Effects of ionic strength on the chromophores of dissolved organic matter.

This study examined effects of variations of the ionic strength (IS) on the absorbance of dissolved organic matter (DOM). The measurements performed for DOM of allochthonous (Suwannee River humic and fulvic acids, SRHA and SRFA) and autochthonous (Pony Lake fulvic acid, PLFA) origin showed that increases of IS (which was controlled by additions of sodium perchlorate) from 0.001 to 0.3 mol/L were accompanied by increases of the absorbance of DOM. The extent of the increase of DOM absorbance observed at increasing IS was consistently greater at higher pH values, and it followed the order of PLFA < SRFA < SRHA. The absolute values of the spectral slopes of the log-processed absorbance spectra of DOM calculated for a 350 to 400 nm wavelength range decreased proportionally to the logarithm of IS values. This result was hypothesized to be indicative of the deprotonation of the DOM chromophores at increasing IS values, which was supported by model calculations showing that values of the spectral slopes were nearly linearly correlated with the extent of IS-induced deprotonation of the operationally defined phenolic groups in DOM.