The formation kinetics of iodinated phenolic compounds: Extension to humic substances

The nature, extent and rate of reaction between iodine and humic substances (HS) are studied. The hypothesis of the investigation is that the rate law for the fixation of I2(aq) onto HS resembles, in form, that for the iodination of phenol. The specific objectives used to test this hypothesis are: (i) verification of the rate law for the iodination of phenol and substituted phenolic compounds, and identification of the reaction products; and (ii) investigation of the reaction kinetics between I 2(aq) and HS.;The chemical kinetics between I2(aq) and various substituted phenolic compounds, acting as analogs for HS, are investigated. The rate is determined as a function of, pH, I2(aq) and I-(aq) concentration. A mechanism is postulated, based on experimental results, reaction product analysis and computer modeling using available kinetic data. A linear free energy relationship (LFER) is constructed based on the experimentally determined rates of the substituted phenolic compounds and using a substituent constant ( s+ ), previously established for electrophilic aromatic substitution reactions. The LFER for iodination of the phenolic compounds is valid over three orders-of-magnitude. Similar rate measurements are made on well-characterized and extracted samples of humic and fulvic acids. The iodination rates of the HS are close to the LFER established for the simple phenolic compounds.;This study provides strong evidence that it is the phenolic regions of natural HS that are responsible for iodine consumption and that the rates of reaction between iodine and HS are fundamentally similar to those of the model phenol compounds.;Interactions of iodine with humic materials are important to understand, due primarily to anthropogenic inputs of the radioactive isotopes 129I and 131I. Both isotopes are uranium fission products and are released from nuclear reactors during accident conditions ( 131I) or waste disposal (129I). Iodine-129 (t 1/2 = 16 million years) persists in the environment and may tend to migrate readily with water through soil because of its predominantly anionic form, I- or IO3-, and its poor sorption to most soil minerals. Iodine is believed to react readily with natural organic matter, affecting its volatility and reactivity.