Undissolved humic acid (HA) has a great retention effect on the migration of hexavalent chromium [Cr(VI)] in soil, and HA functional groups play a predominant role in this process. However, the coupled mode between Cr(VI) retention and HA functional groups reaction is still unclear. In this study, it was found that a fair amount of Cr on HA existed in the forms of ion exchangeable and binding Cr(VI) during the reaction resulting from the ion exchange adsorption and complexation of Cr(VI). According to the results of two-dimensional correlation spectroscopic analysis (2DCOS), HA functional groups participated in the reaction with Cr(VI) in the order of carboxyl ≈ chelated carboxyl > phenol > polysaccharide > methyl, and all the functional groups were more likely to be located at aromatic domains. Based on the results of XPS spectra, rather than to be oxidized by Cr(VI), carboxyl more tended to be complexed by chromium, which is regarded as the precondition for Cr(VI) reduction. Phenol, polysaccharide and methyl with distinct reaction activities successively acted as major electron donors for Cr(VI) reduction in different reaction stages. Consequently, it was determined that the retention of Cr(VI) by undissolved HA followed an adsorption-complexation-reduction mechanism, and based on this, a multi-step kinetic model with multiple types of complexation/reduction sites was developed to simulate the retention processes resulting in a much better fitting effect (R² > 0.99) compared with traditional first-order and second-order kinetic models (R² < 0.95). This demonstrated that the multi-step kinetic model is of great potential in accurately simulating the migration and transformation of Cr(VI) in soil environment.

不溶性腐殖酸（HA）对六价铬[Cr（VI）]在土壤中的迁移具有很大的保留作用，而HA官能团在该过程中起主要作用。但是，Cr（VI）保留和HA官能团反应之间的耦合模式仍不清楚。在这项研究中，发现在离子交换吸附和络合Cr（VI）的反应过程中，HA上有大量Cr以离子交换和结合Cr（VI）的形式存在。根据二维相关光谱分析（2DCOS）的结果，HA官能团以羧基≈螯合羧基>苯酚>多糖>甲基的顺序参与了与Cr（VI）的反应，并且所有官能团均更有可能位于芳香区域。根据XPS光谱的结果，而不是被Cr（VI）氧化，羧基更容易被铬络合，这被认为是Cr（VI）还原的前提。具有不同反应活性的苯酚，多糖和甲基先后在不同反应阶段充当还原Cr（VI）的主要电子供体。因此，可以确定未溶解的HA对Cr（VI）的保留遵循吸附-络合-还原机理，并在此基础上，开发了具有多种类型的络合/还原位点的多步动力学模型以模拟保留产生更好的拟合效果的过程（具有不同反应活性的多糖和甲基先后在不同的反应阶段中充当了Cr（VI）还原的主要电子供体。因此，可以确定未溶解的HA对Cr（VI）的保留遵循吸附-络合-还原机理，并在此基础上，开发了具有多种类型的络合/还原位点的多步动力学模型以模拟保留产生更好的拟合效果的过程（具有不同反应活性的多糖和甲基先后在不同的反应阶段中充当了Cr（VI）还原的主要电子供体。因此，可以确定未溶解的HA对Cr（VI）的保留遵循吸附-络合-还原机理，并在此基础上，开发了具有多种类型的络合/还原位点的多步动力学模型以模拟保留产生更好的拟合效果的过程（ 与传统的一阶和二阶动力学模型相比，R ² > 0.99）（R ²  <0.95）。这表明，多步动力学模型在准确模拟土壤环境中Cr（VI）的迁移和转化方面具有巨大的潜力。