Characterisation of NOM by adsorption parameters and effective diffusivities

Adsorption equilibria were studied with RO isolates for both activated carbon and activated alumina. Maximum carbon loadings on the order of 65 mg/g DOC were obtained, while the maximum loadings of activated alumina were about 30 mg/g DOG. In the latter case the slope of the isotherms was generally steeper, indicating weaker adsorbabilities of some of the NOM compounds on the metal oxide adsorbent. Since NOM must be regarded as a multi-component system with respect to adsorption, the data were evaluated by the adsorption analysis procedure. The results were used for a theoretical comparison of NOM adsorption based on the same initial concentration of all samples. The method of colloidal titration was applied to the RO isolates in the range pH = 3-8. Poly-DADMAC was used as a cationic polymer, and a streaming current instrument was used for end point detection. The results which are termed anionic colloidal charge (ACC) values increase with increasing pH for pH = 3-6 while they are about constant for pH = 6-8. At the samples' original pH, the specific ACC values found range from 5.2 to 7.6 meq/g DOG. Mass transfer coefficients for NOM adsorption on granular activated carbon were determined for both RO and EVAP isolates, respectively. The data were used to derive effective diffusivities for each sample. The results differ by a factor of about 2 with the Maridalsvann sample having the highest and the Humex sample having the lowest diffusivity. Furthermore, the results for the EVAP isolates are in 8 of 9 cases higher than for the RO samples. Based on a comparison with known macromolecules, average molecular weight values were estimated to be on the order of 700-4800 Dalton. (C) 1999 Elsevier Science Ltd.