Preparation And Properties Of Lignin/Biochar/PLA Composite Coated Sustained-release Fertilizer

Over the past few decades,the use of chemical fertilizers has increased dramatically to meet the growing demand for food from the world’s growing population.At the same time,concerns about the environment,health and sustainable use of resources have been increasing.Therefore,minimizing fertilizer losses,increasing fertilizer utilization and improving soil pollution while ensuring food production are important tasks in the current innovation of agricultural fertilizer science and technology.Slow-release fertilizer(SRF)is receiving more and more attention because of its advantages of high fertilizer utilization efficiency,sustainable nutrient supply and low application cost.Coating is a common method to synthesize SRF.Among them,synthetic polymer coated SRFs are constrained by complicated preparation process,non-renewable substrate material,high cost and energy consumption.To address these shortcomings,a novel modified lignin/biochar-coated slow-release fertilizer was developed in this paper,and its performance as a green and efficient slow-release fertilizer was investigated.Firstly,the hydrophobic modified lignin was prepared by a series of esterification reactions using sulfonated lignin as raw material,and the reaction law was elucidated to prepare hydrophobic degradable modified lignin.Then,poplar biochar with layered porous structure was prepared by high temperature pyrolysis after lignin removal by alkali cooking method.Finally,the above two materials are compounded in proportion to polylactic acid as the binder to prepare the coating material and then coated with urea to obtain the slow release fertilizer.The prepared film-coated slow-release fertilizer has the characteristics of good film-coating effect,degradability and excellent slow-release effect.The main research contents and results are as follows.(1)The inexpensive industrial sodium lignosulfonate was used as raw material,which was treat by after acid precipitation,filtration,washing and drying and purification.The purified lignin(SL)was esterified with four acyl chlorides,acetyl chloride(AC-C2),octanoyl chloride(OC-C8),dodecanoyl chloride(LC-C12)and hexadecanoyl chloride(PC-C16),respectively,with the pyridine as catalyst.Then four kind of hydrophobic lignin were prepared.The chemical reaction characteristics and physical properties of the four modified lignans were analyzed by comparing their contact angles,FTIR and 1H NMR,and it was found that among the four modified lignans,the hydrophobicity increased with the increase of the chain length of the acyl chloride,and the hydrophobic property of the hexadecanoyl chloride was the strongest.(2)Delignified poplar wood chips(DW)were obtained by boiling with NaOH/Na2SO3 solution and bleaching with H2O2 solution.The prepared DW was carbonized at high temperature under inert gas to obtain carbonized DW(CDW),and activated CDW(ACDW)was prepared by KOH activation.The prepared ACDW has a unique layered porous structure with a significant increase in mesopore and micropore pore volume and a high specific surface area of 970.52 m2·g-1.In addition,the KOH activation treatment increased the oxygen-containing functional groups on the surface of biochar.The Cr(Ⅵ)adsorption experiments showed that the maximum adsorption capacity of ACDW for Cr(Ⅵ)could reach 288.9 mg·g-1,which was 16 times higher than that of poplar biochar without any treatment.It showed the great potential of this biochar as a water or soil amendment,and provided a theoretical basis for the subsequent preparation of encapsulated slow-release fertilizer.(3)Five different hydrophobic envelope materials(PPAC)were prepared by mixing 6%PCSL and 30%PLA 1:1 and adding different ratios of ACDW.The results showed that when the biochar content was increased to 3%,the contact angle of the membrane decreased from 109.92° to 90.72°,and the membrane surface was slightly rough.SEM showed a uniform porous structure on the surface,indicating a good distribution of biochar.When the biochar content was higher,its good hydrophilicity and excessive pore volume would lead to an increase in the swelling performance of the membrane material,which was not conducive to the slow release of fertilizer.(4)A PCSL/ACDW/PLA coating solution was used to coat the urea using the dip coating method,and the mass ratio of the three components in the coating solution was 1:1:5 for PCSL:ACDW:PLA.the coating material had good coating ability when coating urea,and the coating layer was thin and uniform with smooth surface.the EDS results showed that there was a small amount of nitrogen,which proved that the coating was good sealing property.The Cr(Ⅵ)adsorption experiments on the prepared wrapped fertilizer showed that the adsorption amount reached 40.39 mg·g-1 at pH 5,showing its good adsorption performance.The release time reached 25 h in still water and 35 days under the conditions of soil column drenching,indicating its excellent slow-release performance.Zero-level kinetics,first-level kinetics,Higuchi kinetics,and Korsmeyerpeppas kinetics were also fitted to the slow-release process of film-coated urea.The results showed that the slow release process of film-coated urea in water and soil was dominated by both diffusion and solubilization.