Removal of microcystin variants with powdered activated carbon

Hepatotoxic microcystins resulting from cyanobacteria blooms occur in drinking water sources worldwide. Their removal in the water treatment process is essential if safe drinking water is to be produced. The presence of microcystins in raw water is problematic to the water treatment plant operator, as they are not removed by conventional water treatment practices (coagulation, flocculation, sedimentation and filtration). Powdered activated carbon (PAC) has been shown to have the potential to remove microcystin-LR; however, its success is dependent on the application of a sufficient dose for the production of water that is safe to consume. Currently there is no method available for determining PAC doses for the removal of microcystin-LR and little information exists on the removal of other microcystin analogues. In previous work the Homogeneous Surface Diffusion Model (HSDM) was successfully used to predict the adsorption of earthy-musty tastes and odours onto PAC. In this study, adsorption experiments were performed to determine whether the HSDM could be applied to two microcystin analogues: microcystin-LR (mLR) and microcystin-LA (mLA). The HSDM was used to predict the kinetics of mLR and mLA adsorption onto a wood-based PAC from a spiked sample of reservoir water. From this information PAC doses required to remove the toxic compounds were predicted for a range of conditions. These two variants were more difficult to remove than microcystin-RR (mRR) and microcystin-YR (mYR). The ease of removal was found to be in the following order: mRR > mYR > mLR > mLA.