Competitive interaction of natural organic matter and herbicides during adsorption to preloaded and non-preloaded granular activated carbon

Natural organic matter (NOM) preloading and the competitive interaction of NOM and herbicides during adsorption to granular activated carbon (GAC) were studied through a partitioning approach. Preloading reduced partition coefficient (K_D) values in single solute experiments with the adsorption of atrazine and MCPA to GAC. Competitive interaction was studied through NOM dilution experiments with non-preloaded and preloaded GAC. For both atrazine and MCPA, NOM competed significantly at raw water concentrations (approx.5 mg1^-1 dissolved organic carbon). The K_D of atrazine increased gradually with NOM dilution, while no significant increase in K_D for MCPA was found until NOM dilution reached almost equimolar concentrations. Under treatment plant conditions NOM preloading will occur but there are still sites available for herbicide adsorption. However, NOM can compete for the remaining sites and it is likely that the character and concentration of NOM is critical for competitive interaction of herbicides for adsorption sites on GAC in a water treatment plant.