Characterization of DOM as a function of MW by fluorescence EEM and HPLC-SEC using UVA, DOC, and fluorescence detection

Abstract

To investigate the composition of dissolved organic matter (DOM) as a function of apparent molecular weight (MW) by rapid analytical methods, high performance liquid chromatography (HPLC)-size exclusion chromatography (SEC) was conducted with sequential on-line detectors consisting of UV, fluorescence, and quantitative DOC measurement. Fluorescence excitation-emission matrix (EEM) spectrophotometry was used to select wavelengths for the HPSEC on-line fluorescence system. The chosen peak maxima locations of excitation-emission wavelengths were 278-353 nm for protein-like substances and 337-423 nm for fulvic-like substances based on an analysis of EEM spectra for various samples and reference materials. This system provides quantitative and qualitative information on the specific MW components of DOM, including proportion of DOC (by DOC measurement), aromaticity (by comparison of UV and DOC measurements), and chemical properties (by fluorescence measurement). It further allows determination of organic matter characteristics (e.g., fulvic-like, protein-like, and polysaccharide-like substances) as a function of MW. Three types of samples (Irvine Ranch ground water (IRWD-GW), Barr Lake surface water (BL-SW), and Hawaii wastewater secondary effluent) were analyzed by the HPSEC-UVA-fluorescence-DOC system. These results were compared with fluorescence EEM for samples fractionated by HPLC-SEC. The DOM fraction in the high apparent MW range (over 10,000g/mol) consisted of polysaccharide-like substances for IRWD-GW and a mixture of polysaccharide-like/protein-like substances for BL-SW and wastewater secondary effluent. Minimal amounts of fulvic-like substances were found in the wastewater secondary effluent sample. The DOM fractions in a medium apparent MW range (5000-1000 g/M) showed higher aromaticity (fulvic in character) than any other fractions for all samples. For the DOM fraction in the low apparent MW range (below 680 g/M), additional aliphatic organic matter was found in IRWD-GW, while BL-SW contained protein-like processes. DOM plays an important role in drinking water and wastewater treatment processes. An enhanced HPSEC technique with multiple on-line detectors enables a better understanding of quantitative and qualitative DOM properties and can help to design and optimize water/wastewater treatment facilities.