Effects of natural organic matter model compounds on the transformation of carbon tetrachloride by chloride green rust

Abstract

Interest has grown in the use of reactive minerals for natural and engineered transformation of ground water contaminants. This study investigated how the structural properties of 10 model compounds representing natural organic matter (NOM) influenced their adsorption to chloride green rust (GR-Cl), and how this adsorption affected rate constants for transformation of carbon tetrachloride (CT) by GR-Cl. The affinity of benzoic acid, phthalic acid, trimesic acid, pyromellitic acid, and mellitic acid for the GR-Cl surface generally increased in the order of increasing number of carboxylic acid functional groups, increasing acidity of these functional groups, and increasing charge density. For NOM model compounds that had phenolic functional groups ( p-hydroxybenzoic acid, α-resorcylic acid, and caffeic acid), the affinity for the GR-Cl surface was greatest for caffeic acid, which had two adjacent phenolic functional groups. Some NOM model compounds had experimentally determined Langmuir maximum adsorption capacities ( q max−Langmuir) greater than those calculated based on external surface area measurements and the size of the NOM model compound, suggesting adsorption to internal as well as external sites at the GR-Cl surface for these compounds. Rate constants for CT transformation by GR-Cl generally decreased as the affinity of the NOM model compounds (estimated by Langmuir K values) increased, but there was no statistically significant correlation between Langmuir parameters (i.e., K and q max−Langmuir) and rate constants, perhaps due to significant adsorption of some NOM model compounds to sites that were not accessible to CT, such as interlayer sites. Unlike the other NOM model compounds, caffeic acid, which adsorbed to a significant extent to the GR-Cl surface, increased the rate constant for CT transformation. The influence of NOM on rate constants for CT transformation by green rusts should be considered in ground water remediation planning.