Source: Dissertation Abstracts International, Volume: 76-07(E), Section: B.

Advisers: William A. Mitch; Joseph J. Pignatello.

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Hazardous Organic Contaminants have been released to the environment via human activities during their applications as pesticides, flame-retardants, and explosives. These compounds may be bioaccumulative and toxic, and therefore pose risks to ecosystems and human life. They also tend to associate with black carbons in soils and sediments. Black carbons are a complex mixture produced by the incomplete combustion of fossil fuels or biomass and constitute 10 to 30 % of the total organic carbon in soils and sediments. They are the ultimate sink for many hazardous organic contaminants. Traditionally, environmental engineers assume that black carbons serve as passive sorbents, sequestering contaminants from biotic or abiotic transformation via aqueous phase reactions. However, chemical reactions mediated by black carbons are possible. Sulfides generated from biological sulfate reduction often co-occur with black carbons at concentrations up to 5 mM in soils and sediments. Sulfides are known to serve as both strong reductants and nucleophiles for contaminant destruction in the aqueous phase. However, their reactivity towards sorbed contaminants has not been characterized. I postulated that the surface of black carbons could enhance the reactivity of sulfides in contaminants transformation.

Therefore, whether black carbons shield or enhance the degradation of sorbed contaminants in the presence of sulfides is addressed in this thesis from three perspectives. First, I demonstrated that sorption to black carbon facilitates the sulfide-mediated degradation of two non-aromatic, nitrated compounds, nitroglycerin and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), both of which occur in estuarine sediments at naval bombing ranges. Destruction increased with sulfide and graphite concentrations, yielding relatively harmless products on the timescale of hours. To distinguish the reaction pathways, I then developed an electrochemical cell, which physically separated the hydrogen sulfide and target contaminants in separate cells, thereby preventing nucleophilic substitution reactions, but kept the cells electrically connected via sheet graphite electrodes, enabling redox reactions. I demonstrated that nitroglycerin destruction proceeded by a reduction, while RDX destruction appeared to involve a surface-assisted nucleophilic substitution reaction in the presence of black carbon and sulfides.

Second, I characterized the properties of black carbon responsible for promoting its reactivity by distinguishing two hypotheses: a) that oxygenated functional groups (i.e., quinones) in black carbons promote degradation of sorbed contaminants by hydrogen sulfide, or 2) that highly conductive graphitic regions facilitate electron transfer reactions from hydrogen sulfides. By developing techniques to tune each property without affecting the other, I demonstrated that the electrical conductivity of black carbon, rather than quinone functional groups, was responsible for the reactivity of black carbon with respect to RDX destruction. The results suggested that black carbons promoted the formation of reactive sulfur-based nucleophiles via oxidation of hydrogen sulfide, and that these nucleophiles were responsible for the destruction of RDX via a nucleophilic substitution reaction.

Third, I evaluated the applicability of black carbon-mediated reactions to a wide variety of contaminant structures, including nitrated and halogenated aromatic compounds, heterocyclic aromatic compounds, and halogenated alkanes. These are common structures in pesticides, flame-retardants, and personal care products. Apart from mentioned nitrated aliphatic compounds (i.e., nitroglycerin and RDX), transformation by sulfides appeared to be limited to nitrated aromatics. While both nitro- and halonitroaromatic compounds underwent black carbon-mediated reactions with hydrogen sulfide, product analysis indicated that the nitro group was the primary site of attack, forming an aniline derivative via reduction of the nitro group. I then demonstrated that the reactivity of nitrated aromatics towards reduction in the presence of black carbons and sulfides correlated with the calculated energy of the lowest unoccupied molecular orbital, suggesting that electron transfer to the target compound is rate-limiting. Observed destruction rates also correlated with black carbon conductivity. However, other reduced sulfur species (i.e., sulfite and thiosulfate) did not reduce nitroaromatics, even though they have a lower solution reduction potential than hydrogen sulfide.

In summary, my thesis work alters the predominant view of black carbons as non-reactive sorbents in sediments. The results indicate that sorption to black carbons promotes the chemical transformation of nitrated alkanes and aromatics by hydrogen sulfide. However, the pathways by which different nitrated organics react varied. The research lays the groundwork for understanding the natural attenuation of nitrated organic contaminants in soils and sediments, and for the application of black carbons as sediment amendments to foster the in-situ, abiotic transformation of contaminants.