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Purpose: Precipitation of dissolved organic matter (DOM) by multivalent cations is important for biogeochemical cycling of organic carbon. We investigated to which extent cation bridges are involved in DOM precipitation and how cross-links by cations and water molecule bridges (WaMB) stabilize the matrix of precipitated DOM. Materials and methods: DOM was precipitated from the aqueous extract of a forest floor layer adding solutions of Ca(NO3)2, Al(NO3)3 and Pb(NO3)2 with different ini...Purpose: Precipitation of dissolved organic matter (DOM) by multivalent cations is important for biogeochemical cycling of organic carbon. We investigated to which extent cation bridges are involved in DOM precipitation and how cross-links by cations and water molecule bridges (WaMB) stabilize the matrix of precipitated DOM. Materials and methods: DOM was precipitated from the aqueous extract of a forest floor layer adding solutions of Ca(NO3)2, Al(NO3)3 and Pb(NO3)2 with different initial metal cation/C (Me/C) ratios. Precipitates were investigated by differential scanning calorimetry before and after aging to detect cation bridges, WaMB and restructuring of supramolecular structure. Results and discussion: 25-67precipitation efficiency of cations increased in the order Ca < Al < Pb, while the cation content of precipitates increased in the order Pb < Ca < Al. The different order and the decrease in the WaMB transition temperature (T*) for Al/C>3 is explained by additional formation of small AlOOH particles. Thermal analysis indicated WaMB and their disruption at T* of 53O C-65O C. Like cation content, T* increased with increasing Me/C ratio and in the order Ca < Pb < Al for low Me/C. This supports the general assumption that cross-linking ability increases in the order Ca < Pb < Al. The low T* for high initial Me/C suggests less stable and less cross-linked precipitates than for low Me/C ratios. Conclusions: Our results suggest a very similar thermal behavior of OM bound in precipitates compared to soil organic matter and confirms the relevance of WaMB in stabilization of the supramolecular structure of cation-DOM precipitates. Thus, stabilization of the supramolecular structure of the DOM precipitates is subjected to dynamics in soils.» weiterlesen» einklappen

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