Complexation of Arsenite with Humic Acid in the Presence of Ferric Iron

In the presence of iron (Fe), dissolved organic matter (DOM) may bind considerable amounts of arsenic (As), through formation of Fe-bridged As−Fe-DOM complexes and surface complexation of As on DOM-stabilized Fe-colloids (collectively referred to as As−Fe-DOM complexation). However, direct (e.g., chromatographic and spectroscopic) evidence and fundamental kinetic and stability constants have been rarely reported for this As−Fe-DOM complexation. Using a size exclusion chromatography (SEC)-UV-inductively coupled plasma mass spectrometry (ICP-MS) technique, arsenite (AsIII)-Fe-DOM complexation was investigated after adding AsIII into the priorly prepared Fe-DOM. A series of evidence, including coelution of As, Fe, and DOM from the SEC column and coretention of As, Fe, and DOM by 3 kDa MWCO centrifugal filtration membrane, demonstrated the occurrence of AsIII−Fe-DOM complexation. The kinetic data of AsIII−Fe-DOM complexation were well described by a pseudofirst order rate equation (R2 = 0.95), with the rate constant (k′) being 0.17 ± 0.04 1/h. Stability of AsIII−Fe-DOM complexation was characterized by apparent stability constant (Ks) derived from two-site ligand binding model, with log Ks ranging from 4.4 ± 0.2 to 5.6 ± 0.4. Considering the kinetics (within hours) and stability (similar to typical metal-humates) of AsIII−Fe-DOM complexation, this complexation needs to be included when evaluating As mobility in Fe and DOM rich environments.