Cr K-Edge X-Ray Absorption Spectroscopy Analysis of the Oxidative Polymerization Products of Hydroquinone in the Presence of Cr(VI).

Polyphenol polymerization has been considered as one of the abiotic humification processes in soils. Hydroquinone (H₂Q), a low molecule weight of polyphenol, is commonly used in medicine, cosmetic, and pesticide productions and is regarded as an important precursor of humic substances in soils. It is well-known that Mn-oxides (birnessite, δ-MnO₂), one of the most important abiotic redox-activated minerals in soils and sediments, can convert H₂Q to humic substance-like material via oxidative polymerization reaction. However, few studies have been carried out about the effects of other oxidants, such as Cr(VI), on polyphenol polymerization. Cr(VI), a strong oxidant, may be distributed in the environments upon inappropriate handling and disposal of Cr-containing wastes. Interactions of H₂Q with Cr(VI) may occur naturally, and investigations of the reactive pathway of H₂Q and Cr(VI) as compared with that in Mn-oxides/ H₂Q systems may be helpful to clarify the reactive mechanisms of oxidation or oxidative polymerization when organic compounds and oxidants co-exist in the ecosystems. In the study, the extended X-ray absorption for fine structures (EXAFS) was used to investigate the bonding configurations of Cr on the organic polymers upon interactions of H₂Q with Cr(VI). The analyses of the redox products of H₂Q and Cr(VI) were also performed by Cr K-edge X-ray absorption spectroscopy (XAS) to examine Cr species using X-ray absorption near-edge structure (XANES) technique. Results showed that Cr(VI) was capable of oxidizing H₂Q at pH 3, leading to the formation of 1,4-benzoquinone (BQ). During 75 min reaction, the solution color was gradually changed from light yellow to dark yellow-brown, and precipitates were formed along with an increase in solution pH to 7.3. The color change suggested that a portion of small organic molecules, formed due to oxidation of H₂Q by Cr(VI), may be polymerized to larger molecules. Linear combination fitting of XANES spectra suggested that the precipitate consisted mainly of Cr(OH)₃·nH₂O which could also be confirmed by EXAFS spectra, showing that chromium was coordinated to 6 O atoms in the first shell. Further study, e.g., EXAFS spectra fitting for the second coordination shell, is required to evaluate the roles of Cr atoms and the possible formations of Cr-Cr or Cr-C bonds in the precipitates obtained from the oxidative polymerization of organic molecules by Cr(VI).