13C-CP/MAS-NMR Spectroscopy of Chemically Modified and Unmodified Synthetic and Natural Humic Acids

In a former study we investigated the influence of phenolic OH groups on the complexation behavior of humic acids (HAs) with uranium(VI) using chemically modified synthetic HA (type M1) as well as modified natural HAs (Aldrich and Kranichsee) with blocked phenolic OH groups. These modified HAs were synthesized by permethylation of the original HAs with diazomethane, which converts carboxyl groups to methyl esters and acidic (phenolic) OH groups to methyl ethers and subsequent alkaline saponification of the permethylated HAs. This results in the hydrolysis of ester groups, whereas phenolic OH groups remain blocked.
In continuation, we performed in the present study comparative 13C-CP/MAS-NMR measurements of the modified and unmodified HAs type M1, Aldrich and Kranichsee to verify that there are no significant structural changes of the HAs during their modification. We also studied NMR spectra of alkaline treated HAs type M1, Aldrich and Kranichsee to determine potential structural changes of the HAs which may occur during the alkaline treatment of the permethylated HAs. In addition to the modified and unmodified HAs with carbon-13 at natural abundance we investigated 13C-labeled modified HAs type M1 (permethylated, phenolic OH groups blocked, partly methylated) that were synthesized using [13C]diazomethane as methylation reagent. These studies were performed to verify the modification process and to determine which functional groups of the HAs show the highest affinity for methylation with diazomethane.
From our studies we conclude that only carboxyl and acidic (phenolic) OH groups are methylated with diazomethane, whereby carboxyl groups show the highest affinity for methylation. The alkaline hydrolysis of diazomethylated HAs results in HAs with blocked phenolic OH groups. No significant changes of the overall HA structure occur. This method offers the possibility to synthesize selective modified HAs that can be used to study the influence of various functional groups on the complexation behavior of HAs with metal ions.