Evaluation and modelling of dissolved organic matter reactivity toward As(III) and As(V) – implication in environmental arsenic speciation.

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 As(III) and As(V), 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 As(III) binding on SRHA occurred through direct SRHA-As(III) binding and that neither Na nor Ca presence modify this mechanism. As(V) 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 As(V)-SRHA binding was significantly enhanced, Ca acting as an efficient cationic bridge through the formation of an SRHA-Ca-As(V) 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.