Sodium perchlorate

Biotransformation and kinetics

Administrative data

Endpoint:

biotransformation and kinetics

Type of information:

experimental study

Adequacy of study:

supporting study

Data source

Reference

Reference Type:

publication

Title:

DEGRADATION KINETICS OF PERCHLORATE IN SEDIMENTS AND SOILS

Author:

Tan K, Anderson TA and Jackson WA

Year:

2004

Bibliographic source:

Water..4ir: cmd Soit Pollution 151: 245-259, 2004.

Materials and methods

Test guideline

Qualifier:

no guideline followed

GLP compliance:

no

Type of medium:

terrestrial

Test material

Test material information

Constituent 1

Reference substance name:

Perchlorate

IUPAC Name:

Perchlorate

Results and discussion

Transformation products:

not measured

Applicant's summary and conclusion

Executive summary:

Perchlorate-reducing bacteria are believed to be distributed ubiquitously in the environment. Microcosm studies using site sediment and water collected from four natural habitats near NWIRP, McGregor have indicated that rapid intrinsic bio-remediation is possible in the stream sediments which are continuously or intermittently exposed to perchlorate. Microcosm treatments using soil from another perchlorate­contaminated site (LHAAP) also were capable of perchlorate degradation although a long lag period (up to 60 days) may be necessary, depending on the environmental conditions. Intrinsic perchlorate degradation rates ranged from 0.13 to 0.46 day^-' for four sediments, corresponding to a half-life (t1/2= 0.693/k) range of 1.4 days to 5.0 days, with variation in rates depending mainly on the organic substrate availability.

Although it is clear that nitrate does interfere with perchlorate degradation, to date the pathway and mechanism involved is poorly understood. Most, but not all, perchlorate reducers could use nitrate as an electron acceptor and some denitrifying bacteria are capable of perchlorate degradation. Recently, some authors held the view that separate terminal reductases capable of reducing other electron acceptors were responsible for perchlorate and nitrate degradation in an isolated bacterial strain. However, the strain grew more rapidly with nitrate. The current results support this hypothesis. Based on those results, it could be extrapolated that the microorganisms in the sediments and soil from this study can use both perchlorate and nitrate as alternative electron acceptors, depending on the relative electron equivaience ratio. In the presence of relatively high nitrate concentration, the bacteria will preferentially use nitrate as an electron acceptor because growth on nitrate is much faster. After nitrate has been depleted, perchlorate reductase will function to use perchlorate as an electron acceptor. Thus, the presence of nitrate only affects the lag time of perchlorate degradation under the assumption that organic substrate availability is not a limiting factor. The fact that perchlorate degradation rate (Kmy) of HW84 Sidestream remained almost constant even if the NO₃^--N

was lowered to 1.0 mg/L, compared to 7.6 mg/L in the control treatment, also supported this assumption. If only one enzyme was involved, perchlorate and nitrate would become competitive inhibitors, and we should see a significant increase in perchlorate degradation rate because of the depletion of the competitive electron acceptor nitrate. From the point view of thermodynamics in terms of energy yield of electron acceptors, perchlorate's energy yield (Gibb's free energy per electron equivalent deltaG'_o=-112.1kJ/e^-) as an electron acceptor is similar to that of nitrate (deltaGo = -112.2kJ/e^-), when hydrogen is used as the electron donor. This implies that denitrification is not more energy-favorable than perchlorate degradation. The preference of perchlorate to NO₃^- as electron acceptor should be associated with a different enzyme involved which lowered the activation energy. Further research should be conducted to fully understand the metabolism involved. The presence of nitrate may explain the persistence of perchlorate in the environment, especially when perchlorate concentration is considerably lower than that of nitrate (i.e. most groundwaters), which may require a relatively longer lag time for perchlorate degradation to happen. Our studies also indicate that higher organic substrate availability can shorten the lag time of both perchlorate and nitrate degradation.

An attempt was made to correlate environmental conditions (i.e., organic sub­strate availability (represented by TVS), and/or nitrate concentration) with degradation rates and lag times. In general, no correlations were found, probably due to limited data as well as the complexity of the environmental system. Perchlorate degradation in the sediments and soil is affected by numerous environmental conditions (i.e., substrate, perchlorate concentration, population of perchlorate reducers, nitrate), and other factors.

Information generated from this study is useful in understanding the fate of perchlorate in sediment and soil, and highlighting the potential rote of intrinsic degradation or natural attenuation of perchlorate degradation.