Environ. Sci. Technol. 1998, 32, 2883-2886 of oxygen and the formation of intermediate products. The Humic Substance Formation via the second step (B) is the eventual decay of the long-lived Oxidative Weathering of Coal intermediates and the formation of CO2 (4). Therefore, it is important to simultaneously measure the O2 consumption rate and the CO2 formation rate to understand the overall SOOBUM CHANG* AND oxidation process. Also, to understand the fate of oxidized ROBERT A. BERNER coal in natural environments, the physical and chemical Department of Geology and Geophysics, Yale University, properties of the intermediate products in water should be P.O. Box 208109, New Haven, Connecticut 06520 ascertained. Formation of humic substance by oxidative weathering of low-rank coals such as lignite is well-known. Weathered lignite is called leonardite, and up to 85% of its organic matter Oxidative weathering of sedimentary organic matter in the is alkali-extractable (5). Due to its high humic substance Earth's surficial environment is one of the major processes content, leonardite has been studied and used for various in the geochemical carbon cycle on geological time scales. agricultural and environmental applications, including ion It has been assumed in most geochemical models that exchange and complexation with heavy metal ions (6). On there is complete oxidation of sedimentary organic matter the other hand, there have been no field observations only to CO2. However, studies have shown that humic regarding extensive humic substance formation via oxidative substances can be produced via the oxidation of coal. We weathering of higher-rank coals such as bituminous coal. have determined the aqueous oxidation kinetics of pyrite- However, it has been known that humic substances can be free bituminous coal at 24 and 50 °C by using a dual- produced in the laboratory via the oxidation of higher-rank ° coals. When bituminous coal is treated with various oxidizing cell flow-through method. At 24 C, dissolved carbon is agents such as hydrogen peroxide, nitric acid, or heated - removed from the coal water system mainly in the form of oxygenated air, so-called ªregeneratedº humic substances CO2 and is equivalent to 30-50% of the consumed are formed (7, 8). Considering the relatively large amounts oxygen. The remaining 50-70% of the consumed oxygen of sedimentary organic matter exposed to the earth's surficial is retained on the coal surface in the form of insoluble environment, it is important to ascertain if humic substances organic oxidation products. Formation of greater proportions can be formed, not only as the products of lignite weathering of dissolved organic oxidation products is expected but also as weathering products of higher-rank coals and under natural conditions where water-rock contact time possibly kerogen in black shales. is much longer than in our experiments (18-25 h). We report the results of flow-through experiments which FTIR analysis indicates marked increases in carbonyl were performed to investigate the kinetics of the aqueous ° oxidation of pyrite-free bituminous coal. Oxygen consump- groups for coal oxidized in oxygenated water at 50 C. tion rates are compared with rates of oxidation product Both dissolution of the solid oxidation products and the formation, and the implication of results for humic substance oxygen consumption rate should be accelerated by formation is discussed. an increase in pH. Materials and Methods In this study, a pyrite-free bituminous coal was used as an Introduction example of organic matter in sedimentary rocks. The Oxidative weathering of sedimentary organic matter in the bituminous coal sample R-57 from Utah was purchased from continental surficial environment is one of the major D. J. Mineral Kit Co. in Montana. The elemental composition processes in the geochemical carbon cycle (1, 2). Sedimen- of the coal is shown in Table 1. The analytical methods used tary organic carbon is mainly in the form of kerogen to determine the pyrite sulfur, acid-extractable iron, inorganic disseminated in black shales and as coals. Complete carbon, and organic carbon content are described elsewhere oxidation to CO2 of sedimentary organic matter exposed to (9-12). Total sulfur oxyanion content (sulfate and sulfite earth's surficial environments has become one of the major and thiosulfate) in the output solution was monitered by assumptions in the studies of geochemical cycles. The using a Dionex Ion Chromatography System and was less weathering of organic matter is especially important because than 1% of the total oxygen consumption. Therefore, the it is one of the major controls of the atmospheric oxygen oxygen consumption rate obtained with our experiments is level through geologic time (1, 3). Despite its importance, not affected by the oxidation of pyrite or organic sulfur and the aqueous oxidation rate of organic matter has not been is defined as the oxidation rate of coal organic matter alone. determined at ambient temperature. This paper presents Because the purpose of this study is to understand the long- an initial attempt to attack this problem. Because of difficulty term effect of atmospheric oxygen on the chemistry of coal in obtaining pyrite-free marine kerogen, we chose to study weathering, coal samples were preoxidized by grinding and low-sulfur coal as representing one type of sedimentary then stored in air for more than 15 days. organic matter. In this way, the uptake of O2 via pyrite Figure 1 shows the dual-cell flow-through apparatus used oxidation could be avoided. in this study. The principle and technical details of the single- On the basis of the oxidation studies of coal in air, it has cell flow-through method are described elsewhere and will been believed that the weathering of coal occurs in two steps: not be discussed extensively (13, 14). The advantage of this dual-cell apparatus is that one can measure a relatively small + 9A8 9B8 Ccoal O2 intermediates CO2 difference between two large values of oxygen concentration The first step (A) of the overall reaction is the consumption in a blank cell and in a reactor cell. In this way, the effect of minor fluctuations in the oxygen level in the water supplied * Corresponding author e-mail: [email protected]; to both cells can be canceled out when the reaction rate is phone: (203)432-3182; fax: (203)432-3134. calculated. The input solution was prepared by using oxygen S0013-936X(98)00250-8 CCC: $15.00 1998 American Chemical Society VOL. 32, NO. 19, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2883 Published on Web 08/27/1998 TABLE 1. Elemental Composition of Bituminous Coal wt % organic carbon 71-73 carbonate carbon 0.4 nitrogen 1.7 total sulfur 0.5 pyrite sulfura <0.01 acid extractable iron 0.06 a Total reduced inorganic sulfur. FIGURE 2. Input system. photochemical reaction and microbial oxidation did not significantly affect rates of oxygen consumption during the experiments. Dissolved oxygen content was measured by Winkler titration (15). Dissolved CO2 content was determined by acid-base titration (16). Fourier transform infrared (FTIR) spectra were obtained using a Bio-Rad (Digilab) FTS 175 system. KBr pellets with the 2% coal sample were used to obtain the spectra. Both oxidized and unoxidized coal samples were prepared at the same time. This was done by using a KBr pellet frame made of two superimposed identical pieces of paper with each piece containing two 6 mm windows. The paper sample holders were placed between two 30 mm stainless steel disks, and the assembly was subjected to 25 000 kg for 3 min. The spectra of the two KBr pellets were measured using dry air as the background. For each measurement, the spectra are recorded using a resolu- tion of 2 cm-1 and by coadding 32 wavenumber scans FIGURE 1. Dual-cell flow-through apparatus. (interferograms). For each sample, the spectra are obtained every 0.4 mm across the KBr pellet for the total of 11-13 supply cartridges in which the oxygen level of the injected measurements (or a total of 352-416 scans). The average water is controlled by a diffusive process (see Figure 2). In of the measurements is calculated for both unoxidized and the reactor cell, oxidation reactions between powdered oxidized samples. The ratio spectra are obtained by dividing bituminous coal and dissolved oxygen occur and, as a result, the averaged spectra of the oxidized sample by the average the oxygen concentration decreases. Solutions coming out for the unoxidized sample. Although there is probably a of both cells are collected separately in 65 mL syringes for small difference between the thicknesses of the two KBr dissolved oxygen analysis. After being passed through the pellets, the baseline of the ratio spectra is usually flat, which syringes, solutions are collected for flow rate measurements. indicates the difference in KBr pellet thickness does not affect To prevent possible loss of dissolved oxygen, chromatography the ratioed spectra. The amount of dissolved organic carbon tubing (PEEK, poly-ether-ether-ketone) was used wherever (DOC) in solution was measured by using a Shimadzu Total possible. Organic Carbon Analyzer (model TOC-5000A). Each solution Experiments were performed at 24 and 50 °C. Each sample (5 mL) for DOC determination was acidified by adding experiment was performed by changing input oxygen 75 µL of 2 N HCl and purged with nitrogen for 10 min right concentrations and waiting generally a few thousand hours; before analysis to remove dissolved inorganic carbon. for a steady state. After a steady state of constant oxygen level was reached, oxygen consumption rates were deter- Results and Discussion mined. One or two CO2 titrations were performed following Experimental conditions and other parameters for two kinetic the oxygen consumption rate measurements.