Displacement effect of NOM on atrazine adsorption by PACs with different pore size distributions.

This study investigated displacement of atrazine by the strongly competing fraction of natural organic matter (NOM) in batch and continuous-flow powdered activated carbon/PAC) adsorption systems. Due to the displacement effect, atrazine adsorption capacity in a continuous flow PAC/microfiltration (MF) system, where the carbon retention time is greater than the hydraulic retention time, decreased with time or NOM throughput. The capacity was lower than that measured in a batch reactor or predicted by the equivalent background compound-ideal adsorbed solution theory (EBC-IAST) method. A mathematical model previously developed to simulate the adsorption process in the PAC/MF system was modified to take into account the displacement effect. Two types of PACs were tested using a range of influent atrazine concentrations and carbon doses. The extent of atrazine displacement by NOM was found to depend on the type of PAC, while the rate of displacement was a function of PAC type as well as carbon dose. The PAC lost its adsorption capacity for atrazine faster at a lower carbon dose. PAC B, which has a higher percentage of mesopores, lost more atrazine adsorption capacity but at a slower rate than PAC A.