Extraction and characterization of ternary complexes between natural organic matter, cations, and oxyanions from a natural soil

•Humic acid from a natural soil and its ternary complexes were characterized by SEC-ICP-MS.•Intact ternary complexes were extracted directly from the soil and characterized.•Complexes in the soil extract were similar to those of the purified humic acid.•The amount of As observed bound in these complexes is relatively small but merits further study.

Natural organic matter (NOM) can have a significant influence on the mobility and fate of inorganic oxyanions, such as arsenic and selenium, in the environment. There is evidence to suggest that interactions between NOM and these oxyanions are facilitated by bridging cations (primarily Fe3+) through the formation of ternary complexes. Building on previous work characterizing ternary complexes formed in the laboratory using purified NOM, this study describes the extraction and characterization of intact ternary complexes directly from a soil matrix. The complexes are stable to the basic extraction conditions (pH 12) and do not appear to change when the pH of the extract is adjusted back to neutral. The results suggest that ternary complexes between NOM, cations, and inorganic oxyanions exist in natural soils and could play a role in the speciation of inorganic oxyanions in environmental matrices.

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