| Fulvic acid(FA)is an important component of humic acid(HA).It is widely used in the production of sustainable agriculture due to its small molecular weight,water solubility,strong chelating ability and high content of active functional groups.The traditional fulvic acids are mainly extracted from non-renewable weathering coal,lignite and other mineral resources.However,fulvic acid has a poor synergistic effect on fertilizer nutrients and stimulates crop growth due to the complicated extraction process and active groups are easily destroyed.Paper-making wastewater is regarded as one of the most serious environmental pollution.How to realize the reuse of substances in papermaking wastewater and reduce environmental pollution is an urgent problem to be solved in the paper industry.Previous work has verified that the pulp black liquor contains fulvic acid substance,and it is not clear whether this substance has similar chemical structure and biological activity with that of natural fulvic acid.In order to solve the above problems,FA1 and FA2 were extracted and purified following the standard procedure recommended by International Humic Substances Society(IHSS)from leonardite and the pulp black liquid,respectively.Various characteristic methods and seed germination test were employed to compare the differences in chemical structure and physiological activity between FA1 and FA2.Meanwhile,FA2 was chelated with Ca2+to prepare calcium fulvic acid(FA2-Ca)and explored the effect on plant growth.The research not only solved the problem of wastewater treatment in papermaking enterprises,but also reduced its environmental pollution.It also provides some theoretical basis and technical support for the study of fulvic acid in papermaking black liquor in the future.The main results are as follows:(1)FA1 and FA2 were characterized by element analysis,Fourier transform infrared(FT-IR)spectroscopy,13C-nuclear magnetic resonance(13C-NMR)spectroscopy and Hnuclear magnetic resonance(1H-NMR)spectroscopy.Both FA1 and FA2 contained the same element and specific functional groups,but the elemental ratio and functional group content are slightly different.FA2 contained more C and less O than FA1.The N and S content of FA2 was about three times higher than that of FA1.The H content of FA2 almost doubled than that of FA1.The high content of sulfur and nitrogen indicated that FA2 contained a large number of biological macromolecules(peptides and polysaccharides).The results of FT-IR and NMR showed that FA2 had more hydroxyl groups,carboxyl groups and small molecular active groups than FA 1.The results of chemical oxygen demand(COD)and ammonia nitrogen(NH3-N)show that the COD and NH3-N content of FA2 is greatly reduced compared with the pulp black liquor,and it meets the agricultural water standard and can be directly applied.(2)The rice seed germination test indicated that FA1 and FA2 had a significant effect on promoting rice seed germination within an appropriate concentration range.The optimum concentration for FA1 and FA2 was 5 mg/L and 60 mg/L,respectively.Compared with the control,the rate of germination treated with FA1(5 mg/L)and FA2(60 mg/L)were increased by 5.6%and 9.5%,respectively.The results of amylase activity assay showed that the main mechanism of FA1 and FA2 promoting seed germination was to increase the activity of amylase,accelerate the decomposition of endosperm and granules,and provide energy for germination.The results of root tip section showed that FA1 and FA2 not only stimulated the growth of single cell in root system,but also promoted cell division and cell number,thus promoting the growth of the whole root system of plant.(3)The effects of the chelation of FA2 and Ca2+were investigated under different reaction conditions.The results showed that when the reaction temperature was 40℃,the reaction time was 120 min,the pH was 5,and the concentration of Ca2+(Ca(NO3)2)was 0.1 mol/L,the chelation rate rate of Ca2+and FA2 was achieved by 87%.The chelate was characterized by a series of characterization methods;the results showed that the chelating reaction of FA2 with Ca2+was mainly on phenol hydroxyl group,alcohol hydroxyl group and carboxyl group.(4)The results of hydroponic test showed that FA2-Ca significantly promoted the growth and development of plants.Compared with the control,the application of calcium fulvic acid chelate increased rice height by 11%,fresh weight of root and aboveground by 64%and 39%,respectively and dry weight of root and aboveground by 50%and 22%,respectively.The calcium accumulation of rice when treated with calcium fulvic acid chelate was 47%higher than that of the control. |
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