Characterization of dissolved organic carbon in prairie surface water using FTIR spectroscopy
Abstract
Surface water reservoirs in rural areas of the Canadian prairies often have poor water quality due to contamination by dissolved organic carbon (DOC). DOC can activate growth of microorganisms in water distribution systems and form disinfection by-products (DBPs) in water treatment. The presence of microbiological contaminants and DBPs are potentially harmful to human health. Therefore, rapid and simple methods for DOC characterization are needed to evaluate potential water sources and to assist in understanding how best to remove DOC.
The first objective of this project was to develop a sample preparation and analysis procedure utilizing Fourier Transform Infrared Spectroscopy (FTIR) to characterize the DOC content of water samples. The second objective was to compare FTIR analysis results for fractionated and un-fractionated water samples to more convention DOC characterization methods (such as UV, SUVA and specific THMFP). The third objective was to demonstrate the application of the procedure to source water assessment and water treatment process evaluation by characterizing the DOC content of several typical treated and un-treated prairie water samples at several locations in Saskatchewan.
In the first phase of the study prepared samples of known DOC concentration were separated into six fractions (hydrophobic acid (HPOA), hydrophobic neutral (HPON) and hydrophobic base (HPOB); and hydrophilic acid (HPIA), hydrophilic neutral (HPIN) and hydrophilic base (HPIB)) using resin fractionation techniques. FTIR and conventional UV spectroscopic measurements, DOC concentration, and trihalomethane formation potential (THMFP) measurements were taken on the un-fractionated samples and their fractions.
A water matrix interference problem in the FTIR measurements was overcome by depositing a residue of dry solids from the aqueous solution containing the DOC on a
flat, organic compound free and non-infrared absorptive gold plated slide before analysis. This simple evaporation procedure developed for concentrating water samples successfully deposited a solid residue for FTIR scanning. Scanning of the solid residue of each sample at multiple locations successfully produced a spectrum of average results suitable for interpretation.
Each organic fraction separated from the prepared samples of known DOC was then assessed using FTIR analysis. Comparison of the spectra from the resin adsorption fractions gave an indication of the relationship between functional groups and the hydrophobic/hydrophilic nature of the DOC. The results suggest that the hydrophobic fractions contain more aromatic functional groups. This demonstrates that the FTIR spectra can provide information regarding the hydrophobic/hydrophilic nature of the DOC as an alternative to the resin separation procedure.
The sample preparation and FTIR analysis procedure was then used to characterize the DOC content of source and treated waters at several locations within Saskatchewan. The results of these initial investigations indicate the method can effectively identify the major organic functional groups present in source waters and the changes in the major functional groups that occur as the water is subjected to water treatment unit operations and processes. Further, the presence of several key functional groups is related to an increase in THMFP.