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| Mechanism behind the influence of metal(hydr)oxides on humification of lignin |
| Received:June 28, 2019 |
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| KeyWord:goethite;bayerite;δ-MnO2;lignin;humification |
| Author Name | Affiliation | E-mail | | WANG Nan | College of Agriculture, Jilin Agricultural Science and Technology University, Jilin 132101, China | | | WANG Shuai | College of Agriculture, Jilin Agricultural Science and Technology University, Jilin 132101, China | | | XU Jun-ping | College of Agriculture, Jilin Agricultural Science and Technology University, Jilin 132101, China | | | WANG Yu | College of Agriculture, Jilin Agricultural Science and Technology University, Jilin 132101, China | | | DOU Sen | College of Resources and Environmental Sciences, Jilin Agricultural University, Changchun 130118, China | dousen1959@126.com |
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| Abstract: |
| Metal(hydr) oxides have a catalytic role in lignin humification, and the related abiotic mechanisms remain to be revealed. The method of liquid shake flask culture was adopted; during the process, goethite, bayerite and δ-MnO2 were added to the fluid medium containing lignin, and then a microbial suspension was inoculated for the culture period of 110 d. Precipitates were collected dynamically, and the organic components attached on the metal(hydr) oxides were removed using H2O2 solution and their structural characteristics were analyzed by scanning electron microscopy(SEM), Fourier transform infrared spectroscopy(FTIR), and X-ray diffraction analysis(X-ray diffraction, XRD). The results showed that the proportion of small particles was increased, and their edges were sharpened after the participation of goethite in lignin humification. Hydrated hydroxyl groups on the surface of goethite could form H-bonds with the inoculated strains and their transformed lignin, enhancing the degree of association between surface hydrated hydroxyl groups(-OH2+) and O-H of crystal H2O. Protonation of Fe-OH enhanced the vibration frequency of Fe-O. Ligand exchange reactions and H-bonds were the main mechanisms by which goethite catalyzed the humification of lignin; After participating in lignin humification, crystal particles of bayerite had a clearer profile, and particles of smaller size had a tendency to aggregate. Al-OH on the surface of bayerite could combine with the inoculated strains and their transformed lignin through H-bonds to enhance the vibration frequency of hydrated hydroxyl groups; simultaneously, a part of OH groups of bayerite could also be replaced by the carboxyl groups from microbial strains and their transformed lignin, which could weaken Al-O bonds in AlO4 tetrahedron and AlO6 octahedron. The vibration frequency of Al-OH2+ was enhanced by the water bridges and the formation of a chelate complex. Hydrophobic interaction was the main mechanism of δ-MnO2 involvement in lignin transformation. After the transformation, the particle binding strength of δ-MnO2 tended to weaken, edges of micro-nanospheres were sharpened, and surface crystal H2O tended to get removed. Protonation of Mn-OH increased the vibration frequency of O-Mn-O. All the above three metal(hydr) oxides catalyze the humification of lignin by the action of surface hydroxyl groups and metal oxygen-containing groups, without themselves undergoing any phase change. |
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