Persistent organic pollutant destruction by sulfides and black carbons

Persistent organic pollutant destruction by sulfides and black carbons

Hydrophobic persistent organic pollutants (POPs), particularly those with nitro or halogen substituents, remain a critical and on-going, global concern. Although the production of polychlorinated biphenyls (PCBs) and chlorinated hydrocarbon pesticides (e.g., lindane) is being phased out worldwide, production of high volumes of a host of structurally similar compounds continues. As a result of their hydrophobic characteristics, these compounds tend to associate with sediments or bioaccumulate to high concentrations in the fatty tissues of organisms at the top of the food chain, including humans. Certain POPs are associated with toxicity, including carcinogenicity (e.g., PCBs) and liver toxicity (e.g., PBDEs).

Recent research indicates the dominant role that black carbons occurring in sediments, such as soots and chars, play in sorbing POPs. Indeed, activated carbon, a synthetic black carbon, has been applied to sediments to sequester POPs, dramatically reducing their bioavailability to benthic species. While their sorptive role is well-recognized, black carbons also exhibit reactive functional groups, such as quinones. Preliminary evidence indicates that black carbons may mediate reactions involving sulfides resulting in POP destruction. Sulfides are powerful reductants and nucleophiles, occurring naturally under the sulfate-reducing conditions prevailing in sediments, particularly in marine systems.

Sulfide destruction of the military explosive, RDX, by sulfides in the presence of black carbons.

Like TNT, the explosive compound RDX is a common contaminant of military bombing ranges, such as the Vieques bombing range in Puerto Rico. More effective in-situ RDX remediation techniques are needed. Via current in-situ techniques, including anaerobic bioremediation, reduction by zero-valent iron, and Fe(II) bound to magnetite, RDX destruction generates undesirable nitrosated byproducts such as hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX), and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX). In the case of military bombing ranges, RDX remediation using zero-valent iron cutoff trenches downstream of RDX plumes may not be feasible because the locations of the unexploded ordinance (UXO) serving as sources of RDX plumes are poorly characterized.

We report that RDX was rapidly destroyed by sulfides in the presence of black carbon, forming nitrite and formaldehyde, rather than toxic nitrosated reduction products. Although traditionally viewed as inactive sorbents, black carbons have been noted to participate in the destruction of certain contaminants, such as azo dyes, via quinonoid functional groups. However, in our experiments sulfide modification of quinonoid functional groups did not seem to be involved. Although at least 1.2 mM sulfides were needed for the reaction to proceed, these concentrations are found in certain marine sediments, together with black carbon. Compared with current in-situ remediation techniques which generate nitrosated byproducts, such as bioremediation and zero-valent iron cutoff trenches, this in-situ, abiotic remediation technique may be an attractive alternative. This technology may be particularly pertinent for unexploded ordinance (UXO) sites in marine areas (e.g., Vieques, Puerto Rico), where alternative in-situ technologies requiring excavations (e.g., zero-valent iron cutoff trenches) may be dangerous. The co-occurrence of sulfides and black carbons in these areas may provide a protective blanket underneath bombing ranges, where the locations of UXO are often poorly characterized.

For more information, see:

Kemper, J.M.; Ammar, E.; Mitch, W.A. Abiotic degradation of RDX in the presence of hydrogen sulfide and black carbon. Environ. Sci. Technol., 2008, 42 (6), 2118-2123. (Full Text)

Xu, W.; Dana, K.E.; Mitch, W.A. Black-carbon mediated destruction of nitroglycerin and RDX by hydrogen sulfide: relevance to in-situ remediation. Environ. Sci. Technol., 2010, 44, 6409-6415.(Full Text)