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Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy
Published on: September 17, 2017
Mechanism study of humic acid functional groups for Cr(VI) retention: Two-dimensional FTIR and 13C CP/MAS NMR
Jia Zhang1, Linpeng Chen1, Huilin Yin1
1Beijing Key Laboratory of Water Resources & Environmental Engineering, China University of Geosciences, Beijing 100083, China.
Abstract:
Undissolved humic acid (HA) is known to substantially effect the migration and transformation of hexavalent chromium [Cr(VI)] in soils. The mechanisms of Cr(VI) retention in soils by undissolved HA have been reported; however, past studies are inconclusive about the types of HA functional groups that are involved in Cr(VI) retention and the retention mechanisms. Utilizing a two-dimensional correlation spectroscopy (2DCOS) analysis for FTIR and 13C CP/MAS NMR, this study investigated the variations of HA function groups and molecular structures after reactions with aqueous Cr(VI) under different pH conditions. Based on the changing sequence of functional groups interpreted from the 2DCOS results, a four-step mechanism for Cr(VI) retention was determined as follows: (1) electrostatic adsorption of Cr(VI) to HA surface, (2) complexation of adsorbed Cr(VI) by carboxyl and ester, (3) reduction of complexed Cr(VI) to Cr(III) by phenol and polysaccharide, and (4) complexation of reduced Cr(III) by carboxylic groups. These functional groups that are involved in Cr(VI) retention were determined to occur in aromatic domains.
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