ACTIVATED CARBON ADSORPTION OF HUMIC SUBSTANCES: EFFECT OF MOLECULAR SIZE AND HETERODISPERSITY (DESORPTION, ULTRAFILTRATION, MACRO MOLECULES, DIFFUSION, ELECTROSTATICS)

For activated carbon to be an effective water treatment process for the removal of selective organic compounds, a thorough understanding of the adsorption and diffusion of humic substances is required, because such macromolecular organic matter is ubiquitous in aqueous environments. This dissertation addresses the role of physical and chemical properties of the heterogeneous activated carbon on the adsorption process by examining the relationship between humic macromolecular size and adsorbent pore size and relating the adsorptive behavior to a polymer adsorption model.; Using ultrafiltration, a wide distribution of molecular size (MS) is found, with the median MS increasing in the following order: aquatic humic acid < aquatic fulvic acid (TURN) soil fulvic acid < commercial humic acid; this rank order is consistent with MS evaluation by UV-VIS spectroscopy. The MS distribution is influenced by pH and ionic strength in a manner consistent with polyelectrolyte solution theory.; The adsorption equilibrium of the heterodisperse humic substances exhibits a dependence on experimental methodology, resulting in a set of nonunique isotherms. Modifying the isotherms to account explicitly for the adsorbent dose by expressing the equilibrium condition as the amount of unadsorbed solute per unit adsorbent mass, rather than as solution concentration, results in unique isotherms independent of experimental methodology. The adsorption isotherms of the MS fractions, expressed on an adsorbent mass basis, display an inverse dependence on MS. However, normalizing on the basis of available adsorbent surface area results in very similar adsorptive behavior for all MS fractions, indicating similar chemical composition. The effects of adsorbent charge and solution ionic strength conform to adsorption electrostatic principles. No measureable desorption is observed upon reducing the solution concentration.; Examination of the rate behavior over extended time periods reveals a long-term removal of humic substances by activated carbon. This long-term behavior does not conform to a single-domain diffusion analysis, and suggests that hindered diffusion or adsorbed-phase macromolecular reorientation may occur.; In summary, it appears that accounting for size and charge effects offers the potential of greatly improving the understanding of both the equilibrium and the rate of adsorption of humic matter by activated carbon.