Adsorption competition onto activated carbon, studied by magic-angle spinning NMR

The competition between water and two phosphorus-containing compounds for adsorption sites on two activated carbon materials has been studied by H-2 and P-31 magic-angle spinning (MAS) NMR as a function of time after addition of the second adsorbate. Both H-2 and P-31 spectra show two bands, one close to the resonance of the relevant liquid compound and the other (assigned to molecules in micro- and meso-pores) shifted by 6-9 ppm to low frequency. Addition of the phosphorus-containing compounds to carbons with pre-adsorbed water rapidly causes displacement of most of the water from the strong adsorption sites. Other changes occur over a period of up to 24 h. Addition of water to carbons with pre-adsorbed phosphate causes some transfer of phosphate from the macropore/surface sites to the micro/mesopores although these were already heavily occupied by phosphate.