The distribution of charge and acidic functional groups in natural organic matter: the dependence on molecular weight and pH
Ritchie, Jason Duane
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The Suwannee River natural organic matter (SRNOM) was fractionated by preparative size-exclusion chromatography (SEC) into seven molecular weight (MW) fractions. The SRNOM and its MW fractions were subsequently analyzed for their concentrations of acidic functional groups by direct titrations, average MWs and MW distributions by semi-analytical SEC, and charge-to-MW distributions by capillary electrophoresis. Carboxyl concentrations in the MW fractions were inversely proportional to their average MWs. Conversely, the phenolic concentrations, though smaller than the carboxyl concentrations, were proportional to average MWs. Hysteresisthe non-overlap between sequential forward and reverse titrationswas observed for the SRNOM and its MW fractions, where the reverse titrations predicted a greater concentration of carboxylic acid groups than the forward titration. Because hysteresis is thought to be caused by the base-catalyzed hydrolysis of esters, this suggests that ester groups in the SRNOM are distributed over all MWs. Data for direct titrations, MW distributions, and capillary electrophoresis were evaluated by a computational scheme that solves for the most probable distribution of acidic functional groups and charges on solutes in the SRNOM and the MW fractions as a function of pH. Depending on the MW ranges of the samples, solutes in the SRNOM and the MW fractions are predicted to have from one to a maximum of 25 carboxyl groups per solute. Most phenolic groups are predicted to be on solutes that have a minimum of two carboxyl groups. At low pH, all samples have high relative abundances of solutes with the lowest charges. The charges of solutes are predicted to increase with increasing pH due to the sequential ionization of acidic functional groups. Depending on the MW ranges of the samples, the maximum probable charges of solutes in the SRNOM and the fractions at high pH are -12 to -30. By knowing the most probable distribution of charge and abundances of acidic functional groups, researchers will make better estimates of thermodynamic parameters and models that describe equilibria between metals and natural organic matter in the environment.