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 dissolvedorganicmatter (DOM) flushed into source water, which could be further changedduring 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 spectrometryon a controlled study which used the leachates of black and whiteashes (collected immediately after the 2013 Rim Fire, CA) along withunburned white fir (Abies concolor)and ponderosa pine (Pinus ponderosa). Results showed that the assigned features in DOM from burned asheshad a smaller molecular weight (MW), contained more unsaturated, aromatic,and oxidized and less fraction of labile compounds, and were enrichedwith nitrogen, relative to unburned litters. After alum coagulation,we observed the disappeared features with higher MW and S/C and N/Catom ratios than the resistant features. During the following disinfections,the disappeared features serving as the potential DBP precursors hadhigher MW and unsaturation degree, contained more S- and N-containingfunctionalities, and were more aromatic, compared to the resistantfeatures. Our study suggested organic compounds with small MW andhigh aromaticity as the targets for water facilities to improve post-firedrinking water quality. Wildfirepromotes the generation of compounds with smallmolecular weight and high aromaticity, which should be targeted toimprove the post-fire drinking water quality.