Synthesis And Agricultural Application Of Lignin-based Hydrogel

Shortage of water resources,desertification of soil and excessive use of fertilizers are the main contributing factors in degradation of cultivated land,and the application of polymer hydrogel represents an important approach to soil restoration.Polymer hydrogels can not only improve the efficiency of water use in agricultural soil and reduce water loss and restore fertilizer in the soil so that less fertilizer is needed,but also have the advantages of being degradable,environmentally friendly and non-toxin.For all these attributes,they are being widely used in soil improvement and regulation.In this paper,sodium lignosulfonate?L?,sodium alginate?SA?and konjac flour?KJ?were selected as the polymer materials for a new green hydrogel.Lignin is a type of aromatic organic material with high polymerization degree,which has the characteristics of abundant sources,regeneration,biodegradable,low price and abundant functional groups.Sodium alginate is a natural polysaccharide carbohydrate extracted from seaweed and is a natural hydrogel.However,pure sodium alginate hydrogel is not appropriate for direct application in agriculture due to its low water absorption,poor structure and easy decomposition.Therefore,in this paper,a new degradable lignin-based hydrogel?L/KJ/SA hydrogel?was synthesized by using lignin as the precursor of sodium alginate,using konjac with long chain structure and calcium chloride as the crosslinking agent.Optimum conditions of L/KJ/SA hydrogel were studied,the possible structure and cross-linking mechanism were discussed,the preliminary exploration of the application of L/KJ/SA hydrogel in agriculture was preliminaries explored,and the performance and application prospect of L/KJ/SA hydrogel were comprehensively evaluated.In view of the beneficial physical structure and water absorption characteristics of L/KJ/SA hydrogel,we further applied it to soil moisture and nutrient maintenance.The optimal ratio between lignin,sodium alginate,konjac flour and anhydrous calcium chloride should be1:2.75:5:0.1 to synthesize the optimum L/KJ/SA hydrogel which has the maximum water absorption 41.23 g·g^-1.Infrared spectroscopy?IR?revealed that carboxyl stretching vibration characteristic of lignin and sodium alginate and syringa group vibration were found in the structure of the L/KJ/SA hydrogel,indicating the success of the liquid phase cross-linking.SEM analysis showed that the three-dimensional network structure of L/KJ/SA hydrogel was constituted by a large number of irregularly lamellar structures and filled with a large number of pores.In view of the good effect of L/KJ/SA hydrogel on water and fertilizer conservation,we further verified its effect on the growth of tobacco plants under extreme drought conditions.Adding L/KJ/SA hydrogel to soil can improve the water holding capacity of the soil to different degrees and soil can contain more water with more use of the L/KJ/SA hydrogel.L/KJ/SA hydrogel could also reduce the saturated hydraulic conductivity of soil:when more L/KJ/SA hydrogel was used,barrier to vertical water infiltration was stronger,and soil retained more water.L/KJ/SA hydrogel had the greatest capacity of retaining soil moisture between 0.3MPa-1.50 MPa,and could increase the available water content of soil,which prolonged the supply of water demand for plant growth.L/KJ/SA hydrogel could improve the p H value of soil and reduce the salt content in the soil.L/KJ/SA hydrogel could reduce the leaching loss of25.81%-38.83%nitrate nitrogen,18.96%-32.35%ammonium nitrogen,14.6%-28.22%total phosphorus and 16.92-26.89%available potassium in soil.Soil burial experiments showed that L/KJ/SA hydrogel had good soil degradability,degrading only 20%in 120 days of soil burial,and the degradation cycle paralleled at least one of the growth cycle of crops.In view of the advantageous effect of L/KJ/SA hydrogel on water and fertilizer conservation,we further verified its effect on the growth of tobacco plants under extreme drought conditions.L/KJ/SA hydrogel could promote the growth of tobacco by increasing plant height,stem width and plant weight to a certain extent and improve tobacco quality under drought stress.L/KJ/SA hydrogel could improve the net photosynthetic rate of tobacco plants,effectively replenish missing water in tobacco,slow down the degree of stomatal closure,thereby increase intercellular CO₂concentration in tobacco leaves,inhibit the transpiration rate of tobacco leaves,and maintain strong growth and active biological activity of plants.L/KJ/SA hydrogel could effectively reduce the accumulation of proline and sugar in tobacco plants,reduce the effect of drought stress,and prolong the growth time of tobacco plants in extreme drought conditions for 9-14 days.In this study,a L/KJ/SA hydrogel with green,degradable,water and fertilizer retaining function was synthesized through a simple liquid phase crosslinking polymerization.The results showed that the synthesis and application of L/KJ/SA hydrogel could improve the effective utilization of lignin and sodium alginate.Its optimized ratio,possible structure and cross-linking mechanism laid a certain foundation for its application in dry soil improvement and should have broad utilization prospects.