Synthesis and Characterization of Humic/Melanin-like Compounds by Oxidative Polymerization of Simple Aromatic Precursors

Abstract

Dissolved organic matter (DOM) is a complex mixture of naturally occurring organic molecules originating from multiple marine and terrestrial sources. DOM plays a significant role in water quality by affecting the photochemistry, trace metal transport, and acidity in aquatic systems. Understanding the chemical composition of DOM helps interpret the links between its optical properties and molecular structures. Currently, the molecular origins of the optical properties of DOM are not well-defined. In this study, we oxidize and initiate the polymerization of melanin precursors 1,8-dihydroxy naphthalene and 5,6-dihydroxy indole by the addition of hydrogen peroxide and/or with ultraviolet irradiation. Our goal is to evaluate the possibility of reproducing the optical signatures of DOM from simple aromatic precursors. Optical characterization shows an extreme shift of the absorbance to a featureless trend and broad fluorescence peaks (350–500 nm) like DOM. Electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) mass spectrometry show evidence of oligomers with varying degrees of oxidation. The combination of our results shows that about 1–4 units of melanin oligomers with varying degrees of oxidation mimic the optical properties of DOM. Overall, our results strongly support the idea that simple precursors form oligomeric chromophores mimicking DOMs optical properties through simple oxidative steps.