Underwater unexploded ordnance (UXO) casings eventually corrode and release their munitions and propellants into the overlying waters. Over time biological, chemical, and physical processes transform these munitions constituents into other compounds that have different transport and toxicity properties depending on the ecosystem. Previous studies have been conducted on the transport and transformation of munitions compounds in coastal aquatic systems. In the area of photolysis, recent work has suggested that the rate of transformation of compounds such as 2,4,6-trinitrotoluene (TNT) is rapid in seawater, but very little work has been completed to determine what products are formed in a saltwater environment and the rate at which those products undergo photochemical decay. Rapid transformation rates of some munitions in marine waters through photolysis, dilution, and metabolism suggest that these munitions may not pose a long-term concern. However, if the transformation products are more toxic than the parent compound, the liability of the Department of Defense (DoD) remains. In addition, if the transformation pathways and rates are not fully characterized in marine systems, the lack of scientific information could lead to poor remediation and management decisions.
The objective of this work was to improve the understanding of photolysis as a mechanism to transform chemical constituents released from munitions. Specifically, the goals were to determine how photolysis rates of 2,6-dinitrotoluene (2,6-DNT), 2,4-dinitrotoluene (2,4-DNT), and TNT were impacted by water source and specific dissolved materials (nitrate, dissolved organic matter [DOM]) and to determine the photoproducts and their photolysis rates.
Munitions constituents were dissolved in various natural and laboratory waters and in some cases were augmented with nitrate or DOM. The solutions were irradiated in thermostated quartz optical cells in a Suntest CPS+ solar simulator. High performance liquid chromatography (HPLC) was used to measure concentrations. Photolysis products were extracted using solid phase extraction and eluded using various solvents. The eluents were analyzed using liquid chromatography/mass spectrometry (LC/MS) to identify the products.
The impact of water type on photolysis was tested using four near-shore seawater samples, four estuary or river water samples, and ultrapure water. The decline in DNT or TNT concentration due to photolysis was modeled as a first-order process. The photolysis rate constants for each munitions constituent showed little variation in the four seawater samples tested, which had similar salinity and DOM. These rate constants differed by as much as 50% from those found using estuary and river water samples, which have lower salinity and higher DOM. The addition of DOM as humic acid to ultrapure water increased the 2,4-DNT and 2,6-DNT rate constants. The addition of humic acid sodium salt to seawater increased the photolysis rate constants for 2,6-DNT, but it decreased the rate constants for 2,4-DNT. The addition of nitrate enhanced photolysis in ultrapure water, but did not affect photolysis in seawater. For these experiments, the proof-of-concept was achieved in that differences were found in different water types, and one factor affecting the rates is dissolved organic matter. Future work should include testing waters from other Navy locations and determining what additional solutes alter photolysis rates.
The products formed through DNT photolysis included dinitrobenzyl alcohols (DNBOHs), dinitrobenzaldehydes (DNBCHOs), and 2-amino-4-nitrobenzoic acid (2A4NBA). When photolyzing these products, the rate constants for DNBOHs and DNBCHOs were larger than those of their parent DNT, but 2A4NBA was not photolyzed. For these experiments, the proof-of-concept was achieved in that the photolysis mechanism is more clearly understood. Future work should determine other photoproducts and their photolysis rates.
This research has shown that munitions constituents will photolyze at different rates in natural waters that vary in salinity and DOM. Upon photolysis, some compounds are produced that photolyze faster than their parent compound and are less likely to accumulate in environment, while other products do not photolyze and will accumulate if not transformed by other mechanisms. These results have increased our understanding of photodegradation mechanisms of munitions and will help DoD assess the environmental impact of UXO on coastal ranges and dumping sites and plan for the future use of these ranges.