Seawater leaching investigation of polychlorinated biphenyls from solid matrices

Leaching behaviors for polychlorinated biphenyls in solid materials (PCBs-ISM) found onboard ex-Navy vessels have been investigated under laboratory-simulated shallow water (artificial reef) conditions. The following broad classes of shipboard solids were the focus of this study: electrical cable, felt gasket, bulkhead insulation, foam rubber, black rubber, and paint. Experiments were performed at 25 degrees Celsius to approximate sea-surface temperatures found in United States coastal areas (shallow/reef conditions). Average leach rates were determined by continuously exposing the solids to seawater under dynamic laboratory-simulated reef conditions for 14-16 months. The PCB mass released from each solid was determined from the PCB mass present in the seawater volume surrounding the solid at sequential time intervals over the course of the total exposure or leaching time. In general, results of these leaching experiments indicate that PCB release from shipboard solid matrices is very low and attenuated to varying degrees as a function of solid matrix type. The shipboard-solid-specific leaching behaviors were quantified as a function of time to evaluate temporal effects related to shipboard solid conditioning and PCB depletion. None of the materials tested exhibited a constant PCB release rate; rather, the leach rates were variable and changed as a function of exposure time. This type of behavior is characterized by an initial leach rate increase, followed by a leach rate maximum, beyond which the leach rate decreases monotonically over a significantly longer timeframe. The required exposure time to reach a maximum leach rate was variable for each solid, from immediate (days) to gradual (months), suggesting differing degrees of initial conditioning for each solid upon seawater exposure. Leach rates observed over the course of each leaching experiment for any given solid were observed to span several orders of magnitude. Upon comparing the leach rate behaviors observed for different shipboard solids, these differences also spanned several orders of magnitude, indicating that PCB release is also affected to varying degrees by different solid matrices. Although these warm temperature leach rates were the primary focus of the effort reported here, these studies were complemented by a set of experiments performed to investigate thermodynamic effects for lower temperatures expected at greater ocean depths (4 degrees Celsius). These cold temperature leach rate behaviors and relative leach rate rankings were similar to the 25 degrees Celsius leach rate results. However, 4 degrees Celsius leach rate magnitudes were significantly suppressed and slower initial kinetics were observed during the initial conditioning period. This temperature dependence is significant for the sinking of vessels as artificial reefs in cold shallow water, in addition to the sinking of vessels in deeper ocean environments. PCB leach rate results and data treatments will be presented, with particular emphasis on using these empirical data in risk assessments evaluating the re-use of ex-Navy vessels for constructing artificial reefs.