《Table 1–Reaction rate constants of abiotic humification.》

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《Mechanism of oxidation and catalysis of organic matter abiotic humification in the presence of MnO_2》

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m:the number of CO2molecules released per molecule of FLA formed；k1and k2represent the kinetic constants of the two steps in the formation of FLAs and HLAs，respectively.R2was the fitting correlation coefficient for k1，and k2was calculated according to th

The related m value was determined and gave a CO2release amount of 0.73–1.87 mg/mg DOC of FLAs formed.In addition，the related kinetic equations were determined using a kinetic model，and the reaction rates were evaluated in terms of k1and k2.The calculations of k1and k2echoed the calculations of previous studies（Zhang et al.，2015），and the corresponding reaction constants are summarized in Table 1.The obtained results show that the formation of FLAs and HLAs respectively followed pseudo-second-order and zero-order kinetics models well.Accordingly，based on the results shown in Table 1，combined with the previous analysis of the increases in the DOM content up to the levels in MR6，the amount of Mn O2was sufficient for oxidizing HS precursors and forming FLAs，resulting in a lower reaction rate，which can be seen from the variations in k1.Note that in the presence of a considerable amount of Mn O2，k1remained stable，which can be seen from the changes in the equilibrium constant from MR2 to MR5.However，as the concentration of DOM increased further，k1was reduced by roughly one order of magnitude below that of system MR5，demonstrating that the presence of an inadequate amount of Mn O2impeded the formation of FLAs.Thus，Mn O2was strongly involved in the formation of FLAs，and the amount of Mn O2had a profound influence on the behavior and fate of the FLAs.In contrast，k2remained nearly constant in the various systems，indicating that the reduction products of Mn O2，Mn（III）and Mn（II），were formed during the formation of FLAs but had little impact on the formation of HLAs，and possibly functioned as catalysts to facilitate the production of HLAs.