Predicting the removal of atrazine by powdered and granular activated carbon

Abstract

The general objective of this research was to develop and test bench scale procedures for the prediction of atrazine removal in full scale and pilot scale powdered activated carbon (PAC) and granular activated carbon (GAC) adsorption processes.

To quantify the competitive effect of background organic matter (BOM) on the adsorption capacity of PAC for atrazine, the equivalent background compound (EBC) method was tested, in which the mixture of competing BOM compounds was represented by one hypothetical equivalent background compound (EBC). Using the Freundlich isotherm equation and the ideal adsorbed solution theory (IAST), a search routine was developed to determine the adsorption characteristics of the EBC. A comparison with experimental data showed that the EBC method successfully described adsorption equilibria of micropollutants over a wide range of initial concentrations. Furthermore, it was discovered that the capacity of activated carbon for a target compound that was present at trace levels in natural water was directly proportional to the initial target compound concentration as confirmed by experimental and mathematical modeling results for atrazine and other micropollutants.

The pseudo single-solute homogeneous surface diffusion model (HSDM) was employed to describe the rate of atrazine adsorption on PAC. A combination of the EBC method and the pseudo single-solute HSDM successfully predicted atrazine removal in batch reactors over a large range of initial concentrations. Furthermore, reasonable agreement was obtained between the predicted and measured full scale removal of atrazine.

Rapid small-scale column tests (RSSCTs) were judged to be most useful for predicting the initial performance of adsorbers that contained virgin GAC. However, results from the current study showed that RSSCTs could not predict atrazine removal after operating times that exceeded 3.5 to 7 months. Similarly, RSSCTs were not useful to predict the remaining life of GAC adsorbers after long preloading times.

To predict the remaining life of operating GAC filters, the pseudo single-solute HSDM, which required the determination of equilibrium and kinetic parameters for preloaded GAC, proved to be the most promising technique.