A PREDICTIVE MODEL FOR CHLOROFORM FORMATION FROM HUMIC SUBSTANCES

In 1979 the United States Environmental Protection Agency (USEPA) amended the Interim Drinking Water Standard to establish a MCL of 100 (mu)g/l of TTHMs, because of the carcinogenic nature of chloroform. During nationwide studies conducted by the EPA, it was found that chloroform was the predominant THM found in municipal water distribution systems. Earlier it was determined that humic substances (i.e., humic and fulvic acid) represent the major natural organic contaminants in natural waters as well as the principal precursor for THM formation. This research was conducted to develop a multiple parameter model for predicting chloroform formation, based on the type of precursor, initial concentration of precursor as TOC, initial chlorine-to-TOC ratio, temperature and pH.; Humic and fulvic acids were extracted from a Washington Peat Soil by the alkaline-acid separation technique. Aqueous solutions of either humic or fulvic acid were prepared at the appropriate TOC and pH, chlorinated with sufficient sodium hypochlorite solution to achieve the desired chlorine-to-TOC ratio, and incubated at the desired temperature in 120 ml serum vials. At designated reaction times the chloroform formation reaction was terminated with an excess of sodium thiosulfate, and resulting chloroform concentrations were determined by the liquid-liquid extraction method using a gas chromatograph with a linearized electron capture detector. The particular parameters investigated were: humic and fulvic acid, TOCs of 2.5, 5.0 and 10.0 mg/l, chlorine-to-TOC ratios of 2, 4 and 6, pHs of 5.5, 7.0 and 8.5, and temperatures of 10(DEGREES), 20(DEGREES) and 35(DEGREES)C.; Data generated during the experimental phase of the research was analyzed in relation to the importance of the TOC and chlorine-to-TOC ratio parameters by using a multiple linear regression computer program. The analysis also focused on the effect of reaction temperature, pH, as well as the type of precursor on the rate of chloroform formation and chloroform yield. The model was developed using humic acid extracted from a Washington Peat Soil as the "basic" precursor. This enabled the initial development of a humic acid model which was subsequently modified for application to fulvic acid.; Based on this research, it can be concluded that the reaction pH and temperature have a significant effect on the rate of chloroform formation as well as the chloroform yield. Also, it was found that the chloroform yield was directly proportional to the TOC with a doubling in TOC resulting in a doubling in chloroform yield. The effect of the chlorine-to-TOC ratio was determined to have a significant effect up to a ratio of 6 to 1, with the effect becoming insignificant at greater ratios. Finally it was found that the model could be readily applied to modeling chloroform formation in natural water, based on an evaluation of data taken from the literature. Model application to natural waters was accomplished by incorporating two precursor-related parameters into the humic acid model. These parameters enabled calibration of the model for a specific water with given precursor characteristics.