Effect of molecular size and charge on biofilm sorption of organic matter

Packed-bed reactors were used to examine the effects of molecular size and charge on biofilm sorption of organic compounds selected for their similarity to the size and charge ranges of aquatic natural organic matter (NOM). In addition, the sorbate compounds were chosen for nonbiodegradability, which was verified by BOD tests. In experiments using uncharged NOM-surrogate compounds it was found that as sorbate molecular size increased, biofilm sorption decreased. Also, negatively charged NOM-surrogate compounds were found to be less well sorbed to biofilm than neutral molecules. Apparently biofilm sorption of small molecules is enhanced by diffusion of the sorbate into the porous biofilm matrix; whereas larger sorbate molecules probably accumulate at the biofilm-water interface because they are excluded from the biofilm pores. The observed decrease in biofilm sorption of anionic organic molecules compared to removal of neutral sorbates was thought to be the result of electrostatic repulsion between the negatively-charged sorbate and the negatively-charged biofilm particularly slowing diffusion of the sorbate molecules within the biofilm.