Chemistry of the Naturally Formed Chlorinated Hydrocarbons in Soils and Aquatic Systems
Organo-Cl compounds have been used extensively for years in agricultural and industrial applications. Because of their long residence time in the environment, these compounds threaten to contaminate aquatic and soil systems. On the other hand, several living organisms were found to produce organo-Cl compounds using enzymatic processes and inorganic-Cl in soils and aquatic systems. Although experimental evidence for the occurrence of small chain chlorinated alkanes in soil and marine systems exists, direct evidence for the persistent and less-volatile organochlorine compounds is not available. Using in-situ synchrotron based spectroscopic methods, in addition to other conventional laboratory techniques (isotopic and chromatographic methods), we are examining the chemical forms of Cl in soil and aqueous organic molecules, and the biogeochemical processes involved in their formation. Improved understanding on their geochemistry is useful for the evaluation of the toxicity of both natural and man-made organo-Cl compounds, and to take remedial strategies for the treatment of chlorinated hydrocarbons in contaminated systems.
Since the X-ray absorption spectra (specifically, the near-edge X-ray absorption fine structure, or NEXAFS) of organo-Cl compounds are significantly different from those of inorganic forms, the chemical forms of Cl can be identified directly in complex organic matrices without any sample preparation. For instance, Cl in -I (inorganic chloride, Cl -) and +VII (perchlorate, ClO4-) oxidation states exhibit absorption edges at 2820.7 eV and 2830.2 eV, respectively, with those of intermediate oxidation states in between these two energies (energy calibrated against KCl at 2822.8 eV). In contrast to these inorganic Cl species, Cl connected to C atoms in organic compounds exhibits intense low-energy features corresponding to the 1s → p* and s* electronic transitions of the C-Cl bonds well below the Cl-main absorption edge. Further, Cl atoms connected to aromatic and cyclic C exhibited these features ~0.5 eV higher than those of aliphatic mono-chlorinated compounds, and the spectral resolution at these energies is sufficient enough to distinguish different compounds.
When compared to these structural models, the humic substances (both fulvic and humic acids) isolated from river water, soil, and peat exhibit an intense absorption edge at ~ 2821 (±0.1) eV. Their spectral-fit with those of inorganic chloride, and aliphatic and aromatic organo-Cl compounds indicate that these isolated humics primarily contain aromatic organo-Cl, with < 30% aliphatic organo-Cl. Inorganic Cl - is also found to be one of the abundant Cl-species in some of the isolated molecules. Different chemical fractions of humic substances (e.g. humic and fulvic acids) also exhibit the same forms of organo-Cl compounds, with differences in their total concentration. In contrast to the isolated humics and organics in soil samples, the fresh-plant materials of different species (Sequoia sp., Liquidambar sp., Eucalyptus sp.) exhibit spectral features corresponding to the inorganic-Cl - only. The senile plant leaves either attached to the plant or collected on soil surfaces also showed spectral features of inorganic Cl - and aromatic Cl. However, the highly humidified plant leaves of all the examined plant species showed intense spectral features at 2821.1 (± eV, which corresponds to the aromatic organo-Cl.
The results presented here, for the first time, directly document the occurrence of the stable naturally produced, aromatic organo-Cl compounds in terrestrial ecosystems. In addition, our studies show that they are formed at rapid rates in the environment. We are continuing these studies to understand the processes involved in their formation and their role in the metal and mineral surface complexation reactions. We are also using the same methods in exploring the biotic and abiotic transformations of chlorinated hydrocarbons in the environment.