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 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 should be P.O. Box 208109, New Haven, Connecticut 06520 ascertained. Formation of humic substance by oxidative weathering of low-rank such as is well-known. Weathered lignite is called leonardite, and up to 85% of its 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 , or heated - removed from the coal water system mainly in the form of oxygenated air, so-called “regenerated” 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 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 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. Additional experiments are shown in Table 2. Figure 3 shows the plot experiments were performed to test the effect of light and of O2 consumption rates (black symbols) and CO2 production microbes on the oxygen consumption rate. To test the effect rates (white symbols) versus O2 concentration. All data are of light on the reaction rate, an experiment was performed from COAL25-127 and were collected between 2000 and under continual light conditions until the oxygen level 5900 h of total duration (see Table 2). No significant reached a long-term steady state, after which an aluminum correlation was found between the CO2 production rate and foil cover was placed on the reactor cell to block the light. the oxygen level. On the other hand, a positive correlation In another experiment, the effect of microbes on the oxygen is shown between the O2 consumption rate and the oxygen consumption rate was tested by injecting 275 µM mercuric concentration. These results show that the CO2 production chloride over a 15 h period after reaching a steady state. In rate is fairly constant and independent of either oxygen level both experiments, reaction rates were monitored for 200- or O2 consumption rate, which strongly suggests that the 300 h after the tests were started. The results indicate oxygen consumption reaction and CO2 formation reaction

2884 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 19, 1998 TABLE 2. Experimental Conditions for Kinetic Experiments COAL25-127 COAL50-001 temperature (°C) 24.0 ( 0.5 50.0 ( 0.1 flow rate (mL/min) 0.32-0.40 0.40-0.85 initial sample weight (g) 35.0 35.0 final sample weight (g) 34.5 33.5 initial surface area (m2/g) 3.93 0.71 final surface area (m2/g) 6.92 1.06 duration (h) 8300 4300 % OC oxidizeda 0.88 0.80 a OC oxidized, % of organic carbon oxidized ) (integrated amount of O2 consumption in water)/(initial organic C content) × 100.

FIGURE 4. Results of FTIR analysis. All samples are from COAL50- 001. Samples were dried and stored in a vacuum at room temperature. A coal-KBr mixture was prepared by mixing a 40 mg sample of coal and 2000 mg of KBr powder, and only 50 mg of the mixture was used for each analysis: (A) spectrum obtained from starting material FIGURE 3. CO2 production rate and O2 consumption rate vs [O2]. All (i.e., coal before the experiment), (B) spectrum obtained from final data are from COAL25-127: (black symbols) O2 consumption rate material (i.e., coal after the experiment), and (C) ratio spectrum (error ) 2 standard deviations) and (white symbols) CO2 production -1 (B/A). Note that CO2 peaks around 2400 cm are due to incomplete rate (errors are based on the analytical error of 5 µM in dissolved purging and should be disregarded. CO2 measurements).

functional groups. If the CO2 production rate is proportional are separate kinetic processes under the given experimental to the amount of reactants available (i.e., oxygen-containing conditions. functional groups on the coal surface), it is possible that the Figure 4 shows FTIR spectra of the initial (before experi- CO2 production rate is eventually affected by the oxygen ment, marked as A) and final (after experiment, marked as consumption rate over a much longer time scale because B) coal samples for COAL50-001. The ratio spectrum of faster oxygen consumption could increase the amount of B/A was obtained to estimate the relative changes in the oxygen-containing functional groups available for the de- amount of organic functional groups. The result is shown composition reaction. Simultaneously, these oxygen- as an absorbance ratio spectrum (C). The major peak containing functional groups formed via oxidative weathering - centered at 1730 cm 1 indicates a marked increase of carbonyl of coal could be involved in various chemical reactions until groups such as esters, , aldehyde, and ketone they are decomposed to CO2 and other products. groups. A relatively small but significant increase at around The ratio of the CO2 production rate to the O2 consumption - 1570 cm 1 is assigned to a carboxylate (carboxylic salts) band. rate may be higher than 30-50% in natural subsurface An increase in the hydroxyl group either in phenolic or in environments where the water flow rate is substantially slower - carboxylic forms (around 3500 cm 1), a decrease in aliphatic than the ones used in the kinetic experiments. On the other - C-H stretching peaks (between 2800 and 3000 cm 1), and hand, a slower flow rate may instead result in higher -1 an increase in ether bonding (between 1100 and 1400 cm ) concentrations of DOC (as opposed to CO2) due to longer are also identified. Increases in the levels of oxygen- contact time between the groundwater and solid oxidation containing functionalities and decreases in aliphatic C-H products. The residence time of water in the reactor cell peaks strongly indicate that substantial amounts of solid was only 18-25 h in most of the flow-through experiments. organic oxidation products are formed during the experiment Batch experiments performed in our laboratory indicate that and retained on the coal surface. the DOC content of aerated water in contact with bituminous Blank and reacted solutions collected from 24 and 50 °C coal increases steadily as a linear function of time up to 7 experiments were analyzed for DOC contents. No significant years (17). Therefore, one can speculate that in subsurface amount of DOC was found in any of these samples (results environments where groundwater flow is slow, oxygen not shown). The detection limit of the DOC analyzer (10 consumption via coal oxidation could be balanced by -12 -1 µmol of DOC/L) is equivalent to 1.4 × 10 mol of DOC g dissolved CO2 and DOC liberated to solution. s which is relatively insignificant compared with CO2 It should be noted that the pH of the input solution used production rates (Figure 4). Therefore, under our experi- in our study was in the range of 5-6 due to the equilibration mental conditions, oxidation products are in the form of of the solution with atmospheric CO2. Because pH is a major either dissolved CO2 or insoluble solids retained on the coal factor affecting the dissolution of humic substances (18), surface. For the 24 °C experiment, CO2 production rates future research should focus on the effect of pH on DOC were 30-50% of the oxygen consumption rates, which means formation due to oxidative weathering of coal at various that the remaining 50-70% of the oxygen consumed is oxygen concentrations. Our preliminary results indicate a retained on the coal surface in the form of oxygen-containing very large acceleration of oxidative coal weathering at pH 13

VOL. 32, NO. 19, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2885 (19). If the DOC production rate is faster at higher pH, (5) Broughton, P. L. J. Sediment. Petrol. 1972, 42, 356-358. regenerated humic substances mobilized in the form of DOC (6) Westall, J. C.; Jones, J. D.; Turner, G. D.; Zachara, J. M. Environ. - can be quantified in terms of humic and fulvic . Sci. Technol. 1995, 29, 951 959. The role of microbes in coal oxidation was not covered (7) Calemma, V.; Iwanski, P.; Rausa, R.; Giradi, E. Fuel 1994, 73, 700-707. in this study. In natural environments, microbes can enhance (8) Sigh, K. P. Chem. Eng. J. 1996, 63, 189-194. oxidation rates of sedimentary organic matter by producing (9) Canfield, D. E.; Raiswell, R.; Westrich, J. T.; Reaves, C. M.; Berner, oxidizing agents such as peroxides and by using them to R. A. Chem. Geol. 1986, 54, 149-155. attack organic structures of coal (20-22). Therefore, oxida- (10) Berner, R. A. Am. J. Sci. 1970, 268,1-23. tion rates measured in this study are probably slower than (11) Raiswell, R.; Buckley, F.; Berner, R. A.; Anderson, T. F. J. Sediment. the rates found in nature. Petrol. 1988, 58, 812-819. - Finally, organic matter in coal is of terrestrial origin and (12) Krom, M. D.; Berner, R. A. J. Sediment. Petrol. 1983, 53, 660 663. has significantly different chemical structures compared with (13) Lasaga, A. C. J. Geophys. Res. 1984, 89, 4009-4025. most kerogen types of aquatic (lacustrine and marine) origin. (14) Nagy, K. L.; Lasaga, A. C. Geochim. Cosmochim. Acta 1992, 56, For example, coal contains greater amount of aromatic 3093-3111. carbon and phenolic functional groups than aquatic (15) Strickland, J. D. H.; Parsons, T. R. A practical handbook of (23). Therefore, it is imperative to study the oxidative seawater analysis, 2nd ed.; Bulletin of Fisheries Research Board weathering of aquatic kerogens to fully understand the of Canada, 167; Fisheries Research Board of Canada, 1972; 310 weathering of sedimentary organic matter exposed to earth’s pp. (16) Stumm, W.; Morgan, J. J. Aquatic chemistry, 2nd ed.; Wiley- surficial environments. Interscience: New York, 1981; 780 pp. (17) Chang, S. Coal weathering and the geochemical carbon cycle. Acknowledgments Ph.D. Thesis, Yale University, New Haven, CT, 1997. We are grateful to Ian MacInnis, John Hedges, Tony Lasaga, (18) Hedges, J. I.; Oades, J. M. Org. Geochem. 1997, 7/8, 319-361. Phil Ihinger, and Steve Petsch for assistance and helpful (19) Chang, S.; Berner, R. A. Unpublished results. discussions. Acknowledgment is made to the donors of the (20) Faison, B. D.; Woodward, C. A.; Bean, R. M. Appl. Biochem. Biotechnol. 1990, 24/25, 831-841. Petroleum Research Fund, administered by the American (21) Crawford, D. L.; Nielsen, E. P. Appl. Biochem. Biotechnol. 1995, Chemical Society, for partial support of this research under 54, 223-231. Grant 29132-ACS. (22) Ralph, J. P.; Graham, L. A.; Catcheside, D. E. A. Appl. Microbiol. Biotechnol. 1996, 46, 226-232. Literature Cited (23) Mann, A. L.; Patience, R. L.; Poplett, I. J. F. Geochim. Cosmochim. - (1) Berner, R. A. Palaeogeogr., Palaeoclimitol., Palaeoecol. 1989, Acta 1991, 55, 2259 2268. 75,97-122. (2) Hedges, J. I. Mar. Chem. 1989, 39,67-93. Received for review March 16, 1998. Revised manuscript (3) Berner, R. A.; Canfield, D. E. Am. J. Sci. 1989, 289, 333-361. received June 1, 1998. Accepted June 15, 1998. (4) Nelson, C. R. Chemistry of coal weathering. Coal Sci. Technol. 1989, 14,1-32. ES9802504

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