Change of Three-Dimensional Excitation-Emission Matrix Fluorescence Spectra of Commercial Humic Acid by Ultrafine Bubble Ozonation and Biodegradation

Abstract

A solution of commercial humic acid （80 mg/L, 28.5 mg－C/L） was subjected to a biodegradation test following ultrafine bubble ozonation. Semi-batch ozonation for 20 min （80 mg－O₃/L） removed 48% of the total organic carbon （TOC）, decreased the color unit from 950 to less than 50, and increased the ratio of biochemical oxygen demand （BOD₅） to TOC from 0.16 to 0.73 in the solution. Activated sludge microorganisms removed 30% of TOC from the untreated solution, but caused no marked decolorization. A 14－day biodegradation test following the 20－min ozonation revealed the TOC removal of more than 90%, but the color unit remained as 75, suggesting accumulation of colored metabolites. Three-dimensional excitation-emission matrix fluorescence spectral analysis revealed that ozonation shifted peaks at Ex200－300/Em450－600 and Ex450/Em540 in the humic acid solution, respectively, to peaks at Ex200－250/Em300－450 and Ex320/Em430. During biodegradation, those peaks decreased, but a peak at Ex230/Em450 remained, presumably representing colored metabolites.