Molecular Transformation of Dissolved Organic Matter during the Processes of Wildfire, Alum Coagulation, and Disinfection Using ESI(−) and ESI(+) FT-ICR MS

Forest fires alter the quality of terrestrial dissolved
organic
matter (DOM) flushed into source water, which could be further changed
during alum coagulation and serve as potential disinfection byproduct
(DBP) precursors in water facilities. To unveil this molecular transformation,
we applied 12T Fourier transform ion cyclotron resonance mass spectrometry
on a controlled study which used the leachates of black and white
ashes (collected immediately after the 2013 Rim Fire, CA) along with
unburned white fir (Abies concolor)
and ponderosa pine (Pinus ponderosa). Results showed that the assigned features in DOM from burned ashes
had a smaller molecular weight (MW), contained more unsaturated, aromatic,
and oxidized and less fraction of labile compounds, and were enriched
with nitrogen, relative to unburned litters. After alum coagulation,
we observed the disappeared features with higher MW and S/C and N/C
atom ratios than the resistant features. During the following disinfections,
the disappeared features serving as the potential DBP precursors had
higher MW and unsaturation degree, contained more S- and N-containing
functionalities, and were more aromatic, compared to the resistant
features. Our study suggested organic compounds with small MW and
high aromaticity as the targets for water facilities to improve post-fire
drinking water quality.