Preparation And Characteristics Of Different Lignite Humic Acids

Lignite reserves are abundant in China,but its effective utilizations are low.Humic acids（HAs）,the important component of lignites,have been widely used in many fields,i.e.agricultural,medical and industrial fields.Therefore,the extraction of HAs from lignites is one of the effective ways to improve lignite ultilizations.However,both the extraction process conditions and properties of HAs extracted from different lignites are different,and how to improve the HAs yield and researchs on its properties have always been the research hotspot.Therefore,the Yiliang lignite,Hulunbeier lignite,Wuhai leonardite and Lishi leonardite were selected as raw materials in this paper,response surface methodology（RSM）was used to optimize the extraction process conditions of HAs to improve HAs yield,and ligintes with low HAs content were oxidized by hydrogen peroxide（H₂O₂）solution to improve HAs content.The properties of HAs extracted from oxidized and raw lignites were sudied using modern instrumental analyses,which could be provide a reference for the HAs applications.The main research contents and results are as follows:（1）The effects of interaction between factors on the HAs yield were investigated by RSM,and the optimal extraction conditions of HAs were obtained,the results are as follows:（1）Yiliang lignite HAs（YLHA）:Extraction time was 4.1 h,concentration of sodium hydroxide（NaOH）solution was 0.39 mol/L,liquid-solid ratio（ratio of NaOH solution volume to lignite mass）was 32 m L/g,and YLHA yield was 40.87%.（2）Hulunbeier lignite HAs（HBHA）:Extraction time was 5.6 h,concentration of NaOH solution was 0.31 mol/L,liquid-solid ratio was 20 m L/g,and HBHA yield was34.19%.（3）Lishi leonardite HAs（LSHA）:Extraction time was 4.4 h,concentration of NaOH solution was 0.30 mol/L,liquid-solid ratio was 21 m L/g,and LSHA yield was52.77%.（4）Wuhai leonardite HAs（WHHA）:Extraction time was 3.1 h,concentration of NaOH solution was 0.21 mol/L,liquid-solid ratio was 12 m L/g,and WHHA yield was 69.52%.（2）The four HAs obtined under the optimal extraction conditions were characterized.The results indicated that:（1）Four HAs are terrestrial HAs and contain low authochthonous components,and the types of functional groups in four HAs are similar.（2）The aromaticity,resistance to thermal decomposition,mean thickness of stacking microcrystalline structures and average number of C planes in the stacking microcrystalline structures of four HAs are WHHA>LSHA>HBHA>YLHA.（3）The hydrophilic-hydrophobic index(f_h/h),relative contents of total acid groups and carboxyl groups are YLHA>LSHA>HBHA>WHHA.（4）The relative content of phenolic hydroxyl is YLHA>HBHA>LSHA>WHHA,the molecular weight is WHHA>HBHA>LSHA>YLHA,the spacing between C planes of four HAs is close,and their values are 3～4（?）.（3）The lignites with low HAs content（YL and HB）were oxidized using H₂O₂solution,and the effects of different oxidation conditions on HAs content were investigated.The results are as follows:（1）The oxidation conditions of YL:Concentration of H₂O₂ solution was 6%,oxidization time was 3 h,liquid-solid ratio（ratio of H₂O₂ solution volume to lignite mass）was 3 m L/g,and the HAs content was45.43%.（2）The oxidation conditions of HB:Concentration of H₂O₂ solution was 30%,oxidization time was 6 h,liquid-solid ratio was 3 m L/g,and the HAs content was40.88%.Results indicate that the young lignites are oxidized more easily.（4）The HAs extracted from oxidized lignites were characterized and compared with raw lignites HAs,and the results are as follows:HAs extracted from oxidized lignites contained more total acidic groups,carboxyl groups and aliphatic carbon,and their f_h/hvalues were higher and thermal oxidation properties were better than those of HAs extracted from raw lignites,the compositions of polycyclic aromatics and fluorophore types in oxidized lignite HAs were similar to those in raw lignite HAs.However,the resistance to thermal decomposition of oxidized lignite HAs was worse than that in raw lignite HAs.Above results indicate that H₂O₂ oxidation operation increases the content of oxygen-containing functional groups and biological activity of HAs.