of pore structure in pores of less than 16 - 20 Ǻ. Good color and TOC removals are, however, possible with Special "A", as its limited pore structure is specific to adsorption of the fragments responsible for the bulk of overall solution color and TOC.</p>