Study On The Process And Mechanism Of Industrial Lignin Promoting Humification Of Pulp Mill Sludge Composting

In recent years,with the rapid development of paper industry in China,the discharge of solid waste such as paper mill sludge also increases year by year,which brings greater environmental pressure to enterprises and the society.Composting is considered as an environment-friendly solid waste treatment technology,which has the advantages of simple operation and reutilization of products.However,the composting of industrial sludge has many problems,such as low utilization rate of organic matter and poor quality of composting products.The use of additives in compost can effectively increase content of humus substance（HS）so as to improve the quality of compost products.In this paper,we studied the formation and change of HS during the composting of paper mill sludge.Two industrial lignin,alkali lignin（AL）and sodium lignosulfonate（SLS）,were added respectively to comprehensively investigate their influence on the humification process,and the mechanism of the two additives effecting the formation of HS was explored.Firstly,the changes of temperature,p H,organic matter（OM）,HS and other physicochemical parameters with the composting time were analyzed.The results showed that AL and SLS increased the temperature and p H in the composting process,accelerating the degradation of organic nitrogen.At the end of composting,HS content in AL(44.43g·kg^-1)and SLS(45.87g·kg^-1)treatments were significantly higher than that in control(34.36g·kg^-1).The degree of polymerization（DP）of the compost products was also significantly different,which increased from 0.67 in control to 0.70（AL）and 1.14（SLS）,indicating that the addition of SLS was more conducive to improving the humification degree of the paper mill sludge compost.The microbial community analysis showed that AL and SLS additives significantly changed the microbial community structure during the composting process.They significantly increased the relative abundance of thermophilic microorganisms Bacillus and Symbiobacterium as well as the maturity-related strains Truepera,Moheibacter,Pusillimonas and Halomonas.Redundancy analysis（RDA）showed that temperature and p H were the main factors affecting the microbial community in AL and SLS treatments.The addition of AL and SLS adjusted the physicochemical parameters such as temperature and p H,affecting the microbial community structure,and thus accelerated the humification process.The mechanism of the two additives promoting the HS formation was investigated by Ultraviolet-visible（UV-Vis）,three-dimensional excitation and emission matrix（3D-EEM）,Fourier transform infrared（FTIR）and Pyrolysis gas chromatography/mass spectrometry（Py-GC-MS）.Ultraviolet spectrum parameters,such as A_260-280 and E₂₅₃/E₂₀₃,indicated that the HS of the AL and SLS treatments had more stable structure and higher oxidation degree.3D-EEM showed that the AL and SLS accelerated degradation of proteins and formation of humic acids during the thermophilic phase.The parallel factor（PARAFAC）analysis further indicated that the addition of AL and SLS,especially during the thermophilic phase,promoted the oxidation of low-molecular lignin structural units to quinone-like substances.FTIR results showed that AL and SLS promoted the accumulation of aromatic compounds and carboxylic acids,and 2D-FTIR-COS further found that quinone-like substances were the most active components in the AL and SLS treatments.Py-GC-MS results showed that AL increased the contents of polysaccharides,methoxy phenols and aromatic hydrocarbons,while SLS promoted the oxidation of polysaccharides and methoxy phenols and the formation of refractory aliphatics.RDA further pointed that the oxidative degradation of polysaccharides and methoxyphenol as well as the accumulation of unsaturation were the main factors that improved the degree of humification.In conclusion,the addition of Al and SLS on the one hand provided the precursors of polyphenols and promoted the rapid evolution of quinones-like substances;On the other hand,these two additives also increased the relative abundance of microorganisms associated with composting maturity,thus promoting the oxidative degradation of lignocellulose,and ultimately promoting the HS formation.This work provided a new direction and theoretical basis for the comprehensive utilization of waste in papermaking industry.