Co-Precipitations of Dissolved Organic Carbons with Different Ratios of Fe/Al in an Acidic Solution.

Dissolved organic carbon (DOC), a broad classification of organic molecules, is of operational definition of those dissolved materials which can pass through a 0.45 or 0.22 £gm pore-size membrane filter. Bearing abundant polar molecules, such as oxygen, DOC have been found capable of binding and modifying the surface properties of the soil minerals, regulating indirectly the mobility and bioavailability of organic and inorganic contaminants. On the other hand, DOC can associate intimately with polyvalent metals, forming DOC-metal precipitates at an acidic solution. This reaction may alter biogeochemical carbon cycle in soils if the dynamic unstable DOC can be fixed by metals against further leaching, runoff, and biodegradation. Therefore, it is vital important to understand the formations and stabilities of DOC-metal precipitates in the soils. In this study, effects of specific multivalent cations, such as Al and Fe(III), on the structures of DOC derived from two high organic-containing soils of Changhua (CHA) and Yangminshan (YS) of Taiwan were systematically examined at pH 3~6. Results showed that a higher metal/C ratio was favorable for the formation of insoluble metal-DOC co-precipitates, and the particle sizes of the precipitates increased with a decrease in solution pH. In the presence of Fe(III) and Al, the highest amount of CHA-DOC/metals co-precipitates would be produced at pH 3~4.5; however, a higher pH, i.e., pH 6, was required for the YS-DOC samples. The diagrams of XRD showed that the 2-line ferrihydrite existed in YS DOC-Fe/Al co-precipitates, but the structures of CHA DOC-Fe/Al co-precipitates were amorphous. FTIR analyses demonstrated that carboxyl and phenolic groups of DOC might be the binding sites for the metals. X-ray absorption spectroscopy (XAS) was also used to determine the structures of DOC-Fe/Al co-precipitates. The spectra of Fe-EXAFS indicated that the coordination environment of Fe in the YS DOC-Fe/Al co-precipitates with different Fe/(Fe+Al) molar ratios (i.e., 0.25, 0.50, 0.75, and 1) were similar, but the coordination number of Fe in CHA DOC-Fe/Al co-precipitates increased with an increase in Fe concentrations. Further study is required to clarify the effects of Al or the compositions of DOC on the structures and coordination environments of Fe in the DOC-Fe/Al co-precipitates.