Mn-Catalyzed Oxidation of Multiple-Ringed Aromatics

Abstract

The role of manganese (Mn) oxides in catalyzing the abiotic oxidation of polynuclear aromatic compounds (PNAs) has been investigated by monitoring the oxidation of dihydrodiol and dione PNAs and corresponding reduction of Mn in an aqueous environment. 1,4-, 1,3-, and 2,3-naphthalenediol, 1,4-naphthoquinone, and l,4-dihydroxy-9,10-anthracenedione (Quinizarin) were oxidized in an oxic, aqueous environment, where each of these organic compounds is soluble in water, except Quinizarin. A mathematical model is presented, which describes the redox reactions, and suggests that electron transfer/organic release from the oxide surface is rate limiting with the reaction apparently proceeding through a free-radical formation. Results indicate significant oxidation of the organics, that the oxidation of the diols is kinetically more favorable than that of the dione, and that lower molecular weight PNAs containing diol and dione functional groups are susceptible to abiotic-catalyzed oxidation, thereby suggesting that progressively more complex PNA molecules may also be susceptible to oxidation.

Mn-Catalyzed Oxidation of Naphthalenediol

“…Following previous research [5,21,22], Whelan and Sims [15] have suggested that the reductive-dissolution/organic-oxidation process could move through a free-radical formation prior to polymerization. Stone and Morgan [23] have mechanistically described this reductivedissolution/oxidation process in four steps: 1) precursor-complex formation (i.e., reductant adsorption), 2) electron transfer, 3) release of oxidized organic product, and 4) release of reduced metal ion.…”

“…The results in Figures 4 and 6 assume that separate organic controls are indicative of the initial concentration in the reaction vessel. Because separate reaction vessels are employed for the organic analyses (i.e., separate samples, controls, and blanks without subsampling), this assumption may be violated when variability between the controls are too great [15,16].…”

“…Reagent-grade chemicals were used in these experiments, including 2,3-naphthalenediol (Aldrich 98%). All standards, controls, and blanks were prepared in the same matrix as the samples following the same procedures outlined by Whelan and Sims [15].…”

“…The experimental design, following the procedures of Whelan and Sims [15] and Whelan [16], focused on monitoring Mn and the parent organic substrate using Inductively Coupled Plasma and High Performance Liquid Chromatography (HPLC), respectively. The Mn particles were prepared using a procedure based on Murray [17] and Godtfredsen [18].…”

“…They have suggested that the process is surface chemically controlled and not transport controlled. Whelan and Sims [15], therefore, assumed that electron transfer/release of aromatic organic is the rate-limiting step and reduced the four steps outlined above to the following equations, accounting for proton consumption: >MnIV(OH)2 + 2 Nap-(OH)2 < > >MnIV(0-Nap-OH)2 + 2 H20 (1) k-i k7 >MnIV(0-Nap-OH)2 + n H+ -> Mn2+ + H20 + oxidized products (2) where k1 and k2 are rate constants in the forward direction and k_-, is a rate constant in the reverse direction. Assuming that the only species that contribute to the surface mass balance equation are >MnIV(OH)2 and >MnIV(0-Nap-OH)2 [23,24], and that other competing anions and cations are not considered, the surface mass balance equation is simplified to:…”