First determination of Cu adsorption on soil humin
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Humic substances are heterogeneous mixtures of organic compounds occurring in huge amounts mainly in waters, soils, sediments and organic wastes. They are formed during the decay of living organisms. They play a very important role in many environmental processes including carbon sequestration, water cleaning and retention, soil erosion, fertility and pollutant retention. However, due to their complex nature, humic substances are still poorly characterized and much less known than living matter. Humin is the most insoluble and, in turn, the least understood fraction of humic substances. To our knowledge, no information is currently available on the adsorption and desorption behaviors of metal ions on soil humin. Here, we report the adsorption and desorption properties of Cu(II) on humin and humic acids isolated from a forest soil in northeast China using the batch equilibration method. Solid-state 13C cross-polarization magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) spectroscopy was used to characterize and compare the chemical structures of humin and humic acid. The batch experiments’ results show that humin has a lower adsorption capacity and higher adsorption reversibility for Cu(II) than humic acid. The adsorption isotherms well fitted both the Langmuir and Freundlich equations. Humin, therefore, plays an important role in controlling the fate, transport and bioavailability of Cu(II) in the environment. The 13C CPMAS NMR spectra showed that compared with humic acid, humin was higher in alkyl C, carbohydrate C and phenolic C and was lower in methoxyl C, aryl C and carbonyl C. These findings mean that humin was less alkylated, more aliphatic and more hydrophobic.
KeywordsHumin Humic acid Cu(II) Adsorption Desorption 13C NMR
This work was financially supported by the National Basic Research Program of China (2009CB118600), the National Agricultural Department Public Benefit Research Foundation (201103003), the Postdoctoral Project of Jilin Province (Grant No. 01912) and the Postdoctoral Project of Northeast Agricultural Research Centre of China (Grant No. 00225). The authors wish to thank Ming Qin and Yong Wang for assistance in the laboratory work, and Dr. Zijiang Jiang for his technical support in solid-state 13C CPMAS NMR spectroscopy. We would also like to express our great appreciation to two anonymous reviewers and Dr. Eric Lichtfouse, the Editor-in Chief, for their valuable comments and suggestions.
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