Characterisation of soda lignins as raw materials soil improving materials

Thesis (MScFor)--Stellenbosch University, 2025.

Ngqebane, A. 2025. Characterisation of Soda Lignins as Raw Materials Soil Improving Materials. Unpublished masters thesis. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/68d1ec10-e216-441f-a825-fa656647cfe3

ENGLISH ABSTRACT: The rising need for food, fuelled by the fast-growing population, has led to the adoption of intensive farming methods to achieve high crop production. However, these methods—such as frequent ploughing and the heavy use of chemical fertilizers, pesticides, and herbicides—have seriously damaged soil health. Common issues include soil erosion, loss of organic content, nutrient depletion, compaction, salinization, acidification, and disturbances in microbial communities. A more sustainable approach involves using organic fertilizers, which can boost soil structure and fertility by increasing soil organic matter, nutrient levels, and microbial diversity. One key component of soil organic matter is lignin, a major structural polymer in plant cell walls. Lignin plays a crucial role in the formation of stable humic substances that enhance soil structure, store carbon, and retain nutrients. However, its high carbon-to-nitrogen (C/N) ratio (e.g., 201.8 for Lignorganic Soda [LO] and >320 for Sappi Soda [SS]) limits its direct use as a soil amendment. To address this, a process called ammoxidation—using oxygen and ammonia—can introduce nitrogen into lignin, creating a lignin-based soil fertilizing material. This process produces nitrogen-enriched lignin (N-lignin), which mineralizes gradually in the soil, providing long-lasting benefits. In this study, two types of soda lignin, L0 and SS, were characterized, treated with ammoxidation, and tested for their potential as soil amendments. The raw materials were analysed to understand their properties and how these influence nitrogen incorporation during ammoxidation. LO was found to contain higher phenolic hydroxyl groups (1.443 mmol/g) compared to SS (1.088 mmol/g), making it more reactive. Structural analysis using FTIR showed that while LO and SS had similar structural features, their differences in syringyl and guaiacyl ratios, as well as aliphatic side chain content, affected their behaviour during ammoxidation. Ammoxidation was performed in a 5-liter stirred autoclave at 80°C under 8-bar oxygen pressure, using 7 wt.% ammonia for four hours. The resulting N-lignins had nitrogen contents of 1.74% (N-LO) and 0.75% (N-SS), with corresponding C/N ratios of 21.93 and 47.21. Only N-LO met the acceptable C/N range (20–30) for use as a soil amendment. Both N-LO (pH 7.36) and N-SS (pH 7.77) slightly exceeded the ideal pH range (5–7). However, the N-lignins enhanced the ability of sandy soil to retain water, indicating their potential for agricultural applications. A four-week plant trial using white mustard (Sinapis alba) seedlings evaluated the performance of N-lignins. The trial demonstrated that N-SS has a potential to support plant growth effectively, achieving an average plant height of 59.7 mm and a mean stem diameter of 1.421 mm. N-SS also resulted in a fresh mass yield of 0.19 t/ha. While N-LO-treated plants underperformed, this could be linked to its raw material properties, which require further optimization. These results highlight the potential of N-SS in promoting plant growth and its viability provided that nitrogen incorporation is improved. N-LO in its current form proved to inhibit plant growth due to the presence of high contents of sugar components, which have been shown, particularly in lignosulfonates, that they produce growth inhibiting substances.

AFRIKAANSE OPSOMMING: Die toenemende behoefte aan voedsel, aangevuur deur die vinnig groeiende bevolking, het gelei tot die aanvaarding van intensiewe boerderymetodes om hoë gewasproduksie te behaal. Hierdie metodes— soos gereelde ploeg en die swaar gebruik van chemiese kunsmis, plaagdoders en onkruiddoders— het egter grondgesondheid ernstig beskadig. Algemene kwessies sluit in gronderosie, verlies van organiese inhoud, voedingstofuitputting, verdigting, versouting, versuring en versteurings in mikrobiese gemeenskappe. 'n Meer volhoubare benadering behels die gebruik van organiese kunsmis, wat grondstruktuur en vrugbaarheid kan bevorder deur grondorganiese materiaal, voedingstofvlakke en mikrobiese diversiteit te verhoog. Een sleutelkomponent van grondorganiese materiaal is lignien, 'n belangrike strukturele polimeer in plantselwande. Lignien speel 'n deurslaggewende rol in die vorming van stabiele humiese stowwe wat grondstruktuur verbeter, koolstof stoor en voedingstowwe behou. Sy hoë koolstof-tot-stikstof (C/N) verhouding (bv, 201.8 vir Lignorganic Soda [LO] en >320 vir Sappi Soda [SS]) beperk egter die direkte gebruik daarvan as 'n grondwysiging. Om dit aan te spreek, voer 'n proses genaamd ammoxidation— using suurstof en ammoniak—can stikstof in lignien in, wat 'n lignien-gebaseerde grondbemestingsmateriaal skep. Hierdie proses produseer stikstof-verrykte lignien (N-lignien), wat geleidelik in die grond mineraliseer, wat langdurige voordele bied. In hierdie studie is twee tipes koeldranklignien, L0 en SS, gekenmerk, met ammoksidasie behandel en vir hul potensiaal as grondwysigings getoets. Die grondstowwe is ontleed om hul eienskappe te verstaan en hoe dit stikstofinkorporasie tydens ammoksidasie beïnvloed. Daar is gevind dat LO hoër fenoliese hidroksielgroepe (1,443 mmol/g) bevat in vergelyking met SS (1,088 mmol/g), wat dit meer reaktief maak. Strukturele analise met behulp van FTIR het getoon dat terwyl LO en SS soortgelyke strukturele kenmerke gehad het, hul verskille in siringiel- en guaisielverhoudings, sowel as alifatiese sykettinginhoud, hul gedrag tydens ammoksidasie beïnvloed het. Ammoksidasie is uitgevoer in 'n 5-liter-geroerde outoklaaf by 80°C onder 8-staaf suurstofdruk, met 7 gew.% ammoniak vir vier uur. Die resulterende N-ligniene het stikstofinhoud van 1,74% (N-LO) en 0,75% (N-SS) gehad, met ooreenstemmende C/N-verhoudings van 21,93 en 47,21. Slegs N-LO het die aanvaarbare C/N-reeks (20–30) ontmoet vir gebruik as 'n grondwysiging. Beide N-LO (pH 7.36) en N- SS (pH 7.77) het die ideale pH-reeks (5–7) effens oorskry. Die N-ligniene het egter die vermoë van sanderige grond verbeter om water te behou, wat hul potensiaal vir landboutoepassings aandui. ’n Plantproef van vier weke wat wit mosterd (Sinapis alba) saailinge gebruik het, het die prestasie van N-ligniene geëvalueer. Die proef het getoon dat N-SS 'n potensiaal het om plantgroei effektief te ondersteun, wat 'n gemiddelde planthoogte van 59,7 mm en 'n gemiddelde stamdeursnee van 1,421 mm bereik. N-SS het ook gelei tot 'n vars massa-opbrengs van 0,19 t/ha. Terwyl N-LO-behandelde plante onderpresteer het, kan dit gekoppel word aan sy grondstof eienskappe, wat verdere optimalisering vereis. Hierdie resultate beklemtoon die potensiaal van N-SS in die bevordering van plantgroei en die lewensvatbaarheid daarvan, mits stikstofinkorporasie verbeter word. N-LO het in sy huidige vorm bewys dat dit plantgroei inhibeer as gevolg van die teenwoordigheid van hoë inhoud van suikerkomponente, wat getoon is, veral in lignosulfonate, dat hulle groei-inhiberende stowwe produseer.

Masters