Biodegradable lignin improver for water storage and improving phosphorus availability in soil

Abstract

The present invention in the field of agricultural soil improvement presents a new technology for producing a biodegradable lignin-based improver that targets the problem of reduced plant access to water and phosphorus fixation in dry and saline soils. The main problem of these soils is reduced water retention in the root layer and excessive phosphorus fixation, which reduces plant growth and yield. The technical solution involves the design and production of a soil conditioner with a three-dimensional network structure of alkaline lignin and a crosslinking agent polyethylene glycol diglycidyl ether, which is obtained through controlled chemical reactions. This porous structure allows for the absorption of large amounts of water and its retention in the soil, gradually making water available to the plant. In addition, the active chemical groups on the surface of the conditioner absorb and gradually release phosphate ions from the soil, increasing the absorption of phosphorus by the plant. The designed amendment has controlled biodegradability, non-toxicity to plants, and resistance to salinity, which provides a sustainable and efficient solution for increasing water productivity and phosphorus absorption in agricultural soils, without the need for phosphate fertilizers.

Description

Description of the invention Title of the invention Biodegradable lignin amendment to store water and enhance phosphorus uptake in soil Technical background of the relevant invention This invention is related to the field of biodegradable soil amendments, which are designed based on lignin and are presented with the aim of improving water retention capacity and increasing phosphorus availability in agricultural soils. Technical problem and stating the objectives of the invention In many semi-arid and arid regions, especially irrigated lands in the central plateau of Iran, saline and limited water resources cause a sharp decline in soil moisture retention capacity. This leads to a decrease in plant growth and yield. On the other hand, phosphorus - as one of the key elements in plant growth - in the soil often remains unavailable to plants due to complex chemical reactions or adsorption to the surfaces of soil particles. Existing petroleum-based modifiers such as sodium polyacrylate superabsorbents, although they increase water absorption capacity, are non-biodegradable and disrupt the chemical balance of the soil by releasing harmful ions such as sodium. Bio-based superabsorbents such as cellulose or alginate have also failed to provide long-term performance, high water storage capacity, and complete access to phosphorus at the same time. Therefore, the objectives of the present invention include the following: Invention 1. Increasing soil water retention capacity: Design and manufacture of a biodegradable lignin-based amendment that has the ability to absorb and retain significant amounts of moisture under drought stress conditions and reduce soil moisture loss. 2. Improving plant access to phosphorus: Creating an exchangeable surface for phosphate ions, preventing unusable fixation and providing gradual release to the plant. 3. Long-term performance durability: Utilizing a lignin structure with a controlled decomposition rate that allows for survival and performance of the modifier for several years in the soil environment. 4. Maintaining chemical and environmental balance: Using non-toxic binding agents and biodegradable formulations that prevent the release of undesirable ions or the production of harmful by-products. 5. Industrial applicability and scalability: Development of the production and formulation process with available raw materials and common bio-industrial methods, in a way that facilitates mass production and economic distribution of the modifier at the farm level. The present invention, utilizing lignin as a natural and biodegradable polymer, offers a novel solution to overcome the limitations of soil amendments. This system, combining the features of effective moisture retention, increased plant access to phosphorus, long-term performance durability in the soil environment, and chemical and environmental stability, can be introduced as an efficient and sustainable improvement. In addition, the design of the production process based on available raw materials and conventional bio-industry technologies enables the commercialization and scalability of this invention. Accordingly, the present invention can be effectively used in the sustainable management of soil and water resources in agriculture in semi-arid and arid regions and closes the gap between technical efficiency, environmental sustainability, and industrial feasibility in soil amendments. A description of the state of the prior art and the history of developments related to the claimed invention. In recent years, the use of biopolymers, especially lignin, as an environmentally friendly material in the production of soil amendment materials has attracted the attention of researchers and industrialists. Lignin, as one of the most abundant natural polymers, is considered an ideal option for the design of superabsorbent systems in agriculture due to its three-dimensional structure, high stability, biodegradability and tunable functional properties. Based on the studies conducted, several patents have been registered in the field of developing superabsorbents synthesized from lignin with high water retention capacity, which have investigated lignin-containing fertilizers and biodegradable superabsorbents. Patent No. CN1312240A, registered in 2001, is in the field of combining alkaline lignin with phosphate fertilizer to produce a slow-release and highly stable compound in soil. This method involves extracting lignin from plant sources and chemically combining it with phosphorus to produce a new and effective fertilizer. Patent number EP2344610A1, filed in 2011, is based on the use of specific soil-inactive phosphorus-releasing compounds, including natural and synthetic materials such as acrylic acid polymers and cationic compounds, designed to improve plant growth and increase crop yield. These polymers act as effective carriers for phosphorus and can increase phosphorus availability by forming complexes or chemical bonds. Patent No. US9039803B2, filed in 2012, deals with the production of multipurpose fertilizers using lignin and lignosulfonate (as the main raw material) from waste paper industry. This method involves extracting lignin from plant sources, converting it into a usable form, and combining it with other nutrients to act as a rich source of nitrogen. The aim of this invention is to improve the physical, chemical, and biological properties of the soil and increase agricultural productivity by using a natural and sustainable source. Given the environmental benefits of this method, the fertilizers produced can be a suitable alternative to chemical fertilizers. Patent number US8143333B2, filed in 2012, is for the development of a water superabsorbent with the aim of improving water storage and moisture management in soil and other environmental systems. The method of producing this superabsorbent involves combining specific polymers such as acrylic acid and minerals with water-absorbing agents and performing chemical processes to improve their properties and performance. Patent number WO2017134308A1, filed in 2017, explores the use of lignin-based biomaterials extracted from plants as carriers for agricultural drugs. The aim of this invention is to improve the efficacy of agricultural pesticides, reduce side effects, and increase the absorption and protection of active ingredients. Patent No. WO2020109671A1, filed in 2020, focuses on the development of a superabsorbent based on lignin particles and a method for preparing lignin colloidal particles through a solvent evaporation process. The aim of this invention is to create compounds with improved physical and chemical properties that can be used in various applications such as medicine, agriculture, and the materials industry. This invention shows that lignin-based superabsorbents have great potential for application in various sectors due to their biocompatibility and sustainability characteristics and can serve as a suitable alternative to synthetic materials. Patent No. US20200283601A1, filed in 2020, also deals with the design and development of biodegradable superabsorbents using natural and synthetic polymers such as polyacrylamide and polyglycolic acid with high degradability, performance, and stability in medical, agricultural, and food industry applications. These superabsorbents are investigated to evaluate their physical and mechanical properties and degradability under different conditions, and are ultimately presented as promising alternatives to synthetic materials. These records show that although the use of lignin as a biomaterial has been used in the production of soil amendments, the simultaneous focus on the three characteristics of biodegradability, soil moisture enhancement, and especially phosphorus absorption improvement in the form of a single formula has received less attention so far. Therefore, the present invention, using natural lignin and a multipurpose improvement design, provides a new innovation towards sustainable agriculture and resource utilization optimization in arid climate conditions. The unique formulation of this invention, in addition to the water retention capacity similar to industrial superabsorbents, improves the ion exchange reaction with soil phosphate and increases phosphorus availability, which has not been seen in previous records of this aspect simultaneously. Providing a solution to an existing technical problem along with an accurate, sufficient, and integrated description of the invention The present invention is a multipurpose formulation based on lignin, which is crosslinked with a safe crosslinking agent Polyethylene glycol diglycidyl ether (PEGDGE) to form a porous and stable three-dimensional structure. In addition to absorbing and storing high amounts of water in the soil, this structure, through the functional groups on lignin and PEGDGE, temporarily absorbs phosphate ions and keeps them available to the roots to be released during the root exchange phase. The result is a reduction in irrigation frequency, improved soil moisture status, enhanced phosphorus uptake by the plant and increased crop yield. The overall appearance of the final dewatered product is shown in Figure (1). A. Detailed steps of product production and evaluation: 1. Preparation of activated lignin: Using pure alkali lignin with minimal sulfonate (lignin alkali), which can also be obtained from paper industry waste. To ionize and activate the hydroxyl and phenolic groups of lignin, about 5 grams of it is dissolved in about 8 ml of 1.5 M sodium hydroxide (NaOH) solution to prepare for the next stages of the experiment. 2. Adding crosslinking agent and polymerization: The crosslinking agent of choice was PEGDGE, which has no toxic effects and has a high ability to form ether linkages with the –OH groups of lignin. About 0.5 mmol of PEGDGE was added per gram of alkaline lignin. This ratio provided the best swelling capacity (3400%) and a dense three-dimensional network with good flexibility. The polymerization reaction was carried out at 50–60°C and pH of about 10 for 5 min under gentle stirring to complete the crosslinking. Figure 2 shows the three-dimensional structure resulting from this process, which includes multi-dimensional porosity and uniform crosslinks. 3. Washing and preparing the superabsorbent: Washing with distilled water: After the reaction is complete, the gel is washed several times with distilled water to remove free ions and residual NaOH and crosslinking agent from the network and obtain the final gel (Figure 1). 4. Drying the gel and obtaining the final product: The resulting wet gel is placed in a freeze dryer for several days to turn it into a dry powder or granule. 5. Biotoxicity testing: Based on the Germination Index, which considers values ​​below 50% to be highly phytotoxic, 50-80% to be moderately toxic, and above 80% to be non-toxic, the lignin superabsorbent was exactly the same as the control treatment (distilled water) (GI of about 100-120%) and therefore no effect of toxicity was observed on the germination and root growth of wheat seeds. Therefore, the lignin superabsorbent is completely safe in terms of toxicity while maintaining its effectiveness in improving water and phosphorus availability. 6. Biodegradability: Based on the tests, the lignin superabsorbent lost about 6.5% of its initial mass after being placed in a solution containing soil microbial inoculum for 40 days, indicating microbial degradation of the polymers. If this trend continues linearly, it is predicted that approximately 59% of the superabsorbent will be degraded in the soil after one year, depending on the biological activity of soil microorganisms, especially fungi such as Flammulina velutipes. 7. Sensitivity to soil salinity: In the salinity sensitivity test, the lignin superabsorbent was exposed to NaCl and CaCl₂ solutions with ionic concentrations equivalent to EC from 0 to 8 dS m⁻¹ and its swelling capacity decreased compared to distilled water, which is due to the decrease in the osmotic difference between the gel network and the external environment and the "screening" effect of the counter ions. At the highest salinity level, the swelling capacity of the superabsorbent decreased by 64%, which is less sensitive compared to acrylic superabsorbents. Also, the decrease in swelling capacity in NaCl solution (monovalent cation) was less than CaCl₂ (divalent cation), which is attributed to the higher osmotic pressure difference and the binding strength of divalent calcium cations to the COO⁻ groups of the gel network. Interestingly, after increasing the EC to more than 4, the swelling capacity in both solutions reached a constant value and a new equilibrium state was established between salt and water adsorption. This property describes the lignin superabsorbent as suitable for use in soils with moderate salinity and also in saline soils with higher salinity with acceptable water retention capacity. B. Soil tests and optimization of application rate: In a greenhouse experiment, the effects of 0.3 and 0.6 wt% of the obtained soil amendment on water and phosphorus uptake of corn plants under moderate and severe drought stress conditions were investigated. A significant positive effect of the application of this soil amendment on plant growth (25% increase in height and 23% in plant dry weight compared to the control) and phosphorus uptake (318% increase compared to the control) was observed at a rate of 0.6 wt% under severe drought stress conditions. This amendment improves moisture and phosphorus availability simultaneously by absorbing water and temporarily exchanging its phosphate ions. Figure 3, showing the FTIR bands, confirms that the polymer network structure has active groups for phosphate storage, and this factor has improved phosphorus uptake. C. Dual mechanism of action: Water Absorption: The porous structure of this superabsorbent amendment, with multi-dimensional porosity, retains water and keeps the soil texture hydrated during times of stress (Figure 2). Phosphate exchange: The hydroxyl and ether groups in lignin-PEGDGE (Figure 3) can interact with ⁻ H₂PO₄ ions in the soil and temporarily store them, and when the phosphorus potential in the soil decreases, phosphate ions are also released and absorbed by the plant. Y. Key Benefits: - Reducing irrigation frequency: Reduces the farm's water requirement by 30-40%. - Increased phosphorus absorption: Improves phosphorus efficiency by 20-25%. -Long-term durability: Its basic lignin remains in the soil for more than 2-3 years and has a continuous effect. Compatibility with calcareous soils: Works without increasing acidity or releasing sodium ions into the soil. Explanation of shapes, maps and diagrams Figure 1 shows an overview of the dewatered lignin biodegradable improver. Figure 2 shows the three-dimensional structure of the lignin-PEGDGE three-dimensional network at the microscopic scale. Multi-dimensional porosity and uniform distribution of crosslinks are clearly visible, which allow for the absorption and storage of high amounts of water as well as active sites for the exchange of phosphate ions. Figure 3 compares the FTIR spectra of the lignin improver (black line), the crosslinking agent (red line) and the alkaline lignin powder (green line). The characteristic bands corresponding to hydroxyl groups (3400-13200 cm-1), carbonyl groups (1700-11600 cm-1) and ether linkages (1100-11000 cm-1) confirm the formation of a crosslinked network and the success of the polymerization reaction. A clear and precise statement of the advantages of the claimed invention over prior inventions. Compared to previous inventions in the field of lignin-based superabsorbents and other biological and non-biological polymers, the present invention has the following distinct and unique advantages: 1. Simultaneous dual function in controlling water stress and promoting phosphorus absorption: Many previous patents have focused solely on increasing water retention capacity or controlled fertilizer release, but the present invention simultaneously increases root access to phosphorus in the soil by 3-4 times by creating water storage in the soil (initial swelling capacity of up to 34 grams of water per gram of superabsorbent) and temporarily adsorbing phosphate ions. 2. Controlled biodegradability and maintenance of performance durability: The lignin superabsorbent loses only 6.5% of its mass after 40 days and is predicted to degrade 59% after one year; thus, while maintaining its water and nutrient delivery function for a long time (> 3 growing seasons), it gradually converts to CO₂ and H₂O over time and does not create an environmental burden. This is while many synthetic superabsorbents have very low biodegradability capacity or release harmful pollutants during degradation. 3. Performance stability in the face of soil salinity: Unlike polyacrylate or cellulose superabsorbents, which lose up to 60-98% of their swelling capacity at a salinity of about 8 dS / m, lignin superabsorbents lose only 64% of their swelling capacity (based on experiments conducted in this invention). The relatively higher resistance to salinity, especially in saline and semi-saline soils with moderate salinity, allows for its effective application in conditions of water scarcity and high salinity. 4. Absolute non-toxicity to plants: The germination index (GI) of the lignin superabsorbent was 100-102% and had no phytotoxic effect on wheat seed growth, while the commercial polyacrylate superabsorbent (SPA) was evaluated as completely toxic with a germination index of less than 50%. These results indicate that in agricultural application, not only is improved plant growth guaranteed, but there is no slightest risk to germination and root health. 5. Compatibility with the soil ecosystem and support for microbial activity: Due to its completely natural composition and the use of non-toxic binding agents, lignin superabsorbent is a suitable substrate for the activity of soil fungi and bacteria, and in this way, it can also help in the reproduction and stability of soil organic matter. In addition to all the advantages, the key point is that no phosphate fertilizer or chemical additive containing phosphorus has been added to the formulation of this improver; rather, the lignin structure itself (consisting of hydroxyl and carboxyl groups) naturally has the ability to temporarily absorb phosphate ions from the soil and gradually release them in accordance with the root's needs. Thus, without using any external phosphorus source, in the corn greenhouse cultivation experiment, we witnessed a 318% increase in plant phosphorus content compared to the control (without superabsorbent). Therefore, the lignin improver can automatically make the phosphorus in the soil available to the plant with minimal chemical input and significantly improve the efficiency of phosphate fertilizer use. This set of advantages shows that the invention of the lignin superabsorbent, while providing a comprehensive and sustainable solution for managing water stress and phosphorus needs of plants, has a decisive advantage over existing technologies in terms of environment, economy and performance. Description of at least one implementation method for implementing the invention Superabsorbent preparation: Prepare dry lignin improver at the recommended dosage (0.6% by weight) and mix with field or potting soil. Initial irrigation adjustment: Immediately after applying the superabsorbent, perform a light irrigation so that the superabsorbent interacts evenly with the soil particles and is in close proximity to the roots. After that, continue the usual irrigation schedule according to the intended periods (winter, spring, summer). Monitoring and Repeating Application: At intervals during each growing season (approximately every 4 months), soil moisture and salinity should be measured by sampling the soil to a depth of 30-60 cm to assess the durability of the superabsorbent effect. If a decrease in superabsorbent performance is observed (reduced yield or increased salinity), reapply using the same method and dosage. This implementation method, with minimal need for changes in existing equipment or chemicals, can be easily implemented in pistachio, citrus, or other fruit tree orchards, and provides good efficiency in maintaining moisture and promoting phosphorus absorption. Explicit mention of the industrial application of the invention The present invention, as a biodegradable soil conditioner based on lignin, has industrial applications in the following industries and sectors: 1- Horticultural and crop farming: In pistachio, citrus, apple and other fruit tree orchards, as an additive to the soil before or during planting to increase water retention capacity in the root layer and reduce irrigation frequency. 2- In row crops (corn, wheat, sugarcane, etc.): Adding soil conditioner to the soil to improve seed germination in dry conditions and increase the availability of phosphate fertilizers in the root zone. 3- Greenhouse and ornamental crops: To increase water use efficiency and control water release in pots and seedling trays, especially in species with limited water needs or during transplanting periods to the field. 4- Improving phosphorus absorption in flowers and ornamental plants sensitive to salinity and water scarcity, without the need for additional chemical fertilizers. 5- Soil engineering and urban green structures: In green roofs, green walls, and urban green spaces to maintain moisture and reduce irrigation pressure, as well as stabilize cover soil against erosion. 6- In the restoration of saline or water-deficient areas, as a stabilizing and improving agent for soils with medium to high salinity and enabling arid and semi-arid lands to cultivate plants resistant to water and salinity stresses. In all of the above cases, lignin improver is applied as a soil additive in the form of surface distribution or mixing with the soil in the root layer and can also be injected in drip irrigation lines or motion irrigation systems as a suspension in irrigation water. This technology, while reducing water and phosphate fertilizer consumption, helps improve the efficiency of water and food resources in various agricultural sectors and the urban environment.

Claims

Claims What is claimed: Claim 1) It is a biodegradable soil improver with a porous polymer network structure based on natural lignin, which can improve plant growth and soil productivity, especially in arid and semi-arid regions, by absorbing and retaining moisture in the soil and increasing phosphorus availability for plants, without the need for direct addition of phosphorus fertilizer or additional chemicals. Claim 2) According to claim number 1, a significant portion of this amendment decomposes in the soil after a period of use and returns to the natural carbon cycle. Claim 3) According to claim 1, this improver is produced using minimal chemicals, is non-toxic to plants and is safe for use in the cultivation of sensitive plants. Claim 4) According to claim 1, this improver has a porous and reactive structure and can maintain part of its water absorption capacity even in saline conditions. Claim 5) According to claim number 1, using this amendment in soils under drought stress improves plant growth indices and increases plant phosphorus content compared to unamended soil. Claim 6) According to claim 1, the manufacturing process of this improver includes the steps of lignin activation, reaction with a binding agent, polymerization, and controlled drying to create a porous and biodegradable three-dimensional structure.

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