## 3 Results and Discussions
### Analysis of Changes in Chemical Forms for Heavy Metals  
Figure 11 shows the changes in the chemical morphology of manganese before and after sorption of DX under different conditions. In the original sample of DX, the contents were ranked as Fe-Mn bound state (5.41 mg:g\({}^{-1}\)) > residual state (0.69 mg:g\({}^{-1}\)) > exchangeable state (0.50 mg:g\({}^{-1}\)) > organic matter bound state (0.24 mg:g\({}^{-1}\)) > carbonate bound state (0.13 mg:g\({}^{-1}\)). The Fe-Mn oxide bound state accounts for more than 70% of the total manganese mass in DX and is the part of Fe\({}_{2}\)O\({}_{3}\) and MnO\({}_{2}\), etc. that generates soil nodules in a relatively stable form. The exchangeable state is the part that changes more before and after sorption, which generally refers to heavy metals exchanged and adsorbed on soil clay minerals and other components, such as iron hydroxide, manganese hydroxide and humus. It is the most sensitive to changes in the soil environment and is most readily absorbed by crops. Moreover, heavy metals in the residual (8-10%) are stably present in the lattice of quartz and clay minerals, etc., which have little effect on soil heavy metal migration and bioavailability. Both the carbonate-bound state and the organic matter-bound state accounted for less than 4%, and hardly changed significantly before and after sorption. The carbonate-bound state is sensitive to soil environmental conditions. As the soil pH decreases, a large amount of released ionic state heavy metals are taken up by the crop. The organic matter bound state is the one that enters the organic matter in different forms and then undergoes chelation to form chelated salts or sulfides. In general, this form is not easily absorbed and used by organisms.  
The change in the percentage of manganese morphology after sorption, net of DX background values, is shown in Figure 12. During the sorption of Mn(II) by DX sediments, both the residual state and the organic matter-bound state accounted for a relatively low proportion of the total heavy metal chemistry and did not change significantly. Compared to single sorption, the proportion of exchangeable state gradually decreased from 38% to 29% after reusing DX sediments sorption, but the proportion of Fe-Mn oxidation state increased by 4%. The reason for this phenomenon is the oxidation of a part of previously adsorbed Mn(II) by the action of microorganisms to form manganese oxides, which combine with new heavy metal ions to generate soil nodules. The hydroxide of Mn(II) combined with CO\({}_{2}\) in the air to generate new carbonate groups, resulted in a rise of about 6% in the carbonate-bound state after repeated sorption. In addition, the percentage of the Fe-Mn oxidation state and the exchangeable state changed significantly after the addition of NaN\({}_{3}\). With the increase of NaN\({}_{3}\) concentration, the proportion of Fe-Mn oxidation state decreased, while the proportion of exchangeable state increased. When 50 mM NaN\({}_{3}\) was added, the proportion of the Fe-Mn oxidation state decreased from 54% to 43%, and the exchangeable state increased by 10.80%. This evidence proves that the inhibitory  
Figure 10: Effect of adding sodium azide to inhibit microbial action on manganese sorption.  
effect of NaN\({}_{3}\) on microorganisms is the main reason for the decrease of Mn(II) sorption efficiency. Microorganisms play a key role in heavy metal sorption, especially in Fe-Mn oxide generation. Under the action of microorganisms, the easily released Mn(II) in the exchangeable and carbonate-bound states in the sediment can gradually form a relatively stable Fe-Mn-bound state, thus stabilizing mineralization in a long-term natural cycle. Therefore, microorganisms in sediments have a positive impact on inhibiting the migration of heavy metals in soils.