Influence Of Soil Humic Acids On The Formation And Transformation Of Iron Oxides

Iron oxides are important components with various species and existed extensively in soil of Southern China. Their structure and properties are feasible to change with environment conditions. Iron oxides tend to form associations with humic acids in soil which will affect their formation and transformation process. This study on the transformation of iron oxides in the presence of humic acids can not only clarify the mechanism of iron oxides formation in soil, but provide certain explanation for the formation of soil aggregations, the cycle of chemical element in soil.In this work, we applied X-ray diffraction (XRD), Microscope scanning methods (SEM), Fourier transform infrared adsorption spectrum (FT-IR), and Brunauer-Emmett-Teller (BET) analysis to characterize the crystal structure, phase components, and morphology of iron oxides in the presence of humic acids. The main results are shown as follows:1. At pH4, the formation of crystalline hematite was significantly promoted by humic acids. The XRD diffraction peak of hematite in the presence of HA was stronger than that without HA. Besides, the morphology of iron oxides was completely changed in the presence of HA. The particle was ellipsoidal with diameter of50-100nm when HA was added. However, the morphology of products was low crystalline and irregular chunks without HA. With the increase of pH, the promoted formation of crystalline iron oxides by HA decreased. At pH>6, this promotion by HA disappeared.2. The crystallization of products was enhanced with the increasing of temperature. The difference of products between with and without HA was the largest when temperature was heated to70℃. Under the condition of80℃, the crystallinity of iron oxides was the highest in all the systems. When temperature was lowered to40℃and50℃, iron oxides with high crystallinity could be obtained.3. The types of humic acids also affected the formation and transformation of iron oxide. Compared with HA, lower concentration of FA can speed up the transformation of soil iron oxides because FA possessed more carboxylic groups and phenolic groups than HA, and the total cidity of FA exhibited higher than that of soil HA.4. The concentration of humic acids had a significant effect on the crystallinity but limited influence on the morphologies of the iron oxides. The BET of the samples dropped firstly following with an increase when the concentration of HA was increased.5. The results of FT-IR spectra of samples showed that the vibrating peaks of phenolic hydroxyl and carboxyl functional moved with different degree after humic acid was added. Because the reaction between phenolic groups and carboxylic groups of humic acid and iron oxides occurred, leading to the formation of stable chelating structure.