| Interaction of Ca~(2+) and soil humic acid characterized by a joint experimental platform of potentiometric titration,UV–visible spectroscopy,and fluorescence spectroscopy |
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| 摘 要： | Rocky desertification has become a major environmental issue in the karst region of southwestern China.Karst rocky desertification was more severe in regions of limestone soil than in adjacent regions of other soils,despite the relatively higher soil organic matter(SOM)content in limestone soil. The underlying mechanism remains ambiguous. We speculated that the geochemical characteristics of limestone soils in the karst region plays an essential role, especially the high calcium content of limestone soil. To test this hypothesis, we collected limestone soil samples from a limestone soil profile in the southwestern China karst region and extracted humic acid(HA) from these limestone soil samples. We investigated the interaction of Ca~(2+)and three HA samples on a joint experimental platform, which consists of an automatic potentiometric titrator, a UV–visible spectrometer, and a Fluorescence spectrometer. HA solutions were titrated by Ca~(2+)and optical spectra of the HA solutions were monitored during the titration experiments. The results indicated that:(1) the interaction of Ca~(2+)and HA is a combined process of adsorption and complexation. Adsorption dominated the overall distribution behavior of Ca~(2+), which could be fit by Langmuir and Freundlich isotherm models.Complexation was distinguished only when the concentration of Ca~(2+)is low;(2) the changes of UV–visible spectroscopy and excitation–emission matrix fluorescencespectroscopy spectra of HA samples when they were binding with Ca~(2+)implied the apparent molecular size and structure of HA became larger and more complex;(3) the combination of Ca~(2+)and HA plays an important role in the SOM preservation of limestone soils but the stability of the Ca–HA association was relatively weak. The present study draws attention to maintaining the relatively higher Ca~(2+)concentration in limestone soils in ecologic restoration attempts in karst regions.
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