Here, we systematically compared the photoactivity and photobleaching behavior between dissolved black carbon (DBC) from rice straw biochar and leached dissolved organic carbon (LDOC) from rice straw compost using complementary techniques. The Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis showed that DBC was dominated by polycyclic aromatic (55.1%) and tannin-like molecules (24.1%), while LDOC was dominated by lignin-like (58.9%) and tannin-like molecules (19.7%). Under simulated sunlight conditions, DBC had much higher apparent quantum yields for 3DOM* and 1O2 but much lower apparent quantum yields for •OH than LDOC. After a 168 h irradiation, the total number of LDOC formulas identified by FT-ICR MS decreased by 40.1% with concurrent increases in O/C and H/C ratios and also decreases in double bond equivalence minus oxygen (DBE - O) and average molecular weight identified by gel permeation chromatography. However, despite the large decreases in UVA254 and DOC, the total number of DBC formulas decreased only by 12.0% with nearly unchanged O/C ratio, DBE - O values, molecular weight distribution, and benzenepolycarboxylic aromatic condensation (BACon) index regardless of the decreased percentage of condensed aromatic carbon (ConAC %). Compared with LDOC, the photolysis of DBC was much less oxidative and destructive mainly via breakup of a small portion of the highly condensed aromatic rings, probably accompanied by photodecarboxylation.
Keywords: FT-ICR MS analysis; comprehensive structural characterization; condensed aromatic rings; photoactivity; photobleaching.