In this AM251 research, the DOC-based isotherms were calculated by actual measurement of the DOC of each sample, as opposed to being derived from converting/correlating UVA254 data with DOC data. As illustrated in Fig. 7, the DOC-based isotherms predicted a greater degree of fractional solid phase sorption per dose for SW-HA relative to SW-FA, whereas the opposite effect was observed for the UVA254- and THMFP-based isotherms. As a result, mass-based DOC measurement can never reveal that SW-FA is more efficiently removed than SW-HA as demonstrated in the UVA254- or THMFP-based isotherms (Fig. 7).
There are multiple factors that contribute to the inability of DOC-based isotherms to predict UVA254- or THMFP-based isotherms for SW-FA and SW-HA. Firstly, DOC is generally expressed as a mass-based concentration, not a molar-based concentration. Conceptually, if two HS solutions of equal volume are compared—one FA and one HA—each having 100 molecules (i.e., the same molar-based concentration but not the same mass-based concentration) and one-half of the molecules from each solution are sorbed onto activated carbon, the 50 molecules of HA remaining in solution will always have a greater carbon content measured by DOC than 50 molecules of FA because HAs have a greater molecular weight and more carbon atoms than FAs. Therefore, a greater carbon mass of the HA will have been sorbed compared to the FA, when in actuality the same number of moles will have been removed.