Environ. Sci. Technol. 2005, 39, 3368-3373
Controlled Release of Nitrate and Sulfate to Enhance Anaerobic Bioremediation of Phenanthrene in Marine Sediments YINJIE J. TANG,† SHELLY CARPENTER,‡ JODY DEMING,‡ AND B A R B A R A K R I E G E R - B R O C K E T T * ,† Departments of Chemical Engineering and Oceanography, University of Washington, Seattle, Washington 98195
Experimental measurements demonstrated that rates of in situ microbial anaerobic biodegradation of phenanthrene in undisturbed marine sediments were enhanced when controlled-release electron acceptors (i.e., nitrate and sulfate (for comparison)) were employed. The experimental method used whole and interval cores injected with radiolabeled 14C-phenanthrene, which were incubated, sacrificed, and processed for 14CO2 recovery to determine degradation rates. Nitrocellulose and CaSO4 were formulated to release the electron acceptors into the sediments at rates consistent with bacterial utilization (i.e., that avoided inhibition observed previously at high soluble nitrate (although not sulfate) concentrations). The controlled-release of both compounds, measured in collateral experiments, enhanced the natural anaerobic phenanthrene biodegradation rates by factors up to 2-3. Biodegradation via sulfate reduction was most rapid in the early stages (24 days) of experiments, consistent with reports that many marine bacteria in submerged sediments are sulfate reducers. In comparison, in longer experiments (after 42 days), the anaerobic biodegradation rates were observed at least as high with the addition of nitrocellulose (over 40% of added phenanthrene recovered as 14CO2). Both nitrate and sulfate reduction was observed during anaerobic incubation, although the presence of nitrate seemed to reduce the sulfate reduction. The studied forms of the controlled-release nitrate and sulfate may provide capping amendments to decontaminate marine harbor sediments.
Introduction In some estuaries of Puget Sound, submerged marine sediments are severely contaminated by toxic polyaromatic hydrocarbons (PAHs) owing to the long use of creosote for wood preservation (1). A temporary solution for large area contamination is capping with clean dredge spoils (1, 2). While this method largely avoids contaminant dispersal, capping prevents the resupply of nutrient and electron acceptors from the water column and adversely affects the benthic ecosystem. In situ bioremediation, defined as inplace contaminant degradation by a variety of microorgan* Corresponding author phone: 01-206-543-2250; e-mail:
[email protected]. † Department of Chemical Engineering. ‡ Department of Oceanography. 3368
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 9, 2005
isms, is used in areas where the degradation rate is fast enough to meet EPA requirements (1-3). However, bioremediation is slow when insufficient oxidizers (or electron acceptors) limit the rate of degradation by microbial consortia (4, 5) especially at depths below the sediment-water interface (6, 7). After capping, the polluted sediments are separated from seawater (more like an artificial closed system). In this case, electron acceptors may eventually become very limited in such a closed system, leaving the PAHs in capped sediment well-preserved and undegraded. The addition of controlledrelease electron acceptors to enhance in situ biodegradation may be an effective way to ensure degradation of the PAHs capped sediment sites. Such an in situ bioremediation process could lead to both reduced thickness of the cap (a potential cost savings) and decreased risk of the release of soluble PAH into the seawater (through the cap) since PAH would be remineralized instead by an active bioremediation layer under the cap. Aged PAHs, not available for microbial action, would likely remain undegraded in the capped sediments; on the other hand, if ever becoming available, aged PAHs would be subject to degradation rather than to harmful release to the overlying seawater. Table 1 shows two important electron acceptors and their concentration profiles measured in our research site. Nitrate provides nearly as much energy for mineralization as oxygen; sulfate has a low free energy of reaction but is the predominant subsurface oxidant (4, 5). Adding either soluble sulfate or nitrate to act as electron acceptors under a variety of conditions (8, 9) accelerates anaerobic microbial degradation. However, to our knowledge, the effectiveness of amendments added below the capping material has not been studied systematically. Additionally, in open environmental site remediation (e.g., groundwater or submerged marine sediments), the repeated addition of soluble electron acceptors is required to maintain effective concentrations. Such treatments are costly and may cause toxicity to microorganisms and higher benthic organisms (9). A potential way to balance economics and ecological health is to provide a continuous, controlled release of electron acceptors directly into the sediment subsurface using a means that maintains effective concentrations for weeks to months. Two compounds, gypsum (CaSO4‚2H2O) and nitrocellulose, can slowly release sulfate and nitrate, respectively. Gypsum has a solubility of 14 mM in 25 °C water, within the range studied by Rothermich et al. (10). Nitrocellulose (cellulose nitrate) has three reactive NO2 groups per sugar unit (Figure 1) (11). It can slowly release nitrate and nitrite via hydrolysis or microbial transformation. The initial extraction of 1 g of nitrocellulose in 100 mL of water provides 0.29 mM nitrate in solution (12, 13). Under anaerobic conditions, nitrocellulose can be degraded to nitrate, nitrite, sugar, H+, and NH4+ that can be used by bacteria as electron acceptors and carbon sources (14, 15). The objective of this study is to measure the effectiveness of the controlled release of sulfate and nitrate, placed at depth, to enhance the rate of anaerobic biodegradation in undisturbed marine sediments. Since phenanthrene (a three-ring PAH) is designated an indicator PAH in Eagle Harbor, WA, our research site, 14C-phenanthrene was used as a radiolabeled tracer to sensitively monitor bacterial mineralization rates. Whole as well as interval cores of undisturbed sediment from Eagle Harbor were injected, incubated at 13 °C, and sacrificed to follow the time course of nitrocellulose and CaSO4 enhancement of anaerobic biodegradation as a function of depth. Sediment core methods measure fresh 10.1021/es040427w CCC: $30.25
2005 American Chemical Society Published on Web 03/22/2005
TABLE 1. Comparison of Electron Acceptor Characteristics in Marine Sediments electron acceptor
O2
NO3-
SO42C14 H10 + 8.25SO42- + 9H2O f 14HCO3- + 4.125HS+4.125H2S + 1.625H+ -1.51 0.06
Chemistry and Thermodynamics
phenanthrene mineralization reaction stoichiometrya
C14 H10 + 16.5O2 f 14CO2 + 5H2O
C14 H10 + 13.2H+ + 13.2NO3- f 14CO2 + 11.6H2O + 6.6N2
∆GΘ (kcal/equiv)b microbial yield to biomassb
-25.28 0.33
-23.73 0.31
occurrence depth range (cm)c background concentration range (µmol/L)c
0-3 0-1 cm: 30-300
0-3 0-3 cm: 3∼20
0 to >12 0-3 cm: ∼2.0 × 104
1-3 cm: < 3 below 3 cm: ∼0 limited by aqueous solubility
below 3 cm: ∼0
3-6 cm: ∼2.5 × 104 6-9 cm: ∼2.5 × 104 up to 30
Measurements Pertinent to Electron Acceptors in Eagle Harbor
apparent inhibition concn (mM)d
