Anaerobic Electron Acceptor Amendment for Aromatic Pollutant Biodegradation in Sediments

Recent studies in our laboratory and others have demonstrated conclusively that polycyclic aromatic hydrocarbons (PAHs) and other unsubstituted aromatics are biodegradable under anaerobic conditions. This important result suggests that PAH contamination in anaerobic environments (such as sediments) may be more amenable to biological treatment than once thought. In previous work, we postulated that the availability of anaerobic electron acceptors limits degradation of aromatic pollutants in sediment systems and that amendment with anaerobic electron acceptors could be exploited to remediate contaminated sediments. We termed this the Anaerobic Electron Acceptor Amendment (AEAA) process. This research consisted of two phases: Phase 1 was to quantify nitrate and sulfate reduction rates for test sediments from the study sites. These experiments consisted of incubations with test sediments amended with varying levels of nitrate or sulfate and incubated under field temperatures. In Phase 2 we amended sediment from the field sites with nitrate or sulfate and incubated them in the field over time to assess electron acceptor, sediment organic matter (SOM), and pollutant transformations. Electron acceptor utilization and SOM degradation rates were measured at temperatures that represented a year round range: 4° C (winter), 15° C (fall/spring), and 22° C (summer) to determine the temperature dependence for electron acceptor utilization coupled to SOM degradation. Concurrent with this work, we developed a diffusive flux model of electron acceptor transport in sediments. These results were used to develop comparisons of cost effectiveness between the AEAA process and other treatment technologies. The diffusive flux model of nitrate and sulfate release from the microcosms showed that the process of bioturbation has a dramatic effect on the loss of electron acceptor out of the cap due to the greatly increased advective flux. Preliminary cost analysis suggests that SOM-degradation related electron acceptor utilization does not represent an insurmountable cost barrier. Costs based on utilization rates predicted in this study suggest that the AEAA process may be competitive with existing sediment treatment technologies. INTRODUCTION Contaminated sediments are a substantial management problem in terms of disposal and treatment costs, as well as from a human health standpoint. Current estimates of contaminated sediment disposal requirements range from 14 to 28 million cubic yards (cy) per year (NRC, 1997). At estimated treatment costs ranging upwards from $50 to well over $150/cy, this represents a $1-3 billion annual cost. Generally, treatment technologies costing more than $50/cy remain prohibitively expensive (NRC, 1997). With limitations on monetary resources, cheaper alternatives are needed for contaminated sediment treatment. Now that ocean disposal of contaminated sediment is no longer permitted, the primary treatment options are sediment confinement and/or removal; either by capping with clean sediment, transfer to confined disposal basins, or transfer to landfills. Nearly all present chemical and biological treatment technologies proposed for sediment treatment are too expensive to compete with sequestration techniques (NRC, 1997). In the case of Polycyclic Aromatic Hydrocarbons (PAHs), one of the most common organic sediment pollutants (NRC, 1997), bioremediation-based treatment scenarios suffer from several potential drawbacks. Although most PAHs with fewer than five rings can be degraded aerobically, most contaminated sediments are anaerobic. Aerobic treatment of PAHs is problematic in the sediment environment because of the buoyancy and low solubility of oxygen. We have demonstrated previously that PAHs are transformed under strictly anaerobic conditions in marine sulfateand nitrate-reducing enrichments (Rockne and Strand, 1998), denitrifying enrichments (Rockne and Strand, 2001), and by microbial isolates (Rockne et al., 1999; Rockne et al., 2000). However, PAH biodegradation in oxygen-limited conditions is at present poorly understood and likely limited by availability of anaerobic electron acceptors (Rockne, 1997). Anaerobic electron acceptors such as nitrate or sulfate do not have several of the problems associated with aerobic treatment, and they may potentially be cheaply amended to contaminated sediments. There are two potential delivery methods for the blending of electron acceptor salts with contaminated sediments: in situ injection and ex situ chemical blending. The first technique involves injection of salt slurries into contaminated sediments, a technique that has been used to reduce toxicity in PAH-contaminated sediment in Hamilton Harbor, Ontario by Environment Canada and Golder and Associates. Ex situ chemical blending could easily be combined with hydraulic dredging techniques currently employed for contaminated sediment removal processes (NRC, 1997). Sediment is removed in slurry by hydraulic dredging and is typically transported to containment facilities such as underwateror nearshoreconfined disposal facilities (CDFs), or for landfilling. The contaminated sediment slurry is typically pumped either directly from the contaminated site or transported by barge to the facility and pumped into the CDF. At this point, an anaerobic electron acceptor slurry could be mixed with the dredge slurry and become buried under subsequent sediment additions. Final closure of the CDF would permanently shut out oxygen, negating the possibility of aerobic degradation of the contaminants. With the addition of sulfateor nitrate-salts however, anaerobic degradation of the contaminants could occur over time. Although the rates of anaerobic PAH degradation are slower than aerobic rates (Rockne and Strand, 1998), extended time periods would be available for biodegradation of the contaminants in either a confined disposal system or in situ. The cost of a potential new sediment treatment technology is critical to its practical use. As mentioned above, treatment costs above $50/cy are probably prohibitively expensive for large volume treatments. In an analysis of nitrate amendment dosing requirements, Rockne (1997) reported nitrate-loading requirements of 0.1% sodium nitrate salt by weight (approximately 1 lb/cy) for biodegradation of PAHs in creosote-contaminated marine sediments at Eagle Harbor, Puget Sound Washington, USA. With advances in precision dredging technology and slurry pumping (NRC, 1997), costs for anaerobic electron acceptor amendment have the potential to be relatively low. Because of the high levels of sediment organic matter (SOM) typically found in contaminated sediments, anaerobic electron acceptor levels are typically limiting to oxidation processes (Rockne, 1997). For example, even in highly contaminated systems like Eagle Harbor, total PAH concentrations were only a few percent of the total organic matter (Rockne, 1997). Whether the SOM is refractory or represents a sizable electron acceptor sink is a critical question in understanding the dynamics of anaerobic biodegradation processes in contaminated systems. The extent t degraded by bacteria, and what ty refractory, is critical to understanding sink. A major question of the propo metabolism of potentially more labil biodegradation of the organic pollutan