Modeling Of Molecular Structures Of Humic Acids And Biochars, And Sorption Mechanisms Of DBSA On Humic Acids And Biochars

This paper used an integrated experimental methods to analyze the structural characteristics of alluvial soil humic acid (HA) under the same amount of nitrogen and phosphate fertilizers and three kinds of biochars under different pyrolysis temperatures, build the2D structural models of soil HA and corn straw biochars and optimize the3D structures of HA and biochars with the quantum chemistry method to gain the most optimal and stable3D conformations. Merging the3D structures of HA and biochars with Dodecylbenzene sulfonic acid (DBSA) to simulate the sorption process and analyze the sorption mechanisms. It is a method to study the interaction mechanisms of DBSA with HA and biochars from molecular level and provide the theory basis for experimental researches. At the same time, soils, HAs and biochars were used to sorb DBSA to understand the sorption characteristics and sorption abilities. The sorption mechanisms of DBSA on HAs and biochars were studied to provide the theory basis for reducing the harmful effects of DBSA to the environment. At last, the pendant drop method was used to test the interfacial tension of HA solutions on the interface of air-water and study the sorption behaviors of HA solutions on the interface of air-water. It can give the theory guide for the application of HA as a surfactant.(1) Elemental analysis, micro infrared spectroscopy, and C-13NMR spectroscopy were used to study the structural characteristics of HAs which were extracted from the soils with the application of cake fertilizer, green fertilizer and straw fertilizer. After a long term fertilization of23years, the structures of HAs had some obvious changes. The HA with the application of cake fertilizer had the highest contents of H and N, the ratios of H/C and (N+O)/C were also the biggest. Cake fertilizer made the HA had the most of amino compounds, carboxylic acids, alkyl carbon and alkoxy carbon. Green fertilizer made the HA had the biggest O and O/C, meanwhile, the hydroxyl and carbonyl carbon content were the highest.(2) Different types of experimental methods were used to study the physical, chemical properties and structural characteristics of corn straw, sludge, and poplar leaf biochars at300℃,500℃,700℃. There are some significant differences in the scanning electron microscope of three kinds of biochars, but all of them will form the particle mixtures of varying sizes. At the same time, pyrolysis with high temperatures destroyed the pore structures of biochars. The ash contents of these biochars were high, which were as high as67.48-83.98%in the sludge biochar. With the increase of the pyrolysis temperatures, the yields of biochars decreased. The biochars were neutral to alkaline. The biggest surface area was251.11m2g for700℃straw biochar. The C content in corn straw and poplar leaf biochars will increase with the increase of the pyrolysis temperatures, but it deduced in the sludge biochars. There were decline trends for the contents of H, N, O and H/C, O/C,(N+O)/C in three kinds of biochars. The results of micro IR spectroscopy showed that there were some significant differences existed in three kinds of biochars and the same type of biochar under different pyrolysis temperatures. The contents of OH、C=O and CH2groups in sludge biochars were smaller than other biochars. Higher pyrolysis temperature made the aromatic carbon increased, C=O, OH and aliphatic hydrocarbon contents increased in biochars. The solid state C-13NMR spectra of300℃biochars were notably different from that of500℃and700℃biochars. High pyrolysis temperatures made the alphatic carbon and alkoxy carbon of biochars decreased and the aromatic carbon increased.(3) Elemental analysis, Curie point pyrolysis gas chromatography-mass spectrometry and C-13NMR spectroscopy were used to analyze the structures of soil HA,300℃and500℃corn straw biochars to build the2D structural models. Experimental and simulated elemental compositions, intensity distribution of carbon groups, and C-13NMR spectra were compared to confirm the validity of the2D structural models. The application of Hyperchem software can transform the2D structural models to3D structural models and optimize the3D structural models by molecular mechanics and molecular dynamics methods to get the models with the lowest energies and most stable confirmations. The conceptual structural models of soil HA,300℃and500℃corn straw biochars were C85H97N9O43, C78H68N2O25, C59H29NO10, respectively. For soil HA and300℃corn straw biochar, the models with-4charges were the most stable which had the lowest energies. The optimized model which did not have negative charge of500℃corn straw biochar was the most stable. Deprotonation process is an endothermic process.(4) Quantum chemistry was used to model the sorption of DBSA on HA and biochars. During the sorption process, the formation of H bond and hydrophobic interaction were the main sorption mechanisms for the sorption of DBSA on HA and300℃corn straw biochar. For700℃corn straw biochar, it is hydrophobic interaction which made the sorption happened. HA had the highest bending capacity with DBSA.(5) The sorption behaviors of HA solution on the interface of air-water showed that the interfacial tension decreased with the increase of the HA concentrations. HA is a natural surfactant. The relation curve of interfacial densities and concentrations of HA solution fit well with Langmuir equation and HA could form expanded film on the interface of air-water. The mean molecular areas of HA were very big.(6) Mollisol soil, alluvial soil, mollisol soil HA, alluvial soil HA,300℃,700℃corn straw biochars, and300℃,700℃poplar leaf biochars were chosen as the sorbent to sorb DBSA and study the sorption characteristics. Then sorption mechanisms were discussed by using BR. spectroscopy, C-13NMR spectroscopy and electron spin resonance spectra (ESR). The sorption rates were relatively fast for700℃com straw biochars and poplar leaf biochars which can get equilibrium in36hours. DBSA can reach equilibrium in48hours for soils and soil HAs. The equilibrium sorption mass of HAs and biochars were significantly greater than soils’ at low concentration. The biggest equilibrium sorption mass is37.52mg/g for300℃poplar leaf biochar. The sorption isotherms fit the Freundlich model very well for soils and HAs, and300℃corn straw biochar’s sorption isotherm fit the linear model. For other biochars, the sorption isotherms fit Langmuir model. The sorption capacity of DBSA on HAs was bigger than biochars’. The sorption mechanisms for HAs and DBSA were the formation of H bond and covalent bond, charge transfer, hydrophobic interaction, and Van der Waal’s force. For biochars and DBSA, the sorption mechanisms were anion exchange, formation of H bond and covalent bond, charge transfer, hydrophobic interaction, and Van der Waal’s force.