Evaluation and modelling of dissolved organic matter reactivity toward AsIII and AsV – Implication in environmental arsenic speciation

• AsIII and AsV binding on SRHA studied by dialysis, fluorescence and modelling.
• AsIII binding by direct interaction whatever the presence of mono or divalent cations.
• AsV binding by direct interaction when alone or in presence of monovalent cation.
• AsV binding by cationic bridge when in presence of divalent cation.
• New conditional constant for SRHA, AsIII and AsV, ternary complex SRHA–AsV–Ca.

Many studies have been carried out to identify dissolved organic matter–trace metals interactions, as organic matter (OM) was demonstrated to be a governing parameter of metals speciation. Concerning arsenic (As), such OM–As studies are scarce and concluded that, when As binding occurred, it was probably through cationic bridges or, in some cases, directly. Yet, analytical proofs remained complex to obtain. In this work, As binding with Suwanee River Humic Acid (SRHA), as an example of dissolved organic matter, was studied, considering both AsIII and AsV, at various pH and in absence/presence of Na and Ca. Dialysis, fluorescence measurements and PHREEQC modelling were performed to identify and characterize the mechanisms at work for the various performed experiments. It was observed that AsIII binding on SRHA occurred through direct SRHA–AsIII binding and that neither Na nor Ca presence modify this mechanism. AsV appeared to be also bound by SRHA through direct interaction, but suffered from the competition of Na for the SRHA binding sites. Oppositely, in presence of Ca, the overall AsV–SRHA binding was significantly enhanced, Ca acting as an efficient cationic bridge through the formation of an SRHA–Ca–AsV ternary complex. All the obtained data were satisfactorily simulated using a unique set of binding parameters which can therefore be implemented in any speciation code to better address As behaviour in environmental conditions.

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