The main objective of this research was to explore the fluorescence quenching mechanism of humic substance (Suwannee River natural organic matter, (SWNOM)) to amino acids (tryptophan, tyrosine) and protein (bovine serum albumin, (BSA)) by multi-spectroscopic methods. The locations of the peak of tryptophan, tyrosine, and BSA from the Parallel Factor Analysis were at Ex/Em 280/356 nm, 275/302 nm, and 280/344 nm, respectively. For SWNOM, two peaks appeared at Ex/Em of 240/448 nm, and 350/450 nm. Static quenching was the dominant quenching mechanism between BSA and SWNOM, whereas, no quenching was observed between tryptophan or tyrosine and SWNOM. Fourier-transform infrared spectroscopy and thermodynamic calculation demonstrated that hydrogen bonding and van der Waals force are the potential binding forces of BSA-SWNOM complex, as a result of rearrangement in the secondary polypeptide carbonyl hydrogen bonding network of BSA. This rearrangement led to the conformational change in BSA that induced quenching of BSA fluorescence by SWNOM.

Fluorescence excitation–emission matrix (EEM) spectroscopy is often used to determine the levels of trihalomethane (THM) precursors in natural organic matter. However, humic substances are known to quench the fluorescence of amino acids and proteins. To date, none of the EEM-based models for predicting THM formation potential (THMFP) have explicitly accounted for these quenching effects. Thus, we investigated the importance of correcting for fluorescence quenching during THMFP prediction. Fluorescence titration experiments revealed that the correction improved the accuracy of THM prediction. EEM-based models using the corrected fluorescence intensity displayed the highest accuracy (R2 > 0.99; mean absolute error 8.1 μg/L and 13.9 μg/L for chloroform and bromoform, respectively) among models using individual parameters of EEM intensity, dissolved organic carbon (DOC), ultraviolet absorbance at 254 nm (UV254), specific UV254 (SUVA254) and differential ultraviolet absorbance at 272 nm (ΔUV272). Thus, EEM-based models require both the fluorescence intensity of a humic-like component and the corrected fluorescence intensity of a protein-like component for accurate THMFP prediction, for both chlorination and bromination processes. We also found it to be unnecessary to combine DOC with EEM intensity in terms of prediction accuracy, as long as the fluorescence quenching correction is applied.

Fluorescence quenching of proteinaceous substances by natural organic matter is a well-known phenomenon, but there are no known methods for correcting it. The main objective of this research was to develop an empirical equation to correct the fluorescence quenching of different concentrations of bovine serum albumin (BSA – 0.15, 0.25, 0.5, 0.75, 1, 1.25 μmol/L (μM)) by Suwannee river natural organic matter (SWNOM - 0,2,4,6,8,10 mg-C/L) using the fluorescence titration method. The excitation emission matrix (EEM) data were analyzed by parallel factor analysis with inner filter effect removal. With increasing SWNOM concentration, BSA peak intensity quenching was in the range 29–85%, with a linear relationship for increment of either BSA or SWNOM concentration. A higher ratio of SWNOM to BSA resulted in greater BSA peak intensity quenching. The unquenched BSA peak (BSA (RU)) is given by the empirical equation. The calculated unquenched BSA peak intensities using the empirical equation agreed well with the actual unquenched peak values (R2 = 0.98, mean absolute error = 0.33 RU). The equation is expected to help in rapid estimation of the quenching effect of SWNOM on BSA.

Flow conditions are known to affect nitrogen transformation in freshwater environments. However, these effects have mainly been examined in interaction with the biofilm present on bed sediment. Therefore, this study investigated the relationship between the degree of turbulence and nitrification in a suspended phase of the water column. All experiments were performed in batch reactors equipped with stirrers set at six different rotational speeds varying from 15 to 1100 rpm, which correspond to Reynolds number (Re) from 540 to 39,500. The results showed that an increase in Reynolds numbers had effects on both ammonium and nitrite oxidation processes. The optimum ammonium oxidation rate constant (k) was at laminar condition (Re = 2000), although the specific growth rate of the ammonium oxidizing bacterium Nitrosomanas europaea () were equal in all experimental conditions. On nitrite oxidation process, at Reynolds number up to approximately 4000 both nitrite oxidation rate constant k and specific growth rate of the nitrite oxidizing bacterium Nitrobacter winogradskyi () were stimulated. At Reynolds numbers greater than Re 10,800, turbulence had an adverse effect that inhibited nitrite oxidation process. Thus, the present study emphasizes the importance on including turbulence as one of important factors which influences the efficiency of nitrification in freshwater column.

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