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

• Published in: Environmental science & technology 2011-04, Vol.45 (8), p.3210-3216
• Main Authors: Liu, Guangliang, Fernandez, Aymara, Cai, Yong
• Format: Article
• Language: English
• Subjects: Arsenic; Arsenites - chemistry; Binding sites; Chromatography; Environmental Pollutants - chemistry; Humic Substances - analysis; Ions; Iron; Iron - chemistry; Kinetics; Mass spectrometry; Membranes; Models, Chemical; Plasma
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es102931p

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 (K s ) derived from two-site ligand binding model, with log K s 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.