Removal of the Cyanotoxin Microcystin-LR from Drinking Water Using Granular Activated Carbon

Cyanotoxins, such as microcystin-LR (MC-LR), produced by harmful algae blooms are becoming a chronic issue drinking water utilities must manage. The removal of MC-LR by powdered activated carbon has been studied, but comparatively little information exists for granular activated carbon (GAC) treatment. Since many utilities already use GAC to treat other seasonal contaminants or to mitigate disinfection by-product formation, there is need to evaluate its performance. In this study, we present equilibrium isotherms and rapid small-scale column test (RSSCT) breakthrough curves for MC-LR adsorption onto GAC. Isotherms were completed for six different GAC from four carbon sources (hardwood, lignite, bituminous coal, and coconut) in the absence and presence of natural organic matter (NOM). The proportional diffusivity RSSCTs compared MC-LR and NOM breakthrough for three GAC sources (bituminous coal, lignite, and hardwood) at pH 7.8 and 9.5. The results indicate variations in GAC properties with material type, namely mesopore volume, affect adsorption capacity and time to breakthrough. MC-LR breakthrough followed the isotherm capacity trends and demonstrated breakthrough of MC-LR with the bituminous coal GAC an order of magnitude faster than for the lignite and hardwood GAC, reflecting their mesoporous surface areas. The influence of pH conducted on the bituminous GAC demonstrated little change in time to breakthrough; however, the slope of the breakthrough at pH 9.5 was more gradual than at pH 7.8, reflecting pH-dependence MC-LR competition with NOM. These findings indicate utilities should utilize mesoporous carbon in their current GAC process if they anticipate needing to remove cyanotoxins like MC-LR.