Molecular-level transformations of dissolved black carbon in UV-based advanced oxidation processes

Area of Science:

• Environmental Chemistry
• Water Treatment Technologies
• Organic Geochemistry

Background:

• Dissolved black carbon (DBC) from biochar is prevalent in aquatic environments and acts as a free radical scavenger in advanced oxidation processes (AOPs).
• The molecular transformations of DBC in different UV-AOPs (UV/H₂O₂, UV/PDS, UV/Chlorine) are not well understood, hindering effective water treatment strategies.

Purpose of the Study:

• To elucidate the molecular-level transformations of DBC derived from wheat biochar under various UV-AOP conditions.
• To compare the dominant oxygenation reactions and by-product formation across UV/H₂O₂, UV/PDS, and UV/Chlorine treatments.

Main Methods:

• Characterization of DBC using UV-Vis and fluorescence EEM spectroscopy to monitor structural changes.
• High-resolution analysis using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to identify molecular transformations.
• Mass difference analysis based on 23 reaction types to quantify oxygenation processes and identify specific reaction pathways.

Main Results:

• UV-AOP treatments led to decreased SUVA₂₅₄ values and reduced fluorescence intensity, indicating DBC transformation.
• Significant oxygenation of DBC was observed in all UV-AOPs, with distinct dominant reactions: hydroxylation (+O) in UV/H₂O₂, di-hydroxylation (+2O) in UV/PDS, and tri-hydroxylation (+3O) in UV/Chlorine.
• UV/Chlorine treatment resulted in the formation of 1194 chlorine-containing by-products (Cl-BPs) of unknown structures.

Conclusions:

• This study provides a comprehensive molecular understanding of DBC transformations in different UV-AOPs, highlighting the role of specific radical species.
• The findings contribute to understanding the fate of DBC in water treatment and offer insights into inhibiting the formation of undesirable by-products.
• DBC's susceptibility to oxygenation and its role as a by-product precursor are critical considerations for optimizing AOPs.