COMPETITION AMONG H2 UTILZERS DURING REDUCTIVE DEHALGENATION OF CHLORINATED SOLVENTS

초록

In order to utilize reductive dehalogenation of tetrachoroethene (PCE) and trichloroethene (TCE) effectively for engineered remediation or to evaluate natural attenuation, a better understanding of the competition between organisms for H2, a major substrate for dehalogenation, is needed. The slowest step in the process is transformation of the intermediates cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) to ethene. A numerical model that describes the reductive dehalogenation of cDCE to ethene, including competitive inhibition between cDCE and VC (Haston, 1999) as well as competition for H2 between different microbial populations, was developed. Also included is the kinetics of fermentation of various electron donor to produce H2 through the various stages of transformation. The model was evaluated using experimental results. From a comparison of model and experimental results (Yang and McCarty, 1998), it was determined that yeast extract (YE), an important nutrient commonly added for reductive dehalogenation, served as an important source of H2. The addition of 2 mg of YE supplied sufficient H2 for the complete dehalogenation of 5 umol of cDCE while two-thirds of the H2 from YE was converted into CH4. Interestingly, the addition of 30 umol of benzoate along with the YE did not shorten the time required for dehalogenation, the extra H2 it provided was used by H2-utilizing methanogens, which outgrew dehalogenators when this excess in H2was available. Also, the presence of homoacetogens greatly affected the H2 flow to dehalogenators. When H2 gas was supplied in excess along with YE, model predictions, assuming no homoacetogens were present, indicated H2-utilizing methanogens would consume most of H2 produced from YE, and thus, no dehalogenation would occur. However, when homoacetogenesis was included in the model, homoacetogens were predicted to out-compete the H2-utilizing methanogens, reducing their growth rate. This helped dehalogenators obtain a larger p