Autoxidized Hydroquinone Mimics the Optical Properties of Chromophoric Dissolved Organic Matter

Abstract

One of the most significant knowledge gaps in aquatic chemistry is the chemical composition of chromophores and fluorophores in dissolved organic matter (DOM). While optical measurements have been employed for decades to track DOM quantity and quality, the specific molecular moieties responsible for DOM absorbance and fluorescence remain largely unresolved. In this study, it is demonstrated that autoxidized hydroquinone can reproduce the absorbance and fluorescence spectral shape of humic substance isolates of aquatic, microbial, and terrestrial origin. Furthermore, spectra of autoxidized hydroquinone show responses similar to those of DOM to borohydride reduction and pH titration, two treatments thought to probe the role of charge-transfer interactions. Data presented herein suggest that hydroquinone and p-benzoquinone (formed from hydroquinone autoxidation) interact to form quinhydrone-type charge-transfer contacts that exhibit optical properties highly similar to those of DOM. The similarities observed between DOM and autoxidized hydroquinone support the idea that the charge-transfer model is a viable explanation for DOM photophysical processes. While it is often assumed that the complex mixture of components in DOM is responsible for its optical properties, this study demonstrates that these optical properties can be replicated with a comparatively simple chemical system.