Sensitivity of Aromatic Carbon in Sugarcane Mill Mud to Photolysis within the Visible Range

Abstract

Direct and indirect photolysis are the primary fate pathways of natural organic matter (NOM) impacting the global carbon cycle in surface soils.  For NOM, opposite trends in structural changes are often observed between photolytic and dark aerobic transformations.  Industrial organic byproducts are generated under more controlled conditions than NOM, and their photochemical fate is largely unknown.  This study employed optical methods (fluorescence, UV/visible) to trace the changes in conjugated structures of sugar processing byproduct (mill mud) for 2 wk.  After an initial (≈1 wk) lag period, photolysis became dominant over dark reactions. Photolysis increased the aromaticity (higher emission wavelength peaks) and estimated amount (higher fluorescence intensity) of chromophores, and the effects were greater for pristine than aged mill mud.  However, field aging of mill mud transformed reactive chromophores into stable aromatic structures with characteristic ≈400 nm emission that were resistant to photolysis or dark reaction.  Factory mill mud showed similar fate pathways as NOM.  Anaerobic incubation in dark formed absorbance peaks at 280 and 320 nm typically observed in NOM.  Slope ratios for those peaks were decreased by photolysis, indicating the formation of aromatic structures.  In summary, visible range irradiation is sensitive to conjugated double bonds, and could be used to add values to industrial byproducts, and to evaluate their fate as a soil amendment.