Environ. Sci. Technol. 1998, 32, 2883-2886
Humic Substance Formation via the Oxidative Weathering of Coal SOOBUM CHANG* AND ROBERT A. BERNER Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, Connecticut 06520
Oxidative weathering of sedimentary organic matter in the Earth’s surficial environment is one of the major processes in the geochemical carbon cycle on geological time scales. It has been assumed in most geochemical models that there is complete oxidation of sedimentary organic matter only to CO2. However, studies have shown that humic substances can be produced via the oxidation of coal. We have determined the aqueous oxidation kinetics of pyritefree bituminous coal at 24 and 50 °C by using a dualcell flow-through method. At 24 °C, dissolved carbon is removed from the coal-water system mainly in the form of CO2 and is equivalent to 30-50% of the consumed oxygen. The remaining 50-70% of the consumed oxygen is retained on the coal surface in the form of insoluble organic oxidation products. Formation of greater proportions of dissolved organic oxidation products is expected under natural conditions where water-rock contact time is much longer than in our experiments (18-25 h). FTIR analysis indicates marked increases in carbonyl groups for coal oxidized in oxygenated water at 50 °C. Both dissolution of the solid oxidation products and the oxygen consumption rate should be accelerated by an increase in pH.
Introduction Oxidative weathering of sedimentary organic matter in the continental surficial environment is one of the major processes in the geochemical carbon cycle (1, 2). Sedimentary organic carbon is mainly in the form of kerogen disseminated in black shales and as coals. Complete oxidation to CO2 of sedimentary organic matter exposed to earth’s surficial environments has become one of the major assumptions in the studies of geochemical cycles. The weathering of organic matter is especially important because it is one of the major controls of the atmospheric oxygen level through geologic time (1, 3). Despite its importance, the aqueous oxidation rate of organic matter has not been determined at ambient temperature. This paper presents an initial attempt to attack this problem. Because of difficulty in obtaining pyrite-free marine kerogen, we chose to study low-sulfur coal as representing one type of sedimentary organic matter. In this way, the uptake of O2 via pyrite oxidation could be avoided. On the basis of the oxidation studies of coal in air, it has been believed that the weathering of coal occurs in two steps: A
B
Ccoal + O2 98 intermediates 98 CO2 The first step (A) of the overall reaction is the consumption * Corresponding author e-mail:
[email protected]; phone: (203)432-3182; fax: (203)432-3134. S0013-936X(98)00250-8 CCC: $15.00 Published on Web 08/27/1998
1998 American Chemical Society
of oxygen and the formation of intermediate products. The second step (B) is the eventual decay of the long-lived 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 oxidation process. Also, to understand the fate of oxidized coal in natural environments, the physical and chemical properties of the intermediate products in water should be 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 is alkali-extractable (5). Due to its high humic substance content, leonardite has been studied and used for various agricultural and environmental applications, including ion exchange and complexation with heavy metal ions (6). On the other hand, there have been no field observations regarding extensive humic substance formation via oxidative weathering of higher-rank coals such as bituminous coal. However, it has been known that humic substances can be produced in the laboratory via the oxidation of higher-rank coals. When bituminous coal is treated with various oxidizing agents such as hydrogen peroxide, nitric acid, or heated oxygenated air, so-called “regenerated” humic substances are formed (7, 8). Considering the relatively large amounts of sedimentary organic matter exposed to the earth’s surficial environment, it is important to ascertain if humic substances can be formed, not only as the products of lignite weathering but also as weathering products of higher-rank coals and possibly kerogen in black shales. We report the results of flow-through experiments which were performed to investigate the kinetics of the aqueous oxidation of pyrite-free bituminous coal. Oxygen consumption rates are compared with rates of oxidation product formation, and the implication of results for humic substance formation is discussed.
Materials and Methods In this study, a pyrite-free bituminous coal was used as an example of organic matter in sedimentary rocks. The bituminous coal sample R-57 from Utah was purchased from D. J. Mineral Kit Co. in Montana. The elemental composition of the coal is shown in Table 1. The analytical methods used to determine the pyrite sulfur, acid-extractable iron, inorganic carbon, and organic carbon content are described elsewhere (9-12). Total sulfur oxyanion content (sulfate and sulfite and thiosulfate) in the output solution was monitered by using a Dionex Ion Chromatography System and was less than 1% of the total oxygen consumption. Therefore, the oxygen consumption rate obtained with our experiments is not affected by the oxidation of pyrite or organic sulfur and is defined as the oxidation rate of coal organic matter alone. Because the purpose of this study is to understand the longterm effect of atmospheric oxygen on the chemistry of coal weathering, coal samples were preoxidized by grinding and then stored in air for more than 15 days. Figure 1 shows the dual-cell flow-through apparatus used in this study. The principle and technical details of the singlecell flow-through method are described elsewhere and will not be discussed extensively (13, 14). The advantage of this dual-cell apparatus is that one can measure a relatively small difference between two large values of oxygen concentration 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 to both cells can be canceled out when the reaction rate is calculated. The input solution was prepared by using oxygen VOL. 32, NO. 19, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Elemental Composition of Bituminous Coal wt % organic carbon carbonate carbon nitrogen total sulfur pyrite sulfura acid extractable iron a
71-73 0.4 1.7 0.5
