Exploring the ability of cations to facilitate binding between inorganic oxyanions and humic acid

The mobility and fate of inorganic oxyanions in the environment can be greatly influenced by interactions with natural organic matter (NOM). There is increasing evidence that this interaction between two anionic species is facilitated by cationic bridges, but detailed mechanistic studies into this system are challenging due to the heterogeneous nature of NOM. This work examines the ability of cations (Fe^III, Cr^III, Al^III, or Ga^III) to form ternary complexes with Suwannee River humic acid (SRHA) and the oxyanions of As (As^III and As^V) and Se (Se^IV and Se^VI). Complexes were characterized by separating SRHA from unbound species using size exclusion chromatography coupled to ICP-MS to determine its metal content. Unlike Cr^III and Fe^III, the post-transition metal ions Al^III and Ga^III proved ineffective at forming ternary complexes with any of the oxyanions, although minor complexation was observed with Ga^III, suggesting that electrostatic interactions are not the primary driving force behind the stabilization of these ternary complexes. The results also show differences in the behavior of Fe^III and Cr^III that may indicate that the two cations stabilize the ternary complexes by different mechanisms.