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# Chlorothalonil binding to dissolved humic substances isolated from a Massachusetts cranberry bog

Abstract (Summary)
The fungicide chlorothalonil (1,3-tetrachloroisophthalonitrile) is widely used in cranberry production in Massachusetts. Its partition constant, $\rm K\sb{DOC},$ with aquatic humic substances isolated from cranberry bog water was assessed in this study. Elemental analysis of humic substances indicated differences in C, H, N, and O that were consistent with published elemental ratios of aquatic humic substances. Infrared spectra were also similar with respect to C=O stretching of carboxyl and ketonic groups and showed evidence of aromatic C=C stretching. Ultrafiltration of humic acids showed 53% greater than 10,000 molecular weight cut off (MWCO). The fulvic acid fraction had 71% less than 10,000 MWCO. UV absorbance ratios at 400 and 254 nm were 0.23 and 0.16 for humic and fulvic acid, respectively indicating a more aromatic structure than Aldrich humic acid from soil. Base titration showed a charge at pH 8 of 7.5 and 13 meq g-$\rm C\sp{-1}$ for humic and fulvic acid, respectively. Measurements of the TCIN Henry's constant were made using gas-purge techniques. The value obtained, $5 \times 10\sp{-5}$ kPa $\rm m\sp3$ $\rm mol\sp{-1}$ (s = $1.7 \times 10\sp{-5},$ n = 4), was consistent with calculated values from solubility and vapor pressure. Log $\rm K\sb{DOC}$ values for fulvic and humic acid measured using gas-purge techniques were 4.3 (s = 0.6) and 4.5 (s = 0.8), respectively. These values were greater than reported $\rm K\sb{OC}$ values by as much as 1.5 orders of magnitude, suggesting that $\rm K\sb{OC}$ values may underestimate TCIN solubility enhancement in cranberry bog water. TCIN binding to aquatic humic substances corresponds to increased solubility in aqueous systems and potentially toxic levels in the presence of sorbent. Solubility in water, based on the measured $\rm K\sb{DOC}$ values, could increase by 200% with waters containing 35 mg $\rm l\sp{-1}$ DOC. Measured DOC of cranberry bog waters was in the 1 to 30 mg $\rm l\sp{-1}$ range.
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