Determination of Organically Combined Chlorine in High Molecular Weight Aquatic Organics Mark P. McCahill and Lawrence E. Conroy Chemistry Department, University of Minnesota, Minneapolis, Minn. 55455
Walter J. Maier” Civil and Mineral Engineering Department, University of Minnesota, Minneapolis, Minn. 55455 ~
A procedure for measuring organically combined chlorine (TOC1) in water or wastewaters is described. TOCl is measured after preconcentrating the sample, photolyzing the organics to mineralize the chlorine, and measuring inorganic chloride. The results show that chlorination of wastewater treatment plant effluents leads to significant accumulation of organically combined chlorine. From 0.0025 to 0.007 mol of TOCl per mol of TOC is produced.
Production of low molecular weight organic compounds containing covalently bound chlorine during chlorination of water and wastewater is well documented (1 1. The presence of these compounds is of concern because some are carcinogenic to mammals and toxic to aquatic biota. Haloforms and chlorophenols are representative of the types of compounds generally detected. Unfortunately, analytical techniques suitable for low rnolecular weight and volatile compounds (gas chromatography, GC-mass spectroscopy, liquid chromatography) are not applicable to large, nonvolatile compounds. High molecular weight compounds such as lignins, humic acids, and polymeric nucleic acids are present in natural waters and have reactive sites for chlorine addition. The ultimate fate of such organochlorine compounds is not known, but degradation could lead to formation of low molecular weight organochlorine compounds. Development of a photochemical oxidation technique to measure the total organic chlorine (TOCI) of aqueous organics was the purpose of this study. Measurement of TOCl is preferred because specific compound identification of naturally occurring mixtures of high molecular weight organics is not feasible. A method for (determination of TOCl, by adsorbing organics on activated charcoal, washing to remove free C1- ions present, followed by pyrohydrolyzing to liberate organic chlorine as HCl, and determining C1 microcoulometrically, has been described ( 2 ) ;the method has several disadvantages. Activated carbon adsorption is not as effective for polar organics as for nonpolar organiics. Fractionation of high molecular weight material to characterize TOCl as a function of the size oforganics cannot be conveniently done. A large quantity of activated carbon must be combusted with the organic matter of interest. Chlorinated and brominated nonpolar hydrocarbons in water have been measured by neutron activation analysis (3). This method is very sensitive but requires complete elimination of inorganic halide by extracting the organics with a nonpolar solvent. An elegant method for measuring the concentration of chlorinated and brominated organic compounds using an element-specific microwave plasma detector coupled with a GC separation has been reported ( 4 ) . This method is limited by the GC pre-separation method to volatile compounds. The photochemical method offers the advantages of measuring all chlorinated organics or TOCl determination of molecular size fractionated organics; the oxidation technique can be used with any preconcentration technique. ‘FOCI measurements 5,hould be useful for evaluation of water disinfection techniques and chlorination of wastewater treatment plant effluents with the intention of minimizing TOC1. 0013-936X/80/0914-0201$01.00/0
Materials a n d M e t h o d s
Water samples were photolyzed to oxidize organic material and mineralize covalently bonded chlorine to C1- ion. A General Electric AH6 high-pressure mercury lamp was used as a source of ultraviolet (UV) radiation. The optical output of the AH6 lamp is rated a t 195 W. Samples (20 mL) were irradiated in quartz tubes (set 1 to 2 cm from the lamp) under reflux condensers. Concentrated HNO:j (0.3 mL) (electron grade) was added to the samples before irradiation, and samples were photolyzed for 1 h to convert organics to C O P and organic chlorine to C1- ion. The C1- present before (usually not detectable) and after irradiation was quantitated using an Orion Model 94-17A C1- specific ion electrode, and results were checked by the standard additions method. Standard C1- solutions were prepared on a Millipore “Milli-Q Reagent Grade Water System”. Except as noted, environmental samples were prepared for analysis by filtering through a Millipore 0.45-pm filter to remove suspended solids (operationally defined as soluble material). Samples were stored at 5 “C. Organic constituents were concentrated on a Millipore PSAC 09005 ultrafiltration membrane, which has a nominal molecular weight cut-off of 1000; material of this size or larger is retained by the membrane while smaller molecules pass through the membrane. The ultrafiltrations were run in a Millipore 90-mm ultrafiltration cell fitted with an external reservoir. One to two and one-half liter samples were concentrated to 1/10 or 1/25 of their initial volume; then the 100-mL concentrate was washed with 600 mL of Milli-Q water a t constant sample volume to reduce C1- ion concentration in the concentrate. One set of samples was washed with 600 mL of NaN03 solution (200 mg/L) to see if the presence of Nos- ions affects C1- ion removal from the concentrate. Model compounds were photolyzed to test the effectiveness of the UV irradiation procedure; the solutions were prepared from AR grade reagents except as noted in Table I. Total organic carbon (TOC) was measured with a Beckman 915A total and inorganic carbon analyzer. All determinations of TOC and TOCl were done in duplicate. The precision of measurement of TOCl is determined by the recovery of organic chlorine in the preconcentration and washing step and the measurement of C1-. Recovery ofchlorine-containing compounds depends on the effectiveness of the membrane for retention of the organic molecules and is therefore an operational parameter. The precision of C1measurement by specific ion electrode is &2‘% with a lower limit of detection of 0.5 mg/L. Repeat determinations on 7 different samples ranging to 3.25 mg/L gave a standard deviation of 0.2 mg/L. Comparison of CI- released by photolysis of three reagent grade dichlorobenzoates with their respective elemental composition gave a standard deviation of 3.~5r~~. A similar comparison of 7 compounds, which include practical grade reagents, gave a standard deviation of 6.6%.
@ 1980 American Chemical Society
Results a n d Discussion Model compound study results, shown in Table I. indicate that organic chlorine is recovered nearly quantitatively from a wide variety of organochlorine compounds as C1- under the Volume 14, Number 2, February 1980
201
Table 1. Test Results Using Model Compounds tesl compd concn, mglL
compound
3,5dichlorobenzoic acid 2,5dichlorobenzoic acid 2,4dichlorobenzoic acid 2,4dichlorophenoxyacetic acid &chlorophenol trichloroacetic acid trichloroacetic acid chloroform 8-chloroxanthine 5-chlorouracil 3-chloropropionic acid a
*
Table 111. Mississippi River Water at Saint Anthony Falls a
measured TOCLa mglL Ci-
100 100 100 100 126 58.5 120 147 100 100 100
% ci recovery
35 36 36 34 37 32.5 60 79 21.5 24.5 34
96 98 98 107 107 87 78 61 113 101 104
a Free CI- before irradiation of samples below detection limit for all model compounds. Denotes practical grade reagents.
Table It. Mineralization of Organic Carbon by UV Irradiation sample
TOC before irradiation, mglL
Mississippi River water (concd) 2,4dichlorobenzoic acid 2,4-dichlorophenoxyacetic acid &chlorophenol chloroform trichloroacetic acid
50 100 100 126 147 120
TOC after irradiation, mglL
0 0 0
