Removal of aquaculture therapeutants by carbon adsorption: 2: Multicomponent adsorption and the equilibrium behaviour of mixtures

This paper presents experimental data on the batch equilibrium adsorption behaviour of binary and ternary mixtures of aquaculture therapeutants (Malachite Green, Chloramine-T, and Oxytetracycline) and dissolved organic carbon (DOC) onto the coal-based granular activated carbon 207EA (Sutcliffe-Speakman). These data indicate directly the relative strengths of adsorption of some of the components under the conditions studied and confirm that DOC is generally less strongly adsorbed than most of the therapeutants at the concentrations of interest. This paper also reviews existing multicomponent equilibrium adsorption models and concludes that, for aquaculture applications, the Ideal Adsorbed Solution Theory (IAST) with the Freundlich isotherm is the model that is most suitable. This model was then tested using the experimental mixture data. The results show that IAST with the Freundlich isotherm is very successful at predicting the reduction in adsorption of DOC caused by the presence of competing therapeutants. It tends, however, to underestimate the amounts adsorbed of the therapeutants, particularly when these are present at higher concentrations. In general, the drop in DOC removal caused by the presence of competing therapeutants is quite small. IAST is clearly a good model to use in the design of batch carbon adsorption treatments for therapeutant removal.