Kinetics and modeling of reductive dechlorination at high PCE and TCE concentrations

Abstract

Two biokinetic models employing the MichaelisMenten equation for anaerobic reductive dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE) were developed. The models were compared with results from batch kinetic tests conducted over a wide range of PCE and TCE concentrations with two different dechlorinating cultures. One model applies Michaelis-Menten kinetics with competitive inhibition among chlorinated aliphatic hydrocarbons (CAHs), while the other model includes both competitive inhibition and Haldane inhibition at high CAH concentrations. Model simulations with competitive inhibition simulated the experimental results well for PCE concentrations lower than 300 AM. However, simulations deviated from the experimental observations for PCE or TCE concentrations greater than 300 -400 AM. The kinetic model that incorporated both competitive and Haldane inhibitions better simulated experimental data for PCE concentrations near the solubility limit (1000 AM), and TCE concentrations at half its solubility limit (4000 AM). Based on the modeling analysis of the experimental results, the PM culture (Point Mugu, CA) had very high Haldane inhibition constants for cis-1,2-dichlororethylene (c-DCE) and vinyl chloride (VC) (6000 and 7000 AM, respectively), indicating very weak Haldane inhibition, while the EV culture (the Evanite site in Corvallis, OR) had lower Haldane inhibition constants for TCE, c-DCE, and VC of 900, 750, and 750 AM, respectively. The BM culture (a binary mixed culture of the PM and EV cultures) had transformation abilities that represented the mixture of the EV and PM cultures. Model simulations of the BM culture transformation abilities were well represented by separate rate equations and model parameters for the two independent cultures that were simultaneously solved. Modeling results indicated that a combination of competitive and Haldane inhibition kinetics is required to simulate dechlorination over a broad range of concentrations up to the solubility limit of PCE and half the solubility limit of TCE.

Experimental Evaluation and Mathematical Modeling of Microbially Enhanced Tetrachloroethene (PCE) Dissolution

“…Several studies with pure cultures (see Table S1, Supporting Information) and with mixed dechlorinating consortia (e.g., [17][18][19][20][21][22][23][24] have shown that bacterial PCE dechlorination occurs at or near saturated PCE concentrations. For instance, Desulfuromonas michiganensis strain BB1 was reported to dechlorinate at saturated PCE concentrations and to grow in the presence of PCE DNAPL (4).…”

“…Several mixed culture studies suggest that PCE dechlorination occurs at high, even saturated, concentrations of PCE (e.g., [17][18][19][20][21][22][23][24]. The ability of these mixed cultures to dechlorinate at elevated PCE concentrations may be due to (i) the presence of dechlorinating strains acclimated to high PCE concentrations, or (ii) dechlorinators that exhibit increased PCE tolerance as members of mixed microbial communities (e.g., biofilms).…”

“…Self-inhibition at high PCE concentrations was modeled after Luong (43). Unlike other self-inhibition models (e.g., Haldane) that have been used successfully to model the gradual decline in transformation rates as substrate concentration increases (22), experimental results presented here suggest that dechlorination ceased when PCE concentrations exceeded a maximum tolerable level for the tested cultures. The selected self-inhibition model better represents this terminal inhibition concentration and eliminates the need to quantify another kinetic parameter (i.e., inhibition constant) (43).…”

“…Competitive inhibition was modeled following Yu and Semprini (22) and product inhibition was neglected (30). Self-inhibition at high PCE concentrations was modeled after Luong (43).…”

“…is the competitive inhibition coefficient for chlorinated ethene c, which accounts for competition between PCE and TCE for reducing equivalents (22), where…”