Inner filter effect, suspended solids and nitrite/nitrate interferences in fluorescence measurements of wastewater organic matter

Abstract

In this study, it was assessed the effectiveness to correct for inner filter effect (IFE) the fluorescence spectra of several wastewaters (i.e., primary, secondary and tertiary wastewater effluents) and wastewater-impacted surface waters using a common method based on UV absorbance measurements. In samples of secondary/tertiary wastewater effluents and surface waters, IFE was severe at excitation wavelengths <240 nm, and it was low (4–11%) at excitation wavelengths >340 nm. On the contrary, IFE has always been significant in primary wastewater effluents. After IFE correction, linear relationship was observed between fluorescence and absorbance in dilution series across the full excitation-emission matrix (EEM), although some distortions were still present. Particularly, experimental data showed the presence of static/dynamic quenching of fluorescence due to nitrite/nitrate, which cannot be corrected by IFE correction methods. Indeed, after addition of different nitrate/nitrite concentrations in wastewater (3–40 mg/L as N), the estimated static/dynamic quenching error (QE) after IFE correction was often >20% for tyrosine and tryptophan-like fluorescence measured at excitation <240 nm. However, the QE was low (<5–10%) for fluorescence measured at excitation >240 nm. Overall, the QE increased with the increase of nitrite/nitrate concentration in wastewater. Total suspended solids (TSS) (i.e., particulate organic matter) in water produced intense fluorescence peaks in the tyrosine-like and tryptophan-like region of EEM, and TSS increased the absorbance values at all the excitation wavelengths of the UV–visible absorption spectra in unfiltered samples compared to 0.7 μm filtered samples. On the contrary, tertiary effluents employing full scale sand filtration (TSS < 2–4 mg/l) had similar UV absorbance and fluorescence spectra to 0.7 μm filtered samples. Finally, it was observed that uncorrected fluorescence intensities in the humic-like region of EEM were similar in both filtered and unfiltered samples, and it was independent of TSS concentration, dilution factor and water quality.