Study of the role of sulfur functionalities in humic acids for uranium(VI) complexation

Humic substances influence the speciation and migration behavior of toxic and radiotoxic metal ions, such as actinides, in the environment. Depending on their origin, humic substances contain different amounts of sulfur ranging from 0.1 to 3.6% and 0.5 to 1.43% in soil and aquatic humic substances, respectively [1]. Reduced, intermediately oxidized and highly oxidized sulfur functionalities occur in humic substances [1]. Although sulfur functionalities occur in small concentrations, they can play an important role in the complexation of selected metal ions. Knowledge about the impact of sulfur functionalities on the metal ion complexation by humic substances and their significance compared to oxygen- and nitrogen-containing functional groups helps to improve complexation models for humic substances. Consequently, this contributes to a more reliable geochemical modeling of the interaction processes between humic matter and metal ions in the environment.
In the present work sulfur-containing humic acid (HA) model substances have been synthesized to study the influence of sulfur functionalities on the metal ion complexation by HA. HA type M1-S with different sulfur contents was synthesized from xylose, phenylalanine, glycine, and cysteine and the resulting products were characterized. The identity of the sulfur species was determined by X-ray photoelectron spectroscopy (XPS). XPS results of HA M1-S with 1.94 wt-% sulfur indicate the occurrence of at least two different sulfur species. About 82% of sulfur occurs in form of reduced sulfur species (e.g., thiols, dialkylsulfide and/or disulfides) and about 18% of sulfur is attributed to sulfoxides.
The uranium(VI) complexation of HA with different sulfur contents (0, 1.94 and 3.94 wt-%) has been studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) and TRLFS with ultrafast pulses. Applying the metal ion charge neutralization model [2] comparable complexation constants with a mean value of logβ0.1M = 6.3 ± 0.1 were determined by TRLFS. However, with increasing sulfur content of the HA an increase in the uranium(VI) loading capacities from 26 ± 4 to 37 ± 2% (pH 3.8) was observed. This indicates that with increasing amounts of sulfur functionalities more sites become available for the complexation of uranium(VI) by HA.