Study On The Potential Interval Of Redox Functional Groups Of Humic Acids

In contaminated environments,insoluble heavy metals and organic pollutants of large size trapped in soil micropores(under 2.5 nm)are hard to be removed by bulk humic substances(HS)or microorganisms poorly accessible to pollutants.In order to solve the problem that pollutants in soil micropores are difficult to remove,we need to study the influence of low molecular weight fractions(LMWF)of humic acids(HA).Since HA LMWF can approach into micropores to react with pollutants by changing their valence and solubility,the application of HA LMWF will have a constant benefit to remediation of contaminated soil sites especially in a long-time scale.Recently,HA LMWF obtained from dialysis demonstrate a stronger redox capacity than bulk HA,and the redox capacity is found out to be related to the relative fluorescence intensity(RFI)of HA LMWF by a fluorescence spectra method.The continuous disaggregation of HA LMWF from bulk HA molecules in rapid groundwater flow area provides a possibility by HA LMWF to influence the transformation and transport of the pollutants trapped in soil micropores.In this study,the concentration and molecular weight of three standard humic acids were graded,and the redox capacity and structural characteristics of each sample mentioned above was studied to establish the correlation between them.The concentration gradient of humic acid and the redox capacity of humic acid showed a strong correlation,the potential range in which the reactive functional group in humic acid can be mostly reduced is-0.4 V to-0.69 V.It is 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 redox capacity of HA LMWF per gram carbon were up to 206 times greater than bulk LHA.Similar trends were found for other two standard HA LMWF samples.Fluorescence spectroscopy was used to characterize the changes in fluorophores of HA LMWF stripped in process.We found that an excitation/emission(Ex/Em)position representing a quinone-like fluorophore in all native HA LMWF state samples was located at Ex/Em=260-270 nm/455-495 nm.The quinone-like fluorophore in native HA LMWF can be further reduced into a fluorophore A and a fluorophore B(Ex/Em=300-305 nm/420-430 nm and Ex/Em=215-225 nm/415-430 nm)related to the quinonoid ?-?*transition and benzenoid ?-?*transition,respectively.Moreover,the relative fluorescence intensities of these two fluorophores(A and B)have strong correlations with redox capacities of HA LMWF,confirming a significant role of quinone-like fluorophores in redox properties of HA.Therefore,we can estimate the redox capacity of HA by measuring the relative fluorescence intensity of quinone fluorophores in HA,which has great guiding significance for the practical application of soil remediation engineering.