Converting insolubles from biorefinery lignin stream for use as slow-release fertilizer

Abstract

Lignin is a biopolymer naturally found in the cell walls of plants and can be valorized in various
applications including coatings, resins, slow-release fertilizers, etc., due to its favorable
properties such as biodegradability and high carbon content.
This thesis explores the properties and the use of biorefinery lignin as a coating material for
slow-release fertilizer formulations. Various pre-treatment strategies were implemented,
including enzymatic polymerization, thermal drying, and acid precipitation to alter the
solubility, molecular weight, and hydrophobicity of the polymer.
The results showed that laccase-mediated polymerization significantly increased the molecular
weight of lignin from an average molecular weight of 1.2 kDa to 18 kDa at 2688 uL/g enzyme
dosage. Elevated drying temperatures reduced lignin solubility in water and alkali conditions,
while acid precipitation using 0.5 M H₂SO₄ further increased hydrophobicity by forming water-
insoluble spheres. The results, also, suggested that PEG8000 significantly improved the
mechanical strength and adhesion of lignin coatings during the formulation testing and the final
coating composite successfully delayed the dissolution of both NaCl and NH₄NO₃ granules in
aqueous environments by 1 and 1.5 hours, respectively.
This work demonstrated the feasibility of converting industrial lignin waste into a high-value
product for agricultural use. Future studies could potentially explore other formulations to
optimize the system parameters and extend the research towards long-term soil testing, pilot-
scale planning, and evaluating compatibility with other fertilizers to accomplish commercial
implementation and contribute to a more circular economy.

Popular Abstract

Modern agriculture heavily relies on fertilizers to supply crops with necessary nutrients and increase productivity. However, conventional fertilizers tend to release nutrients too quickly and contribute to losses through leaching into groundwater and/or evaporation into the atmosphere. As a result, these phenomena contribute to various environmental issues, such as eutrophication, soil acidification, and greenhouse gas emissions. In recent years, one sustainable alternative that emerged is lignin-based fertilizers that aim to release nutrients gradually in order to maximize nutrient efficiency and minimize the environmental footprint.

Lignin is an abundant polymer naturally found in plants and is a common byproduct of the pulp & paper and biorefinery industries. Currently, most of the industrial lignin is burned for energy, failing to take advantage of its potential as a high-value and renewable material for various applications. Lignin exhibits several advantages, making it a sustainable alternative to conventional fertilizer coatings, such as a high carbon content, biodegradability and a hydrophobic nature. This thesis focuses on characterizing biorefinery lignin and developing a slow-release fertilizer coating using lignin as the main component.

The research explored several techniques to characterize and modify lignin’s properties for coating applications. Initially, enzymatic polymerization with laccase L371 was performed to increase the molecular weight of lignin in order to process the material on the pilot-scale and generate appropriate lignin fractions for the intended application. Subsequently, thermal drying and acid treatment were tested to unravel lignin's behaviour in terms of solubility and hydrophobicity, allowing the production of solid and water-insoluble macrospheres.

Polyethylene glycol (PEG8000) was combined with lignin to improve the flexibility of the coating, and the final mixture was used to cover granules of sodium chloride (NaCl) and ammonium nitrate (NH₄NO₃), using a simple dip-coating technique. The rate of nutrient release was evaluated in water and the results suggested that the coated granules dissolved much more slowly than the uncoated granules. Overall, the lignin-PEG8000 coatings successfully delayed nutrient release, compared to the rapid dissolution of the uncoated granules in water.

While these lab-scale experiments unravelled the potential of using lignin-based coatings, further work is necessary to evaluate their performance under real soil conditions and assess lignin´s compatibility with other fertilizer types. Moreover, in further research, other formulations could be evaluated for pilot-scale production. In the future, lignin-based fertilizers could help enhance nutrient retention in the soil, reduce waste, and contribute to sustainable agricultural practices.

• author: Alefantou Christopoulou, Faidra Eleni
• supervisor: Roya Rezaei; Anu Suonpää
• organization: Biotechnology (MSc); Biotechnology (M.Sc.Eng.)
• course: KBTM01 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Agriculture and Food Sciences
• keywords: lignin, slow-release fertilizer, biorefinery, hydrophobicity, polymerization, biotechnology
• language: English
• id: 9210054
• date added to LUP: 2025-08-18 12:56:21
• date last changed: 2025-08-18 12:56:21