Lignin-based hybrid materials for agrochemical applications and Cr(VI) water remediation

This study focuses on the sustainable transformation of lignin—an abundant byproduct of the paper-pulp and bioethanol industries—into valuable materials with applications in water remediation and agrochemicals. With approximately 100 million tons of technical lignins produced globally each year, there is an urgent need for innovative and eco-friendly uses for this underutilized resource. This research project explores the potential of lignin to address environmental challenges, particularly in the reduction of toxic hexavalent chromium (Cr(VI)), a threatening environmental contaminant. We assessed the effectiveness of a Kraft softwood lignin (HMW) and a hardwood lignin (EH) in reducing Cr(VI) to the less toxic Cr(III) and lowering chromium concentrations by adsorption mechanisms in contaminated water. The study also investigates the role of lignin functionalization on Cr(VI) remediation, testing acetylated (Ac_Lig) and phosphorylated (P_Lig) lignin derivatives. When comparing the performance of HMW with the functionalized lignins (Ac_Lig and P_Lig), it became evident that the hydroxyl phenolic groups present in lignin play a pivotal role in the adsorption and reduction of metal ions. Additionally, the hybrid material HMW@Fe3O4, combining lignin with magnetite, demonstrated superior performance, reducing Cr(VI) to Cr(III) in just 2 hours. The study also highlights the exceptional chromium adsorption capacity of EH, with up to 60% adsorption, attributed to its higher surface area and syringyl unit content. These findings demonstrate lignin potential as a sustainable biosorbent for environmental remediation. We further explored hybrid materials containing lignin and zinc oxide, which showed promise for agricultural applications, particularly in stimulating plant growth. We successfully synthesized hybrid materials containing zinc oxide (ZnO) nanoparticles, demonstrating their versatility for agrochemical applications. Synthesis employed a scalable, environmentally friendly process, producing materials with varying zinc content. The materials were studied from both inorganic and organic perspectives, ensuring a comprehensive understanding of their properties and behaviours. They were analysed by means of several techniques, ranging from PXRD, ICP-AES, TEM-EDS, SEM-EDS, DLS, to TGA, DSC, Pyrolysis GC-MS and SEC. TEM studies confirmed the size and morphologies of the crystalline nanoparticles, aiding in optimizing synthesis strategies for reproducibility and scalability. We also explore the use of the liquid lignin formulation Solargo™, that offers potential advantages for foliar applications. In vivo trials on tomato seedlings treated with HMW@ZnO showed reduced germination times and increased dry weight, supported by ICP-AES analysis. The incorporation of lignin enables a slow-release mechanism, preventing heavy metal accumulation and reducing potential plant toxicity. Zinc distribution analysis highlighted beneficial effects in stems with no significant increase in overall zinc content compared to controls. In vitro studies demonstrated superior antibacterial activity of HMW@ZnO against Bacillus subtilis, a Gram-positive bacterium commonly found in the rhizosphere, outperforming controls and ZnO alone. By integrating ZnO into a lignin matrix, these materials not only enhance functionality but also add value to lignin, a renewable biopolymer, aligning with sustainable agricultural practices. These findings establish a foundation for further studies, exploring these hybrid materials as sustainable fertilizers and pesticides, reducing reliance on harmful agricultural chemicals. The research project further explored the possibility to obtain lignin-based hybrid materials, containing molybdate phases combined with zinc or copper. These materials hold promise for improving plant health through two main pathways. First, they could provide essential micronutrients that support plant growth and well-being. The incorporation of lignin in the matrix plays a critical role in enabling a slow-release mechanism for these nutrients, ensuring a gradual supply over time. This slow-release function is particularly important, as without it, heavy metals could accumulate in the plant, potentially leading to toxicity and negative environmental consequences. A second promising application of these hybrid materials lies in their use as pesticides for crop protection. Studies exploring the antifungal properties of molybdenum inorganic phases in the literature have shown encouraging results, indicating the potential for these materials to offer an environmentally friendly alternative to traditional chemical pesticides. The inorganic phases were synthesized using a sustainable and easy procedure, making it suitable for in-field applications. This method ensures that the materials are not only effective but also environmentally friendly, in line with the increasing demand for "green" chemistry in industrial applications. This research demonstrates that it is possible to obtain crystalline molybdate phases within the lignin matrix under mild and environmentally friendly conditions. Furthermore, the research explored the use of a mechanochemical approach to provide an alternative method of synthesis for the studied hybrid materials. This approach offers several benefits, including potentially lower energy consumption and no or little solvent employed, which could further enhance the sustainability of the material synthetic process. It is a versatile technique applicable to a broad spectrum of chemical reactions and materials, and it simplifies the scaling up process . Initially, the mechanochemical synthesis of HMW@ZnO was explored, with results compared to those from wet synthesis. The materials were synthesized with different zinc content, and successful mechanochemical scale-up was achieved. Similarly, HMW@Cu₂O was synthesized based on previous studies that highlighted its promising properties for agrochemical applications, making it valuable to investigate the mechanochemical reduction of Cu(II) to Cu(I). Finally, a material containing both cuprite and zincite crystals in the same biopolymeric matrix was synthesized under simple and sustainable conditions. The formation of both inorganic phases was confirmed, and EDS-Mapping analysis demonstrated a homogeneous distribution of the two metals within the matrix. Biopolymer analyses were conducted for all hybrid materials to ensure that the synthetic conditions did not negatively affect the organic component of the composite. TEM studies were conducted to analyse the crystalline nanoparticles formed and assess the similarity of the materials produced by wet or mechanochemical synthesis. To conclude, the ability to integrate these phases into a lignin-based matrix not only enhances their functionality but also adds value by utilizing lignin, a renewable and abundant biopolymer. Ultimately, this study opens up new opportunities for developing sustainable, efficient, and cost-effective materials for agricultural applications, contributing to the future of sustainable farming practices.