Tải bản đầy đủ - 0 (trang)
5 Kinetics and Threshold Levels of PCB Organohalide Respiration

5 Kinetics and Threshold Levels of PCB Organohalide Respiration

Tải bản đầy đủ - 0trang

570



H.D. May and K.R. Sowers



chlorinated ethenes, depending on the specific congeners. Third, PCBs accumulate and remain relatively stable in soils and aqueous sediments because of their

low vapor pressures. As a result of these characteristics the time required for a

PCB-contaminated site to recover cannot yet be predicted due in part to a lack of

quantitative information on rates of PCB dechlorination in the pore water phase.

Although rates of reductive dechlorination in sediments depend upon the specific activities and abundance of organohalide respiring microbes, in situ activity will also be influenced by the aqueous concentrations of the PCB congeners.

Several published reports suggest that substrates in nonaqueous phase solids or

liquids are unavailable for microbial uptake (Zhang et al. 1998). In early studies,

attempts to estimate dechlorination rates and the minimal threshold concentrations

for organohalide respiration of PCBs involved adding Aroclors above the aqueous saturation range to sediment microcosms and assaying the rates of reductive

dechlorination (Fish 1996; Rhee et al. 2001; Cho et al. 2002, 2003; Abramowicz

et al. 1993). The minimal threshold Aroclor concentration for reductive dechlorination in these studies ranged from 10 to 40 mg kg−1. The range of threshold

values observed in the reports are a reflection of the specific indigenous dechlorinating populations, the different Aroclors added (Aroclor 1242 or 1248) and

the sediment characteristics from different sources, which would affect the bioavailability. In contrast, Payne et al. (2011, 2013) observed dechlorination with

as low as 1.3 mg kg−1 weathered PCBs in sediments after bioaugmentation with

“D. chlorocoercia” strain DF-1, which indicated that low concentrations of PCBs

typically observed in the environment were indeed available for direct microbial

uptake. As these studies indicate, a major challenge with relating dechlorination

rate to PCB concentration in sediment has been accounting for bioavailability differences caused by the association of PCBs to different types of organic matter

(Ghosh et al. 2003). Perhaps a more appropriate metric that accounts for bioavailability to organisms in different sediment matrixes is to measure the dissolved

concentrations of PCBs in the pore water (Peijnenburg and Jager 2003; Friedman

et al. 2009).

Lombard et al. (2014) took advantage of recent advances in the use of polymer phase passive samplers for measurement, and for passive dosing of compounds, to measure PCB dechlorination rates at low, environmentally relevant

aqueous concentrations. Dechlorination rates of 2,3,4,5-tetrachlorobiphenyl to

2,3,5-trichlorobiphenyl by “D. chlorocoercia” strain DF-1 were measured over

a range of 1–500 ng L−1 in sediment-free medium using a steady-state concentration of cells (106 cells mL−1). The dechlorination rates of 2,3,4,5-tetrachlorobiphenyl over a range of initial concentrations were linear indicating first order

rate kinetics (Fig. 24.3). In addition, a minimum concentration threshold for

2,3,4,5-tetrachlorobiphenyl dechlorination was not detected with the size of inoculum used. Previous studies (Fish 1996; Rhee et al. 2001; Cho et al. 2002, 2003)

also reported first order rate kinetics, but the apparent minimal threshold was

several orders of magnitude greater since measurment included both PCBs in the

pore water and those adsorbed to sediment. Furthermore, Lombard et al. (2014)

observed higher rates up to 1000 fold more than reported previously. These rate



24 “Dehalobium chlorocoercia” DF-1—from Discovery to Application



571



Fig. 24.3  Accumulation

rate of product PCB 23aq

(2,3,5-trichlorobiphenyl)

plotted against concentration

of the substrate PCB 61aq

(2,3,4,5-tetrachlorobiphenyl),

a normal scale b

logarithmic scale at aqueous

concentrations of: 2.9,

2.9 × 10−1, 7.2 × 10−2,

2.9 × 10−2, 7.2 × 10−3 and

2.9 × 10−3 nM. Reprinted

with permission from

(Lombard et al. 2014).

Copyright 2014 American

Chemical Society



variations can be explained in part by differences in number and types of dechlorinating microorganisms present. Cho et al. (2003) reported that a fivefold difference in rates observed between two independent studies was negligible; when

rates were normalized to the number of microorganisms, slope variations could be

attributed to the cell (or more specifically the enzyme) affinity for a specific substrate. However, rate differences might also be explained by differences in buffering capacity of the associated sediments. Since only total PCBs were measured

and the pore water PCB concentrations were unknown in these earlier studies,

the kinetics of dechlorination for the bioavailable fraction of PCBs could not be

determined.

Lombard et al. (2014) observed no net population growth of “D. chlorocoercia” strain DF-1 at a cell density of approximately 1 × 106 cells mL−1 and

2,3,4,5-tetrachlorobiphenyl concentrations ranging from 1 to 500 ng L−1. The

thermodynamic cell yield of “D. chlorocoercia” strain DF-1 based on the estimated cell yield from oxidation of formate (Heijnen and van Dijken 1992) would

require 2.4 × 10−8 mol of 2,3,4,5-tetrachlorobiphenyl reduction to support one

doubling of 6 × 107 “D. chlorocoercia” strain DF-1 cells in a 50 mL microcosm.

At the highest PCB concentration tested in the study only 3.3 × 10−11 mol of

2,3,4,5-tetrachlorobiphenyl was dechlorinated, which is consistent with the lack



572



H.D. May and K.R. Sowers



Fig. 24.4  Simulation of

dechlorination profiles

for bioavailable PCBs in

sediment for different cell

densities based on aqueous

phase dechlorination

rates for PCB 61

(2,3,4,5-tetrachlorobiphenyl).

Reprinted with permission

from (Lombard et al. 2014).

Copyright 2014 American

Chemical Society



of detectable growth. The combined results of these studies suggest that bioavailability was not a factor in the apparent inhibition of dechlorinating activity at relatively high PCB concentrations observed in earlier studies, but rather was due to

low numbers of indigenous organohalide respiring microorganisms. Based on the

dechlorination rate for 2,3,4,5-tetrachlorobiphenyl by 106 cells mL−1, the estimated dechlorination rates were extrapolated for a range of organohalide respiring

population densities in a typical organic sediment matrix (Fig. 24.4). The results

show that although dechlorination likely occurs with low cell numbers, the rates

would be too low for short-term detection in many environments. Higher concentrations of PCBs or alternative organohalide substrates are required for sustained

growth of the organisms to reach population levels where substantial dechlorination can be observed within days or months. These kinetics and threshold data

indicate that “D. chlorocoercia” strain DF-1, and presumably other PCB organohalide respiring bacteria, are capable of dechlorinating PCBs at environmentally

relevant concentrations that are typically below saturation in water. Furthermore,

sufficient cell numbers of “D. chlorocoercia” strain DF-1 reductively dechlorinate

substantial levels of PCBs in days or months rather than years or decades. Using

passive sampling to measure the dissolved aqueous concentrations of different

PCB congeners and rates of PCB desorption from the sediment matrix, combined

with knowledge of the congener specificity of the organohalide respiring bacteria

used for bioaugmentation, it may be possible to project the rate and threshold levels of PCB dechlorination for a specific sediment site.



24.6 Dechlorination and Degradation of PCBs

in Contaminated Sediments and Soils

Aerobic bioaugmentation studies with bacteria, fungi, and plants to degrade PCBs

in soils and sediments have been done on a laboratory scale, but these processes

generally attack PCBs without substitutions in the 2,3 and 3,4 positions and



24 “Dehalobium chlorocoercia” DF-1—from Discovery to Application



573



have a limited capacity to attack highly chlorinated congeners often found in the

environment (Abraham et al. 2002). In contrast, microbial reductive dechlorination attacks more extensively chlorinated PCB congeners with flanked chlorines

ultimately complementing the aerobic process by removing 2,3 and 3,4 substitutions. Prior to identification and isolation of PCB respiring bacteria, attempts to

treat PCB contaminated sediments by bioaugmentation with PCB-enriched sediment yielded mixed results. Wu and Wiegel (1997) observed no significant stimulation and Bedard et al. (1997) observed only slight stimulation of weathered

Aroclor 1260 dechlorination in Housatonic River sediment bioaugmented with

PCB-enriched sediment slurries, although this activity could be increased by addition of 2,3,4,5,6-pentachlorobiphenyl as a biostimulant. Natarajan et al. (1997)

reported dechlorination of weathered Aroclor 1242 and 1248 in Raisin River sediment microcosms inoculated with microbial granules from an upflow anaerobic

sludge blanket digestor. However, the granules were not grown or maintained

with PCBs or other organochlorides. The study did not rule out the possibility that

the observed activity resulted from hydrogen generated from fermentative bacteria in the granules, which would stimulate indigenous PCB dechlorinating bacteria, rather than bioaugmentation with organohalide respiring bacteria. At the time

these reports were published PCB respiring bacteria had not yet been identified as

Dehalococcoides and related bacteria within the Chloroflexi. More recently, the

commercial PCB mixture Aroclor 1260 was reported to be significantly dechlorinated by a consortium consisting of one or more phylotypes within the Chloroflexi

enriched from sediment microcosms (Fagervold et al. 2007), sediment-free microcosms (Bedard et al. 2007) and by an individual strains of D. mccartyi (Fennell

et al. 2004; LaRoe et al. 2014; Adrian et al. 2009; Wang et al. 2014). The different dechlorination patterns observed in the environment have been attributed to the

diversity of indigenous PCB dechlorinators that attack different chlorine substituted positions within a congener. This conclusion is further supported in a report

by Fagervold et al. demonstrating that the overall pathway for dechlorination of

spiked Aroclor 1260 could be altered by adding different combinations of PCB

respiring bacteria to sediment microcosms (Fagervold et al. 2011). Since PCBs

generally accumulate in soils and sediments the challenges for their remediation

differ from those developed for volatile halogenated contaminants such as chlorinated ethenes in groundwater. Efforts to enhance the dechlorination of PCBs in

contaminated sediments with strain DF-1 and other organohalide respiring bacteria

are discussed below.



24.6.1 Biostimulation

There have been efforts to identify factors affecting dechlorination and degradation activities in laboratory microcosms (Abramowicz et al. 1993; Tiedje et al.

1993; Berkaw et al. 1996; Wu et al. 1996; Cho et al. 2004) with the goal of accelerating the natural processes in the environment. Although PCB respiring bacteria



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

5 Kinetics and Threshold Levels of PCB Organohalide Respiration

Tải bản đầy đủ ngay(0 tr)

×