DIRECT MEASUREMENTS OF ARSENIC COMPLEXATION WITH DISSOLVED ORGANIC MATTER

Arsenic is a carcinogen found in drinking water supplies worldwide. The bioavailability of arsenic in natural waters can be limited by binding to dissolved organic matter (DOM) [1], which is a complex mixture of labile and recalcitrant carbon compounds. It is important to understand DOM parameters influencing arsenic binding to DOM to predict bioavailability and toxicity of arsenic in these systems. Here, we explore the metal-DOM complexes in coal-derived DOM and study the arsenic binding to this DOM source in a simulated experiment by direct measurements of the complexes.

Direct measurements of metal-DOM were made by coupled high-performance liquid chromatography (HPLC), size-exclusion column (SEC), and fluorescence detector (FLD) and inductively coupled plasma mass spectrometer (ICP-MS). To represent groundwater DOM, we used aqueous extracts of coals from Hot Springs County, Arkansas. By HPLC-SEC-FLD, we characterized sizes and fluorescence of DOM fractions. HPLC-SEC-ICP-MS was used to measure metals in initial DOM solutions, namely vanadium, chromium, manganese, iron, zinc, and arsenic. Metal-DOM complexes were confirmed if the retention times of the DOM fraction and the metal peak were identical. Our method offers an improvement by directly measuring metal-DOM complexes over a range of molecular sizes without the intrinsic assumptions that the physical separation like dialysis and sequential filtration require.

To study prevailing arsenic binding sites, the DOM solution was adjusted to 25 mg C/L and spiked with sodium meta-arsenite to reach concentrations from 25 to 500 ppb. We distinguished arsenic-DOM from free arsenic by comparing the measurements from HPLC-SEC-FLD and HPLC-SEC-ICP-MS applied in parallel. Our initial results show that the lowest molecular weight fraction of DOM (~538 Da) was associated with the complexed arsenic. Fluorescence intensity of this fraction decreased after arsenic addition (25 ppb), meaning that coordination is likely. This research opens a new perspective in the studies of metal environmental fate based on the direct measurement. The results can be further applied in the health-associated studies for understanding the toxicity effects of arsenic-contaminated water in the presence of DOM.

[1] Pothier et al. Sci. Total Environ. 2020, 716, 137118