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

Abstract The mobility and fate of inorganic oxyanionsin 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 complexeswith Suwannee River humic acid (SRHA) and the oxyanionsof 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 chromatographycoupled 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 complexeswith 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 complexesby different mechanisms.