Biosynthetic melanin with excellent performance can be used for heavy metal adsorption

Abstract

Melanin is a complex polymer composed of indole and/or phenolic monomers, which show the potential to adsorb heavy metals in water due to their numerous functional groups. However, the artificial synthesis of melanin is difficult and the natural sources are limited. In this study, we screened the filamentous fungus Stachybotrys sp. HSS-1 to produce 11.56 g/L melanin M1 after fermentation optimization. M1 was identified as eumelanin using UV spectroscopy, elemental analysis, HPLC-MS, infrared spectroscopy, and nuclear magnetic resonance. Melanin M1 nano-materials showed the ability to adsorb the heavy metals Cd2+, Cu2+ and Pb2+ in water; in particular, the adsorption upper limit of Pb2+ reached 45 mg/g. Melanin M1 nano-materials also exhibited excellent thermal stability and radiation resistance, with a residual rate of 96.46% after heating at 100 °C for 2 h and more than 98% after 60 min of UV irradiation and 5 d of sunlight exposure. In addition, melanin M1 nano-materials strongly inhibited Staphylococcus aureus, Bacillus megaterium, and Salmonella, and the scavenging rates of 1 g/L M1 on superoxide radicals and hydroxyl radicals in the solution reached 97.33% and 98.59%, respectively. Therefore, melanin M1 nano-materials can not only adsorb heavy metals but also show the ability to resist the complex natural environment. Overall, this study developed high-yield, stable microbial melanin nano-materials that was shown to be an excellent adsorbent for heavy metal removal from water.