EVALUATING THE IMPACT OF PH AND MOLECULAR CHARACTERISTICS ON THE FORMATION OF AS-DOM COMPLEXES

Arsenic (As) contamination is a worldwide environmental health issue due to its toxicity and numerous origins. The toxicity of As contamination is dependent on the its mobility. Complexation of As with dissolved organic matter (DOM) decreases the toxicity by reducing the amount of free As; DOM properties influence As-DOM complexation. Our work assesses how DOM characteristics and pH affect As-DOM complex formation while using a novel approach to determine As-DOM complexation.

We used two types of DOM: Suwanee Reverse Osmosis Humic acid and DOM derived from two distinct coal samples representing diverse molecular characteristics. DOM molecular weights were characterized using high performance liquid chromatography coupled with a size exclusion column (HPLC-SEC) and a fluorescence and absorbance detector. We investigated aromaticity, carboxylic, and phenolic functional groups; these molecular structures are potentially responsible for As-DOM mechanisms. Carboxylic and phenolic functional group content were quantified through potentiometric acid base titrations, while aromaticity was assessed through specific ultraviolet absorbance (SUVA₂₅₄), with an Agilent UV-VIS.

As-DOM complexation was determined through a series of dialysis experiments prepared with various concentrations of sodium metaarsenite (5 – 500 µg/L) and pH conditions (3-10). Once equilibrium was reached, As concentrations outside and inside the dialysis tube were analyzed through inductively coupled plasma mass spectrometry (ICP-MS). In parallel, As-DOM solution was analyzed with HPLC-SEC-ICP-MS which directly measures free and complexed As. By combining these methods, we received a novel data on mechanisms of As-DOM complexation, conditional distribution coefficients D_OM by dialysis experiment, and molecular weight of As-DOM complexation sites. Results were used to build isotherms based on As concentration and pH. To investigate the impact of molecular characteristics and establish trends in As-DOM complexation, we compared D_OM values to molecular characteristics of DOM from different origin. Our results evaluate trends in As-DOM complexation with direct evidence of complexation furthermore contributing to our current understanding of As cycling.