Differentiating humic and algal surface active substances in coastal waters by their pH-dependent adsorption behaviour

Sinusoidal alternating current (ac) voltammetry is a long-established and very sensitive technique for the determination of surface active substances (SAS) in natural waters. However, different classes of SAS cannot be distinguished from the shape of their ac current vs. potential ( i – E ) curves alone. Here we describe an effective method for quantifying the mixed pools of marine algal exudates and aquatic humic substances present in coastal waters. The method involves monitoring the adsorption kinetics of naturally occurring SAS at two different pH values, here chosen to be pH 3.5 and 7.0. We found that a simple rectangular hyperbola to describe the surface coverage of the mercury drop versus time yielded an excellent fit from which a kinetic adsorption constant k (s −1 ) could be derived. In addition, k increased linearly with concentration of the adsorbing species. Calibration plots of k vs. concentration for two model substances, i.e. Nordic Reservoir NOM and diatom culture exudates, were obtained. The value of k was found to be a sensitive function of pH between pH 3.5 and 7.0 for humic substances, but not for phytoplankton exudates. This provided the basis for resolving the concentrations of ‘humic-like’ and ‘algal-like’ SAS in natural samples collected along a transect extending from the mouth of a humic-rich river (S = 5) to the far field region of the river plume (S > 34). Concentration–salinity relationships indicated a terrestrial source of humic SAS and a marine source of algal SAS, as expected. Calculated dissolved organic carbon ([DOC]) concentrations were in good agreement with [DOC] measured directly.