19 F NMR analysis. Concentration for the internal standard (TFE) solution was set to 2.3 mM by transferring 0.42 mL TFE into 1 L H2O. The NMR analyte consists of 0.25 mL D2O, 0.25 mL filtrate and 0.2 mL TFE. Quantification was performed according to the literature. Integrated area under the characteristic peaks of the terminal CF3 groups for PFOA (-80 ppm) was compared relative to the TFE peaks (-76 ppm). Residual PFOA concentration after adsorption was calculated from the linear calibration equation obtained.