Adsorption of Dissolved Organic Carbon Extracted from Sewage Sludge on Montmorillonite and Kaolinite in the Presence of Metal Ions

The adsorption of dissolved organic carbon extracted from sewage sludge (SSDOC) and its complexes with Cu, Pb, and Cd by montmorillonite and kaolinite was measured at 283, 298, and 308 K. Experiments were conducted at 2 g L(-3) clay suspension concentration in an ionic background of 50 mM NaCIO4. Sufficient metal ion con centrations were present to saturate the clays (92 and 5 cmol(c) kg(-1) for montmorillonite and kaolinite, respectively). The pH value of the suspensions was adjusted to 5.5 prior to initiation of the adsorption experiments. Kinetic studies conducted under aseptic conditions indicated that within 2 h the adsorption of the SSDOC by the clays reached an equilibrium value. Adsorption of SSDOC was found to be reversible with respect to small changes in the concentration of the adsorbate thus meeting the criteria for thermodynamic equilibrium. Rapid adsorption of the SSDOC suggests that adsorption was restricted to the external surfaces of the layer silicates. Statistical analysis of the data showed that all of the isotherms were described by a linear isotherm model. The average isosteric heat of adsorption <(Delta H)over bar>(qc)) was estimated to be O +/- 2 kJ mol(-1) SSDOC, within experimental error, for each of the clay metal combinations. The low values for <(Delta H)over bar>(qc) suggests that adsorption of SSDOC by montmorillonite and kaolinite is the result of an increase in the configurational entropy of the clay-metal-SSDOC system. Pooled adsorption data across temperatures yielded a single regression line for each clay-metal-SSDOC system with the following order of slopes and calculated average free energy of adsorption <(Delta G)over bar>(ads)): Pb > Cu > Cd. The observed order of the values for <(Delta H)over bar>(ads) follow the same trend for the formation constants of metal humic complexes. This observation suggests that the favor able increase in <(Delta G)over bar>(ads) is the result of changes in the conformation of the SSDOC making adsorption more favorable. When the value for <(Delta G)over bar>(ads) was computed on the basis of external clay surface area the differences between the two clays decreased markedly, suggesting that adsorption was restricted to the external surfaces of the minerals.