Reducing capacities in continuously released low molecular weight fractions from bulk humic acids

Humic substances are important redox-active organic compounds with various kinds of functional groups in different molecular sizes and connections. They can be formed during the degradation of biomass and biowaste in the environment. In soil environments, insoluble heavy metals and large size organic pollutants trapped in soil micropores (under 2.5 nm) are hard to be removed by bulk HS or microorganisms poorly accessible to pollutants. Recently, low molecular weight fractions (LMWF) of humic acids (HA) obtained from dialysis demonstrate a stronger reducing capacity than bulk HA, and the reducing capacity is found out to be related to the relative fluorescence intensity of LMWF HA by a fluorescence spectra method. The continuous disaggregation of LMWF HA from bulk HA molecules in rapid groundwater flow area provides a possibility by LMWF HA to influence the transformation and transport of the pollutants trapped in soil micropores. Therefore, the objective of this study is to explore the mathematical relationship between reducing capacity of LMWF HA and relative fluorescence intensity of quinone moieties in LMWF HA. We found that Leonardite humic acids (LHA) 3500-LMWF (under 1.25 nm) only account for 0.40%-1.53% of the total organic carbon of bulk HA. However, the reducing capacity of LMWF HA per gram carbon were up to 206 times greater than bulk LHA. Similar trends were found for other two standard LMWF HA samples. Fluorescence spectroscopy was used to characterize the changes in fluorophores of LMWF HA stripped in process. We found that an excitation/emission (E_x/E_m) position representing a quinone-like fluorophore in all native LMWF HA state samples was located at 260-270/455-495 nm. The quinone-like fluorophore in native LMWF HA can be further reduced into a fluorophore A and a fluorophore B (E_x/E_m at 300-305/420-430 nm and 215-225/415-430 nm) related to the quinonoid π-π* transition and benzenoid π-π* transition, respectively. Moreover, the relative fluorescence intensities of these two fluorophores (A and B) have strong correlations with reducing capacities of LMWF HA, confirming a significant role of quinone-like fluorophores in redox properties of HA. LMWF HA can enter micropores to react with pollutants, the application of biomass will have a constant benefit in remediation of contaminated soil site especially in a long time scale.