Fungal Melanin and its Composites for Water Remediation and Wood Treatment

Abstract

Melanin is a pigment that is found in most organisms. In nature, many different sources of melanin are available, with the most common ones being the ink of cephalopods, sheep wool, human hair and different types of bacteria and fungi. Structural studies suggest that melanin has diverse and heterogeneous polymeric structures, and has slightly different compositions, sizes, colors and functions depending on its sources. During the past decade, melanin has attracted increasing attention for use in organic semiconductors and bioelectronics, drug delivery, photo-protection and environmental remediation processes. Although considerable advances in these fields have been achieved, real-world applications of melanin are still scarce, due to the limited and expensive source of natural melanin. In addition, its insolubility in water and many organic solvents as well as its diverse structure and properties makes it more difficult to study and to use in practical applications. Recent biotechnological advances from our research group and others have allowed a relatively large-scale production of fungal melanin, which could replace current expensive commercial melanin. Fungal melanin is soluble in aqueous solutions, facilitating its use, either directly or as a component in composites with other natural polymers. In this thesis, fungal melanin and composites with other biodegradable polymers were investigated for water remediation and wood protection, with the aim to bring fungal melanin closer to practical and sustainable applications. It is well known that heavy metal and organic dye pollution in water resources has become a severe environmental and public health problem worldwide due to rapid urbanization and industrialization. Thereby, enhanced treatments are urgently required with respect to eco-friendliness, filtration efficiencies and low operational costs. Within the scope of the doctoral thesis, first, fungal melanin was incorporated into polymeric nanofibrous membranes via an 4 electrospinning method to obtain stable and highly porous filtration systems, which were used to absorb heavy metal ions from water. Adsorption assays were performed on both raw melanin and melanized membranes. At physiotoxic concentrations of Pb2+, Cd2+, Ni2+ and Cr3+, fungal melanin was able to remove more than 90% of heavy metals in single-component solutions, showing a strong affinity for binding Pb2+ a low affinity for essential metals such as Ca2+ and Zn2+. The metal adsorption profiles also showed that melanized membranes were able to maintain the adsorption capacity of raw melanin. Thus, these membranes can be efficiently used as filtration membranes for the removal of heavy metals. Secondly, fungal melanin was used to develop melanized-cationic cellulose nanofiber foams that can successfully remove crystal violet, a common organic dye used in textile industry, reducing its concentrations in water down to the sub-ppm level. The foam can be recycled several times while retaining its adsorption/desorption properties, demonstrating a high practicability for adsorbing the cationic dye crystal violet. This work highlights the opportunity to combine both advanced features of sustainable polymers such as cellulose and the unique properties of fungal melanin for manufacturing bio-hybrid composites for water purification. Besides being an effective scavenger for toxic chemicals, fungal melanin can be used directly as an antimicrobial agent for wood protection. When melanin was used together with plant oils for wood treatment, a double impregnation process resulted in an accelerated and superior antibacterial response when compared to oil(s) without melanin. A significant antifungal effect was also obtained, i.e. the lethal effect against Chaetomium globosum was 90% on walnut wood after 2 weeks incubation. In addition, the treated wood samples absorbed less water than the untreated ones and resulted in a higher dimensional stability of treated wood at different humidity conditions, highlighting the practicability of this approach for wood protection. Overall, the knowledge obtained in this doctoral thesis helps to better understand the functions and applicability of fungal melanin and opens avenues for future research to design new mate-rials and systems that can be applied in real life.