Humic substance formation via the oxidative weathering of coal

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 pyrite-free bituminous coal at 24 and 50 degrees C by using a dual-cell flow-through method. At 24 degrees 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 degrees C. Both dissolution of the solid oxidation products and the oxygen consumption rate should be accelerated by an increase in pH.