EFFECT OF DISSOLVED ORGANIC MATTER FLUORESCENCE ON IRON (III) COMPLEXATION

Depending on the source and chemical composition, such as functional group characteristics, the complexation capacity of DOM can change significantly. Here, we explore how DOM of different origins, and thus different chemical properties, influence iron (III) binding in water. We hypothesized that the fluorescence properties of DOM can predict iron (III) complexation capacity.

We tested DOM from three different origins: two leachates of coals from Hot Springs County, and rural Pulaski County, Arkansas; and Suwannee River natural organic matter. Several analytical techniques were used to determine Fe-DOM interactions. The fluorescence properties of DOM samples were characterized by high-performance liquid chromatography coupled to size exclusion column (HPLC-SEC) with fluorescence detector. Each DOM fraction was characterized by fluorescence at excitation/emission (Ex/Em) wavelengths to characterize fractions of the DOM related to humic acids (HA, Ex/Em= 325/452 nm), fulvic acids (FA, Ex/Em= 337/423 nm) and polysaccharides (PS, Ex/Em=278/353 nm).

The total concentration of iron in Hot spring, Suwannee, and Pulaski samples were 8.0, 0.16 and 0.40 ppm, respectively. The concentration of complexed iron (III) by Hot Spring and Pulaski was maximum at initial iron (III) concentration of 200 ppm and reached 98 ppm and 90 ppm, respectively. For Suwannee, the maximum Fe-DOM complexation was reached at initial iron (III) concentration of 150 ppm and resulted in 87 ppm of complexed iron (III). The highest fluorescence intensity in all DOM samples was observed in HA wavelength range; and Hot Spring DOM characterized by the highest value of fluorescence peak height of 5.4 LU. The fluorescence intensities of Suwannee DOM were the lowest. Our results show, that higher fluorescence intensity of DOM corresponds to their higher complexation capacity. After addition of iron (III), the fluorescence signal reduced for HA, FA, and PS in Hot Spring and Suwannee DOM; and for FA and PS in Pulaski DOM. No decrease in fluorescence intensity of HA was observed in Pulaski DOM. Fluorescence signal quenching can possibly suggest the fractions participating in the iron (III) complexation. Results of this research can be used for predicting the bioavailability of iron (III) based on the extent of metal-DOM complexion.